The Younger Dryas is considered the archetypal
millennial-scale climate change event, and identifying its cause is
fundamental for thoroughly understanding climate systematics during
deglaciations. However, the mechanisms responsible for its initiation remain
elusive, and both of the most researched triggers (a meltwater pulse or a
bolide impact) are controversial. Here, we consider the problem from a
different perspective and explore a hypothesis that Younger Dryas climate
shifts were catalysed by the unusually sulfur-rich 12.880 ± 0.040 ka BP
eruption of the Laacher See volcano (Germany).
We use the most recent chronology for the GISP2 ice core ion dataset from the
Greenland ice sheet to identify a large volcanic sulfur spike coincident with
both the Laacher See eruption and the onset of Younger Dryas-related cooling
in Greenland (i.e. the most recent abrupt Greenland millennial-scale cooling
event, the Greenland Stadial 1, GS-1). Previously published lake sediment and
stalagmite records confirm that the eruption's timing was indistinguishable
from the onset of cooling across the North Atlantic but that it preceded
westerly wind repositioning over central Europe by ∼ 200 years. We
suggest that the initial short-lived volcanic sulfate aerosol cooling was
amplified by ocean circulation shifts and/or sea ice expansion, gradually
cooling the North Atlantic region and incrementally shifting the midlatitude
westerlies to the south. The aerosol-related cooling probably only lasted
1–3 years, and the majority of Younger Dryas-related cooling may have been
due to the sea-ice–ocean circulation positive feedback, which was
particularly effective during the intermediate ice volume conditions
characteristic of ∼ 13 ka BP. We conclude that the large and
sulfur-rich Laacher See eruption should be considered a viable trigger for
the Younger Dryas. However, future studies should prioritise climate
modelling of high-latitude volcanism during deglacial boundary conditions in
order to test the hypothesis proposed here.
Introduction
The Younger Dryas (YD) climate anomaly occurred during the last deglaciation
and is often described as a brief return to near-glacial conditions in
northern Europe. Research now indicates that the YD was indeed characterised
by cold conditions across the North Atlantic and Europe (Carlson et al.,
2007; von Grafenstein et al., 1999) but also by a southward-shifted westerly
wind belt over Europe (Bakke et al., 2009; Brauer et al., 2008; Lane et al.,
2013a; Baldini et al., 2015b), a southward-shifted Intertropical Convergence
Zone (Shakun et al., 2007; Chiang and Bitz, 2005; Bahr et al., 2018),
increased moisture across the southwest of North America (Polyak et al.,
2004; Asmerom et al., 2010), and potential warming in parts of the Southern
Hemisphere (Bereiter et al., 2018; Kaplan et al., 2010).
A common explanation for the YD involves meltwater-induced weakening of
Atlantic Meridional Overturning Circulation (AMOC) (Berger, 1990; Alley,
2000; Broecker et al., 2010; Johnson and McClure, 1976; Schenk et al., 2018).
Initial support for this theory included elevated δ18O values in
Gulf of Mexico sediment dating from the early YD (implying that meltwater was
rerouted elsewhere) (Broecker et al., 1988; Flower and Kennett, 1990; Teller, 1990). The meltwater was originally
proposed to have travelled to the North Atlantic via the St Lawrence Valley,
but results have so far revealed only limited geological evidence of a
massive flux of freshwater coincident with the YD initiation (Broecker,
2006a; Rayburn et al., 2011). The freshwater pulse may have followed another
route to the ocean, and other research has proposed the Mackenzie Valley
(Condron and Winsor, 2012; Murton et al., 2010) as a possible alternative and
the Fennoscandian Ice Sheet as an alternate source (Muschitiello et al.,
2015). Very recent surface exposure ages suggest that the route to the North
Atlantic from Lake Agassiz was indeed free of ice before the YD initiation
(Leydet et al., 2018), coinciding with evidence of freshening of the Gulf of
St Lawrence (Levac et al., 2015), illustrating that meltwater could indeed
have followed the St Lawrence Valley route to the North Atlantic.
Nevertheless, the issue concerning the routing of a meltwater pulse
illustrates a key uncertainty inherent in the meltwater pulse hypothesis.
Another complication is that efforts to model the AMOC response to freshwater
inputs under deglacial boundary conditions have yielded equivocal results
(e.g. Meissner, 2007). Consequently, although meltwater forcing remains the
most widely researched cause of the YD, it is not universally accepted. Broad
agreement does exist that AMOC weakening was associated with the YD onset
(Lynch-Stieglitz, 2017), but the driver of this weakening remains unclear.
(a) Map with the locations of sites discussed and
(b) an isopach map of the Laacher See tephra fall deposits across
central Europe. Semi-transparent white areas in (a) demarcate
continental glaciers, and the reconstruction and map are adapted from Baldini
et al. (2015b). The sites shown are Laacher See volcano (yellow triangle);
LG, La Garma Cave; MFM, Meerfelder Maar; LA, Lake Ammersee; PC, El Pindal
Cave; LK, Lake Kråkenes; LL, Lake Lucerne; GS, Gerzensee; NGRIP, NGRIP
ice core. The dashed line in (b) is the outer detection limits of
the distal tephra layers (adapted from Bogaard and Schmincke, 1985). Isopach
line labels in (b) are in centimetres.
Other recent research has proposed that a large impact event, or events, over
North America may have triggered the YD (Firestone et al., 2007; Kennett et
al., 2009). The Younger Dryas Impact Hypothesis (YDIH) is supported by the
discovery of iridium, shocked quartz, platinum, and millions of tons of
impact spherules at the YD boundary layer (Kennett et al., 2009; Wittke et
al., 2013; Wu et al., 2013). Wolbach et al. (2018a) use charcoal and soot
evidence from 152 sedimentary sequences and elevated concentrations of
platinum at 26 sites (including Greenland) to argue that a cometary impact at
the beginning of the YD triggered the largest wildfires of the Quaternary,
consuming 9 % of global terrestrial biomass. The YDIH has proven
remarkably controversial, and different researchers suggest either
terrestrial origins for the same evidence, that the evidence is not unique to
the YD boundary, or that the YD boundary layer was misidentified (e.g. Pinter
et al., 2011; van Hoesel et al., 2015; Holliday, 2015; Scott et al., 2017;
Daulton et al., 2017). Finally, still other research proposes that the YD
resulted from internal oceanic processes and that no external forcing was
required to trigger the observed climate shifts (Sima et al., 2004).
Here, we summarise but do not argue extensively for or against any of these
established hypotheses. Instead, we investigate the hypothesis that the YD
was triggered by the ∼ 12.9 ka BP eruption of the Laacher See
volcano, located in the East Eifel Volcanic Field (Germany). Earlier research
briefly alluded to the eruption as a possible causative mechanism for the YD
(e.g. Berger, 1990; van den Bogaard et al., 1989). However, because the meltwater pulse hypothesis was already
popularised and because the effects of volcanic eruptions on climate would
escape detailed quantification until after the 1991 Pinatubo eruption, the
concept of the ∼ 12.9 ka BP Laacher See eruption as a YD trigger
never gained traction. Importantly, the concept was effectively dismissed
after lacustrine evidence across central Europe appeared to indicate that the
YD's clearest expression appeared ∼ 200 years after the Laacher See
Tephra within the same sediments (e.g. Brauer et al., 2008, 1999a; Hajdas et
al., 1995). For example, Schmincke et al. (1999) state that “The Younger
Dryas cooling period clearly was not triggered by the Laacher See eruption
(LSE) as formerly thought because it started ca. 180 years after the
eruption”, reflecting the accepted sequence of events at that time. However,
the identification of the Vedde Ash chronostratigraphic unit within
Meerfelder Maar (Germany) lake sediments has improved correlations with
Greenland ice core records, which contain the same ash (Lane et al., 2013a).
This revised chronological framework now strongly suggests that the
12.880 ka BP Laacher See eruption was in fact synchronous with cooling
associated with the YD onset (i.e. the most recent abrupt Greenland
millennial-scale cooling event, the Greenland Stadial 1, GS-1) but preceded
major atmospheric circulation shifts over central Europe (Rach et al., 2014).
Additionally, we utilise ion data from the Greenland ice core GISP2
(Zielinski et al., 1997) on the most recent chronological model for the core
(the GICC05modelext chronology; Seierstad et al., 2014) to identify a large
volcanogenic sulfate spike whose timing coincides with both the Laacher See
eruption and the initiation of GS-1-related cooling. We suggest that the
initial, short-lived volcanogenic aerosol cooling triggered a sea-ice–AMOC
positive feedback that caused both basin-wide cooling and the dynamical
climate shifts most closely associated with the YD. Should future research
confirm a volcanically forced YD, it would strengthen our understanding of
the processes operating during deglaciations, and underscore the strong role
that volcanic aerosols can play in moderating global climate. Although more
research is clearly needed to thoroughly investigate this hypothesis, the
apparent coincidence of a large, very sulfur-rich eruption with the beginning
of YD cooling is compelling and well worth exploring further.
Background
Laacher See volcano, Germany, is situated in the East Eifel Volcanic Field,
which is part of the West European rift system (Baales et al., 2002; de Klerk
et al., 2008) (Fig. 1). The LSE occurred at
∼ 12.880 ± 0.040 ka BP based on the position of tephra within
regional varved lake sequences (e.g. Wulf et al., 2013; Brauer et al., 1999a;
Lane et al., 2015; Bronk Ramsey et al., 2015), consistent with radiocarbon
(12.934 ± 0.165 cal ka BP; Baales et al., 2002) and
40Ar /39Ar (12.9 ± 0.50 ka BP; van den Bogaard, 1995)
ages for the eruption; the absolute age of the eruption is therefore well
constrained. The eruption was one of the largest in Europe during the late
Quaternary and dispersed over 20 km3 of pumice and ash over
> 230 000 km2 of central Europe and beyond (Baales et al.,
2002; Bogaard and Schmincke, 1985). The eruption consisted of alternating
Plinian and phreatomagmatic phases; Plinian columns exceeded 20 km height
and injected ash and volcanic gas into the stratosphere (Harms and Schmincke,
2000). Direct effects of the LSE included ash deposition, acid rain,
wildfires, and increased precipitation, all of which could have affected the
local and far-field ecology and cultures at the time (de Klerk et al., 2008;
Baales et al., 2002; Engels et al., 2015, 2016).
The SO2 yield of climatologically significant late
Holocene eruptions (Shinohara, 2008). Red circles represent petrologic
estimates of SO2 release, and orange triangles represent estimated
actual values, calculated from either satellite or ice core data (Shinohara,
2008). The grey bar represents the range of values for the Laacher See
eruption as suggested by Textor et al. (2003), Schenk et al. (2018), and Bahr
et al. (2018), and the blue triangle in the
Laacher See column represents the total estimated SO2 emitted by
the Laacher See eruption (83.6 Mt) assuming that the actual SO2
emitted is 20.9 times the petrologic estimate, calculated here as the mean
value of 17 non-basaltic explosive eruptions (the 16 listed in
Shinohara (2008) plus the AD 1257 Samalas eruption; Vidal et al., 2016).
The distribution of the Laacher See volcanic cloud 6 months after
the ∼ 12.9 ka BP eruption based on existing climate model outputs
(MAECHAM4 model) (Graf and Timmreck, 2001). The figure is redrawn based on
the original in Graf and Timmreck (2001), which contains details regarding
the model and simulation. This simulation assumed that the eruption injected
15 Mt SO2 into the lower stratosphere, ∼ 7× less than
the maximum estimate of Textor et al. (2003a).
The LSE discharged up to 6.3 km3 of unusually sulfate-rich, evolved
phonolite and sulfide-rich mafic phonolite magma from a strongly
compositionally zoned reservoir (Harms and Schmincke, 2000). Estimating
SO2 release from eruptions preserved in the geologic record is
difficult and relies on petrologic comparisons between the sulfur content of
melt inclusions (the magma at depth) and the sulfur content of glass in
erupted products (the degassed magma) (Devine et al., 1984; Scaillet et al.,
2004; Textor et al., 2003; Vidal et al., 2016). Petrologic methods can lead
to underestimations of total sulfur release by up to 2 orders of magnitude
relative to satellite data (Gerlach et al., 1996; Scaillet et al., 2004). For
example, satellite-derived estimates of the 1991 Pinatubo eruption's
SO2 content (15–20 megatonnes (Mt)), are considerably higher than
the 0.11 Mt petrologic estimate (Sheng et al., 2015), and similar
discrepancies between petrologic and satellite-derived estimates of
SO2 emissions exist for other modern eruptions (Scaillet et al.,
2004) (Fig. 2). The consensus is that this excess sulfur is contained in a
sulfur-rich vapour phase that is released prior to and during the eruption
(Gerlach et al., 1996; Wallace, 2001). Petrologic studies by Harms and
Schmincke (2000) suggest that the LSE released 3.8 Mt of SO2, but
considering a sulfur-rich vapour phase they concluded that the eruption
released at least 20 Mt of SO2 into the stratosphere. Textor et
al. (2003) suggested that the eruption released between 6.76 and 104.8 Mt of
SO2, when considering uncertainties regarding the oxidation state
of the vapour phase. Despite these unknowns, the amount of SO2
released by the LSE was considerably higher than the petrologic estimates,
and Schmincke et al. (1999) speculated a maximum release of 300 Mt
SO2, assuming the same relationship between petrologic and observed
values as for the Pinatubo eruption. We utilise a similar but more
comprehensive approach, taking the mean value of the relationship between
petrologic and observed values of the 16 non-basaltic explosive eruptions
catalogued by Shinohara (2008) plus estimated values for the large AD 1257
Samalas eruption (Vidal et al., 2016) to estimate that the LSE released
∼ 83 Mt SO2, although the range in possible values is
substantial. A variety of estimated values clearly exist, but the eruption
almost certainly released more SO2 than the Pinatubo eruption
(∼ 20 Mt) and possibly even more than the 1815 Tambora eruption
(∼ 70 Mt) (Fig. 2). However, unlike the Pinatubo and Tambora
eruptions, sulfate aerosols produced by the LSE were largely restricted to
the Northern Hemisphere (NH) (Fig. 3), leading to strong cooling in the
stratosphere that affected the NH disproportionately (Graf and Timmreck,
2001).
Both Pinatubo and the LSE were magnitude 6 (M6) eruptions, where
“magnitude” is a measure of eruption size referring to the amount of
material erupted (Deligne et al., 2010) on a logarithmic scale. However, the
cooling effects of a volcanic eruption are controlled by the amount of sulfur
released and not necessarily the eruption size (Rampino and Self, 1982). In
general, magnitude and erupted sulfur amounts are well correlated
(Oppenheimer, 2003; Carn et al., 2016), and therefore magnitude is often used
as a surrogate for sulfur yield. However, variability of almost 3 orders of
magnitude exists in the amount of sulfur released amongst equivalently sized
explosive eruptions (Carn et al., 2016), and consequently eruption size is
not the only predictor of total sulfur released. In the case of the LSE, all
the existing evidence suggests that it was anomalously enriched in sulfur
relative to its magnitude (Baales et al., 2002; Scaillet et al., 2004) and
that it therefore should have produced significant NH cooling.
Temperature (a–d) and wind strength (e, f) proxy
records from the North Atlantic and Europe. (a) NGRIP ice core
δ18O (Greenland, GICC05) (Rasmussen et al., 2006). (b)
Precipitation δ18O reconstructed using deep lake ostracods from
Lake Ammersee, southern Germany. (c) Meerfelder Maar (MFM) (Eifel
Volcanic Field, Germany) n-alkanes δDterr (terrestrial
lipid biomarkers) (Rach et al., 2014; Litt et al., 2001). (d) Lake
Lucerne, Switzerland, isoprenoid tetraether (BIT) record (Blaga et al.,
2013). (e) Wind strength as reconstructed using Ti count rate in
Lake Kråkenes (Norway); three-point moving average shown. (f)
Wind strength as reconstructed using MFM varve thickness data (Brauer et al.,
2008; Mortensen et al., 2005). The records are arranged so that cooling is
down for all the records. The LST (vertical red line) is present or inferred
within the MFM, Lake Ammersee, and Lake Lucerne cores. The LST is not evident
in the NGRIP or Lake Kråkenes cores, and the eruption's timing relative
to NGRIP δ18O and Lake Kråkenes Ti is based on layer counting
from the Vedde Ash (“VA”), a tephrochronological marker
(12.140 ± 0.04 ka BP) also found in MFM (Lane et al., 2013a; Brauer
et al., 2008). The published chronologies for Lake Ammersee and Lake Lucerne
were shifted slightly by a uniform amount (0.115 and -0.093 kyr,
respectively) to ensure the contemporaneity of the LSE in all the records.
This adjustment does not affect the LSE's timing relative to the Lake
Ammersee or Lake Lucerne climate records. Published radiometric dates for the
LSE are shown (red circles) with errors, although the absolute age is not as
important as its timing relative to the apparent climate shifts.
Results and discussionThe timing of the Laacher See eruption relative to the Younger
Dryas
Key research using European lake sediment archives containing the Laacher See
Tephra (LST) suggested that the LSE preceded the YD onset by
∼ 200 years (Hajdas et al., 1995; Brauer et al., 1999a). For example,
the LST appears very clearly in the Meerfelder Maar sediment core (Germany),
and it does indeed appear to predate the YD (e.g. Brauer et al., 1999a).
However, the recent discovery of the Icelandic Vedde Ash in Meerfelder Maar
sediments has revised the relative timing of key climate archives around the
North Atlantic (Lane et al., 2013a). It is now apparent that the clearest
hydroclimatic expression of the YD in central Europe lags behind Greenland
cooling associated with GS-1 by 170 years (Fig. 4). Specifically, several
lake sediment cores from different latitudes have revealed that westerly wind
repositioning was time transgressive, with winds gradually shifting to the
south following the initiation of GS-1 cooling (Bakke et al., 2009; Lane et
al., 2013a; Brauer et al., 2008; Muschitiello and Wohlfarth, 2015).
The GISP2 (bi-decadal; Stuiver et al., 1995) (black) and NGRIP
(blue) ice core δ18O records synchronised on the GICC05modelext
chronology (Seierstad et al., 2014). The red bars indicate GISP2
volcanological sulfate record (also synchronised on the GICC05 timescale).
The maximum counting error of the ice core chronology is 0.138 ka at
12.9 ka BP (Seierstad et al., 2014). The best age estimate of the LSE is
shown (red circle) (Brauer et al., 2008; Lane et al., 2015), and a possible
correlation to a synchronous sulfate spike highlighted by the vertical dashed
arrow. The large spike at ∼ 13 ka BP represents a smaller but more
proximal Icelandic eruption (Hekla) associated with a volcanic ash layer
(Mortensen et al., 2005; Bereiter et al., 2018).
Although the LSE preceded the most clearly expressed dynamical climatic
change associated with the YD in central Europe, its timing
(12.880 ± 0.040 ka BP) is indistinguishable from the Greenland
temperature decrease leading into GS-1, beginning at
12.870 ± 0.138 ka BP (Rach et al., 2014; Steffensen et al., 2008;
Rasmussen et al., 2014) (GS-1 is defined as starting at
12.846 ka BP in the NGRIP record, but abrupt
cooling predates this by ∼ 24 years) (Fig. 4). In central Europe,
hydrogen isotope ratios of land plant-derived lipid biomarkers from
Meerfelder Maar confirm that North Atlantic atmospheric cooling began at
∼ 12.880 ka BP (Rach et al., 2014), synchronous with both the onset
of Greenland GS-1-related cooling and the LSE but preceding the atmospheric
response associated with meridionally displaced westerly winds at the same
site (importantly, in Meerfelder Maar (MFM) the first data point suggesting
cooling occurs immediately above the LST; Rach et al., 2014). This
observation is further supported by speleothem δ18O records from
the La Garma and El Pindal caves in northern Spain (Baldini et al., 2015b;
Moreno et al., 2010), Swiss lake sediment δ18O records (Lotter et
al., 1992), Lithuanian lake sediment trace element profiles (Andronikov et
al., 2015), and Swiss lake organic molecule-based (BIT and TEX86)
temperature reconstructions (Blaga et al., 2013), all showing North Atlantic
cooling beginning immediately after the LSE (Fig. 4). Some lacustrine proxy
records suggest that cooling began a few years after the LSE (e.g. the
Gerzensee δ18O stack, van Raden et al., 2013). The existence of a
short lag between the temperature signal and the recording of that signal in
some lacustrine archives may reflect differences in the type of archive used
(i.e. terrestrial versus lacustrine), a decadal-scale residence time of
groundwater feeding certain lakes, or differences in moisture source regions
for different lakes. The trigger responsible for the cooling must coincide
with (or predate) the earliest evidence of the cooling, and a lag must
logically exist between the forcing and any later response (rather than the
effect preceding the cause).
Recent work used volcanic marker horizons to transfer the GICC05modelext to
the GISP2 ice core (previously on the Meese–Sowers chronology) (Seierstad et
al., 2014), thereby improving comparisons with records of volcanism (Fig. 5)
and facilitating a direct comparison between the GISP2 volcanic sulfate
record (Zielinski et al., 1997) and both the GISP2 and the NGRIP
δ18O records (Fig. 5). For the interval around the Younger Dryas
initiation, very little chronological difference between this timescale and
IntCal13 exists (Muscheler et al., 2014), suggesting that the timescale is
robust. We find that a large sulfate spike at 12.867 ka BP in the GISP2
record on the GICC05modelext timescale is contemporaneous with the LSE's
timing based on (i) dates for the eruption based on varved lake deposits
(12.880 ± 0.04 ka BP, Lane et al., 2015) and (ii) layer counting
from the Vedde Ash within NGRIP (Fig. 5); we therefore ascribe this sulfate
spike to the Laacher See eruption. Earlier research also briefly considered
this spike based on absolute dating (Brauer et al., 1999b), but because of
the concept that the LSE preceded the YD boundary by ∼ 200 years,
concluded that an earlier sulfate spike most likely represented the LSE.
Utilising the GICC05modelext chronology (Seierstad et al., 2014) as well as
recent layer counts within the MFM sediment (Lane et al., 2013a), we suggest
that the sulfate spike at 12.867 ka BP represents the LSE, and that the
earlier one tentatively identified by Brauer et al. (1999b) may represent a
small eruption of the Icelandic volcano Hekla, consistent with the
interpretation of Muschitiello et al. (2017). Although the coincidence
between our identified sulfate spike, the date of the LSE, and the onset of
North Atlantic cooling associated with the YD is compelling, detailed
tephrochronological analyses are required to definitively ascribe this
sulfate spike to the LSE.
A complex response of climate to volcanic eruptions
Explosive volcanic eruptions can inject large amounts of sulfur-rich gases as
either SO2 or H2S into the stratosphere, which are
gradually oxidised to form sulfuric acid vapour (Rampino and Self, 1984).
Within a few weeks, the vapour can condense with water to form an aerosol
haze (Robock, 2000), which is rapidly advected around the globe. Once in the
atmosphere, sulfate aerosols induce summer cooling by scattering incoming
solar radiation back to space (Timmreck and Graf, 2006; Baldini et al.,
2015a). An eruption's sulfur content, rather than its explosivity, determines
most of the climate response (Robock and Mao, 1995; Sadler and Grattan,
1999). Furthermore, recent research has also highlighted the role of
volcanogenic halogens, such as Cl, Br, and F, in depleting stratospheric
ozone (Cadoux et al., 2015; Klobas et al., 2017; Kutterolf et al., 2013;
Vidal et al., 2016; LeGrande et al., 2016). Ozone absorbs solar ultraviolet
and thermal infrared radiation, and thus depletion in ozone can result in
surface cooling and would act to amplify the radiative effects of
volcanogenic sulfate aerosols (Cadoux et al., 2015).
Although the radiative effects associated with volcanic aerosols are
reasonably well understood, the systematics of how volcanic eruptions affect
atmospheric and oceanic circulation are less well constrained. Research
suggests that volcanic eruptions affect a wide variety of atmospheric
phenomena, but the exact nature of these links remains unclear. For example,
Pausata et al. (2015) used a climate model to conclude that high-latitude NH
eruptions trigger an El Niño event within 8–9 months by inducing a
hemispheric temperature asymmetry leading to southward Intertropical
Convergence Zone (ITCZ) migration and a restructuring of equatorial winds.
The model also suggests that these eruptions could lead to AMOC shifts after
several decades, consisting of an initial 25-year strengthening followed by a
35-year weakening, illustrating the potential for climate effects extending
well beyond sulfate aerosol atmospheric residence times. Several modelling
studies based on historical data suggest that eruptions may strengthen AMOC
(Ottera et al., 2010; Swingedouw et al., 2014; Ding et al., 2014) but also
increase North Atlantic sea ice extent for decades to centuries following the
eruption due to the albedo feedback and reductions in surface heat loss (Ding
et al., 2014; Swingedouw et al., 2014). Other models suggest that AMOC may
intensify initially but then weaken after about a decade (Mignot et al.,
2011). A modelling study by Schleussner and Feulner (2013) suggested that
volcanic eruptions occurring during the last millennium increased Nordic Sea
sea ice extent, which weakened AMOC and eventually cooled the entire North
Atlantic Basin. Importantly, Schleussner and Feulner (2013) concluded that
short-lived volcanic aerosol forcings triggered “a cascade of sea ice-ocean
feedbacks in the North Atlantic, ultimately leading to a persistent regime
shift in the ocean circulation”. Other research finds that North Atlantic
sea ice growth following a negative forcing weakened oceanic convection and
northward heat export during the Little Ice Age (Lehner et al., 2013).
Quantifying the long-term influences of single volcanic eruptions is
confounded by the effects of subsequent eruptions and other factors (e.g.
solar variability, El Niño events), which can overprint more subtle
feedbacks. For example, model results looking at recent eruptions found
evidence that different types of eruptions can either constructively or
destructively interfere with AMOC strength (Swingedouw et al., 2014).
Therefore, despite increasingly clear indications that volcanic eruptions
have considerable long-term consequences for atmospheric and oceanic
circulation, the full scale of these shifts is currently not well understood
even over the last 2 millennia, and is essentially unknown under glacial
boundary conditions.
However, indications that volcanism may have had particularly long-term
climate effects during the last glacial do exist. Bay et al. (2004) found a
very strong statistical link between evidence of Southern Hemisphere (SH)
eruptions and rapid Greenland warming associated with Dansgaard-Oeschger (DO)
events, although no causal mechanism was proposed. More recent research
determined that every large, radiometrically dated SH eruption (five
eruptions) across the interval 30–80 ka BP occurred within dating
uncertainties of a DO event (Baldini et al., 2015a). The same research also
found a strong statistical correlation between large NH volcanic eruptions
and the onset of Greenland stadials over the same 30–80 ka BP interval
(Baldini et al., 2015a), and proposed that during intermediate ice volume
conditions, a positive feedback involving sea ice extension, atmospheric
circulation shifts, and/or AMOC weakening may have amplified the initial
aerosol injection. This positive feedback continued to operate until it was
superseded by another forcing promoting warming at high latitudes in the NH
or an equilibrium was reached. This also appears consistent with observations
during the YD: (i) a sulfur-rich NH volcanic eruption, (ii) long-term NH
high-latitude cooling, (iii) NH midlatitude westerly wind migration to the
south, and (iv) slow recuperation out of the event following rising
65∘ N insolation and/or a SH meltwater pulse.
The LSE erupted twice the volume of magma as, and potentially injected
considerably more sulfur into the stratosphere than, the AD 1991 Pinatubo
eruption (Baales et al., 2002; Schmincke, 2004). The Pinatubo eruption cooled
surface temperatures by 0.5 ∘C globally, and up to 4 ∘C
across Greenland, Europe, and parts of North America, for 2 years following
the eruption (Schmincke, 2004), while aerosols persisted in the stratosphere
for ∼ 3 years (Diallo et al., 2017). An existing model suggests that the
LSE created a sulfate aerosol veil wrapping around the high northern
latitudes (Graf and Timmreck, 2001) (Fig. 3). The model suggests that NH
summer temperatures dropped by 0.4 ∘C during the first summer
following the eruption, though it assumes that the eruption released
substantially less SO2 (15 Mt SO2, Graf and Timmreck,
2001) than current maximum estimates (104.8 Mt SO2, Textor et al.,
2003) (Fig. 2), which rival the sulfur amount delivered to the stratosphere
by the AD 1815 Tambora eruption. Actual aerosol-induced cooling may far
exceed these estimates, a perspective supported by Bogaard et al. (1989), who
estimate that the environmental impacts of the eruption of the sulfur-rich
phonolite melt may even have exceeded those of the far larger (but silicic)
Minoan eruption of Santorini (∼ 1613 BC). It does not seem
unreasonable that an eruption of this size, sulfur content, and geographic
location, occurring during a transitional climate state, could have catalysed
a significant climate anomaly.
The nature of the positive feedback
Volcanogenic sulfate aerosols typically settle out of the atmosphere within 3
years; aerosol-induced cooling alone therefore cannot explain the YD's
extended duration. We suggest that aerosol forcing related to the LSE
initiated North Atlantic cooling, which consequently triggered a positive
feedback as proposed for earlier Greenland stadials (Baldini et al., 2015a).
Existing evidence strongly suggests that North Atlantic sea ice extent
increased (Bakke et al., 2009; Baldini et al., 2015b; Broecker, 2006b;
Cabedo-Sanz et al., 2013) and AMOC weakened (Broecker, 2006b; McManus et al.,
2004; Bondevik et al., 2006) immediately after the GS-1 onset, and therefore
both could have provided a powerful feedback. The feedback may have resided
entirely in the North Atlantic and involved sea ice expansion, AMOC
weakening, and increased albedo, as previously suggested within the context
of meltwater forcing (Broecker et al., 2010). Alternatively, it has been
noted that hemispherically asymmetrical volcanic sulfate loadings induced
ITCZ migration away from the hemisphere of the eruption (Ridley et al., 2015;
Hwang et al., 2013; Colose et al., 2016), and it is possible that these ITCZ
shifts forced wholesale shifts in atmospheric circulation cells. This
hypothesised mechanism is broadly consistent with that advanced by Chiang et
al. (2014), where a forcing at a high northerly latitude subsequently drives
southward ITCZ migration, which then affects global atmospheric circulation.
Within the context of GS-1, LSE aerosol-related NH cooling could have shifted
the ITCZ to the south, thereby expanding the NH polar cell and shifting the
NH polar front to the south. Sea ice tracked the southward-shifted polar
front, resulting in more NH cooling, a weakened AMOC, and a further southward
shift in global atmospheric circulation cells (Baldini et al., 2015a). Such a
scenario is consistent with recent results based on tree ring radiocarbon
measurements suggesting that GS-1 was not caused exclusively by long-term
AMOC weakening but instead was forced by NH polar cell expansion and
southward NH polar front migration (Hogg et al., 2016).
Our hypothesis that the YD was triggered by the LSE and amplified by a
positive feedback is further supported by modelling results suggesting that a
combination of a moderate negative radiative cooling, AMOC weakening, and
altered atmospheric circulation best explain the YD (Renssen et al., 2015).
AMOC consists of both thermohaline and wind-driven components, and
atmospheric circulation changes can therefore dramatically affect oceanic
advection of warm water to the North Atlantic. Recent modelling suggests that
reduced wind stress can immediately weaken AMOC, encouraging southward sea
ice expansion and promoting cooling (Yang et al., 2016), illustrating a
potential amplification mechanism following an initial aerosol-induced
atmospheric circulation shift. Twentieth-century instrumental measurements
further support this by demonstrating that westerly wind strength over the
North Atlantic partially modulates AMOC (Delworth et al., 2016).
Initiation of the proposed positive feedback would require volcanic aerosols
to remain in the atmosphere for at least one summer season. Evidence based on
the seasonal development of vegetation covered by the LST suggests that the
LSE occurred during late spring or early summer (Schmincke et al., 1999), and
varve studies similarly suggest a late spring or early summer eruption (Merkt
and Muller, 1999). Available evidence therefore suggests that the eruption
occurred just prior to maximum summer insolation values, maximising the
potential scattering effects of the volcanogenic sulfate aerosols. Even if it
were a winter eruption, for historical eruptions similar in magnitude to the
Laacher See eruption, aerosols remained in the atmosphere longer than 1 year,
regardless of the eruption's latitude. For example, aerosols remained in the
atmosphere for ∼ 3 years after the Pinatubo eruption (15∘ N,
120∘ E) (Diallo et al., 2017), which probably released considerably
less SO2 than the LSE (Fig. 2). Measurable quantities of aerosols
remained in the atmosphere for approximately 3 years even after the 1980
Mount St Helen's eruption (46∘ N, 122∘ W) (Pitari et al.,
2016), which produced only 2.1 Mt SO2 (Baales et al., 2002; Pitari
et al., 2016) and erupted laterally (Eychenne et al., 2015). In short, the
LSE eruption probably occurred during the late spring or early summer, but
even if the eruption were a winter eruption, the LSE's aerosols would have
certainly persisted over at least the following summer, with the potential to
catalyse the positive feedback we invoke.
It is worth highlighting that a similar mechanism may have also contributed
to Little Ice Age cooling (Miller et al., 2012), with research suggesting
that a coupled sea-ice–AMOC mechanism could extend the cooling effects of
volcanic aerosols by over 100 years during the Little Ice Age (Zhong et al.,
2011). This perspective is supported by modelling results suggesting that a
large volcanic forcing is required to explain Little Ice Age cooling
(Slawinska and Robock, 2017). Lehner et al. (2013) identify a
sea-ice–AMOC–atmospheric feedback that amplified an initial negative
radiative forcing to produce the temperature pattern characterising the
Little Ice Age. However, Rahmstorf et al. (2015) argue that no identifiable
change in AMOC occurred during the Little Ice Age, and Thornalley et
al. (2018) similarly suggest that AMOC only weakened near the end of the
Little Ice Age. This implies that the drivers of Little Ice Age cooling were
related to sea-ice, atmosphere, or oceanic (other than AMOC, e.g. horizontal
subpolar gyre circulation; Moreno-Chamarro et al., 2017) changes, although
this requires further research to confirm. Large volcanic eruptions in
AD 536, 540, and 547 are hypothesised to have triggered a coupled
sea-ice–ocean circulation feedback that led to an extended cold period
(Buntgen et al., 2016). Recent research also highlights the possibility that
volcanism followed by a coupled sea-ice–ocean circulation positive feedback
triggered hemispheric-wide centennial- to millennial-scale variability during
the Holocene (Kobashi et al., 2017). If a feedback was active following
volcanic eruptions during the 6th century, the Little Ice Age, and the
Holocene in general, the intermediate ice volume and
transitional climate characteristic of the last deglaciation should have
amplified any volcanic aerosol forcing. This perspective is consistent with
previous observations, including those of Zielinski et al. (1996), who noted
that when the climate system is in a state of flux, it is more sensitive to
external forcing and that any post-volcanic cooling would be longer lived.
Importantly, Rampino and Self (1992) stated “Volcanic aerosols may also
contribute a negative feedback during glacial terminations, contributing to
brief episodes of cooling and glacial readvance such as the Younger Dryas
Interval”. Our results are entirely consistent with this perspective, and
here we highlight a sulfur-rich volcanic eruption whose timing coincided with
the onset of YD-related cooling. However, despite increasingly tangible
evidence that eruptions can affect ocean circulation and sea ice extent, the
exact nature of any positive feedback is still unclear. Future research
should prioritise the identification and characterisation of this elusive,
but potentially commonplace, feedback that amplifies otherwise subtle NH
temperature shifts.
Histogram of the frequency of abrupt Greenland cooling events
relative to ice volume and ice volume change. Red Sea sea level (Siddall et
al., 2003) is used as a proxy for ice volume and to determine (a)
the distribution of ice volume conditions associated with all Greenland
cooling events identified by Rasmussen et al. (2014) over the last 120 kyr
(n=55). A Gaussian best-fit curve describes the distribution (grey-filled
curve) (mean =-68.30 m; σ=21.85 m), and sea level during
the GS-1 onset is marked with a solid blue line. (b) The distribution of sea level change rates
(reflecting global ice volume shifts) associated with 55 Greenland cooling
events over the last 120 kyr. This distribution is also described by a
Gaussian best-fit curve (grey-filled curve) (mean =-1.35493; σ=7.19 m ka-1); 21 events are associated with decreasing ice volume,
and 34 are associated with increasing ice volume.
Two 5 kyr intervals of the NGRIP δ18O
record bracketing (a) the ∼ 12.9 ka BP Laacher See eruption
and (b) the ∼ 74 ka BP Toba super-eruption (Rasmussen et
al., 2006). The two eruptions are among the most well-dated eruptions of the
Quaternary. The LST is not apparent in the NGRIP core (although we identify a
candidate sulfate spike in the GISP2 record; see Fig. 5), and the timing of
the LST relative to the NGRIP δ18O record is based on the
difference in ages between the LST and the Vedde Ash found in European lake
sediments (Lane et al., 2013a). The timing of the Toba eruption relative to
NGRIP is based on the position of the most likely sulfate spike identified by
Svensson et al. (2013). Blue arrows indicate the timing of possible SH
cooling events. The timing of Meltwater Pulse-1B is based on Ridgwell et
al. (2012), but the source, timing, and even occurrence of the meltwater
pulse are still debated. The inset panel (c) shows NGRIP
δ18O during the 200 years immediately following the two eruptions
(the sulfate spike T2 is assumed to represent Toba, as suggested by Svensson
et al., 2013).
Sensitivity to ice volume
Magnitude 6 (M6) eruptions such as the LSE are large but not rare; for
example, over the last 2 000 years, 12 M6 or larger eruptions are known
(Brown et al., 2014), but none produced a cooling event as pronounced as the
YD. Abrupt millennial-scale climate change is characteristic of glacial
intervals, and the forcing responsible only appears to operate during
intervals when large ice sheets are present. The lack of large-scale and
prolonged climate responses to external forcing over the Holocene implies
either the absence of the forcing (e.g. lack of large meltwater pulses), a
reduced sensitivity to a temporally persistent forcing (e.g. volcanism), or
the existence of an internal forcing mechanism particular to glacial
climates. The subdued climate response to volcanic eruptions over the recent
past could reflect low ice volume conditions and the absence (or muting) of
the requisite positive feedback mechanism. However, millennial-scale climate
change was also notably absent during the Last Glacial Maximum
(∼ 20–30 ka BP), implying that very large ice sheets also discourage
millennial-scale climate shifts.
The apparent high sensitivity of the climate system to millennial-scale
climate change during times of intermediate ice volume is well documented
(e.g. Zhang et al., 2014, 2017), and here we investigate this further by
examining the timing of Greenland stadials relative to ice volume (estimated
using Red Sea sea level; Siddall et al., 2003). The timings of 55 stadial
initiations as compiled in the INTIMATE (INTegration of Ice core, MArine, and
TErrestrial) initiative (Rasmussen et al., 2014) are compared relative to ice
volume, and indeed a strong bias towards intermediate ice volume conditions
exists, with 73 % of the millennial-scale cooling events occurring during
only 40 % of the range of sea level across the interval from 0 to
120 ka BP (Fig. 6). The distribution of events suggests that the most
sensitive conditions are linked to ice volume associated with a sea level of
-68.30 m below modern sea level. This intermediate ice volume was
commonplace from 35 to 60 ka BP and particularly from 50 to 60 ka BP.
However, over the interval from 0 to 35 ka BP, these ideal intermediate ice
volume conditions only existed during a short interval from 11.8 to
13.7 ka BP, and optimal conditions were centred at 13.0 ka BP (Fig. 6).
These results are broadly consistent with previous research (e.g. Zhang et
al., 2014, 2017), but the timing and duration of the most sensitive interval
of time and the likelihood that a forcing produces a longer-term cooling
event may ultimately depend on a complex interplay between ice volume and
atmospheric CO2 (Zhang et al., 2017). Atmospheric pCO2
during the YD initiation was relatively high (∼ 240 ppmv) and could
therefore affect the timing of ideal conditions for abrupt climate change in
conjunction with ice volume, but their precise interdependence is still
unclear. Finally, a frequency distribution of the sea level change rate
associated with each stadial indicates that whatever mechanism is responsible
for triggering a stadial operates irrespective of whether sea level (i.e. ice
volume) is increasing or decreasing (Fig. 6b). Because active ice sheet
growth should discourage meltwater pulses, this observation seemingly argues
against meltwater pulses as the sole trigger for initiating stadials.
Comparison with the climate response to the Toba super-eruption
Comparing the LSE to another well-dated Quaternary eruption occurring under
different background conditions provides information regarding the nature of
the relevant forcings. The magnitude > 8 (i.e.
∼ 100× greater than the LSE) Toba super-eruption occurred
approximately ∼ 74 ka BP and was the largest eruption of the
Quaternary (Brown et al., 2014). Despite its size, the climate response to
the eruption remains vigorously debated, with some researchers suggesting
that the eruption triggered the transition to Greenland Stadial 20 (GS-20)
(Polyak et al., 2017; Baldini et al., 2015a; Carolin et al., 2013; Williams
et al., 2009) but others arguing for only a small climate response (Haslam
and Petraglia, 2010). However, it does not appear that the eruption triggered
a sustained global volcanic winter (Lane et al., 2013b; Svensson et al.,
2013), and therefore globally homogenous long-term aerosol-induced cooling
probably did not occur. The Toba super-eruption occurred during an orbitally
modulated cooling trend with intermediate ice volume (-71 m below modern
sea level, very near the sea level associated with the most sensitive ice
volume conditions (-68.3 m below modern sea level, as suggested here)),
and it is possible that the eruption expedited the transition to GS-20
through a combination of an initial short-lived aerosol-induced cooling
amplified by a positive feedback lasting hundreds of years.
In this analysis we use the timing of the Toba eruption relative to the NGRIP
record suggested by Svensson et al. (2013) (their NGRIP sulfate spike
“T2”), but the timing remains uncertain. If future research revises the
timing relative to NGRIP, this would necessarily affect the interpretations
discussed below. Assuming that the Toba eruption was responsible for the
sulfate spike T2, the eruption was followed by high-latitude NH cooling
accompanied by southward ITCZ migration and Antarctic warming, consistent
with sea ice growth across the North Atlantic and a weakened AMOC. The
cooling rates into GS-1 (the YD) after the LSE and GS-20 after the Toba
eruption are nearly identical (Fig. 7), consistent with the possibility that
a similar process was responsible for the cooling in both instances. The
cooling rate may reflect the nature of the post-eruptive positive feedback,
which was potentially independent of the initial radiative cooling effects of
the two eruptions, provided that they were large enough to trigger a
feedback. The reconstructed climate responses following the LSE and Toba
eruptions diverge after the achievement of maximum cooling. In the case of
Toba, cold conditions persisted for ∼ 1.7 kyr before the very rapid
temperature increase characteristic of the onset of Greenland Interstadial
(an abrupt Greenland warming event) 19 (GI-19) (Fig. 7). GS-1 ended after
∼ 1.2 kyr, but unlike GS-20 which had stable cold temperatures
throughout, δ18O (Baldini et al., 2015b; Steffensen et al., 2008)
and nitrogen isotope data (Buizert et al., 2014) suggest that the coldest
conditions around the North Atlantic occurred early within GS-1 and were
followed by gradual warming. The LSE and Toba eruptions occurred under
different orbital configurations, and the contrasting topologies
characteristic of GS-1 and GS-20 may reflect differing insolation trends. At
∼ 13 ka BP, summer insolation at 65∘ N was increasing,
rather than decreasing as during the Toba eruption ∼ 74 ka BP. The
Toba eruption may have triggered a shift to the insolation-mediated baseline
(cold) state, whereas the LSE may have catalysed a temporary shift to cold
conditions opposed to the insolation-driven warming trend characteristic of
that time interval, resulting in a short-lived cold event followed by gradual
warming. Radiative cooling events in the Southern Hemisphere (e.g. a SH
eruption, an Antarctic meltwater pulse) may have abruptly terminated both
GS-1 and GS-20; the GS-1 termination is broadly consistent with the putative
timing of Meltwater Pulse 1B (MWP-1B) (Ridgwell et al., 2012), whereas any SH
trigger for the termination of GS-20 is unidentified. MWP-1B (or another SH
cooling event) may have cooled the SH and strengthened AMOC, prompting
northward migration of the ITCZ and NH midlatitude westerlies to achieve
equilibrium with high insolation conditions and thereby rapidly reducing sea
ice extent and warming Greenland. However, this requires further research,
particularly because the source, duration, timing, and even existence of
MWP-1B are still debated. Reduced oceanic salt export within the North
Atlantic subtropical gyre, characteristic of stadials, may have
preconditioned the North Atlantic toward a vigorous AMOC following the
initial migration of atmospheric circulation back to the north (Schmidt et
al., 2006).
The residence time of aerosols within the atmosphere is not critical within
the context of this model provided the positive feedback is activated, and a
sufficiently high aerosol-related cooling over only one summer and one
hemisphere could suffice. The feedback's strength may also depend on the
amount of hemispheric temperature asymmetry caused by the eruption.
Consequently, the high-latitude LSE may actually have induced a stronger
hemispheric temperature asymmetry than the low-latitude Toba eruption,
although the Toba eruption would have resulted in considerably more overall
aerosol-induced cooling. The long-term (e.g. hundreds to thousands of years)
climate response may depend on the climate background conditions; if a NH
eruption occurs during an orbitally induced cooling trend (as may have been
the case for Toba), the eruption may catalyse cooling towards the
insolation-mediated baseline. If the NH eruption occurs during rising
insolation and intermediate ice volume (e.g. Laacher See), we suggest that
any post-eruption feedback will continue until it is overcome by a
sufficiently high positive insolation forcing or a SH cooling event. Although
the size of the two eruptions differed by 2 orders of magnitude, the
residence time of the respective aerosols in the stratosphere was probably
broadly similar: ∼ 3 years for the LSE (similar to that of the Pinatubo
eruption) and ∼ 10 years for Toba (Timmreck et al., 2012; Robock et
al., 2009). Although modelling results suggest that Toba resulted in
∼ 10 ∘C of cooling (Timmreck et al., 2012), compared to
probably around ∼ 1 ∘C for the LSE,
we argue that both eruptions were able to cool climate sufficiently to
trigger sea ice growth, weaken AMOC, and thereby initiate the positive
feedback.
A useful way of visualising this is to consider two extreme scenarios:
(i) very high CO2 concentrations and no ice (e.g. the Cretaceous)
and (ii) very low CO2, low insolation, and very high ice volume
(e.g. the Last Glacial Maximum). An eruption the size of the LSE would
probably not significantly affect climate beyond the atmospheric residence
time of the sulfate aerosols during either scenario. Under the background
conditions characteristic of the first scenario, insufficient aerosol forcing
would occur to trigger ice growth anywhere, and consequently no positive
feedback would result. In the case of the second scenario, the eruption would
cause ice growth and cooling for the lifetime of aerosols in the atmosphere
before conditions returned to the insolation-mediated equilibrium. In
contrast to these extreme scenarios, an injection of volcanogenic sulfate
aerosols into the NH atmosphere would most effectively trigger a feedback
during intermediate CO2 and ice volume conditions; in other words,
during a transition from one ice volume state to another when the climate
system was in a state of disequilibrium. Under these conditions, we suggest
that activation of the feedback would occur even if the sulfate aerosols
settled out of the atmosphere after just 1 year. Therefore, although the Toba
and Laacher See eruptions were of very different magnitudes, the nature of
the positive feedback would depend largely on the background conditions
present during the individual eruptions. We argue that both eruptions were
large enough, and contained enough sulfur, to activate the positive feedback
under their respective background conditions. The positive feedback's
strength was then controlled by background conditions rather than eruption
magnitude (or amount of sulfur released). Furthermore, we predict that the
more asymmetric the hemispheric distribution of the aerosols, the stronger
the feedback. For these reasons, the long-term (centennial- to
millennial-scale) climate response following the extremely large but
low-latitude Toba eruption may have approximated those of the far smaller but
high-latitude Laacher See eruption. Therefore, the long-term climate
repercussions to very large volcanic eruptions may not consist of prolonged
radiative cooling (i.e. “volcanic winter”) but rather of geographically
disparate dynamical shifts potentially not proportional to eruption
magnitude.
Response to other late Quaternary eruptions
The strongest argument against the LSE contributing to YD climate change is
that several other similar or larger-magnitude eruptions must have occurred
during the last deglaciation and that therefore the LSE was not unusual.
However, the LSE (i) was unusually sulfur-rich, (ii) was high-latitude, and
(iii) coincided with ideal ice volume conditions. It is in fact the only
known sulfur-rich, high-latitude eruption coinciding with the most sensitive
ice volume conditions during the last deglaciation. Early work noted that
because the AD 1963 Agung eruption was very sulfur-rich, it forced cooling
of a similar magnitude to the 1815 Tambora eruption, despite Tambora ejecting
considerably more fine ash (∼ 150 times more) into the upper atmosphere
(Rampino and Self, 1982). Although estimates of total sulfur released vary,
it is clear that the LSE was also unusually sulfur-rich (probably releasing
more than Agung (Fig. 2)), so had the potential to exert a considerable
forcing on climate disproportionate to its magnitude. Furthermore, other
eruptions may also have contributed to climate change but current
chronological uncertainties preclude establishing definitive links. For
example, the ∼ 14.2 ka BP Campi Flegrei eruption may have caused some
cooling, but this is impossible to confirm with eruption age uncertainties of
±1.19 ka.
Other large sulfate spikes exist in the GISP2 sulfate record, and in fact the
two spikes at 12.6 and 13.03 ka BP are both larger than the potential LSE
sulfate spike. The size of the sulfate spike is not necessarily proportional
to the size of the eruption, and the sulfate spike at 13.03 ka BP is likely
related to the small eruption of Hekla (Iceland) (Muschitiello et al., 2017),
which deposited sulfate in the nearby Greenland ice but was climatologically
insignificant. Similarly, the spike at 12.6 ka BP may also reflect the
influence of a small but proximal unknown eruption. Alternatively, a large
eruption of Nevado de Toluca, Mexico, is dated to 12.45 ± 0.35 ka BP
(Arce et al., 2003) and could correspond to the large sulfate spike at
12.6 ka BP within the GISP2 ice core. The eruption was approximately the
same size as the LSE, so the lack of long-term cooling may reflect a
different climate response due to the eruption's latitude, which caused a
more even distribution of aerosols across both hemispheres. Although there is
no long-term cooling following the 12.6 ka BP sulfate spike, it is
associated with a short cooling event; therefore, the lack of long-term
cooling at 12.6 ka BP may simply reflect the fact that temperatures had
already reached the lowest values possible under the insolation and carbon
dioxide baseline conditions characteristic of that time.
Compatibility with other hypotheses
A substantial amount of research has now characterised the YD climate, and
several hypotheses exist attempting to explain the excursion. The most
well-researched hypothesis involves a freshwater pulse from the large
proglacial Lake Agassiz substantially reducing AMOC strength (Alley, 2000;
Broecker, 1990; Broecker et al., 2010). A weakened AMOC would result in a
colder North Atlantic, a southward displaced ITCZ, and Antarctic warming, all
features characteristic of the YD. However, direct geological evidence of a
meltwater pulse originating from Laurentide proglacial lakes coincident with
the YD onset remains elusive (Broecker et al., 2010; Petaev et al., 2013).
Recent research suggests that meltwater from Lake Agassiz could have had an
ice-free route to the North Atlantic at the time of the YD initiation (Leydet
et al., 2018), revising earlier ice sheet reconstructions suggesting that the
Laurentide Ice Sheet blocked the most obvious meltwater pathways to the North
Atlantic (Lowell et al., 2005). Although direct evidence that a meltwater
pulse occurred along this route immediately preceding the YD's onset is
limited, some sediment core evidence suggesting that floods through the St
Lawrence Valley coincided with the YD initiation does exist (Levac et al.,
2015; Rayburn et al., 2011). A northward meltwater flow path along the
MacKenzie Valley to the Arctic Ocean is also possible (Murton et al., 2010;
Not and Hillaire-Marcel, 2012; Tarasov and Peltier, 2005). This is supported
by both geological and modelling evidence, which suggests that a freshwater
pulse into the Arctic Ocean could have weakened AMOC by
> 30 %, considerably more that the < 15 %
resulting from a similar pulse injected into the North Atlantic (Condron and
Winsor, 2012). Still other evidence implicates meltwater from the
Fennoscandian Ice Sheet (FIS) as the YD trigger (Muschitiello et al., 2015,
2016), highlighting meltwater provenance as a key uncertainty intrinsic to
the meltwater hypothesis. Intriguingly, Muschitiello et al. (2015) find that
the FIS-derived meltwater pulse ended at 12.880 ka BP, coinciding with the
LSE. It is therefore conceivable that the meltwater pulse resulted from
insolation-controlled melting of the FIS, until the cooling associated with
the LSE and associated positive feedback reversed this trend, though
currently this remains speculative. Finally, evidence from a sediment site SW
of Iceland very strongly suggests that cooling into stadials precedes
evidence of iceberg discharges and that therefore North Atlantic
iceberg-related surface water freshening was not the trigger for stadial
events (Barker et al., 2015). Despite these issues, a meltwater pulse is
still widely considered as the leading hypothesis for the initiation of the
YD, although the meltwater source, destination, and duration are still
vigorously debated.
Iridium-rich magnetic grains, nanodiamonds, and carbon spherules coinciding
with a carbon-rich black layer at approximately 12.9 ka BP were interpreted
by Firestone et al. (2007) as evidence that an extraterrestrial impact (or
impacts) may have contributed to the YD. The consequences of the impact
(possibly a cometary airburst) were hypothesised to have destabilised the
Laurentide Ice Sheet, cooled NH climate, and contributed to the megafaunal
extinction characteristic of the period (Firestone et al., 2007; Kennett et
al., 2009). The discovery of significant amounts of impact-derived spherules
scattered across North America, Europe, Africa, and South America at
∼ 12.80 ± 0.15 ka BP further supports the Younger Dryas Impact
Hypothesis (YDIH) (Wittke et al., 2013), as does apparently
extraterrestrially derived platinum, found initially in Greenland ice (Petaev
et al., 2013) and subsequently globally (Moore et al., 2017), coincident with
the YD onset. However, other research questions the evidence of an impact,
focussing on perceived errors in the dating of the YD boundary layer
(Holliday, 2015; Meltzer et al., 2014), the misidentification of
terrestrially derived carbon spherules, shocked quartz, and nanodiamonds as
extraterrestrial (Pinter et al., 2011; van Hoesel et al., 2015; Tian et al.,
2011), the non-uniqueness of YD nanodiamond evidence (Daulton et al., 2017),
and inconsistencies regarding the physics of bolide trajectories and impacts
(Boslough et al., 2013). Despite these criticisms, independent researchers
have linked spherules found within the YD boundary layer to an impact
(LeCompte et al., 2012) and have identified the source of the spherules as
northeastern North America (Wu et al., 2013), not only supporting the
hypothesis but also suggesting an impact site proximal to the southern margin
of the Laurentide Ice Sheet (Wu et al., 2013). Most recently, Wolbach et
al. (2018a, b) catalogue extensive evidence of global wildfires they argue
were ignited by the impact of fragments of a > 100 km diameter
comet. According to Wolbach et al. (2018a, b), 9 % of global biomass
burned, considerably more than that following the end of the Cretaceous
impact event. Despite this growing catalogue of evidence of an impact event,
the YDIH remains extremely controversial. Possibly due to this controversy,
the majority of research relating to the YDIH has focussed on verifying the
evidence of an impact, and the possible climate repercussions stemming from
an impact remain relatively poorly constrained.
Of these proposed triggers, only the Laacher See eruption is universally
accepted as having occurred near the YD–Allerød boundary, and strong
evidence now exists dating the eruption precisely to the onset of YD-related
North Atlantic cooling (GS-1). However, we emphasise that these hypotheses
are not mutually exclusive. For example, meltwater releases into the North
Atlantic and Arctic Ocean undoubtedly did occur during the last deglaciation,
and a meltwater pulse and a volcanic eruption occurring within a short time
of each other could conceivably result in an amplified climate response.
Similarly, available evidence suggests that an impact event may have occurred
within a few decades of the LSE, and in fact Wolbach et al. (2018) date the
impact to 12.854 ± 0.056 ka BP based on a Bayesian analysis of 157
dated records interpreted as containing evidence of the impact. This date is
just 26 years after the 12.880 ka BP LSE (and well within dating
uncertainty) and the two events could therefore have both contributed to
environmental change associated with the YD. Conversely, the LSE could also
be directly responsible for some of the evidence of an impact, a possibility
that has not been investigated thoroughly due to the widespread, but
incorrect, concept that the eruption predates the evidence of an impact by
∼ 200 years. An impact event (independently or in conjunction with YD
cooling) is suggested to explain North American megafaunal extinctions
(Firestone et al., 2007; Wolbach et al., 2018a, b), though recent research
has built a compelling case that anthropogenic factors such as overhunting
and disease were largely responsible for the demise of many species of large
mammalian fauna (Sandom et al., 2014; Bartlett et al., 2016; van der Kaars et
al., 2017; Cooper et al., 2015; Metcalf et al., 2016). Notably, megafaunal
extinction dates appear correlated with the timing of human colonisation of
the Western Hemisphere (Surovell et al., 2016), a result inconsistent with an
impact-driven extinction. However, even if future research confirms that
megafaunal extinctions were caused by human activity rather than a bolide
impact, this does not reduce the YDIH's explanatory power as a YD trigger.
Again, it is conceivable that the LSE, a bolide impact, and/or a meltwater
pulse occurred within a short interval, reinforcing each other. A similar
eruption–impact mechanism has been proposed for major mass extinctions
through geological time (Tobin, 2017; White and Saunders, 2005; Keller,
2005), but these involved long-term flood basalt (e.g. the Deccan Traps)
emplacement rather than a single, short-lived explosive eruption. The
probability of multiple triggers occurring within a short time of each other
is low, but the possibility cannot be excluded outright.
A key issue with the YDIH, however, is that YD-type events apparently
occurred during previous terminations, such as TIII, (Broecker et al., 2010;
Cheng et al., 2009; Sima et al., 2004) that are not reasonably attributable
to other extraterrestrial impacts. However, TIII would also likely have
experienced at least one high-latitude Northern Hemisphere M> 6 volcanic eruption. Termination II lacked a YD-type event,
possibly because extremely rapid sea level rise (and ice volume decreases)
and very high insolation levels (Carlson, 2008) discouraged a strong
post-eruptive positive feedback and/or shortened the “window” of ideal ice
volume conditions. Another issue with the YDIH is that the YD was simply not
that anomalous of a cold event, and therefore does not require an unusually
powerful trigger. Over the last 120 kyr, 26 Greenland Stadial (GS) events
occurred (Rasmussen et al., 2014), of which the YD was the most recent. This
does not exclude the possibility that the YD was forced by an impact event,
but the most parsimonious explanation is that most stadial events had similar
origins, implying a much more commonplace trigger than an impact.
Furthermore, nitrogen isotopes suggest that Greenland temperature was
4.5 ∘C colder during the Oldest Dryas (18 to 14.7 ka BP) than the
YD (Buizert et al., 2014), again suggesting the transition to the YD does not
require an extreme forcing. It is also worth noting that the YD may actually
represent a return to the insolation-mediated baseline, and that potentially
the Bølling–Allerød (B-A) warm interstadial (i.e. GI-1, from 14.642 to
12.846 ka BP), immediately preceding the YD, was the anomaly (Thornalley et
al., 2011; Sima et al., 2004). Although an in-depth discussion of the B-A is
outside the scope of this study, the B-A may represent an interval with a
temporarily invigorated AMOC (an “overshoot”) (Barker et al., 2010), which,
after reaching peak strength, began to slow down back towards its glacial
state because of the lack of a concomitant rise in insolation (Knorr and
Lohmann, 2007; Thornalley et al., 2011). The rate of this slowdown was
independent of the final trigger of the YD, and indeed much of the cooling
back to the glacial baseline was achieved by ∼ 13 ka BP.
Consequently, only a small “nudge” may have been required to expedite the
return to the cold baseline state, consistent with a very sulfur-rich
volcanic eruption (occurring every few hundred years) but not necessarily
with a rare, high-consequence event. In other words, the B-A may represent
the transient anomaly, and the conditions within the YD represented typical
near-glacial conditions, obviating the need for an extreme YD triggering
mechanism. However, the gradual cooling after the B-A differs from the rapid
cooling observed at around 12.9 ka BP clearly visible in the NGRIP
δ18O and nitrogen isotope data as well as in numerous other North
Atlantic records, raising the possibility that the LSE (or other external
forcing) expedited the final cooling into the YD. It is also possible,
however, that the increased cooling rate reflects non-linearity inherent in
Atlantic oceanic circulation, potentially linked to transitions across key
thresholds between different modes of oceanic circulation (e.g. Ganopolski
and Rahmstorf, 2001).
A possible scenario is therefore that many glacial terminations were
characterised by B-A-type events that were then followed by gradual cooling
back to near-glacial conditions. This gradual cooling was sometimes
interrupted and accelerated by YD-type events forced by high-latitude
volcanism, possibly linked to crustal stresses induced by deglaciation
(Zielinski et al., 1996). Some deglaciations did not experience a YD-type
event, and we speculate that this was perhaps due to short-lived ideal ice
volume conditions not coinciding with an eruption. We stress, however, that
deglaciations were also characterised by large meltwater pulses, and
consequently the presence of a YD-type event during multiple deglaciations is
consistent with either mechanism. Additionally, the absence of a YD-type
event during some terminations may simply reflect the absence of preceding
B-A-type event and the presence of a very long stadial interval which then
transitioned directly into an interglacial (Cheng et al., 2009; Carlson,
2008). Terminations lacking a YD-type event tend to have a strong insolation
forcing that may have suppressed AMOC throughout the termination, preventing
the development of a B-A-type event and decreasing the possibility of a
YD-type event following after peak B-A-type warming (Wolff et al., 2010;
Barker et al., 2010, 2011).
Conclusions
We propose that the unusually sulfur-rich 12.880 ± 0.040 ka BP
Laacher See volcanic eruption initiated GS-1 cooling and the atmospheric
reorganisation associated with the YD event. Recent revisions to the
chronological framework of key European climate archives now strongly suggest
that the onset of GS-1-related North Atlantic cooling occurred simultaneously
with the LSE. We have identified a large volcanic sulfur spike within the
GISP2 ion data (Zielinski et al., 1997) on the recent GICC05modelext
chronology that coincides with both the onset of GS-1 cooling as recorded
within the same ice core and the date of the LSE. Lipid biomarker hydrogen
isotope ratios from Meerfelder Maar further corroborate that GS-1 atmospheric
cooling began at 12.880 ka BP, coincident with the Laacher See Tephra
within the same sediment but preceding the larger dynamical atmospheric
response associated with the YD in central Europe by ∼ 170 years (Rach
et al., 2014). Aerosol-induced cooling immediately following the eruption may
have caused a positive feedback involving sea ice expansion and/or AMOC
weakening, as previously proposed for other Greenland stadials over the
interval 30–80 ka BP (Baldini et al., 2015a), the 6th century AD (Buntgen
et al., 2016), the Little Ice Age (Zhong et al., 2011; Miller et al., 2012),
and the Holocene in general (Kobashi et al., 2017). Viewed from this
perspective, the YD was simply the latest, and last, manifestation of a last
glacial stadial.
The strongly asymmetric nature of the sulfate aerosol veil released by the
LSE cooled the Northern Hemisphere preferentially, inducing a strong
hemispheric temperature asymmetry and potentially triggering a cascade of
dynamical climate shifts across both hemispheres, including a
southward-shifted ITCZ and Hadley cells. Intermediate ice volume conditions
around ∼ 13 ka BP, driven by rising insolation during the Last
Glacial termination, may have promoted a positive feedback following the
LSE's injection of between 6.76 and 104.8 Mt of SO2 into the
stratosphere (Textor et al., 2003). This is also consistent with observations
that YD-type events were not unique to the last deglaciation but existed
during older deglaciations as well (Broecker et al., 2010). At least one
high-latitude M6 eruption likely occurred during most deglacial intervals,
and therefore GS-1 and earlier YD-type events may simply reflect the
convergence of large, sulfate-rich high-latitude NH eruptions with
intermediate ice volume conditions. NH continental ice sheet decay induced
continental lithospheric unloading and may have triggered high-latitude NH
volcanism (Zielinski et al., 1997; Sternai et al., 2016), highlighting the
intriguing possibility that eruptions such as the LSE were not randomly
distributed geographically and temporally but instead were intrinsically
linked to deglaciation. This perspective is strongly supported by a previous
observation that the three largest eruptions (including the LSE) in the East
Eifel Volcanic Field (Germany) were all associated with warming during
glacial terminations and the reduction in ice mass in northern Europe (Nowell
et al., 2006).
The hypothesis that the Laacher See eruption triggered the YD is testable.
Detailed tephrochronological studies of the ice containing the sulfate spike
identified here could confirm the source of the sulfate, and, if it is
confirmed as the LSE rather than a smaller Icelandic eruption, this would
provide an important step towards attributing GS-1 and the YD to volcanic
forcing (although evidence of GS-1 cooling already seems to occur immediately
above the LST in central Europe). Similarly, volcanic sulfate “triple”
isotope ratios of sulfur and oxygen provide information regarding the
residence time of volcanic plumes in the stratosphere. The majority of
atmospheric processes encourage mass-dependent fractionation; however, rare
mass-independent fractionation processes produce isotope ratios that do not
behave according to predictions based on mass-dependent processes (Martin et
al., 2014). Historical volcanic eruptions where sulfate aerosols reached the
stratosphere have been successfully identified in ice cores (Baroni et al.,
2008; Savarino et al., 2003), indicating that the technique is effective at
distinguishing large explosive eruptions from smaller local ones. This
technique could also determine if the sulfate in the potential LSE sulfate
spike reached the stratosphere. Although this would not necessarily confirm
the LSE as the source, it would strongly suggest that the sulfate was derived
from a climatologically significant eruption rather than a smaller Icelandic
one. Most importantly, more climate modelling studies of high-latitude,
large, and sulfur-rich eruptions under deglacial boundary conditions are
needed to constrain the climate effects of volcanism during deglaciation. The
role of halogen emissions are particularly understudied, and modelling
efforts should also quantify their effects on climate. Finally, accurate
dating of other large volcanic eruptions during intermediate ice volume
conditions is key to testing the link between volcanism and other Greenland
stadials; this information could eventually also support, or refute, the
LSE's role in triggering the YD.
More research is clearly necessary to better characterise the sensitivity of
Last Glacial climate to volcanic eruption latitude, sulfur content, and
magnitude. Due to perceived chronological mismatches, the concept that the YD
was triggered by the LSE is vastly understudied compared to both the Younger
Dryas Impact Hypothesis and the meltwater forcing hypothesis. However, the
concept that the LSE triggered the YD has clear advantages compared to other
hypotheses: (i) there is no disagreement that the eruption occurred,
(ii) available evidence suggests that the eruption occurred synchronously
with the initiation of YD cooling, (iii) a volcanic trigger is consistent
with the relatively high frequency of similar, and often more severe, cooling
events during intermediate ice volume conditions, (iv) volcanic aerosol
cooling followed by a prolonged positive feedback has been implicated in
other cooling events, and (v) events similar to the YD occurred over other
deglaciations, supporting a relatively commonplace trigger such as volcanism.
Future research may well demonstrate that the Laacher See eruption did not
play any role in catalysing the YD, but the coincidence of a large,
high-latitude, and anomalously sulfur-rich eruption with the initiation of
YD-related cooling merits serious further consideration.
Data availability
The previously published datasets used here are available
from the source publications, from the original authors, or from open access
databases (NOAA's National Climatic Data Center, PANGAEA,
etc.).
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
We thank two anonymous reviewers and David Pyle, Xu Zhang, Alan Condron, and
Evžen Stuchlik for constructive comments that greatly improved the
manuscript. We also thank David Thornalley for his editorial handling and
suggestions, which also helped improved the manuscript. Early versions of the
manuscript benefitted from critical comments by Bob Hilton, Erin McClymont,
and Lisa Baldini. Ed Llewellin is thanked for comments that helped finalise
the manuscript. We also thank Dirk Sachse for useful discussions and
Paul Mayewski for providing the GISP2 ion datasets. Edited by: David Thornalley Reviewed by: Alan
Condron, Xu Zhang, David Pyle, and two anonymous referees
ReferencesAlley, R. B.: The Younger Dryas cold interval as viewed from central
Greenland, Quaternary Sci. Rev., 19, 213–226, 10.1016/S0277-3791(99)00062-1, 2000.
Andronikov, A. V., Rudnickait, E., Lauretta, D. S., Andronikova, I. E.,
Kaminskas, D., Sinkunas, P., and Melesyte, M.: Geochemical evidence of the
presence of volcanic and meteoritic materials in Late Pleistocene lake
sediments of Lithuania, Quatern. Int., 386, 18–29, 2015.
Arce, J. L., Macias, J. L., and Vazquez-Selem, L.: The 10.5 ka Plinian
eruption of Nevado de Toluca volcano, Mexico: Stratigraphy and hazard
implications, Geol. Soc. Am. Bull., 115, 230–248, 2003.Asmerom, Y., Polyak, V. J., and Burns, S. J.: Variable winter moisture in the
southwestern United States linked to rapid glacial climate shifts, Nat.
Geosci., 3, 114–117, 10.1038/Ngeo754, 2010.Baales, M., Joris, O., Street, M., Bittmann, F., Weninger, B., and Wiethold,
J.: Impact of the late glacial eruption of the Laacher See volcano, Central
Rhineland, Germany, Quaternary Res., 58, 273–288,
10.1006/qres.2002.2379, 2002.Bahr, A., Hoffmann, J., Schönfeld, J., Schmidt, M. W., Nürnberg, D.,
Batenburg, S. J., and Voigt, S.: Low-latitude expressions of high-latitude
forcing during Heinrich Stadial 1 and the Younger Dryas in northern South
America, Glob. Planet. Change, 160, 1–9,
10.1016/j.gloplacha.2017.11.008, 2018.
Bakke, J., Lie, O., Heegaard, E., Dokken, T., Haug, G. H., Birks, H. H.,
Dulski, P., and Nilsen, T.: Rapid oceanic and atmospheric changes during the
Younger Dryas cold period, Nat. Geosci., 2, 202–205, 2009.Baldini, J. U. L., Brown, R. J., and McElwaine, J. N.: Was millennial scale
climate change during the Last Glacial triggered by explosive volcanism?,
Sci. Rep., 5, 17442, 10.1038/srep17442, 2015a.Baldini, L. M., McDermott, F., Baldini, J. U. L., Arias, P., Cueto, M.,
Fairchild, I. J., Hoffmann, D. L., Mattey, D. P., Müller, W., Nita, D.
C., Ontañón, R., Garciá-Moncó, C., and Richards, D. A.:
Regional temperature, atmospheric circulation, and sea-ice variability within
the Younger Dryas Event constrained using a speleothem from northern Iberia,
Earth Planet. Sc. Lett., 419, 101–110, 10.1016/j.epsl.2015.03.015,
2015b.Barker, S., Knorr, G., Vautravers, M. J., Diz, P., and Skinner, L. C.:
Extreme deepening of the Atlantic overturning circulation during
deglaciation, Nat. Geosci., 3, 567–571, 10.1038/ngeo921, 2010.
Barker, S., Knorr, G., Edwards, R. L., Parrenin, F., Putnam, A. E., Skinner,
L. C., Wolff, E., and Ziegler, M.: 800,000 years of abrupt climate
variability, Science, 334, 347–351, 2011.Barker, S., Chen, J., Gong, X., Jonkers, L., Knorr, G., and Thornalley, D.:
Icebergs not the trigger for North Atlantic cold events, Nature, 520,
333–338, 10.1038/Nature14330, 2015.Baroni, M., Savarino, J., Cole-Dai, J., Rai, V. K., and Thiemens, M. H.:
Anomalous sulfur isotope compositions of volcanic sulfate over the last millennium
in Antarctic ice cores, J. Geophys. Res., 113, D20112, 10.1029/2008JD010185, 2008.Bartlett, L. J., Williams, D. R., Prescott, G. W., Balmford, A., Green, R.
E., Eriksson, A., Valdes, P. J., Singarayer, J. S., and Manica, A.:
Robustness despite uncertainty: regional climate data reveal the dominant
role of humans in explaining global extinctions of Late Quaternary megafauna,
Ecography, 39, 152–161, 10.1111/ecog.01566, 2016.Bay, R. C., Bramall, N., and Price, P. B.: Bipolar correlation of volcanism
with millennial climate change, P. Natl. Acad. Sci. USA, 101, 6341–6345,
10.1073/pnas.0400323101, 2004.Bereiter, B., Shackleton, S., Baggenstos, D., Kawamura, K., and Severinghaus,
J.: Mean global ocean temperatures during the last glacial transition,
Nature, 553, 39–44,
10.1038/nature25152, 2018.
Berger, W. H.: The Younger Dryas cold spell – a quest for causes, Glob.
Planet. Change, 89, 219–237, 1990.Blaga, C. I., Reichart, G. J., Lotter, A. F., Anselmetti, F. S., and Damste,
J. S. S.: A TEX86 lake record suggests simultaneous shifts in temperature in
Central Europe and Greenland during the last deglaciation, Geophys. Res.
Lett., 40, 948–953, 10.1002/grl.50181, 2013.Bogaard, P. and Schmincke, H. U.: Laacher See Tephra – a widespread
isochronous Late Quaternary tephra layer in central and northern Europe,
Geol. Soc. Am. Bull., 96, 1554–1571,
10.1130/0016-7606(1985)96<1554:Lstawi>2.0.Co;2, 1985.
Bondevik, S., Mangerud, J., Birks, H. H., Gulliksen, S., and Reimer, P.:
Changes in North Atlantic radiocarbon reservoir ages during the Allerod and
Younger Dryas, Science, 312, 1514–1517, 2006.Boslough, M., Harris, A. W., Chapman, C., and Morrison, D.: Younger Dryas
impact model confuses comet facts, defies airburst physics, P. Natl. Acad.
Sci. USA, 110, E4170–E4170, 10.1073/pnas.1313495110, 2013.Brauer, A., Endres, C., Gunter, C., Litt, T., Stebich, M., and Negendank, J.
F. W.: High resolution sediment and vegetation responses to Younger Dryas
climate change in varved lake sediments from Meerfelder Maar, Germany,
Quaternary Sci. Rev., 18, 321–329, 10.1016/S0277-3791(98)00084-5, 1999a.
Brauer, A., Endres, C., and Negendank, J.: Lateglacial calendar year
chronology based on annually laminated sediments from Lake Meerfelder Maar,
Germany, 17–25, 1999b.Brauer, A., Haug, G. H., Dulski, P., Sigman, D. M., and Negendank, J. F. W.:
An abrupt wind shift in western Europe at the onset of the Younger Dryas cold
period, Nat. Geosci., 1, 520–523, 10.1038/ngeo263, 2008.Broecker, W. S.: Salinity history of the Northern Atlantic during the last
deglaciation, Paleoceanography, 5, 459–467, 10.1029/Pa005i004p00459,
1990.Broecker, W. S.: Was the younger dryas triggered by a flood?, Science, 312,
1146–1148, 10.1126/science.1123253, 2006a.Broecker, W. S.: Abrupt climate change revisited, Glob. Planet. Change, 54,
211–215, 10.1016/j.gloplacha.2006.06.019, 2006b.Broecker, W. S., Andree, M., Wolfli, W., Oeschger, H., Bonani, G., Kennett,
J., and Peteet, D.: The chronology of the last deglaciation: Implications to
the cause of the Younger Dryas Event, Paleoceanography, 3, 1–19,
10.1029/Pa003i001p00001, 1988.Broecker, W. S., Denton, G. H., Edwards, R. L., Cheng, H., Alley, R. B., and
Putnam, A. E.: Putting the Younger Dryas cold event into context, Quaternary
Sci. Rev., 29, 1078–1081, 10.1016/j.quascirev.2010.02.019, 2010.Bronk Ramsey, C., Albert, P. G., Blockley, S. P. E., Hardiman, M., Housley,
R. A., Lane, C. S., Lee, S., Matthews, I. P., Smith, V. C., and Lowe, J. J.:
Improved age estimates for key Late Quaternary European tephra horizons in
the RESET lattice, Quaternary Sci. Rev., 118, 18–32,
10.1016/j.quascirev.2014.11.007, 2015.
Brown, S., Crosweller, H., Sparks, R. S., Cottrell, E., Deligne, N.,
Guerrero, N., Hobbs, L., Kiyosugi, K., Loughlin, S., Siebert, L., and
Takarada, S.: Characterisation of the Quaternary eruption record: analysis of
the Large Magnitude Explosive Volcanic Eruptions (LaMEVE) database, J. Appl.
Volcanol., 3, 5, 2014.
Buizert, C., Gkinis, V., Severinghaus, J. P., He, F., Lecavalier, B. S.,
Kindler, P., Leuenberger, M., Carlson, A. E., Vinther, B., Masson-Delmotte,
V., White, J. W. C., Liu, Z. Y., Otto-Bliesner, B., and Brook, E. J.:
Greenland temperature response to climate forcing during the last
deglaciation, Science, 345, 1177–1180, 2014.Buntgen, U., Myglan, V. S., Ljungqvist, F. C., McCormick, M., Di Cosmo, N.,
Sigl, M., Jungclaus, J., Wagner, S., Krusic, P. J., Esper, J., Kaplan, J. O.,
de Vaan, M. A. C., Luterbacher, J., Wacker, L., Tegel, W., and Kirdyanov, A.
V.: Cooling and societal change during the Late Antique Little Ice Age from
536 to around 660 AD, Nat. Geosci., 9, 231–236, 10.1038/NGEO2652, 2016.
Cabedo-Sanz, P., Belt, S. T., Knies, J., and Husum, K.: Identification of
contrasting seasonal sea ice conditions during the Younger Dryas, Quaternary
Sci. Rev., 79, 74–86, 2013.
Cadoux, A., Scaillet, B., Bekki, S., Oppenheimer, C., and Druitt, T. H.:
Stratospheric ozone destruction by the Bronze-Age Minoan eruption (Santorini
Volcano, Greece), Sci. Rep., 5, 12243, 2015.Carlson, A. E.: Why there was not a Younger Dryas-like event during the
Penultimate Deglaciation, Quaternary Sci. Rev., 27, 882–887,
10.1016/j.quascirev.2008.02.004, 2008.Carlson, A. E., Clark, P. U., Haley, B. A., Klinkhammer, G. P., Simmons, K.,
Brook, E. J., and Meissner, K. J.: Geochemical proxies of North American
freshwater routing during the Younger Dryas cold event, P. Natl. Acad. Sci.
USA, 104, 6556–6561, 10.1073/pnas.0611313104, 2007.Carn, S. A., Clarisse, L., and Prata, A. J.: Multi-decadal satellite
measurements of global volcanic degassing, J. Volcanol. Geotherm. Res., 311,
99–134, 10.1016/j.jvolgeores.2016.01.002, 2016.Carolin, S. A., Cobb, K. M., Adkins, J. F., Clark, B., Conroy, J. L., Lejau,
S., Malang, J., and Tuen, A. A.: Varied response of western Pacific hydrology
to climate forcings over the Last Glacial period, Science, 340, 1564–1566,
10.1126/science.1233797, 2013.
Cheng, H., Edwards, R. L., Broecker, W. S., Denton, G. H., Kong, X. G., Wang,
Y. J., Zhang, R., and Wang, X. F.: Ice age terminations, Science, 326,
248–252, 2009.
Chiang, J. C. H. and Bitz, C. M.: Influence of high latitude ice cover on
the marine Intertropical Convergence Zone, Clim. Dynam., 25, 477–496, 2005.Chiang, J. C. H., Lee, S. Y., Putnam, A. E., and Wang, X. F.: South Pacific
Split Jet, ITCZ shifts, and atmospheric North-South linkages during abrupt
climate changes of the last glacial period, Earth Planet. Sc. Lett., 406,
233–246, 10.1016/j.epsl.2014.09.012, 2014.Colose, C. M., LeGrande, A. N., and Vuille, M.: Hemispherically asymmetric
volcanic forcing of tropical hydroclimate during the last millennium, Earth
Syst. Dynam., 7, 681–696, 10.5194/esd-7-681-2016, 2016.Condron, A. and Winsor, P.: Meltwater routing and the Younger Dryas, P. Natl.
Acad. Sci. USA, 109, 19928–19933, 10.1073/pnas.1207381109, 2012.
Cooper, A., Turney, C., Hughen, K. A., Brook, B. W., McDonald, H. G., and
Bradshaw, C. J. A.: Abrupt warming events drove Late Pleistocene Holarctic
megafaunal turnover, Science, 349, 602–606, 2015.Daulton, T. L., Amari, S., Scott, A. C., Hardiman, M., Pinter, N., and
Anderson, R. S.: Comprehensive analysis of nanodiamond evidence relating to
the Younger Dryas Impact Hypothesis, J. Quaternary Sci., 32, 7–34,
10.1002/jqs.2892, 2017.de Klerk, P., Janke, W. F., Kuehn, P., and Theuerkauf, M.: Environmental
impact of the Laacher See eruption at a large distance from the volcano:
Integrated palaeoecological studies from Vorpommern (NE Germany),
Palaeogeogr. Palaeocl., 270, 196–214, 10.1016/j.palaeo.2008.09.013,
2008.Deligne, N., Coles, S., and Sparks, R.: Recurrence rates of large explosive
volcanic eruptions, J. Geophys. Res., 115, B06203, 10.1029/2009JB006554 2010.Delworth, T. L., Zeng, F., Vecchi, G. A., Yang, X., Zhang, L., and Zhang, R.:
The North Atlantic Oscillation as a driver of rapid climate change in the
Northern Hemisphere, Nat. Geosci., 9, 509–512, 10.1038/ngeo2738, 2016.
Devine, J. D., Sigurdsson, H., Davis, A. N., and Self, S.: Estimates of
Sulfur and Chlorine Yield to the Atmosphere from Volcanic-Eruptions and
Potential Climatic Effects, J. Geophys. Res., 89, 6309–6325, 1984.
Diallo, M., Ploeger, F., Konopka, P., Birner, T., Muller, R., Riese, M.,
Garny, H., Legras, B., Ray, E., Berthet, G., and Jegou, F.: Significant
contributions of volcanic aerosols to decadal changes in the stratospheric
circulation, Geophys. Res. Lett., 44, 10780–10791, 2017.
Ding, Y. N., Carton, J. A., Chepurin, G. A., Stenchikov, G., Robock, A.,
Sentman, L. T., and Krasting, J. P.: Ocean response to volcanic eruptions in
Coupled Model Intercomparison Project 5 simulations, J. Geophys. Res.-Ocean.,
119, 5622–5637, 2014.Engels, S., van Geel, B., Buddelmeijer, N., and Brauer, A.: High-resolution
palynological evidence for vegetation response to the Laacher See eruption
from the varved record of Meerfelder Maar (Germany) and other central
European records, Rev. Palaeobot. Palyno., 221, 160–170,
10.1016/j.revpalbo.2015.06.010, 2015.
Engels, S., Brauer, A., Buddelmeijer, N., Martin-Puertas, C., Rach, O.,
Sachse, D., and Van Geel, B.: Subdecadal-scale vegetation responses to a
previously unknown late-Allerod climate fluctuation and Younger Dryas cooling
at Lake Meerfelder Maar (Germany), J. Quaternary Sci., 31, 741–752, 2016.
Eychenne, J., Cashman, K., Rust, A., and Durant, A.: Impact of the lateral
blast on the spatial pattern and grain size characteristics of the 18 May
1980 Mount St. Helens fallout deposit, J. Geophys. Res.-Sol. Ea., 120,
6018–6038, 2015.Firestone, R. B., West, A., Kennett, J. P., Becker, L., Bunch, T. E., Revay,
Z. S., Schultz, P. H., Belgya, T., Kennett, D. J., Erlandson, J. M.,
Dickenson, O. J., Goodyear, A. C., Harris, R. S., Howard, G. A., Kloosterman,
J. B., Lechler, P., Mayewski, P. A., Montgomery, J., Poreda, R., Darrah, T.,
Hee, S. S. Q., Smitha, A. R., Stich, A., Topping, W., Wittke, J. H., and
Wolbach, W. S.: Evidence for an extraterrestrial impact 12,900 years ago that
contributed to the megafaunal extinctions and the Younger Dryas cooling, P.
Natl. Acad. Sci. USA, 104, 16016–16021, 10.1073/pnas.0706977104, 2007.Flower, B. P. and Kennett, J. P.: The Younger Dryas Cool Episode in the Gulf
of Mexico, Paleoceanography, 5, 949–961, 10.1029/Pa005i006p00949, 1990.
Ganopolski, A. and Rahmstorf, S.: Rapid changes of glacial climate simulated
in a coupled climate model, Nature, 409, 153–158, 2001.
Gerlach, T. M., Westrich, H. R., and Symonds, R. B.: Preeruption vapor in
magma of the climactic Mount Pinatubo eruption: Source of the giant
stratospheric sulfur dioxide cloud, in: Fire and Mud: eruptions and lahars of
Mount Pinotubo, Phillipines, edited by: Newhall, C. G. and Punongbayan, R.
S., University of Washington Press, Seattle, 415–433, 1996.Graf, H. F. and Timmreck, C.: A general climate model simulation of the
aerosol radiative effects of the Laacher See eruption (10,900 BC), J.
Geophys. Res.-Atmos., 106, 14747–14756, 10.1029/2001jd900152, 2001.Hajdas, I., IvyOchs, S. D., Bonani, G., Lotter, A. F., Zolitschka, B., and
Schluchter, C.: Radiocarbon age of the Laacher See Tephra:
11,230 ± 40 BP, Radiocarbon, 37, 149–154, 1995.Harms, E. and Schmincke, H. U.: Volatile composition of the phonolitic
Laacher See magma (12,900 yr BP): implications for syn-eruptive degassing
of S, F, Cl and H2O, Contrib. Mineral. Petrol., 138, 84–98,
10.1007/Pl00007665, 2000.Haslam, M., and Petraglia, M.: Comment on “Environmental impact of the
73 ka Toba super-eruption in South Asia” by M. A. J. Williams, S. H.
Ambrose, S. van der Kaars, C. Ruehlemann, U. Chattopadhyaya, J. Pal and P. R.
Chauhan, Palaeogeogr. Palaeocl., 296, 199–203,
10.1016/j.palaeo.2010.03.057, 2010.Hogg, A., Southon, J., Turney, C., Palmer, J., Ramsey, C. B., Fenwick, P.,
Boswijk, G., Friedrich, M., Helle, G., Hughen, K., Jones, R., Kromer, B.,
Noronha, A., Reynard, L., Staff, R., and Wacker, L.: Punctuated Shutdown of
Atlantic Meridional Overturning Circulation during Greenland Stadial 1, Sci.
Rep., 6, 25902, 10.1038/Srep25902, 2016.Holliday, V. T.: Problematic dating of claimed Younger Dryas boundary impact
proxies, P. Natl. Acad. Sci. USA, 112, E6721, 10.1073/pnas.1518945112,
2015.Hwang, Y. T., Frierson, D. M. W., and Kang, S. M.: Anthropogenic sulfate
aerosol and the southward shift of tropical precipitation in the late 20th
century, Geophys. Res. Lett., 40, 2845–2850, 10.1002/Grl.50502, 2013.Johnson, R. G. and McClure, B. T.: Model for Northern Hemisphere continental
ice sheet variation, Quaternary Res., 6, 325–353,
10.1016/0033-5894(67)90001-4, 1976.Kaplan, M. R., Schaefer, J. M., Denton, G. H., Barrell, D. J. A., Chinn, T.
J. H., Putnam, A. E., Andersen, B. G., Finkel, R. C., Schwartz, R., and
Doughty, A. M.: Glacier retreat in New Zealand during the Younger Dryas
stadial, Nature, 467, 194–197, 10.1038/nature09313, 2010.
Keller, G.: Impacts, volcanism and mass extinction: random coincidence or
cause and effect?, Aust. J. Earth Sci., 52, 725–757, 2005.Kennett, D. J., Kennett, J. P., West, A., Mercer, C., Hee, S. S. Q., Bement,
L., Bunch, T. E., Sellers, M., and Wolbach, W. S.: Nanodiamonds in the
Younger Dryas Boundary Sediment Layer, Science, 323, 94–94,
10.1126/science.1162819, 2009.
Klobas, J. E., Wilmouth, D. M., Weisenstein, D. K., Anderson, J. G., and
Salawitch, R. J.: Ozone depletion following future volcanic eruptions,
Geophys. Res. Lett., 44, 7490–7499, 2017.Knorr, G. and Lohmann, G.: Rapid transitions in the Atlantic thermohaline
circulation triggered by global warming and meltwater during the last
deglaciation, Geochem. Geophys. Geosy., 8, Q12006, 10.1029/2007GC001604, 2007.Kobashi, T., Menviel, L., Jeltsch-Thömmes, A., Vinther, B. M., Box, J.
E., Muscheler, R., Nakaegawa, T., Pfister, P. L., Döring, M.,
Leuenberger, M., Wanner, H., and Ohmura, A.: Volcanic influence on centennial
to millennial Holocene Greenland temperature change, Sci. Rep., 7, 1441,
10.1038/s41598-017-01451-7, 2017.
Kutterolf, S., Hansteen, T. H., Appel, K., Freundt, A., Kruger, K., Perez,
W., and Wehrmann, H.: Combined bromine and chlorine release from large
explosive volcanic eruptions: A threat to stratospheric ozone?, Geology, 41,
707–710, 2013.Lane, C. S., Brauer, A., Blockley, S. P. E., and Dulski, P.: Volcanic ash
reveals time-transgressive abrupt climate change during the Younger Dryas,
Geology, 41, 1251–1254, 10.1130/G34867.1, 2013a.Lane, C. S., Chorn, B. T., and Johnson, T. C.: Ash from the Toba
supereruption in Lake Malawi shows no volcanic winter in East Africa at 75
ka, P. Natl. Acad. Sci. USA, 110, 8025–8029, 10.1073/pnas.1301474110,
2013b.Lane, C. S., Brauer, A., Martín-Puertas, C., Blockley, S. P. E., Smith,
V. C., and Tomlinson, E. L.: The Late Quaternary tephrostratigraphy of
annually laminated sediments from Meerfelder Maar, Germany, Quaternary Sci.
Rev., 122, 192–206, 10.1016/j.quascirev.2015.05.025, 2015.LeCompte, M. A., Goodyear, A. C., Demitroff, M. N., Batchelor, D., Vogel, E.
K., Mooney, C., Rock, B. N., and Seidel, A. W.: Independent evaluation of
conflicting microspherule results from different investigations of the
Younger Dryas impact hypothesis, P. Natl. Acad. Sci. USA, 109, E2960–E2969,
10.1073/pnas.1208603109, 2012.LeGrande, A. N., Tsigaridis, K., and Bauer, S. E.: Role of atmospheric
chemistry in the climate impacts of stratospheric volcanic injections, Nat.
Geosci., 9, 652, 10.1038/ngeo2771, 2016.
Lehner, F., Born, A., Raible, C. C., and Stocker, T. F.: Amplified Inception
of European Little Ice Age by Sea Ice-Ocean-Atmosphere Feedbacks, J. Clim.,
26, 7586–7602, 2013.Levac, E., Lewis, M., Stretch, V., Duchesne, K., and Neulieb, T.: Evidence
for meltwater drainage via the St. Lawrence River Valley in marine cores from
the Laurentian Channel at the time of the Younger Dryas, Glob. Planet.
Change, 130, 47–65, 10.1016/j.gloplacha.2015.04.002, 2015.Leydet, D. J., Carlson, A. E., Teller, J. T., Breckenridge, A., Barth, A. M.,
Ullman, D. J., Sinclair, G., Milne, G. A., Cuzzone, J. K., and Caffee, M. W.:
Opening of glacial Lake Agassiz's eastern outlets by the start of the Younger
Dryas cold period, Geology, 46, 155–158, 10.1130/G39501.1, 2018.
Litt, T., Brauer, A., Goslar, T., Merkt, J., Balaga, K., Muller, H.,
Ralska-Jasiewiczowa, M., Stebich, M., and Negendank, J. F. W.: Correlation
and synchronisation of Lateglacial continental sequences in northern central
Europe based on annually laminated lacustrine sediments, Quaternary Sci.
Rev., 20, 1233–1249, 2001.Lotter, A. F., Eicher, U., Siegenthaler, U., and Birks, H. J. B.:
Late-glacial climatic oscillations as recorded in Swiss lake sediments, J.
Quaternary Sci., 7, 187–204, 10.1002/jqs.3390070302, 1992.Lowell, T., Waterson, N., Fisher, T., Loope, H., Glover, K., Comer, G.,
Hajdas, I., Denton, G., Schaefer, J., Rinterknecht, V., Broecker, W., and
Teller, J.: Testing the Lake Agassiz meltwater trigger for the Younger Dryas,
Eos, Transactions American Geophysical Union, 86, 365–372,
10.1029/2005EO400001, 2005.Lynch-Stieglitz, J.: The Atlantic Meridional Overturning Circulation and
Abrupt Climate Change, Ann. Rev. Mar. Sci., 9, 83–104,
10.1146/annurev-marine-010816-060415, 2017.
Martin, E., Bekki, S., Ninin, C., and Bindeman, I.: Volcanic sulfate aerosol
formation in the troposphere, J. Geophys. Res.-Atmos., 119, 12660–12673,
2014.McManus, J. F., Francois, R., Gherardi, J. M., Keigwin, L. D., and
Brown-Leger, S.: Collapse and rapid resumption of Atlantic meridional
circulation linked to deglacial climate changes, Nature, 428, 834–837,
10.1038/nature02494, 2004.Meissner, K. J.: Younger Dryas: A data to model comparison to constrain the
strength of the overturning circulation, Geophys. Res. Lett.,
34, L21705, 10.1029/2007GL031304, 2007.Meltzer, D. J., Holliday, V. T., Cannon, M. D., and Miller, D. S.:
Chronological evidence fails to support claim of an isochronous widespread
layer of cosmic impact indicators dated to 12,800 years ago, P. Natl. Acad.
Sci. USA, 111, E2162–E2171, 10.1073/pnas.1401150111, 2014.Merkt, J. and Muller, H.: Varve chronology and palynology of the Lateglacial
in Northwest Germany from lacustrine sediments of Hamelsee in Lower Saxony,
Quatern. Int., 61, 41–59, 10.1016/S1040-6182(99)00016-6, 1999.Metcalf, J. L., Turney, C., Barnett, R., Martin, F., Bray, S. C., Vilstrup,
J. T., Orlando, L., Salas-Gismondi, R., Loponte, D., Medina, M., De Nigris,
M., Civalero, T., Fernandez, P. M., Gasco, A., Duran, V., Seymour, K. L.,
Otaola, C., Gil, A., Paunero, R., Prevosti, F. J., Bradshaw, C. J. A.,
Wheeler, J. C., Borrero, L., Austin, J. J., and Cooper, A.: Synergistic roles
of climate warming and human occupation in Patagonian megafaunal extinctions
during the Last Deglaciation, Sci. Adv., 2, e1501682, 10.1126/sciadv.1501682, 2016.Mignot, J., Khodri, M., Frankignoul, C., and Servonnat, J.: Volcanic impact
on the Atlantic Ocean over the last millennium, Clim. Past, 7, 1439–1455,
10.5194/cp-7-1439-2011, 2011.Miller, G. H., Geirsdóttir, Á., Zhong, Y., Larsen, D. J.,
Otto-Bliesner, B. L., Holland, M. M., Bailey, D. A., Refsnider, K. A.,
Lehman, S. J., Southon, J. R., Anderson, C., Björnsson, H., and
Thordarson, T.: Abrupt onset of the Little Ice Age triggered by volcanism and
sustained by sea-ice/ocean feedbacks, Geophys. Res. Lett., 39, L02708,
10.1029/2011GL050168, 2012.Moore, C. R., West, A., LeCompte, M. A., Brooks, M. J., Daniel Jr, I. R.,
Goodyear, A. C., Ferguson, T. A., Ivester, A. H., Feathers, J. K., Kennett,
J. P., Tankersley, K. B., Adedeji, A. V., and Bunch, T. E.: Widespread
platinum anomaly documented at the Younger Dryas onset in North American
sedimentary sequences, Sci. Rep., 7, 44031, 10.1038/srep44031, 2017.
Moreno-Chamarro, E., Zanchettin, D., Lohmann, K., and Jungclaus, J. H.: An
abrupt weakening of the subpolar gyre as trigger of Little Ice Age-type
episodes, Clim. Dynam., 48, 727–744, 2017.Moreno, A., Stoll, H., Jimenez-Sanchez, M., Cacho, I., Valero-Garces, B.,
Ito, E., and Edwards, R. L.: A speleothem record of glacial
(25–11.6 kyr BP) rapid climatic changes from northern Iberian Peninsula,
Glob. Planet. Change, 71, 218–231, 10.1016/j.gloplacha.2009.10.002,
2010.
Mortensen, A. K., Bigler, M., Gronvold, K., Steffensen, J. P., and Johnsen,
S. J.: Volcanic ash layers from the Last Glacial Termination in the NGRIP ice
core, J. Quaternary Sci., 20, 209–219, 2005.Murton, J. B., Bateman, M. D., Dallimore, S. R., Teller, J. T., and Yang, Z.
R.: Identification of Younger Dryas outburst flood path from Lake Agassiz to
the Arctic Ocean, Nature, 464, 740–743, 10.1038/Nature08954, 2010.
Muscheler, R., Adolphi, F., and Knudsen, M. F.: Assessing the differences
between the IntCal and Greenland ice-core time scales for the last 14,000
years via the common cosmogenic radionuclide variations, Quaternary Sci.
Rev., 106, 81–87, 2014.Muschitiello, F. and Wohlfarth, B.: Time-transgressive environmental shifts
across Northern Europe at the onset of the Younger Dryas, Quaternary Sci.
Rev., 109, 49–56, 10.1016/j.quascirev.2014.11.015, 2015.Muschitiello, F., Pausata, F. S. R., Watson, J. E., Smittenberg, R. H.,
Salih, A. A. M., Brooks, S. J., Whitehouse, N. J., Karlatou-Charalampopoulou,
A., and Wohlfarth, B.: Fennoscandian freshwater control on Greenland
hydroclimate shifts at the onset of the Younger Dryas, Nat. Commun.,
6, 8939, 10.1038/ncomms9939, 2015.Muschitiello, F., Lea, J. M., Greenwood, S. L., Nick, F. M., Brunnberg, L.,
MacLeod, A., and Wohlfarth, B.: Timing of the first drainage of the Baltic
Ice Lake synchronous with the onset of Greenland Stadial 1, Boreas, 45,
322–334, 10.1111/bor.12155, 2016.Muschitiello, F., Pausata, F. S. R., Lea, J. M., Mair, D. W. F., and
Wohlfarth, B.: Enhanced ice sheet melting driven by volcanic eruptions during
the last deglaciation, Nat. Commun., 8, 1020, 10.1038/s41467-017-01273-1,
2017.Not, C. and Hillaire-Marcel, C.: Enhanced sea-ice export from the Arctic
during the Younger Dryas, Nat. Commun., 3, 647, 10.1038/ncomms1658, 2012.
Nowell, D. A. G., Jones, M. C., and Pyle, D. M.: Episodic Quaternary
volcanism in France and Germany, J. Quaternary Sci., 21, 645–675, 2006.Oppenheimer, C.: Ice core and palaeoclimatic evidence for the timing and
nature of the great mid-13th century volcanic eruption, Int. J. Climatol.,
23, 417–426, 10.1002/Joc.891, 2003.
Ottera, O. H., Bentsen, M., Drange, H., and Suo, L. L.: External forcing as a
metronome for Atlantic multidecadal variability, Nat. Geosci., 3, 688–694,
2010.
Pausata, F. S. R., Chafik, L., Caballero, R., and Battisti, D. S.: Impacts of
high-latitude volcanic eruptions on ENSO and AMOC, P. Natl. Acad. Sci. USA,
112, 13784–13788, 2015.Petaev, M. I., Huang, S. C., Jacobsen, S. B., and Zindler, A.: Large Pt
anomaly in the Greenland ice core points to a cataclysm at the onset of
Younger Dryas, P. Natl. Acad. Sci. USA, 110, 12917–12920,
10.1073/pnas.1303924110, 2013.Pinter, N., Scott, A. C., Daulton, T. L., Podoll, A., Koeberl, C., Anderson,
R. S., and Ishman, S. E.: The Younger Dryas impact hypothesis: A requiem,
Earth Sci. Rev., 106, 247–264, 10.1016/j.earscirev.2011.02.005, 2011.
Pitari, G., Di Genova, G., Mancini, E., Visioni, D., Gandolfi, I., and
Cionni, I.: Stratospheric aerosols from major volcanic eruptions: a
composition-climate model study of the aerosol cloud dispersal and e-folding
time, Atmosphere-Basel, 7, 75, 2016.Polyak, V. J., Rasmussen, J. B. T., and Asmerom, Y.: Prolonged wet period in
the southwestern United States through the Younger Dryas, Geology, 32, 5–8,
10.1130/G19957.1, 2004.
Polyak, V. J., Asmerom, Y., and Lachniet, M. S.: Rapid speleothem delta C-13
change in southwestern North America coincident with Greenland stadial 20 and
the Toba (Indonesia) supereruption, Geology, 45, 843–846, 2017.Rach, O., Brauer, A., Wilkes, H., and Sachse, D.: Delayed hydrological
response to Greenland cooling at the onset of the Younger Dryas in western
Europe, Nat. Geosci., 7, 109–112, 10.1038/NGEO2053, 2014.Rampino, M. R. and Self, S.: Historic eruptions of Tambora (1815), Krakatau
(1883), and Agung (1963), their stratospheric aerosols, and climatic impact,
Quaternary Res., 18, 127–143, 10.1016/0033-5894(82)90065-5, 1982.Rampino, M. R. and Self, S.: Sulfur-rich volcanic-eruptions and stratospheric
aerosols, Nature, 310, 677–679, 10.1038/310677a0, 1984.
Rampino, M. R. and Self, S.: Volcanic winter and accelerated glaciation
following the Toba super-eruption, Nature, 359, 50–52, 1992.Rasmussen, S. O., Andersen, K. K., Svensson, A. M., Steffensen, J. P.,
Vinther, B. M., Clausen, H. B., Siggaard-Andersen, M. L., Johnsen, S. J.,
Larsen, L. B., Dahl-Jensen, D., Bigler, M., Rothlisberger, R., Fischer, H.,
Goto-Azuma, K., Hansson, M. E., and Ruth, U.: A new Greenland ice core
chronology for the last glacial termination, J. Geophys. Res.-Atmos., 111,
D06102, 10.1029/2005JD006079, 2006.Rasmussen, S. O., Bigler, M., Blockley, S. P., Blunier, T., Buchardt, S. L.,
Clausen, H. B., Cvijanovic, I., Dahl-Jensen, D., Johnsen, S. J., Fischer, H.,
Gkinis, V., Guillevic, M., Hoek, W. Z., Lowe, J. J., Pedro, J. B., Popp, T.,
Seierstad, I. K., Steffensen, J. P., Svensson, A. M., Vallelonga, P.,
Vinther, B. M., Walker, M. J. C., Wheatley, J. J., and Winstrup, M.: A
stratigraphic framework for abrupt climatic changes during the Last Glacial
period based on three synchronized Greenland ice-core records: refining and
extending the INTIMATE event stratigraphy, Quaternary Sci. Rev., 106, 14–28,
10.1016/j.quascirev.2014.09.007, 2014.Rayburn, J. A., Cronin, T. M., Franzi, D. A., Knuepfer, P. L. K., and
Willard, D. A.: Timing and duration of North American glacial lake discharges
and the Younger Dryas climate reversal, Quaternary Res., 75, 541–551,
10.1016/j.yqres.2011.02.004, 2011.Renssen, H., Mairesse, A., Goosse, H., Mathiot, P., Heiri, O., Roche, D. M.,
Nisancioglu, K. H., and Valdes, P. J.: Multiple causes of the Younger Dryas
cold period, Nat. Geosci., 8, 946–980, 10.1038/NGEO2557, 2015.Ridgwell, A., Maslin, M., and Kaplan, J. O.: Flooding of the continental
shelves as a contributor to deglacial CH4 rise, J. Quaternary Sci., 27,
800–806, 10.1002/jqs.2568, 2012.Ridley, H. E., Asmerom, Y., Baldini, J. U. L., Breitenbach, S. F. M., Aquino,
V. V., Prufer, K. M., Culleton, B. J., Polyak, V., Lechleitner, F. A.,
Kennett, D. J., Zhang, M., Marwan, N., Macpherson, C. G., Baldini, L. M.,
Xiao, T., Peterkin, J. L., Awe, J., and Haug, G. H.: Aerosol forcing of the
position of the intertropical convergence zone since AD1550, Nat. Geosci., 8,
195–200, 10.1038/ngeo2353, 2015.
Robock, A.: Volcanic eruptions and climate, Rev. Geophys., 38, 191–219,
2000.Robock, A. and Mao, J. P.: The volcanic signal in surface temperature
observations, J. Clim., 8, 1086–1103,
10.1175/1520-0442(1995)008<1086:Tvsist>2.0.Co;2, 1995.Robock, A., Ammann, C. M., Oman, L., Shindell, D., Levis, S., and Stenchikov,
G.: Did the Toba volcanic eruption of similar to 74 ka BP produce
widespread glaciation?, J. Geophys. Res.-Atmos., 114, D10107, 10.1029/2008JD011652, 2009.Sadler, J. P. and Grattan, J. P.: Volcanoes as agents of past environmental
change, Glob. Planet. Change, 21, 181–196,
10.1016/S0921-8181(99)00014-4, 1999.Sandom, C., Faurby, S., Sandel, B., and Svenning, J. C.: Global late
Quaternary megafauna extinctions linked to humans, not climate change, P.
Roy. Soc. B-Biol. Sci., 281, 20133254,
10.1098/rspb.2013.3254, 2014.Savarino, J., Bekki, S., Cole-Dai, J. H., and Thiemens, M. H.: Evidence from
sulfate mass independent oxygen isotopic compositions of dramatic changes in
atmospheric oxidation following massive volcanic eruptions, J. Geophys.
Res.-Atmos., 108, 4671, 10.1029/2003JD003737, 2003.
Scaillet, B., Luhr, J. F., and Carroll, M. R.: Petrological and
volcanological constraints on volcanic sulfur emissions to the atmosphere,
in: Volcanism and the Earth's Atmosphere (Geophysical Monograph Series),
edited by: Robock, A. and Oppenheimer, C., 11–40, 2004.Schenk, F., Väliranta, M., Muschitiello, F., Tarasov, L., Heikkilä,
M., Björck, S., Brandefelt, J., Johansson, A. V., Näslund, J.-O., and
Wohlfarth, B.: Warm summers during the Younger Dryas cold reversal, Nat.
Commun., 9, 1634, 10.1038/s41467-018-04071-5, 2018.Schleussner, C. F. and Feulner, G.: A volcanically triggered regime shift in
the subpolar North Atlantic Ocean as a possible origin of the Little Ice Age,
Clim. Past, 9, 1321–1330, 10.5194/cp-9-1321-2013, 2013.
Schmidt, M. W., Vautravers, M. J., and Spero, H. J.: Rapid subtropical North
Atlantic salinity oscillations across Dansgaard-Oeschger cycles, Nature, 443,
561–564, 2006.
Schmincke, H. U.: Volcanism, Springer-Verlag, Berlin, 327 pp., 2004.Schmincke, H. U., Park, C., and Harms, E.: Evolution and environmental
impacts of the eruption of Laacher See Volcano (Germany) 12,900 a BP,
Quatern. Int., 61, 61–72, 10.1016/S1040-6182(99)00017-8, 1999.Scott, A. C., Hardiman, M., Pinter, N., Anderson, R. S., Daulton, T. L.,
Ejarque, A., Finch, P., and Carter-champion, A.: Interpreting palaeofire
evidence from fluvial sediments: a case study from Santa Rosa Island,
California, with implications for the Younger Dryas Impact Hypothesis, J.
Quaternary Sci., 32, 35–47, 10.1002/jqs.2914, 2017.Seierstad, I. K., Abbott, P. M., Bigler, M., Blunier, T., Bourne, A. J.,
Brook, E., Buchardt, S. L., Buizert, C., Clausen, H. B., Cook, E.,
Dahl-Jensen, D., Davies, S. M., Guillevic, M., Johnsen, S. J., Pedersen, D.
S., Popp, T. J., Rasmussen, S. O., Severinghaus, J. P., Svensson, A., and
Vinther, B. M.: Consistently dated records from the Greenland GRIP, GISP2 and
NGRIP ice cores for the past 104 ka reveal regional millennial-scale delta
O-18 gradients with possible Heinrich event imprint, Quaternary Sci. Rev.,
106, 29–46, 10.1016/j.quascirev.2014.10.032, 2014.
Shakun, J. D., Burns, S. J., Fleitmann, D., Kramers, J., Matter, A., and
Al-Subary, A.: A high-resolution, absolute-dated deglacial speleothem record
of Indian Ocean climate from Socotra Island, Yemen, Earth Planet. Sc. Lett.,
259, 442–456, 2007.Sheng, J. X., Weisenstein, D. K., Luo, B. P., Rozanov, E., Arfeuille, F., and
Peter, T.: A perturbed parameter model ensemble to investigate Mt.
Pinatubo's 1991 initial sulfur mass emission, Atmos. Chem. Phys., 15,
11501–11512, 10.5194/acp-15-11501-2015, 2015.Shinohara, H.: Excess Degassing from Volcanoes and Its Role on Eruptive and
Intrusive Activity, Rev. Geophys., 46, RG4005, 10.1029/2007RG000244,
2008.Siddall, M., Rohling, E. J., Almogi-Labin, A., Hemleben, C., Meischner, D.,
Schmelzer, I., and Smeed, D. A.: Sea-level fluctuations during the last
glacial cycle, Nature, 423, 853–858, 10.1038/Nature01690, 2003.
Sima, A., Paul, A., and Schulz, M.: The Younger Dryas – an intrinsic feature
of late Pleistocene climate change at millennial timescales, Earth Planet.
Sc. Lett., 222, 741–750, 2004.Slawinska, J. and Robock, A.: Impact of volcanic eruptions on decadal to
centennial fluctuations of Arctic sea ice extent during the last millennium
and on initiation of the Little Ice Age, J. Climate, 31, 2145–2167, 10.1175/jcli-d-16-0498.1, 2017.Steffensen, J. P., Andersen, K. K., Bigler, M., Clausen, H. B., Dahl-Jensen,
D., Fischer, H., Goto-Azuma, K., Hansson, M., Johnsen, S. J., Jouzel, J.,
Masson-Delmotte, V., Popp, T., Rasmussen, S. O., Rothlisberger, R., Ruth, U.,
Stauffer, B., Siggaard-Andersen, M. L., Sveinbjornsdottir, A. E., Svensson,
A., and White, J. W. C.: High-resolution Greenland Ice Core data show abrupt
climate change happens in few years, Science, 321, 680–684,
10.1126/science.1157707, 2008.
Sternai, P., Caricchi, L., Castelltort, S., and Champagnac, J. D.:
Deglaciation and glacial erosion: A joint control on magma productivity by
continental unloading, Geophys. Res. Lett., 43, 1632–1641, 2016.Stuiver, M., Grootes, P. M., and Braziunas, T. F.: The GISP2 δ18O
climate record of the past 16,500 years and the role of the Sun, ocean, and
volcanoes, Quaternary Res., 44, 341–354, 1995.Surovell, T. A., Pelton, S. R., Anderson-Sprecher, R., and Myers, A. D.: Test
of Martin's overkill hypothesis using radiocarbon dates on extinct megafauna,
P. Natl. Acad. Sci. USA, 113, 886–891, 10.1073/pnas.1504020112, 2016.Svensson, A., Bigler, M., Blunier, T., Clausen, H. B., Dahl-Jensen, D.,
Fischer, H., Fujita, S., Goto-Azuma, K., Johnsen, S. J., Kawamura, K.,
Kipfstuhl, S., Kohno, M., Parrenin, F., Popp, T., Rasmussen, S. O.,
Schwander, J., Seierstad, I., Severi, M., Steffensen, J. P., Udisti, R.,
Uemura, R., Vallelonga, P., Vinther, B. M., Wegner, A., Wilhelms, F., and
Winstrup, M.: Direct linking of Greenland and Antarctic ice cores at the Toba
eruption (74 ka BP), Clim. Past, 9, 749–766, 10.5194/cp-9-749-2013,
2013.
Swingedouw, D., Mignot, J., Labetoulle, S., Guilyardi, E., and Madec, G.:
Initialisation and predictability of the AMOC over the last 50 years in a
climate model, Clim. Dynam., 42, 555–556, 2014.Tarasov, L., and Peltier, W. R.: Arctic freshwater forcing of the Younger
Dryas cold reversal, Nature, 435, 662–665, 10.1038/Nature03617, 2005.Teller, J. T.: Meltwater and precipitation runoff to the North Atlantic,
Arctic, and Gulf of Mexico from the Laurentide Ice Sheet and adjacent regions
during the Younger Dryas, Paleoceanography, 5, 897–905,
10.1029/Pa005i006p00897, 1990.Textor, C., Sachs, P. M., Graf, H.-F., and Hansteen, T. H.: The 12,900 years
BP Laacher See eruption: estimation of volatile yields and simulation of
their fate in the plume, Geol. Soc., London, Special Publications, 213,
307–328, 10.1144/gsl.sp.2003.213.01.19, 2003.
Thornalley, D. J. R., Barker, S., Broecker, W. S., Elderfield, H., and
McCave, I. N.: The Deglacial Evolution of North Atlantic Deep Convection,
Science, 331, 202–205, 2011.Thornalley, D. J. R., Oppo, D. W., Ortega, P., Robson, J. I., Brierley, C.
M., Davis, R., Hall, I. R., Moffa-Sanchez, P., Rose, N. L., Spooner, P. T.,
Yashayaev, I., and Keigwin, L. D.: Anomalously weak Labrador Sea convection
and Atlantic overturning during the past 150 years, Nature, 556, 227–230,
10.1038/s41586-018-0007-4, 2018.Tian, H., Schryvers, D., and Claeys, P.: Nanodiamonds do not provide unique
evidence for a Younger Dryas impact, P. Natl. Acad. Sci. USA, 108, 40–44,
10.1073/pnas.1007695108, 2011.Timmreck, C. and Graf, H.-F.: The initial dispersal and radiative forcing of
a Northern Hemisphere mid-latitude super volcano: a model study, Atmos. Chem.
Phys., 6, 35–49, 10.5194/acp-6-35-2006, 2006.Timmreck, C., Graf, H. F., Zanchettin, D., Hagemann, S., Kleinen, T., and
Kruger, K.: Climate response to the Toba super-eruption: Regional changes,
Quatern. Int., 258, 30–44, 10.1016/j.quaint.2011.10.008, 2012.
Tobin, T. S.: Recognition of a likely two phased extinction at the K-Pg
boundary in Antarctica, Sci. Rep., 7, 16317,
2017.
van den Bogaard, P., Schmincke, H. U., Freundt, A., and Park, C.: Thera and
the Aegean World, Third International Congress, Santorini, Greece,
463–485, 1989.van den Bogaard, P.: 40Ar /39Ar ages of sanidine phenocrysts
from Laacher See Tephra (12,900 Yr Bp): chronostratigraphic and petrological
significance, Earth Planet. Sc. Lett., 133, 163–174,
10.1016/0012-821x(95)00066-L, 1995.van der Kaars, S., Miller, G. H., Turney, C. S. M., Cook, E. J., Nurnberg,
D., Schonfeld, J., Kershaw, A. P., and Lehman, S. J.: Humans rather than
climate the primary cause of Pleistocene megafaunal extinction in Australia,
Nat. Commun., 8, 14142,
10.1038/ncomms14142, 2017.van Hoesel, A., Hoek, W. Z., Pennock, G. M., Kaiser, K., Plumper, O.,
Jankowski, M., Hamers, M. F., Schlaak, N., Kuster, M., Andronikov, A. V., and
Drury, M. R.: A search for shocked quartz grains in the Allerod-Younger Dryas
boundary layer, Meteorit. Planet. Sci., 50, 483–498, 10.1111/maps.12435,
2015.van Raden, U. J., Colombaroli, D., Gilli, A., Schwander, J., Bernasconi, S.
M., van Leeuwen, J., Leuenberger, M., and Eicher, U.: High-resolution
late-glacial chronology for the Gerzensee lake record (Switzerland): δ18O correlation between a Gerzensee-stack and NGRIP, Palaeogeogr.
Palaeocl. Pt. B, 391, 13–24, 10.1016/j.palaeo.2012.05.017, 2013.Vidal, C. M., Métrich, N., Komorowski, J.-C., Pratomo, I., Michel, A.,
Kartadinata, N., Robert, V., and Lavigne, F.: The 1257 Samalas eruption
(Lombok, Indonesia): the single greatest stratospheric gas release of the
Common Era, Sci. Rep., 6, 34868, 10.1038/srep34868, 2016.
von Grafenstein, U., Erlenkeuser, H., Brauer, A., Jouzel, J., and Johnsen, S.
J.: A mid-European decadal isotope record from 15,500 to 5,000 years B.P,
Science, 284, 1654–1657, 1999.Wallace, P. J.: Volcanic SO2 emissions and the abundance and
distribution of exsolved gas in magma bodies, J. Volcanol. Geotherm. Res.,
108, 85–106, 2001.
White, R. V. and Saunders, A. D.: Volcanism, impact and mass extinctions:
incredible or credible coincidences?, Lithos, 79, 299–316, 2005.Williams, M. A. J., Ambrose, S. H., van der Kaars, S., Ruehlemann, C.,
Chattopadhyaya, U., Pal, J., and Chauhan, P. R.: Environmental impact of the
73 ka Toba super-eruption in South Asia, Palaeogeogr. Palaeocl., 284,
295–314, 10.1016/j.palaeo.2009.10.009, 2009.Wittke, J. H., Weaver, J. C., Bunch, T. E., Kennett, J. P., Kennett, D. J.,
Moore, A. M. T., Hillman, G. C., Tankersley, K. B., Goodyear, A. C., Moore,
C. R., Daniel, I. R., Ray, J. H., Lopinot, N. H., Ferraro, D.,
Israde-Alcantara, I., Bischoff, J. L., DeCarli, P. S., Hermes, R. E.,
Kloosterman, J. B., Revay, Z., Howard, G. A., Kimbel, D. R., Kletetschka, G.,
Nabelek, L., Lipo, C. P., Sakai, S., West, A., and Firestone, R. B.: Evidence
for deposition of 10 million tonnes of impact spherules across four
continents 12,800 y ago, P. Natl. Acad. Sci. USA, 110, E2088–E2097,
10.1073/pnas.1301760110, 2013.
Wolbach, W. S., Ballard, J. P., Mayewski, P. A., Adedeji, V., Bunch, T. E.,
Firestone, R. B., French, T. A., Howard, G. A., Israde-Alcantara, I.,
Johnson, J. R., Kimbel, D., Kinzie, C. R., Kurbatov, A., Kletetschka, G.,
LeCompte, M. A., Mahaney, W. C., Melott, A. L., Maiorana-Boutilier, A.,
Mitra, S., Moore, C. R., Napier, W. M., Parlier, J., Tankersley, K. B.,
Thomas, B. C., Wittke, J. H., West, A., and Kennett, J. P.: Extraordinary
Biomass-Burning Episode and Impact Winter Triggered by the Younger Dryas
Cosmic Impact approximate to 12,800 Years Ago. 1. Ice Cores and Glaciers, J.
Geol., 126, 165–184, 2018a.
Wolbach, W. S., Ballard, J. P., Mayewski, P. A., Parnell, A. C., Cahill, N.,
Adedeji, V., Bunch, T. E., Dominguez-Vazquez, G., Erlandson, J. M.,
Firestone, R. B., French, T. A., Howard, G., Israde-Alcantara, I., Johnson,
J. R., Kimbel, D., Kinzie, C. R., Kurbatov, A., Kletetschka, G., LeCompte, M.
A., Mahaney, W. C., Melott, A. L., Mitra, S., Maiorana-Boutilier, A., Moore,
C. R., Napier, W. M., Parlier, J., Tankersley, K. B., Thomas, B. C., Wittke,
J. H., West, A., and Kennett, J. P.: Extraordinary Biomass-Burning Episode
and Impact Winter Triggered by the Younger Dryas Cosmic Impact approximate to
12,800 Years Ago. 2. Lake, Marine, and Terrestrial Sediments, J. Geol., 126,
185–205, 2018b.
Wolff, E. W., Chappellaz, J., Blunier, T., Rasmussen, S. O., and Svensson,
A.: Millennial-scale variability during the last glacial: The ice core
record, Quaternary Sci. Rev., 29, 2828–2838, 2010.Wu, Y. Z., Sharma, M., LeCompte, M. A., Demitroff, M. N., and Landis, J. D.:
Origin and provenance of spherules and magnetic grains at the Younger Dryas
boundary, P. Natl. Acad. Sci. USA, 110, E3557–E3566,
10.1073/pnas.1304059110, 2013.Wulf, S., Ott, F., Slowinski, M., Noryskiewicz, A. M., Drager, N.,
Martin-Puertas, C., Czymzik, M., Neugebauer, I., Dulski, P., Bourne, A. J.,
Blaszkiewicz, M., and Brauer, A.: Tracing the Laacher See Tephra in the
varved sediment record of the Trzechowskie palaeolake in central Northern
Poland, Quaternary Sci. Rev., 76, 129–139,
10.1016/j.quascirev.2013.07.010, 2013.
Yang, H., Wang, K., Dai, H., Wang, Y., and Li, Q.: Wind effect on the
Atlantic meridional overturning circulation via sea ice and vertical
diffusion, Clim. Dynam., 46, 3387–3403, 10.1007/s00382-015-2774-z, 2016.Zhang, X., Lohmann, G., Knorr, G., and Purcell, C.: Abrupt glacial climate
shifts controlled by ice sheet changes, Nature, 512, 290–294,
10.1038/nature13592, 2014.Zhang, X., Knorr, G., Lohmann, G., and Barker, S.: Abrupt North Atlantic
circulation changes in response to gradual CO2 forcing in a glacial
climate state, Nat. Geosci., 10, 518–523,
10.1038/ngeo2974, 2017.Zhong, Y., Miller, G. H., Otto-Bliesner, B. L., Holland, M. M., Bailey, D.
A., Schneider, D. P., and Geirsdottir, A.: Centennial-scale climate change
from decadally-paced explosive volcanism: a coupled sea ice-ocean mechanism,
Clim. Dynam., 37, 2373–2387, 10.1007/s00382-010-0967-z, 2011.
Zielinski, G. A., Mayewski, P. A., Meeker, L. D., Whitlow, S., and Twickler,
M. S.: A 110,000-Yr Record of Explosive Volcanism from the GISP2 (Greenland)
Ice Core, Quaternary Res., 45, 109–118, 1996.Zielinski, G. A., Mayewski, P. A., Meeker, L. D., Gronvold, K., Germani, M.
S., Whitlow, S., Twickler, M. S., and Taylor, K.: Volcanic aerosol records
and tephrochronology of the Summit, Greenland, ice cores, J. Geophys.
Res.-Ocean., 102, 26625–26640, 10.1029/96jc03547, 1997.