CPClimate of the PastCPClim. Past1814-9332Copernicus PublicationsGöttingen, Germany10.5194/cp-14-1893-2018A stalagmite test of North Atlantic SST and Iberian hydroclimate linkages
over the last two glacial cyclesA stalagmite test of North Atlantic SSTDennistonRhawn F.rdenniston@cornellcollege.eduHoutsAmanda N.AsmeromYemaneWanamaker Jr.Alan D.HawsJonathan A.PolyakVictor J.ThatcherDiana L.Altan-OchirSetsenhttps://orcid.org/0000-0003-3036-0392BorowskeAlyssa C.BreitenbachSebastian F. M.https://orcid.org/0000-0001-9615-2065UmmenhoferCaroline C.RegalaFrederico T.BenedettiMichael M.BichoNuno F.Department of Geology, Cornell College, Mount Vernon, Iowa 52314, USADepartment of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USADepartment of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa 50011, USADepartment of Anthropology, University of Louisville, Louisville, Kentucky 40208, USAInstitute for Geology, Mineralogy, and Geophysics, Ruhr-University, 44801 Bochum, GermanyDepartment of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USAAssociação de Estudos Subterrâneos e Defesa do Ambiente, Torres Vedras, PortugalDepartment of Earth and Ocean Sciences, University of North Carolina Wilmington, Wilmington, North Carolina 28403, USAInterdisciplinary Center for Archaeology and Evolution of Human Behaviour (ICArEHB), Universidade
do Algarve, Faro, Portugalcurrent address: Department of Earth Sciences, University of New
Hampshire, Durham, New Hampshire 03824, USAcurrent address: Department of Geosciences, École Normale
Supérieure, PSL Res. Univ., Paris, Francecurrent address: Department of Ecology and Evolutionary Biology,
University of Connecticut, Storrs, Connecticut 06269, USARhawn F. Denniston (rdenniston@cornellcollege.edu)11December20181412189319136November201724November201726November201827November2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://cp.copernicus.org/articles/14/1893/2018/cp-14-1893-2018.htmlThe full text article is available as a PDF file from https://cp.copernicus.org/articles/14/1893/2018/cp-14-1893-2018.pdf
Close coupling of Iberian hydroclimate and North Atlantic sea
surface temperature (SST) during recent glacial periods has been identified
through the analysis of marine sediment and pollen grains co-deposited on the
Portuguese continental margin. While offering precisely correlatable records,
these time series have lacked a directly dated, site-specific record of
continental Iberian climate spanning multiple glacial cycles as a point of
comparison. Here we present a high-resolution, multi-proxy (growth dynamics
and
δ13C, δ18O, and δ234U
values) composite stalagmite record of hydroclimate from two caves in western
Portugal across the majority of the last two glacial cycles (∼220 ka).
At orbital and millennial scales, stalagmite-based proxies for hydroclimate
proxies covaried with SST, with elevated δ13C,
δ18O, and δ234U values and/or growth hiatuses
indicating reduced effective moisture coincident with periods of lowered SST
during major ice-rafted debris events, in agreement with changes in
palynological reconstructions of continental climate. While in many cases the
Portuguese stalagmite record can be scaled to SST, in some intervals the
magnitudes of stalagmite isotopic shifts, and possibly hydroclimate, appear
to have been somewhat decoupled from SST.
Introduction
The Portuguese continental margin is an important location for understanding
variations in paleoceanographic conditions over orbital and millennial scales
(Hodell et al., 2013; Voelker and de Abreu, 2011). Here, marine sediments
record basin-wide oceanographic signals, while co-deposited pollen grains
track coeval vegetation changes occurring across Iberia. Integrated analysis
of these proxies has revealed a close coupling of North Atlantic SST,
regional climate, and Iberian ecosystems during the last three glacial
cycles, including changes in vegetation dynamics (Sánchez Goñi et
al., 2002, 2008; Tzedakis et al., 2004; Roucoux et al., 2006; Martrat et al.,
2007; Naugthon et al., 2007), atmospheric circulation (Sánchez Goñi
et al., 2013), and fire frequency (Daniau et al., 2007). One commonly applied
palynological metric is the abundance of temperate tree pollen, which rises
during warm and wet conditions associated with both interglacials and
Greenland interstadials, concomitant with shifts in Iberian margin SST
(Sánchez Goñi et al., 2002; Tzedakis et al., 2004; Combourieu-Nebout
et al., 2009; Fletcher et al., 2010; Chabaud et al., 2014). However, the
nature of such land–sea connections is partially obscured by the size of
catchments from which the pollen is derived, with some reaching into central
Iberia and spanning a range of environmental settings subject to varying
climatic influences (Martin-Vide and Lopez-Bustins, 2006; Naughton et al.,
2007) (Fig. 1).
Average annual precipitation (mm) of the Iberian Peninsula for years
1901–2009 CE (GPCC v. 6; Schneider et al., 2014) relative to cave study
sites (white stars: GLC – Gruta do Casal da Lebre; BG – Buraca
Gloriosa). Rectangle denotes location of northwest Spain cave sites (NWSCs)
(Moreno et al., 2010; Stoll et al., 2013); FM – Fuentillejo maar (Vegas
et al., 2010) and GT – Gitana Cave (Hodge et al., 2008); VC – Villars
Cave (Genty et al., 2003) located just north of map. Also shown are locations
of marine cores discussed in the text and GNIP stations at Porto, Vila Real, and
Portalegre. Bathymetric contours shown in grey (m). Location of currents
after Voelker et al. (2010).
Testing the links between terrestrial and marine systems benefits from
continental climate archives that provide precisely dated and high-resolution
rainfall-sensitive time series spanning tens of millennia, but such records
remain rare in Iberia, particularly near the west Iberian margin (Fletcher et
al., 2010; Moreno et al., 2012; Stoll et al., 2013). Here we present a
composite stalagmite record of four proxies for hydroclimate – growth
dynamics and δ13C, δ18O, and
δ234U values – spanning the majority of the last and
penultimate glacial cycles (∼220 ka) at two cave sites in western
Portugal. These time series offer a rare site-specific continental record
capable of examining the coherence of SST controls on Iberian climate and
ecosystem dynamics across glacial and interglacial periods. The new record
provides a continental perspective of hydroclimate dynamics linked to
regional oceanographic conditions.
Samples and regional settingEnvironmental setting
We report the analysis of five stalagmites (BG41, BG66, BG67, BG611, BG6LR)
from Buraca Gloriosa (BG; 39∘32′ N, 08∘47′ W;
420 m a.s.l.) and one stalagmite (GCL6) from Gruta do Casal da Lebre (GCL;
39∘18′ N, 9∘16′ W; 130 m a.s.l.), two caves in
western Portugal (Fig. 1). Environmental conditions in BG and GCL are well
suited for speleothem paleoclimate reconstruction (see below). BG and GCL are
located within the Meso-Mediterranean bioclimatic zone that dominates much of
Iberia (Fig. 1). This region is characterized by strong seasonality with
warm, dry summers and cool, wet winters (Fig. 2) associated with the winter
westerlies (Blanco Castro et al., 1997). In contrast, the Atlantic zone,
north of the Douro River, is cooler, wetter, and less strongly seasonal. In
the Pleistocene, the transition between these zones likely shifted southward
with Mediterranean-type vegetation restricted to refugia (Rey Benayas and
Scheiner, 2002).
Oxygen isotopic composition of precipitation versus rainfall amount
(a) and air temperature (b). Data collected at IAEA/GNIP
site in Porto, Portugal (see Fig. 1 for location) for 1988–2004. Oxygen
isotope data represent multiyear averages of monthly means. The two other
closest GNIP stations in Portugal – Vila Real and Portalegre (see Fig. 1) –
share similar relationships between precipitation oxygen isotopic composition
and air temperature (+0.27 ‰ ∘C-1, r2=0.76 and
+0.26 ‰ ∘C-1, r2=0.69, respectively) to that of
Porto (+0.21 ‰ ∘C-1). The relationship between
precipitation oxygen isotopic composition and monthly precipitation amount is
-3.5 ‰ 100 mm-1 month-1 (r2=0.64),
-3.7 ‰ 100 mm-1 month-1 (r2=0.49), and
-1.6 ‰ 100 mm-1 month-1 (r2=0.62) for the three sites,
respectively. Note that the right-hand y axis in panel (a) is inverted in
order to illustrate the inverse nature of rainfall and precipitation oxygen
isotopic composition.
Over interannual scales, the hydroclimate of Iberia is tightly coupled with
the winter North Atlantic Oscillation (NAO) (Fig. 3), an atmospheric dipole
that strongly influences precipitation across much of western Europe and that
more broadly reflects the strength and positioning of the Azores high-pressure system, which steers storm tracks contained within the westerlies
into or north of Iberia (e.g., Trigo et al, 2002; Paredes et al., 2006;
Trouet et al., 2009; Cortesi et al., 2014). The NAO is typically measured as
the NAO index, which is calculated using atmospheric pressure differences
between Iceland and Lisbon (or the Azores) (Barnston and Livezey, 1987). The
nature of the influence of the NAO varies across Iberia, but it is strongly
correlated with rainfall in western Portugal (Fig. 3), with a positive NAO
index associated with a steeper pressure gradient and elevated Iberian
aridity. Iberian precipitation has also been linked to SST in regions ranging
from the western North Atlantic to the Iberian margin (Lorenzo et al., 2010)
where ocean circulation is dominated by the south-flowing Portugal Current
and the near-coastal, north-flowing Iberian Poleward Current, two systems
that transport pollen from river mouths along the continental shelf (Fig. 1).
Iberian rainfall anomalies associated with the North Atlantic
Oscillation. Composites of November–March precipitation anomalies
(mm month-1) during (a) positive and (b) negative NAO
winters for the period 1901–2012. Positive–negative NAO winters were
determined using the December–March Hurrell principal-component-based NAO
index (CDG, 2018) as winters with NAO values in the highest–lowest
decile of all winters. The PC-based NAO index represents the time series of
the leading empirical orthogonal function of SLP anomalies over the Atlantic
sector at 20–80∘ N, 90∘ W–40∘ E. Precipitation
anomalies are based on the GPCC precipitation version 7 at 0.5∘
spatial resolution (Schneider et al., 2014). Yellow stars denote cave sites
in this study. BG: Buraca Gloriosa; GCL: Gruta do Casal da Lebre.
Cave settings
Buraca Gloriosa cave is located near the town of Alvados, 30 km from the
Atlantic Ocean, within Middle Jurassic limestones of the Estremadura
Limestone Massif (Rodrigues and Fonseca, 2010), a topographically distinct
region in central Portugal (Fig. 1). The ∼35 m long cave is accessed
through a single small (∼0.5 m2) entrance at the top of a
collapse at the base of a 30 m high escarpment (Fig. 4). The cave is well
decorated although little active growth is occurring today. Vegetation above
the cave is primarily shrubs, small trees, and mosses hosted by a thin
(0–10 cm) and highly organic soil layer.
Profile and map views of Buraca Gloriosa (top) and Gruta do Casal da
Lebre (bottom). Entrance denoted by arrow (top panel) and filled square
(bottom panel). Red stars denote locations of stalagmites used in this
study.
Gruta do Casal da Lebre overlooks the coastal town of Peniche and is hosted
by Upper Jurassic limestones. The cave is 130 m long and contains a single
1 m2 entrance that opens onto a 7 m vertical shaft (Fig. 4). This
entrance has been closed with a solid metal door in recent decades in order
limit access to the cave, and this modification has likely reduced air
exchange in GCL relative to its original state. Like BG, GCL hosts little
active calcite deposition, but contains numerous fossil stalagmites and
stalactites. The vegetation over the cave has been replaced in recent decades
by stands of eucalyptus that grow in thin (< 1–5 cm) clay-rich
soils.
Pollen sources
Pollen deposited on the west Iberian margin is sourced primarily from
vegetation inhabiting the watersheds of the major west-flowing stream systems
draining Portugal and Spain, which are (from north to south) the Douro,
Tagus, and Sado rivers. The areas encompassed by these streams are large
(79 000, 81 000, and 7650 km2, respectively) and span a variety of
elevations. The Tagus and Sado are primarily responsible for pollen deposited
southwest of Portugal, while the Douro plays an important role in delivering
pollen to the more northwesterly sites (Fig. 1). Prevailing wind patterns
likely prevent substantial transport of pollen from Iberia to the western
Portuguese margin (Naughton et al., 2007). The pollen data presented here
were collected in three closely spaced cores from the southwest Iberian
margin: MD01-2443, 250–194 ka (Roucoux et al, 2006; Tzedakis et al., 2004);
MD01-2444, 193–136 ka (Margari et al., 2010, 2014); MD95-2042, 141–1 ka
(Sánchez Goñi et al., 2008, 2013) (Fig. 1). They are integrated here
into a single time series.
Materials and methodsEnvironmental monitoring
Environmental conditions were measured at both cave sites over a multiyear
period, with data recorded in 2 h intervals near the areas where the
stalagmites were deposited. Temperature and relative humidity were obtained
using HOBO U23 automated sensors, while barometric pressure was recorded with
HOBO U20L loggers. Drip rates were monitored at BG with Stalagmate acoustic
drip counters (Collister and Mattey, 2008).
a Present defined as the year 1950 CE. b Initial 230Th/232Th atomic ratio of 13.5 (±6.75) ppm
used to correct for unsupported 230Th in BG stalagmites. GCL
stalagmites use 4.4 (±2.2) ppm. c Errors at 2 SD level.
Uranium-series dating
Stalagmite chronologies were constructed with a total of 69 230Th
dates obtained at the University of New Mexico (Table 1) using the methods of
Asmerom et al. (2010). For dating of stalagmite carbonate, powders ranging
from 100–200 mg were weighed, dissolved in 15N
nitric acid, spiked with a mixed
229Th–229Th–236U tracer, and processed using
column chemistry methods. U and Th fractions were dissolved in 5 mL of
3 % nitric acid and transferred to analysis tubes for measurement on a
Thermo Neptune MC-ICP-MS. U and Th solutions were aspirated into the Neptune
using a Cetac Aridus II low-flow desolvating nebulizer and run as static
routines. All isotopes of interest were measured in Faraday cups, except for
234U and 230Th, which were measured in the secondary
electron multiplier (SEM). Gains between the SEM and the Faraday cups were
determined using standard solutions of NBL-112 for U and an in-house
230Th–229Th standard for Th that was measured after every
fifth sample; chemistry blanks reveal U and Th blanks below 20 pg. Ages are
reported using 2 standard deviation errors.
For BG stalagmites, corrections were made for unsupported 230Th
using a 230Th/232Th ratio of 13.5 ppm (±50 %), a value
determined from isotopic analysis of cave drip water. To obtain this value,
108 mL of drip water was transferred into six 30 mL Teflon beakers. These
beakers were fluxed in 6N HCl for an hour, rinsed, and
heated gently on a hot plate until approximately 1–2 mL of fluid remained in
each. All solutions were then combined into a single 30 mL Teflon beaker,
spiked with the same tracer described above (which contains HF), fluxed, and
then taken to complete dryness. The resulting precipitate was dissolved with
15N HNO3, dried down, dissolved again in 7N HNO3, and
processed with the same column chemistry methods used for the stalagmite
samples. We lack independent constraints on the initial Th ratio for the GCL
stalagmite and thus apply the default value of 4.4 ppm (±50 %).
This difference in the initial Th ratio impacts the corrected ages of GCL6 by
0.5–3.0 kyr relative to the value used for BG and thus does not
meaningfully influence our interpretations.
COPRA-derived age models for BG and GCL stalagmites. Black lines
represent mean of calculated age models, while red lines denote 95 %
confidence intervals. See Table 1 for specific ages and isotopic ratios.
Orange square represents a “dummy age” that was included in order to
extrapolate below the hiatus, which is only possible with at least two dated
points. The bottom of BG611 was based on linear extrapolation through dated
intervals. Distances for BG66 were measured relative to the topmost section of
the interval for which stable isotopes were obtained, and not relative to the cap
of the stalagmite (see Fig. 6).
Age models were developed via multiple polynomial interpolations between
dated intervals using the COPRA age-modeling software (Breitenbach et al.,
2012) (Fig. 5). Aside from providing age models, COPRA also yields mean
modeled stable isotope values and confidence intervals (Supplement Fig. S1).
Here we rely primarily on the original δ18O and
δ13C values because COPRA-derived median values reflect
statistically robust variations, but reduce to some degree the range of
isotopic variability. For COPRA, a dummy age was included in the age model
for BG41 in order to extrapolate below the hiatus, which is only possible
with at least two dated points. The value of this dummy age was based on the
assumption that it maintains a stratigraphically correct slope (i.e., higher
sections of the stalagmite represent younger material). The dummy age was
applied a conservative error, meaning that it was as large as possible
without causing stratigraphic inversion with respect to the bounding ages.
Stable isotope ratios
A total of 1510 stable isotope analyses were performed on calcite samples
milled from the central axis of each stalagmite. After milling, powders were
weighed (∼200µg) and transferred to reaction vessels that
were flushed with ultrapure helium. Samples were then digested using
> 100 % H3PO4 and equilibrated overnight (∼16 h) at 34 ∘C before being analyzed. Isotopic ratios were
measured using a GasBench II with a CombiPal autosampler coupled to a Thermo
Finnigan Delta Plus XL mass spectrometer at Iowa State University. A
combination of internal and external standards was run after every fifth
sample, as well as before and after each batch, in order to ensure
reproducibility. Oxygen and carbon isotope ratios are presented in parts per
mil (‰) relative to the Vienna Pee Dee Belemnite carbonate standard
(VPDB). Average precision for both δ13C and
δ18O analyses is better than ±0.1 ‰ (1σ).
For isotopic analyses of soil organic matter and vegetation collected from
above the caves, samples were dried, crushed, and transferred to tin boats.
Carbon isotopic ratios were measured using a Thermo Finnegan Delta Plus XL
mass spectrometer in continuous-flow mode coupled with a Costech elemental
analyzer. Caffeine (IAEA-600), cellulose (IAEA-CH-3), and acetanilide
(laboratory standard) isotopic standards yielded an average analytical
uncertainty for carbon of ±0.09 ‰ 1σ (VPDB). Drip water
samples were measured using a Picarro L2130-i isotopic liquid water analyzer,
with autosampler and ChemCorrect software. Each sample was measured six
times, with only the last three injections used to determine isotopic values
in order to minimize memory effects. Three reference standards (VSMOW,
IAEA-OH-2, IAEA-OH-3) were used for regression-based isotopic corrections and
to assign the data to the appropriate isotopic scale. Reference standards
were measured at least once every five samples. The average analytical
uncertainty for δ18O measurements was ±0.1 ‰
1σ (VSMOW).
BG and GCL stalagmites and U/Th ages. Red lines denote stable
isotope sampling transects. Blue and white scale bars (cm) define
differential enlargement of each stalagmite. Black arrows represent intervals
excluded from this study due to evidence of open system behavior. Sections
without arrows or transect lines are older than the interval examined in this
study. The impact of recrystallization in stalagmite cores was assessed by
parallel sampling transects (parallel red lines on BG66 and GCL6) and
demonstrated consistent stable isotopic values and trends (Fig. S7).
Stalagmite mineralogy and fabrics
The calcite comprising the BG samples ranges across a variety of fabrics
including a faster-growing, white, fibrous form and a slower-growing, dense,
clear structure (Fig. 6; Fig. S2). In some samples, sharp changes between the
two forms within the same growth horizons mark intervals of recrystallization
during which U/Th ages are highly inconsistent, and these
intervals were excluded from our data set. BG6LR, which grew discontinuously
over much of the last glacial cycle, suffered from alteration of early and
middle Holocene material, which was therefore excluded from this analysis.
BG67 is characterized primarily by fibrous calcite that has been
recrystallized to clear, dense calcite in a narrow band descending through
its core. U/Th dates from the fibrous calcite on the margins of the
growth surface reveal open system behavior and thus this portion of BG67 was
excluded. Recrystallization is evident in portions of GCL6 (particularly just
above its base) and BG66 but the consistency of U/Th dates and the
trends in stable isotopes suggest that this alteration may have occurred soon
after original deposition. We tested whether these altered sections retain
reliable paleoclimatic information by analyzing stable isotopes along partial
transects located just outside the zones of recrystallization (Fig. 6).
Because stable isotopic values and trends between these transects were
consistent (within the analytical errors), we retained these sections in the
time series. Growth position changed at numerous times in several of these
stalagmites, and our sampling strategy accounted for these changes so as to
consistently collect samples for stable isotopic analysis from the top
surface (cap) of each stalagmite rather than the margins.
ResultsEnvironmental monitoring
Temperature and relative humidity collected inside both caves document
environmental conditions over a multiyear period. Relative humidity remained
largely stable at ∼100 % in both caves. Temperatures, while
different at the two sites, exhibited similar seasonal variability that
approximates the mean average temperature of the region (14.2±0.4∘C at BG and 16.2±0.3∘C at GCL for
August 2012–January 2018) (Fig. 7).
Temperature and relative humidity variations from (a) Buraca Gloriosa and (b) GCL. Drip rate from Buraca
Gloriosa and precipitation variability (c) from Monte Real, Portugal
(35 km from BG). Temperature sensor in GCL was changed in November 2014 and
the sensitivity of the new instrument varies slightly from the original.
Drip water was collected at BG both over the course of minutes during site
visits on four separate occasions (November 2014, October 2015, March 2016,
January 2018) and as months-long integrated samples. A total of 25 drip water
samples were analyzed for stable isotopic values. Drip water δ18O values range from -2.4 ‰ to -4.6 ‰ , with a
mean of -3.8±0.8 ‰ (Supplement Table S1), although as the
timing of site visits varied, this value clearly is impacted by seasonal
controls on precipitation (and thus infiltration) oxygen isotope values. Drip
rates were measured for much of the period spanning June 2014 to January 2018
(for a total of ∼36 months) and exhibit seasonal variations tied to the
winter wet and summer dry seasons, as well as individual rain events
(Fig. 7).
U–Th dates and age models
234U–230Th dating of BG and GCL stalagmites reveals
growth across approximately three-quarters of the last 220 ka, with periods
of deposition interrupted by numerous hiatuses of varying length, with the
longest gaps from 160–147, 97–87, 72–60, 41–36, 32–30, and 17–15 ka
(Figs. 5 and 6; Fig. S3). These features, coupled with repeated changes in
growth direction and high 232Th abundances in select sections,
complicate construction of a chronology in some intervals. Macroscopic
petrographic discontinuities suggest the presence of several short-lived
hiatuses, but these were included as gaps in the age models only where
U/Th dates reveal an identifiable temporal offset. For example, the
marine isotope stage (MIS) 6–5e boundary recorded by stalagmite BG67 is marked
by both a change in drip position and a sharp transition from dense, clear
calcite to a white, fibrous form. Taken together, it is clear that a hiatus
of some duration occurred at this time. However, these isotope data are
presented as being uninterrupted given the continuity of δ18O
values and no U/Th evidence for a long-lived hiatus (Fig. 6).
Assessing equilibrium in speleothem 18O and 13C values
We used two approaches to assess the fidelity of BG–GCL carbon and
oxygen isotopes as records of past environmental variability. First, Hendy
tests, in which stalagmite isotopic ratios must satisfy two criteria in order
to be considered as having crystallized near isotopic equilibrium with cave
drip water (Hendy, 1971), were performed for each stalagmite. The first half
of the Hendy test involves analysis of multiple isotopic analyses performed
on samples drilled at increasing distance from the central growth axis along
the same series of growth layers. The conceptual justification for this
approach is that drip water, and thus speleothem calcite, 18O values
should remain constant down the stalagmite flanks because 16O
preferentially lost to CO2 outgassing is replenished by
CO2 hydration and hydroxylation reactions. Progressive
18O enrichment associated with kinetic effects tied to Rayleigh
distillation suggests isotopic disequilibrium. No such consistent trends
toward elevated oxygen isotopic ratios are found (Fig. 8), and thus the
BG and GCL stalagmites appear to satisfy the first criterion of the Hendy test.
Hendy tests of BG and GCL stalagmites. (a) Covariance plots of
carbon and oxygen isotopic ratios. Correlation coefficients (r2 values)
are listed for each plot. High positive correlations have been identified as
an indicator of nonequilibrium crystallization. (b) Oxygen (blue)
and carbon (green) isotopic variations along the same growth layers with
distance (listed in the upper left corner of each panel) from the stalagmite
central growth axis. Progressive increases in δ18O values have
been interpreted to reflect disequilibrium crystallization. Limitations of
the Hendy tests are discussed in the text.
The second portion of the Hendy test is based on the degree of covariation of
carbon and oxygen isotopic ratios. Oxygen isotopic ratios of speleothem
calcite reflect those of infiltrating fluids, which are generally close to
the 18O values of meteoric precipitation, and in many
locations are linked to climate (air temperature, moisture source,
seasonality of precipitation, or rainfall amount; Lachniet, 2009).
Interpreting changes in oxygen isotope composition at BG and GCL during intervals
of profound climatic change such as the last glacial period is
complicated by the multiple factors that influenced δ18O
values of precipitation at these sites, including shifts in moisture source.
The potential exists for rainfall in Iberia to be derived from atmospheric
moisture sources that change on synoptic and seasonal scales (Moreno et al.,
2014; Gimeno et al., 2010, 2012) as well as in response to changing glacial
boundary conditions (Florineth and Schlüchter, 2000; Kuhlemann et al.,
2008; Luetscher et al., 2016). In addition, strong but opposite correlations
exist in modern precipitation between rainwater δ18O values
and (i) the regional air temperature (r=+0.8) and (ii) rainfall amount
(r=-0.8), both of which are related to the strong seasonality of
precipitation associated with Meso-Mediterranean climates (IPMA, 2016).
Correlations between carbon and oxygen isotope ratios are presented in
Fig. 8. Three stalagmites – BG6LR, BG66, and BG67 – show strong
correlations between 13C and 18O (r2=0.6), while the
other three samples lack a strong correlation. If one considers the second
criterion of the Hendy test, the nature of equilibrium crystallization in
stalagmites BG6LR, BG66, and BG67 would be considered suspect. It must be
noted, however, that the reliability of the Hendy test has been questioned
because (1) equilibrium may be maintained in some portions of a stalagmite
but not others, (2) growth layers thin progressively down the sides of the
stalagmite, making it difficult to restrict samples to the same material, and
(3) equilibrium covariation of carbon and oxygen isotope ratios may be
the direct or indirect result of climatic variability (Dorale and Liu, 2009;
Lechleitner et al., 2017). We therefore interpret both isotope ratios and
their covariation as environmental signals.
Hydroclimate proxiesCarbon isotopes
Interpreting speleothem δ13C variability in a climatic context
requires understanding, or at least constraining, the origins of these
isotopic shifts. Stalagmite δ13C values reflect two primary
inputs: CO2 derived from the atmosphere and/or soil zone and
bicarbonate derived from the dissolution of bedrock carbonate. Speleothem
13C values reflect the type (C3 vs. C4) and
density of vegetation over the cave, both of which are impacted by changes in
air temperature and/or precipitation. The average δ13C value
of biogenic CO2 in the soil zone is tied to the ratio of plants
utilizing the C3 (average δ13C-26 ‰)
versus C4 (average δ13C-14 ‰)
photosynthetic pathways (Deines, 1980; von Fischer et al., 2008). Similarly,
vegetation density and soil respiration rates over the cave impact the
relative contribution of atmospheric CO2 (preindustrial
δ13C-6 ‰ to -7 ‰; Francey et al., 1999)
compared to soil-derived CO2 (Hellstrom and McCulloch, 2000;
Genty et al., 2003). Phanerozoic bedrock δ13C values range
from -4 ‰ to +8 ‰ (Saltzman and Thomas, 2012), but
these values are static and do not contribute to temporal variability in
stalagmite carbon isotopic ratios.
Superimposed on these inputs are secondary effects capable of influencing the
δ13C values of drip water in the epikarst or cave. When voids
in the bedrock are not fully saturated, CO2 degassing from
infiltrated water may occur in the epikarst. This preferential loss of
12CO2 (that may result in crystallization of calcium carbonate
– so-called prior calcite precipitation) enriches the residual solution in
13C, a signal that can be transferred into underlying stalagmites
(Baker et al., 1997). Once the solution enters the cave, equilibrium
fractionation between dissolved carbon species may be disrupted owing to
issues surrounding CO2 degassing under low-drip-rate conditions
(Breitenbach et al., 2015) or by disequilibrium processes occurring during
carbonate crystallization (Mickler et al., 2004; Fairchild et al., 2006).
Importantly, δ13C values reflect local infiltration rather
than (pan-)regional atmospheric conditions as in the case of
δ18O. This difference between the two proxies offers the
opportunity to investigate environmental changes at different spatial scales.
Terrestrial deposits preserving pollen spectra spanning substantial portions
of the last glacial cycle from western Iberia are rare (Gómez-Orellana et
al., 2008; Fletcher et al., 2010; Moreno et al., 2012), and thus pollen in
marine sediments represents a particularly important continental climate
record. Pollen samples obtained from the Iberian margin contain small
percentages of Poaceae, the family including the majority of
C4 plants, demonstrating a persistent and overwhelming majority of
C3 (largely shrub and arboreal) vegetation throughout the last
glacial cycle, including between Greenland stadials (GS) and interstadials
(GI) and across Heinrich stadials (HS) (d'Errico and Sánchez Goñi,
2003; Tzedakis et al., 2004; Desprat et al., 2006; Sánchez Goñi et
al., 2008, 2013; Margari et al., 2014). In the absence of changes in
vegetation type, shifts in the source of carbon found in cave drip water
therefore likely originated with the density of vegetation and/or soil
respiration rates (Genty et al., 2003). Decreases in these values are
generally associated with decreases in temperature and/or increases in
aridity, such as have been inferred from Iberian pollen spectra to
characterize Iberia during GS, HS, and glacial maxima (Sánchez Goñi
et al., 2008; Margari et al., 2014). Complementing these effects are
increases in the contribution of bedrock carbon, as well as prior calcite
precipitation, reflecting a combination of longer residence times of
infiltrating solutions and desaturation of voids in the epikarst above the
cave, both of which are consistent with more arid climates (Baker et al.,
1997; Genty et al., 2003). Thus, we interpret the carbon isotopic values of
the BG–GCL record as primarily a local (hydro)climate proxy, with higher
δ13C values indicative of a cooler, drier climate. Integrating
the GCL6 δ13C record into the BG time series is complicated by
the slightly different bedrock δ13C values of the host rocks
(Table S1) and what may have been distinct vegetation types and cave
hydrologies at each cave when GCL6 was being deposited (187–160 ka).
However, similar δ13C values during their period of overlap
(187–185 ka) suggests that the two records can be consolidated (see below).
A test of equilibrium crystallization in the modern system can be constructed
by comparing modeled stalagmite isotopic values to recently deposited
calcite. The carbon isotopic composition of speleothem calcite is the result
of a complex series of reactions that have been addressed in a number of
studies (Hendy, 1971; Mühlinghaus et al., 2007; Dreybodt, 2008). For
13C in BG stalagmites, we use the equations of Li et al. (2014),
which factor in the two primary sources of carbon (soil CO2 and
bedrock carbonate), the proportion of carbon derived from each source, and
temperature-induced fractionation of carbon isotopes between dissolved carbon
species:
13Ccalcite=f1×[13Cls-(13CCO2(g)+9.48×103/T-23.89+13CCO2(g)+9.48×103/T+0.049T-37.72)],
where f1 is the fraction of bicarbonate from limestone (ls), and T is temperature (∘K).
We assume the most straightforward and simple situation: the system remains
closed to soil CO2 after entering the epikarst, and bedrock
carbonate contributes 50 % of carbon to drip water bicarbonate
(f1=0.5). We apply the average cave temperature of 14.4 ∘C and
the measured 13C values of BG bedrock and the overlying
vegetation–soil of +3±1 ‰ and
-28±1 ‰, respectively. This approach, while certainly overly
simplified for the BG cave system, yields modeled stalagmite
δ13C values averaging -7.7±1 ‰, similar to
calcite crystallized on two glass slides installed at the site of two
actively growing stalagmites in the loft area of BG, which yielded
δ13C values of -8.4±1.2 ‰ .
Oxygen isotopes
The origins of BG and GCL isotopic variability appear
more complex for oxygen than for carbon. Like δ13C values,
local δ18O minima mark interstadials and interglacials.
Analysis of modern precipitation data reveals equally strong, albeit inverse,
correlations between precipitation δ18O and both amount
(r=-0.8) and air temperature (r=+0.8) effects, likely owing to the
dominance of cool season precipitation in annual water budgets (IAEA/WMO,
2016) (Fig. 2). Based on these relationships, it remains possible that
changes in air temperature, overall precipitation, and/or precipitation
seasonality could impact the δ18O values of effective
moisture. That air temperature is likely not a prominent driver of stalagmite
oxygen isotopic variability is supported by two observations, however. First,
the slopes of the air temperature–δ18O relationships
(‰ ∘C-1) at the three
GNIP stations located closest to BG and GCL (Porto, Vila Real, and
Portalegre) are nearly identical (average for the three sites 0.25±0.03 ‰ ∘C-1) but opposite in sign to the calcite–water
temperature dependence of oxygen isotopic fractionation
(-0.2 ‰ ∘C-1) (Kim and O'Neil, 1997) (slopes of
precipitation amount /δ18O are -1.6, -3.5, and
-3.7 ‰ 100 mm-1 month-1, respectively). In the simplest
sense, therefore, a 1 ∘C increase in mean annual air temperature
(and thus also cave temperature) would increase precipitation
δ18O values by approximately the same amount that the water
temperature effect would lower stalagmite calcite δ18O values.
In this simplified scenario, the net effect is a stalagmite record that is
negligibly influenced by multi-decadal- to centennial-scale temperature changes
alone. Secondly, the observed shift toward lower stalagmite δ18O values during interstadials and interglacials, periods of elevated
mean annual temperature, demonstrates that the observed positive correlation
between precipitation δ18O and air temperature is not a
dominant feature over millennial timescales. For example, the 3.5 ‰
decrease in δ18O values between MIS 6 and MIS 5e (136–128 ka)
(Fig. 9) can only be partially accounted for by the ∼1 ‰ ice-volume-related decrease
in North Atlantic surface water δ18O
values (Schrag et al., 1996). Other factors, such as kinetics associated with
humidity and wind speed at the point of evaporation (Grootes et al., 1993),
temperature and source of atmospheric moisture (Herbert et al., 2001), and
cloud evolutionary pathways (Rozanski and Araguás, 1995), also need to be
considered but cannot account for the entirety of this shift. Because of the
narrow continental shelf in central Portugal, the LGM shoreline was located
close to the modern shoreline, thereby minimizing continental effects, and
the magnitude of the impacts of wind speed and ocean temperature do not
appear sufficient to account for the observed stalagmite δ18O
variability. Thus, the decrease in stalagmite δ18O between the
penultimate glacial and last interglacial suggests that stalagmite oxygen
isotope ratios are primarily recording (pan-)regional hydroclimate rather
than temperature. The origin of the anomalously low δ18O
values during GI 1 (dated here from 14.5–13.9 ka) is unclear
(unfortunately no other BG or GCL stalagmite also spans this interval) but
reinforces this inverse relationship between mean annual temperature and
stalagmite oxygen isotope ratios.
BG–GCL stalagmite isotopic time series. Carbon (a) and
oxygen (b) isotopes, with each stalagmite presented in a different
color. δ234U values (yellow circles) for BG6LR are plotted
against carbon isotope ratios (plots showing the δ234U and
δ13C values of the other stalagmites are presented in the
Supplement). U/Th ages (with 2 SD errors) are also shown.
The “?” at the MIS 6–5e transition denotes uncertainties associated with
the continuity of this interval.
Speleothem oxygen isotopic ratios were modeled using the paleotemperature
equation of Kim and O'Neil (1997), which requires measurements of water
(cave) temperature and drip water δ18O values. The resulting
δ18O model value of -3.1±1.0 ‰ is nearly
identical to the glass-plate-grown calcite value of -3.0±0.6 ‰.
It should be noted, however, that assessing equilibrium crystallization in
modern calcite–drip-water pairs at BG is complicated by the low temporal
resolution associated with integrated, months-long drip water samples,
variable timing of drip water collecting trips, and any seasonal biases in
calcite crystallization that at present remain poorly constrained.
Replication between stalagmites of similar age is arguably the single most
reliable method for evaluating the impacts of climate versus secondary
influences, including evaporation and kinetic effects (Denniston et al.,
1999; Mickler et al., 2004), on stalagmite isotopic ratios (Dorale and Liu,
2009; Denniston et al., 2013). When presented as an integrated data set, the
BG–GCL stalagmite carbon and oxygen isotopic time series spans the majority
of the last 220 ka (Fig. 9), although stalagmites spanning the same periods
of time are restricted to 187–185, 111–104, 83–81, 78–73, and 58–53 ka.
Because these intervals are short and because the temporal resolution varies
substantially between stalagmites, replication tests based on these intervals
are of limited utility. However, within the age uncertainties, 18O
and 13C values and trends are similar, suggesting that oxygen and
carbon isotopic ratios track environmental, rather than drip-specific,
variables. The three exceptions in which coeval samples do not replicate well
are 13C values offset by 3 ‰ from 83–81 ka and by
4 ‰ from 58–53 ka and 18O values offset by 1 ‰
from 111–104 ka (Figs. 9, S4).
δ234U values
δ234U values (calculated as the difference between the
age-corrected 234U/238U ratio of a sample and the secular
equilibrium 234U/238U ratio) of speleothem carbonate have also
been used as a proxy for paleoprecipitation (Hellstrom and McCulloch, 2000;
Oster et al., 2012; Plagnes et al., 2002; Polyak et al., 2012; Zhou et al.,
2005). 234U exists in the stalagmite crystalline lattice due to
incorporation from cave drip water and through in situ production from the decay
of 238U. Alpha recoil displaces 234U from its lattice
position, increasing its susceptibility to leaching by infiltrating waters,
meaning that 234U is selectively mobilized relative to
238U in cave drip water (Chabaux et al., 2003; Oster et al., 2012).
The flux of infiltrating fluids is therefore tied to δ234U
values of drip water, and thus stalagmite carbonate, such that decreases in
effective precipitation and/or bedrock dissolution rate, both of which are
tied to increased aridity, are associated with elevated speleothem
δ234U values (Hellstrom and McCulloch, 2000; Plagnes et al.,
2002; Polyak et al., 2012).
As differences in δ234U values between stalagmites may arise
from distinct infiltration pathways (Zhou et al., 2005), complicating the
integration of δ234U values from multiple stalagmites into a
single cohesive data set, we restrict our analysis to stalagmite BG6LR,
which represents the longest individual stalagmite record of the BG–GCL time
series. While the number of δ234U measurements is small
compared to stable isotopic values, the temporal density of the former is
sufficient to demonstrate the utility of δ13C and δ18O values as paleohydroclimate proxies (Fig. 9). Decreased
precipitation or effective moisture is associated with elevated stalagmite
δ13C, δ18O, and δ234U values.
The relationships between δ13C and δ234U values
in all BG and GCL stalagmites are presented in Fig. S5.
Environmental conditions at BG and GCL and links to Iberian margin SST
The previously discussed tests for isotopic equilibrium, including the
reproducibility of carbon and oxygen isotope ratios between coeval BG and GCL
stalagmites, support the notion that their δ13C and
δ18O values may be integrated into cohesive time series
reflecting paleohydroclimatic conditions and used to assess links between
continental climate and SST (Fig. 10). Over the last several glacial cycles,
oceanographic conditions along the western Iberian margin varied at
millennial and orbital timescales in close correlation with Greenland air
temperature and North Atlantic conditions and circulation (Roucoux et al.,
2005; Daniau et al., 2007; Sánchez Goñi et al., 2008; Darfeuil et
al., 2016). Abrupt changes in SST reflect a balance between southward
expansion of subpolar waters and northward migration of subtropical water
masses (de Abreu et al., 2003). During the particularly cold conditions
characterizing HS and GS, Iberian margin SST decreased by up to 9 ∘C
(to as much as 13 ∘C below present values; de Abreu et al., 2003),
with these changes helping to position the Arctic or subarctic front at ∼39∘ N, the same latitude as BG and GCL. These cold surface waters
reduced the production and transport of atmospheric moisture to Iberia
(Eynaud et al., 2009; Voelker and de Abreu, 2011) and would have thereby
influenced the timing of speleothem growth and carbon and oxygen isotopic
values in BG and GCL stalagmites. Indeed, the composite BG–GCL record
documents coherence, at both orbital and millennial scales, between
Portuguese hydroclimate, vegetation, and Iberian margin SST during the last
two glacial cycles (Figs. 10 and 11). In an attempt to quantify this
covariance, we binned the SST and stalagmite stable isotope data into
century-long intervals. The relatively short record of BG41 was not included,
and age models for stalagmites BG66 and GLC6 were increased by 4.0 kyr and
1.3 kyr, respectively, to improve correlation with the SST chronology. The
resulting inverse correlation between SST and carbon and oxygen is strong
(r=-0.55 and -0.52, respectively; p<0.0001) (Fig. S6).
Comparison of Portuguese stalagmite hydroclimate proxies with
regional and global climate records from the last two glacial cycles.
(a) Ice-rafted debris abundance from North Atlantic ODP Site 980
(McManus et al., 1999 using Hulu Cave timescale as presented in Barker et
al., 2011); (b) composite BG–GCL stalagmite carbon isotopic time
series with NH summer insolation (Berger an Loutre, 1991);
(c) carbon isotopic time series from Villars Cave, southern France
(Genty et al., 2003, 2006); (d) alkenone-based Iberian
margin SST reconstruction (core MD01-2443; Martrat et al., 2007);
(e) temperate forest pollen abundance from three closely spaced
cores (MD01-2443, 250–194 ka, Roucoux et al, 2006, and Tzedakis et al., 2004;
MD01-2444, 194–136 ka, Margari et al., 2010, 2014; MD95-2042, 136–1 ka,
Sánchez Goñi et al., 2008, 2013); (f) NGRIP (0–122 ka)
(North Greenland Ice Core Project members, 2004) and synthetic Greenland
oxygen isotopic record (Barker et al., 2011); (g) marine isotope
stages.
Iberian margin SST (red) versus stalagmite carbon (black;
a) and oxygen (blue; b) isotopes. Numbers denote select GI
events using the stratigraphic nomenclature of Rasmussen et al. (2014).
Growth intervals
The single most fundamental prerequisite to speleothem deposition is
infiltration of surface waters, and thus the timing of stalagmite growth can
reflect changes in mean hydroclimatic state. Deposition of multiple BG
stalagmites was punctuated by hiatuses spanning similar time intervals
(although the precise ages of the onset and/or termination of the hiatuses
are distinct), a relationship that suggests links to changes in hydroclimate
rather than random drip-site-specific variability.
Hiatuses in some BG samples coincide with HS1, HS3, HS4, and HS6, and pollen
spectra independently suggest increased aridity during HS and glacial maxima.
Decreases in arboreal pollen abundance and concomitant increases in
drought-tolerant vegetation coincide with periods of reduced SST. Vegetation
patterns during maximal IRD deposition on the Iberian margin reveal not only
dramatically reduced forest cover but also a pronounced expansion of
semidesert plants (e.g., Sánchez Goñi et al., 2000; Roucoux et al.,
2005; Naughton et al., 2009). These changes mark the long hiatus between HS7
and HS6 (71–59 ka), which overlaps some of the coldest SSTs of the
last 70 ka as reconstructed using
U37K′
at core MD95-2042 (Darfeuil et al., 2016) (Figs. 10, 12). An absence of BG
stalagmite deposition from ∼160–149 ka occurs at the same time as
massive seasonal discharges from the Fleuve Manche (Channel River) and the coldest
continental climates and SSTs (157–154 ka) of the last 220 ka, as
determined from pollen and foraminifera from core MD01-2444 (Margari et al.,
2014; Fig. 1).
BG–GCL stalagmite carbon isotopic time series and Iberian margin
SST. Light blue vertical rectangles denote North Atlantic cold events (some
of which are labeled). Several interruptions in stalagmite growth coincide,
within the errors of the stalagmite chronologies, with periods of depressed
SST. Question mark at MIS 6–5e transition denotes visible hiatus not
resolvable by U/Th dates.
Whether hiatuses in BG speleothem deposition are a result of pronounced
reductions in precipitation, an extension of below-freezing temperatures that
limited infiltration (Vaks et al., 2013; Fankhauser et al., 2016), or
variations in infiltration pathway–drip position is ambiguous. Pollen
transfer functions from MD95-2042 suggest winter temperatures dropped below
0 ∘C during HS and annual precipitation was reduced by up to
50 % (from 800 to 500–400 mm during HS3, HS4, and HS5) (Sánchez
Goñi et al., 2002). Applying this temperature reconstruction to western
Portugal is complicated, however, by the broad area across which these pollen
grains were sourced. Permafrost reconstructions (Vandenberghe et al., 2014)
of Iberia argue against the hypothesis that continuous subzero temperatures
inhibited infiltration and stalagmite growth. We thus suggest that the
hiatuses observed at BG and GCL were driven largely by reductions in
precipitation.
Other western European cave records also share similar growth histories. For
example, stalagmites from Villars Cave, southwestern France (Genty et al.,
2003, 2010; Wainer et al., 2011), and from multiple caves in northern Spain
(Stoll et al., 2013) (Fig. 1) are also punctuated by hiatuses during HS. For
example, at or near HS7, stalagmite hiatuses were formed at Villars Cave
(78–76 ka), in northern Spain (∼75 ka), and BG (80–78 ka). No
stalagmite deposition has been identified at BG from 71–60 ka or Villars
cave from 67–62 ka, a period that includes HS6. Finally, HS1 is marked by a
hiatus in northern Spain (18–15.5 ka) and at BG (17–15 ka). While the
timing of these hiatuses is not identical and not all hiatuses at Villars
Cave and the Spanish caves are coincident with those at BG, the substantial
degree of overlap suggests a common origin. Stoll et al. (2013) noted that
stalagmite deposition and/or elevated growth rates in northern Spain
stalagmites occurred during periods of high Northern Hemisphere summer
insolation or during GI, while hiatuses occurred during periods of low
insolation and low SST (< 13.7 ∘C). The BG record supports
the hypothesis that growth interruptions are related to SST controls on
regional atmospheric moisture availability, although the impact of insolation
is not clear.
BG–GCL stable isotopic and δ234U variability
Stalagmite δ13C and δ18O values covary with
changes in SST at orbital timescales. The offset between interglacial and
glacial isotopic values averages ∼3 ‰ for δ18O
and ∼7 ‰ for δ13C values (Fig. 10). Stalagmite
δ234U values also preserve these changes in aridity.
Millennial-scale changes are also recorded in stalagmite carbon isotope
ratios, with shifts of 3 ‰–7 ‰ associated with GI–GS
transitions and oxygen isotopic changes of ∼1 ‰–2 ‰. The large
swing in δ18O values during the transition from GI-1 to the
Younger Dryas (YD) (∼5 ‰ from 14.0–13.5 ka) is anomalous.
Given that the change in δ13C values at this time
(6 ‰) is consistent with other GI transitions, the hydroclimatic
implications of this interval require additional study. Similarly, oxygen and
carbon isotopic variability is pronounced during the late Holocene portion of
the BG record. The origin of this high variability is unclear. Replication of
the Holocene portion of this record is currently underway and will help address this
question (Thatcher et al., 2018).
Where growth is continuous during HS, the link between stalagmite isotopic
variations and SST changes is clearly visible (Fig. 11). Prominent positive
carbon isotopic excursions define the YD, HS2, HS5, HS6, and HS8, consistent
with diminished concentrations of arboreal pollen in cores from the Iberian
margin, and serve to document particularly cold and dry conditions at these
times (Sánchez Goñi et al., 2000, 2008; Roucoux et al., 2006).
Reduced stalagmite δ13C values mark periods of enhanced
effective moisture from 170–160 and 145–135 ka, tracking peaks in
temperate tree pollen and alkenone-based SST. The BG record reveals a
pronounced increase in stalagmite δ13C values during the YD,
at odds with the plateau in SST observed in some Portuguese coastal margin
sediments at this time. However, a higher-resolution SST record reveals a
pronounced drop in SST (Rodrigues et al., 2010), well matched with the BG
isotopic profile and the stalagmite record from Villars Cave.
Hydroclimatic shifts associated with GS and GI are most clearly expressed
during MIS 5a and 5b in the BG carbon isotope record (Fig. 11). Other
European stalagmite records have identified GI–GS events from the last
glacial period (Genty et al., 2003; Spötl et al., 2006; Boch et al.,
2011; Moseley et al., 2014) (Fig. 10), but the level of resolution recorded
in the BG–GCL time series has not been clearly identified previously in
western Iberia. A carbon isotope time series (albeit with low temporal
resolution) of a flowstone from southeastern Spain does not present clear
evidence of either GI or most HS during the last glacial cycle, although it
does contain a clear expression of HS11 (Hodge et al., 2008) (Fig. 1). And
while some Iberian lakes and peat bogs document environmental changes
concurrent with HS, no single record, including one of the longest – the 50 ka time series from the Fuentillejo maar, south-central Spain – contains a
consistent signal for all HS (Vegas et al., 2010; Moreno et al., 2012)
(Fig. 1). GS–GI oscillations during MIS 3 are not clearly defined in BG
stalagmites, likely owing to insufficient temporal resolution, although the
BG records do share a resemblance to reconstructed SST variability (Fig. 11).
Whether the apparent inconsistent linkages between Iberian margin SST and
Iberian hydroclimate are due to the limitations of these proxies,
region-specific responses to SST variations, or a changing influence of SST
on precipitation is unclear. However, other points of divergence between SST
and the BG and GCL records exist. For example, some marine cores reveal a
prominent spike in forest taxa occurring at the start of interglacials,
decreasing thereafter for the next 5–10 kyr (Tzedakis et al, 2004; Desprat
et al., 2007) (Fig. 10). This early interglacial peak is a common feature in
several time series, including the Antarctic δD (Petit et al., 1999)
and CH4 records (Loulergue et al., 2008), and in stalagmite
isotopic ratios from the eastern Mediterranean (Bar-Matthews et al., 2003)
and southern France (Couchoud et al., 2009) (Fig. 10). The BG–GCL
δ13C and δ18O records lack this feature,
although the previously discussed issues surrounding the continuity of the
MIS 6–5e transition may complicate identifying it.
Stalagmite δ13C and δ18O values are lower
during GI 20–22 (MIS 5a–4; 84–72 ka) than in either the Holocene or MIS 5e
(Figs. 10 and 12), and BG6LR δ234U values support this
observation. This interval is of particular interest given that Atlantic
forest pollen, which has been used as a proxy for air temperature, was
decoupled from SST across northwestern Iberia during cold events (C18–C20)
(Rousseau et al., 2006; Rasmussen et al., 2014). This decoupling is
interpreted as reflective of a weakened control of SST on Iberian atmospheric
temperature that, in turn, enhanced transport of atmospheric vapor to the
high latitudes, amplifying the production of ice sheets in the early stages of
the last glacial cycle (Sánchez Goñi et al., 2013). This process has
also been demonstrated for an earlier interglacial (MIS 19; Sánchez
Goñi et al., 2016). Other offsets include (1) the gradual change in BG
δ13C and δ18O values across the MIS 8–7
boundary, in contrast to the sharp rise in SST at this time, (2) the
anomalously large δ13C response to ice-rafting event C24
(111–108 ka), and (3) the persistence of low δ13C values as
SST decreased from 205–187 ka (Figs. 11 and 12).
The mechanism linking SST and Iberian hydroclimate over millennial timescales remains unclear. The NAO exerts a strong control over Iberian
precipitation, and previous studies have suggested that GS, GI (Moreno et
al., 2002; Sánchez Goñi et al., 2002; Daniau et al., 2007), and HS
(Naughton et al., 2009) were characterized by distinct NAO modes. The
dynamics of the NAO and Azores high-pressure system prior to the historical
era are only beginning to be understood (Trouet et al., 2009; Olsen et al.,
2012; Wassenburg et al., 2013), and the BG–GCL record cannot address this
question independently. However, rainfall variability in eastern Iberia is
less closely tied to the NAO than is western Iberia and instead reflects
other climatic phenomena including the El Niño–Southern Oscillation
(Rodó et al., 1997), helping to produce an east–west precipitation
gradient. Additional high-resolution speleothem records from central and
eastern Iberia could therefore provide a more robust test of the underlying
drivers of millennial-scale hydroclimatic changes during recent glacial
periods.
Conclusions
The BG–GCL composite speleothem record demonstrates that the hydroclimate
and vegetation dynamics in west-central Portugal tracked Iberian margin SST
over orbital and millennial scales during the past two glacial cycles.
Enhanced aridity characterized HS, as evidenced by elevated carbon and
oxygen isotopic ratios and/or hiatuses in stalagmite growth, consistent with
other regional stalagmite time series. GI–GS variability expressed in the
Iberian margin SST record and in co-deposited pollen spectra is also present
in the BG–GCL time series and is particularly well defined in MIS 5a and
5b. Understanding differences between the structures of the stalagmite and
SST records during some time intervals will require the development of
speleothem records from central and southern Iberia.
Use of the following data sets is gratefully
acknowledged: Global Precipitation Climatology Center data by the German
Weather Service (DWD) accessed through http://gpcc.dwd.de (last
access: 28 March 2016); NAO index data provided
by the Climate Analysis Section, NCAR, Boulder, USA (Climate
Data Guide, 2018) – updated
regularly and accessed through
https://climatedataguide.ucar.edu/climate-data/hurrell-north-atlantic-oscillation-nao-index-pc-based
(last
access: 21 May 2018).
The supplement related to this article is available online at: https://doi.org/10.5194/cp-14-1893-2018-supplement.
RFD and JAD designed the experiment. Laboratory analyses were performed by RFD, ANH, YA, ADW, VJP, SAO,
and ACB. Fieldwork was conducted by RFD, ANH, ADW, JAH, DLT, ACB, FTR, MRB, and NB. Data were analyzed by RFD, ANH, YA, ADW, JAH, DLT, SAO, ACB, SFMB,
CCU, and MMB. Manuscript was written by RFD with contributions from all authors.
The authors declare that they have no conflict of
interest.
Acknowledgements
This work was supported by the Center for Global and Regional Environmental
Research, Cornell College (to Rhawn F. Denniston), and the US National
Science Foundation (grant BCS-1118155 to Jonathan A. Haws, BCS-1118183 to
Michael M. Benedetti, and AGS-1804132 to Caroline C. Ummenhofer). Field
sampling was performed under the auspices of IGESPAR (to Jonathan A. Haws) and
Associação de Estudos Subterrâneos e Defesa do Ambiente. Brandon
Zinsious and Stephen Rasin contributed to fieldwork at BG, and Zachary
LaPointe assisted with radioisotopic analyses; Suzanne Ankerstjerne performed
stable isotope measurements. This paper
benefitted tremendously from discussions with Maria F. Sánchez Goñi,
David Hodell, and Chronis Tzedakis. We thank five anonymous reviewers who
substantially improved this paper's scope and clarity through detailed
and thoughtful assessments. Stable and U-series isotope data are available at
the NOAA National Centers for Environmental Information website.
Edited by: Nathalie Combourieu
Nebout Reviewed by: five anonymous referees
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