The precise environmental mechanisms controlling
Quaternary glacial cycles remain ambiguous. To address this problem, it is
critical to better comprehend the drivers of spatio-temporal variability in
ice-sheet evolution by establishing reliable chronologies of former
outlet-glacier advances. When spanning multiple glacial cycles, such
chronologies have the capacity to contribute to knowledge on the topic of
interhemispheric phasing of glaciations and climate events. In southern
Argentina, reconstructions of this kind are achievable, as Quaternary
expansions of the Patagonian Ice Sheet have emplaced a well-preserved
geomorphological record covering several glacial cycles. Moreover, robust
ice-sheet reconstructions from Patagonia are powerful barometers of former
climate change, as Patagonian glaciers are influenced by the Southern
Westerly Winds and the Antarctic Circumpolar Current coupled to them. It is essential to better constrain former shifts
in these circulation mechanisms as they
may have played a critical role in pacing regional and possibly global
Quaternary climate change. Here, we present a new set of cosmogenic
10Be and 26Al exposure ages from pre-Last Glacial Cycle moraine
boulder, glaciofluvial outwash cobble, and bedrock samples. This dataset
constitutes the first direct chronology dating pre-Last Glacial Maximum (LGM) glacier advances in
northern Patagonia and completes our effort to date the entire preserved
moraine record of the Río Corcovado valley system (43∘ S,
71∘ W). We find that the outermost margins of the study site depict at
least three distinct pre-Last Glacial Cycle stadials occurring around
290–270, 270–245, and 130–150 ka. Combined with the local LGM
chronology, we discover that a minimum of four distinct Pleistocene stadials
occurred during Marine Isotope Stages 8, 6, and 2 in northern
Patagonia. Evidence for Marine Isotope Stage 4 and 3 deposits were
not found at the study site. This may illustrate former longitudinal and
latitudinal asynchronies in the Patagonian Ice Sheet mass balance during these
Marine Isotope Stages. We find that the most extensive middle-to-late Pleistocene
expansions of the Patagonian Ice Sheet appear to be out of phase with local
summer insolation intensity but synchronous with orbitally controlled
periods of longer and colder winters. Our findings thus enable the exploration of
the potential roles of seasonality and seasonal duration in driving the southern
mid-latitude ice-sheet mass balance, and they facilitate novel
glacio-geomorphological interpretations for the study region. They also
provide empirical constraints on former ice-sheet extent and dynamics that
are essential for calibrating numerical ice-sheet and glacial isostatic
adjustment models.
Introduction
Understanding the environmental mechanisms driving the Quaternary build-up
and collapse cycles of Earth's major ice sheets remains a focus of ongoing
research. Key to this research is the establishment of detailed chronologies
offering direct dating of major ice-sheet expansion and recession events
(Darvill et al., 2016). Numerical glacier chronologies are most informative when
presenting unambiguous dating of ice-sheet margins spanning the entirety of
the Last Glacial Cycle (LGC: ∼ 125–15 ka) and, ideally, earlier
Quaternary glacial cycles (Kaplan et al., 2010; Schaefer et al., 2015). To
the east of the Patagonian Andes, the Argentinian foreland contains one of
the most complete and well-preserved sequences of Quaternary glacial
deposits in the world (Clapperton, 1993). This unique geomorphological
record provides the opportunity to reconstruct and date glacier fluctuations
of the formerly 2500 km long (from north to south) Patagonian Ice Sheet
(PIS) over several glacial cycles (Mercer, 1976; Hein et al., 2011). Furthermore,
the PIS occupied a key mid-latitude position in the ocean-dominated Southern
Hemisphere (SH) and was the only ice mass to fully intersect the
precipitation-bearing Southern Westerly Wind belt, a fundamental feature of
the southern mid-latitude climate system. Hence, reconstructing the timing
of PIS expansion and recession events during several glacial cycles can
provide a rare insight into southern mid-latitude Quaternary climate evolution
(Davies et al., 2020).
Palaeo-climate proxy records such as those retrieved from ice- and
ocean-sediment cores indicate that some of the most potent Quaternary
cooling events occurred during the middle Pleistocene (Parrenin et al., 2013;
Shakun et al., 2015), a period (∼ 800–130 ka; Hughes et al., 2020)
characterised by 100 kyr pacing of glacial–interglacial cycles.
Paradoxically, there is a dearth of directly dated terrestrial glacial
records for these glacial events (Hughes et al., 2020). In Patagonia, bracketing
ages for pre-LGC glaciations have been produced in some places (e.g. near Lago
Argentino; 50.5∘ S) with 40Ar /39Ar dating, K-Ar dating, and
palaeomagnetic measurements on lava flows interbedded between
Pliocene/Pleistocene glacial till units (Mercer, 1976; Sylwan et al., 1991;
Meglioli, 1992; Singer et al., 2004). For instance, Mercer (1976) used such
methods to constrain the likely timing of the Greatest Patagonian Glaciation
to between 1.2 and 1.0 Ma in the Río Gallegos (52∘ S) and
Lago Buenos Aires (46.5∘ S) regions. While such studies have been
instrumental in establishing the broad chronological framework for
Patagonian glaciations (Rabassa and Coronato, 2009), direct numerical dating
of individual outlet-glacier advances is now required to develop a detailed
knowledge of pre-Last Glacial Maximum (LGM) and pre-LGC ice-sheet activity in Patagonia.
In Patagonia, direct numerical dating of pre-LGC glacial advances,
achievable using terrestrial cosmogenic nuclide (TCN) surface exposure
dating, has been limited to two valleys of eastern central Patagonia (Kaplan
et al., 2005; Hein et al., 2009, 2011, 2017; Cogez et al., 2018). Throughout the rest of
the southern mid-latitudes (23–66∘ S), terrestrial glacial
records yielding TCN exposure ages that pre-date the LGC have been reported
using moraine boulders in New Zealand (Putnam et al., 2013), the southern central
Andes (Terrizzano et al., 2017), and Tasmania (Barrows et al., 2002; Kiernan et al., 2004,
2010, 2014, 2017; Colhoun and Barrows, 2011; Augustinus et al., 2017). Because
pre-LGC moraines and boulders are often poorly preserved, the majority of
these investigations have identified the approximate timing of individual
pre-LGC glacial advances with small (n<5 per moraine) and
scattered sets of ages. Consequently, robust and direct numerical
chronologies of pre-LGC glacial expansion events remain particularly scarce
in the southern mid-latitudes. Patagonian investigations have shown that TCN
exposure dating can yield direct ages for the deposition of pre-LGC
moraine-outwash complexes preserved in the Argentinian steppe (e.g. Hein et al.,
2017). While moraine boulders are well suited to dating glacier advances of
the LGC, Hein et al. (2009) revealed that it can be more appropriate to target outwash terraces in eastern
Patagonia for dating pre-LGC glacial margins
because their low-gradient surfaces are less prone to degradation via
gravity-driven diffusion when compared to steep-sided and unconsolidated
moraines. On pre-LGC moraines, such degradation more frequently causes
moraine surface lowering and boulder exhumation post-deposition, leading to
significant exposure-age scatter and underestimations of moraine formation
age. In contrast, surface cobbles from well-preserved outwash plains tend to
display comparatively less post-depositional scatter and are thought to date
the glacial event more precisely (Hein et al., 2009). This approach has been used
successfully to constrain the timing of glacial advances in several
locations in central and southern Patagonia (Darvill et al., 2015; Hein et al., 2009,
2011, 2017; Mendelová et al., 2020). However, to date, no pre-LGC glacial
events have been dated using this technique in northern Patagonia.
Consequently, we adopt this approach here and target both moraine boulders
and outwash surface cobbles to constrain the timing of pre-LGC glacial
advances in northeastern Patagonia.
We focus our investigation on the Río Corcovado (RC) valley system
(43∘ S; 71∘ W), formerly host to a major outlet glacier
of the PIS in northeastern Patagonia (Caldenius, 1932). We present a
multi-glacial-cycle 10Be and 26Al TCN exposure-age chronology from the
three stratigraphically oldest glacial moraine/outwash complexes deposited
by the RC outlet glacier. This study builds on our earlier work (Leger et al.,
2020, 2021a, b) to compile a geochronological reconstruction that spans the
entire preserved moraine-outwash record of the RC valley system. We use our
cosmogenic nuclide-derived chronology to test whether major middle
Pleistocene advances previously dated in central Patagonia (Hein et al., 2009,
2017) coincide with moraines found in its northeastern sector (this study),
and thus whether such advances depict strong regional climate signals
expressed across much of the palaeo-ice front. We then compare the different
magnitudes of middle Pleistocene Patagonian glaciations relative to other
formerly glaciated regions of the world and thereby assess the presence of
former interhemispheric and inter-regional (a)synchronies in ice-sheet
behaviour. Finally, we evaluate the synchronicity of PIS expansion events
with other global and regional palaeo-climate proxy records and explore the
potential roles of various insolation parameters in controlling
middle-to-late Pleistocene climate variations in Patagonia and the southern
mid-latitudes.
Setting
Our study site focuses on the ice-marginal environment of the former PIS
during Quaternary ice-age maxima, at the latitude of the major
eastward-flowing Palena outlet glacier (43∘ S; Leger et al., 2020,
2021a). This up-to-100 km-long outlet glacier diverged towards the eastern
mountain front into three glaciers occupying the Río Frío valley
to the northeast, the Río Huemul (RH) valley to the east, and the RC
valley to the south (Fig. 1; Leger et al., 2021a). The local valleys are
characterised by a westward-sloping bed topography such that the former
outlet glaciers flowed up along reverse-sloping beds. The surface geology of
the study area is characterised by well-preserved and distinct sequences of
ice-contact glaciogenic deposits interspersed among their associated
glaciolacustrine and glaciofluvial sediment-landform assemblages (Leger et al.,
2020). Terminal moraine complexes indicate that the RC and RH outlet
glaciers re-converged to deposit moraine sequences located approximately 60 km to the east of the Argentinian town of Corcovado (43∘54′ S; 71∘46′ W), into an environment presently climatically
defined as the semi-arid Patagonian steppe (Fig. 1; Garreaud et al., 2013). The local
aridity (∼570 mm a-1; Fick and Hijmans, 2017; Fig. 1a)
results from the Patagonian Andes acting as a powerful orographic barrier to
the dominant Southern Westerly Winds (Fig. 1a), causing a potent west–east
rain-shadow effect (Garreaud et al., 2013; Fig. 1a). Along the flowlines of the
RC outlet glacier, the ice eroded bedrock mainly sourced from the Mesozoic
north Patagonian batholith towards the west (Hervé et al., 2017) along with
formations of Jurassic/Cretaceous volcanic and sedimentary units located
farther east (Haller et al., 2003). Moraine-outwash complexes studied here thus
contain a mixture of rock types, including quartz-bearing leuco-monzonite,
granite, and white granodiorite that are well suited to TCN exposure dating
using in situ 10Be and 26Al radionuclides.
Ice-sheet-wide geomorphological mapping of glacial sediment–landform
assemblages has been conducted by Caldenius (1932), Glasser and Jansson
(2008), and Davies et al. (2020), while the first detailed glacial
geomorphological map specific to the RC and RH valleys was generated by
Leger et al. (2020). These maps enabled the identification of at least eight
stratigraphically distinct moraine-outwash complexes deposited by the RC
outlet glacier (Fig. 1c). This study focuses on dating results from the
three stratigraphically oldest glacial margins: i.e. the RC 0 (outermost), RC I,
and RC II (Fig. 1c) moraine-outwash complexes. Directly inboard of these
margins, the RC outlet glacier deposited another series of at least five
younger moraine complexes (RC III–VII moraines) that were previously dated to between
26.4±1.4 and 19.9±1.1 ka using TCN exposure dating and
Bayesian age modelling (Leger et al., 2021a). The latter moraine belts reflect
the local Last Glacial Maximum (LGM) expansions of the PIS.
(a) 1 km2 spatial resolution total
annual precipitation (mm a-1) and mean annual
windspeed (m s-1) land data (1970–2000 mean) from the
WorldClim version 2.1 dataset (Fick and Hijmans, 2017) overlaid on maps of
southernmost South America, with the location of the study area highlighted by a black
box. (b) Adapted from Leger et al. (2020, 2021a, b): approximate former extent of the PIS during the LGM. Locations of major PIS paleo-outlet glaciers are
designated as follows: BI: Bahía Inútil, EM: Estrecho de Magallanes, SS: Seno
Skyring, BV: Bella Vista, TDP & UE: Torres del Paine & Última
Esperanza, LA: Lago Argentino, LV: Lago Viedma, LC/P: Lago
Cochrane/Pueyrredón, LBA: Lago Buenos Aires, LP/LGV: Lago Palena/General
Vintter. NPI, SPI, and CDI stand for the North, South, and Cordillera Darwin
Patagonian ice fields, respectively. A -125 m (brown-coloured) topographic
contour roughly simulating former coastline locations at the LGM is
displayed (after Lambeck et al., 2014).
Bathymetric data were acquired from the General Bathymetric Chart of the
Oceans (GEBCO). (c) Glacial geomorphology of the study area indicating
stratigraphic relationship of moraine-outwash complexes preserved, sample
site location, and former ice-flow direction. RC and RH stand for the Río
Corcovado and Río Huemul outlet glaciers. * Other TCN dating samples, as
published in Leger et al. (2021 a, b).
Methods
Preliminary identification and mapping of the major glacial sediment–landform
assemblages studied here was carried out using remotely sensed imagery and
topographic data. These geomorphological interpretations were then
ground checked during a 5 week field season during the 2020 austral summer.
Details of the geomorphological mapping methodology and criteria, and a
complete map of the study area, are given by Leger et al. (2020). To date the RC
0–II glacier advances, we sampled both outwash surface cobbles and moraine
boulders. Specifically, we measured 10Be in five cobbles from the
outermost proglacial outwash surface sampled, here termed the “Tecka
outwash”, which we correlate stratigraphically with the RC 0 advance (Fig. 1c). We sampled five and six cobbles respectively from the RC I and RC II
outwash terraces, six moraine boulders located along a single broad ridge of
the RC II moraine complex, and one ice-moulded bedrock surface located
between the RC I and II margins (Figs. 1–3). Additionally, we measured
26Al in 5 of these 23 samples to determine whether they present simple
or complex exposure/burial histories. The chosen moraine boulders are large
(90–200 cm high) and rounded to sub-rounded granodiorite and granite
erratics embedded in the RC II moraine crest (Fig. 3). Where found, the top
2–5 cm of boulder surfaces exhibiting smooth rock fragments protruding from
more eroded surrounding surfaces were sampled using a hammer, chisel, and angle
grinder.
To date the formation of proglacial outwash plains, quartz-bearing and
fluvially rounded cobbles (4–10 cm diameter; crushed whole) were sampled
from well-preserved terraces characterised by low surface gradients
(∼0.5–1∘), following the procedure
established by Hein et al. (2009, 2011) (Fig. 2a–d). Surface ventifacts and
desert varnish, indicative of prolonged cobble exposure, were considered a
required sampling criterion. Sample preparation and cosmogenic isotope ratio
measurements were conducted at three different laboratories: the Cosmogenic
Isotope Analysis and Accelerator Mass Spectrometer (AMS) facilities of the
Scottish Universities Environmental Research Centre (SUERC) (East Kilbride,
UK), the French National Cosmogenic Nuclides Laboratory (LN2C) and the ASTER
AMS facility of the European Centre for Research and Teaching in
Environmental Geosciences (CEREGE, Aix-en-Provence, France), and the
University of Edinburgh's Cosmogenic Nuclide Laboratory (Edinburgh, UK).
Sample details and nuclide concentrations are displayed in Table 1. Details
relevant to sample location, photographs, preparation, and wet chemistry are
provided in the Supplement.
10Be and 26Al exposure ages were calculated using the online
calculator formerly known as the CRONUS-Earth online calculator version 3
(Balco et al., 2008). Rock density was assumed to be 2.65 g cm-3 and the
elevation flag was standard. For 10Be exposure-age calculations, we used the
NIST_27900 standardisation (equivalent to 07KNSTD within rounding error) and the
central Patagonia production rate (Kaplan et al., 2011) re-calculated from the
ICE-D online database (http://calibration.ice-d.org/, last access: 29 March 2022). This
local production rate yields a current sea-level high-latitude production
rate of 3.96±0.2410Be atoms g-1 yr-1 using Lm scaling,
versus 4.09±0.1910Be atoms g-1 yr-1 for the global
mean of Borchers et al. (2016). Details regarding specific AMS standards, nominal
10Be /9Be ratios, and the 10Be half-life employed are included in the Supplement. 26Al exposure ages were calculated using the
Purdue Z92-0222 AMS standardisation (equivalent to KNSTD within rounding error) and
the default CRONUS-Earth online calculator's global production rate of
Borchers et al. (2016), as the central Patagonia production rate does not feature
26Al /27Al measurements. We, however, consider it reasonable to use both
the central Patagonia and global production rates when comparing 10Be and
26Al ages, as their respective 10Be calibrations agree within error,
with reported LSDn scaling-fitting parameter values of 0.823±0.067
and 0.846±0.040, respectively (calibration: ICE-D online database).
All exposure ages are displayed in Table 2. Below, we discuss exposure ages
calculated using the time-dependent LSDn scaling scheme of Lifton et al. (2014)
with 1σ analytical uncertainties. Exposure ages are between 6 %–9 %
and 0.6 %–1% older when using the St and Lm scaling schemes of Lal (1991) and
Stone (2000), respectively. The 10Be New Zealand production rate (Putnam
et al., 2010), often used for Patagonian TCN exposure-age datasets due to its
southern mid-latitude location (44∘ S), decreases our exposure
ages by between 1.7 % and 1.5 % using LSDn scaling, which is less than the
1σ analytical uncertainties (3.5 %–1.6 %).
Field photographs of (a–d) proglacial outwash plains and
outwash surface cobbles sampled for TCN exposure dating and (e, f) the
ice-moulded bedrock surface sampled (RC20-01). Ages displayed are 10Be exposure ages ±1σ analytical uncertainties. Further
sample coordinates and characteristics are presented in Table 1.
Field photographs of moraine granite and granodiorite
boulders sampled for TCN exposure dating. Ages displayed are
10Be exposure ages ±1σ analytical uncertainties.
ResultsGeomorphology
In the RC valley, the three most distal preserved moraine complexes (RC
0–II) are morphologically distinct from the younger LGM moraines (RC
III–VII). They display greater relief (70–90 m) than the RC III–VII
moraines (10–40 m; Leger et al., 2021a), are more subdued, and have
broader-crested ridges. Along our sampling transect, the three moraine
complexes are 14 km (RC 0), 12 km (RC I), and 5 km (RC II) more distal than
the outermost-dated LGM moraine (RC III). The sampled RC II moraine is a
∼1 km-wide multi-ridge complex presenting a flat-topped,
subdued crest surface elevated up to 45 m above its ice-distal outwash
plain. The ice-distal slope gradient is variable but does not exceed
5∘ – a relatively low grade compared to the slopes of the younger,
better preserved LGM moraines, which feature slope gradients of
9–19∘ (Leger et al., 2021a). The RC I moraine complex features slope
gradients (>5∘) and surface geomorphologies for
individual moraine ridges comparable to the RC II moraine belt, but it is much
wider in places (up to 4.6 km) and exhibits a greater concentration of
distinct hummocky ridges and moraines. The subdued moraine geomorphologies
characteristic of the RC 0–II margins are suggestive of progressive lateral
slope downwasting and post-depositional moraine surface erosion of fine
material (Clapperton, 1993; Glasser and Jansson, 2008; Hein et al., 2017).
Geomorphological observations thus suggest the RC II and older moraine
complexes were deposited significantly prior to the local LGM. In the
adjacent RH valley, the outermost glacial margin consists of a
∼2 km-wide belt of kettle-kame topography displaying a
chaotic hummocks-and-hollows configuration, and was formerly termed the
“Tecka drift” (Haller et al., 2003; Leger et al., 2020). The sampled Tecka outwash
surface marks the distal limit of this kettle-kame deposit (Fig. 1c). Based
on geomorphological mapping and moraine-complex traceability across the RC
and RH valleys, the Tecka limit (RH valley) is assumed here to correspond
stratigraphically to the RC 0 advance, for which no well-preserved outwash
plain surface could be found in the RC valley (Fig. 1c).
The outwash surfaces sampled in this study all present long-term
preservation properties (Fig. 2). They display low-gradient surfaces
(0.5–1∘), desert pavements, sparse vegetation cover, and
preserved braided palaeo-meltwater channels suggestive of limited surface
erosion/deflation (Fig. 2a, d; Leger et al., 2020). These glaciofluvial terraces
are composed of sorted and fluvially rounded sands and gravel deposits. Near
the locations of outwash cobble sampling, the outwash surfaces display no
detectable signs of post-deposition activation and/or incision by
post-glacial hydrology. Where checked, modern soil thickness varied between
0 and 20 cm. This is comparable to observations made further south in the
Lago Pueyrredón (47.3∘ S) and Lago Belgrano (47.5∘ S) valleys (Hein et al., 2009; Mendelová et al., 2020). At the locations of
sampling, these terraces occur adjacent to moraine complexes and mark their
distal limits (Fig. 1c), thus enabling the unambiguous stratigraphic
association of an outwash deposit with a specific glacial advance.
TCN exposure dating sample details and nuclide concentrations.
All cobble samples were crushed whole, without
prior cutting. Sample preparation and wet chemistry were conducted at three
different laboratories: Υ samples: the University of
Edinburgh's Cosmogenic Nuclide Laboratory (Edinburgh, UK), * samples: the
Scottish Universities Environmental Research Centre (SUERC) (East Kilbride,
UK), Δ samples: the French National Cosmogenic Nuclides
Laboratory (LN2C) of the European Centre for Research and Teaching in
Environmental Geosciences (CEREGE, Aix-en-Provence, France). Nuclide ratios
in Υ and * samples were measured at the SUERC AMS
facility (East Kilbride, UK), while nuclide ratios in Δ
samples were measured at the ASTER AMS facility (CEREGE, Aix-en-Provence,
France). SiO2 is the chemical
formula of silica (quartz). # Total
27Al mass from both the carrier
(27Al concentration: 982±0.1µg g-1) and the sample, determined by
ICP-OES. The Be carrier solution used at LN2C has a
9Be concentration of 3025±9µg g-1, while carrier solutions used at SUERC
and the University of Edinburgh's Cosmogenic Nuclide Laboratory have
9Be concentrations of 849±12 and 1000 µg g-1, respectively. Conversions of isotope ratios to 10Be and
26Al concentrations were conducted
following standard equations.
TCN exposure-age chronology
The exposure-age results for the 23 samples are presented in Table 2. Four
of the five cobbles sampled on the outermost Tecka/RC 0 outwash surface
(Fig. 1c) range from 252.0±6.3 ka to 284.1±7.0 ka and afford
a mean 10Be exposure age of 268.7±14.4 ka (arithmetic mean ±1σ standard deviation). One older age (19RHS12: 311.5±8.1 ka) lies outside the remaining population's 95 % (2σ)
confidence level and is rejected as a statistical outlier. The remaining
exposure-age population is relatively well clustered given the landform age,
but still displays a >1 mean square weighted deviation (MSWD)
value of 4.38 (>k=2.63). Such a MSWD value is indicative of
greater exposure-age scatter than can be predicted by 1σ analytical
uncertainties alone (Jones et al., 2019).
The five 10Be ages from the RC I outwash surface cobbles range from 185.0±3.0 to 257.4±6.6 ka. Although the age population presents
no obvious statistical or stratigraphical outliers, the exposure ages are
poorly clustered and present a high MSWD value of 33.6 (>k=2.41), indicating significantly more exposure-age scatter than solely
predicted by analytical uncertainties. From the 26Al measured in two
samples (RC20-13 and RC20-15) from this population, the resulting
26Al /10Be concentration ratios are 6.4±0.3 and 6.0±0.6, respectively. These ratios are consistent with the canonical
26Al /10Be surface spallation production ratio of ∼6.75,
the value currently used in the CRONUS-Earth online calculator (Balco et al.,
2008), and are here interpreted as indicating a single, continuous exposure
history (within uncertainty) post erosion (Granger and Muzikar, 2001; Balco
and Rovey, 2008; Fig. S1 in the Supplement).
The ice-moulded bedrock surface (RC20-01), located inboard of the RC I
moraine and outboard of the RC II moraine, yields a 10Be exposure age of
185.7±5.0 ka, an 26Al exposure age of 181.1±8.5 ka, and
an 26Al /10Be concentration ratio of 6.8±0.3, which is
consistent with continuous exposure post erosion of the surface.
The six 10Be ages from the RC II outwash surface cobbles range from 120.6±2.0 to 146.6±4.3 ka, with a mean exposure age of 131.3±11.1 ka. The age population is tightly clustered considering the
analytical uncertainties associated with the TCN exposure dating of pre-LGC
landforms, and thus features no obvious statistical or stratigraphical
outliers. However, the population still yields a MSWD value of 11.4
(>k=2.26), diagnostic of some non-analytical exposure-age
scatter in the 10Be age distribution.
Three of the six 10Be ages from the RC II moraine boulders range from
125.5±3.4 to 161.9±3.6 ka and afford a mean exposure age
of 145.3±18.4 ka. Younger ages from the remaining three boulders
(RC20-03, 10, 11) plot outside the remaining boulder population's 95 %
confidence level and the 2σ envelope associated with the youngest RC
II outwash cobble (RC20-07). We consider 10Be inheritance an unlikely
source of exposure-age scatter compared to the high potential for boulder
exhumation causing young outliers. Moreover, the RC II outwash and RC II
moraine belt are geomorphologically likely to represent the same glacier
expansion event. These three younger boulder ages, which furthermore would
indicate a glaciation occurring during the warmer MIS 5 interglacial (a less
probable alternative), were thus rejected as stratigraphical and statistical
outliers (Table 2). The three remaining moraine boulder ages still exhibit a
high MSWD value of 28.7 (>k=3.0), indicating that the
remaining dataset displays substantially more exposure-age scatter than can
be predicted solely by analytical uncertainties. The two oldest sampled
boulders, RC20-04 (161.9±3.6 ka) and RC20-05 (148.6±3.0 ka),
yield 26Al exposure ages of 143.6±5.4 and 137.5±5.4 ka, corresponding to 26Al /10Be concentration ratios of 6.3±0.3
and 6.5±0.3, respectively. Such ratios are interpreted here as
indicating continuous exposure post erosion of the sampled boulders.
TCN exposure ages and summary statistics.
10Be and
26Al exposure ages were calculated
using the online calculator formerly known as the CRONUS-Earth online
calculator version 3 (Balco et al., 2008). Rock
density is assumed to be 2.65 g cm-3 and the elevation
flag is the standard. All samples were collected in 2020.
10Be ages reported here are
calculated using the central Patagonia production rate (Kaplan
et al., 2011; re-calculated from the
ICE-D online database:
http://calibration.ice-d.org/, last access: 29 March 2022), while
26Al ages are calculated here using
the global 26Al production rate of
Borchers et al. (2016). The AMS standardizations
employed for calculator input data are NIST_27900
(10Be) and Purdue Z92-0222
(26Al). Scaling schemes: St is the
time-independent version of Lal (1991) and Stone (2000), Lm is the
time-dependent version of Lal (1991) and Stone (2000), and LSDn is the
time-dependent scheme of Lifton et al. (2014).
Ages are reported with 1σ analytical and external
uncertainties, the latter including production rate and scaling
uncertainties in addition to analytical ones. Summary statistics were
calculated for each dated landform using only
10Be exposure ages and LSDn
scaling. This includes arithmetic means with the 1σ SD, 1σ propagated (from individual ages)
analytical uncertainty, and propagated 1σ analytical plus
production rate uncertainty (PR%). Summary statistics for sets of outwash
surface cobble ages also display the oldest cobble exposure age, which is
considered a better minimum-age estimate of outwash deposit formation here.
Summary statistics also include MSWDs and peak ages calculated using
standard equations as described by Jones et al. (2019). Ages in bold are here interpreted as the most appropriate summary
ages per landform and are the ones used throughout the paper for discussion
(see justification in Sect. 4.3).
TCN exposure-age scatter sources
We consider post-depositional shielding of TCN exposure dating samples by
vegetation and/or snow to be negligible given the semi-arid and windy
conditions (annual 1970–2000 mean wind speed of ∼5.3 m s-1; WorldClim 2.1 data; Fick and Hijmans, 2017) that characterise the
study site and more generally the Argentinian steppe foreland (Garreaud et al.,
2013; Hein et al., 2009, 2017). Modern daily winter (June–August) precipitation
is estimated at 2.2 mm near the sampling site (Fick and Hijmans, 2017),
which would represent between 20 and 50 cm thick snowfalls given various
snow densities. Snow cover 75–150 cm thick persisting for 4
months of the year is typically required to reduce nuclide production by
∼5 % (snow density range: 0.16–0.33 g cm-3; Dunai,
2010). Snow accumulation at the study site is thus considered too low to
impact exposure ages beyond analytical uncertainties at the 105 year
timescale. Cosmogenic nuclide inheritance from previous exposure has been
shown to be negligible in other eastern Patagonian valleys (e.g. Douglass et al.,
2007; Hein et al., 2009) and is considered unlikely for our samples based on the
long (>80 km) distances separating glacial deposits and source
regions. Such transport distances should have enabled efficient glacial
erosion of transported clasts. For moraine boulder samples, we expect the
exposure-age scatter to originate mainly from rock surface
erosion and boulder exhumation through moraine surface deflation and lateral
creep (Putkonen and Swanson, 2003; Briner et al., 2005; Hein et al., 2017).
For all outwash surface cobbles sampled, total rock-surface erosion is
considered negligible due to the same reasons described for moraine boulder
samples, but also due to the fluvially rounded and polished nature of the target
cobbles. Such an interpretation is further supported by the analysis of
10Be concentrations in a proglacial outwash depth profile of MIS 8 – old
sediments deposited more than 65 km east of the closest bedrock source
region, in an eastern Patagonian setting similar to our study site (Hein et al.,
2009). Results from this analysis indicate that nuclide inheritance is
negligible in outwash deposits of the Río Blanco and Hatcher units in
the Lago Pueyrredón valley (47.5∘ S). We do, however, expect
exposure-age scatter to reflect cobble exhumation via a combination of
outwash surface deflation (Hein et al., 2009; Darvill et al., 2015) and near-surface
turbation (e.g. cryoturbation) caused by possible local development of soil
permafrost during cold intervals (Trombotto, 2008). We thus assume
post-depositional disturbance to be the predominant cause of cobble exhumation and
young apparent exposure ages (Phillips et al., 1990; Hein et al., 2011), and thus
consider the oldest cobble exposure age a better minimum-age estimate of
outwash-plain stabilisation following glacier-front retreat, with the
exception of obvious statistical outliers (Table 2, Fig. 4).
(a) Kernel density plots adapted from IceTEA tools for
exposure dating outputs (Jones et al., 2019) and
summary statistics for dated landforms. Each thick red curve represents the summed
probability distribution for the exposure-age population, excluding
outliers, while thin red curves depict Gaussian curves for individual
samples. Outliers are denoted by dashed grey curves. Dashed black and
purple vertical lines denote the population's arithmetic mean and peak ages,
respectively. (b) Same kernel density plots as (a) but visualised on a single
timescale to enable better comparison of different landform ages and the
relative exposure-age scatter between surface outwash cobble and moraine
boulder samples. Panel (b) also features the global LR04 stack of benthic
δ18O records of Lisiecki
and Raymo (2005) that are commonly used to define the timing of Marine Isotope
Stages and are denoted by alternating white and blue vertical bands here.
DiscussionTCN exposure age interpretationsThe Tecka/RC 0 outwash cobble exposure ages
Surface cobbles from the Tecka/RC 0 outwash (Fig. 1c) yield a mean exposure
age of 268.7±14.4 ka (n=4), excluding one statistical outlier with
an age of 311.5±8.1 ka, and display a discernible amount of
exposure-age scatter (MSWD: 4.38>k). Following the rationale
outlined in Sect. 4.3, we consider the outwash's oldest cobble (284.1±7.0 ka) to be the closest minimum-age estimate for the RC 0 advance
(Figs. 4, 5). The relatively tight cluster of remaining exposure ages (284±7–252±6 ka) supports this interpretation (Fig. 4).
At the sampling sites, the outwash terrace surface displays preserved
braided meltwater channels that suggest minimal outwash surface deflation
post deposition. We therefore assess the possible impact of cryoturbation
alone on exposure-age underestimation and estimate broadly the potential
magnitude of former cobble exhumation through soil. To do so, we model the
constant exhumation of all sampled cobbles (excluding outliers) through a
given soil horizon (density: 1.3 g cm-3) and calculate the resulting
exposure-age bias caused by cosmic-ray attenuation with depth (Gosse and
Phillips, 2001; see the Supplement). We acknowledge that cobble
upfreezing was unlikely to have been continuous and of a similar magnitude for
all cobbles. We assume that some of this variability is considered by
reporting a population mean exposure age post simulation that matches the
original age of the oldest cobble, although this assumption undoubtedly
yields uncertainties. Using such simulations, we calculate that a soil
thickness of ∼12 cm would have been required for the Tecka/RC 0 cobbles to yield a mean 10Be exposure age (283.1±15.0 ka:
+5.7 %) that approximately matches that of the oldest original surface
cobble. This estimated magnitude of cobble upfreezing is plausible given our
observations of modern soil thicknesses (0–20 cm) near to the sampling
locations. Therefore, despite local semi-arid conditions, cobble exhumation
due to cryoturbation through a realistic soil thickness can potentially
explain the observed exposure-age scatter (Fig. 5). Such results may also
support the hypothesis that outwash surface erosion and deflation was
minimal at this location.
In summary, our interpretations suggest that the exposure ages from the
Tecka outwash place the timing of the RC 0 advance at around 290–270 ka,
during the Marine Isotope Stage (MIS) 8 (300–243 ka; Lisiecki and
Raymo, 2005) interval (Figs. 4, 6).
Illustration of the impact of combined outwash–plain
surface deflation and near-surface turbation on the exposure-age scatter for the
outwash surface cobbles sampled. The relationship between depth and nuclide
concentration is illustrated by the decay curve (purely schematic) at the
bottom left-hand corner of the diagram, which demonstrates that radionuclide
production in rock minerals reduces with depth below the surface due to
cosmic-ray attenuation. The attenuation effect causes cobbles that have
experienced some exhumation post deposition due to the effect of upfreezing
(dashed vertical arrows; scenario 1) to display exposure-age scatter,
causing landform deposition age underestimation. Cobbles that have remained near
the outwash surface since deposition (scenario 2) will display less/no
scatter and provide a better estimate of the true minimum outwash deposition age. Cobble
upfreezing is considered to mainly occur within the soil horizon
(brown coloured) due to the impact of potential permafrost formation. Soil
thickness is locally estimated to reach a maximum of ∼20 cm.
The outwash surface photograph focuses on one of the surface cobbles sampled
(yellow circle) and enables the assessment of the surface cobble density,
roundness, size, vegetation type, and cover density that are characteristic
of outwash plain surfaces sampled within the context of this investigation.
The RC I outwash cobbles and the RC I outermost advance
Surface cobbles from the RC I outwash are poorly clustered (MSWD: 33.6>k) and afford a mean exposure age of 214.0±29.5 ka and
an oldest cobble age of 257.4±6.6 ka (Table 2, Fig. 4). As
previously justified (Sect. 4.3), we consider the population's oldest
cobble (257.4±6.6 ka) as the closest minimum-age estimate for the RC
I outwash deposition. Simulations of constant cobble upfreezing through soil
(Sect. 5.1.1) demonstrate that a mean exposure age (257.2±35.3 ka:
+20 %) similar to that of the oldest original cobble is obtained when
simulating constant exhumation through a ∼39 cm thick soil
horizon. Although reasonable, this average soil thickness is greater than
modern observations (0–20 cm). For these cobbles, three distinct outwash
surface locations were sampled. They display near identical surface
morphologies, vegetation cover and apparent modern soil thicknesses (Fig. 2d). It is thus challenging to assess whether spatially variable
cryoturbation could have contributed to the observed scatter. This may
indicate that, additionally to cobble upfreezing, outwash surface deflation
played, in this instance, an important role in causing significant
exhumation of certain RC I cobbles post deposition. This hypothesis is
supported by the observation that RC I cobble exposure ages display three
distinct age groups that correspond with the three different sampling
locations (Table 1, Fig. S3 in the Supplement). The first group (sample RC20-16) yields an
exposure age of 257.4±6.6 ka, while the second (RC20-12 and 13) and
third (RC20-14 and 15) groups display mean exposure ages of 222.4±20.2 and 191.5±9.3 ka, respectively. We note that the 1σ
standard deviations associated with these age groups do not overlap. Thus,
it seems plausible that outwash surface deflation was greater at the
locations of the second and third sample groups, which, combined with clast
upfreezing, could therefore explain the substantial exposure-age scatter
observed in the 10Be age distribution of the RC I outwash cobbles.
An alternative hypothesis to explain the large RC I exposure-age scatter
would be to argue that the RC I outwash could be a composite glaciofluvial
deposit formed by meltwater runoff associated with numerous distinct RC
outlet-glacier advances. While we commonly expect younger glaciofluvial
sedimentation to bury previous deposits, certain cobbles associated with the
oldest advances could have been remobilised by either meltwater or ice
during younger advances and re-deposited towards the outwash surface. The
overlap between the youngest RC I outwash cobble age (185.0±3.0 ka)
and the deglacial 10Be age (185.7±5.0 ka) obtained from the
ice-moulded bedrock surface sampled directly inboard of the RC I limit could
support this hypothesis. Moreover, the RC I moraine belt is a wider
(∼4 km) complex that constitutes more ice-contact hummocks and
hummocky ridges, suggestive of a greater glaciogenic sediment volume than the
RC 0 and RC II moraine complexes (Fig. 6a). This geomorphological
distinction could signal that a higher number of individual outlet-glacier
advances have reached the RC I margin.
Establishing which of the previous hypotheses are more likely to be valid
would require a more detailed chronological and sedimentological analysis
through a depth profile of the RC I outwash deposit. However, regardless of
which mechanism is responsible for the observed exposure-age scatter, we
argue that the oldest exposure age from the RC I outwash suggests that a separate
extensive advance of the RC outlet glacier occurred at around 270–245 ka,
during the latest stage of the MIS 8 interval (Figs. 4, 6).
The sampled bedrock surface and the innermost RC I advances
The sampled ice-moulded bedrock's position and elevation (1026 m a.s.l.) suggest that
it was exposed after deglaciation from the RC I moraine–outwash complex and
prior to the RC II advance (Figs. 1c, 6a). Its 10Be age of 185.7±5.0 ka is thus interpreted as a minimum-age estimate of ice-front retreat
after the innermost RC outlet-glacier advance had reached the RC I moraine
complex. The older RC I outwash-cobble mean exposure age (214.0±29.5 ka) brings further evidence for this stratigraphic relationship. The
26Al /10Be ratio (6.8±0.3) from this surface bedrock sample
does not indicate a complex exposure-burial history (within uncertainty),
thus suggesting that the RC glacier did not advance beyond and bury the bedrock
for a prolonged and >100 kyr period (Fig. S1 in the Supplement). This ratio also
demonstrates efficient subglacial erosion of the bedrock surface, which
created a fresh surface free of inherited nuclides when last covered by ice.
Unlike other samples, we noticed signs of surface erosion on the bedrock
outcrop (Fig. 2e). The surface demonstrated a lack of fluvio-glacial polish
preservation and showed signs of homogeneous granular disintegration to a
depth that proved challenging to quantify in the field. The coarser-grained
and flat nature of this rock surface may have caused it to be more subject
to frost wedging than rounded moraine boulders. We thus suspect a certain
degree of exposure-age underestimation from this bedrock sample (RC20-01).
Applying the rock-surface erosion rate of 0.2 mm kyr-1 estimated for
semi-arid central Patagonia (Douglass et al., 2007; Hein et al., 2017) increases its
age by 3 % to 191.3±5.3 ka, which is within the 1σ
analytical uncertainty of the exposure age. Subsequently, this minimum
deglacial exposure age suggests that the youngest outlet-glacier advance to have
reached the RC I moraine complex had to have occurred prior to ∼190 ka. This interpretation is coherent with RC I outwash cobble exposure ages,
and together indicate that the RC I complex was most likely deposited prior to
the MIS 6 (191–130 ka; Lisiecki and Raymo, 2005) cold interval. Because
palaeo-climate proxy records such as the Antarctic atmospheric temperature and
global SST suggest a return to warmer, near-interglacial conditions between
the MIS 7d interstadial (∼ 220–230 ka) and ∼190 ka (Fig. 8; Parrenin et al., 2013; Shakun et al., 2015), it is possible that the
innermost RC I advance occurred during or prior to the MIS 7d interstadial.
To summarise, outwash cobble and bedrock ages suggest that outermost RC I
glaciogenic deposits were likely deposited by a MIS 8 glacier advance
(∼ 270–245 ka), as portrayed by the oldest outwash cobble age
(Figs. 6b, 7). On the other hand, the innermost and youngest outlet-glacier
advance to have reached the multi-ridge, 4 km-wide RC I moraine complex
(Figs. 1c, 6a) may have been significantly younger, and perhaps occurred
during the MIS 7d interstadial. However, the latter part of this
interpretation remains unclear due to the lack of chronological data for
the innermost RC I deposits in combination with the analytical uncertainties and
the large scatter displayed by outwash cobble exposure ages from this
margin. More chronological constraints are thus required to test this
hypothesis.
The RC II exposure ages
As for the older RC I and Tecka outwash cobble samples, we argue that the
observed scatter (MSWD: 11.4>k) in the RC II outwash cobble
exposure ages (mean: 131.3±11.1 ka) likely originates from cobble
exhumation and minor outwash-surface deflation (Fig. 5). This interpretation
is reinforced by cobble ages displaying a younger mean age and a tighter
cluster than moraine boulders from the stratigraphically equivalent glacial
margin (RC II; Table 2, Fig. 4). Consequently, the oldest surface cobble
(146.7±4.3 ka) is deemed a better minimum-age estimate for the RC II
glacial advance. We find that a similar mean exposure age (145.5±12.6 ka:
+11 %) to that of the oldest cobble can be obtained when
simulating the constant exhumation of all cobbles (see Sect. 5.1.1) through a 24 cm thick soil horizon. This estimate, although highly uncertain, is
consistent with modern soil thickness observations near sampling locations
(0–20 cm). This simulation suggests that, despite local semi-arid
conditions, cobble upfreezing through a realistic soil thickness can cause
the observed exposure-age scatter leading to the age underestimation of
outwash plain formation. In summary, the RC II outwash cobble ages seem to
suggest a minimum-age estimate of ∼145 ka. Therefore, we
argue that the RC II limit reflects an extensive expansion of the local PIS
outlet glacier that occurred towards the latest stage of MIS 6 (Figs. 4, 6, 7).
Following outlier removal (n=3), the RC II moraine boulder mean exposure
age (n=3; 145.3±18.4 ka) overlaps the RC II mean outwash cobble
age within the 1σ analytical uncertainty. The boulder population also
displays two ages that are consistent with the set of outwash cobbles (125.5±3.4 and 148.6±3.0 ka). These results support our
stratigraphic interpretation that these two landforms were deposited by the
same outlet-glacier advance. Because we sampled rounded boulders and
specifically targeted raised rock fragments presenting polished surfaces, here we
consider boulder surface erosion to be negligible, in contrast to the
sampled ice-moulded bedrock surface, where granular disintegration was
ubiquitous. The population's oldest boulder (RC20-04: 161.9±3.6 ka)
displays an ∼15 kyr older exposure age than the oldest RC II
outwash surface cobble. Cosmogenic nuclide inheritance from
pre-glacial-transport exposure is considered unlikely given the >80 km distance to the source region and the sub-rounded morphology of the
boulders sampled, indicative of subglacial clast erosion (Fig. 3b,c). We
instead propose that this older boulder may reflect at least one earlier glacial
advance to the same terminal position, or may have been re-deposited during
the RC II advance following exposure during previous glacier advances
that reached proximal inboard ice-front positions. The 26Al /10Be
concentration ratio from this sample (6.3±0.3) does not suggest a
prolonged period of boulder burial (Fig. S1 in the Supplement). We therefore hypothesise
that previous PIS advances might have occurred locally during earlier MIS 6 cold
intervals (e.g. MIS 6c, e), while the youngest penultimate MIS 6 glacial maximum
is more accurately dated by the RC II surface outwash cobbles, as the
glaciofluvial terraces likely remained active until the end of the
glaciation.
Overall, our results from the RC II limit (n=12 ages) indicate that
outwash surface cobble ages display less post-depositional scatter than
moraine boulder ages, with MSWD values of ∼11 vs.
∼30, respectively. This comparison thus agrees with results
of Hein et al. (2009, 2017) suggesting that well-rounded cobbles sampled from the
surface of former proglacial outwash plains are better suited for numerically
dating pre-LGC glaciations in semi-arid, undisturbed environments such as the
Argentinian steppe of Patagonia. However, even if semi-arid conditions
usually limit the soil thickness to <20 cm on outwash plains of the
Argentinian steppe, our simulations show that potential cobble exhumation
due to cryoturbation alone can cause meaningful landform-age
underestimations for pre-LGC glacial deposits. This source of
post-depositional exposure-age scatter is thus important to consider when
interpreting outwash surface cobble exposure ages. Another advantage of
targeting outwash cobbles is their abundance in formerly active
glaciofluvial environments, while moraine boulders on pre-LGC margins are
often sparse and, where found, are highly eroded. This enables rigorous
selection of the cobbles that are most suitable for TCN exposure dating. On the
other hand, obtaining exposure ages from both moraine boulders and outwash
cobbles can provide additional insight into former glacier activity that may
not be evident without the combined approach.
(a) Glacial geomorphological map focused on the RC 0 to RC
II moraine-outwash sequence around the location of TCN exposure dating
samples, as highlighted by white dots (with the exception of Tecka/RC 0 outwash
samples, Fig. 1c). Displayed exposure ages are
10Be ages ±1σ analytical uncertainties, calibrated
with LSDn scaling and the central Patagonia production rate (Kaplan et al., 2011). Ages in red were interpreted as
outliers. “Bed” stands for “bedrock” and depicts the ice-moulded bedrock
surface sampled. Elevation data are provided by the 30 m resolution DEM from
the ALOS World 3D missions (AW3D30, version 2.2; JAXA;
https://www.Eorc.jaxa.jp/ALOS/en/aw3d30/, last access: 29 March 2022) with a shaded
relief background and elevation contour lines at 15 m intervals. (b)
Simplified and smoothed elevation profile graph (DEM data: AW3D30) drawn
along the black line denoted in panel (a), along with TCN exposure dating results (10Be) from the RC 0–III
moraine-outwash dataset after interpretation of the best minimum-age
estimate for each dated landform. Palaeo-ice surface elevation and slope
gradients are hypothetical and purely schematic. * Stratigraphic order of
events (1: oldest, 5: youngest).
Comparisons with palaeo-climate and global cryosphere contextsThe MIS 8 glaciations
Our chronology suggests that two of the most extensive PIS advances
preserved at the study site (RC 0 and RC I outermost) occurred during MIS 8,
at around 290–270 and 270–245 ka, respectively (Figs. 6, 7). These
expansion events are coeval within dating uncertainties, with glacial
advances recorded by the two largest outlet glaciers of the former PIS in
central Patagonia. There, the penultimate glacial advances in both the Lago
Buenos Aires valley (46.5∘ S; Hein et al., 2017; Fig. 1) and the Lago
Pueyrredón valley (47.5∘ S; Hein et al., 2009) are thought to have
occurred during MIS 8 (Fig. 8i, h). Altogether, these three pre-LGC
Patagonian TCN chronologies indicate that the MIS 8 glacial events were some
of the PIS's most extensive Pleistocene glaciations. These advances likely
affected the former PIS across a large latitudinal range, as they can now be
traced across multiple valleys from central to northern Patagonia. The
geographical ubiquity of these events is diagnostic of a strong climatic
forcing, while their timing coincides with some of the lowest middle
Pleistocene δ deuterium values recorded over Antarctica at around
280–270 ka, which are associated with a maximum MIS 8 Antarctic cooling of
-8.5∘C relative to pre-industrial modern temperatures (Parrenin
et al., 2013; Fig. 8). The RC 0 advance may also be coeval with a severe peak in
the dust flux recorded in the EPICA Dome C Antarctic ice core at ∼270 ka (Lambert et al., 2008; Fig. 8). Isotopic composition analyses have
demonstrated that Antarctic ice-core dust concentrations are positively
correlated to outwash-plain activity in southern Patagonia, and thus may
directly reflect PIS outlet-glacier expansion events (Sugden et al., 2009;
Koffman et al., 2021).
During MIS 8, Northern Hemisphere (NH) mid-latitude summer insolation
intensity reached two distinct minima at 280 and 255 ka (Berger and
Loutre, 1991; Fig. 8). These troughs are synchronous with peaks in the SH
mid-latitude seasonality curve indicative of colder winters and warmer
summers in Patagonia at those times (Fig. 8d; Darvill et al., 2016). Importantly,
the signal of NH mid-latitude summer insolation intensity also mirrors
orbitally controlled SH seasonal duration (Fig. 8c; Denton et al., 2021). Hence,
Patagonia was experiencing longer, colder winters and shorter, warmer
summers around 280 and 255 ka. In contrast, SH mid-latitude summer
insolation intensity reached maximum values at 280 and 255 ka but
minimum values at 292 and 268 ka, which are within the exposure-age scatter
and analytical uncertainties associated with the dating of the RC 0 and
outermost RC I advances (this study) and the MIS 8 advances of the Lago
Buenos Aires and Lago Pueyrredón outlet glaciers (Hein et al., 2009, 2017). Therefore, due to current TCN exposure dating uncertainties, it
remains a challenge to determine whether the MIS 8 Patagonian glaciation was
coeval with a SH or a NH mid-latitude summer insolation intensity minimum.
However, more accurate dating of younger PIS outlet-glacier advances
occurring during the global LGM (MIS 2; e.g. Peltier et al., 2021), including the RC
outlet glacier (RC III–VII; Leger et al., 2021a), has shown that these later SH
glacier advances – and SSTs and Antarctic atmospheric temperature
depressions – were anti-phased with SH summer insolation intensity, and were
instead coeval with high SH seasonality and winter duration. Consequently,
it is plausible that the PIS advanced at ∼280 ka (RC 0) and
∼255 ka (RC I), when SH summer insolation intensity was high
but SH winters were cold and long. The proposed timing for the RC 0
advance is consistent with other SH palaeo-climate proxy records which
suggest that peak MIS 8 cooling occurred relatively early during the glacial
cycle (i.e. 280–270 ka; Fig. 8), in contrast with the later MIS 6 and MIS 5d to
MIS 2 glaciations, which are characterised by maximum global cooling
that occurred just prior to sudden glacial terminations (Hughes et al., 2020).
Paradoxically, MIS 8 displays a relatively weak global cooling signal in
several records such as benthic δ18O (Hughes et al., 2020), and is
thought to have been characterised by the lowest sea-level depression
relative to other middle-Pleistocene 100 kyr glacial intervals (-93 m; Spratt
and Lisiecki, 2016). Moreover, glacial terrestrial records from different
regions demonstrate contrasting ice-sheet and glacier behaviours during MIS
8. Indeed, although stratigraphic evidence for MIS 8 glaciations is often
elusive, some has been reported for the Fennoscandian and British-Irish ice
sheets (Beets et al., 2005; Davies et al., 2012; White et al., 2010; 2017) and former ice
sheets and glaciers overprinting Poland (Krznanian glaciations; Lindner and
Marks; 1999), Siberia and Russia (Astakhov et al., 2011, 2016), central and
southern Europe (e.g. Preusser et al., 2011), and the West Coast Range of Tasmania
(Colhoun and Barrows, 2011; Kiernan et al., 2010). In North America,
chronological constraints for the stratigraphic evidence of pre-MIS 6 glacial
deposits are, in most cases, too imprecise to determine the Marine Isotope
Stage to which they relate (Hughes et al., 2020). The review by Hughes et al. (2020)
concludes that robust chronological evidence of MIS 8 glaciations is rare,
and, where it is found, it is often characterised by margins located inboard of
MIS 6 limits. In general, direct chronological evidence of MIS 8 glaciations
more extensive than MIS 6-2 advances has only been found in Russia (east of
the Urals; Astakhov et al., 2016) and Patagonia. Therefore, the extensive MIS 8
glaciation of Patagonia implies a strong regional cooling signal (Hein et al.,
2017).
It has been noted in previous work that the major eastern Patagonian
valleys, including the RC valley (Figs. 6, 7), feature records of
progressively decreasing glacier extents as the glacier advances get younger
(Singer et al., 2004; Kaplan et al., 2009; Cogez et al., 2018). This pattern may not be
entirely climate related. Instead, it might be caused by a combination of
long-term erosion of the accumulation area and focused subglacial erosion
along former glacial valleys that resulted in glacial overdeepening (Kaplan
et al., 2009). During Quaternary glacial cycles, selective subglacial erosion –
shown to be most effective along the flowlines of warm-based PIS outlet
glaciers and near the centre of the ice sheet, where the ice was thickest
(Clapperton, 1983) – has caused rapid and zonally asymmetric overdeepening of the
main Patagonian valleys (Rabassa and Clapperton, 1990). In the past
∼1.2 Myr, this erosive process has been estimated to cause
>1000 m of bedrock elevation lowering in certain Patagonian
valleys (Singer et al., 2004). Kaplan et al. (2009) argued that such overdeepening
would likely have overcome the effects of tectonic- and glacial-isostatic-adjustment-related uplift. For a given location and identical climate
forcing, erosion-driven bed lowering may have led to progressively more
negative glacier mass balances during each major PIS expansion event,
causing less extensive glacier advances with time. Moreover, while it was more
effective towards the former ice divide, subglacial erosion would also have
increased retrograde basal slope gradients and thus augmented basal shear
stress during subsequent glacier advances. These processes could therefore
partly explain the observed pattern of decreasing outlet-glacier extent
through time in eastern Patagonia (Fig. 6). Consequently, although the
geographically widespread MIS 8 advances of the PIS were most likely the
result of strong climate forcing, their outboard positions relative to MIS
6-2 deposits do not necessarily indicate colder regional conditions during
MIS 8 than during the younger advances.
Palaeo-glacial reconstructions for the RC, RH, RF, and Lago
Palena/General Vintter (LP/LGV) valleys for each of the advances/stillstands and
deglacial events interpreted in this investigation (a–c). Panel (d) displays
the ice extent associated with the RC III moraine complex, as described in
detail in a companion publication (Leger et al.,
2021a). The latter reconstruction includes the formation of glaciolacustrine
phase one (here termed “PL1”; Leger et al., 2021a).
The ice-sheet and mountain glacier models were digitised manually in ArcGIS.
Apart from when they are delineated by confidently mapped moraine limits
(e.g. the RC, RH, and LP/LGV moraine sequences),
the geographical locations of ice margins are inferred from the topography.
Topographic data are from the AW3D30 DEM. “OC”
and “MB” denote the type of rock samples used
for TCN exposure dating of the represented event, and stand for
“outwash cobbles” and
“moraine boulders”, respectively. Yellow stars
indicate the approximate sample locations for TCN exposure dating of each
represented event, while each panel features the number of
10Be exposure ages produced (n=x) at the bottom left-hand corner (this excludes
26Al ages). There are no previously
published glacier chronologies for the RH, RF, and LP/LGV valleys. Relative
ice extents in those neighbouring valleys are thus inferred from our RC
chronology and cross-valley comparisons of moraine set numbers, preservation,
and morphostratigraphy. Hence, these inferences yield some uncertainties.
Proglacial lake volume estimates were computed from DEM data (AW3D30).
The MIS 6 glaciation
TCN exposure ages from the RC II deposit suggest that a major expansion of
the PIS occurred during MIS 6 and towards the latter part of the glacial
interval at around 130–150 ka (Figs. 4, 6, 7). Our chronology suggests that
maximum PIS expansion occurred just prior to the rapid global warming
leading to MIS 5e, one of the warmer middle-to-late Pleistocene interglacial
periods (Fig. 8). This is consistent with global sea-level reconstructions
suggesting that the second greatest global ice volume of the past 800 kyr,
associated with a sea-level minimum of -125 m, was reached towards the end
of MIS 6, at ∼ 140–135 ka (Spratt and Lisiecki, 2016). At
that time, mean atmospheric temperatures over Antarctica are estimated to
have reached their second lowest values of the past 800 kyr (-9.3∘C relative to the pre-industrial period; Fig. 8). Global cooling during MIS 6
was more persistent than during other middle Pleistocene glacial cycles.
Indeed, this cycle is associated with four major Antarctic
atmospheric temperature minima spread over ∼50 kyr and occurring at
around ∼185, ∼165, ∼155, and
∼140 ka (Parrenin et al., 2013; Fig. 8). As observed for the MIS 8
interval, these peaks in atmospheric cooling occurred a few kyr after NH
summer insolation intensity minima associated with SH seasonality and
winter duration maxima. Global SSTs and atmospheric CO2 concentrations
remained consistently low for the same 50 kyr time window between
∼190 and ∼140 ka (Bereiter et al., 2015; Shakun et al.,
2015). The persistence of the cold phase throughout MIS 6 may have
facilitated multiple expansions of the PIS at this time. In our 10Be
chronology, this may be reflected in the potential recycling and
re-deposition of one RC II moraine boulder sample, yielding a 10Be
exposure age of 162±4 ka during the younger penultimate advance
dated by the RC II outwash cobbles (∼ 130–150 ka).
Evidence for several distinct MIS 6 glaciations has been reported from other
regions of the world. For the European Ice Sheet, for instance,
stratigraphic investigations have led to the dating, and naming, of two
glaciations within MIS 6: the Late Saalian Drenthe glaciation
at 175–155 ka and the Warthe Stadial between 150 and 135 ka (Toucanne
et al., 2009; Margari et al., 2014). More generally, MIS 6 glacial deposits are
ubiquitous near former glacial margins and are documented by a wider body of
evidence than other pre-LGC glaciations (e.g. Rinterknecht et al. 2012; Putnam et al.,
2013; Kiernan et al., 2014; Evans et al., 2019; Fernandes et al., 2021), suggesting that
the greatest global ice volume of the middle Pleistocene was likely reached
during that interval (Hughes et al., 2020). In Patagonia, however, our chronology
is amongst the first published datasets offering the direct and robust dating of
a MIS 6 glacial expansion event. Indeed, the well-studied Lago
Pueyrredón and Lago Buenos Aires pre-LGC moraine-outwash records do not
provide clear chronological evidence for MIS 6 advances (Hein et al., 2009,
2017). However, this may just imply that the MIS 2 advances of the Lago
Pueyrredón and Lago Buenos Aires outlet glaciers were more extensive
than any MIS 6 glacial events. Therefore, just as for the MIS 8 glaciation, MIS 6
expansions of outlet glaciers may have been ubiquitous throughout the former
PIS. However, the current lack of robust pre-LGC glacier chronologies in
Patagonia prevents this hypothesis from being tested adequately.
Synthesis: the timing of Patagonian glaciations
Along with companion publications focusing on the LGC (Leger et al., 2020,
2021a), our findings illustrate that the preserved sequence of
moraine-outwash complexes in the RC valley system records a minimum of four
major PIS expansion events occurring during MIS 8, 6, and 2. In Antarctic
atmospheric paleo-temperature proxy records, global SST records, and global
atmospheric CO2 records, the strongest minima between today and 330 ka are all recorded by the RC moraine-outwash complexes, with the exception of
the 70–60 ka (MIS 4) cold interval, for which no preserved moraine-outwash
complex could be found at the study site. However, we note that a major
advance of the PIS near this time did occur on the western side of the Andes
at 42∘ S (García et al., 2021; Gómez et al., 2022). There, Garcia
et al. (2021) dated glacial sediments on the Isle of Chiloe to 57.8±4.7 ka, or early MIS 3, with a somewhat smaller advance occurring during MIS 2
at 26.0±2.9 ka. The lack of a corresponding MIS 3/4 moraine east of
the mountains suggests that the PIS behaved asynchronously between its
western and eastern margins at this time. In southeastern Patagonia,
advances more extensive than MIS 2 were also dated to MIS 4 and 3 using TCN
exposure dating (e.g. Darvill et al., 2016; García et al., 2018; Peltier et al., 2021). We
argue that while the RC outlet glacier most likely advanced during the MIS 4
and 3 cooling events, these intervals were relatively short lived, impeding
the PIS from becoming locally thick enough to allow the RC outlet glacier to
advance along its retrograde bed slope and generate an extensive advance.
Very likely, in addition, all glaciogenic deposits associated with MIS 3/4
advances may have been eroded during the younger MIS 2 advances. This may
suggest that steady cooling over periods > 15–20 kyr is an
important pre-requisite for northeastern PIS outlet glaciers to reach a mass
balance positive enough to enable highly extensive advances.
Vertical plot comparing this investigation's TCN
10Be chronology to other Patagonian
outlet-glacier TCN chronologies, to insolation intensity variations, and to
palaeo-climate proxy records over the past 350 kyr. (a) Stable isotope
δ deuterium record (orange curve) from
the EPICA Dome C (EDC) ice core, 75∘ S, East Antarctica, and
Antarctic atmospheric temperature (overlying dashed black curve) from a
stack of isotopic temperature reconstructions from five different ice cores
(EDC, Vostok, Dome Fuji, TALDICE, and EDML) (Parrenin et al., 2013), with the most potent cooling events of the past 350 kyr highlighted by black circles. TCN exposure ages from the RC valley
interpreted as the best minimum-age estimate for each reconstructed glacial
event are displayed (red dots) with analytical (black horizontal bars) and
external (red horizontal bars) uncertainties. (b) Global stack (n=49 cores) of paired planktonic δ18O-derived sea surface temperature (SST) data with
Marine Isotope Stage numbers indicated (Shakun et al., 2015). (c) Dark green line: summer (mid-June) insolation
intensity for 60∘ N (Berger and Loutre, 1991). Dashed black curve:
seasonal duration at 60∘ S (insolation threshold = 300 W m-2), after Huybers and Denton (2008) and Denton et al. (2021). (d) Southern Hemisphere
seasonality at 30∘ S, 40∘ S, and 60∘ S,
calculated by subtracting June from December insolation intensity values
(Berger and Loutre, 1991) such that increasing seasonality indicates colder
winters and warmer summers (after Darvill et al.,
2016). (e) Composite record of atmospheric CO2
concentrations from several Antarctic ice cores including
EDC, WAIS, Vostock, Siple Dome, TALDICE, EDML, and LAW Dome (Bereiter et al., 2015). (f) Dust flux record from the EDC
ice core (Lambert et al., 2008). (g–i) Kernel
density distribution curves for TCN exposure ages from the RC 0 to RC II
moraine-outwash records (this study: g), the Hatcher moraine-outwash
complex in the Lago Cochrane/Pueyrredón valley (Hein et al., 2009; h), and the Moreno 2/3
moraine-outwash limits in the Lago General Carerra/Buenos Aires valley
(Hein et al., 2017; i).
Climatic and orbital drivers of SH glaciations – hypotheses
Our chronology implies that the most extensive middle Pleistocene PIS
expansion events (MIS 8, 6, and 2) preserved match the strongest minima in
Antarctic atmospheric temperatures (Parrenin et al., 2013), global oceanic SSTs
(Shakun et al., 2015), and global atmospheric CO2 concentrations (Bereiter et al.,
2015) (Fig. 8). They also appear to occur around the timing of minima in NH
summer insolation intensity (60∘ N) and maxima in SH seasonality,
while being out of phase with the mid-latitude SH summer insolation intensity
(Fig. 8c). However, one must note that this statement can only be advanced
with confidence for the local MIS 2 expansions of the PIS. For the local MIS
8 and MIS 6 glaciations, this observation is based on current knowledge of
10Be production rates and the assumptions made in this paper, and does
not take into account the full exposure-age range covered by dating
analytical uncertainties. The first observations of the asynchrony between
SH mid-latitude glacier advances/recessions and local summer insolation
intensity (e.g. Mercer, 1976; Denton et al., 1999) encouraged the hypothesis that
major glacial-termination-inducing warming signals were primarily controlled
by the NH summer insolation intensity driving the NH glacial mass balance, and were
propagated from the Northern to the Southern Hemisphere through oceanic and
atmospheric circulation shifts (e.g. Broecker, 1998; Denton et al., 2010). Some of
these interhemispheric climate-transmission mechanisms, such as the impact
of the abrupt weakening of the Atlantic meridional overturning circulation
in the North Atlantic following the last deglaciation, have been well
documented (Barker et al., 2009; Denton et al., 2010). However, there is still a great
deal of debate regarding the dominant trigger mechanisms responsible for
global ice-sheet expansion/retreat cycles (Putnam et al., 2013). Indeed, the
above hypothesis entails the dilemma that while the NH summer insolation
intensity would be a key driver of the NH ice-sheet mass balance, it would
have little impact on mass fluctuations of SH mid-latitude ice
sheets and glaciers (Denton et al., 2021). This problem invites the consideration
of alternative hypotheses, such as the potential roles of SH seasonality and
seasonal duration in controlling global ice-sheet mass fluctuations.
García et al. (2018) noted that the extensive advances of the Torres del
Paine (51∘ S) and the Última Esperanza (51.5∘ S)
outlet glaciers at 48 ka (MIS 3) coincide with a minimum in
obliquity-modulated high-latitude winter insolation intensity. The authors
therefore hypothesise that colder SH winters during periods of high
obliquity may favour SH mid-latitude glaciation by promoting local
atmospheric cooling, Southern Ocean stratification, and expanded Antarctic
sea ice. Since we observe that middle-to-late Pleistocene PIS expansion
events appear to occur during periods of longer, colder winters and shorter,
warmer summers, we suggest that other PIS chronologies (including this
study) may support the hypothesis of García et al. (2018).
Alternatively, the Zealandia Switch hypothesis (Denton et al., 2021) focuses on the SH
seasonal duration aspect of orbitally controlled insolation as a potential
driver of late-Quaternary glaciation. By pacing the seasonal cycle of the
weakened and northward-migrated Southern Westerly Winds during winter versus the
strengthened and southward-migrated Southern Westerly Winds during summer,
this hypothesis implies that SH seasonal duration might be a key driver of
global climate change during middle-to-late Pleistocene glacial/interglacial
cycles. Using global climate model simulations, the authors show that a
reduced SH summer duration and increased SH winter duration could cause
millennial-scale equatorward migrations of the Southern Westerly Winds and
its coupled Antarctic Circumpolar Current. Such a zonal wind configuration
would induce a northward migration of the subtropical front and a
latitudinally restricted subtropical gyre system in the Indo-Pacific Ocean
basins (Denton et al., 2021). The bathymetry of the Australia/Zealandia continent is such that
a restricted Indo-Pacific subtropical gyre could decrease the Indonesian
Throughflow and restrict the westward Agulhas Leakage into the South Atlantic
Ocean (Lorrey and Bostock, 2017). A reduced Aghulhas Leakage has
implications for the Atlantic salt input, resulting in a potential weakening
of the Atlantic Meridional Overturning Circulation (Caley et al., 2012; De Boer et al.,
2013). The authors thus argue that such a southern oceanic and atmospheric
circulation configuration could promote a global glacial mode (Denton et al.,
2021) that would greatly affect the SH and thus Patagonian glacier mass balance.
Moreover, variations in the intensity and latitudinal position of the
Southern Westerly Winds influence the Southern Ocean upwelling of deep-stored
CO2 and nutrients (Anderson et al., 2009; Ai et al., 2020). This mechanism is
thought to represent a major contribution to variations in global
atmospheric CO2 concentrations over millennial timescales (Sarmiento
and Toggweiler, 1984). Former equatorward shifts and weakening of the
Southern Westerly Winds could have decreased Southern Ocean upwelling and
enhanced CO2 storage in the deep ocean, thus causing delayed global
cooling amplification (Sime et al., 2013). Conversely, a poleward migration and
strengthening of the Southern Westerly Winds is thought to amplify global
warming via increasing Southern Ocean upwelling, CO2 outgassing, and
increasing CO2 atmospheric concentrations. This hypothetical CO2
capacitor storage-and-release mechanism may explain the highly amplified
warming observed during major glacial terminations (Anderson et al., 2009; Denton
et al., 2021). This positive feedback mechanism may partly justify why
middle-to-late Pleistocene global oceanic and atmospheric temperature
variations often reached their most extreme values with a notable delay
(a few kyr) relative to the NH mid-latitude summer insolation intensity.
To summarise, empirical data, including this study, suggest that major
middle-to-late Pleistocene cycles of SH mid-latitude glaciers and ice sheets
appear to be coeval with NH glaciers and ice sheets but out of phase with
local mid-latitude summer insolation intensity (Putnam et al., 2013; Peltier et al.,
2021), traditionally thought to be the primary driver of middle-to-late
Pleistocene glacial expansion/demise cycles (Mercer, 1984). However, recent
hypotheses suggest that this paradox might be explained by the potentially
more dominant climate-forcing effect of the seasonality and
seasonal-duration expressions of SH insolation through their impact on
atmospheric temperature, the position of the Southern Westerly Winds belt
and Subtropical Front, Antarctic sea ice extent, and Southern Ocean
stratification and CO2 storage-and-release feedback mechanisms. Testing
the above hypotheses and determining which of seasonality or seasonal
duration played a primary role in driving SH climate and glacial variations
during the middle-to-late Pleistocene remains a major challenge and
represents a key avenue for future research.
Conclusions
We provide geomorphological and direct chronological evidence for at least
three extensive, stratigraphically distinct pre-Last Glacial Cycle advances
of the Río Corcovado glacier, a major outlet of the former northern
Patagonian Ice Sheet. TCN surface exposure dating of outwash surface cobbles
yields best age range estimates for the RC 0, RC I, and RC II advances of
290–270, 270–245, and 150–130 ka, respectively.
Together with evidence we have reported in Leger et al. (2021a, b), our complete
chronology reveals a minimum of four distinct expansion events of
northeastern Patagonian Ice Sheet outlet glaciers that occurred during the
MIS 8, MIS 6, and MIS 2 intervals.
Our dataset presents the first robust direct dating evidence of MIS 6
glaciations in Patagonia.
We did not find evidence for MIS 4 and 3 deposits in the RC valley
geomorphological record. However, extensive glaciations did occur at these
times at the former western margin of the ice sheet, and in other, more
southern, Patagonian regions. This highlights longitudinal and latitudinal
asynchronies in the former Patagonian Ice Sheet mass balance during these
stages. We suggest that MIS 4 and 3 outlet-glacier advances likely occurred
in northeastern Patagonia but were of a relatively short extent, and their
associated deposits were likely eroded by the younger, more extensive MIS 2
glaciation.
We interpret that major middle-to-late Pleistocene Patagonian Ice Sheet
expansions may have occurred in synchrony with the greatest cooling events
recorded in global SST and Antarctic atmospheric temperatures, with the most
prominent peaks in Antarctic ice-core dust concentrations, and with major
minima in global CO2 atmospheric concentrations. We observe a
particularly good correlation between Antarctic atmospheric temperature
signals and southern mid-latitude ice-sheet volume fluctuations over
several glacial cycles.
Our results suggest that Patagonian glaciations were more extensive during
MIS 8 than during MIS 6-2, while global ice volumes and sea level depression
were not particularly high during MIS 8. This implies an interhemispheric
asynchrony in the relative volume and extent of Earth's major ice sheets
during MIS 8. However, we acknowledge that a larger MIS 8 eastern Patagonian
Ice Sheet is not necessarily indicative of a more potent local
climate-forcing event than during younger glaciations (MIS 6-2), as glacial
erosion, which greatly influences the local topography, may play a role in
controlling ice-sheet extent.
Major middle-to-late Pleistocene advances of the Patagonian Ice Sheet appear
to have occurred out of phase with local summer insolation intensity but in
synchrony with orbitally controlled periods of longer and colder winters. We
hypothesise that seasonality and seasonal duration may exert more control
over the southern mid-latitude ice-sheet mass balance than mid-latitude summer
insolation intensity over millennial timescales. This may be a consequence
of the possible effect of seasonal duration variability on the strength and
migration of the Southern Westerly Winds, and the impact of these migrations
on global climate and Southern Ocean circulation and CO2 upwelling, as
proposed by the Zealandia Switch hypothesis.
Our findings corroborate work by Hein et al. (2009, 2017) and indicate that, in
eastern Patagonia, pre-LGC outwash surface cobbles yield exposure ages
displaying less post-depositional scatter than moraine boulders from the
same margins, and hence facilitate more accurate estimations of the
approximate timing of pre-LGC glacial advances.
Data availability
All data associated with the production of new 10Be and 26Al TCN
exposure ages, including sample characteristics and preparation protocols,
are provided in the manuscript tables and figures and the Supplement. Further inquiries can be directed to the corresponding author.
The data needed to re-calculate the 10Be and 26Al TCN exposure ages
presented in this paper using the online calculator formerly known as the
CRONUS-Earth online calculators version 3 is freely available online, as
long as the original publication is cited when used and/or referred to. This
includes a text file with data blocks in the correct input format and
arranged by the landforms studied in this investigation. The data are located in
an open-access Mendeley Data online repository accessible using this DOI: 10.17632/gg4b3sh9k2.1 (Leger et al., 2022).
The supplement related to this article is available online at: https://doi.org/10.5194/cp-19-35-2023-supplement.
Team list
ASTER Team: Georges Aumaître (Aix-Marseille Université, CNRS, Coll.
France, IRD, INRAE, CEREGE, Aix-en-Provence, 13545, France), Fawzi Zaidi
(Aix-Marseille Université, CNRS, Coll. France, IRD, INRAE, CEREGE,
Aix-en-Provence, 13545, France), and Karim Keddadouche (Aix-Marseille
Université, CNRS, Coll. France, IRD, INRAE, CEREGE, Aix-en-Provence,
13545, France).
Author contributions
TPML co-conceived the study with ASH, raised the finances for
fieldwork, conducted fieldwork with help from PTG, wrote the NERC NEIF
grant application together with ASH, conducted result analyses and
interpretations, wrote the manuscript and supplementary materials with feedback provided by ASH and RGB and later by AR, IS,
and DF in a second time, and generated maps and figures. TCN
exposure-age analyses, including wet chemistry and AMS measurements, were
conducted by AR and DF at SUERC and TPML, and by IS and
the ASTER Team at CEREGE and the ASTER AMS.
Competing interests
The contact author has declared that none of the authors has any competing interests.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
We express our gratitude towards all individuals who contributed to the
https://crowd.science/ (last access: 29 March 2022) crowdfunding campaign that provided us with the means
to conduct a field expedition to the study site in January and February
2020. We thank all local landowners who authorized access and sampling on
their properties, in particular the Estancia Tecka (Chubut Province,
Argentina) for enabling access to their land and private roads. We thank
Elaine McDougall and Allan Davidson for their invaluable laboratory work
enabling efficient quartz isolation and purification from our rock samples.
Finally, we are thankful to the three reviewers for providing valuable
feedback and comments which greatly improved the final manuscript.
Financial support
This study was conducted within the context of a University of Edinburgh
E3 Doctoral Training Partnership Ph.D. studentship (award code:
NE/L002558/1) awarded by the Natural Environment Research Council (NERC) and
the University of Edinburgh School of GeoSciences to Tancrède P. M. Leger Fieldwork
(1 February 2020) was partly supported by a crowd-funding campaign through the
Crowd.Science fundraising platform (https://crowd.science, last access: 29 March 2022) and a British
Society for Geomorphology Postgraduate Research Grant award (BSG-2019-04)
awarded to Tancrède P. M. Leger. TCN exposure dating laboratory analysis and AMS
measurements were funded by a NERC National Environmental Isotope Facility
(NEIF) grant (2245-0320) awarded to Andrew S. Hein and Tancrède P. M. Leger in July
2020. For the purpose of open access, the author has applied a creative
commons attribution (CC BY) licence to any author accepted manuscript
version arising.
Review statement
This paper was edited by Denis-Didier Rousseau and reviewed by three anonymous referees.
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