Search

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.

The timing and magnitude of Holocene glacial oscillations in most currently ice-free areas of Antarctica remain unknown. This work focuses on the recent deglaciation in the northern sector of the Fildes Peninsula, King George Island, northern Antarctic Peninsula. The ice cap covering ca. 90% of the island has receded since the Last Glacial Maximum and exposed ca. 29 km2 of ice-free land. We reconstruct its glacial history based on a dataset of 12 36Cl exposure ages obtained through cosmic-ray exposure (CRE) dating of moraine boulders, polished surfaces and erratic boulders surrounding the peninsula's northern plateau. Results reveal that the deglaciation of the northern Fildes Peninsula took place during the Holocene Thermal Maximum at 7–6 ka, when warm conditions promoted a massive glacial retreat. The present arrangement of ice-free areas was in place by 6 ka. Small cirque moraines suggest the subsequent occurrence of favourable climate conditions for glacial expansion fed by intense snow deflation at 4.6 and 1 ka at the foot of the northern plateau. The deglaciation pattern of the Fildes Peninsula resulted from the combined shrinkage of different ice masses, rather than of the long-term retreat of the King George Ice Cap. No evidence of glacier expansion during more recent cold periods (i.e. the Little Ice Age) was found. These results fit well with regional deglacial histories inferred from lacustrine sediments and raised beaches and complement the existing chronological framework to help better understand the peninsula's Holocene geoecological dynamics.

Mid-latitude mountain glaciers are sensitive to local summer temperature changes. Chronologies of past glacier fluctuations based on the investigation of glacial landforms therefore allow for a better understanding of natural climate variability at local scale, which is relevant for the assessment of the ongoing anthropogenic climate warming. In this study, we focus on the Holocene, the current interglacial of the last 11 700 years, which remains a matter of dispute regarding its temperature evolution and underlying driving mechanisms. In particular, the nature and significance of the transition from the early to mid-Holocene and of the Holocene Thermal Maximum (HTM) are still debated. Here, we apply an emerging approach by combining in situ cosmogenic 10Be moraine and 10Be–14C bedrock dating from the same site, the forefield of Steingletscher (European Alps), and reconstruct the glacier's millennial recession and advance periods. The results suggest that, subsequent to the final deglaciation at ∼10 ka, the glacier was similar to or smaller than its 2000 CE extent for ∼7 kyr. At ∼3 ka, Steingletscher advanced to an extent slightly outside the maximum Little Ice Age (LIA) position and until the 19th century experienced sizes that were mainly confined between the LIA and 2000 CE extents. These findings agree with existing Holocene glacier chronologies and proxy records of summer temperatures in the Alps, suggesting that glaciers throughout the region were similar to or even smaller than their 2000 CE extent for most of the early and mid-Holocene. Although glaciers in the Alps are currently far from equilibrium with the accelerating anthropogenic warming, thus hindering a simple comparison of summer temperatures associated with modern and paleo-glacier sizes, our findings imply that the summer temperatures during most of the Holocene, including the HTM, were similar to those at the end of the 20th century. Further investigations are necessary to refine the magnitude of warming and the potential HTM seasonality.

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 resolve conundrums on 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 its coupled Antarctic Circumpolar Current. Former shifts in these circulation mechanisms are essential to better constrain, 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-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 the outermost margins of the study site depict at least three distinct pre-Last Glacial Cycle glaciations occurring at 284 ± 7 ka, 257 ± 7 ka, and 147 ± 4 ka. Combined with the local LGM chronology, we discover that a minimum of four distinct Pleistocene glaciations occurred during Marine Isotope Stages eight, six, and two in northern Patagonia. Evidence for stage four and three deposits were not found at the study site, which illustrates former longitudinal and latitudinal asynchronies in Patagonian Ice Sheet mass balance during these stages. We find 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 to explore the potential roles of seasonality and seasonal duration in driving southern mid-latitude ice-sheet mass balance and facilitate novel glacio-geomorphological interpretations for the study region. They also provide empirical constraints of former ice-sheet extent and dynamics that are essential to calibrating numerical ice-sheet and glacial isostatic adjustment models.

Cosmogenic nuclide dating of glacial landforms on the Kerguelen Archipelago (49°S, 69°E) gives the opportunity to study multi-millennial glacier fluctuations within the sub-Antarctic sector of the Indian Ocean. We here dated such geomorphic features to provide time constraints over the last 17,000 years using in situ-produced 36Cl in three glacial valleys: Val Travers valley, Ampere Glacier valley and Arago Glacier valley. For the first time, a combination of in situ-produced 36Cl and 10Be dating and 26Al/10Be ratios analysis was performed in the quartz-bearing syenite boulders of the Arago Glacier site. In addition, a Bayesian approach was computed to obtain a better constraint on moraine dating. Glacial advances occurred during the Late Glacial at 16.0 ± 1.9 ka and at 12.9 ± 1.7 ka in Val Travers, and at 13.6 ± 1.8 ka in Arago Glacier valley, probably linked to the Heinrich Stadial 1 and/or Antarctic Cold Reversal events, respectively. This suggests that all glaciers at this latitude were broadly sensitive to the large-scale climatic signal of the Antarctic Cold Reversal. So far, no Early nor Mid-Holocene moraines have been found in the glacial valleys on Kerguelen, indicating that the glaciers had probably receded significantly during these periods. This is in agreement with previously determined 14C ages from peat bogs, which suggest extensive deglaciation during several millennia of the Holocene period. Samples from glacially-polished bedrock surfaces (ranging from ∼4.4 ka to ∼14 ka) at Ampere Glacier site also suggest that this valley was ice free for several millennia during the Holocene. Finally, glaciers seem to have re-advanced only during the Late Holocene, especially within the last millennium, at ∼1 ka, ∼ 430 yr and ∼300 yr. A comparison of this new dataset with the available 10Be ages from other southern mid-latitude regions during the Holocene allows the identification of three different glacier evolution patterns. We suspect that variations of Kerguelen glaciers, which are located in the Southern Indian Ocean, were controlled by the combined effects of sea surface temperature related to the variations of the Antarctic Polar Front and fluctuations of precipitation related to long-term variations of the Southern Annular Mode.

Mid-latitude mountain glaciers sensitively respond to local summer temperature changes. Chronologies of past glacier fluctuations based on the investigation of glacial landforms therefore allows for a better understanding of warm-season climate variability at local scale. In this study, we focus on the Holocene, the current interglacial of the last 11,700 years, which remains matter of dispute regarding its temperature evolution and underlying driving mechanisms. In particular, the nature and significance of the transition from the early to mid-Holocene and of the Holocene Thermal Maximum (HTM) are still debated. Here, we apply a new approach by combining in situ cosmogenic 10Be moraine and 10Be-14C bedrock dating from the same site, the forefield of Steingletscher (European Alps), and reconstruct the glacier’s millennial recession and advance periods. The results suggest that subsequent to the final deglaciation at ~10 ka, the glacier was mostly smaller than its 2000 CE extent until ~3 ka, followed by the predominant occurrence of glacier advances until the end of the Little Ice Age in the 19th century. These findings agree with existing proxy records of Holocene summer temperature and glacier evolution in the Alps, showing that glaciers throughout the region retreated beyond modern extents for most of the Early and mid-Holocene. This implies that at least the summer climate of the HTM was warmer than that of the end of the 20th century for several millennia. Further investigations are necessary to refine the magnitude of warming and the potential HTM seasonality.

International audience

International audience; La chronologie relative des fluctuations des paléoglaciers d’Auvergne au cours du dernier cycle glaciaire n’a pas bénéficié de l’apport des nouveaux outils géochronologiques comme les Nucléides Cosmogéniques Terrestres (NCT) ou la Luminescence Optiquement Stimulée (OSL) jusqu’à aujourd’hui. Appliqués aux formations glaciaires des massifs du Cantal et de l’Aubrac, ces outils ont le potentiel de lever les incertitudes qui existent autour de l’âge du Maximum Glaciaire Würmien (MGW) et de la déglaciation. Nous présentons les âges d’exposition aux rayonnements cosmiques obtenus dans ces deux massifs. Les formes et formations glaciaires datées par 10Be, 26Al et 36Cl sont reconnues sur le terrain et bien identifiées dans le cadre chronologique relatif construit à partir d’observations morpho-stratigraphiques. Ces données permettent de proposer un cadre chronologique absolu des fluctuations glaciaires, dont les avancées glaciaires majeures, du Cantal et de l’Aubrac : MGW, Récurrence et stades internes sont synchrones des épisodes climatiques les plus rigoureux, enregistrés par ailleurs à l’échelle régionale. Cependant, durant le début du Tardiglaciaire, la déglaciation totale de l’Aubrac précède celle du Cantal. Cette divergence entre les deux massifs est contrôlée par l’action conjointe des conditions topographiques et paléoclimatiques, qui mettent en évidences des dynamiques paléoglaciaires contrastées. Enfin, ces résultats alimentent la reconstitution des paléoenglacements à l’aide d’outils de modélisation (GlaRe, Positive Degree-Day), puis la reconstitution des paléogradients climatiques (thermiques et hygrométriques) qui prévalaient lors de ces stades glaciaires. Ces reconstitutions permettent d’explorer différents scénarii paléoclimatiques, notamment la part relative des flux atlantiques et/ou méditerranéens dans l’alimentation des paléoglaciers du Cantal et de l’Aubrac au cours du Dernier Maximum Glaciaire et du Tardiglaciaire.

International audience; L’archipel des Kerguelen constitue un observatoire unique pour l’étude des paléoglaciers. Il s’agit, en effet, de la plus grande surface de terres englacées de la zone subantarctique de l’océan Indien, où de nombreuses formations glacio-géomorphologiques peuvent y être datées. Toutefois, nos connaissances des fluctuations glaciaires sur l’archipel pendant le Tardiglaciaire et l’Holocène restent limitées. C’est pourquoi des échantillons issus de ces morphologies glaciaires ont été collectés dans les vallées glaciaires du glacier Arago, du Val Travers et du glacier Ampère. Les nouveaux âges 36Cl et 10Be de durée d’exposition au rayonnement cosmique présentés dans cette étude permettent de mieux contraindre la chronologie glaciaire existante au cours des derniers 17000 ans. Une avancée glaciaire a pu être mise en évidence durant le Tardiglaciaire il y a 14.4 ka sur le site du Val Travers, probablement liée à l’événement froid de l’Antarctic Cold Reversal. Ces résultats sont cohérents avec ceux précédemment obtenus sur l’archipel et plus généralement dans la zone subantarctique, suggérant que tous les glaciers à cette latitude ont été globalement sensibles à ce signal climatique. L’évolution des glaciers à Kerguelen pendant l’Holocène reste en revanche encore largement méconnue. Des datations radiocarbone d’éléments tourbeux sur le site du glacier Ampère suggèrent que les glaciers étaient certainement moins étendus au début de l’Holocène qu’à la fin de l’Holocène. En outre, aucune avancée du début et du milieu de l’Holocène n’a pu être démontrée jusqu’à présent. Enfin, les glaciers semblent avoir réavancé uniquement lors de la fin de l’Holocène. La comparaison de ce nouveau jeu de données avec des âges 10Be disponibles provenant d’autres régions subantarctiques a permis d’identifier 3 différents schémas possibles d’évolution des glaciers pendant l’Holocène. Parmi ces schémas, l’oscillation des glaciers observée à Kerguelen semble particulièrement originale. Nous discuterons enfin des facteurs climatiques pouvant rendre compte de l’évolution des glaciers à Kerguelen (température, précipitations).