CR1.1

IODP Expedition 379 to the Amundsen Sea continental rise recovered latest Miocene-Holocene sediments from two sites on a drift in water depths >3900m. Sediments are dominated by clay and silty clay with coarser-grained intervals and ice-rafted detritus (IRD) (Gohl et al. 2021, doi:10.14379/iodp.proc.379.2021). Cobble-sized dropstones appear as fall-in, in cores recovered from sediments >5.3 Ma.  We consider that abundant IRD and the sparse dropstones melted out of icebergs formed due to Antarctic ice-sheet calving events. We are using petrological and age characteristics of the clasts from the Exp379 sites to fingerprint their bedrock provenance. The results may aid in reconstruction of past changes in icesheet extent and extend knowledge of subglacial bedrock.

Mapped onshore geology shows pronounced distinctions in bedrock age between tectonic provinces of West or East Antarctica (e.g. Cox et al. 2020, doi:10.21420/7SH7-6K05; Jordan et al. 2020, doi.org/10.1038/s43017-019-0013-6). This allows us to use geochronology and thermochronology of rock clasts and minerals for tracing their provenance, and ascertain whether IRD deposited at IODP379 drillsites originated from proximal or distal Antarctic sources. We here report zircon and apatite U-Pb dates from four sand samples and five dropstones taken from latest Miocene, early Pliocene, and Plio-Pleistocene-boundary sediments. Additional Hf isotope data, and apatite fission track and ⁴⁰Ar/³⁹Ar Kfeldspar ages for some of the same samples help to strengthen provenance interpretations.

The study revealed three distinct zircon age populations at ca. 100, 175, and 250 Ma. Using Kolmogorov-Smirnov (K-S) statistical tests to compare our new igneous and detrital zircon (DZ) U-Pb results with previously published data, we found strong similarities to West Antarctic bedrock, but low correspondence to prospective sources in East Antarctica, implying a role for icebergs calved from the West Antarctic Ice Sheet (WAIS). The ~100 Ma age resembles plutonic ages from Marie Byrd Land and islands in Pine Island Bay.  The ~250 and 175 Ma populations match published mineral dates from shelf sediments in the eastern Amundsen Sea Embayment as well as granite ages from the Antarctic Peninsula and the Ellsworth-Whitmore Mountains (EWM). The different derivation of coarse sediment sources requires changes in iceberg origin through the latest Miocene, early Pliocene, and Plio/Pleistocene, likely the result of changes in WAIS extent.

One unique dropstone recovered from Exp379 Site U1533B is green quartz arenite, which yielded mostly 500-625 Ma detrital zircons. In visual appearance and dominant U-Pb age population, it resembles a sandstone dropstone recovered from Exp382 Site U1536 in the Scotia Sea (Hemming et al. 2020, https://gsa.confex.com/gsa/2020AM/meetingapp.cgi/Paper/357276). K-S tests yield high values (P ≥ 0.6), suggesting a common provenance for both dropstones recovered from late Miocene to Pliocene sediments, despite the 3270 km distance separating the sites. Comparisons to published data, in progress, narrow the group of potential on-land sources to exposures in the EWM or isolated ranges at far south latitudes in the Antarctic interior.  If both dropstones originated from the same source area, they could signify dramatic shifts in the WAIS grounding line position, and the possibility of the periodic opening of a seaway connecting the Amundsen and Weddell Seas.

How to cite: Siddoway, C., Thomson, S., Hemming, S., Buchband, H., Quigley, C., Furlong, H., Hilderman, R., Robinson, D., Watkins, C., Cox, S., and Licht, K. and the IODP Expedition 379 Scientists and Expedition 382 Scientists: U-Pb zircon geochronology of dropstones and IRD in the Amundsen Sea, applied to the question of bedrock provenance and Miocene-Pliocene ice sheet extent in West Antarctica, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9151, https://doi.org/10.5194/egusphere-egu21-9151, 2021.

Large benthic δ¹⁸O fluctuations, which are caused by deep-ocean temperature and ice-volume changes, are shown on multiple time scales during the early to mid-Miocene (23-14 Myr ago). To understand how these signals are related to orbital changes, it is necessary to disentangle them. Here, we approach this problem by simulating how the Antarctic ice sheet (AIS) responds to typical CO₂ changes during this period. We use the 3D thermodynamical model PISM, forced by climate model output, to conduct both transient and steady-state experiments. Our results indicate that even if equilibrium differences are relatively large (~40 m.s.l.e.), transient AIS variability on orbital time scales (20-400 kyr) still has a much smaller amplitude due to the slow ice-volume response to climatic changes. We analyse our results further using a conceptual model, based on the notion that at any CO₂ level an ice sheet will grow (shrink) by a specific rate towards its smaller (larger) equilibrium size. We show that phases of concurrent ice volume increase and rising CO₂ levels are possible, even though the equilibrium ice volume decreases monotonically with CO₂. When the AIS volume is out of equilibrium with the forcing climate, the ice sheet can still be adapting to a relatively large equilibrium size, although CO₂ is rising after a phase of decrease. A delayed response of Antarctic ice volume to in-sync solar insolation and CO₂ changes can cause discrepancies between Miocene solar insolation and benthic δ¹⁸O variability.

How to cite: Stap, L. B., van de Wal, R. S. W., Sutter, J., Knorr, G., and Lohmann, G.: Simulating the contribution of the Antarctic ice sheet to Miocene benthic δ18O variability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7680, https://doi.org/10.5194/egusphere-egu21-7680, 2021.

The Marine Isotope Substage 11c (MIS11c) interglacial (425 – 395 thousand years before present) is a useful analogue to climate conditions that can be expected in the near future, and can provide insights on the natural response of the Antarctic ice sheets to a moderate, yet long lasting warming period. However, its response to the warming of MIS11c and consequent contribution to global sea level rise still remains unclear. We explore the dynamics of the Antarctic ice sheets during this period using a numerical ice-sheet model forced by MIS11c climate conditions derived from climate model outputs scaled by three ice core and one sedimentary proxy records of ice volume. We identify a tipping point beyond which oceanic warming becomes the dominant forcing of ice-sheet retreat, and where collapse of the West Antarctic Ice Sheet is attained when a threshold of 0.4 ^oC oceanic warming relative to Pre-Industrial levels is sustained for at least 4 thousand years. Conversely, its eastern counterpart remains relatively stable, as it is mostly grounded above sea level. Our results suggest a total sea level contribution from the East and West Antarctic ice sheets of 4.0 – 8.2 m during MIS11c. In the case of a West Antarctic Ice Sheet collapse, which is the most probable scenario according to far-field sea-level reconstructions, this range is reduced to 6.7 – 8.2 m, and mostly reflects uncertainties regarding the initial configuration of the East Antarctic Ice Sheet.

How to cite: Mas e Braga, M., Bernales, J., Prange, M., Stroeven, A. P., and Rogozhina, I.: Sensitivity of the Antarctic ice sheets to the warming of MIS11c, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4076, https://doi.org/10.5194/egusphere-egu21-4076, 2021.

The growth and decay of marine ice sheets act as important controls on regional and global climate, in particular, the behavior of the ice sheets is a key uncertainty in predicting sea-level rise during and beyond this century. The East Antarctic Ice Sheet (EAIS), which contains deep subglacial basins with reverse-sloping, is considered to be susceptible to ice loss caused by marine ice sheet instability. Sediment core offshore Wilkes Subglacial Basin reveals oscillations in the provenance of detrital sediment that have been interpreted to reflect an erosion of Wilkes Basin during interglacial periods MIS 5, MIS 7, and MIS 9 greater than Holocene period (Wilson et al., 2018). The aim of our study is to investigate past climate and environmental changes in the coastal area of the East Antarctic Ice Sheet during MIS 7.5 and 9.3 with the help of a new high-resolution water isotopes record of the TALDICE ice core.

Here we present new δ¹⁸O and δD high resolution (5 cm) records covering the oldest portion of the TALDICE ice core. MIS 7.5 and 9.3 isotopic signal reveals a unique feature, already observed for MIS 5.5, that has not been spotted in other Antarctic ice cores (Masson-Delmotte et al., 2011). Interglacial periods at TALDICE are characterized by a first peak, observed in correspondence to the culmination of the deglaciation event as for all Antarctic cores, followed by a less pronounced isotopic peak (for MIS 5.5 and 9.3) or a plateau (for MIS 7.5) prior to the glacial inception. Several factors might drive this peculiar behavior of the water stable isotopes record, as an increase in temperatures due to a drop in surface elevation or changes in moisture sources.

The new δ¹⁸O and δD high-resolution records for the TALDICE ice core reveal a unique pattern that characterizes interglacial periods at Talos Dome. Taking into account the coastal position of the core and its vicinity to the Wilkes Subglacial Basin we intend to investigate the possible decrease in surface elevation, through the application of the GRISLI ice sheet model (Quiquet et al., 2018), and changes in moisture sources, traceable from the d-excess record.

How to cite: Crotti, I., Landais, A., Stenni, B., Frezzotti, M., Quiquet, A., Prié, F., Minster, B., Dreossi, G., and Barbante, C.: The unique behavior of stable water isotopes profiles during interglacial periods at Talos Dome, Antarctica, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1321, https://doi.org/10.5194/egusphere-egu21-1321, 2021.

In the Antarctic Peninsula (AP), the small ice caps distributed across its periphery and surrounding islands have recorded important ice volume changes since the end of the Last Glacial Cycle. These volume changes have occurred in the form of surface extent shrinking and ice thinning. The latter has been investigated only at a reduced number of locations. In this context, nunataks constitute key spots to assess the environmental evolution of deglaciated areas as they offer the opportunity to track the deglaciation history and reconstruct past ice losses by using Cosmic-Ray Exposure (CRE) dating. Indeed, nunataks are supposed to have played a prominent role in the vegetal and animal colonization of Antarctica.

The South Shetland Islands archipelago is one of the AP areas where past ice thinning has been least investigated, with only one study conducted in King George Island. In order to shed some light on the last deglaciation and its associated ice thinning, we apply ¹⁰Be CRE dating to vertical sequences of glacially polished outcrops on two palaeonunataks and one nunatak distributed across the ice-cap covering part of the Hurd Peninsula (SW of Livingston Island): Reina Sofía Peak (62°40'8" S, 60°22'51" W, 273 m a.s.l.), Moores Peak (62°41'21" S, 60°20'42" W, 407 m a.s.l.) and Napier Peak (62°40'18"S, 60°19'31" W, 382 m a.s.l.).

Most of the resulting exposure ages provided a logical chronological sequence and allowed reconstructing past vertical changes of the ice surface. The uppermost surfaces of the Moores and Reina Sofía peaks became deglaciated during the Last Glacial Maximum (LGM), between ~24 ka and ~20 ka. Following the LGM, between ~20 and ~14 ka (Termination-1), a massive deglaciation occurred. This trend was especially exacerbated at around ~14 ka, triggering the onset of the deglaciation at other nunataks, such as the Napier Peak, in good agreement with the coetaneous global melt-water pulse 1a. From our results, we infer that ice shrinking during the Holocene must have been very limited compared to the post-LGM period.

Nevertheless, some of the exposure ages were either anomalously old or inconsistent, such as those found at the summits of the Reina Sofía and Moores peaks or at the base of the Napier nunatak. These artifacts suggest the occurrence of nuclide inheritance and are indicative of the conservation of previously exposed surfaces. These ages allow to qualitatively infer cold-based regimes characterized by basal ice frozen to bed, with slow mobility and inefficient subglacial erosion due to the gentle slope gradient, not capable of removing inherited nuclides accumulated during former exposure periods. But, as a whole, the dataset adds valuable knowledge on the polythermal character of the Hurd Peninsula Ice cap.

This paper was supported by the project NUNANTAR (02/SAICT/2017 – 32002; Fundação para a Ciência e a Tecnologia, Portugal) and the College on Polar and Extreme Environments (University of Lisbon).

How to cite: Fernández-Fernández, J. M., Oliva, M., Palacios, D., García-Oteyza, J., Navarro, F., and Schimmelpfennig, I.: Post-Last Glacial Maximum ice thinning and glacier dynamics in the Hurd Peninsula ice cap (Livingston Island, South Shetland Islands, Antarctic Peninsula), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9957, https://doi.org/10.5194/egusphere-egu21-9957, 2021.

In this study, we present the stable water isotope record (δ¹⁸O) from an ice core drilled in Palmer Land, the southern Antarctic Peninsula (AP). This unique record, records changes in eastern AP ice shelf melt on the Larsen ice shelves. We show that warm years recorded in the ice core δ¹⁸O record are associated northeasterly winds that pass over the peninsula and subsequently result in foehn-induced surface warming and melt events on the Larsen Ice shelves on the eastern coast. The recent strengthening of westerly winds that circumference Antarctica (positive trend in SAM) and the deepening of the Amundsen Sea Low drives these strong northeasterly winds. We reconstruct the number of yearly melt days on the Larsen B ice shelf using melt days estimates from the published QSCAT/ASCAT dataset. Our record shows that melting on the Larsen B ice shelf since the late 1990s was higher than at any time during the past 388 years. However, periods with a high number of melt days have occurred in the past during the latter parts of the 17^th and 19^th centuries, as well as more recently during the 1940s, which may indicate past foehn-induced ice shelf melting.

How to cite: Emanuelsson, D. and Thomas, E. R.: Regional climate and ice shelf melt captured in an Antarctic Peninsula ice core, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11743, https://doi.org/10.5194/egusphere-egu21-11743, 2021.

Ocean and ice sheet in the West Antarctic sector have witnessed large climate changes during the second half of the 20^th century including a strong and widespread continental warming, important regional changes in sea-ice extent and snow accumulation, as well as a major mass loss from the melting of some ice shelves. However, the potential links between those observed changes are still unclear and instrumental data do not allow determining if they are part of a long-term evolution or specific to the recent decades. In this study, we analyze the climate variability of the past two centuries in the West Antarctic sector by reconstructing the key atmospheric variables (atmospheric circulation, near-surface air temperature and snow accumulation) as well as the sea-ice extent at the annual timescale using a data assimilation approach. To this end, information from Antarctic ice core records (snow accumulation and δ¹⁸O) and tree-ring width sites located in the mid-latitudes of the Southern Hemisphere are combined with the physics of climate models using a data assimilation method. This ultimately provides a complete spatial reconstruction over the west Antarctic region. Our reconstruction reproduces well the main characteristics of the observed changes over the instrumental period. We show that the observed sea-ice reduction in the Bellingshausen-Amundsen Sea sector over the satellite era is part of a long-term trend, starting at around 1850 CE, while the sea-ice expansion in the Ross Sea sector has only started around 1950 CE. Furthermore, according to our reconstruction, the Amundsen Sea Low pressure (ASL) displays no significant linear trend in its strength or position over 1850-1950 CE but becomes stronger and shifts eastward afterwards. The year-to-year sea-ice variations in the Ross Sea sector are strongly related to the ASL variability over the past two centuries, including the recent trends. By contrast, the link between ASL and sea ice the Bellingshausen-Amundsen Sea sector changes with time, being stronger in recent decades than before, Our reconstruction also suggests that the continental response to the variability of the ASL may not be stationary over time, being significantly affected by modification of the mean circulation. Finally, we show that the widespread warming since 1958 CE in West Antarctica is unusual in the context of past 200 years and is explained by both the deeper ASL and the positive phase of the Southern Annular Mode.

How to cite: Dalaiden, Q., Goosse, H., Rezsohazy, J., and Thomas, E. R.: Reconstructing atmospheric circulation and sea-ice extent in the West Antarctic over the past 200 years using data assimilation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9225, https://doi.org/10.5194/egusphere-egu21-9225, 2021.

The regional climate model HIRHAM5 developed for Greenland ice sheet applications has now been updated to also simulate Antarctic conditions. The outputs of the Antarctic runs have been used to force an offline subsurface model, to give estimates of the Antarctic surface mass balance (SMB) from 1980 to 2017.  Here, we compare two different versions of this offline subsurface model and evaluate how they simulate the physical properties of the uppermost part of the Antarctic firn pack. We find that the total calculated SMB of Antarctica is sensitive to the subsurface model variant. One model version uses an Eulerian framework, meaning that mass is advected through layers of fixed mass. When snowfall occurs at the surface, it is added to the first layer and an equal mass from that layer is shifted to the underlying layer. The same goes for each layer in the model column, and vice versa for mass loss. The other model version uses a Lagrangian framework for the layer development. Layers evolve through splitting and merging dynamically based on a number of weighted criteria.

The model simulations are validated against in situ observations of firn temperature and subsurface density. We find a mean temperature bias of 0.42-0.52 ℃  and a mean bias in modelled density of -24.0±18.4 kg m⁻³ and  -8.2±15.3 kg m⁻³ for layers less than 550 kg m⁻³ for the Eulerian and Lagrangian framework, respectively. For layers with a density above 550 kg m⁻³ the bais is -31.7±23.4 kg m⁻³ and -35.0±23.7 kg m⁻³  for the Eulerian and Lagrangian framework, respectively. The modelling framework also  affects the resulting  SMB. The Lagrangian framework,  estimates a total SMB  of 2473.5±114.4 Gt yr⁻¹ while the Eulerian framework results in slightly higher modelled SMB of  2564.8±113.7 Gt yr⁻¹. The majority for this difference in modelled SMB is pinpointed to the  ice shelves (the SMB over grounded ice only  differs  30 Gt yr⁻¹) and  demonstrates the importance of firn modelling in areas with substantial melt. Both the Eulerian and the Lagrangian SMB estimates are within each other's uncertainties and within range of other SMB studies. However, the Lagrangian version has the best statistics for modelling the densities. Given the importance of precipitation to Antarctic SMB, climate models must accurately simulate regional circulation patterns that modulate precipitation rates. We therefore investigate the relationship between SMB in individual drainage basins and the southern annular mode (SAM),  using Monte Carlo simulations. This shows a robust relationship between SAM and SMB in half of the basins (13 out of 27). In general, when SAM is positive there is a lower SMB over the Plateau and a higher SMB on the westerly side of the Antarctic Peninsula, and vice versa when the SAM is negative.

How to cite: Hansen, N., Langen, P. L., Boberg, F., Forsberg, R., Simonsen, S. B., Thejll, P., Vandercrux, B., and Mottram, R.: Antarctic Surface Mass Balance from 1980 to 2017, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8714, https://doi.org/10.5194/egusphere-egu21-8714, 2021.

Due to ocean warming in the Amundsen sea, pine island glacier and thwaites glacier could lose their buttressing ice shelves in the near future. This would lead to glacier retreat through the marine ice sheet instability and could be accelerated by additional cliff calving (marine ice cliff instability). Using the Parallel Ice Sheet Model (PISM-PIK) we investigate this in a regional setup of the West Antarctic Ice Sheet. We remove floating ice in the Amundsen sea and investigate the resulting glacier retreat without additional cliff calving and with cliff calving with a range of maximum calving rates. We find that without additional cliff calving, the removal of the ice shelves in the Amundsen sea leads to a glacier retreat that is equivalent to 0.4-0.6m of sea level rise in 100 years, consistent with earlier simulations of other models (ABUMIP and LARMIP-2). Cliff calving can more than double this number.

How to cite: Schlemm, T. and Levermann, A.: Towards investigating the race of two Marine Ice instabilities: Sheet vs. Cliff, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6252, https://doi.org/10.5194/egusphere-egu21-6252, 2021.

The future retreat rate for marine-based regions of the Antarctic Ice Sheet is one of the largest uncertainties in sea-level projections. The Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) aims to improve projections and quantify uncertainties by running an ensemble of ice sheet models with forcing derived from global climate models. Here, the Community Ice Sheet Model (CISM) is used to run ISMIP6-based projections of ocean-forced Antarctic Ice Sheet evolution. Using several combinations of sub-ice-shelf melt schemes, CISM is spun up to steady state over many millennia. During the spin-up, basal-friction and thermal-forcing parameters are adjusted to optimize agreement with the observed ice thickness. The model is then run forward to year 2500, applying ocean thermal forcing anomalies from six climate models. In all simulations, ocean warming triggers long-term retreat of the West Antarctic Ice Sheet, especially in the Filchner-Ronne and Ross sectors. The ocean-forced sea-level rise in 2500 varies from about 150 mm to 1300 mm, depending on the melt scheme and ocean forcing applied. Further experiments show relatively high sensitivity to the basal friction law, and moderate sensitivity to grid resolution and the prescribed collapse of small ice shelves. The Amundsen sector exhibits threshold behavior, with modest retreat under many parameter settings, but complete collapse under some combinations of low basal friction and high thermal-forcing anomalies. Large uncertainties remain, as a result of parameterized sub-shelf melt rates, simplified treatments of calving and basal friction, and the lack of ice–ocean coupling.

How to cite: Lipscomb, W., Leguy, G., Jourdain, N., Asay-Davis, X., Seroussi, H., and Nowicki, S.: ISMIP6-based projections of ocean-forced Antarctic ice loss using the Community Ice Sheet Model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6722, https://doi.org/10.5194/egusphere-egu21-6722, 2021.