Subglacial Conditions of the Kamb Ice Stream and its Response to Environmental Change

<p>The Siple Coast ice streams, which drain the West Antarctic Ice Sheet into the Ross Ice Shelf, are susceptible to temporal changes in flow dynamics. The Kamb Ice Stream on the Siple Coast, stagnated approximately 160 years ago, thought to partially be the result of basal water diversion. The character of its subglacial environment can exert an important control on long- and short-term ice sheet and ice stream fluctuations. Were the Kamb Ice Stream to reactivate in response to subglacial or future climate change, it would have the potential to contribute more substantially to ice discharge into the Ross Ice Shelf. Therefore, it is important to characterise the present-day subglacial environment and climatic conditions that may reactivate this flow. This study investigates the present-day subglacial conditions of the Kamb Ice Stream and how these conditions may be affected by environmental perturbations. Due to the difficult nature of making direct observations of ice sheet basal conditions, other methods are employed to investigate the response of the Kamb Ice Stream to environmental change. Active source seismic surveying data obtained during the 2015/16 and 2018/19 austral summer seasons provides an instantaneous snapshot of the present-day basal conditions. Flowline and whole-continent numerical ice sheet modelling is used to investigate the longer-term response of the Kamb Ice Stream and the West Antarctic Ice Sheet. Amplitude analysis of seismic lines indicate saturated till beneath the Ross Ice Shelf in the vicinity of the grounding zone, which is supported by retreat rates of the Kamb Ice Stream grounding zone post-stagnation. Seismic reflection imaging suggests potential dewatered till conditions beneath the grounded Kamb Ice Stream. Flowline modelling of the Kamb Ice Stream indicates that changes to the water content of the subglacial sediments appear to be self regulating, with high reversibility over centennial timescales. Oceanic temperature forcings are the key driver of change of the Kamb Ice Stream, and the ice stream is susceptible to topographic pinning points in 2D and lateral drag. Future glaciological change is more likely to occur in response to oceanic than to atmospheric temperature perturbations. Results from 3D continent-wide modelling experiments also find that precipitation increases offset the effect of air temperature perturbations and influence subglacial conditions, indicating more dynamic ice stream behaviour on the Siple Coast. This study has worked to re-enforce and strengthen our existing understanding of the Kamb Ice Stream and its sensitivity to environmental change. Future work using higher-resolution simulations and a higher density of observational data may help refine these results.</p>

Subglacial Conditions of the Kamb Ice Stream and its Response to Environmental Change

<p>The Siple Coast ice streams, which drain the West Antarctic Ice Sheet into the Ross Ice Shelf, are susceptible to temporal changes in flow dynamics. The Kamb Ice Stream on the Siple Coast, stagnated approximately 160 years ago, thought to partially be the result of basal water diversion. The character of its subglacial environment can exert an important control on long- and short-term ice sheet and ice stream fluctuations. Were the Kamb Ice Stream to reactivate in response to subglacial or future climate change, it would have the potential to contribute more substantially to ice discharge into the Ross Ice Shelf. Therefore, it is important to characterise the present-day subglacial environment and climatic conditions that may reactivate this flow. This study investigates the present-day subglacial conditions of the Kamb Ice Stream and how these conditions may be affected by environmental perturbations. Due to the difficult nature of making direct observations of ice sheet basal conditions, other methods are employed to investigate the response of the Kamb Ice Stream to environmental change. Active source seismic surveying data obtained during the 2015/16 and 2018/19 austral summer seasons provides an instantaneous snapshot of the present-day basal conditions. Flowline and whole-continent numerical ice sheet modelling is used to investigate the longer-term response of the Kamb Ice Stream and the West Antarctic Ice Sheet. Amplitude analysis of seismic lines indicate saturated till beneath the Ross Ice Shelf in the vicinity of the grounding zone, which is supported by retreat rates of the Kamb Ice Stream grounding zone post-stagnation. Seismic reflection imaging suggests potential dewatered till conditions beneath the grounded Kamb Ice Stream. Flowline modelling of the Kamb Ice Stream indicates that changes to the water content of the subglacial sediments appear to be self regulating, with high reversibility over centennial timescales. Oceanic temperature forcings are the key driver of change of the Kamb Ice Stream, and the ice stream is susceptible to topographic pinning points in 2D and lateral drag. Future glaciological change is more likely to occur in response to oceanic than to atmospheric temperature perturbations. Results from 3D continent-wide modelling experiments also find that precipitation increases offset the effect of air temperature perturbations and influence subglacial conditions, indicating more dynamic ice stream behaviour on the Siple Coast. This study has worked to re-enforce and strengthen our existing understanding of the Kamb Ice Stream and its sensitivity to environmental change. Future work using higher-resolution simulations and a higher density of observational data may help refine these results.</p>

Marine Palynomorphs from the Plio-Pleistocene interval of the AND-1B Drill-Core McMurdo Sound, Antarctica

<p>The ANDRILL project recovered over 600 m of Plio-Pleistocene sediments within the Ross embayment, Antarctica. These sediments contain a record of local and regional paleoenvironmental conditions and glacial dynamism. They also provide a proxy for ice dynamics of the West Antarctic Ice Sheet (WAIS) during a period when global temperatures were ~3OC higher than modern. This unique record provides an analogue for future global climate change, which is expected to rise by 3OC by the end of the 21st century. Sixty-one samples from the upper 600 m of the AND-1B core were analysed for their palynomorph content yielding 4 to 5380 grains per sample (with an average frequency of 34 grains per gram). Marine palynomorphs including fossil dinoflagellate cysts, acritarchs, and prasinophyte algae were the focus of this study and fluctuations in their abundance and diversity reflect changes in paleoenvironment and glacial dynamics. The upper 600 m can be divided into 6 discrete units based on the palynomorph assemblage: The early-Pliocene (~5.0 – 4.6 Ma. Unit 1) is characterised by relatively high abundances of in situ round brown dinoflagellate cysts, microforaminiferal linings, and Leiosphaeridia, suggesting warmer than modern paleoenvironmental conditions and seasonal ice within the Ross embayment. The WAIS was likely small and highly dynamic during Unit 1. The mid-Pliocene (~4.6 – 3.4 Ma. Unit 2) exhibits relatively high abundances of round brown dinoflagellate cysts, microforaminiferal linings, and scolecodonts. The relatively low abundance of Leiosphaeridia (understood to indicate proximal/seasonal ice) suggests that ice free conditions at the drill site may have existed for up to ~1.2 Ma and that this may be the warmest period recorded in the core. During the warm, mid-Pliocene interval a sudden increase in scolecodonts (fossilized polychaete remains) may give indications into the water depth at the drill site because of their dependence on physical disturbance (decreasing with depth) for population growth. Further study of the scolecodonts is required before confident estimates of water depth can be made. The mid- to late-Pliocene (~3.4 – 2.6 Ma. Units 3, 4 & 5) is characterised by a variable palynomorph assemblage indicating variability in paleoenvironmental conditions, ice cover at the drill site, and ultimately a variable WAIS. A spike in the prasinophyte alga Cymatiosphaera (understood to indicate reduced salinity) at the base of a diatomite unit in the late-Pliocene may be a record of algae thriving in meltwater from the collapse of the WAIS. Further highresolution analysis is needed to help resolve this event. The Quaternary interval (~2.6 Ma and younger. Unit 6) is significantly different from previous units and is dominated by reworked Eocene dinoflagellate cysts and acritarchs (the “Transantarctic Flora”). This interval records a period of significant cooling and glacial expansion and the WAIS likely grew to its modern “polar” state. The WAIS may have undergone several collapses during super-interglacial periods in the Pleistocene but if it did it did not persist in its collapsed state for significant periods of time.</p>

Marine Palynomorphs from the Plio-Pleistocene interval of the AND-1B Drill-Core McMurdo Sound, Antarctica

<p>The ANDRILL project recovered over 600 m of Plio-Pleistocene sediments within the Ross embayment, Antarctica. These sediments contain a record of local and regional paleoenvironmental conditions and glacial dynamism. They also provide a proxy for ice dynamics of the West Antarctic Ice Sheet (WAIS) during a period when global temperatures were ~3OC higher than modern. This unique record provides an analogue for future global climate change, which is expected to rise by 3OC by the end of the 21st century. Sixty-one samples from the upper 600 m of the AND-1B core were analysed for their palynomorph content yielding 4 to 5380 grains per sample (with an average frequency of 34 grains per gram). Marine palynomorphs including fossil dinoflagellate cysts, acritarchs, and prasinophyte algae were the focus of this study and fluctuations in their abundance and diversity reflect changes in paleoenvironment and glacial dynamics. The upper 600 m can be divided into 6 discrete units based on the palynomorph assemblage: The early-Pliocene (~5.0 – 4.6 Ma. Unit 1) is characterised by relatively high abundances of in situ round brown dinoflagellate cysts, microforaminiferal linings, and Leiosphaeridia, suggesting warmer than modern paleoenvironmental conditions and seasonal ice within the Ross embayment. The WAIS was likely small and highly dynamic during Unit 1. The mid-Pliocene (~4.6 – 3.4 Ma. Unit 2) exhibits relatively high abundances of round brown dinoflagellate cysts, microforaminiferal linings, and scolecodonts. The relatively low abundance of Leiosphaeridia (understood to indicate proximal/seasonal ice) suggests that ice free conditions at the drill site may have existed for up to ~1.2 Ma and that this may be the warmest period recorded in the core. During the warm, mid-Pliocene interval a sudden increase in scolecodonts (fossilized polychaete remains) may give indications into the water depth at the drill site because of their dependence on physical disturbance (decreasing with depth) for population growth. Further study of the scolecodonts is required before confident estimates of water depth can be made. The mid- to late-Pliocene (~3.4 – 2.6 Ma. Units 3, 4 & 5) is characterised by a variable palynomorph assemblage indicating variability in paleoenvironmental conditions, ice cover at the drill site, and ultimately a variable WAIS. A spike in the prasinophyte alga Cymatiosphaera (understood to indicate reduced salinity) at the base of a diatomite unit in the late-Pliocene may be a record of algae thriving in meltwater from the collapse of the WAIS. Further highresolution analysis is needed to help resolve this event. The Quaternary interval (~2.6 Ma and younger. Unit 6) is significantly different from previous units and is dominated by reworked Eocene dinoflagellate cysts and acritarchs (the “Transantarctic Flora”). This interval records a period of significant cooling and glacial expansion and the WAIS likely grew to its modern “polar” state. The WAIS may have undergone several collapses during super-interglacial periods in the Pleistocene but if it did it did not persist in its collapsed state for significant periods of time.</p>

The Antarctic Coastal Current in the Bellingshausen Sea

The ice shelves of the West Antarctic Ice Sheet experience basal melting induced by underlying warm, salty Circumpolar Deep Water. Basal meltwater, along with runoff from ice sheets, supplies fresh buoyant water to a circulation feature near the coast, the Antarctic Coastal Current (AACC). The formation, structure, and coherence of the AACC has been well documented along the West Antarctic Peninsula (WAP). Observations from instrumented seals collected in the Bellingshausen Sea offer extensive hydrographic coverage throughout the year, providing evidence of the continuation of the westward flowing AACC from the WAP towards the Amundsen Sea. The observations reported here demonstrate that the coastal boundary current enters the eastern Bellingshausen Sea from the WAP and flows westward along the face of multiple ice shelves, including the westernmost Abbot Ice Shelf. The presence of the AACC in the western Bellingshausen Sea has implications for the export of water properties into the eastern Amundsen Sea, which we suggest may occur through multiple pathways, either along the coast or along the continental shelf break. The temperature, salinity, and density structure of the current indicates an increase in baroclinic transport as the AACC flows from the east to the west, and as it entrains meltwater from the ice shelves in the Bellingshausen Sea. The AACC acts as a mechanism to transport meltwater out of the Bellingshausen Sea and into the Amundsen and Ross seas, with the potential to impact, respectively, basal melt rates and bottom water formation in these regions.

An inventory of supraglacial lakes and channels across the West Antarctic Ice Sheet

Quantifying the extent and distribution of supraglacial hydrology, i.e. lakes and streams, is important for understanding the mass balance of the Antarctic ice sheet, and its consequent contribution to global sea level rise. The existence of meltwater on the ice surface has the potential to affect ice shelf stability and grounded ice flow, through hydrofracturing and the associated delivery of meltwater to the bed. In this study, we systematically map all observable supraglacial lakes and streams in West Antarctica, by applying a semi-automated Dual-NDWI (Normalised Difference Water Index) approach to > 2000 images acquired by the Sentinel-2 and Landsat-8 satellites during January 2017. We use a K-Means clustering method to partition water into lakes and streams, which is important for understanding the dynamics and inter-connectivity of the hydrological system. When compared to a manually-delineated reference dataset on three Antarctic test sites, our approach achieves average values for sensitivity (85.3 % and 77.6 %), specificity (99.1 % and 99.7 %) and accuracy (98.7 % and 98.3 %) for Sentinel-2 and Landsat-8 acquisitions, respectively. In total, we identified 10,478 supraglacial features (10,223 lakes and 255 channels) on the West Antarctic Ice Sheet (WAIS) and Antarctic Peninsula (AP), with a combined area of 119.4 km2 (114.7 km2 lakes, 4.7 km2 channels). 27.3 % of feature area was found on grounded ice, 17.8 % of feature area comprised lakes which crossed the grounding line, while 54.9 % of feature area was found on floating ice shelves. New continental-scale inventories such as these, the first produced for WAIS and AP, are made possible by the recent expansion in satellite data provision. The inventories provide a baseline for future studies and a benchmark to monitor the development of Antarctica’s surface hydrology in a warming world, and thus enhance our capability to predict the collapse of ice shelves in the future. The dataset is available at https://doi.org/10.5281/zenodo.5109856 (Corr et al., 2021).

Stabilizing effect of mélange buttressing on the Marine Ice Cliff Instability of the West Antarctic Ice Sheet

Due to global warming and particularly high regional ocean warming, both Thwaites and Pine Island glaciers in the Amundsen region of the Antarctic Ice Sheet could lose their buttressing ice shelves over time. We analyze the possible consequences using the Parallel Ice Sheet Model (PISM), applying a simple cliff-calving parameterization and an ice-mélange-buttressing model. We find that the instantaneous loss of ice-shelf buttressing, due to enforced ice-shelf melting, initiates grounding line retreat and triggers the marine ice sheet instability (MISI). As a consequence, the grounding line progresses into the interior of the West Antarctic Ice Sheet and leads to a sea level contribution of 0.6 m within 100 a. By subjecting the exposed ice cliffs to cliff calving using our simplified parameterization, we also analyze the marine ice cliff instability (MICI). In our simulations it can double or even triple the sea level contribution depending on the only loosely constraint parameter which determines the maximum cliff-calving rate. The speed of MICI depends on this upper bound on the calving rate which is given by the ice mélange buttressing the glacier. However, stabilization of MICI may occur for geometric reasons. Since the embayment geometry changes as MICI advances into the interior of the ice sheet, the upper bound on calving rates is reduced and the progress of MICI is slowed down. Although we cannot claim that our simulations bear relevant quantitative estimates of the effect of ice-mélange buttressing on MICI, the mechanism has the potential to stop the instability. Further research is needed to evaluate its role for the past and future evolution of the Antarctic Ice Sheet.

Sea-level trends across The Bahamas constrain peak last interglacial ice melt

During the last interglacial (LIG) period, global mean sea level (GMSL) was higher than at present, likely driven by greater high-latitude insolation. Past sea-level estimates require elevation measurements and age determination of marine sediments that formed at or near sea level, and those elevations must be corrected for glacial isostatic adjustment (GIA). However, this GIA correction is subject to uncertainties in the GIA model inputs, namely, Earth’s rheology and past ice history, which reduces precision and accuracy in estimates of past GMSL. To better constrain the GIA process, we compare our data and existing LIG sea-level data across the Bahamian archipelago with a suite of 576 GIA model predictions. We calculated weights for each GIA model based on how well the model fits spatial trends in the regional sea-level data and then used the weighted GIA corrections to revise estimates of GMSL during the LIG. During the LIG, we find a 95% probability that global sea level peaked at least 1.2 m higher than today, and it is very unlikely (5% probability) to have exceeded 5.3 m. Estimates increase by up to 30% (decrease by up to 20%) for portions of melt that originate from the Greenland ice sheet (West Antarctic ice sheet). Altogether, this work suggests that LIG GMSL may be lower than previously assumed.