Bathymetric controls on calving processes at Pine Island Glacier

Pine Island Glacier is the largest current Antarctic contributor to sea level rise. Its ice loss has substantially increased over the last 25 years through thinning, acceleration and grounding line retreat. However, the calving line positions of the stabilizing ice shelf did not show any trend within the observational record (last 70 years) until calving in 2015 led to unprecedented retreat and changed alignment of the calving front. Bathymetric surveying revealed a ridge below the former ice shelf and two shallower highs to the north. Satellite imagery shows that ice contact on the ridge likely was lost in 2006 but was followed by intermittent contact resulting in back stress fluctuations on the ice shelf. Continuing ice shelf flow also led to occasional ice shelf contact with the northern bathymetric highs, which initiated rift formation that led to calving. The observations show that bathymetry is an important factor in initiating calving events

Deep water inflow slowed offshore expansion of the West Antarctic Ice Sheet at the Eocene-Oligocene transition

The stability of the West Antarctic Ice Sheet is threatened by the incursion of warm Circumpolar Deepwater which flows southwards via cross-shelf troughs towards the coast there melting ice shelves. However, the onset of this oceanic forcing on the development and evolution of the West Antarctic Ice Sheet remains poorly understood. Here, we use single- and multichannel seismic reflection profiles to investigate the architecture of a sediment body on the shelf of the Amundsen Sea Embayment. We estimate the formation age of this sediment body to be around the Eocene-Oligocene Transition and find that it possesses the geometry and depositional pattern of a plastered sediment drift. We suggest this indicates a southward inflow of deep water which probably supplied heat and, thus, prevented West Antarctic Ice Sheet advance beyond the coast at this time. We conclude that the West Antarctic Ice Sheet has likely experienced a strong oceanic influence on its dynamics since its initial formation

Bathymetric controls on calving processes at Pine Island Glacier

Pine Island Glacier is the largest current Antarctic contributor to sea-level rise. Its ice loss has substantially increased over the last 25 years through thinning, acceleration and grounding line retreat. However, the calving line positions of the stabilising ice shelf did not show any trend within the observational record (last 70 years) until calving in 2015 led to unprecedented retreat and changed the alignment of the calving front. Bathymetric surveying revealed a ridge below the former ice shelf and two shallower highs to the north. Satellite imagery shows that ice contact on the ridge was likely lost in 2006 but was followed by intermittent contact resulting in back stress fluctuations on the ice shelf. Continuing ice-shelf flow also led to occasional ice-shelf contact with the northern bathymetric highs, which initiated rift formation that led to calving. The observations show that bathymetry is an important factor in initiating calving events

Elevated geothermal surface heat flow in the Amundsen Sea Embayment, West Antarctica

The thermal state of polar continental crust plays a crucial role for understanding the stability and thickness of large ice sheets, the visco-elastic response of the solid Earth due to unloading when large ice caps melt and, in turn, the accuracy of future sea-level rise prediction. Various studies demonstrate the need for precise measurements and estimation of geothermal heat flow (GHF) in Antarctica for better constrained boundary conditions to enhance the ice sheet model performance. This study provides ground-truth for regional indirect GHF estimates in the Amundsen Sea Embayment, which is part of the West Antarctic Rift System, by presenting in situ temperature measurements in continental shelf sediments. Our results show regionally elevated and heterogeneous GHF (mean of 65 mWm−2) in the Amundsen Sea Embayment. Considering thermal blanketing effects, induced by inflow of warmer water and sedimentary processes, the estimated GHF ranges between 65 mWm−2and 95 mWm−2

West Antarctic archipelago covered by cool-temperate forests during the early Oligocene glaciation

The Eocene–Oligocene Transition (~34.4–33.7 Ma) marks a major step in the long-term evolution from the greenhouse climate of the Early Palaeogene to the icehouse regime of the Late Neogene and Quaternary. However, it remains uncertain which landmasses were covered by ice sheets during the Early Oligocene Glacial Maximum (~33.7–33.2 Ma), an interval of peak glaciation inferred from deep-sea benthic foraminifera oxygen isotope records that immediately follows the Eocene-Oligocene Transition. The scarcity of Late Eocene and Early Oligocene continental and shallow-marine records in both Arctic and Antarctic regions has prevented the reconstruction of environmental conditions and ice-sheet extent during the Early Oligocene, which is critical for assessing ice–ocean–atmosphere interactions during early stages of the Cenozoic icehouse. Here, we present the first Early Oligocene shallow-marine record from the Pacific margin of West Antarctica, recovered from the central Amundsen Sea Embayment shelf on RV Polarstern expedition PS104 at Site 21. Marine mudstones recovered at this site document the presence of a vegetated archipelago at a palaeo-latitude of 73.5°S. Pollen assemblages and organic biomarker proxies indicate a cool-temperate Nothofagus-dominated forest situated within a productive marine archipelago. No evidence for marine terminating ice was detected in the cores from Site 21, thus indicating that the West Antarctic Ice Sheet was small or entirely absent during the Early Oligocene

West Antarctic archipelago covered by cool-temperate forests during the early Oligocene glaciation

The Eocene-Oligocene Transition (~34.4–33.7 Ma) marks a major step in the long-term evolution from the greenhouse climate of the Early Palaeogene to the icehouse regime of the Late Neogene and Quaternary. However, it remains uncertain which landmasses were covered by ice sheets during the Early Oligocene Glacial Maximum (~33.7–33.2 Ma), an interval of peak glaciation inferred from deep-sea benthic foraminifera oxygen isotope records that immediately follows the Eocene-Oligocene Transition. The scarcity of Late Eocene and Early Oligocene continental and shallow-marine records in both Arctic and Antarctic regions has prevented the reconstruction of environmental conditions and ice-sheet extent during the Early Oligocene, which is critical for assessing ice–ocean–atmosphere interactions during early stages of the Cenozoic icehouse. Here, we present the first Early Oligocene shallow-marine record from the Pacific margin of West Antarctica, recovered from the central Amundsen Sea Embayment shelf on RV Polarstern expedition PS104 at Site 21. Marine mudstones recovered at this site document the presence of a vegetated archipelago at a palaeo-latitude of 73.5°S. Pollen assemblages and organic biomarker proxies indicate a cool-temperate Nothofagus-dominated forest situated within a productive marine archipelago. No evidence for marine terminating ice was detected in the cores from Site 21, thus indicating that the West Antarctic Ice Sheet was small or entirely absent during the Early Oligocene

Temperate rainforests near the South Pole during peak Cretaceous warmth

The mid-Cretaceous was one of the warmest intervals of the past 140 million years (Myr) driven by atmospheric CO2 levels around 1000 ppmv. In the near absence of proximal geological records from south of the Antarctic Circle, it remains disputed whether polar ice could exist under such environmental conditions. Here we present results from a unique sedimentary sequence recovered from the West Antarctic shelf. This by far southernmost Cretaceous record contains an intact ~3 m-long network of in-situ fossil roots. The roots are embedded in a mudstone matrix bearing diverse pollen and spores, indicative of a temperate lowland rainforest environment at a palaeolatitude of ~82°S during the Turonian–Santonian (93–83 Myr). A climate model simulation shows that the reconstructed temperate climate at this high latitude requires a combination of both atmospheric CO2 contents of 1120–1680 ppmv and a vegetated land surface without major Antarctic glaciation, highlighting the important cooling effect exerted by ice albedo in high-CO2 climate worlds

Temperate rainforests near the South Pole during peak Cretaceous warmth

The mid-Cretaceous period was one of the warmest intervals of the past 140 million years1,2,3,4,5, driven by atmospheric carbon dioxide levels of around 1,000 parts per million by volume6. In the near absence of proximal geological records from south of the Antarctic Circle, it is disputed whether polar ice could exist under such environmental conditions. Here we use a sedimentary sequence recovered from the West Antarctic shelf—the southernmost Cretaceous record reported so far—and show that a temperate lowland rainforest environment existed at a palaeolatitude of about 82° S during the Turonian–Santonian age (92 to 83 million years ago). This record contains an intact 3-metre-long network of in situ fossil roots embedded in a mudstone matrix containing diverse pollen and spores. A climate model simulation shows that the reconstructed temperate climate at this high latitude requires a combination of both atmospheric carbon dioxide concentrations of 1,120–1,680 parts per million by volume and a vegetated land surface without major Antarctic glaciation, highlighting the important cooling effect exerted by ice albedo under high levels of atmospheric carbon dioxide