Forecasting the future by learning from the past

During the Last Glacial Maximum, i.e., the culmination of the last ice age about 20 thousand years ago, the entire seabed in the shallow part of the Bellingshausen Sea (the so called continental shelf) was covered by the Antarctic Ice Sheet. When this ice retreated after the last ice age, it left traces behind on the seafloor in the form of morphological features shaped by the ice and a cover of glacial sediments. Within this deglaciated sea, the glacial sediments eventually got buried below a drape of marine sediments. Today, the marine geology team onboard the RV Polarstern samples these glacial-marine sediment sequences using a gravity corer to reconstruct when and how fast the Antarctic Ice Sheet retreated after the last ice age. This reconstruction is done via three consecutive steps in the laboratory. First, after the recovery of the sediment cores, they are scanned to measure their physical properties, including density, magnetic properties, and sound velocity. Next, the cores are split, their lithological composition is described, and discrete sediment samples are taken, on which various other parameters, such as water content, grain size, etc., are measured. These data are then used to identify the subglacial sediments, which had been deposited under the ice sheet, and the marine sediments deposited on top of them, and locate the transition between them in the core. Finally, calcareous microfossils, such as foraminifera, are extracted from the marine sediments. By measuring the content of radiocarbon in the shells of these microfossils, their age and thus the age of the sediment can be determined. Ultimately, this temporal data is used to identify the age of ice retreat at the core location. By repeating this analysis along a core transect extending through the Bellingshausen Sea from today’s ice sheet front towards the continental slope, the marine geology team can reconstruct the retreat pattern of the Antarctic Ice Sheet after the Last Glacial Maximum. In addition, the PS134 marine geologists analyze fossil diatom (siliceous phytoplankton) assemblages, and other sedimentological and geochemical properties, to reconstruct past environmental changes in response to the ice retreat. The figure below shows a sketch explaining the concept.

Sketch explaining the working strategy of the PS134 marine geology team. Disclaimer: this sketch shows our working principle, not real results, nor the reverse bed slope causing the ice sheet to be particularly vulnerable to global warming. Credits: Matthias Troch.

One of the first geologists to discover evidence of this more extensive and older Antarctic Ice Sheet was Henryk Arctowski, geologist onboard the Belgian Antarctic expedition of 1897-1899. Arctowski was a Polish earth scientist affiliated with the University of Liège (Belgium), and he was hired by commander Adrien de Gerlache to coordinate the scientific work onboard the RV Belgica’s Antarctic expedition. During their exploration of the Gerlache Strait from January 23rd until February 12th 1898, Arctowski described and mapped numerous ancient moraines, i.e., arcuate formations made of debris deposited along the edge of former glaciers and ice sheets. These landforms provided irrefutable evidence for a larger and more extensive glaciation along the Antarctic Peninsula at some time in the past. Unfortunately, back in Arctowski’s days, absolute dating techniques to determine the moraines’s ages were not yet developed.

The marine geology team onboard the PS134 RV Polarstern expedition, from left to right and top to bottom: Lena, Rob, Claus-Dieter, Matthias, James, Johann, Norbert, Zelna, Rosemary, and Kenza.
Henryk Arctowski in his lab onboard the RV Belgica during the Belgian Antarctic expedition of 1897-1899. Credits: Library of Congress, Frederick A. Cook Society.

Both the original research of Henryk Arctowski and today’s investigations by the PS134 team are highly relevant in the context of contemporary and future climate change. Namely, the contribution of ice loss from the Antarctic Ice Sheet to global sea-level rise is projected to reach up to 3.3 meters by the year 2100 in the worst-case scenario (graph below; Bakker et al 2017 Scientific Reports). This contribution is mainly due to the melting of the West Antarctic Ice Sheet, which drains together with the Antarctic Peninsula Ice Sheet into the Bellingshausen Sea. As the West Antarctic Ice Sheet sits predominantly below sea level on the Antarctic continent, it is considered as inherently instable and particularly vulnerable to global warming. A considerable uncertainty, however, remains on how fast the West Antarctic Ice Sheet will recede in the future and contribute to world-wide sea-level rise, as its stability is controlled by a complex interaction between various processes, including destabilization of floating ice shelves (which are buttressing ice resting on land), inflow of warm ocean water, marine ice sheet instability (i.e., fast ice retreat over a reverse bed slope), ice cliff failure, and surface melting on ice shelves triggered by warm air temperatures that results in so called “hydrofracturing”, i.e., fragmentation of ice-shelves caused by the refreezing of the meltwater in crevasses within the ice shelves. By investigating geological archives, earth scientists, such as Henryk Arctowski and the PS134 team onboard the RV Polarstern, aim to constrain past configurations of the Antarctic Ice Sheet, its previous retreat rates, and mechanisms triggering its retreat. Their geological results are eventually used by numerical modelers to calibrate computer simulations and improve quantitative forecasts of future ice sheet change and sea-level rise. Via this interdisciplinary collaboration between earth scientists and numerical modelers, we aim to better forecast the future by learning from the past.

Future global sea-level projections including the uncertain contribution of the West Antarctic Ice Sheet, following RCP8.5 (Representative Concentration Pathway). RCP8.5 is considered the worst-case greenhouse gas concentration trajectory adopted by the Intergovernmental Panel on Climate Change (IPCC). Red curve: West Antarctic Ice Sheet remains intact; yellow curve: mid-range estimate; dark red curve: full West Antarctic Ice Sheet disintegration within a couple of decades causing an additional global sea-level rise of 3.3 meters by the year 2100. Figure from Bakker et al., 2017 (Scientific Reports).

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