Belgica Trough and the West Antarctic Ice Sheet

When the RV Belgica got trapped in the pack ice of the Bellingshausen Sea in the beginning of March 1898, Adrien de Gerlache and his crew had no idea that they actually had got stuck in the ice just above the margin of a huge submarine trough carved into the seafloor about 600 m below their ship. The 75-150 km wide trough, which was named “Belgica Trough” in honour of de Gerlache’s expedition over a century after this event, extends for about 500 km across the continental shelf north of the Bryan Coast and English Coast in West Antarctica, thereby reaching water depths down to 1200 m close to the coast, ca. 600 m on the mid-shelf and ca. 650 m at the shelf edge. Sparse, high-resolution seafloor mapping in this region during less than a handful of research expeditions carried out by various countries (UK, US, Germany, South Korea) since the mid-1990s has revealed the presence of various morphological features (“bedforms”) on the floor of the trough that revealed the “footprint” of an enormous glacier in the past. During ice ages preceding the current warm period bedrock and seabed sediments in the vicinity of today’s coast had been eroded at the base of this so-called “Belgica palaeo-ice stream” before the eroded debris was deposited by the glacier further away from the coast. Thus, the existence of Belgica Trough with its bedforms gives testimony of repeated advances of ice masses grounded on the seafloor across the continental shelf during the past.

Bathymetry of the Bellingshausen Sea shelf with the location of Belgica Trough and topography under the Antarctic Ice Sheet landward of the coast (white lines: present coast lines and ice-shelf fronts, respectively) (Graham et al. 2011, The Cryosphere).

Although similar palaeo-ice stream troughs had been mapped elsewhere on the Antarctic continental shelf, Belgica Trough stands out because of several facts. First, taking into account its combined length, width and water depth, Belgica Trough is the largest palaeo-ice stream trough in Antarctica outside of the Ross Sea embayment. Estimates suggest that the area from which ice fed into the Belgica palaeo-ice stream during the last ice age may have been comparable to that of today’s Antarctic Peninsula Ice Sheet or reached nearly ten times the size of Belgium. Second, Belgica Trough was eroded by ice draining both the West Antarctic Ice Sheet (WAIS) and the Antarctic Peninsula Ice Sheet. The Antarctic Ice Sheet actually comprises three separate ice sheets, whose dynamic behaviour is quite different because of different conditions at their bases. The huge and thick East Antarctic Ice Sheet stores ice corresponding to 52 m of global sea level and rests predominantly above present sea level on the Antarctic continent, while the bed of the thinner WAIS, its “smaller brother”, holds ice corresponding to about 5.3 m of sea level and lies largely below sea level (= “marine based”). The Antarctic Peninsula Ice Sheet, the smallest of the Antarctic ice sheets, stores ice corresponding to just 0.3 m of global sea level and sits almost entirely above sea level on the continent. Third, in contrast to most other Antarctic palaeo-ice stream troughs, such as the Pine Island-Thwaites palaeo-ice stream trough located in the Amundsen Sea embayment directly to the west of the Bellingshausen Sea shelf, no major outlet glaciers drain into Belgica Trough today. This implies that during the present warm time only the tributary glaciers, which once fed into the huge Belgica palaeo-ice stream, remain.

Belgica Trough and its bedforms document the past dynamics of the WAIS and therefore may hold the key for predicting its future behaviour. The WAIS is considered inherently unstable and particularly vulnerable to global warming because its marine-based bed dips towards the interior of the Antarctic continent. Theoretical consideration indicate that under these conditions any retreat of the grounding line, i.e. the ice sheet’s marginal zone, where the ice starts to float, will inevitably lead to a runaway retreat, implying that the grounding line will withdraw even further landward until the entire ice sheet has eventually decayed. Studies of geological archives (e.g., fossil corals in tropical regions) have provided evidence that during warm periods of the past, such as a warm time ca. 125,000 years ago that preceded the last ice age, global sea level was at least 6 m higher than today. This sea-level highstand may have resulted from a “collapse” of the WAIS, with some minor contributions from ice-sheet loss in Greenland and East Antarctica.

Linear furrows (MSGL = Mega-Scale Glacial Lineations) carved by a huge glacier (named “Belgica palaeo-ice stream”) into the seafloor of Belgica Trough during the past ice age (Ó Cofaigh et al. 2005, J. Geophys. Res.).

Taking into account the overall instability of the WAIS, satellite data and in-situ measurements carried out in its Bellingshausen Sea drainage sector and the neighbouring Amundsen Sea drainage sector since the mid-1990s give raise for concern. The data show that over recent decades glaciers in these WAIS sectors have undergone dramatic thinning whilst their flow speeds accelerated and their grounding lines retreated, with grounding lines of Pine Island Glacier, Thwaites Glacier and other glaciers draining into the Amundsen Sea embayment having retreated by up to ca. 1 – 10 km/year. Most scientists think that the main reason for this alarming ice loss, which contributed about 7 mm to global sea-level rise from 2003 to 2019, is the ocean-induced melting of floating ice shelves stabilising the ice streams. This melting is caused by the inflow of relatively warm deep water onto the West Antarctic continental shelf. The reasons for the intensified “pumping” of the deep water onto the shelf and its variability over years and decades are not fully understood, yet, but changes in Southern Hemisphere wind systems due to atmospheric warming in response to increased anthropogenic greenhouse gas emissions are likely. Computer simulations suggest that the contribution of the Antarctic ice sheets to global sea-level rise by the year 2300 will be ca. 1.0 m as long as 21st warming does not exceed +1.5 °C, but will increase to 1.5 m, if this warming reaches +3 °C, with worst-case scenarios for future greenhouse gas emissions predicting a collapse of the WAIS and the marine based parts of the East Antarctic Ice Sheet, resulting in ca. 9.5 m sea-level rise.

Modelled contribution of Antarctic Ice Sheet melting to global mean sea-level rise above present (= Δ (GMSL)) until the year 2300 under the assumption of +1.5 °C (left) and +3.0 °C (right) global atmospheric warming, respectively (black line: mean predicted sea-level rise; blue shading: level of uncertainty, with darker shading illustrating higher probability) (DeConto et al. 2021, Nature).

Given the opportunity that the study of the Belgica palaeo-ice stream history offers for a better understanding of the dynamical behaviour of the WAIS and improving forecasts of global sea-level rise caused by current and future WAIS melting, Belgica Trough is a high-priority research target. The fact that no significant ice shelves exist along its south-western coast in Eltanin Bay alludes to the possibility that this region is a potential analogue for future environmental conditions in other WAIS sectors, such as the Amundsen Sea embayment, where major ice streams still exist but currently undergo major ice loss. In this respect, we may be able to learn more about the future fate of WAIS ice streams, such as Thwaites Glacier and Pine Island Glacier, by investigating the past fate of the ice stream that once occupied the area, where RV Belgica was trapped in the ice 125 years ago.

This blog post was written by Claus-Dieter Hillenbrand, marine geologist at the British Antarctic Survey (Cambridge, UK) and co-investigator of the PS134 marine geology team.

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