by- 23 January 2019 ( last update : 24 January 2019 )
A team involving researchers from six French laboratories, including the Institute of Earth Sciences, has been reconstructing the history of East Antarctica’s erosion for 350 million years (Ma). This study shows that Antarctic erosion is essentially very old, dating back to the Late Paleozoic Ice Age (LPIA), a major glaciation period dated 340-300 Ma. The relief is essentially inherited from this period, while the Antarctic ice cap has had little impact on erosion over the last 30 Ma.
Understanding the history of Antarctic erosion is fundamental to understanding the links between glacial and climatic dynamics, including the future behaviour of the ice cap in response to warming and its impact on sea level. The continental surface’s response to glacial erosion remains largely unknown due to the fact that more than 99% of Antarctica is covered by a 2 to 3 km thick ice cap. Geophysical studies have shown a complex subglacial relief, interpreted either as a result of tectonic rifting episodes or as a result of very significant subglacial erosion.
As part of a Labex OSUG@2020 project supported by the University of Grenoble Alpes, we conducted a thermochronological study on samples collected over 600 km along the coasts of Terre Adélie. Sampling has been carried out with the support of the IPEV since the 2000s in the GEOLETA and ARLITA campaigns. The results of this study show that a homogeneous exhumation of about 4 km took place in East Antarctica between 340 and 300 Ma (Fig. 1). These data exclude exhumation during the Permian rifting (280-250 Ma) or in connection with the rifting prior to the opening of the Southern Ocean (160-90 Ma). Figure 2: Reconstruction of East Antarctica at the LPIA Glacial Maximum (at about 300 MA), based on thermochronological data obtained in Adelie Land (this study) and those obtained elsewhere in Antarctica and Australia. The positions of several large glaciers are proposed according to glacial flow directions and peripheral basins. A large glacier: that of Terre-Adélie-Georges V (TA-GV) explains the significant erosion of East Antarctica, while at 300 Ma the South Pole was located near its current location. This erosion does not seem to have a tectonic cause in view of the absence of deformation of this age on the Adelie Earth scale. However, this erosion is linked to a glacial phase, that of the LPIA, which coincides with the significant reduction in the CO2 content of the atmosphere (between 300 and 600 ppmV), and with the lowering of the sea level by around -30 to -70 m. At that time the South Pole was close to what it is today (Fig. 2). The presence of glacial masses in the Adelie Land is corroborated by peri-glacial basins to the east and north of the studied area, and by paleo-directional radiar glacial flows in relation to it. The strong erosion that is highlighted here shows that the dynamics of glaciers near the pole during the LPIA were comparable to those of temperate glaciers with a "warm base" as they are today in mid-latitude. The end of this glacial episode is very fast at 300 Ma, it is correlated with a continental drift leading to a shift of the pole with respect to the continent.
Such data show that there has been very little erosion at the tertiary level due to the high stability of the Antarctic ice cap, including in the warmest phases since 33 Ma. This is due in particular to an internal sliding mode within the ice cap and the absence of basal friction characteristic of cold base glaciers. This suggests that the Antarctic ice cap may also remain relatively stable in the short term and may contribute to sea level rise in the context of global warming. We now need to validate this model by studying the moraine blocks transported by glaciers at the scale of East Antarctica.
Rolland, Y., Bernet, M., van der Beek, P., Gautheron, C., Duclaux, G., Bascou, J., Balvay, M., Héraudet, L., Sue, C., Ménot, R.-P. (2019) Late Paleozoic Ice Age glaciers shaped East Antarctica landscape, Earth and Planetary Science Letters, doi : 10.1016/j.epsl.2018.10.044.
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