Blue Ice Moraines as an Archive of Past EAIS dynamics: Mt. Achernar as a Case Study in the Central Transantarctic Mountains

Abstract:

Observations from the interior of East Antarctica are essential for placing direct constraints on the ice sheet’s history over multiple glacial cycles, which also can be used to test numerical modeling of its past dynamics. In particular, laterally extensive, blue ice or ablation moraines are important archives of the former behavior of the EAIS and WAIS during at least the Pleistocene and Holocene. We can now quantify changes in the former ice surfaces using such deposits, which have been studied for decades, but have lacked chronological information. We are carrying out ¹⁰Be-²⁶Al-³He dating and provenance initiatives at Mt. Achernar, near the head of the Law Glacier, where there is a well-preserved archive of ice sheet history extending spatially over 5-10 km and temporally over the last few hundred thousand years, during which time the climate swung between full glacial and warm interglacial changes. Here, concentric moraines are continuous and well preserved, and the entire complex is no higher than about ~30 meters above the modern EAIS surface. The cosmogenic ages steadily progress away from the EAIS, over 10³ to 10⁵ timescales. In addition, agreement of ¹⁰Be and ²⁶Al concentrations indicate that, at least over the long term, blue ice deposits at Mt Achernar do not have a complicated history of burial and re-exposure. This is consistent with the inferred process of blue ice moraine formation that involves debris coming up from below and accumulating on the surface, when ice encounters the Transantarctic Mountains. Based on our findings we conclude that the interior of EAIS has been relatively stable for the last few 100 kyr, with ice surface elevation changes on the order of tens of meters, including 20-30 meters since the LGM. In a net sense, the EAIS has also been getting slightly lower over the last half million years or so. We hypothesize that if the interior of the EAIS had undergone major lowering or more pronounced surface changes over the time represented, we would not observe the well-preserved continuous blue ice moraines and associated overall progression in ages with distance from the EAIS.