Presence of diatomaceous mud beneath stratified diamicton in the southwestern Ross Sea and its implication

Abstract

In the western Ross Sea it has generally been considered that a thick ice sheet was grounded to the sea floor during the last glacial maximum (LGM) and the retreat of the ice sheet did not begin until ~13 ky BP. A gravity core (DG12-GC06) from the southern Drygalski Trough in the southwestern Ross Sea, however, raises a question about the stability of the Ross Ice Sheet during the last glacial period. 'Typical' Ross Sea sequence of LGM to Holocene age is composed of diatomaceous mud and underlying diamicton. The core sediment from the southern Drygalski Trough also contains post-glacial diatomaceous mud ('upper' mud) and glacial stratified diamicton, but the diamicton layers are overlain by another, 'lower' diatomaceous mud layers. The presence of 'lower' mud beneath diamicton indicates a seasonally open marine environment prior to/during the last glacial event in the southwestern Ross Sea. Radiocarbon dates of acid-insoluble organic matter (AIO) from the 'lower' mud range from ~32 to 39 ky BP. AIO radiocarbon age of surface sediment is ~4ky BP and corrected age based on surface sediment age is ~28 to 35 ky BP. However, this kind of age correction is ratified only when composition of organic matter of surface sediment is similar to that of the 'lower' mud. We applied ramped pyrolysis technique for radiocarbon dating of the 'lower' mud to date younger organic matter fraction, which produced ages of 22~23 ky BP. It seems that ice sheet on the western Ross Sea retreated at ~24 ky BP and advanced at ~21 ky BP. In order to preserve the 'lower' mud, we infer that the second advance featured a thinner ice sheet than the initial advance.