[M-IS06] 南大洋・南極氷床が駆動する全球気候変動

[MIS06-16] Reconstruction of ice retreat history in Pine Island Bay, West Antarctica, based on sedimentary Be-10 records

*菅沼 悠介^1,2、Gerhard Kuhn³、Claus-Dieter Hillenbrand⁴、Albert Zondervan⁵ (1.国立極地研究所、2.総合研究大学院大学・極域科学専攻、3.Alfred Wegener Institute (AWI)、4.British Antarctic Survey (BAS)、5.GNS Science, National Isotope Centre)

キーワード：氷床底湖、Be-10、氷床後退、西南極

Reconstruction of the glacial dynamics of the Antarctic ice sheets during the past by studying records from their margin is essential to evaluate their stability and to anticipate their contribution to future sea level rise. Recently, the first direct evidence for a paleo-subglacial lake on the Antarctic continental shelf was reported from a small bedrock basin in Pine Island Bay, West Antarctica, based on a distinct sediment facies and geochemical pore water signatures, i.e. low chloride concentrations, in a marine sediment core (PS69/288) (Kuhn et al., 2017). These data indicate that the sediment in the lower part of the core was deposited under a low-energy subglacial lake setting. Here we report further evidence for a paleo-subglacial lake based on changes in Be-10 concentrations in the sediments. A significant down-core decrease in the Be-10 concentration indicates very limited input of meteoric Be-10 to the sediments in the lower part of the core, suggesting a depositional environment that was isolated from the open ocean. This is consistent with the proposed subglacial lake setting. In detail, the Be-10 concentration shows a further drop within a sand, silt and mud interval from ca. 580 to 470 cm core depth that was interpreted to have been deposited during the transition from the subglacial lake to a sub-ice shelf cavern by grounding line retreat in that area at about 11 kyrs B.P. (Hillenbrand et al., 2013, Kuhn et al., 2017). The lowered Be-10 concentration at the base of this interval probably results from the dominant supply of sediment that had been deeply buried under the West Antarctic Ice Sheet (WAIS) for a very long time. Above a minor up-core increase from 464 to 324 cm, the Be-10 concentration decreases again at about 260 cm. This decrease may correspond to three possible factors: 1.) increased supply of sediments from below the ice sheet (possible meltwater plumes), 2.) an episode of permanent sea-ice cover, or 3.) a re-advance of the ice shelf. Above 260 cm the Be-10 concentration increases significantly toward the top of the core, indicating that an open marine setting had established at the core site. We also report that new data from marine sediment cores (PS69/295, PS75/166, and PS75/214) in other parts of Pine Island Bay. These data provide new insight into a more dynamic behavior of the WAIS in Pine Island Bay during Holocene. Overall, the Be-10 concentration of the sediments is a powerful tool to study paleo-subglacial lakes in Antarctica and processes of ice sheet to ice shelf transition during the subsequent deglaciation.