[M-IS10] Global climate change driven by the Southern Ocean and the Antarctic Ice Sheet

[MIS10-P04] Reconstruction of the palaeoceanographic environment based on biological production in the northern Weddell Sea since the last glacial period

*Aya Osanai¹, Minoru Ikehara², Asuka Yamaguchi³, Stephen P. Obrochta⁴ (1.Kochi University graduate School of Integrated Arts and Sciences, 2.Marin Core Research Institute, Kochi University, 3.Atmosphere and Ocean Research Institute, The University of Tokyo, 4.Akita University graduate School of International Resource Science)

Keywords:Southern Ocean, biological Productivity, last glacial, diatom ooze

The Antarctic Cryosphere is known as a region that is sensitive to global environmental changes and is an important area for understanding the global climate system. Analysis of sediments in the Southern Ocean, a large reservoir of negative heat and CO₂, can reveal the evolution of the sedimentary and marine environments. The Southern Ocean is home to the Antarctic Circumpolar Current (ACC), the largest surface current system on Earth, which circulates eastward around the Antarctic Continent and influences global heat transport and biological production. The Weddell Gyre, which is the largest circulation current south of the southern limit of the ACC, is located between 55°S-60°S and 60°W-30°W in the surface layer of the Weddell Sea in the Atlantic secter of the Southern Ocean. The ACC is driven by westerly winds, the Weddell Gyre by westerly winds in the north, and coastal winds from Antarctica in the south, and it is known that the expansion, contraction, and north-south shift of the basin occur with climate events such as warming and cooling in the Antarctic cryosphere. The Southern Boundary of the ACC (SB) and the northern part of the Weddell Gyre have the Weddell-Scotia Confluence (WSC), which is the boundary between each basin. Until now, few studies have been conducted to clarify the evolution of the sedimentary environment at the boundary between the WSC, ACC, and the Weddell Gyre. Therefore, this study aims to reconstruct the evolution of sedimentary environments over the past 30,000 years using a piston core collected from the modern SB basin during the Hakuho-Maru KH-19-6 Leg 4 cruise.
Piston core KH-19-6-PC07 (58°45.0446'S,18°26.4848'W,4401 m water depth) was collected at the South American-Antarctic Ridge (ASSR), which is the divergent boundary between the South American and Antarctic plates. Visual core description and smear slide observations confirm that the main lithology of the PC07 core is diatomaceous silt. Radiocarbon dating of the bulk organic carbon indicates a sedimentary field with a high sedimentation rate, and a sedimentary environment with high primary production, mainly of diatoms, is expected. Therefore, we attempted to reconstruct the depositional environment with a focus on biological production. At present, total organic carbon (TOC) content, total nitrogen (TN) content, stable isotope ratio of organic carbon (δ¹³C_org), nitrogen isotope ratio (δ¹⁵N), and inorganic geochemical data by X-ray fluorescence core scanner (ITRAX) have been obtained. The TOC is about 0.22 %, δ¹³C_org is about -23 ‰, and δ¹⁵N is about +4.6 ‰. Br/Ti ratio, which is considered to be an indicator of biological production, was found to increase locally during the Holocene and the early part of the last glacial stage. Although δ¹³C_org tends to be relatively high and δ¹⁵N tends to be relatively low during periods of high bioproduction in the Southern Ocean,δ¹³C_org and δ¹⁵N show no correlation. Based on these results, we discuss the evolution of the palaeoceanographic environment from the last glacial to the Holocene, focusing on the changes in biological production in the Southern Ocean.