[M-IS05] Global climate change driven by the Southern Ocean and the Antarctic Ice Sheet

[MIS05-P18] Quantification of water mass age of Antarctic Bottom Water off Cape Darnley in East Antarctica, using chlorofluorocarbon and sulfur hexafluoride

*Yoshihiko Ohashi¹, Michiyo Yamamoto-Kawai¹, Kazuya Kusahara², Kenichi Sasaki², Narimi Baba¹, Naoyuki Tamura¹, Natsumi Nojiro¹, Kay I. Ohshima³ (1.Tokyo University of Marine Science and Technology, 2.Japan Agency for Marine Earth Science and Technology, 3.Institute of Low Temperature Science, Hokkaido University)

Keywords:Antarctic Bottom Water , Transient tracer, Water mass age

Antarctic Bottom Water (AABW), which is formed by the mixing of Dense Shelf Water (DSW) and Circumpolar Deep Water (CDW), spreads throughout the global ocean. This process has a crucial role in transporting heat, freshwater, and carbon from the continental shelf to the deep ocean. Off Cape Darnley (CD) in East Antarctica, a newly formed AABW was found (Ohshima et al., 2013). However, the spreading of AABW and its time scale off CD have not been clearly understood. To quantitatively understand its time scale, it is effective to use transient tracers of chlorofluorocarbon (CFC) and sulfur hexafluoride (SF₆). We carried out CTD observation and water sampling in the Indian sector of the Southern Ocean, including off CD, in the summers of 2019 and 2020. CFC-12 and SF₆ of the samples were measured by an analytical system based on Bullister and Wisegarver (2008). Additional data observed in 2013 (MR12-05) and 2016 (WHP I08S) were obtained from the Clivar and Carbon Hydrographic Data Office. A numerical experiment of transient tracers was also performed using a coupled ocean-sea ice-ice shelf model.

The model results showed that transient tracer concentrations of AABW can be explained by a two-endmember mixing of DSW formed in the previous winter and very old CDW (almost no CFC/SF₆). Based on the model results, the water mass age (the elapsed time since the water is in contact with the atmosphere) of AABW was quantified from observed concentrations of CFC/SF₆. Along the western canyons of CD and ~3500 m isobath near the bottom, the water mass age of AABW (~0–3 years) was younger than those on the eastern side and in the western offshore (~5–15 years). This distribution of new AABW is roughly consistent with the suggested pathway of new CD Bottom Water in the previous study. The old AABW on the eastern side and in the western offshore of CD reflect the effects of AABW inflow through the Princess Elizabeth Trough and from the Weddell Sea, respectively. Transient tracer measurements provide quantitative information on the time scale of AABW spreading.