[M-IS08] Global climate change driven by the Southern Ocean and the Antarctic Ice Sheet

[MIS08-13] Estimation of age and anthropogenic CO₂ content of Antarctic Bottom Water using chlorofluorocarbon and sulfur hexafluoride

★Invited Papers

*Yoshihiko Ohashi¹, Michiyo Yamamoto-Kawai², Kazuya Kusahara³, Ken’ichi Sasaki³, Kay I. Ohshima⁴ (1.The University Centre in Svalbard, 2.Tokyo University of Marine Science and Technology, 3.Japan Agency for Marine-Earth Science and Technology, 4.Hokkaido University)

Keywords:Antarctic Bottom Water, Age, Anthropogenic CO2, Chlorofluorocarbon, Sulfur hexafluoride

Antarctic Bottom Water (AABW) is formed by the mixing of warm Circumpolar Deep Water (CDW) and cold Dense Shelf Water. Through the formation and spreading of AABW, the anthropogenic CO₂ (C_ant) is transported from the ocean surface to the abyss and C_ant is stored in the abyssal ocean for long periods. AABW plays an important role in the long-term C_ant sequestration. In this study, data of chlorofluorocarbon and sulfur hexafluoride observed from 2005 to 2020 in the Southern Ocean (SO) were used to investigate the age (the time elapsed since the AABW was formed) and C_ant content of AABW. In the coastal regions of the Cosmonaut, Cooperation, D’Urville, and Somov Seas and near the Pacific-Antarctic Ridge, the age of AABW was < 5 years, reflecting the spread of newly formed AABW. Higher C_ant inventory in AABW was detected in the Atlantic Ocean sector of the SO such as the Weddell and Lazarev Seas (55 ± 43 mol m⁻²) and the Cosmonaut and Cooperation Seas (43 ± 18 mol m⁻²) than those in the other regions. The amount of C_ant in AABW in this region (11 PgC) was estimated to be 85% of the total C_ant in AABW of the SO, suggesting the Atlantic Ocean sector as a major player to C_ant sequestration in the abyssal ocean. The high C_ant inventories in the Weddell and Lazarev Seas and the Cosmonaut and Cooperation Seas are attributed to the small effect of dilution by Lower CDW and the presence of new AABW, respectively. The high C_ant inventories also corresponded to the thick layer and high formation rate of AABW. The total amount of C_ant in AABW in the SO (13 PgC) was roughly estimated to be one-fifth of the total C_ant in the whole water column in the SO. This study highlights that AABW formation, transport, and storage processes play important roles in the total C_ant sequestration in the SO.