[A-CC29] Ice cores and paleoenvironmental modeling

[ACC29-03] Impacts of changes in physical and biogeochemical processes in the Southern Ocean on the ocean carbon cycle during glacial periods

★Invited Papers

*Hidetaka Kobayashi¹, Akira Oka¹, Akitomo Yamamoto², Ayako Abe-Ouchi¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Japan Agency for Marine-Earth Science and Technology)

Keywords:ocean carbon cycle, glacial-interglacial cycle, Southern Ocean, iron gertilization

Analyses of air trapped in ice cores have shown that atmospheric carbon dioxide concentrations (pCO2) during glacial periods are about 90 ppm lower than that during interglacial periods over the past 800,000 years. This change is recognized to mainly arise from changes in the ocean carbon cycle, but the detailed mechanism behind is not yet clarified. This study focuses on changes in physical and biological processes in the Southern Ocean, which have not been adequately considered, and assesses their quantitative contribution to changes in the global ocean carbon cycle. Paleo reconstructions suggest that colder and saltier waters occupied the deep ocean during the Last Glacial Maximum (LGM) than the pre-industrial, especially in the Southern Ocean, where deep-sea salinity was very high. It is also known that dust deposition to the surface has increased during the LGM compared to the pre-industrial. Recent modeling study has shown that the supply of highly soluble iron from glaciogenic dust can lead to carbon sequestration and lower dissolved oxygen concentrations in the deep ocean by increasing the efficiency of the ocean biological pumps. Based on these findings, we performed numerical experiments under glacial climate using a global-scale three-dimensional ocean general circulation model that considers strong salinity stratification and iron fertilization by glaciogenic dust in the Southern Ocean. By considering the above two processes, biological production increases in the sub-Antarctic region, and the vertical mixing between the surface and the deep ocean is weakened in the Southern Ocean. As a result, deep water contains lower oxygen, lighter stable carbon isotope ratios (δ13C), and older radiocarbon ages (14C ages) were more pronounced compared to the pre-industrial experiment. We successfully reproduced glacial deep water characteristics consistent with paleo proxy data. Furthermore, these changes in the Southern Ocean resulted in increased concentrations of dissolved inorganic carbon throughout the Southern Ocean to the deep ocean. This was accompanied by amplification of carbonate compensation, the sedimentation-dissolution feedback of carbonate sediments, resulting in a further increase in carbon sequestration and realistic changes in atmospheric pCO2 (∼77 ppm). Our results demonstrate the importance of the Southern Ocean in reproducing atmospheric pCO2 at the LGM and contribute to the understanding of the mechanisms of variability in the ocean carbon cycle on a glacial-interglacial scale.