Distribution of mantle 3He in the Indian Ocean simulated by an Ocean General Circulation Model

Daisuke Tsumune, Ctr Res Inst Electic Power Ind, Abiko, Japan, Frank Bryan, National Center for Atmospheric Research, Climate and Global Dynamics, Boulder, United States, Keith T Lindsay, NCAR, Boulder, CO, United States, Kazuhiro Misumi, CRIEPI, Abiko, Chiba, Japan, Takaki Tsubono, Central Research Institute of Electric Power Industry, Abiko, Japan, Naoto Takahata, Atmosphere and Ocean Research Institute University of Tokyo, Kashiwa, Japan, Hajime Obata, The University of Tokyo, Atmosphere and Ocean Research Institute, Kashiwa, Japan and Jun Nishioka, Hokkaido University, Institute of Low Temperature Science, Sapporo, Japan
Abstract:
3He and 4He are supplied to the ocean from mid-ocean ridges due to hydrothermal activity. The 3He/4He isotope ratio from the Earth’s mantle is about 8 times larger than that of the atmosphere. δ3He, defined as the isotopic ratio of He relative to that of the atmosphere, is useful as a passive tracer of the abyssal circulation and good proxy for understanding of biogeochemical process of trace metals, such as iron. δ3He simulations were carried out to estimate the 3He flux and to investigate the deep circulation pattern in the Indian Ocean using the CESM1.2 ocean component (POP) with finer resolution than previous studies of He isotopes. We employed the Newton-Krylov solver to efficiently spin up 3He and 4He tracers. 3He flux is portioned geographically as a function of ridge positions, lengths, and spreading rates, based on the OCMIP protocol. We decomposed the total global mantle 3He flux into 10 regional source areas to investigate the contribution of interbasin transport to the deep 3He distribution in the Indian Ocean.

The simulated δ3He distribution with 30% of 3He flux specified by the OCMIP protocol (1000 mol/year in global ocean) is in good agreement with the observationally estimated inventory in the Indian Ocean. The 3He flux is 14 mol/year for the Indian Ocean ridge sources. The distribution of simulated δ3He is in good agreement with observations, with a maximum δ3He at the depth of 2500m. Mantle 3He was transported from the southeast Pacific Ocean and the Southern Ocean to the Indian Ocean due to the long resident time (>2,000 years). We should consider the remote effect of other basin for mantle 3He to investigate the biogeochemical process of mantle iron, whose residence time is shorter (100-200 years).

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