Quantifying He fluxes from the mantle using multi-tracer data assimilation

摘要

A global, coarse-resolution ocean model previously fitted to geostrophic shear estimates and to data of 10 hydrographic parameters and tracers has been used to simulate the ³He and ⁴He distributions resulting from the release of mantle helium from mid-ocean ridges. The model is in very good agreement with ¹⁴C and chlorofluorocarbon data and has realistic global ocean overturning strength as well as water mass formation and transport rates. It is found that previously published global mantle ³He fluxes are too high by a factor of about 2. In the model, optimal agreement of modelled δ³He with data is achieved for a global flux of 450 ± 50 mol ³He yr⁻¹. The formulation of He source strengths proportional to ridge spreading rates appears compatible with data. A model/data misfit analysis shows significant and large-scale δ³He underestimation in the southwestern Pacific centred over the Lau Backarc Basin (approx. 179° W/20° S). These misfits disappear in a set-up with 30 of the 450 mol yr⁻¹ global total ³He flux released in the Lau Basin over a depth range between 1250 and 2500 m. Such He flux contributions are missing in present mantle He source compilations. Hydrothermal fluxes of other trace elements and isotopes (TEI) can be calculated from He fluxes on the basis of TEI : He ratios measured close to the sources.This article is part of the themed issue ‘Biological and climatic impacts of ocean trace element chemistry’.