Abstract
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’.
Highlights
One of the exciting discoveries of the GEOTRACES observational programme are the pronounced deep-ocean trace element plumes originating from hydrothermal vents along geologically active ocean ridges found in all ocean basins
Hydrothermal source strengths of other trace elements or isotopes (TEI) can be calculated from TEI : He concentration ratios measured near the sources
The deep-ocean mantle He can be regarded as a reference tracer for other non-conservative TEIs released by hydrothermal activity, and results obtained from the evaluation of He data help the interpretation of the more complicated biogeochemically active trace elements, such as Fe, Zn, Mn and Al
Summary
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. 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. 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.
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