Objective estimates of mantle ³He in the ocean and implications for constraining the deep ocean circulation

Hydrothermal vents along the ocean's tectonic ridge systems inject superheated water and large amounts of dissolved metals that impact the deep ocean circulation and the oceanic cycling of trace metals. The hydrothermal fluid contains dissolved mantle helium that is enriched in ³He relative to the atmosphere, providing an isotopic tracer of the ocean's deep circulation and a marker of hydrothermal sources. This work investigates the potential for the ³He/⁴He isotope ratio to constrain the ocean's mantle ³He source and to provide constraints on the ocean's deep circulation. We use an ensemble of 11 data-assimilated steady-state ocean circulation models and a mantle helium source based on geographically varying sea-floor spreading rates. The global source distribution is partitioned into 6 regions, and the vertical profile and source amplitude of each region are varied independently to determine the optimal ³He source distribution that minimizes the mismatch between modeled and observed δ³He. In this way, we are able to fit the observed δ³He distribution to within a relative error of ∼15%, with a global ³He source that ranges from 640 to 850 mol yr⁻¹, depending on circulation. The fit captures the vertical and interbasin gradients of the δ³He distribution very well and reproduces its jet-sheared saddle point in the deep equatorial Pacific. This demonstrates that the data-assimilated models have much greater fidelity to the deep ocean circulation than other coarse-resolution ocean models. Nonetheless, the modelled δ³He distributions still display some systematic biases, especially in the deep North Pacific where δ³He is overpredicted by our models, and in the southeastern tropical Pacific, where observed westward-spreading δ³He plumes are not well captured. Sources inferred by the data-assimilated transport with and without isopycnally aligned eddy diffusivity differ widely in the Southern Ocean, in spite of the ability to match the observed distributions of CFCs and radiocarbon for either eddy parameterization.