Salt fingers contribute substantially to diapycnal oxygen transport into the oxygen minimum zone of the eastern South Pacific

Abstract

Oxygen minimum zones are large-scale subsurface oxygen-deficient layers experiencing rapid reconfiguration under climate change, partly driven by changes in ventilation whose mechanisms remain poorly understood. Along the eastern South Pacific oxygen minimum zone, a subsurface salinity maximum associated with equatorial subsurface water creates thermohaline gradients that trigger salt-finger instabilities across the lower oxycline. Here we show, using high-resolution microstructure, velocity, and oxygen observations, that these instabilities enhance diapycnal mixing and drive significant oxygen fluxes into the oxygen-deficient layer. While the upper oxycline is dominated by shear-driven turbulence, strong stratification suppresses vertical mixing, resulting in low diffusivities where sharp oxygen gradients are present, whereas salt fingering below yields high diffusivities where gradients are weaker, leading to comparable oxygen fluxes across both boundaries. The widespread occurrence of salt-finger-favorable conditions across the eastern South Pacific suggests that this mechanism plays a persistent and previously unrecognized role in the oxygen minimum zone ventilation.