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

Abstract Oxygen Minimum Zones (OMZs) are large-scale, oxygen-deficient layers that are expanding under ongoing ocean deoxygenation. Yet, the physical mechanisms that sustain their vertical ventilation remain incompletely understood. At the southern extent of the Eastern South Pacific OMZ, off central Chile, a pronounced subsurface salinity maximum associated with Equatorial Subsurface Water creates strong thermohaline gradients that trigger salt-finger instabilities at the lower oxycline. Here, we combine high-resolution microstructure, velocity, and oxygen observations to show that these double-diffusive instabilities substantially enhance diapycnal mixing and drive upward oxygen transport into the OMZ core. While the upper oxycline is dominated by shear-driven turbulence with sharper gradients and lower diffusivities, salt fingering in the lower oxycline produces diffusivities up to two orders of magnitude greater. As a result, diapycnal oxygen fluxes across the lower oxycline rival those at the upper boundary. The widespread prevalence of salt-finger-favorable conditions across the Eastern South Pacific suggests that this mechanism plays a persistent and previously underrecognized role in OMZ ventilation. These results highlight the importance of double-diffusive mixing in shaping oxygen distributions and underscore its relevance for predicting future deoxygenation trends.