The vertical transport of sediment in marine settings can be influenced by a settling-driven convective instability and velocity shear. We conduct three-dimensional numerical simulations to investigate vertical sediment transport out of a surface mixed layer under different shear strengths. We show how this transport is determined by a competition between the growth of the settling-driven convective instability (Rayleigh-Taylor) and the stratified shear instabilities, and provide implications for marine carbon dioxide removal. In weak shear scenarios (characteristic velocity difference $\mathrm{\Delta }U<0.05\mathrm{m}$ ${\mathrm{s}}^{-1}$), the Rayleigh-Taylor instability drives enhanced vertical sediment transport compared to Stokes particle settling; the sediment transport is weakly sensitive to the strength of shear, with the slowest effective settling velocity in the absence of shear. In the presence of strong shear scenarios $(\mathrm{\Delta }U>0.05\mathrm{m}$ ${\mathrm{s}}^{-1})$, stratified shear instabilities grow rapidly and suppress the Rayleigh-Taylor instability, and the effective settling of particles is significantly reduced. Furthermore, with an increase in the strength of shear instabilities, the effective particle settling velocity and the rate of mass transport decrease. We show how these estimates for particle transport out of the surface mixed layer relate to the limited potential for atmospheric carbon dioxide draw down via approaches that rely on particle dissolution in the surface ocean.

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• Carbon sequestration
• Kelvin-Helmholtz instability
• Mixing in geophysical flows
• Particle-laden flows
• Rayleigh-Taylor instability