Ocean based CO2 removal with enhanced mineral weathering: the effective settling of milled particles from particle-laden flows

The vertical transport of sediment from particle-laden flows is an important natural phenomenon in marine settings. One application, for example, is the settling of milled rock in the ocean for the purposes of atmospheric CO2 removal during the rock dissolution process, a method known as enhanced mineral weathering. In stratified, dynamic marine settings, sediment vertical transport can be enhanced by a settling-driven convective instability (Rayleigh-Taylor). The presence of a horizontal velocity shear can further influence this vertical transport. We conduct numerical simulations to investigate vertical sediment transport in the presence and absence of shear. We show how this transport is determined by a competition between the Rayleigh-Taylor and the stratified shear instability (Kelvin-Helmholtz). When the Kelvin-Helmholtz instability grows rapidly and suppresses the Rayleigh-Taylor instability, the effective settling velocity is significantly reduced. On the other hand, if the Rayleigh-Taylor instability dominates and completely inhibits the Kelvin-Helmholtz instability, the effective settling velocity is enhanced due to the additional energy input by shear. We explore the parameter space of these regimes and interpret their physics. Findings are put in specific context with simple dissolution time scales for enhanced mineral weathering approaches to oceanic carbon capture.