Reactive transport modeling of the effects of seafloor sediment hydrodynamics on ocean alkalinization

Coastal environments are pivotal in the global carbon cycle. Introducing alkaline materials, like olivine-rich rocks, for enhanced weathering and ocean alkalinity enhancement (OAE) holds promise for atmospheric carbon sequestration. Material weathering is incomplete in the water column but occurs significantly after deposition on the ground. This study elucidates the intricate geochemical processes that occur after deposition of the introduced olivine along coastal seabeds, focusing on the impact of mixing zones between terrestrial groundwater and saltwater in the sediment. These zones, where diverse water compositions converge, may promote rock dissolution, influencing OAE. The collective interaction of these factors with OAE remains insufficiently explored. 
Utilizing a 2D modeling approach with FEFLOW coupled with piChem software, our research comprehensively simulates dynamic coastal systems. The model, incorporating multi-component transport, assesses factors like flow rates, groundwater and seawater composition, alkaline material concentration, and sediment permeability, impacting carbon sequestration efficacy. Results showcase olivine settling dynamics, revealing varying times for different-sized grains to reach the seafloor. Notably, 10 µm olivine grains take about a month to settle in 1000 m water depth, while 100 µm grains settle within days. Preliminary findings highlight substantial mineral weathering on the seafloor, emphasizing hydrological conditions' significant influence. Discussions focus the implications of alkalinity transfer into the sediment, crucial for understanding overall process efficiency. This ongoing research emphasizes the need for a holistic understanding of geochemical dynamics in coastal environments to optimize carbon sequestration through OAE.