Silicate alteration in marine sediments: kinetics, pathway, and dependency

Over its long geological history, the overall habitability of Earth has been governed by the chemical alteration of silicate minerals, a reaction that buffers pCO2 and climate. While terrestrial silicate weathering is widely appreciated, marine silicate weathering and reverse weathering (or marine silicate alteration, MSiA, altogether), has long been considered insignificant in the big picture. This paradigm is challenged by recent work that suggests reverse weathering, as an oceanic Si sink, could be three times higher than previously thought. The latest estimates of marine silicate weathering showing its CO2-fixing capacity could be 82% of that of its terrestrial counterpart. Though potentially significant, these estimates are associated with large uncertainties and untested assumptions. In particular, information about the exact chemical pathway of MSiA, kinetics, and the environmental dependency is missing. To fill these gaps, I will provide the first comprehensive assessment of MSiA by quantifying its rates through both laboratory experiments and field observations. While the former constrains how MSiA initiates, the latter represents the million-year quasi-steady state condition in nature. Reproducing the conditions for MSiA in the laboratory is undeniably challenging due to the required multi-year incubation under up to 340 times atmospheric pressure and near-frozen conditions, which I can reproduce with a novel apparatus. Circulation of modified seawater with realistically slow flow will be maintained to derive MSiA rates through continuous fluid composition monitoring. Together with the rates estimated from field observations, I will evaluate the dependency of MSiA on environmental factors, such as the type/quality of silicates and organic matter. The project will be transformative in our understanding of the coupling between Si and C cycles, and thus provide fundamental knowledge for predicting Earth responses to a likely hotter and wetter future.