The role of silicate alteration in regulating marine carbon cycling

Alteration of silicate phases in marine sediments is important for the global budget of carbon, silicon, and many other elements. Despite the growing recognition of in-situ marine silicate alteration (dissolution and formation), its global significance and controlling factors are still poorly understood. By compiling data from scientific drilling and applying numerical modelling, we investigate the interactions between two connected feedback loops: the particulate organic-inorganic carbon loop and the forward-reverse silicate weathering loop. By simulating early diagenetic sequences in the top tens of meters of sediments, we examined the saturation state and interactions of a wide range of silicate minerals. When organic matter is degraded at moderate rates, iron- and sulfate-reduction elevate porewater pH and create a condition that favours reverse weathering driven by the formation of smectite-group clay phases. Our simulations estimate an up to 10 % increase in DIC flux towards the oxic surface sediments with no substantial change in the alkalinity flux. On the other hand, fast organic matter degradation acidifies pore fluids in extended methanogenic zones and induces dissolution of silicates. Dissolution of several reactive silicate phases (mica, amphibole, and pyroxene groups) buffers porewater pH by effectively converting dissolved CO₂ to carbonate alkalinity and results in marine silicate weathering. Consequently, the total alkalinity flux towards oxic sediments increases by as much as 11 % under this condition. The analyses of a global database of pore fluid composition suggest dominant weathering of K- and Mg-containing silicate minerals that can be identified visually. Higher-than-seawater Mg concentrations were observed in almost all sites where total alkalinity is >56 meq/L, with Mg accounting for up to 40 % of the measured alkalinity. Modelling of this data compilation points to dissolution of Mg- and K-rich mica-group silicates as the primary cause for the elevated total alkalinity. The degree of TA excess seems to increase at sites with greater burial thermal history below the sulfate reduction zone with two trends observed among different continental margins. Such a relationship is however not always statistically significant due to the limited number of study sites in certain margins.

Organic matter degradation determines the overall level and flux of DIC as well as dissolved CO₂ in pore fluids, with DIC speciation is modulated through silicate alteration and carbonate authigenesis. As demonstrated by our numerical modelling, increasing organic matter degradation rate four-fold leads to 6.1 times higher fluxes of DIC and total alkalinity towards the oxic sediments, with marine silicate alteration contributing ca. 10-11 % of the changes in DIC fluxes (for reverse weathering) and total alkalinity fluxes (for silicate weathering). We further show that the amounts of DIC sequestered as authigenic carbonate in the methanogenic sediments due to marine silicate weathering could be substantially lower than previous estimates but depends highly on the type of reactive silicate phases weathered. As expected, reverse weathering increases fluxes of dissolved CO₂. However, an even greater increase in dissolved CO₂ flux is predicted when organic matter is rapidly degraded, despite the pH buffering from active marine silicate weathering. The CSi coupling has the potential to modify fluxes of DIC, total alkalinity, and likely other major dissolved constituents between marine sediments and the overlying bottom seawater. However, interpreting the convoluted feedback between organic matter degradation, marine silicate alteration, and carbonate authigenesis requires integrated efforts that take field observations, theoretical consideration, and laboratory constraints into consideration.