The role of marine silicate alteration in regulating carbon cycling

Abstract

Despite the growing recognition of in-situ silicate alteration (dissolution and formation) in marine sediments, its global significance and controlling factors are still poorly understood. By compiling information from scientific ocean drilling programs and applying numerical modelling, we aim to 1) provide constraints on the environmental parameters of silicate dissolution in marine sediments, 2) identify silicate phases responsible for the hyper porewater alkalinity (>56 meq/L) commonly observed from productive continental margin sediments, and 3) investigate the interplay between silicate dissolution, clay formation, and carbonate authigenesis as well as their effect on marine carbon cycling.  Through numerical modelling, we show that alkaline conditions resulting from combined iron and sulfate reduction favour formation of smectite group clay minerals, while the acidic conditions arising from organic matter fermentation promote dissolution of saponite and several mica-group silicates. This result resonates with previous observations of reverse weathering (i.e. clay formation) in shallow iron- and/or sulfate reducing sediments, while silicate weathering (i.e. silicate dissolution) has been reported deeper in methanogenic sediment columns.  Using pore fluid composition data, we show that marine silicate weathering is primarily driven by dissolution of K- and Mg-containing silicate minerals. Especially, higher-than-seawater Mg concentrations were observed in almost all sites that have hyper alkalinity and the weathering process contribute more than one-third of the measured alkalinity. No apparent difference was observed for porewater Ca concentrations when comparing sites with and without hyper alkalinity, which hints for complicated feedbacks through authigenic carbonate formation.  The global dataset analysed revealed that sites with high alkalinity correspond to locations with a medium distance from shore. While such a pattern cannot be easily explained by supply of organic matter nor by silicate phases alone, we interpret this observation to be the result of sediment maturity. Our inference is further strengthened by observations of higher alkalinity at sites with greater thermal history within the methanogenesis zone, a factor that measures how much time and temperature a sediment parcel has experienced under subsurface conditions. Collectively, we conclude that substantial dissolution of marine silicate phases occurs when the sediments have been transported some distance offshore and buried below sulfate reduction zone for a prolonged period and/or experience sufficiently high geothermal heating. We simulated alteration of silicate and carbonate phases within a complete early diagenetic sequence to understand how dissolved carbon is converted to alkalinity under variable organic matter degradation rates. We show that authigenic carbonate formation is effective in control downcore DIC/alkalinity level with a moderate organic matter degradation rate. Only a very limited amount of carbonic acid produced by reverse weathering can diffuse away from sediments. Under a scenario with fast organic matter fermentation, dissolution of silicates (such as phlogopite) becomes the only buffer for porewater pH that converts most of the dissolved inorganic carbon produced from organic matter fermentation to carbonate alkalinity. Consequently, marine weathering sustained by silicate mineral dissolution increases the alkalinity production by as much as 16%, with most of the alkalinity leaking to surface oxic sediments instead of being sequestrated as carbonate minerals.