Dissolved inorganic carbon and alkalinity fluxes from coastal marine sediments: model estimates for different shelf environments and sensitivity to global change

[摘要] We present a one-dimensional reactive transport model toestimate benthic fluxes of dissolved inorganic carbon (DIC) and alkalinity(A_T) from coastal marine sediments. The model incorporates thetransport processes of sediment accumulation, molecular diffusion,bioturbation and bioirrigation, while the reactions included are the redoxpathways of organic carbon oxidation, re-oxidation of reduced nitrogen, ironand sulfur compounds, pore water acid-base equilibria, and dissolution ofparticulate inorganic carbon (calcite, aragonite, and Mg-calcite). Thecoastal zone is divided into four environmental units with differentparticulate inorganic carbon (PIC) and particulate organic carbon (POC)fluxes: reefs, banks and bays, carbonate shelves and non-carbonate shelves.Model results are analyzed separately for each environment and then scaledup to the whole coastal ocean. The model-derived estimate for thepresent-day global coastal benthic DIC efflux is 126 Tmol yr⁻¹, based ona global coastal reactive POC depositional flux of 117 Tmol yr⁻¹. ThePOC decomposition leads to a carbonate dissolution from shallow marinesediments of 7 Tmol yr⁻¹ (on the order of 0.1 Pg C yr⁻¹. Assumingcomplete re-oxidation of aqueous sulfide released from sediments, theeffective net flux of alkalinity to the water column is 29 Teq. yr⁻¹,primarily from PIC dissolution (46%) and ammonification (33%).Because our POC depositional flux falls in the high range of global valuesgiven in the literature, the reported DIC and alkalinity fluxes should beviewed as upper-bound estimates. Increasing coastal seawater DIC to whatmight be expected in year 2100 due to the uptake of anthropogenic CO₂increases PIC dissolution by 2.3 Tmol yr⁻¹and alkalinity efflux by4.8 Teq. yr⁻¹. Our reactive transport modeling approach not only yieldsglobal estimates of benthic DIC, alkalinity and nutrient fluxes undervariable scenarios of ocean productivity and chemistry, but also providesinsights into the underlying processes.

[学科分类] 地球化学与岩石