Diffusive cation fluxes in deep-sea sediments and insight into the global geochemical cycles of calcium, magnesium, sodium and potassium

• A review of the global biogeochemical cycles of major cations – calcium, magnesium, sodium and potassium
• Extended higher-resolution flux estimates of major cations in marine sediments than previous work has allowed
• Diffusive fluxes of major cations within marine sediments should be included in the global budgets of the cations

The biogeochemical cycles of the ocean's major cations — calcium, magnesium, sodium and potassium — are linked to the ocean's alkalinity budget through terrestrial weathering and the subsequent formation and burial of calcium carbonate in the ocean. Chemical reactions within ocean sediments play a critical role in the biogeochemical cycles of the major cations, as indicated by geochemical gradients (both positive and negative) in the concentrations of these cations within the pore-fluid system (i.e. fluid trapped between sediment particles). Here we review the biogeochemical cycles of calcium, magnesium, and sodium, and provide new estimates of the diffusive fluxes of these cations within marine sediments to explore the importance of these sedimentary processes. We quantify these fluxes by compiling a global database of pore fluids from the various Ocean Drilling Programs (Deep Sea Drilling Program — DSDP, Ocean Drilling Program — ODP, International Ocean Drilling Program — IODP), comprising nearly 700 locations, which allows a wider geographic coverage and therefore better integrated flux estimates than previous work has allowed. The myriad of subseafloor chemical reactions that may influence the concentrations of the major cations in pore fluids include authigenic carbonate precipitation, carbonate dissolution, clay mineral formation, and ion exchange; as previous work has shown, we confirm that these integrated fluxes are globally significant. Because the DSDP/ODP/IODP cores begin sampling one meter below the sediment–water interface, additional studies of the processes within the top meter are needed to accurately calculate total cation fluxes across the sediment–water interface. Delineating the various processes that control the major cation chemistry of seawater over geologic time scales remains critical for understanding the operation of the CO2 silicate-weathering thermostat on geologic timescales.