LARGE-SCALE GROUNDWATER FLOW AND THE MAJOR ION CHEMICAL BUDGETS OF THE OCEAN

Submarine groundwater discharge has been defined to include such large-scale flow processes as geothermal convection and sediment compaction in continental shelves. We suggest that this flow is an overlooked control on the major ion chemistry of the ocean. Conventional ocean chemical budgets are constructed using river discharge, mid-ocean ridge (MOR) convection, off-axis alteration of oceanic crust, and CaCO₃ production. These budgets are difficult to balance and are subject to persistent uncertainty. We synthesized data from 17 passive continental margin basins to calculate a range of estimated groundwater and chemical fluxes from continental shelves, considering five major ions (Ca²⁺, Na⁺, K⁺, Mg²⁺, and Cl^-). Chemical compositions of four likely basinal fluid archetypes were synthesized from the literature. Net chemical fluxes were calculated assuming seawater was the starting composition for groundwater discharging from the basins. When extrapolated globally, volumetric groundwater flux estimates were comparable to those for MOR axial circulation. In mass balance models, we found that passive continental margins have the potential to remove Mg²⁺ in quantities that balance inputs from the other chemical sources for three out of four considered fluid archetypes. We suggest that passive margins also contribute significant quantities of Ca²⁺ and K⁺ to the oceans, which may, in combination with off-axis alteration of oceanic crust, balance chemical budgets. These contributions likely vary in response to long-term changes in sea level and the prevalence of passive continental margins over geologic time.