Alkalinity and carbon fluxes from coastal aquifers to the ocean via submarine groundwater discharge

The coastal ocean links land and sea through rivers and submarine groundwater discharge, which contribute to the coastal carbon budget. Groundwater discharge, including fresh groundwater and recirculated seawater, remains poorly constrained globally. Here, we compile a global dataset of coastal groundwater chemistry and estimate fluxes of dissolved inorganic carbon and total alkalinity. Using conceptual reaction models, we analyze the alkalinity-carbon relationship to identify dominant processes. These patterns reflect carbonate dissolution and precipitation, and organic matter remineralization under oxic and anoxic conditions, indicating that coastal aquifers function as geochemical reactors. Recirculated seawater sampled inland is more enriched than nearshore groundwater, consistent with longer residence time and enhanced water-rock interaction. Groundwater contributes 3–7 percent of riverine dissolved inorganic carbon flux, equivalent to 2.7–2.9 and 2.2-2.4 trillion moles per year of dissolved inorganic carbon and alkalinity, depending on lithology and redox conditions. This emphasizes the importance of incorporating groundwater fluxes into Earth system models. Submarine groundwater discharge delivers non-negligible fluxes of dissolved inorganic carbon and total alkalinity to the coastal ocean, indicating that subsurface processes may influence marine carbon cycling, based on global groundwater chemistry data.