Alkalinity and carbon production in coastal aquifers via fresh and saline groundwater discharges into the coastal ocean

The coastal ocean is a critical interface between land and sea, where rivers and submarine groundwater discharge (SGD) transport dissolved carbon and alkalinity into the marine environment. While riverine inputs have been well quantified, SGD contributions, both fresh and recirculated seawater, remain poorly constrained globally. Here, we compile a global dataset of coastal groundwater chemistry and quantify the dissolved inorganic carbon (DIC) and total alkalinity (TA) fluxes via SGD. Analyzing the TA–DIC relationship reveals consistent slope patterns. The reactions include carbonate dissolution/precipitation, sulfate reduction/denitrification, and CO2 addition from organic matter deposition or respiration. These findings imply that aquifers function as geochemical reactors. Long-residence saline SGD is systematically more enriched than nearshore SGD, using distance from shore as a proxy for water-rock interaction time. Accounting for lithology and redox conditions, we estimate that SGD delivers ~2.7-2.9 Tmol·yr⁻¹ of DIC and 2.2-2.4 Tmol·yr⁻¹ of TA, comparable to rivers, with DIC often exceeding TA due to biogeochemical processes. Despite uncertainties in the saline SGD components, our results establish a critical framework to quantify SGD fluxes and emphasize their importance. Incorporating SGD into Earth system models is critical for understanding coastal buffering capacity, carbon fluxes, and feedbacks to climate change and ocean acidification.