Abstract

Coastal ecosystems play a major role in marine carbon budgets, but substantial uncertainties remain in the sources and fluxes of coastal carbon dioxide (CO 2 ). Here, we assess when, where, and how submarine groundwater discharge (SGD) releases CO 2 to shallow coastal ecosystems. Time-series observations of dissolved CO 2 and radon ( 222 Rn, a natural groundwater tracer) across 40 coastal systems from 14 countries revealed large SGD-derived CO 2 fluxes. The mean groundwater partial pressure of CO 2 was 35 times higher than surface seawater. The mean SGD-derived CO 2 flux was 148 ± 226 millimoles per square meter per day (mmol m −2 day −1 ), resulting in a mean water-air CO 2 flux of 80 ± 133 mmol m −2 day −1 . Tidal rather than diel cycles drove CO 2 enrichment in most ecosystems. Tidally driven SGD was the primary CO 2 source in mangroves, salt marshes, tidal flats, estuaries, and canals. Overall, we expand current knowledge of marine carbon cycles by demonstrating SGD as an important source of CO 2 that requires inclusion in coastal carbon budgets.