Large Blue Carbon Export Through Submarine Groundwater Discharge: Insights From a Marsh‐Creek Basin System

Abstract Salt marshes are crucial for blue carbon sequestration and the mitigation of climate change. While submarine groundwater discharge (SGD) is increasingly recognized as an important pathway for carbon export in salt marsh tidal creek systems, current research predominantly relies on point‐scale, time‐series observations of creek water. Investigations employing extensive spatial coverage and depth‐resolved sampling within creek networks remain limited. This study implemented a sampling campaign across spring‐neap tidal cycles encompassing a wide area of a tidal creek and collecting groundwater samples from multiple depths at the creek bottom within the North Inlet‐Winyah Bay National Estuarine Research Reserve System in South Carolina, United States. By employing radium isotopes ( 226 Ra and 228 Ra) as tracers, we quantified SGD and associated carbon fluxes and elucidated the biogeochemical transformation processes occurring within the marsh‐creek basin. The results reveal substantial blue carbon exports via creek bottom SGD: a dissolved inorganic carbon (DIC) flux of 241 ± 61 mmol/m 2 /d and net negative fluxes of dissolved organic carbon (−22 ± 5 mmol/m 2 /d) and total nitrogen (−0.98 ± 0.27 mmol/m 2 /d). DIC as the predominant form of carbon export via SGD is comparable to the carbon sequestration capacity of primary productivity and surpasses the net carbon burial of the marsh. A comparative analysis across the globe indicates that tidal creeks are a significant component influencing the coastal carbon budget balance process. These findings further highlight the critical role of groundwater‐transported carbon fluxes in the coastal carbon cycle, particularly within tidal creek basins, and emphasize the necessity of integrating groundwater processes into blue carbon assessments to effectively address climate change.