Groundwater Carbon Exports Exceed Sediment Carbon Burial in a Salt Marsh

Abstract

Salt marshes can sequester large amounts of carbon in sediments, but the relation between carbon storage and exportation remains poorly understood. Groundwater exchange can flush sediment carbon to surface waters and potentially reduce storage. In this study, we estimated groundwater fluxes and associated carbon fluxes using a radon (222Rn) mass balance and sediment carbon burial rates using lead (210Pb) in a pristine salt marsh (North Inlet, SC, USA). We used δ13C to trace carbon origins. We found that groundwater releases large amounts of carbon to the open ocean. These groundwater fluxes have the potential to export 7.2 ± 5.5 g m−2 of dissolved inorganic carbon (DIC), 0.2 ± 0.2 g m−2 of dissolved organic carbon (DOC) and 0.7 ± 0.5 g m−2 of carbon dioxide (CO2) per day. The fluxes exceed the average surface water CO2 emissions (0.6 ± 0.2 g m−2 day−1) and the average sediment carbon burial rates (0.17 ± 0.09 g m−2 day−1). The δ13C results suggest that groundwater carbon originated from salt marsh soils, while the sediment carbon source is derived from salt marsh vegetation. We propose that the impact of salt marshes in carbon cycling depends not only on their capacity to bury carbon in sediments, but also on their high potential to export carbon to the ocean via groundwater pathways.