Carbon dioxide degassing and inorganic carbon export from a marsh‐dominated estuary (the Duplin River): A marsh CO2 pump

Abstract

We investigated the seasonal changes and the interactions of the CO2 system in a marsh‐dominated estuary, the Duplin River, on Sapelo Island, Georgia. Surface water pCO2, total alkalinity (TA), and total dissolved inorganic carbon (DIC) showed a pronounced seasonal progression in this system. The estuary released 256‐306 g C m‐2 (of water) yr‐1 of CO2 to the atmosphere and exported 109 g C m‐2 (of water) yr‐1 of DIC to adjacent coastal waters. There was a clear seasonal pattern in both fluxes with spring minima, and late summer and fall maxima. Release of inorganic carbon as a respiratory product from surrounding salt marshes is the primary process supporting these two carbon export terms. Concurrently, TA was exported from the Duplin River mainly in summer and fall as a result of anaerobic respiration in marshes. Based on data from the upper Duplin River, export of inorganic carbon from salt marshes was 156 g C m‐2 (of marsh) yr‐1. Extrapolation of this value to all southeastern U.S. salt marshes indicates that they cumulatively export 0.7 X 1012 g inorganic carbon annually to coastal waters. This export rivals that of riverine inorganic carbon flux. We propose that the CO2 fixation of marsh grasses and the subsequent export of inorganic and organic carbon is one major mechanism that causes the marsh‐influenced nearshore and offshore waters in the southeastern U.S. to be annual net sources of atmospheric CO2. We also suggest that this process provides an efficient and unique means for ocean carbon sequestration of atmospheric CO2.