Evaluating the Roles of Intertidal Salt Marshes to the Coastal CO2 System and Coastal Carbon Budget

Aleck Zhaohui Wang1, Kevin D Kroeger2, Neil K Ganju3, Meagan Eagle Gonneea2 and Sophie N Chu4, (1)Woods Hole Oceanographic Institution, Marine Chemistry and Geochemistry, Woods Hole, MA, United States, (2)U.S. Geological Survey, Woods Hole Coastal and Marine Science Center, Woods Hole, MA, United States, (3)U.S. Geological Survey, Woods Hole, MA, United States, (4)Massachusetts Institute of Technology/Woods Hole Oceanographic Institution Joint Program, Woods Hole, MA, United States
Abstract:
Interestingly, highly productive intertidal salt marshes export alkalinity, dissolved inorganic carbon (DIC) and hydrogen ion to the ocean. This creates an apparent paradox in that salt marshes are both an acidifying and alkalizing source to the ocean. Limited studies suggest that the marsh DIC and alkalinity export via tidal exchange may play critical roles in coastal carbon cycling and carbonate chemistry, but the current estimates are still far too uncertain to be conclusive. This is largely due to the lack of high-resolution measurements that can capture true variability of DIC and alkalinity and hydrodynamics of coast-marsh exchange. This study uses in situ sensors to obtain much needed high-resolution measurements along with modeling to significantly improve the assessment of DIC export fluxes from intertidal salt marshes over minutes to annual scales. The results show that coastal salt marshes may provide significant buffering capacity against coastal ocean acidification, despite the fact that these coastal wetlands also release hydrogen ion to the coastal ocean. There is an apparent decoupling between DIC and alkalinity exports, where DIC is exported throughout the year, while most of the alkalinity signal occurs in the summer. Production of alkalinity is a sign of intensive anaerobic respiration in marsh sediments. A significant portion of the total alkalinity is non-carbonate alkalinity. The new estimate of annual area marsh DIC export is several fold higher than previous estimates, primarily due to improvements in the measurement resolution and water flux budget. This result implies that the marsh DIC export is one of the largest DIC fluxes along the U.S. East Coast. The study also indicates more than 40% of marsh net community production may be exported as DIC (and alkalinity) to the coastal ocean, the largest term in the marsh carbon budget. Globally, the marsh DIC export may play a significant role in the carbon cycle.
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