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A "Marsh CO2 Pump" for Transferring Carbon from Air to Sea
Wang, Z.A. and Cai, W.-J.  2004.  Carbon dioxide degassing and inorganic carbon export from a marsh-dominated estuary (the Duplin River): A marsh CO2 pump.  Limnology and Oceanography 49: 341-354.

We have written previously about the ability of tidal marshes to sequester large amounts of carbon in the sediments beneath them and how rising sea levels augment this phenomenon in reviews of the studies of Choi et al. (2001) and Hussein and Rabenhorst (2002).  We here describe another more effective means by which salt marshes extract carbon from the air and export it to the sea for burial.

What was done
Wang and Cai conducted an investigation of seasonal changes and interactions of various forms of carbon in a marsh-dominated estuary, the Duplin River, on Sapelo Island, Georgia, USA.  This work led them to propose "a 'marsh CO2 pump' concept regarding carbon fixation and subsequent export to adjacent waters," which set of linked processes they describe as "an efficient and unique means for ocean carbon sequestration of atmospheric CO2."  At one end of this carbon pump, as they describe it, "a large amount of atmospheric CO2 is fixed by the marsh primary producers," while at the other end "inorganic and organic carbon is exported to the ocean."

What was learned
Combining their results with the results of several other investigators, the authors prepared an annual budget of various carbon fluxes for all southeastern US salt marshes.  The end result was a net primary production (CO2 fixation) flux of 9.3 x 1012 g C yr-1, a CO2 return (degassing) flux to the atmosphere of 1.7-2.2 x 1012 g C yr-1, and a total flux of dissolved organic plus inorganic carbon to the sea of 1.9-5.7 x 1012 g C yr-1, which export flux amounts to something on the order of 20 to 60% of net primary production, totally dwarfing the 1-2% of net primary production that is buried in sediments beneath the marshes each year.

What it means
Wang and Cai state that "this marsh-regulated biological pump mechanism may have important implications for global carbon cycling and CO2 budgets if a large fraction of global coastal wetlands and their adjacent water systems behaves similarly," which is a logical assumption.  There is also reason to believe that the preponderance of terrestrial vegetation behaves likewise, exporting dissolved organic and inorganic carbon to soils and ultimately streams and rivers that make their way to the sea.  In addition, there is every reason to believe that the aerial fertilization effect of the ongoing rise in the air's CO2 content is continually enhancing this terrestrial carbon pump, and that it may be partially responsible for the increase in riverine transport of dissolved organic carbon (Worrall et al., 2004) and total alkalinity Raymond and Cole, 2003) that is manifesting itself in various rivers around the world.

Choi, Y., Wang, Y., Hsieh, Y.-P. and Robinson, L.  2001.  Vegetation succession and carbon sequestration in a coastal wetland in northwest Florida: Evidence from carbon isotopes.  Global Biogeochemical Cycles 15: 311-319.

Hussein, A.H. and Rabenhorst, M.C.  2002.  Modeling of nitrogen sequestration in coastal marsh soils.  Soil Science Society of America Journal 66: 324-330.

Raymond, P.A. and Cole, J.J.  2003.  Increase in the export of alkalinity from North America's largest river.  Science 301: 88-91.

Worrall, F., Harriman, R., Evans, C.D., Watts, C.D., Adamson, J., Neal, C., Tipping, E., Burt, T., Grieve, I., Monteith, D., Naden, P.S., Nisbet, T., Reynolds, B. and Stevens, P.  2004.  Trends in dissolved organic carbon in UK rivers and lakes.  Biogeochemistry 70: 369-402.

Reviewed 2 March 2005