Organic Carbon Isotope Systematics of Coastal Marshes

Measurements of nitrogen, organic carbon and δ13C are presented forSpartina-dominated marsh sediments from a mineral marsh in SW Netherlands and from a peaty marsh in Massachusetts, U.S.A. δ13C of organic carbon in the peaty marsh sediments is similar to that ofSpartinamaterial, whereas that in mineral marshes is depleted by 9–12‰. It is argued that this depletion in δ13C of organic matter in marsh sediments is due to trapping of allochthonous organic matter which is depleted in13C. The isotopic composition and concentration of organic carbon are used in a simple mass balance to constrain the amount of plant material accumulating in marsh sediments, i.e. in terms of the so-called net ecosystem production. Net ecosystem production ( 2–100 g C m−2year−1) is a small fraction (1–5%) of plant production ( 2000 g C m−2year−1). This small amount of plant material being preserved is nevertheless sufficient to support marsh-accretion rates similar to the rate of sea-level rise.