Organic carbon isotope systematics of coastal marshes

Measurements of nitrogen, organic carbon and delta(13)C are presented for Spartina-dominated marsh sediments from a mineral marsh in SW Netherlands and from a peaty marsh in Massachusetts, U.S.A. delta(13)C Of organic carbon in the peaty marsh sediments is similar to that of Spartina material, whereas that in mineral marshes is depleted by 9-12 parts per thousand. It is argued that this depletion in delta(13)C of organic matter in marsh sediments is due to trapping of allochthonous organic matter which is depleted in C-13. 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 (similar to 2-100 g C m(-2) year(-1)) is a small fraction (1-5%) of plant production (similar to 2000 g C m(-2) year(-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. (C) 1997 Academic Press Limited.