C25 and C30 biogenic alkenes in sediments and detritus of a New England salt marsh

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As part of a geochemical study of C25 and C30 biogenic alkenes in estuarine environments, distributions of these compounds in detritus and sediments collected from a New England salt marsh (Round Swamp on Conanicut Island in Narragansett Bay, Rhode Island) have been determined. The alkene assemblages detected, consisting primarily of four acyclic C25 dienes and trienes and a C30 bicyclic diene, qualitatively resemble those previously reported for other sediments in which anoxic conditions were prevalent. These similarities exist despite significant differences in the principal sources of sedimentary organic matter, suggesting that the occurrence of these specific alkenes is more likely associated with an in situ process common to anoxic environments than with a direct input from a specific source. Size fractionation (> 840 μm and < 840 μm to 1·2 μm) of marsh detritus revealed that the larger size fraction, consisting primarily of decaying Spartina debris, contains significant amounts of alkenes. This result, together with alkene subsurface profiles which show high surface concentrations decreasing to near-background levels by 20 cm, suggest that anaerobic bacteria are mediating in situ production of these compounds. Previous studies of bacterial hydrocarbons have not reported the presence of these C25 and C30 alkenes, although similar compounds have been isolated from several species of methanogenic bacteria. However, attempts to induce alkene synthesis by decomposing Spartina anaerobically in the laboratory were unsuccessful. In light of this result, the exact source of alkenes in marsh sediments remains uncertain. The absence from marsh sediments of other C25 alkenes whose sedimentary distributions had been previously correlated with the presence of marine (planktonic) organic matter implies the existence of different origins for structurally related constituents of this hydrocarbon series. © 1985.

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Estuarine, Coastal and Shelf Science