Isotopic constraints on the sources and associations of organic compounds in marine sediments

Abstract

To provide a new perspective on the fate of both natural organic matter and hydrophobic organic contaminants (HOCs) in marine sediments, we have investigated the relationship between radiocarbon (14C) age and the different modes of association in aquatic sediments and soils. Radiocarbon is a sensitive tracer of OM provenance, with variations in its natural abundance reflecting the age and origin of material. The main objective has been to determine the significance of these associations, and to assess how they affect the transport, bioavailability, preservation and residence times of organic compounds in the environment. Our results indicate that the majority of HOCs that persist in marine sediments are solvent-extractable and incorporation into insoluble sediment residues is not quantitatively significant. For pristine sediments, systematic variations in 14C content are observed between different chemically defined sedimentary organic fractions. These variations are dependent on organic matter inputs and/or the affects of diagenesis. Our observations also provide evidence for the protection of labile marine carbon by chemical binding. (cont.) Finally, the persistence of n-alkanes from biogenic sources compared to those derived from petroleum indicates that protective matrix association can play a crucial role in determining the long-term fate of a compound. Overall, it is clear that both natural organic compounds and HOCs can undergo very different fates depending on their mode of introduction to, and physical disposition in environmental matrices.