Relationships between carbon isotopic composition and mode of binding of natural organic matter in selected marine sediments

We have investigated the relationships between radiocarbon (14C) and stable carbon (13C) isotopic composition and the different modes of binding of organic matter (OM) in surficial sediments selected from near-shore and continental margin sites that vary in terms of input and depositional conditions. To improve our understanding of the entire OM pool, isotopic analysis of sedimentary sub-fractions, as opposed to individual compounds, was performed. This was achieved by sequentially treating sediments via solvent extraction to examine unbound compounds, followed by saponification to cleave ester linked moieties. Isotopic analysis was performed on the bulk sediment and resulting residues. The molecular composition of the extracts was examined using gas chromatography/mass spectrometry (GC/MS) and the relative contributions of terrestrial and marine biomarkers were assessed. Radiocarbon abundances (Δ14C) of the bulk sediment reflect a mixture of modern, pre-aged and fossil carbon. Offsets in Δ14C between the bulk sediment and sediment residues demonstrate varying associations of these carbon pools. For the majority of sites, a negative offset between extracted (EX-RES) and saponified (SA-RES) sediment residues results from the removal of relatively 14C-rich material during saponification. Saponification extracts (SAEs) are mainly composed of short chain (n-C12 to n-C24) alkanoic acids with an even/odd dominance, indicating a predominantly marine algal or microbial source. This provides evidence for the protection of labile marine carbon by chemical binding. This study bridges the gap between molecular level and bulk OM analysis of marine sediments.