Quantifying early diagenesis of fatty acids in a rapidly accumulating coastal marine sediment

Abstract

We report here the results of a quantitative study designed to (i) evaluate the rates of early diagenesis of unbound fatty acids in an organic-rich, coastal marine sediment and (ii) address the relative importance of incorporation of bacterial biomass within the sedimentary organic matter surviving microbially mediated early diagenesis. Radiochronometric tracer studies at the sampling site, Cape Lookout Bight, N.C., U.S.A., prove that sediment accumulation has occurred at a constant rate of 10.3 ± 1.7 cm yr −1 since at least 1971. Higher plant and labile plankton-derived organic materials driving early diagenesis appear to have accumulated at constant annual rates since at least the mid 1970s. Change in the unbound fatty acid concentrations are thus known to result primarily from diagenetic processes. While these data cannot discriminate between the diagenetic processes of (a) complete remineralization and (b) incorporation into a “non-extractable” bound fatty acid fraction, it is nonetheless significant that the measured decrease could account for as much as 8.2% of the production ΣCO 2 and CH 4 at this site (assuming complete remineralization to these end products). These results also indicate the following reactivity relationships: unsaturated fatty acids > branched fatty acids > saturated fatty acids. Furthermore, within the saturated fatty acid fraction, medium chain length compounds (C 14–C 1) are degraded at “apparent” rates 6–7 times faster than long chain length compounds (C 20–C 34). Results of kinetic modelling indicate that no simple relationship exists between remineralization rates and molecular weight (or carbon chain length) and suggest that the preferential preservation of terrestrially derived long chain length fatty acids results from their inclusion into microbially inaccessible matrices.