Early diagenesis and vertical distribution of organic carbon and total nitrogen in recent sediments from southern Chilean fjords (Boca del Guafo to Pulluche Channel)

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Eleven surface sediment cores were taken with a Rumohr corer during the oceanographic cruise Cimar 8 Fiordos (July 2002; between the Corcovado Gulf and Pulluche Channel). These cores were used to determine the vertical distribution of organic carbon, total nitrogen, and their atomic ratio (C/N) for use as a diagenesis indicator. The grains observed were mostly clay-silt in four of the sediment cores and more heterogeneous in the other seven cores. Organic carbon and total nitrogen concentrations were higher in the four clay-silt cores than in the sandy cores, although in terms of their vertical distribution, both concentrations were relatively homogenous at most stations. Nevertheless, exponential decreases characteristic of first-order diagenetic degradation were observed in cores from three stations. The C:N ratio fluctuated between 7 and 10, indicating that the organic material in the sediments was mostly marine in origin. Values were lower at more oceanic stations and greater at more coastal stations; the contribution of terrigenous materials was greater at the latter. We inferred a slower break down of organic carbon as compared to total nitrogen from a steady-state first-order degradation kinetics model, that was applied to stations where both fractions had exponential vertical distributions. Remineralization percentages were between 23 and 34% (organic carbon) and 33 and 43% (total nitrogen) and accumulation percentages were between 77 and 66% (organic carbon) and 67 and 57% (total nitrogen).