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

Abstract

Eleven surface sediment cores were taken with a Rumohr corer during the océanographie 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).