How depositional conditions control input, composition, and degradation of organic matter in sediments from the Chilean coastal upwelling region

Abstract

In a comprehensive study, we compared depositional conditions, organic matter (OM) composition, and organic carbon turnover in sediments from two different depositional systems along the Chilean continental margin: at ∼23° S off Antofagasta and at ∼36° S off Concepción. Both sites lie within the Chilean coastal upwelling system and have an extended oxygen minimum zone in the water column. However, the northern site (23° S) borders the Atacama Desert, while the southern site (36° S) has a humid hinterland. Eight surface sediment cores (up to 30 cm long) from water depths of 126–1350 m were investigated for excess 210Pb ( 210Pb xs) activity, total organic and total inorganic carbon concentrations (TOC and TIC, respectively), C/N-ratios, organic carbon isotopic compositions (δ 13C), chlorin concentrations, Chlorin Indices (CI), and sulfate reduction rates (SRR). Sediment accumulation rates obtained from 210Pb-analysis were similar in both regions (0.04–0.15 cm yr −1 at 23° S, 0.10–0.19 cm yr −1 at 36° S), although total 210Pb xs fluxes indicated that the vertical particle flux was higher at 36° S than at 23° S. We propose that sediment focusing in isolated deposition centers led to high sediment accumulation rates at 23° S. Furthermore, there were no indications for sediment mixing at 23° S, while bioturbation was intense at 36° S. δ 13C-values (−24.5‰ to −20.1‰ vs. VPDB) and C/N-ratios (molar, 8.6–12.8) were characteristic of a predominantly marine origin of the sedimentary OM in both investigated areas. The extent of OM alteration in the water column was partly reflected in the surface sediments as chlorin concentrations decreased and C/N-ratios and CI increased with increasing water depth of the sampling site. SRR were lower at 23° S (areal SRR 0.12–0.60 mmol m −2 d −1) than at 36° S (areal SRR 0.82–1.18 mmol m −2 d −1), which was partly due to the greater water depth of most of the sediments investigated in the northern region and consistent with a lower quality of the sedimentary OM at 23° S. Reaction rate constants for TOC degradation that were obtained from measured SRR ( k SRR; 0.0004–0.0022 yr −1) showed a good correspondence to k TOC that were derived from the depth profiles of TOC (0.0003–0.0014 yr −1). Both, k SRR and k TOC, reflect differences in OM composition. At 36° S they were related to the degradation state of bulk OM (represented by C/N-ratios), whereas near 23° S they were related to the freshness of a small fraction of labile OM (represented by CI). Our study shows that although rates of organic carbon accumulation were similar in both investigated sites, the extent and kinetics of organic carbon degradation were closely linked to differing depositional conditions.