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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 similar to 23 degrees S off Antofagasta and at similar to 36 degrees S off Concepcion. 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 degrees S) borders the Atacama Desert, while the southern site (36 degrees 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 Pb-210 (Pb-210(xs)) activity, total organic and total inorganic carbon concentrations (TOC and TIC, respectively), C/N-ratios, organic carbon isotopic compositions (delta C-13), chlorin concentrations, Chlorin Indices (CI), and sulfate reduction rates (SRR). Sediment accumulation rates obtained from Pb-210-analysis were similar in both regions (0.04-0.15 cm yr(-1) at Pb-230(xs) 0.10-0.19 cm yr(-1) at 36 degrees S), although total Pb-210(xs) fluxes indicated that the vertical particle flux was higher at 36 degrees S than at 23 degrees S. We propose that sediment focusing in isolated deposition centers led to high sediment accumulation rates at 23 degrees S. Furthermore, there were no indications for sediment mixing at 23 degrees S, while bioturbation was intense at 36 degrees S. delta C-13-values (-24.57 parts per thousand to -20.1 parts per thousand 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 degrees S (area] SRR 0.12-0.60 rarnol m(-2) d(-1)) than at 36 degrees 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 degrees 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 degrees S they were related to the degradation state of bulk OM (represented by C/N-ratios), whereas near 23 degrees 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. (c) 2006 Elsevier Inc. All rights reserved.