Organic matter composition and sulfate reduction rates in sediments off Chile

Various organic geochemical parameters and rates of sulfate reduction (SRR) were determined in four sediment cores off the coast of central Chile. To examine the effect of organic matter composition on the sulfate reduction rates, we estimated the marine and terrestrial contributions to the organic matter fraction using C/N ratios, carbon isotopic composition, protein and chlorin concentrations. Whereas three cores, one from the Bay of Concepción, one at the entrance of the Bay and one on the shelf appear to be similarly dominated by marine organic matter input, the organic fraction of another core from the shelf is strongly influenced by terrestrial organic matter. This is demonstrated by higher C/N ratios, lighter 13Corg values, lower protein and lower chlorin concentrations. Additionally, the distribution of sulfate reduction activity with depth at this station differed considerably from the other stations. The marine influenced stations exhibited distinct near-surface peaks of sulfate reduction rates with quasi-exponentially decreasing rates at depth. Sulfate reduction rates at the station influenced by terrestrial organic material exhibited an attenuated near-surface peak and relatively constant rates with depth. Using sulfate reduction rates as a measure of organic carbon reactivity, we were unable to identify differences with respect to degradation kinetics between the terrestrially dominated and marine dominated shelf sediments. It was therefore proposed that the marine organic matter being degraded through the sulfate reduction process is diluted by non-reactive components, including terrestrially derived organic matter leading to the observed sulfate reduction rate distribution.