Organic matter composition and sulfate reduction rates in sediments o Chile

Various organic geochemical parameters and rates of sulfate reduction (SRR) were determined in four sediment cores o€ the coast of central Chile. To examine the e€ect 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 Concepcio n, 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 in ̄uenced by terrestrial organic matter. This is demonstrated by higher C/N ratios, lighter Corg values, lower protein and lower chlorin concentrations. Additionally, the distribution of sulfate reduction activity with depth at this station di€ered considerably from the other stations. The marine in ̄uenced stations exhibited distinct near-surface peaks of sulfate reduction rates with quasi-exponentially decreasing rates at depth. Sulfate reduction rates at the station in ̄uenced 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 di€erences 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. # 2000 Elsevier Science Ltd. All rights reserved.