Sulfate reduction in shelf sediments in the upwelling region off Central Peru

Abstract

Concentration profiles of sulfate, hydrogen sulfide, ferrous sulfide, elemental sulfur, and pyrite are presented from five stations from the upwelling region off Peru near 15°S together with profiles of water content, density and organic content. Four of the stations were located within the oxygen minimum zone (80–500 m depth) and one was located at 2650 m depth. Sulfate reduction rates were determined from radiotracer measurements on all stations by acid distillation followed by chromium reduction. Reduced35S recovered as acid volatile sulfide (H2S and FeS) was compared to35S recovered as chromium reducible sulfur (S° and FeS2). In the top 10 cm of the sediment 15–40% of35Sred was recovered as acid volatile sulfide, deeper into the sediment it was less than 10%. Maximum reduction rates of 68–312 nmol cm−3 day−1 were observed within the top 4 cm of sediment from stations in the oxygen minimum zone. Integrating sulfate reduction rates over whole sediment cores under the oxygen depleted water column gave an average rate of 20 ± 11mmol SO42− m−2 day−1 (n = 3). This corresponds to the mineralization of 9–29% of the planktonic primary production in the overlying water column.