Stable isotope studies of the carbon, nitrogen and sulfur cycles in the Black Sea and the Cariaco Trench

Abstract

Samples for stable isotope studies of possible chemosynthesis in anoxic basins were collected in 1986 in the Cariaco Trench and May 1988 in the Black Sea. POM (particulate organic matter) collected at oxic/anoxic interfaces in the water column showed no distinctive carbon or nitrogen isotopic compositions that could be associated with chemosynthetic bacteria. Carbon and nitrogen isotopic compositions at POM concentration maxima near the top of the sulfide zone were −23%o and 4.5%o, respectively, in both the Black Sea and the Cariaco Trench. Measurements of dissolved inorganic carbon (DIC) in the Black Sea indicated that carbon respired during decomposition at depth had an isotopic composition of −23%o and was isotopically similar to phytoplankton, with no distinctive component that could be attributed to chemosynthetic carbon. These results indicate that either the biomass of chemosynthetic bacteria in the oxic/anoxic interface zones is low relative to sinking phytoplankton or that chemoautotrophic bacteria have isotopic compositions similar to those of phytoplankton. In the uppermost 50 m of sulfidic waters in the Black Sea, sulfide isotopic compositions changed significantly in a region of sulfide consumption, increasing up to 5%o vs deep-water background values of −40.5%o. These increases in sulfide isotopic compositions may be due to sulfide oxidation mediated by MnO 2 or oxygen, but are not consistent with sulfide oxidation by photosynthetic bacteria. Growth experiments with sulfate-reducing bacteria suggested that part of the increase in sulfide isotopic compositions could be due to rapid rates of sulfate reduction in the oxic/anoxic interface regions.