Carbon and sulfur isotopic compositions of organic carbon and pyrite in sediments from the Transvaal Supergroup, South Africa

Abstract

Sedimentary organic matter and pyrite from parts of the lower Transvaal Supergroup and few samples from the Olifantshoek Group have been studied for their carbon and sulfur isotopic compositions, respectively. Carbon isotopic compositions for minimally altered organic matter range from −43 to −31‰. Sulfur isotope values for sedimentary pyrite display values between −1.3 to +23.6‰. Results from this and previously published work are being interpreted with respect to past biochemical pathways. Carbon isotope data indicate autotrophic carbon fixation as the process of primary production. In addition, products of secondary biological reworking contributed to the total organic carbon content. Sulfur isotope data indicate bacterial sulfate reduction as the main process. However, changes in sulfate availability and/or the rate of sulfate reduction may have caused the observed range in isotopic compositions.