Marine Carbon-Sulfur Biogeochemical Cycles during the Steptoean Positive Carbon Isotope Excursion (SPICE) in the Jiangnan Basin, South China

Abstract

Global occurrences of Steptoean Positive Carbon Isotope Excursion (SPICE) during Late Cambrian recorded a significant perturbation in marine carbon cycle, and might have had profound impacts on the biological evolution. In previous studies, SPICE has been reported from the Jiangnan slope belt in South China. To evaluate the bathymetric extent of SPICE, we investigate the limestone samples from the upper Qingxi Formation in the Shaijiang Section in the Jiangnan Basin. Our results show the positive excursions for both carbonate carbon (δ13C) and organic carbon (δ13Corg) isotopes, as well as the concurrent positive shifts in sulfur isotopes of carbonate associated sulfate (CAS, δ34SCAS) and pyrite (δ34Spyrite), unequivocally indicating the presence of SPICE in the Jiangnan Basin. A 4‰ increase in δ13Ccarb of the Qingxi limestone implies the increase of the relative flux of organic carbon burial by a factor of two. Concurrent positive excursions in δ34SCAS and δ34Spyrite have been attributed to the enhanced pyrite burial in oceans with extremely low concentration and spatially heterogeneous isotopic composition of seawater sulfate. Here, we propose that the seawater sulfur isotopic heterogeneity can be generated by volatile organic sulfur compound (VOSC, such as methanethiol and dimethyl sulfide) formation in sulfidic continental margins that were widespread during SPICE. Emission of 32S-enriched VOSC in atmosphere, followed by lateral transportation and aerobic oxidation in atmosphere, and precipitation in open oceans result in a net flux of 32S from continental margins to open oceans, elevating δ34S of seawater sulfate in continental margins. A simple box model indicates that about 35% to 75% of seawater sulfate in continental margins needs to be transported to open oceans via VOSC formation.