Evidence of a large delta (super 13) C (sub carb) and delta (super 13) C (sub org) depth gradient for deep-water anoxia during the Late Cambrian SPICE event

The Upper Cambrian Steptoean Positive Carbon Isotope Excursion (SPICE, ca. 497 to 493 Ma) is characterized by a globally observed positive delta (super 13) C excursion of approximately 4 ppm to 6 ppm. Late Cambrian strata of South China, consisting of thick carbonate rocks with an excellent fossil record, provide key sedimentary successions for characterizing the SPICE and understanding its paleoenvironmental significance. Here we present high-resolution paired delta (super 13) C (sub carb) and delta (super 13) C (sub org) data from three broadly regional late Cambrian (Furongian) sections that were deposited along a shelf margin-slope transect in South China. Our data exhibit an apparent paleobathymetry-dependent delta (super 13) C (sub carb) and delta (super 13) C (sub org) gradient, showing decreasing delta (super 13) C values with increasing water depth. The changes of delta (super 13) C in South China are in contrast to comparable facies-dependent delta (super 13) C data from the central Missouri (USA) intrashelf basin, which show increasing delta (super 13) C values with increasing water depth, but in a shallower paleoenvironmental system. The facies-dependent carbon-isotopic data from South China suggest enhanced burial rates of organic carbon, and may have important implications for understanding changes in paleo-redox conditions and their potential link to the mass extinction.