Reconstructing marine redox conditions for the transition between Cambrian Series 2 and Cambrian Series 3, Kaili area, Yangtze Platform: Evidence from biogenic sulfur and degree of pyritization

摘要

The transition between Series 2 and Series 3 of the Cambrian Period was a time of environmental perturbations and biological innovations. A generally positive sulfur isotopic composition of sedimentary pyrite with δ~(34)S_(pyrite) values between +1.7 and +37.3‰ is consistent with a biological origin via reduction of strongly ~(34)S enriched seawater sulfate. No substantial changes in the redox conditions of the water column are discernible from the DOP record, and the sulfide sulfur isotopic composition is independent ofDOP, in particular an excursion towards less ~(34)S enriched values immediately preceding the proposed level of the Cambrian Series 2 to Cambrian Series 3 transition. This level is also characterized by a negative carbonate carbon isotope excursion, believed to reflect the transgressive flooding of the shelf with basinal anoxic waters. Low DOP values would suggest that these might have been ferruginous rather than sulfidic.