Marine carbon and sulfur cycling across the Ediacaran-Cambrian boundary in Tarim Block and its implications for paleoenvironmental changes

The Ediacaran-Cambrian transition was a critical period of Earth history because of the coincidence among climate change, bioevolution, and tectonic activity. The nature of carbon and sulfur-cycling processes spanning the Ediacaran-Cambrian boundary remains debatable. This study considers high-resolution stable isotope chemostratigraphy of the Sugetbrak and Yutixi sections of the Keping area of the Tarim Basin, northwestern China. The sections display a pronounced negative delta C-13(carb) shift (the Basal Cambrian Carbon Isotope Excursion) that spans the Ediacaran-Cambrian boundary. Deposits overlying the negative excursion are characterized by heavier delta C-13(carb) values ranging from -7.8 parts per thousand to +4.1 parts per thousand. The biostratigraphically calibrated delta C-13(carb) profiles likely reflect widespread perturbation of the global oceanic carbon cycle and associated mass extinction and subsequent biological evolution. The d34Spy profiles of the studied sections display a wide distribution of values, ranging from -16.4 parts per thousand to 23.8 parts per thousand. The variable delta S-34(py) signatures of these sections are partially controlled by microbial sulfate reduction rates, which are sensitive to local sulfate concentration, availability of organic matter and the marine redox state. The strong coupling between enhanced organic carbon and pyrite burial, the appearance of Cambrian-type animals and a shift to a more oxidizing marine environment might suggest a temporal causality of higher oxygen levels of the atmosphere-ocean realm with bio-events during the Ediacaran-Cambrian transition. The development of ecosystems across the Ediacaran-Cambrian transition may have significantly impacted the oceanic carbon and sulfur biogeochemical processes in Earth's history and, in turn, affecting themselves.