Biogeochemical approach to understanding phosphogenic events of the terminal Proterozoic to Cambrian

The Precambrian-Cambrian (PC/C) transition is one of the most remarkable intervals in Earth history. Here, we report the S-isotopic compositions of trace sulfates in apatites of two phosphorite deposits spanning the PC/C boundary on the Yangtze Platform, southern China. A comparison of these S-isotope data with published delta(13)C profiles from contemporaneous carbonates suggests that current views concerning phosphogenesis during this time interval need to be reassessed. In the case of the Late Sinian phosphogenic event, which led to the formation of the Doushantuo phosphorites, an excessive development of microorganisms and the operation of the Fe-redox pump may have been primarily responsible for the processes of phosphorite formation. This suggests predominantly geobiodynamic controls rather than tectonic forcing. By contrast, the deposition of the Early Cambrian Zhongyicun phosphorites was probably caused by a breakup of the pre-existing supercontinent and a concomitant transgression, with upwelling nutrient-rich waters triggering a rapid diversification of small shelly invertebrates on the newly generated continental shelves. (C) 2000 Elsevier Science B.V. All rights reserved.