A deep marine organic carbon reservoir in the non-glacial Cryogenian ocean (Nanhua Basin, South China) revealed by organic carbon isotopes

The late-Cryogenian warm (non-glacial) interval (c.660-c.650 Ma) is potentially of great significance to the co-evolution between life and the surface environment during the emergence of animal life on Earth. In this study, three high-resolution organic carbon isotopic (delta C-13(org) records for the Datangpo/Xiangmeng Formation on the Yangtze Craton are presented. The data derive from drill cores representing different depositional settings at Daotuo (slope setting), Minle (shallow-water basin), and Xiangtan (basin), respectively. The Daotuo and Minle samples exhibit an overall increase of 6-8%o as well as significant isotopic fluctuations following the Tiesi'ao/Sturtian glaciation, while samples from the deeper Xiangtan section show relatively muted fluctuations (+/- 1 parts per thousand) and no overall trend over the same interval. These findings can be plausibly explained by a much longer residence time for marine organic matter, which may have acted as a redox buffer against oxygenation and climate change. The build-up and eventual oxidation of a sub-pycnocline organic carbon reservoir in the redox stratified non-glacial ocean could help to explain the extreme positive and negative carbon isotope perturbations, respectively, in time-equivalent shallow-marine carbonate Platform successions from Mongolia, Australia and Namibia.