THE EVOLVING REDOX LANDSCAPE OF THE NANHUA BASIN ACROSS THE NEOPROTEROZOIC/CAMBRIAN TRANSITION

The South China Craton has played a critical role in refining our understanding of the co-evolution of life and surface environments in the Late Neoproterozoic and Early Paleozoic. From fossils of early metazoan embryos to the many examples of exceptional preservation throughout the Cambrian Explosion, South China holds an outstanding record of animal evolution across this critical transition. Similarly, rocks preserved in South China hold key insights into the changing ocean chemistry that accompanied this extraordinary time. Work form Sahoo and others (2016, Geobiology) used redox sensitive metal enrichments in the Ediacaran Doushantuo Formation to demonstrate that the redox state of Late Neoproterozoic oceans was highly dynamic, rather than stably oxygenated or anoxic as had been suggested previously.

To address how basinal water chemistry continued to evolve into the Phanerozoic, we have investigated the Ediacaran/Cambrian Liuchapo and Jiumenchong formations as captured by drill core from eastern Guizhou. The sampled interval contains an enigmatic, widespread horizon that is strongly enriched in Ni and Mo. We have taken a multi-proxy approach to investigate this layer and its implications for strata above and below (i.e., whether these extreme enrichments and the adjacent strata carry straightforward global-scale paleoceanographic information). Our Fe speciation data indicate that conditions were sulfidic throughout much of the studied interval. While trace metal concentrations are dramatically enriched in the Ni/Mo interval, their concentrations return to modest levels above it, where they are stable for ~10 m before decreasing along with indications of changing local redox. This trend suggests that the source of metals to the Ni/Mo layer did not persist after the deposition of the layer itself. Further, by the time this source was exhausted, the background reservoir of these metals was low, sufficient only to provide small enrichments. The essential implication is that deep ocean anoxia and euxinia, as for the Proterozoic, were regular, if not dominant, features of the Cambrian world. As such, fundamental changes in global marine redox were not found in association with the explosion of Cambrian animal life.