On the origin of Shuram carbon isotope excursion in South China and its implication for Ediacaran atmospheric oxygen levels

Abstract

The Ediacaran Shuram excursion marks one of the largest carbon isotope excursions in Earth’s history, but its origin remains enigmatic. In this study, we examine a carbonate section from Yangtze Gorges Area, South China that archives the Ediacaran Shuram excursion (i.e., “DOUNCE” event in South China), and report its carbon (δ13C), oxygen (δ18O), calcium (δ44/40Ca) together with published radiogenic strontium (87Sr/86Sr) isotope values. We find that the negative δ13C excursion coincides with decreases of δ18O and δ44/40Ca values, along with a positive shift in 87Sr/86Sr ratios. Two different but internally-coherent mechanisms can attribute to our observations—extensive diagenetic alteration under different fluid-buffered systems and coastal processes, and we speculate that both of them may have played important roles in shaping the observed isotope patterns. Using a coupled C-O-Sr-Ca model, we find that these two processes both require an isotopically (13C, 18O, 44Ca) depleted exogenous fluid source (meteoric water or groundwater), potentially generated by oxidation of lithospheric organic carbon. Our hypothesis is consistent with the unique hydrogeological condition in Yangtze Gorge area during the middle Ediacaran. Further, we use an organic matter and pyrite oxidation model to provide a new conservative constraint for middle Ediacaran oxygen partial pressure (pO2). A 10–20% present atmospheric level (PAL) of oxygen concentration is estimated if the Shuram excursion can be more confidently linked to meteoric water or groundwater impacts.