Ediacaran negative C-isotopic excursions associated with phosphogenic events: Evidence from South China

Abstract

The Yangtze platform of South China preserves relatively continuous strata of the Ediacaran Period and has been one of the key areas to explore Neoproterozoic Earth history. In this study, high-resolution carbon isotope (δ13C) and oxygen isotope (δ18O) data of 701 samples were reported for sedimentary rocks spanning the entire Ediacaran Period using a continuous drill core collected from Wangji, South China. The C-isotopic data of the Doushantuo Formation are consistent with previous work, showing three negative δ13C excursions that can be correlated intra-basinally. A large negative δ13C excursion from +8.16‰ to −7.20‰ identified at the top of the Doushantuo Formation, which is equivalent to the “Shuram Negative Excursion”, allows us to correlate the δ13C profile globally. In the overlying Dengying Formation, we identified two distinct δ13C intervals with an average value of +3.32‰ and of +0.40‰, respectively. As well, we observed a negative δ13C excursion in the middle Dengying Formation, which has not been reported previously.Three major negative δ13C excursions, two in the Doushantuo Formation and one in the Dengying Formation, are tightly associated with phosphorite deposits. The temporal correlation between the negative δ13C excursions and the phosphorite deposits suggests a potential link between carbon and phosphorus cycling in the Ediacaran oceans. We argue that multiple periods of strong upwelling may have played an important role in C and P cycling in the Ediacaran oceans. Oxidative processes of the 13C-depleted dissolved organic carbon (DOC) transported into oxic near-surface waters by upwelling resulted in the negative δ13C excursions, and contributed to synchronous burial of abundant phosphorites in the inner shelf areas. The different magnitudes of the negative δ13C excursions observed in the Doushantuo and Dengying formations may reflect spatial changes of DOC reservoir and variations of upwelling intensity and mixing processes.