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

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 (delta C-13) and oxygen isotope (delta O-13) 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 delta C-13 excursions that can be correlated intra-basinally. A large negative delta C-13 excursion from +8.16 parts per thousand to -7.20 parts per thousand identified at the top of the Doushantuo Formation, which is equivalent to the Shuram Negative Excursion, allows us to correlate the delta C-13 profile globally. In the overlying Dengying Formation, we identified two distinct delta C-13 intervals with an average value of + 3.32 parts per thousand and of + 0.40 parts per thousand, respectively. As well, we observed a negative delta C-13 excursion in the middle Dengying Formation, which has not been reported previously. Three major negative delta C-13 excursions, two in the Doushantuo Formation and one in the Dengying Formation, are tightly associated with phosphorite deposits. The temporal correlation between the negative delta C-13 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 C-13-depleted dissolved organic carbon (DOC) transported into oxic near-surface waters by upwelling resulted in the negative delta C-13 excursions, and contributed to synchronous burial of abundant phosphorites in the inner shelf areas. The different magnitudes of the negative delta C-13 excursions observed in the Doushantuo and Dengying formations may reflect spatial changes of DOC reservoir and variations of upwelling intensity and mixing processes.