Ediacaran Marine Carbon,Phosphorus,and Sulfur Cycle

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,we present high-resolution inorganic carbon isotopic(?13Ccarb)and oxygen isotopic(?18Ocarb)records from a continuous drill core from the Yangtze platform of South China,named Wangji drill core.The studied interval spans the Cryogenian Nantuo Formation,the Ediacaran Doushantuo and Dengying formations,and the base of the Cambrian Yanjiahe Formation.The C-isotopic data of the Doushantuo Formation are consistent with previous work,showing three negative?13Ccarb excursions that can be correlated intra-basinally.A large negative ?13Ccarb excursion from+8.16‰ to-7.20‰ identified at the top of the Doushantuo Formation,which is equivalent to the "Shuram-Wonoka Negative Excursion",allows us to correlate the ?13Ccarb profile globally.In the overlying Dengying Formation,we identified two distinct 813Ccarb intervals with an average value of+3.32‰ and of+0.40‰,respectively.As well,we observed a negative ?13Ccarb excursion in the middle Dengying Formation,which has not been reported previously.Three major negative ?13Ccarb excursions,two in the Doushantuo Formation and one in the Dengying Formation,are tightly associated with phosphorite deposits.The temporal correlation between the negative ?13Ccarb 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 ?13Ccarb excursions,and contributed to synchronous burial of abundant phosphorites in the inner shelf areas.The different magnitudes of the negative ?13Ccarb excursions observed in the Doushantuo and Dengying formations may reflect spatial changes of DOC reservoir and variations of upwelling intensity and mixing processes.Remarkable ?13Ccarb excursions accompanied by invariant organic carbon isotopic(?13Corg)values have been observed in many late Neoproterozoic successions.This decoupled relationship between ?13Ccarb and ?13Corg in the Precambrian differs from most Phanerozoic records which have been substantially imprinted by aqueous photosynthesis,and it has been tentatively interpreted as resulting from exotic organic matter inputs or diagenesis.In this study,we also present high-resolution ?13Corg records of Wangji drill core.Intriguingly,the ?13Corg profile exhibits a co-varying trend with the?13Ccarb profile,and the coupled relationship between ?13Ccarb and ?13Corg is rare in geological records of the late Neoproterozoic.The ?13Corg values of the drill core vary from-22.13‰ to-34.12‰,and the overall isotopic difference(?13Ccarb-org)between?13Ccarb and ?13Corg ranges between 19.61‰ and 34.58‰.Most of the ?13Corg and?13Ccarb-org data can be interpreted in terms of oxygenic photosynthesis,probably mainly by eukaryotic algae.However,it is possible that exotic organic matter,most likely DOC from the deep ocean,may also have contributed to the sedimentary organic matter composition of specific intervals studied herein.Moreover,high primary productivity in shallow water may have been sustained for a long period in phosphate-rich conditions,and the organic matter generated by photosynthesis may have been quantitatively sufficient to overwhelm the DOC buffering effect in the total ?13Corg signal throughout the Ediacaran Period.The enhanced burial of organic matter in sediments would gradually result in the accumulation of oxygen,both in the ocean and atmosphere,which would likely have paved the way for macroscopic animal evolution.The mixing of authigenic carbonate,which is generally formed during early diagenesis,has been recently proposed to explain the Precambrian negative ?13Ccarb excursions.The inorganic carbon isotopic values of authigenic carbonate cements could reach-30‰ and to a certain extent modify the ?13Ccarb values of whole rock.To evaluate this authigenic process,we polished several rock samples of Wangji drill core and analyzed their ?13Ccarb values for distinct micro-zones.The micro-zone ?13Ccarb results are well consistent with the whole-rock ?13Ccarb profile.Thus,the ?13Ccarb values of Wangji drill core could reflect the isotopic information of seawater,rather than diagenetic alterations.The micro-zone ?13Ccarb tests make us more confident to reconstruct the Ediacaran carbon cycle using ?13Ccarb and ?13Corg records of Wangji drill core.We also conduct systematic analyses of total carbon contents(TC),total sulfur contents(TS),and pyrite sulfur isotopic(?34Spy)values of Wangji drill core.The carbonates and shales of Wangji drill core display a large range of ?34Spy from-13.7‰to 41.4‰,which might be indicative of fluctuating sulfate concentrations in the Ediacaran ocean.The good correspondence between ?34Spy positive excursion and?13Ccarb positive excursion in the lower Doushantuo Formation implies enhanced synchronous burial of reduced pyrite and organic carbon.The ?34Spy values show large fluctuations within the "Shuram-Wonoka" negative ?13Ccarb excursion in the upper Doushantuo Formation that might be caused by oxidative weathering of pyrites and organic matter on exposed shelfs during regression events.The return to high values of?34Spy within the Dengying Formation is likely to reveal the expansion of marine anoxia,despite the extent that needs further considerations.To summarize,the carbon cycle,the phosphorus cycle,and the sulfur cycle were tightly linked in the late Neoproterozoic oceans,and they might have played a key role in regulating the redox state as well as shaping the ecosphere.