Marine Sulfur Cycling Research During Nantuo Glaciation,Nanhua Basin

Neoproterozoic'snowball Earth'hypothesis suggests that the Marinoan Period ice cap may have covered the entire surface of the Earth,with profound effects on subsequent atmospheric and oceanic oxidation,elements biogeochemical cycling and eukaryotic evolution.For a billion years prior to 720 Ma,eukaryotes experienced a long evolutionary standstill between their origin and dominance in marine planktonic ecosystems,and the"Snowball Earth"Period witnessed a shift from bacterial dominance of marine primary productivity to eukaryotic dominance,one of the most profound ecological changes in Earth's history.This transition also played an important role in regulating the biogeochemical cycling of sulphur and carbon in the water column and laid a solid foundation for subsequent eukaryotic diversification and biological evolution.Rather than being a hindrance to biological evolution,extreme global ice ages in the Neoproterozoic may have been a catalyst for eukaryotic evolution.However,the redox environment of the oceans during this transition,as well as the relationship with rising atmospheric oxygen levels and how eukaryotes survived the extreme environmental conditions of'Snowball Earth',remain scientific questions of general interest to the academic community that have not yet been clearly answered.Sulfur as an important redox-sensitive element in epigenetic environments,whose biogeochemical cycles are closely coupled with carbon,phosphorus and oxygen,and is also one of the elements with a large isotopic fractionation effect,providing important clues in indicating changes in the marine environment during geological history and exploring information on the evolution of early life.Sulphur isotope studies of sulphate or sulphide in sedimentary rocks can indirectly reflect the sulphate concentration of seawater during geological history and thus reveal changes in atmospheric and oceanic oxygen concentrations and the evolution of organisms.However,the apparent lack of primary marine precipitates limited the understanding of the syn-glacial ocean chemistry during the Snowball Earth.Synglacial dolomites in the Nantuo Formation in eastern Guizhou,based on the stratigraphic sequence reflected in the borehole cores and the stratigraphic comparison of the Nantuo Formation in South China,as well as the lithological characteristics of the dolomite,demonstrates that the dolomite was syn-glacial authigenic deposited and that the dolomite contains varying amounts of pyrite,providing ideal materials for exploring the ocean sulfur cycle in the South China Basin during this period.Study area is located in the north-central part of the South China Basin,in a continental slope position during the Nantuo glaciation paleogeography.Nantuo Formation is conformable contact with the overlying Steep Doushantuo Formation and the underlying Datangpo Formation,and the stratigraphy intact and continuous,lithologic dominated by diamictite,interbedded with sandstones and siltstones.According to the rock core characteristics,the study area has experienced four glacial advance and retreat cycles,which can be compared in Nanhua basin.The dolomite of Nantuo formation is located in the middle of the first cycle in the early stage of Nantuo glaciation,lithology is microcrystalline to fine-grained dolomite,with the thickness of about 5.64 m,no obvious boundary with the upper and lower silty mudstone,showing a gradual transition relationship.Pyrite in dolomite predominantly xenomorphic and hypidiomorphic filling between the porous formed by the dolomite crystals,reflecting its formation in the early stages of diagenesis.A bed of synglacial dolomite was collected from a drill core(ZK008)in the Nantuo Formation,Songtao,Guizhou Province,and samples were analyzed by inductively coupled plasma mass spectrometry(ICP-MS)for rare earth and other trace elements.It was found that the rare earth content ranged from 10.33×10^–6 to 67.91×10^–6.After post-Archean Australian shale(PAAS)normalization,the content of Ce anomalies ranged between 0.68 and 0.91(average 0.82);iron concentrations 1 470×10^-6–37 200×10^-6(average 9 902×10^-6);and manganese concentrations4 040×10^–6–17 500×10^–6(average 10 476×10^–6).The high Fe and Mn contents together with very low Ce anomalies in the dolomite indicate that iron-rich,anoxic(ferruginous)conditions were predominant in the Nanhua Basin during the Nantuo glaciation.Comparison with the sedimentary characteristics and Ce anomalies in two other drill cores from Guizhou Province suggest that near-shore seawater was stratified during this period,with oxic conditions at the ocean surface overlying ferruginous water at depth.This implies that subglacial melt water supplied oxygen for the survival of eukaryotes during extreme Marinoan glaciation.The stable carbon and oxygen isotopic compositions of the dolomite samples were analyzed by Delta V Advantage stable isotope mass spectrometer.The results show that the values of?¹⁸O_V-PDB and?¹³C_V-PDB of the samples are ranged from-16.23‰to-5.73‰,and from 8.90‰to-6.22‰,respectively.Compared with the carbon isotopic compositions of carbonate rocks before and after the Marinoan Glaciation,?¹³C_V-PDB values of the dolomite samples in the Nantuo Formation have significantly low negative values.Based on the correlation of carbon and oxygen isotopes and the characteristics of trace elements,it is suggested that the carbon isotopic composition of dolomite in the Nantuo Formation represents the original sedimentary record.Negative carbon isotope excursion during the period was mainly ascribed to the anaerobic oxidation of methane(AOM)by iron and/or manganese oxides under the anoxic and ferruginous conditions.According to the simple isotope mass balance calculation,it is suggested that high fraction of buried organic matters and limited gas exchange betweenair and ocean-gas contribute to the rise of atmospheric oxygen level during the Marinoan Glaciation.Pyrite sulfur isotopes compositions from the dolomite samples of the Nantou Formation were analyzed by LA-MC-ICP-MS in situ analysis,and sulfur isotope mass spectrometry by extraction of carbonate associated sulfate(CAS)from the dolomite.The results show that all samples tested of?³⁴S_Py analytical value was positive,average+28.88‰,median+29.92‰;individual pyrites particle sulfur isotopic composition extremely variable greatly,from 0.46‰to 76.48‰,with a variation of 76.02‰;?³⁴S_CAS values range from 11.44‰to 27.82‰,average+19.85‰,which was much lower than the average sulfur isotopic composition of the pyrite samples.According to pyrite output characteristics in the dolomite,combined with the pyrite sulfur isotope distribution trend with the stratigraphic log,infer that the pyrite samples were formed in the early diagenetic stage,by the sulphide diffusion from the overlying water body into the pore water,and the sulfur isotopic composition can reflect the sulfur isotopic characteristics of the overlying water body.Correlation diagram of paired?³⁴S_CAS and?³⁴S_Py in the dolomite samples of the Nantuo Formation,and pyrite sulfur isotopes along the profile trend analysis,and the correlation analysis between the?³⁴S_V-CDT and?¹³C_V-PDB in the dolomite samples,reflect that the pyrite sulfur isotopic composition of the Nantuo glaciation can be explained by the Rayleigh fractionation model under a closed system,indicates that the Nantuo-age seawater sulphate concentration extremely low,consistent with the anoxic Fe-rich environmental conditions revealed by the combined rare earth elements and carbon isotope geochemistry in the dolomite.Combined analysis revealed that the role of the sulfur cycle in the Nantuo glaciation ocean as follows:During the warm periods,sulphate from stronger subglacial weathering is carried into the glacial ocean via oxygenated subglacial meltwater,resulting in an oxidised surface layer in the Coastal zone and adjacent water;The sulphate diffusing into the deep water column is reduced by bacteria to form hydrogen sulphide,some of which combines with iron in the water column to form pyrite,some of which further diffuses into the lower sediment pores and reacts with iron in the pore water to form pyrite,due to the sulphate concentration relatively high(up to 1 m M),the CAS and pyrite sulfur isotope fractionation values were larger at this point;During the cold period("Snowball Earth"glaciation),as subglacial weathering was greatly reduced,the sulphate flux from glacial meltwater into the ocean was also greatly reduced,and the sulphate concentration in seawater fall to about 65 salt concentrations,Rayleigh fractionation effect resulted in the almost total sulphate reducted to the sulphide,which has been leading to the formation of pyrite that is the same as or ever more than the sulfur isotope of seawater during the glacial period.