Constraints On The Ocean Chemistry Changes During Neoproterozoic In South China

Neoproterozoic(1000-541 Ma)is a critical interval during the evolution of Earth's surface environment and early life.Neoproterozoic Earth may have experienced two ice ages in global scale which was termed "Snowball Earth" events.Snowball Earth conditions have fundamentally changed Earth's surface environments and had important impact on early animal evolution.Consequently,the termination of the Neoproterozoic Snowball Earth provides favorable climate conditions for the dawn of early animals.Ediacaran fauna appeared in the fossil record around 580 Ma ago which may be followed by the "Wengan Biota","Lantian Biota",and ultimately "Cambrian Explosion" of animals.Therefore,it is important to understand co-evolution of environment and early animals during Neoproterozoic.In South China,two Neoproterozoic glacial deposits are well preserved which provide strong evidence for "Snowball Earth" events.There are numerous continuous sedimentary sequences with abundant fossil record in South China.The Ediacaran sedimentary sections in South China contain not only shallow-water facies which are dominated by carbonates but also deep water facies which consist mostly of black shales.It has been well known that the shallow water Doushantuo Formation yield phosphatized "Wengan Biota" which may represent the earliest animal embryos on Earth.For the deep water sedimentary rocks,the black shale-dominated Lantian Formation contain the "Lantian Biota".Therefore,the Ediacaran sedimentary sequences in South China is an excellent laboratory to study ocean and atmospheric chemistry changes in space and time.The samples of this study were collected from two shallow water sections at Wengan and a deep water section in southern Anhui Province.High resolution C-isotopes including ?13Ccarbonate and ?13Corg and O-isotope(?18O)were analyzed for the shallow water Datang and Chuanyuandong sections.In geological time scale,P is an important nutrient and plays a critical role in regulating ocean and atmospheric oxygen.As well,Wengan is one of the most important bases for chemical industries in C hina.The isotopic results show that C-and O-isotopic compositions of the Nantuo glacial deposits record the diagenetic effects rather than the original isotopic compositions of glacial seawater.The isotopic data for the post-Nantuo carbonate and lower phosphorite deposits could represent isotopic compositions of coeval seawater but some samples did not record primary isotopic signals.Rather,the isotopic data from the upper phosphorite deposits and carbonates are found to have approximately recorded the primary seawater values.It appears that the chemical compositions of seawater may have changed during the two phosphogenic episodes.It is concluded that the C-and O-isotopic values must be evaluated for diagenetic effects before they are applied to constrain paleo-environment.The high-resolution deep water black shale samples were collected from a drill core and multiple S-isotopic compositions were analyzed,in addition to total organic carbon(TOC)and pyrite contents.Two episodic increase of TOC and pyrite content suggest that the burial of organic matter and reduced sulfur may have increased episodically in anoxic Ediacaran oceans.In consequence,the enhanced burial of organic matter and reduced sulfur may have produced oxygen into ocean and atmosphere during the Ediacaran period.It appears that there are two increases of ?34S which are consistent with episodic burial of organic matter and reduced sulfur.The ?34S+?33S data imply that little S-isotopic disproportionation were involved in sulfur cycling when the lower Lantian Formation were deposited.However,S-isotopic disproportionation may have increased in changing sulfur cycling when the upper Lantian Formation were deposited.Further study will focus on biogeochemical cycles of sulfur in Ediacaran oceans and their implications for ocean chemistry changes.Finally,the co-evolution of "Lantian Biota" and Ediacaran ocean and atmospheric chemistry is to be fully understood.