Sedimentary Geochemical Study Of The Late Cryogenian-early Cambrian Profiles In South China And Its Geological Implications

In the long geological time,atmosphere and ocean have experienced two important oxidation events which are the Paleoproterozoic Great Oxidation Event and the Neoproterozoic Oxidation Event.These events had a great influence on the origin and evolution of life and the system of atmosphere and ocean.The record of early metazoan fossils and the radiation of marine algae occurred following the Marinoan glaciation in the Neoproterozoic.In order to investigate the relationship between the biological evolution and an ocean oxygenation event in Neoproterozoic,we have collected samples from the three profiles,including the boundary??635 Ma?between the Nantuo and Doushantuo Formations?the Dajiuhu profile?,the boundary??551 Ma?between the Doushantuo and Dengying Formations?the western Sanxia profile?and the boundary between the Dengying and Niutitang Formations?the Siji town profile?.Here,we present the major elements,the trace elements,the TOC contents,C-O-Mo isotopic data of sedimentary rocks collected from these profiles.After discussing the diagenetic and detrital effects,some main results and conclusions are obtained as follow:?1?the profile across the boundary??635 Ma?between the Nantuo and Doushantuo Formations?the Dajiuhu profile?can be distinguished into three intervals:Interval?,the Nantuo Formation beneath the cap carbonate of the Doushantuo Formation,has the high TOC?0.03%?and Mo concentration?5.494 ppm?and similar tendency of the?¹³C values and the?⁹⁸Mo values,which represent the enhanced chemical weathering,the raised nutrient input and the higher primary productivity.Interval?,the cap carbonate,possesses low TOC and Mo concentrations and the consistently diminished carbon composition and the fluctuating?⁹⁸Mo values.Interval?,the sediments above the cap carbonate of the Doushantuo Formation,is characterized by low TOC and Mo concentration,which is similar with the Interval?,but displays decreasing?¹³C values and increasing?⁹⁸Mo values.These features of the Interval?and?were contributed by the end of high intensity chemical weathering,resulting in the atmosphere and shallow-ocean enriching oxygen.The?⁹⁸Mo values elevated until the interval?,indicating that the response of Mo isotope system lagged the shallow-ocean oxygenation.?2?the profiles of the Doushantuo and the Dengying Formation?the Western Sanxia and Siji town profiles?:the Doushantuo Formation from the profile of Western Sanxia has the increasing?¹³C values,?⁹⁸Mo values and the TOC contents?0.02%?comparing with the Doushantuo Formation from the Dajiuhu profile.The above characteristics indicate an improvement of primary productivity and the ocean oxygen content at this interval.Meanwhile,the Ce/Ce*values?0.88?of the carbonates suggest a suboxic palaeoenvironment in this interval.The Dengying Formation from the Western Sanxia profile possesses increasing?¹³C values and TOC contents and decreasing?⁹⁸Mo values,indicating that the reduced ocean oxygen content preserved more the organic carbon.In addition,the Ce/Ce*mean value is 1.0,which implies an anoxic palaeoenvironment in this interval.The Dengying Formation from the Siji town has reduced?¹³C values,and increasing?⁹⁸Mo values and TOC contents,demonstrating the enhanced primary productivity and the raised ocean oxygen contents.Additionally,the Ce/Ce*mean value of this formation is 0.51,implying an oxic palaeoenvironment in this interval.According to the re-establishment of the paleogeography,the paleogeographic environment of the Shennongjia area is a deep basin at 541 Ma.Hence,the deep ocean has been oxidized in the late Ediacaran,which afforded enough oxygen for the Cambrian biological evolution.