Paleo-oceanic Redox Conditions During Deposition Of The Ediacaran Doushantuo Formation In South China

Recent geochemical studies of the Ediacaran Doushantuo Formation in South Chinasuggested that the ocean was strongly stratified, with an oxic surface layer above a euxinicwedge that was sandwiched within ferruginous deep waters. This ocean redox model,however, was derived largely from the data obtained from Yangtze Gorges area that,according to recent paleogeographic reconstruction, were deposited in a restricted intrashelflagoon. In order to test the previous redox model in open-ocean environments, we studied theDoushantuo Formation in slope and basin sections with multiple approaches (include pyritemorphology and size distribution, redox sensitive elements, carbon/oxygen and sulfur stableisotopes analyses). The results are showed as follows:Pyrites in the Doushantuo Formation can be categorized into four genetic typesaccording to their morphology and size distribution: Type A and B representeuhedral/amorphous and framboidal pyrites with early diagenetic origin. While type C and Dhave size distribution that resemble syngenetic pyrites precipitated form euxinic water inmodern environments, and are indicators of water column euxinia. The distribution ofsyngenetic pyrites in studied sections indicates that water column euxinia exist outside theplatform region. And it is more pervasive in lower slope than upper slope. This result issupported by U, Mo and V data. But the redox condition further into the basin is not clear inthis study. FeT/Al data show that Fe is depleted in upper slope and relatively enriched inlower slope, suggesting an iron re-allocation mechanism resemble the modern black sea.In both upper and lower slope sections, temporal changes in euxinic and non-euxinicintervals occur at centimeter to decameter scales, suggesting frequent chemocline fluctuations.Most eukaryotes fossil groups in the Doushantuo Formation are found in non-euxinicintervals, suggesting the distribution of eukaryotes life is controlled by the development ofwater column euxinia. While several opposite cases imply that the existing geochemicalstudies have not yet reached the resolution required to reveal the sub meter redox fluctuationsassociated with the living and burial conditions of the Doushantuo organisms.The correlation of pyrite occurrences and prominentδ13Ccarbshifts in slope sectionssupports the previous idea that the bacterial sulfur reduction (BSR) is an important cause ofwide-spreaded negativeδ13Ccarbin deep water region. The BSR in sediments may have morecontribution to the negativeδ13Ccarbin deep-water environment than those occur in watercolumn, because there are generally more diagenetic pyrites than syngenetic ones in studiedsections. This result implies that theδ13Ccarbgradient in Nanhua Basin is not cause by watercolumn gradient alone, but also strongly influenced by early diagenesis.Fractionation between sulfate and sulfide (Δδ34S) in Nanhua Basin indicate that thesulfate content in seawater is between 50 and 200μM during early time of the Doushantuodeposition and may surpass 200μM later. Intense pyrite burial in anoxic/euxinic region is animportant factor that keep the seawater sulfate content low. Diminishing of these pyrite burialsites during later stage of the Doushantuo deposition may lead to increasing sulfate content inseawater at that time.