The Diagenesis Of Early And Middle Ediacaran Carbonate Sediments In Western Hubei And The Paleoenvironmental Implications Of The Doushantuo Glendonite

Ediacaran Period(ca.635~542 Ma)is a critical transition from Cryogenian Snowball Earth ice age to Cambrian Explosion.So far,there has been abundant data released from sedimentary geochemistry or chemostratigraphy studies aiming at uncovering the changes in marine and atmospheric environments.These changes include significant perturbation of marine inorganic carbon reservoir(e.g.,pronounced excursions in carbonate carbon isotope),temporal and spatial heterogeneity in oceanic redox states,and episodic marine oxidation,particularly in deep ocean.Involved studies are dominantly based on information of depositional processes for the sediments.Whereas,the diagenetic processes and overprints have been relatively less concerned.As a matter of fact,lithified sediments(strata)has undergone series of complicated diagenesis,including early and late diagenetic processes.The alterations and changes in sediments during late burial diagenesis are not only ineffective of paleoenvironmental indication for sedimentation,but also could weaken the robustness of depositional products or proxies in paleoenvironmental implications.In contrast,early diagenesis or authigenesis could occur within the zone from seafloor surface in sediment column to the sediment-water interface.Accordingly,the formation of diagenetic fabrics would be able to record the informations of ambient porewater and even bottom water so as to link with coeval environmental changes.We here focus on two important stratigraphic horizons both characterized by significant paleoclimatic and environmental changes,including ca.635 Ma cap carbonate deposits at the aftermath of Marinoan-age global glaciations and potential horizon correlated to middle Ediacaran glacial event.We intend to investigate the diagenesis of carbonate sediments and discuss its potential paleoclimatic and paleoenvironmental implications.Specifically,comprehensive and multidisciplinary methods were carried out for the basal Ediacaran Doushantuo cap dolostone and middle Doushantuo phosphorous dolostone succession in western Hubei Province.Some new results as follow in this study could sharpen our understanding about relevant diagenetic processes:1.The methane signals with most extremely 13C-depleted isotopic compositions(?13C low to-49‰ VPDB)always exist as sparry calcites finally filling the voids of cracks(e.g.,sheet-crack)and cavities from the Doushantuo cap carbonate in the Yangtze Gorges.In the relative paragenetic sequence of the Doushantuo cap carbonate primarily based on petrography,the calcites with methane signals are identified as the last generation of cements,and the timing of their formation is uncertainly from early diagenesis to late burial diagenesis.Hence,it is not robust as the consequence and evidence of early diagenetic or authigenic anaerobic oxidation of methane in sediments.In the upper unit of the Doushantuo cap dolostone,layers of calcite crystal fans can be identified as early diagenetic or authigenic fabrics.They could nucleate and grow upward at the sediment-water interface with methane-derived isotopic signatures.The occurrence of the calcite crystal fans indicates enhanced anaerobic oxidation of methane in sediment,potentially associated with marine sulfate increase from terrestrial input at the late stage of the cap dolostone deposition.In late burial diagenesis of the Doushantuo cap carbonate,we report the paleotemperatures with carbonaceous material Raman spectroscopy geothermometer generally no higher than 300 ?.The results do not support previous conclusion that the Doushantuo cap dolostone had ever undergone brief hydrothermal activities with high temperature(almost 500 ?).The role of potential hydrothermal events with paleotemperatures lower than 300 ? cannot be excluded in thermal history of the Doushantuo cap dolostone.In addition,the results of Raman geothermometer is compatible with the result of background burial heating of sediments.2.We find the occurrence of silicified glendonite in the middle Ediacaran Dousahntuo Formation on shallow-water inner shelf.The pseudomorph glendonite and its precursor ikaite are evident of early diagenetic or authigenic precipitations.Potential ice-house scenario could be responsible for the formation of Doushantuo glendonite considering about ikaite's favor of near-freezing condition.What is more,relatively high phosphate content of argillaceous dolostone of the middle Doushantuo Formation in Zhangcunping and Huji area could inhibit the nucleation and growth of other euhedral calcium carbonates(e.g.,calcite).On the basis of comprehensive stratigraphic correlations of the Doushantuo Formation,South China,the horizon of silicified glendonite is potentially related to ca.580 Ma Gaskiers glaciation.The original precipitation of ikaite could be facilitated by enhanced anaerobic oxidation of methane and organic matter in sediments,which is indirectly associated with organic burial increase in cold ice age.Furthermore,the occurrence of Doushantuo glendonite is helpful to clarify the temporal relation between some Ediacaran environmental and biotic events.For instance,EN3 or Shuram-equivalent carbon isotope excursion should postdate the Gaskiers glaciation,and the Doushantuo acanthomorphic acritarch assemblage could partially overlap with the Avalon Ediacara fauna in geochronological range.