Geological Records For The Paleo Methane Events In The Critical Time During The Ediacaran-the Early Silurian In South China

Methane,one of the most important carbon reservoirs in carbon cycle,is usually preserved as gas hydrate in the condition of low temperature and high pressure under seafloor.If the stability of gas hydrate decreases and further releasing of abnormally high content of methane（so called methane event）,the anaerobic oxidation of methane will generate a series of changes in elemental and isotopic geochemistry and leave the related geological record.Meanwhile,as one of the super greenhouse gases,methane can seriously affect the climate change.During the long geological history,our Earth has gone through countless times of glaciation-interglaciation cycles.The sea level regressions might trigger multiple methane evens.As one of the major carbon reservoirs,methane would act as an important role in carbon cycle especially during the time of major geological events.To identify the geological records and environmental influence from methane events is of significant to“carbon neutral”and“peak carbon dioxide emissions”.Thus,according to the elemental and isotopic geochemical variations in the modern cold seep environment,we aimed at possible methane events and their geological records in ancient ocean.This study focused on the Ediacaran Doushantuo Formation,the Early Cambrian Yanjiahe Formation and the Late Ordovician Guanyinqiao Bed from South China.Methane events during the mid-late Quaternary at Andaman Sea from the Indian Ocean were also analyzed to make a comparison.Our results show that:1.According to trace elements of pyrites from the Andaman Sea during the mid-late Quaternary,pyrites dominated by anaerobic oxidation of methane（AOM）are enriched in Ni and Co.Negativeδ⁶⁰Ni shift and well negative correlation betweenδ⁶⁰Ni and Ni content indicate that the enrichments of Ni and Co in pyrite are linked with reverse methanogenesis in AOM.2.By investigating trace elements of pyrite from the Doushantuo Formation,the phenomenon that pyrite linked with AOM being enriched in Ni and Co could be an indicator of paleo methane event.3.Besides,according to previous study on Gaskiers glaciation,the interval of EN2should be corresponded with the Gaskiers glaciation.Chemical index of alteration（CIA）for the Doushantuo Formation in South China shows at least three cold periods.These cold periods are distributed in carbon isotopic intervals of mid-upper EN1,EN2 and mid-upper EN3.Appearance of glendonites is found in the cold period of EP1.4.Geochemical investigation for the interval of N2 in the Early Cambrian Yanjiahe Formation indicate that methane event resulted from the cold period that related sea level regression generated negative carbon isotopic shift of N2,negative correlation between carbon and oxygen isotopes and high Ni _EFand Co _EFat the uppermost Yanjiahe Formation.Regression promoted the sulfate input from continent,meanwhile,increased sulfate consumption caused by AOM lead to increased sulfur isotope of CAS and decreased sulfur isotope of chromium reducible sulfur（CRS）.Greenhouse gasses from methane event terminate the cold period,which is evidenced by increased CIA from the overlying Shuijingtuo Formation.5.Extraction of carbonate associated sulfate（CAS）in the interval of N2 from the Early Cambrian Yanjiahe Formation indicate that even by choosing samples with<0.5%w.t.pyrite,oxidation of pyrite still occurred during the CAS extraction.This oxidation may affect accuracy for sulfur isotopes of CAS.Therefore,samples for CAS analyzing should firstly be selected from deep drillcore to avoid contamination from secondary atmospheric sulfate（SAS）.Next,comparing with choosing samples with low pyrite content,it is more important to maintain acid digestion less than 4 hours by centrifugation in order to avoid potential pyrite oxidation.6.In the Guanyiniqao Bed,stable carbon isotopes of carbonate matrix are much lower than those of brachiopods shells.According to low carbon isotope from the overlying black shales,soft sediment deformation and calcitic channel in the Guanyinqiao Bed,light carbon fluid should be generated from AOM and/or OSR occurred in the overlying Longmaxi Formation.Due to different permeability and porosity between the Guanyinqiao limestone and the Longmaxi black shale,diagenetic fluid with light carbon can be migrated downward from the Longmaxi Formation and altered the primary carbon isotopic signals in the Guanyinqiao Bed.Decoupling between U and Mo content indicate that U also went through such migration.Overall,geochemical changes resulted from modern cold seep environment are limited when used in identifying paleo methane events due to diagenetic alteration,compaction,sulfate concentrations in ancient seawater and preservation condition of pyrite.However,this study found geochemical records from paleo methane events.Including（1）negative carbon isotopic shift due to AOM,（2）negative correlation between carbon and oxygen isotopes,（3）enhanced sulfate consumption reflected by sulfur isotopes of CAS and CRS and（4）enrichment of Ni and Co in pyrite or whole rock.Moreover,comparing with climatic changes reflected by CIA,paleo methane events usually occurred at the aftermath of glaciation or glacial maximum.This finding demonstrates that the hypothesis of gas hydrate trigger also works in deep time.If we apply these geochemical records of methane events and combined with coevolution of biology and environment,a significant prediction about climatic change would be made for present day and future.