Co-evolution Of Weathering And Paleomarine During The Ediacaran-Cambrian Transition

Under suitable conditions,the material transported by weathering was deposited,changed the material composition of the ocean and redox media,and also provided material guarantee for the evolution of organisms.It was the link for the exchange of materials and energy in all layers of the earth.At the turn of the Ediacaran-Cambrian,large-scale weathering occurred globally.The carbonate platform of the Yangtze Platform experienced long-term exposure to weathering and erosion,resulting in extensive sedimentary discontinuities and stratigraphic loss.The exposed erosion unconformity was ubiquitous,and some areas also developed karst unconformity.Therefore,the Yangtze platform in Guizhou is a good area for studying the continental weathering during the Ediacaran-Cambrian transition period.In this paper,Liuchapo Formation developed in the deep-water environment belonging to slope face?ZK102 in Daotuo of Songtao?and basin face?ZK901 in the Meilang-Yajin mining area of Tianzhu?during the Ediacaran-Cambrian transition period was taken as the main research object,combining representative outcrops and borehole sections in different sedimentary facies with geochemical data including trace elements and Mg isotopic composition,depicting the coupling relationship between weathering and paleo-oceanic geochemistry,providing dynamics background evidence to reveal the hypoxic event and biological evolution.Combining chronological stratigraphic data such as zircon U-Pb age and Re-Os isotope age,the paper specifically divided sedimentary facies into intra-platform sag facies,slope facies,and basin facies and established precise stratigraphic correlations from Ediacaran to Cambrian.On the basis of stratigraphic correlations,the geochemical characteristics of trace elements were used to reflect the redox environment during this transition period.Based on the coordinated change relationship between trace elements and sedimentary environment,an abnormal change curve of trace elements in the sedimentary strata was constructed.When transitioning from Ediacaran to the Cambrian,the weakly oxidized marine environment of the slope facies gradually converted to the reductive environment,a transient dynamic sulfidation environment also appeared in some areas;the condition in the basin facies was mainly characterized by an anoxic environment,accompanied with the intermittent oxidation,oxygen-depleted environment,and sulfidizing environment might existed locally.Slope facies magnesium isotope composition characteristics showed that the?²⁶Mg value gradually increased from the Ediacaran Doushantuo Formation and the Liuchapo Formation to the bottom of the Cambrian Niutitang Formation.The?²⁶Mg value was larger in the siliceous rocks at the top of Liuchapo Formation and the black shale at the top of Niutitang Formation,indicated that the magnesium isotope could effectively constrain weathering.The weathering from the Ediacaran to Cambrian gradually strengthened,and it was strong during the Ediacaran-Cambrian transition period.Comprehensive analysis of paleo-ocean geochemical evolution and the intensity of continental weathering,large-scale weathering during the Ediacaran-Cambrian transition period promoted ocean current input,organic productivity increased,and a large number of black rock series were deposited.Plenty of organic matter prompted the sulfate reduction to flourish,which generated a large amount of hydrogen sulfide,marine environment changed from an oxidized state to a reduced state,and gradually became a sulfided environment,indicating that the weathering of the Ediacaran-Cambrian transition period had promoted the redox of the ancient ocean effect.The development of vulcanizing body of water leads to the increase of buried organic carbon and pyrite and the decrease of oxygen consumption,which indirectly increase atmospheric oxygen content.Then weathering enhances,accompanied with the increase of terrigenous sulfate and organic matter input,thus influencing on the redox conditions of the ancient ocean.Therefore,there is a coupling relationship between continental weathering varying in intensity and the dynamic changes of redox conditions in ancient oceans through space and time.