Middle Permian Paleo-ocean Environmental Evolution And Cause Mechanisms Of The Capitanian Mass Extinction In The Lower Yangtze Region

The Middle Permian is a very important period in geological history.The global plate structural aggregation and climate change make the living of organisms in constant turmoil.During its late period,it not only occurred a great biotic crisis known as "Capitanian mass extinction event" but also other synchronous geological and environmental events,such as eruption of the Emeishan Large Igneous Province(ELIP),great regression and oceanic anoxia.However,there is still much controversy about them.The first is the failure to form a consensus on the cause mechanism of the Capitanian mass extinction event.Although the large-scale volcanic activity of the Emeishan Large Igneous Province and its resulting climate-ocean environment changes are considered to be the leading cause of extinction,the interaction mechanism and possible geological process between volcanic activity,climate fluctuations and marine environment changes during this period still lacks sufficient evidence of sedimentary geochemistry.Secondly,the formation and development of marine anoxic-euxinic conditions are also considered to be an important cause of this extinction,but the spatial structure,spatio-temporal evolution,driving mechanisms and its causal relationships with this mass extinction still lack more discussions.In this paper,we selected four Middle Permian sections/cores from different sedimentary environments in the Lower Yangtze area as research objects,through comprehensive analysis of sedimentary petrology,chronostratigraphy,organic carbon isotope stratigraphy,major and trace element geochemistry,iron speciation,discussed the response of the late Middle Permian mass extinction and related geological environmental events in deep-water sedimentary environments,reconstructed the temporal and spatial evolution of the Middle Permian paleo-ocean redox conditions in the Lower Yangtze region,and analyzed the influence of paleoclimatic evolution on the upwelling system,revealed the formation mechanism of marine anoxic environment,and discussed the causual relationship between Capitanian bio-environmental events.The main understanding is as follows:1.Through the study of sedimentary geochemistry and chronostratigraphy,it is proposed that the eruption of the Emeishan large igneous province and its accompanied ocean anoxia may be the main cause of the Capitanian biotic crisis.The intensive occurrence of the tuff layers in the upper Kuhfeng(Gufeng)Formation in the Pingdingshan section of Chaohu area and the initial eruption in the Emeishan Large Igneous Province,not only are consistent in time(J.altudaensis to J.xuanhanensis zone,-263-261.5 Ma),but also have obvious affinity in material composition,indicating these tuff layers probably derived from this province.At the same time,the turnover of radiolarians in basinal settings,decline of initially very high primary productivity and demise of shallow water organisms indicate that the biotic crisis in the Lower Yangtze region may begin in the mid-Capitanian.The Emeishan eruptions began immediately before this,indicating a likely causal relationship.A major regression is seen in South China but it did not cause the deep water biotic crisis directly because it occurred later.However,the loss of the shallow-marine habitat area could have exacerbated the crisis for shallow water organisms.The occurrence of marine anoxia could have caused the radiolarian crisis,suggesting a causal relationship between them.The subsequent intensified anoxic-euxinic conditions could have further accelerated decline of primary productivity and demise of shallow water organisms.Therefore,the eruption of the Emeishan Large Igneous Province and its accompanied oceanic anoxia may be the main cause of this biotic crisis.2.In combination with the well-established radiolarian zones and precise radio-isotopic ages of boundary tuffs,a continuously high-resolution Middle Permian?13Corg chemostratigraphic correlation scheme in the Lower Yangtze region(three different water-depths sections/cores)was built.The variation of these ?13Corg records from different environments have very similar trend.The ?13Corg records increased from late Kungurian to early Guadalupian and then decreased to the late Wordian/early Capitanian but had a positive excursion in the Roadian-Wordian boundary.The early-middle Capitanian high positive ?13Corg excursion may be the response of the 'Kamura Event'.However,it does not appear to be a global productivity event,and our box modelling indicates that the coupled ?13C-87Sr/86Sr records may be better explained by a global reduction in erosion and carbonate weathering rates,driven by the establishment of an arid continental interior on Pangaea.The subsequent egative ?13Corg excursion with different magnitudes may be mainly caused by input of isotopically-light carbon sourced from the terrestrial decomposition of organic matter.3.The lower Yangtze region developed a widespread upwelling system during the late Middle Permian,which was characterized by high primary productivity and oxygen minimum zone(OMZ)related anoxic water column condition.The Hexian core has geochemical characteristics similar to those of the modern upwelling system:high organic carbon contents,similar enrichments of bio/redox-sensitive trace metals and bulk sedimentary composition,high Cd/Mo ratios and low Co x Mn values.The exceptional organic enrichment of the Middle Permian Kuhfeng Formation is attributed to elevated primary productivity due to the widespread upwelling system.Detailed geochemical analysis also reveals that redox conditions and marine productivity of this upwelling system have gone through three stages:the pre-extinction early-middle Capitanian is characterized by anoxic conditions with intermittent euxinic episodes and moderate-high productivity;the middle Capitanian is dominated by stronger euxinic conditions and higher productivity;and immediately after the mid-Capitanian,there is change into an oxic shallow water environment with low productivity.Widespread marine anoxic-euxinic conditions resulting from high productivity and enhanced by eruption of the ELIP and great regression could have led to the intensification and expansion of anoxic-euxinic conditions,which may cause it to gradually shoal to shallow water in the middle-late Capitanian and eventually exacerbate the biotic crisis.4.Based on the high-resolution Fe-S-C-RSTEs data of different sections/cores from water-depths and sedimentary environments,the temporal and spatial evolution characteristics of the Middle Permian paleo-ocean redox conditions in the Lower Yangtze region are revealed.The results show that the early-middle Middle Permian(Roadian-Wordian)is dominated by anoxic-ferruginous conditions,and the late Middle Permian(Capitanian)is characterized by more anoxic-euxinic conditions.A synthesis of these varying marine redox conditions points to a highly stratified marine basin,in which euxinic waters dynamically coexisted at intermediate depths with oxygenated surface waters and ferruginous deep waters(OMZ-like redox structure).This OMZ-like redox structure and its chemocline fluctuated with time,and are controlled by the dynamic balance between the marine high active iron and sulfate reservoir.The violent fluctuation of this OMZ-like redox condition could have led to the intensification and expansion of the early-middle Capitanian anoxic-euxinic conditions,which not only restricts the evolution of deep-water organisms,but also threatens the survival of the living benthic organisms.After the mid-Capitanian period,the widespread distribution of euxinic water from the shelf to the slope may further lead to the massive extinction of marine orgainsms.5.The global climate changes during the Middle Permian and its connection with the Capitanian bio-environmental events were explored.The Middle Permian presents a global warming trend from cold to warm,which may have an important impact on the evolution of paleo-upwelling systems and paleo-ocean redox conditions.A geochemical comprehensive study of deep-water slope-basin facies in the Chaohu area of the Lower Yangtze region shows that,during the Roadian to early Capitanian,the global climate was characterized by warm and wet conditions,and a strong upwelling system with anoxic but not euxinic deep-water column conditions developed along the lower Yangtze margin;with the rapid warming during the earlymiddle Capitanian,the upwelling intensity was significantly weakened and gradually stagnated,This slow ocean circulation could have caused the intensification of restricted basin,further enhancing the expansion of the oxygen minimum zone and the euxinic condition.Capitanian global warming may be the result of large-scale volcanic eruptions of the Emeishan Large Igneous Province.It has triggered rapid fluctuations of thermohaline circulation and ocean current through rapid global warming,resulting in the intensification and expansion of anoxic-euxinic conditions and eventually leading to the Capitainan mass extionction.