The Evolution And Dynamics Mechanism Of Paleocean Benthonic Faunas During Late Permian In The Eastern Sichuan Basin

In Late Permian (Changhsingian), the South China was located near the Earth's equator, during which the flourishing of benthonic faunas were promoted owing to the appropriate physicochemical conditions in the ocean. However, the global mass extinction event at the end of the Permian was a devastating blow to benthonic faunas, which wiped out over 90% of species in the ocean and 70% vertebrate families on land and caused a tremendous change in the overall structure of global ecosystem. Currently, there are disputes on the process and pattern of mass extinction at the end of the Permian. With the deepening of studies over the past years, there have been some evidences that the hydrochemical paleocean environment changed before the mass extinction. However, in these studies, the paleocean benthic faunas made different reactions to paleoenvironmental changes and the biological crisis processes under different sedimentary environments may have certain time differences, thereby leading to the absence of complete understanding of paleocean environmental changes in biological crisis event. How did the benthonic faunas in the sea at different depths deal with the paleocean environment pressure? Did the different sedimentary environments in the ocean suffer from the same degree of effects during this biological crisis event at the end of the Permian? What the marine ecological environment in the different sedimentary environments corrections are with the biological crisis during Late Changhsingian. To address these problems, this thesis mainly focuses on the outcrop sections of Changxing formation in the eastern Sichuan Basin and also other investigates the typical sections in South China. Based on historical data analyses, field work and laboratory tests, the evolutionary processes of benthonic faunas in different sedimentary facies are systematically explored on the premise of biostratigraphic correlation. Moreover, based on the research on the interaction and coupling between paleocean physicochemical conditions and biological process during Changhsingian, the dynamic evolutionary process of the benthonic faunas is analyzed to reveal the correlation between evolution of benthic faunas in the Late Permian Changhsingian and biological crisis event at the end of Permian.In this article, by calibrating the conodont zones of different outcrops in the eastern Sichuan Basin, the continuous and complete shallow-water and deep-water sections are selected for further analyses. Then the six types of sedimentary facies and nineteen type's microfacies are classified based on studying the basic characteristics of outcrop sections such as rock type, fabric and lithological sequence, etc. The shallow-water section (Wantanhe, Wuxi, Chongqing, China) were mainly dominated by the reef facies of platform margins in the middle and late Changhsingian stage, and three different types of microfacies were identified, i.e., the reef limestone with sponge skeleton, the microbe-mediated reef limestone with sponge skeleton and the microbialite. The framework of microbialite is mainly made up of the thrombolite formed under the action of benthic microbial communities or constitute by the assemblage of authigenic micrite and bioclastic grains, which was then humped upward to form the microbial mounds with obvious uplifting landform. The creatures such as fusulinid and brachiopods can be observed in the mounds. These mounds differ greatly from the microbialites with finger-like, dendrite and domal structures that were widely distributed in the open platform facies after the mass extinction. The deep-water section (Hongchiba, Wuxi, Chongqing, China) were mainly dominated by the deep shelf-basin facies, where the trace fossils were abundant.According to the types of reef-building organisms, the Changhsingian reefs in shallow-water facies are divided into three types, i.e., the frame reefs formed by the supports of epigenetic frame-building organisms, the microbial mounds formed under the action of benthic microorganisms and with the matrix supporting framework, and the reef knoll, a transitional type. Based on the calibration of biostratigraphy, the organic reef section in Wantanhe, Wuxi, is dissected precisely. Results show that the frame reef and reef-mound were mainly distributed in the Clarkina changxingensis zone in the middle Changhsingian stage; in the late Changhsingian stage, the reef-mound and microbial mound (distributed in the lower Clarkina yini zone) and the microbial mound and microbialite (mainly distributed in the upper Clarkina yini zone) were mainly developed. The epigenetic reef-building biotas such as sponges mainly vanished inside the Clarkina yini zone, which was roughly located below the Hindeodus praeparvus zone. During this period, the ecosystem of bentonic microorganisms was gradually flourishing and occupied the ecological niche of frame reefs, and the gradual succession of the frame-reef ecosystem was finished, which could promote the development of positive landform of platform margins to a certain degree. In the Hindeodus praeparvu zone, the ecosystem of benthic microorganisms eventually collapsed; meanwhile, the quantity of foraminifera faunas in shallow-water facies is decreased rapidly in frequent fluctuations, until the mass extinction. The synergic relationship between the shallow-water benthonic faunas and the paleoenvironmental transition suggesting that the instability of paleoecological environment was developing in shallow-water facies in late ChanghsingianThe bottom of Dalong formation of typical deep-water sedimentary was located in the Clarkina changxingensis zone, which can be classified as a sedimentary system in the middle to late Changhsingian stage. Based on the morphological characteristics, distribution and bioturbation strength of the trace fossils on Hongchiba Section, Wuxi, six types of ichnofabrics in the trace fossils of Dalong Formation can be distinguished through sedimentology analysis. The evolution of ichnofabrics along the vertical direction indicates that the ichnofabrics of Chondrites type-B in the upper part of Clarkina changxingensis zone present the complex tiering structure in shallow stratum. It means that the sign of oxygen deficit has already emerged in deep-water facies during that period. The diverse trace makers in the Clarkina yini zone once flourished again or even evolved, suggesting that the ecological environment in the end of Changhsingian may undergo a temporary improvement; however, due to the fact that these ichnofabrics were all characterized with short colonization windows, this unstable ecological environment was then deteriorated rapidly as the trace fossils in the upper Clarkina yini zone overall disappeared, and eventually the biocoenose communities were extinct.At the turn of Permian and Triassic, the large-magnitude negative shift of carbon isotope emerged all across the world, indicating that a significant change did occur to carbon cycle in the earth's surface system. The variations of marine carbon flux are closely correlated with the evolution of paleoecological environment. Results show that driven by the carbon cycle, marine ecological environment underwent multi-period instabilities in the middle Changhsingian stage. Due to the continuous expansion of paleoecological environment, the biological crisis exhibited multi-stage characteristics, while the benthic communities had obvious synergic characteristics. At the first stage (on the top of the Clarkina changxingensis zone), there was a favorable correlation between Chondrites type-B ichnofabrics in deep-water shelf-bathyal facies and negatively-biased changes of the periodic 513Ccarb oscillations, revealing that the periodic oxygen deficit began to occur in bottom water in deep-water facies zone. At the second stage (in the lower Clarkina yini zone), the changes mainly occurred in shallow-water facies zone. The benthic microbial ecosystem gradually replaced the ecosystem of frame reef and became the primary reef-building biogroups, and 5l3Ccarb began to show fluctuated variations. At the third stage (in the upper Clarkina yini zone), the benthic microbial ecosystem in the shallow-water facies eventually disappeared, and 813CCarb was characterized by oscillatory variations (The average change rate of 513Ccarb are from 5.89%-12.33%) and periodic negatively-biased changes. Meanwhile, the trace makers in deep-water facies zone were almost extinct and 513Ccarb exhibited an obvious negative shift. At the fourth stage (in the upper of Hindeodus praeparvus zone, the main phase of mass extinction), only eurytropic organisms survived in shallow-water facies zone, the quantity of foraminifera faunas are reduced rapidly and 51 Ccarb presented obvious negatively-biased changes. The average change rate of 513Ccarb in platform-marginal facies zone was up to 34.4%, with the average decreasing amplitude of 1.51%o. The micrites in deep-water shelf-bathyal facies zone had abundant iron pyrites indicating a reducing environment. All these results demonstrate that with the mass extinction looming, the environmental pressures in shallow-water and deep-water facies zone increased and the biological crisis was accelerated.Based on above results, this article finally selects the development and evolution of benthonic faunas in different sedimentary facies zone of the study area as the overall research object. By analyzing the multi-environment events, it is proposed that the dynamic elocutionary process of benthonic faunas in the middle and late Changhsingian stage were mainly restricted by oxygen-deficient water masses in deep-water facies, greenhouse effect in shallow-water facies and the decline in pH value of seawater, which the positive feedback reactions were generated and gradually produced greenhouse effect in the earth's surface system. The dynamics mechanism of paleocean benthonic faunas during late Permian may be restrained by the action of volcanic activities. The evolutionary history of benthonic faunas in the Late Permian can fully reflect the interaction between biology and environment when the critical geologic interval was approaching.