Fluctuations Of Redox Conditions And Ecological Responses Across The Permian-Triassic Boundary In Different Paleogeographic Settings Of Middle&Lower Yangtze Region

There are several mass marine extinction events in the earth’s history, and the Permian-Triasssic event is the most extensive, which witnessees the change of marine ecosystem structure from the Paleozoic type to the Modern and Mesozoic one. The research on biological evolution and environment changes during the Permian-Triassic (P/Tr) transition had become one of hottest topics in the geosciences. Until today, however, the cause and mechanism for this event are still controversial, mainly because most previous works focused on a single section and/or an independent area. Thus, it is difficult to detail the geological process in different sedimentary environments during the P/Tr transition, as these sections separate from each other and belong to different paleogeographic backgrounds.Several continuous P/Tr marine sections are well exposed in the north Yangtze region. We selected seven adjacent sections from shallow platform to deep shelf facies, and thus offer a good opportunity to study the sedimentary evolution and ecological responses during the P/Tr transition.The sedimentary sequences vary from different sections during the P/Tr transition in the north Yangtze region. During the Late Permian Changhsingian, a huge set of thick-bedded algae-foraminifer packstone was deposited on the shallow-marine carbonate platform, siliceous wackstone was mainly deposited ahead of the reef slope, and the deep shelf setting is mainly composed of cyclic wackstone-mudstone-clay sediments. After the end-Permian mass extinction, the platform was exposed during a big regression where a hiatus widely occurred. Obvious volcanic clay deposited on the mass extinction boundary that can compare with the volcanic clay in Meishan section (GSSP). In the Early Triassic, the platform started depositing again after a transgression where microbialites widely developed. The microbialites range in thickness from several meters to over ten meters. The microbialites were covered by a dozen-meter thickness of oolitic limestone interbedding with thin layers of vermicular limestone and micrite in the Early Triassic. Different from shallow platform, calcareous mudstone and marlstone accumulated above the volcanic clay, and changed to the thin-bedded micrite and mudstone in the Early Triassic. It is worth noting that allochthonous and autochthonous deposits have been identified in the Changxing formation of the Meishan section B.Pyrite framboids have been widely regarded as an effective authigenic mineral index to quantify redox conditions in both ancient and modern seawater. Thus, our research mainly focused on pyrite framboids to detail the paleoredox changes during the P/Tr transition in the north Yangtze region. Our research reveals that 1) during the Late Permian the shallow platform and some glacis were characterized by a normal and oxic bottom water, and the deep shelf was lower dysoxic or anoxic.2) After the mass extinction, owing to the volcanism, terrestrial clastation and sea-level fluctuation, the paleaoredox was rathe complex. Oxygenation event occurred immediately after the end-Permian mass extinction in deep water and dysoxic or anoxic environment sustained in the shallow platform.3) Then, the result suggests that the oceanic environment returned to the dysoxic conditions in deep water, except the Jingshan area.The environment changes left significant marks on both biological and ecological evolution in different paleogeographical settings during the P/Tr transition. Calcareous algae and benthic foraminfers largely yielded from the Changhsingian packstone on the shallow platform in the area that represent an abundance and high diversity ecological system during the Late Permian. This normal shallow-marine ecosystem was well coincided with the normal and oxic environment inferred by pyrite framboids and trace elements during this period. After the mass extinction, a special ecosystem mainly composed by abundant microgastropods developed in the extra shallow water, suggesting the opportunity species flourished on the shallow platform after the mass extinction. Microbialites also yield an abundant but low diversity ecosystem, which is mainly consisted of cyanobacterial as foundation, and some euryhaline organisms such as microgastropods and ostracodes. This special system was well coincided with the severe environment on the shallow platform in the Early Triassic. Different from those on the shallow platform, the deep shelf ecosystem is consisted of radiolarians and calcareous spheres as planktonic organisms, foraminfers and deep-water sponges as benthic organisms, representing a bottom-dysoxic planktonic ecosystem as a whole. While planktonic organisms were flourishing during the period of the intensive volcanism and enhanced infusing of terrigenous detrital, benthic foraminifers and sponge spicules decreased approaching the extinction horizon, reflecting an intensifying anoxic condition at the end of Permian. Ammonites and bivalves abundantly yielded from mudstone after the mass extinction, suggesting the enough ecological space promoted the opportunity species flourishing after the mass extinction. Rare fossils were found in the thin-bedded micrite and mudstone that may be related with the high-stressed environment in the Early Triassic.In a word, according to the research of stratigraphic sequences, authigenic mineral (pyrite framboid), sedimentary geochemistry, our results reveal that both the sedimentary conditions and redox environments complexly changed from shallow platform to deep shelf during the P/Tr transition. Coincidently, the ecosystem also remarkably changed in different settings during this transition. The mass extinction event mostly destroyed the ecosystem in both shallow and deep water, but also opened sufficient ecological space for survivors to occupy after the extinction.