Permian-Triassic Conodont Correlation From South China And Perigondwana

As the most devastating biotic crisis in earth history, the Permian-Trias sic mass extinction has been investigated using many multi-discipline methods in past 20 years. The eruption of Siberian Large Igneous Provinces (LIP), marine anoxia and global warming have been proposed as causal mechanisms for this crisis. Marine anoxia was widespread at this period and even developed in exceptionally shallow waters. However, exact redox conditions of characteristic shallow water microbialite have been intensely debated. The key dispute is that whether the shallow marine environments was anoxic, fully oxic or something transitional between these states. In order to clarify the date and the debate considering the environment condition of shallow water area at Permian-Triassic Boundary, the size variation of pyrite framboids from South China has been studied calibrated to high resolution conodont biostratigraphy. Since the Permian-Triassic conodont biostratigraphy in Perigondwana is controversial, large conodont samples from Selong section, South Tibet had been collected allowing improved correlation between South China and Perigondwana to be made in this study.Three sections from South China (namely, the Gaohua section, Cili County, Hunan Province; the Xiadaweng section, Guanling County and the Bianyang section, Luodian County, Guizhou Province) and one from Perigondwana (Selong section, South Tibet) have been investigated using high resolution conodont biostratigraphy. Specifically intervals studied in ascending order, H. changxingensis zone in latest Permian, Hindeodus parvus zone and Isarcica isarcica zone in early Triassic from South Chiina and Vjalovognathus carinatus sp. nov. of late Changshingian and H. parvus Zone of earliest Triassic from Selong section, Perigondwana. The biostratigraphic study at Selong section which enable the conodont biostratigraphy correlation between Gondwana and South China to be made precisely. The Permian-Triassic microbialite was partly attributed to latest Changshingian traditionally, but the discovery of H. parvus in the lowermost microbialite let us assign the whole microbialite unit in Gaohua section to the Triassic. The presence of numerous reworked specimens (e.g. M. phosphoriensis-M. rosenkrantzi; J. granti-C. postbitteri postbitteri-C. postbitteri hongshuiensis) in the H. parvus zone from Selong indicated that substantial portions of the Permian stratigraphy (Roadian, Wordian and Capitanian (Middle Permian) and early Wuchiapingian) were initially present in the region prior to deposition of the Selong Formation. The discovery of Vjalovognathus (a cool-temperature tolerant form) in Changshingian strata allowed the youngest Vjalovognathus, V. carinatus sp. nov. to be described, and a possible Permian Vjalovognathus evolutionary trend proposed:in ascending order this is, V. australis (late Sakmarian-early Artinskian), V. shindyensis (early Kungurian), V. nicolli (late Kugurian-Early Roadian) and ultimately V. carinatus sp. nov. (Changshingian). The last species is also found in Zewan Formation (D) in Kashmir allowing correlation of this region with South Tibet.The conodont generic diversity from South China and Selong in South Tibet has been assessed allowing conodont palaeogeographic distribution to be established:Hindeodus, Isarcicella and Clarkina were globally developed. In the latest Permian, the Mesogondolella and Vjalovognathus were only locally presented in Perigondowana area and mixed with small majority of Clarkina. The deep water areas in South China were characterized by a wide range of Clarkina, while shallow water areas were normally dominated by Hindeodus. The sudden input of a large number of Hindeodus into the earliest Triassic, especially H.parvus zone in South China, saw an overturn from Clarkina dominated to Hindeodus-Isarcicella dominated communities. In the Selong section various Hindeodus species occur in the earliest Triassic, similar to the South China record, but in contrast,79% of the early Triassic conodont record is Clarkina. Meanwhile Permian cool water conodont Mesogondolella and Vjalovognathus went extinct in the Permian-Triassic mass extinction event. The geographic size variation of Hindeodus in the H. parvus zone has been studied, both in South China and Perigondwana, specimes show a reduction. This is the first report of the "Lilliput effect" for conodont in higher palaeo-latitude. More specifically, the Selong Hindodus decreased from 1.34 mm2 in the lowermost of H. parvus zone to 0.88 mm2 in the upper part.Petrographic investigation at Gaohua section and Xiadaweng section revealed the microbialite samples from Cili show a history of intense diagenetic modifications with a crystalline fabric of intergrown calcite and dolomite. No pyrite was found in any samples, however iron oxide particles are seen, notably in the microbialite samples, including many with a framboidal morphology allowing their size distribution to be quantified. The pyrite richness in the lower part of Daye Formation varied from modest to high abundant, The mean size of pyrite framboids from microbialite ranged from 7.7 to 9.1 μm. The largest individual framboids often reach diameters of-30 microns. Comparison with modern framboid mean versus standard deviation size plots shows that the Gaohua framboids occur in the mid to upper parts of the dysoxic field, indicating oxygenpoor bottom waters but neither euxinic nor anoxic seafloor conditions. The smallest framboid population was obtained from the vermiculite bed—a burrowed micrite that is interbedded with the oolite that overlies the microbialite, indicating that although benthic oxygen levels were sufficient to allow low intensity bioturbation, dysoxia prevailed at this level. In contrast, the grainstones with giant ooids only contain exceptionally rare framboids, likely suggesting oxic depositional conditions.Permian-Triassic strata at Xiadaweng section is composed of Changxing Formation and Daye Formation. The sediment in Changxing Formation is characterized by yellowish mudstone interrupted by sandstone and black chert. Fossils are extremely abundant and include brachipods, bryozoans, sponges, crinoids, bivalves, trilobites and leaves. These Permian taxia disappear below the Triassic Daye Formation which contains only rare Claraia sp. and Hindeodus. These younger strata consist of laminated micrite interbedded with mudstone. The Petrological study showed a variety of minerals including chlorite and analcite and sometimes magnetite, indicating that the Daye sediment at Xiadaweng contains a volcanic component. Abundant pyrite especially pyrite framboids (although in some intervals framboidals have been oxidesed) are present in Daye Formation. The mean size of the framboids is between 4.7μm-8.5μm with largest example reaching 35p.m. Comparing with framboid mean size versus standard deviation size plots from modern environments shows that anoxic water developed in earliest Triassic at Xiadaweng and was occasionally interrupted by more dysoxic and oxic condition. Overall, a much more oxygen-poor seting than the shallower water areas of South China.Globally, the Mesogondolella and Vjalovognathus conodont lineages went extinct at the end of the Permian ending their more than 40 Myr history. The early Triassic size reduction of Hindeodus species in both South China and Perigondwana and the sudden migrations of Tethyan Clarkina into Perigondwana and the dominated of Hindeodus in earliest Triassic in South China suggests that the rapid global warming rather than the spread of anoxia was the main driver of the conodont changes.