Paleo-oxygenation Facies In Shallow-marine Carbonate Platform Across The Permian-Triassic Boundary In South China: Evidences From Pyrite Framboids And Pyritized Fossils In The Microbialite

Biotic mass extinction near the Permian-Triassic boundary with dramatic environment evolution is a globally epoch-making event, which is considered as the transitionally key point in the process from the destruction of Paleozoic marine ecosystem to the reconstruction of Mesozoic marine ecosystem. Numerous hypotheses have been proposed for this crisis, including extraterrestrial hypothesis and terrestrial hypothesis. At the same time, many researches focused on marine environmental effect caused by this global event from various respects. "Marine enviroments become anoxic after the end-Permian mass extinction" is one of the most widely held opinions.Research on the anoxic event in the geological history can be deduced from different evidences recorded in sedimentary rock. All the information reflecting paleo-redox condition can be attributed to the pale-oxygenation facies. Savrda and Bottjer (1987) suggested that palaeo-oxygenation facies includes the dissolved oxygen content in the bottom water, and the combination of petrological, biological, geochemical characteristics, when sediment deposited. It is an important component of sedimentary facies.The researches on "Permian-Triassic marine anoxic envent" are usually documented by sedimentary facies such as fine lamination, dysaerobic trace fossils, carbon and sulfur-isotope excursions and biomarkers. These provide important evidences for marine environment after the end-Permian mass extinction. But the extent of oxygen deficiency in the bottom water can not be identified by these evidences.However, the size distribution of pyrite framboids has been proved to be a reliable indicator for oxygen level in the bottom water. The size distribution of pyrite framboids is closely related to oxygen deficient extent, not only in various modern marine environments, but also in ancient sediments. So. it is a powerful indicator for oxygen deficient extent in the bottom water.Microbialites formed immediately above the end-Permian mass extinction boundary occur widely in shallow-marine carbonate platform in South China. Large quantities of pyrite framboids preserved in the microbialites provide a very good opportunity for palaeo-oxygenation facies study in shallow-marine carbonate platform after the mass extinction.We choose Laolongdong section, in Chongqing Municipality, and Dajiang section, in Guizhou province, South China, for the research of palaeo-oxygenation facies when microbialite deposited, using the indicator of size distribution of pyrite framboids. Shallow-marine environment is well-oxygenated, as demonstrated by high abundance and diversity of fossils and no pyrite framboid preserved in the bioclastic limestone below the mass extinction boundary in two sections. Large quantities of pyrite framboids, with size distributions comparable with those from modern sediments underlying dysoxic water columns, are preserved in the microbialites immediately above the mass extinction boundary. Detailed study of size distributions of 1374 pyrite framboids found in 9 horizons with the mean diameters from 7.63 to 9.48μm, indicate that the microbialite of Laolongdong section should have been deposited in lower dysoxic water column. In Dajing section,1764 pyrite framboids found in 14 horizons with the mean diameters from 5.26 to 8.3μm, indicate that the water column was anoxic to lower dysoxic, and more deficient in oxygen than Laolongdong section, when the microbialite deposited.By and large, the microbialite was deposited in oxygen-deficient bottom waters. However, shelly benthic fossils interbeded in the microbialite indicate sufficient oxygenation for their growth. The apparent contradiction between richness of pyrite framboids and shelly benthos may be explained by fluctuating marine conditions alternating between short-term of oxygenated state and poorly-oxygenated state In such a turbulent environment, only cyanobacteria and a few foraminifers, ostracodes, gastropods, and worm tubes can survive.Furthermore, a pyritized fauna, including foraminifers, ostracodes, gastropods, and worm tubes, is found in the microbialite in Laolongdong section. The occurrence of abundant pyritized fossils is possibly caused by a high level of dissolved iron in the bottom water. Increased dissolved iron in the water column has important consequences for primary production flourish in large portions of the modern ocean. Microbial flourish, especially the nitrogen-fixing cyanobacteria bloom in the shallow-marine environments after the end-Permian mass extinction, may be promoted by the high level of dissolved iron in the water column.To conclude, the size distributions of pyrite framboids indicate that the widely distributed microbialites in South China should have been the product of widespread dysoxia in shallow-marine carbonate platform immediately after end-Permian mass extinction. The unstable dysoxic shallow-marine may be alternated with periodic oxic events. The occurrence of pyritized fossils may be resulted from high level of dissolved iron in the bottom water when the microbialite deposited.