Mineral And Geochemical Explaination Of Boundary Events

There were several mass extinction events in the earth's history, and the Permian-Triassic event is one of the most extensive, which destroyed 90% marine species, 70% land vertebrate genera and most land vegetation. The events related to mass extinction become hot topics of world science. These studies will help to understand the evolution of life and earth, and supply thoughts and methods to the current environment changes and living conditions of human being.The P-T mass extinction pattern, together with depositional records suggest a series of great geological events abruptly happened at that time, including Siberia basalt volcanic eruption, bolide impact, anoxic event, acid rain, the release of methane hydrate, the rise and fall of sea level, overturn of oceanic seawater, and so on. The key factor in understanding the P-T mass extinction is the relationship between these events and mass extinction. Up to now, volcanic eruption and bolide impact are two main hypotheses. Though many advances are achieved, contradictions and disputes on the records and boundary events are still left.Meishan section (GSSP, Global Stratotype of Permian-Triassic Boundary Section and Point) is one of the most extensive studied P-T boundary sections in the world. Abnormal depositional records, including carbon isotope, oxygen isotope, sulphur isotope, strontium isotope, trace elements and rare earth elements, biomarker, gypsum, fullerene, noble gas, high quartz, microspherule, iridium and other platinum group elements, meteorite fragments, have been searched and studied for many times.The international cooperative item "Cooperative Investigation of Permian-Triassic Events (PTEs) on new core samples from Meishan, South China", led by Prof. Jin from Nanjing Institute of Geology & Palaeontology, CAS and F. Kyte from Institute of Geophysics and Planetary Physics, University of California Los Angeles USA, is intended to clarify these problems on PTEs. They had launched two drills near Meishan section D and obtained fresh samples across P-T boundary. Blind tests and comparisons will be taken and depositional records of bolide impact be concerned.As participants, we carried out systemic study on the Meishan P-T boundary sediments and focused on the mineralogy and organic geochemistry. This paper mainly concerns three aspects: (1) Extracting elemental carbon (or organic carbon) from Meishan P-T boundary sediments, including polycyclic aromatic hydrocarbons (PAHs), black carbon (BC), nanodiamond and fullerene. (2) The examination of pyrite framboids in each bed, including their size and abundance. (3) The search of impact evidences, such as shocked quartz and nanodiamond.Two innovations are made in this paper: (1) First systemic examination of pyrite framboids in Meishan P-T boundary beds, supplying key evidence of dysoxic event; (2) First analysis of BC and combustion-derived PAHs in the P-T boundary, and pointing out intensive wildfires occurred at the end of Permian in Meishan. The detailed results list as below.(1) Sizes and abundances of pyrite framboids were determined from acid residues and in situ on polished sections of the Meishan P-T boundary samples. Abundant fine pyrite framboids occur in pyrite lamina, Beds 25, 26, 28 and 29 in the Meishan PTB with the mean size of 4.6-8.7μm, reflecting the dysoxic depositional environment. The dysoxic event coincided with the mass extinction line (the pyrite lamina, i.e. the bottom of bed 25). Thus, the dysoxic event probably was the main reason for the mass extinction and prolonged recovery of biomass. (2) There exist large quantity of BC and combustion-derived PAHs in the samples of MSC23-2, MSC24c, MSC24ec and bed 26 in the Meishan P-T boundary section. The indicators of BC,BC/TOC,δ¹³C_BV,PAHs revealed frequent wildfires occurred at the end of Permian, most extensive wildfire coinciding with event beds (beds 25 and 26), and long time of wildfire gap in the early Triassic. Event beds wildfire was the last and most extensive at the end of Permian. It was likely a long combustion and may have different phases. The sources of fire probably included land vegetation, peat, coal and methane hydrate. Event beds wildfire accelerated the environment deterioration and the mass extinction. It most likely resulted from the abrupt climate and environment change inducing by violent Siberia basalt volcanic eruption. Abundance with periodical fluctuation of BC and PAHs in Changshing formation vs deficiency with sooth change in upper Yinkeng formation showed the flourish and decline of land ecosystem before and after the mass extinction.(3) The discovery of high quartz, zircon and iron spherule in the event beds (bed 25) discloses the volcanic origin of boundary clay. The discovery of fullerene in the pyrite lamina (the bottom of bed 25), coinciding with the occurrence of dysoxic event and mass extinction, probably indicates the intensive wildfire. Furthermore, nanodiamond didn't extract from Meishan boundary clay.(4) In summary, violent volcanic activities occurred during P-T transition resulted in global environmental changes and intensive wildfires, and consequently dysoxic event occurred. The biomass of land and ocean may be seriously destroyed in the interactive chain.