Research Of Carbon Isotope Excursion In The Late Permian At Xinmin Section, Guizhou, South China

Stable carbon isotope(813C) is widely used in the study of stratigraphic division and comparison, paleo-environment, and geological events. The variation ofδ13C accompanied with environmental changes and life activities, and the fluctuation of carbon isotope composition in stratigraphic units can reflect the perturbation in the global carbon cycle directly or indirectly. The abnormalδ13C is usually related to geological events that had great influence on life evolution such as sea level change, volcanism, ocean anoxia, asteroid impacts and so on, which is often consistent globally or in a wide region.The Permo—Triassic mass extinction was the greatest mass extinction of the Phanerozoic, associated with global carbon cycle disturbance. Although the study of carbon isotope during Permo—Triassic transition has been very thorough, there are still many differences and inadequacies. Those researches are relatively abundant on shallow-water carbonate platform sections in the South China even in the world, but still need more improvement in deep water areas. Some records usually missed or can not be easily identified in shallow water sections due to high degree of stratigraphic condensation and/or subject to limitations of the stratigraphic sequences. The deep water marine PTB sections were well developed in the middle of Guizhou province. The Xinmin section may be one of the best sections for studying the curious geological events during this transition because of its well exposure, uninterrupted sediments, light change of the lithology, well development of claystone and fauna, and clear lithological boundary, all of which may have record much more information than shallow-water sections. Therefore, the study of carbon isotope variations on deep-water section is an integral part to comprehensivly understand the evolution of environmental—biological events in the P—T transitional period.Bulk organic and inorganic isotope compositions and the abundance of acritachs, algae and microbes were examined from the Permo—Triassic boundary at Xinmin section, Guizhou. Based on conodont biostratigraphy and sendimentary characteristics, we study the variations of carbon isotope at Xinmin section, and compare it with that on Global Stratotype Section and Point Meishan section, supplying the carbon isotope data on deep—water sections. We will focus on the aspects that affect the carbon isotope fractionation in order to discuss the great changes of biological—environmental events during the P—T transitional period. And with the variations ofδ13Ccarb andδ13Corg, we will utilize the productivity model to assess the paleo-productivity and organic matter burial ground conditions of deep-water section.(1) Xinmin section and Meishan section can be compared in the same time scale. The beginning ofδ13Ccarb negative excursion below the mass extinction boundary(MEB) in the Late Permian implied that carbon cycle perturbation preceded the mass extinction event itself.813Corg data highlight environment changes in the latest Permian and show the following features:a gradual decreasing phase begins in Clarkina yini zone, and a sharp decreasing phase occurs during Clarkina meishanensis zone, indicating that the marine environment changed from the gradual transformation to mutation process.(2) The clay rocks near PTB in Xinmin section are formed from vocanic felsic rhyolite, generated in within-plate. The volcanic activity in one phase may make significant change ofδ13Corg value in a relatively short time, but no dramatic impact on marine environment for longer time scale. During the period following the mass extinction, the magnitude fluctuation ofδ13Corg was not directly to the volcanic layers. It implied that the ocean carbon cycle system changed greatly after the mass extinction.(3)δ13Corg exhibit a positive peak before the sharp decrease above the event horizon, and the flourishing of green sulfur bacteria(GSB) may result in this positive peak:because we found large amount of frambiodal and granule pyrite. The deep anoxic bottom water upward excursion may cause photic zone euxinia(PZE) which indicate the photic zone layer of the ocean is anoxic and riched in H2S. Then some microbes such as green sulfur bacteria may become dominant in the photic zone through this time, causing this positive peak ofδ13Corg in MEB layer. The temporary peak of 813Corg value exist in Tethyan PTB section, but relatively scare in the pelagic ocean PTB sections, suggesting PZE developed strongly only in the limited sea such as Tethys. However,δ13Corg decreased sharply near the MEB layer in Xinmin section with global consistency, which could be caused by the combination of volcanic activity, marine biological breakdown and a sudden release of methane hydrate.(4) Residual TOC is determined by the combinations of the primary productivity, organic carbon burial fraction(forg) and burial redox conditions. But the data in Xinmin section show that the paleo-productivity plays a dominant role on the value of residual TOC. This conclusion still needs further proof:we found that forg is mainly controlled byδ13Ccarb by Kump's model, in which it concluded increased volcanism leads to only small changes inδ13Ccarb, and set the carbon isotope value of volcanic activity and weathering asδw=-5‰. That is to say, a series of secondary environmental effects caused by volcanism were ignored, such as "volcanic winter", ocean acidification, smoke and other toxic substances and so on, all of which had great impact on the marine environment. However, large-sized volcanism in the Late Permian could result in serious perturbation to marine carbon cycle. Therefore, the model still need to be further improved on calculating organic carbon burial fraction quantitatively or semi-quantitatively during this special geological period.