Microbial Geological Processes And The Associated Biogeochemical Cycles Of Carbon, Nitrogen And Sulfur During The Permian-Triassic Crisis Interval

The most severe mass extinction in Phanerozoic is believed to occur during the Permian-Triassic (P-Tr) transition, ca.252million years ago. Deciphering interactions between the biosphere and natural environments at that time is of significance to understand the ongoing global changes of both organisms and environments in modern days, and to support the sustainable development of the mankind himself. Great achievements were made in past decades on the basis of morphological investigations on metazoans preserved as macro-or micro-fossils in end-Permian rocks, greatly favoring the understanding of the pattern and causes of the largest mass extinction. In contrast with these identifiable faunas due to the distinct morphology and structures, microbes in ancient times were hard to preserve or, if preserved, difficult to identify by morphological examination. This leaves geomicrobes largely unknown on both the composition and roles in Earth history including the P-Tr transition. It is worthy to note that microbes are the critical members involved in interactions of organisms with natural environments, and believed to be the important linkage between environments and other organisms. Microbes are also the dominant primary producers in ocean ecosystems. Variations in both faunas and environmental conditions will leave imprints on microbes in marine and oceans, providing a clue to the deep-time global change. Consequently, the geomicrobes are mainly targeted in this thesis to see if they are sensitive to this crisis and the associated changes in the elemental cycles of carbon, nitrogen and sulfur during P-Tr transition.Of all the microbes, the functional groups are of particular importance. Not only because these groups play an active and dominant roles on the changing environments, but also because they could leave a series of records in ancient rocks, making molecular and isotopic investigations possible. Six sections during P-Tr transition in South China were thus investigated here on the molecular and isotopic levels to explore the components and variations of the geomicrobial functional groups (GFGs) and their roles involved in biogeochemistry cycles of carbon, nitrogen and sulfur. Based on these data, the marine chemistry and its influence on GFGs and faunas were inferred. The temporal relationship of the mass extinctions between marine and terrestrial ecosystems is discussed, and its implication for the causes of the end-Permian mass extinction is finally presented.Based on carbon isotope records, the marine primary productivity is proposed in the thesis to increase greatly immediately after the end-Permian faunal mass extinction, indicating the blooms of some autotrophic GFGs. Lipid biomarker records show distinct increase in the abundance of cyanobacteria, anaerobic prokaryotes and other autotrophic microbes. Nitrogen and sulfur isotope data indicate that aerobic N2-fixing cyanobacteria, anaerobic ammonium-oxidizing bacteria and anaerobic sulfide-oxidizing bacteria are the main autotrophic GFGs in the P-Tr transitional ocean, while denitrifying bacteria, sulfate-reducing bacteria and methanogenic archaea are the main anaerobic GFGs. The carbon isotope records imply the fluctuation in the components of the autotrophic GFGs immediately after the main marine mass extinction. This is consistent with the sulfur isotopic and biomarker records. All these geomicrobiological and biogeochemical records underpin the occurrence of fluctuating oceanic environments at that time.Associated with the faunal mass extinction and the GFGs blooms are the large perturbations in the biogeochemistry cycles of carbon, nitrogen and sulfur. A negative excursion of δ13Ccarb is found to be larger (>3%o) before than after (<2%o) the main marine mass extinction due to the subsequent promotion of the primary productivity. In some sections, the δ13Ccarb values were even observed to keep relatively stable or show a slight increase immediately after the main marine mass extinction. On these points, the well-known negative anomaly in A13Ccarb during the P-Tr transition is not thought to be mainly caused by end-Permian marine mass extinction itself. In fact, the carbon isotope records indicate that the carbon cycle perturbation occurred much earlier, ca.0.5Ma, than the end-Permian main marine mass extinction. The stepwise pre-extinction perturbation of the carbon cycle is ascribed here to the episodic volcanism which in turn caused the collapse of the terrestrial ecosystem in the late Late Permian.Slightly different from the carbon cycle, the nitrogen cycle is observed, by a distinct negative shift in815Norg, to begin the perturbation simultaneously with the end-Permian main metazoan mass extinction. This implies a major change in the marine ecosystem. The δ15Norg varying around0%。 indicates that the bioavailable nitrogen was mainly derived from N2-fixation after the main mass extinction. Deficiency of some other bioavailable nitrogen in the ocean might be caused by denitrification conducted by denitrifying GFGs and anaerobic ammonium oxidation when the oceans suffered from severe anoxia. Denitrification will produce a great amount of N2O, one of the important greenhouse gases, which can be of a great potential contributor to the global warming at that time.The sulfur cycle is found to show several large perturbations during P-Tr crisis. A progressive positive excursion in δ34SCAS is documented to occur from the underlying skeletal limestone to the lower part of the microbialite interval, suggesting the presence of elevated anoxic condition. This anoxic event is observed to bracket, and could be the trigger of, the faunal mass extinction. However, the δ34SCAS records suggest that the most severe anoxic event would occur after the main marine mass extinction, featured by the multiple H2S release into the surface ocean. These anoxic events further intensified the marine mass extinction.Large and fast variations in δ34SCAS documented here suggest that the oceanic sulfate concentration was very low during the P-Tr transition, consistent with the GFGs records and organic geochemistry data. Based on the geochemical modeling proposed here to couple carbon and sulfur cycles, the oceanic sulfate concentration is quantitatively evaluated to be less than15%, even as low as3%, of that in the modern ocean water. This value is exceedingly lower than the previous data reported on the basis of the composition of the mineral inclusions. It is further suggested in the thesis that the declined oceanic sulfate concentration would fuel the bloom of methanogenic GFGs which in turn enhances the flux of CH4release into the surface ocean and the atmosphere. Being the greenhouse gas, CH4released would intensify the global warming at that time.Carbon and sulfur isotopic compositions and organic geochemistry records suggest that collapse of the terrestrial ecosystem would predate that of the marine ecosystem, implying that the end-Permian environmental deterioration initiated on the continent, and subsequently expanded to the ocean. Enhancement of acid rain, UV radiation and atmospheric temperature would be the main reasons to cause the temporal difference of the collapses between the two ecosystems. All these effects would result from the eruption of large volcanism, the main potential trigger of the end-Permian mass extinction. Volcanic activity preceding the main marine mass extinction is also supported by the organic parameter. The ratio of coronene to phenanthrene suggests two episodes of high temperature event during the P-Tr transition which may relate to the coeval volcanic activity. The first episode was documented to start during the deposition of skeletal limestone. That is distinctly prior to the main marine mass extinction but synchronous with the collapse of the terrestrial ecosystem.