Microbialites Around The Permian-Triassic Boundary In South China And South Qinling:Geological Processes Of Microbial Ecosystems

Intensive volcanic activities,rapid temperature rise,extensive oceanic acidification,and anoxic events all occurred in association with the largest end-Permian mass extinction.Microbialites dominated the depleted shallow-marine ecosystems of the Tethys Ocean in the aftermath of end-Permian mass extinction.Previous researches on microbialites mainly focused on their ages,growth pattern,community structure of metazoans,and ecological implications.However,the structure on benthic microbial community and their mineralization remain unclear.In this thesis,the microbialites and calcimicrobes from South China Block and South Qinling micro-plate were studied from sedimentological,mineralogical,palaeobiological,isotope geochemistrical aspects to reconstruct the forming process from the benthic microbial community to microbialites and to understand their controlling factors.Microbialites in the studied intervals are all temporally higher than the conodont Clarkina meishanensis Zone.The main part of microbialites in South China is in the Hindeodous parvus Zone,and corresponds to the low values of ?13Ccarb.The stromatolites in the Qinling micro-plate is located in the Isarcicella isarcica Zone,which is the youngest PTB microbialites to date.The microbialite sequence in South China is characterized by stromatolites-tabular thrombolites-spotted thrombolites-branched thrombolites in ascending order,which implies the deepening in association with the rapid transgression in the early Triassic.The microbialites in the South Qinling area are dominated by stromatolites,which are related to the regional tectonic movement.Massive calcimicrobe Polybessurus occur in the microbialites from South China.Polybessurus are remains of calcified extracellular polymeric substances(EPS)from the coccoid cyanobacteria.Their arrangements largely influence the growth pattern of microbialites.The upright Polybessurus in the stromatolites are well preserved in layers to form a dark micritic lamina.Polybessurus clumps in the thrombolites gradually transform into the lobate structure that subsequently turn to clots in the diagenesis.Therefore,the clot of the thrombolites is a product of diagenesis.The digitate clots are special structures constructed by Polybessurus in the supersaturated seawater,combining organic and inorganic process.Two ways can result in the calcification of Polybessurus.(1)Calcification of EPS driven by photosynthesis of cyanobacteria.The calcification occurred in cells in vivo,and the precipitation of carbonate minerals on EPS increase the preservation potential of these biological structures.(2)Calcification driven by sulfate reduction.EPS of cyanobacteria are degraded by sulfate-reducing bacteria into low-molecular-weight organic compounds,on which carbonate minerals induced by sulfate reduction nucleate.Therefore,this calcification can only mediate precipitation of micrites without preservation of biological structures.Three indispensable controlling factors,biological factors,chemical conditions of seawater,and depositional conditions,resulted in the global occurrence of microbialites around the Permian-Triassic boundary.The onset of PTB microbialites was controlled by extensive regression at the end of the Permian,the big loss of benthic metazoans,and supersaturated seawater respect to carbonate minerals.Regression provides a broad habitat for benthic microbial communities;less predation pressure contributes to the long living of benthic microbial communities;supersaturated seawater is conducive to calcification.The regional changes of bathymetry terminated the development of PTB microbialites,while biological factors and seawater chemical conditions have no considerable change.