Characterizing Microbialites In Critical Transitions Of Earth History

A critical transition in Earth history is defined as a period of great biodiversity or mass extinction.The early Cambrian and Permian-Triassic transition are characterized by the Cambrian Explosion and the end-Permian extinction respectively,which are arguably the two most important transitions of Earth history.Microbialites,widely developing during critical transitions,form a kind of bridge between the organic and inorganic realms,as well as a robust tool for deciphering marine signals in carbonates.They are therefore applied in terms of studying palaeocean evolution.This thesis examines the lower Cambrian microbialite in Tangjiahe site,Guangyuan,and the Permian-Triassic boundary microbialite(PTBMs)in Yudongzi area,Jiangyou.The thesis describes their micro-to mega-structures,and interprets their sedimentary environments and depositional processes,as well as analysis of data sets of C isotopes,major elements,trace elements and rare earth elements in microbialites.The main conclusions are:(1)The Xiannudong Formation of lower Cambrian is the oldest unit bearing acheaocyathan-microbial sediments.Its stratigraphic position,however,is not clear in the new scheme for the Cambrian period that has 4 series and 10 stages,but Xiannudong Formation lies in Series 2.In Tangjiahe site,the ?13C increased from-3.5‰ to 0.5‰ across the boundary between the Xiannudong Formation and the underlying Guojiaba Formation,and continued to rise slowly but with fluctuations during the Xiannudong Formation period.In addition,The Xiannudong Formation yields archaeocyaths fossils including Metacyathellus Lepidus?Erismacoscinus zunyiensis?Qinlingocyathus sp.,Nochoroicyathus sp.?Stillicidocyathus sp.and Dailycyathus sp.,indicating it belongs to the Atdabanian stage.Thus it is concluded that Tangjiahe ?13C curve correlates to CARE positive shift between Terreneuvian and Series 2,suggesting the Xiannudong Formation belongs to Stage 3 and its base might correlate to the top of the Terreneuvian.(2)The Xiannudong Formation microbialites potentially provide evidence for the study of paleo-environment during the main episode of Cambrian Explosion.However,most microbialites therein are not preserved well resulting in calcimicrobial feature and microbial frameworks that are poorly known in the literature.However,the Xiannudong Formation in Tangjiahe site contains a well-preserved Epiphyton biostrome and a Renalcis-Tarthinia bioherm.The former was built by dendritic and fan-shape Epiphyton thalli and is interpreted here as being formed in the upper ramp,while the latter was constructed by fan-shape Renalcis-Tarthinia aggregation and interpreted to have formed in lagoonal conditions protected by oolitic shoals.The Guojiaba Formation,the Xiannudong Formation and the Yanwangbian Formation represent ramp(characterizing by mixed siliciclastic and carbonate sediments),carbonate shelf and river-dominated delta respectively.The Guojiaba Formation ramp transformed into the Xiannudong Formation shelf because of the on-going development of the microbial carbonate factory.Based on the comparison with Cambrian microbialites worldwide,it is concluded that calcimicrobes play the most important role in the formation of early Cambrian reef framework development.Also the microbial carbonate factory drove the growth of early Cambrian carbonate platforms.(3)Three conodont zones are discriminated from Permian-Triassic transition in Yudongzi site,in ascending order,they are:Hindeodus praeparvus zone?Hindeodus parvzus zone and Isarcicella zone.Yudongzi PTBMs are approximately constrained in Hindeodus parvus zone.The microbialite is 11 m thick and consists of layered thrombolites,dendritic thrombolites and layered-meshed thrombolites which are altogether largely recrystallised.Normally the clots in thrombolites are made by a kind of saccular thalli(0.3-0.5 mm across),which have a distinctive outer wall but an ambiguous inner wall,and the inner texture with degrading neomorphism.This character is different from any kind of renalcids.In addition,the saccular thalli wall is surrounded or mounted by micro coccoid microbes and Gakhumella.Framework constructed by saccular thalli is open and filled with peloids(geopetal structures occasionally developed).Given spars filled in space between peloids,it is interpreted that Yudongzi PTBMs formed in a lower subtidal setting with relative high energy.(4)PTBMs are widely adopted as a robust tool in numerous studies for deciphering palaeocean redox condition during Permian-Triassic transition period.However,works with high resolution and multiple proxy are rare,as well as contradiction existed in interpretation by different authors.This thesis adopted three kinds of proxies to decipher redox condition of ambient water of PTBMs,which are 1)trends in abundance and size of shelly fauna of PTBMs;2)total organic carbon abundance(TOC)and total sulfur abundance(TS);3)rare earth elements abundance in clots of PTBMs.The results and suggestions are:1)Increase in size and abundance of shelly fauna in the microbialite sequence implies a well-oxygenated environment during PTBMs growth.2)TOC and TS are rather low as 0.07%and 0.31%respectively.The thesis excludes other possibilities of the low values,which are a)low input of organic materials,b)high sedimentation rate,c)hydrocarbon thermal transformation,d)low iron influx.thus considers PTBMs forming in oxygenated and agitated seawater.3)Despite clots in the most upper sequence being strongly diagenetically altered,the rare earth elements in Yudongzi PTBMs are generally characterized by Ce negative anomaly,depletion of LREE and rich of HREE,indicating a well-oxygenated shallow marine setting,as well as suggesting shallow marine in Tethys was not influenced by anoxic events immediately,at least,was oxygenated during the growth of PTBMs.