Palaeoenvironment Change In Shallow-marine Carbonate Platform Across The Permian-Triassic Boundary In South China And Its Possible Cause

The Permian-Triassic transition is an important turning point in evolution history of the earth.?90%of marine species and?70%of terrestrial species are wiped out from our planet in the greatest mass extinction event during this period.Tropical shallow marine region is one of the hardest hit places in the biotic crisis,because marine lives flourish here.The relationship between the mass extinction event and environmental change in tropical shallow marine region is one of the research hotspots both at home and abroad.A lot of research work has been carried out about the mass extinction event,but study about environmental change in this region is comparatively weak and controversial.Here,we present the major,trace and rare earth elements evidences,pyrite framboid,pyritized fauna and sulfur isotopic compositions of the microbialite sections to document the environmental change in tropical shallow marine region in southern China,and explore the possible cause of environmental change during the Permian-Triassic transition.In order to explore the impact of enhanced terrigenous inputs on environment change in shallow marine region,we analyzed the major,trace,and rare-earth element compositions of Laolongdong section which is located near the Kang-Dian old land and very susceptible to terrigenous fluxes.The major,trace,and rare-earth element profiles for Laolongdong record variations in terrigenous fluxes to a shallow-marine site during the Permian Triassic transition.Given the substantially higher siliciclastic contents of the microbialite?Unit 2?and mudstone?Unit 3?relative to the skeletal limestone?Unit1?,these element rates translate into siliciclastic flux ratios of 3.9×and 88×?Al₂O₃?,3.9×and 86×?Th?,2.4×and 92×?Sc?,and 2.0×and 51×??REE?for Unit 2/Unit 1 and Unit 3/Unit 1,respectively.Thus,these proxies indicate a?2×to 4×increase in siliciclastic influx to the Laolongdong environment from the pre-MEH skeletal limestone to the microbialite?Unit 1 to Unit 2?and a further?20×to 40×increase from the microbialite to the overlying mudstone?Unit 2 to Unit 3?.The sedimentary and geochemical records of the Laolongdong section demonstrate two pulses of the enhanced terrigenous input to the shallow-marine environments on the Yangtze carbonate platform during the Permian-Triassic transition.The first pulse,representing a?2×to?4×increase,coincided with the period of microbialite deposition above the MEH.The second pulse,representing a further?20×to?40×increase,coincided with deposition of mudstones above the microbialite.The two pulses of enhanced terrigenous influx to shallow-marine carbonate environments following the end-Permian mass extinction may have been closely related to contemporaneous changes in terrestrial floras.Paleobotanical studies have identified two stages of destruction of land vegetation during the Pemian-Triassic transition in South China.Although detailed stratigraphic correlations between terrestrial and marine sections are hard to establish owing to differences in lithology and fossil content,present constraints on the timing of land vegetation changes are permissive of correlation with the two pulses of terrigenous inputs to marine sections.Oxygen deficiency in water column is often considered to cause the mass extinction,but there still have many arguments on the palaeoredox change of water column in shallow marine environment.To further explore the palaeoredox change during the Permian-Triassic transition in shallow marine environment,we analyzed the pyrite framboid size of Dajiang section.Whilst the Dajiang section testifies to the expansion of dysoxic waters onto the shallow-marine carbonate platform immediately above the mass extinction boundary in the southern part of South China block,pyrite framboids of the Laolongdong,Xiajiacao,Xiushui and Cili sections suggest a similar event also occurred on the block's northern margins.Oxygen-poor conditions were widespread across shallow-marine carbonate platforms of the equatorial South China block.The co-occurrence of oxygen-poor conditions in shallow-marine environments in diverse palaeogeographic locations shows the global significance of this anoxic event in the Permian-Triassic transition.The intensity of oxygen depletion on South China block is closely coupled to that of the well-known Permian-Triassic?¹³C_carbnegative shift.The close correlation between the intensities of the negative?¹³C_carbshift and oxygen deficiency during the Permian-Triassic transition of South China block suggests that the anoxic events on the shallow-marine carbonate platform may be caused by the upwelling of anoxic deep-ocean waters from Panthalassa.We firstly documented a pyritized fauna in the shallow-marine carbonate environment in South China during the Permian-Triassic transition.In the Laolongdong section,this pyritized fauna shows obvious regularity for its occurrence and disappearance with the microbialite.The occurrence of this pyritized fauna may indicate special geochemical properties of water column in shallow marine region immediately after the end-Permian mass extinction.Below the mass extinction boundary?MEB?,no pyritized fossil is found in the skeletal limestone which preserves a non-pyritized fossil assemblage with high abundance and diversity,such as fusulinid and non-fusulinid foraminifers,calcareous algae,Tubiphytes,crinoids,brachiopods,bryozoans and calcisponges.By contrast,a large number of well-preserved pyritized fossils are found in the microbialite immediately above the MEB.Moreover,pyritization generally appears in almost all metazoan species which proliferated in the aftermath of the mass extinction,such as microconchids,gastropods,foraminifers?Rectocornuspira sp.and Earlandia sp.?,ostracods and bivalves.Similar to their sudden appearances,the mud-limestone above the microbialite witnesses the rapid disappearances of pyritized fossils.Instead some non-pyritized ostracods still appear.The pyritized fauna of the Laolongdong section is an artistic product of the special geochemical properties of seawater immediately after the end-Permian mass extinction.The evidences of U_auth,V_auth,Mo_auth,?³⁴S_CAS,and?³⁴S_pyritedemonstrate that the combination of the dysoxic water column with low sulfate concentration,dissolved iron rich pore water could play an important role in the formation of the pyritized fossil.These special geochemical properties of seawater may play an important role in the microbialite formation.Siberian large igneous province is often considered as the ultimate cause of the mass extinction event,and intermediate-acid volcanic eruptions during the Permian-Triassic transition are often overlooked.In order to explore the relationship between the volcanic activities in southern China and environmental change in shallow marine region,we analyzed the major,trace,and rare-earth element compositions of Cili and Zuodeng section.The Eu anomaly evidences from Zuodeng section reveal an obvious positive Eu anomaly excursion along with mass extinction event in shallow marine environment during the Permian-Triassic transition.The positive Eu anomalies indicate intense hydrothermal activity in shallow marine carbonate platform in Nanpanjiang basin.Because the strong associations between hydrothermal activity and volcanic action in back-arc basin,the tight coupling between the mass extinction event and intense hydrothermal activity in Nanpanjiang basin suggests that dramatic volcanic eruptions caused by plate subduction of Paleo-Tethys Ocean may play an important role in the mass extinction event.The Ce anomaly evidences of Zuodeng section provide new evidence for the redox condition variation and its relation to the mass extinction event in shallow marine environment during the Permian-Triassic transition.The rapid positive Ce/Ce*ratio excursion indicates oxygen deficiency in water column when the microbialite deposited,compared with well-oxygenated environment for bioclastic limestone blow the mass extinction boundary.The Ce/Ce*ratio results of Zuodeng section are consistent with other evidences for palaeoredox change in shallow marine region mentioned above.The close relationship between the volcanic activities and palaeoredox change in shallow marine region indicate that the intense volcanic eruptions in southern China may play an important role in the mass extinction event during the Permian-Triassic transition.The microbialite sections in shallow marine environment act as a bond to reestablish the relation between volcanic eruptions,biological evolution and palaeoenvironmental change in southern China.The intense volcanic eruptions in southern China may play an important role in the two pulses of terrestrial ecosystems collapse,enhanced terrigenous influx and mass extinction.