Geomicrobiological Process And Paleoceanographical Analysis Of Early Triassic Microbialites And Fish-Bearing Calcareous Nodules In South China

Geological microbes,which dominated the ecosystem during the first 5/6 stages of the earth’s life history,are the earliest and most abundant life forms on the earth.Their interaction with the earth’s environment is ubiquitous.Microbial development was inhibited by metazoans for most of the Paleozoic but flourished globally after the Permian-Triassic（P-Tr）mass extinction.The devastating P-Tr mass extinction（250 Ma）is the largest biotic crisis that earth life has suffered during the geological history.Consequently,the Paleozoic marine ecosystem collapsed,and the marine biosedimentary system changed greatly across the P-Tr extinction.Abundant unusual carbonate sediments widely spread over the world during the Early Triassic.A large number of microbe-mediated sedimentary buildups and structures,such as microbialites,ooids and oncoids were widely developed in shallow marine carbonate sedimentary systems in low latitudes during the Early Triassic.Abundant calcareous nodules containing well-preserved fish fossils occur in mudstone/shale successions of Early Triassic age.Recent studies indicated that the formation of calcareous nodules in mudstones is genetically similar to the microbialites,both of which were closely related to microbial activity.Many authors have carried out comprehensive studies on the genesis of the earliest Triassic microbialites,and the results have greatly enhanced our understanding of these unusual sedimentary products after the P-Tr mass extinction.However,previous studies on microorganisms have only stayed on the basis of morphological comparison.There are still serious differences among different scholars on the formation of microbialites or calcareous nodules and the paleoenvironment due to microscopic observation and analysis of rock slices based on the optical microscope and scanning electron microscope.Conventional microscopic analysis methods cannot deeply characterize and analyze the morphology and geochemical composition of these microorganisms due to the limitation of instrument resolution and analysis accuracy.In this paper,the Nano SIMS was used to analyze the morphology and geochemical composition of the samples in addition with conventional optical microscope,electron microscope and micro-Raman to observe and in-situ analyze.Nano SIMS can be sued for in-situ elemental and isotopic analysis with nanoscale high spatial resolution and high precision.These experimental and technical analysis methods can more accurately characterize the microbial fossils with weak signals due to degradation of organic matter in terms of morphology and isotopic composition in fish-bearing calcareous nodules and microbialites to reveal how microbial activities affect the biochemical processes and mechanisms in the marine.Furthermore,there are abundant pyrite framboids which considered to record the most primitive sulfur isotope information in the sedimentary environment due to the short formation time were the best carrier for studying sulfur isotopes.However,the traditional analytical methods cannot obtain the sulfur isotope of pyrite framboids because its particle size is usually less than 10μm.Nano SIMS capable of ultra-high spatial resolution and high precision in-situ isotope analysis is the only choice to solve this problem.Therefore,the microbialites at the bottom of Lower Triassic in Dashiwei section of Leye area,Guangxi and Yudongzi section of Jiangyou area,Sichuan,and fish-bearing calcareous nodules in Qingshan Section of Jurong area,Jiangsu were studied in this paper.The Nano SIMS was used for the in-situ sulfur isotope analysis of pyrite framboids and the analysis of size statistics and morphological of framboidal pyrite was used to reconstruct the redox conditions of ancient ocean.It reveals the precipitation process and the mechanism of carbonate sedimentary involved by microorganisms especially sulfate reducing bacteria in microbialites.The study of the size,morphology and sulfur isotope of pyrite framboids in microbialite and fish-bearing calcareous nodules was used to reveal the changes of ocean redox conditions and the mechanism of sulfate-reducing bacteria in sulfur cycle after the mass extinction.At present,the in-situ high spatial resolution and high precision sulfur isotope analysis of pyrite framboids by Nano SIMS is a new technology.It is necessary to test and optimize the key parameters of this analysis method before this new experimental technology is widely used in the study of paleo-marine environment and geological microbiology analysis aiming to obtain the high spatial resolution and analysis precision.The new understandings and achievements of this paper are as follows:1.The in-situ sulfur isotope analysis method with Nano SIMS 50L established can obtain higher spatial resolution and analysis accuracy by optimizing key parameters.The three faraday cup detection model（FC-FC-FC）can be set to get better analytical results with Cs⁺source.Under the condition of lateral spatial resolution of 3μm,the analysis accuracy of δ³⁴S is better than 0.2‰（1SD）.Under the condition of lateral spatial resolution of 8μm,the external accuracy of polysulfide isotope Δ³³S can reach 0.2‰.Under the condition of lateral spatial resolution of 5μm,the external precision of polysulfide isotope Δ³³S can be better than 0.5‰,which can be applied to precious and tiny samples.Furthermore,a better real value can be obtained by using the linear correction method of double standard sample.2.The statistical results of the particle size of pyrite framboids in lower Triassic in Qingshan Section of Jurong,Jiangsu Province show that the nodule forming layer and its upper and lower layers are all in anoxic condition which provides a suitable sedimentary environment for the preservation of fish fossils and the formation of nodules.In-situ sulfur isotope composition of pyrite framboids by Nano SIMS in fish-bearing calcareous nodules showδ³⁴S range from -26.13‰to-13.19‰with an average of-18.47‰,which is much lower than that of whole-rock pyrite in the same horizon indicating a stronger sulfate reduction process in the nodules.The results of micro-Raman show that the formation of calcite sparry microspheres was related to the process of sulfate-reducing bacteria decomposing organic matter.The abundance of sulfate reducing bacteria can lead to alkaline microenvironment inside the nodules which is conducive to the precipitation of calcium carbonate.The presence of a large number of sulfate-reducing bacteria can also effectively organize the decay of fish fossils at the same time.The formation process of fish-bearing calcareous nodules is roughly as follows:After the fish died,it fell into the stratum sediments and was wrapped by sulfate-reducing bacteria.During the sulfate reduction process,calcium carbonate precipitation was promoted,and finally compacted and formed calcareous nodules.The distribution of pyrite framboids and calcite microspheres,sulfur isotope composition,sedimentary laminae of nodules and laser ablation analysis show that carbonate nodules have very stable chemical properties which indicates growth pattern is as an equilibrium model.3.In-situ sulfur isotope analysis of pyrite framboids with Nano SIMS shows that pyrite framboids in microbialite have very low δ³⁴S values.The values of δ³⁴S is between-55.88‰and-25.91‰,with an average of-41.70‰in Dashiwei Scetion of Leye area,Guangxi.The δ³⁴S values of Yudongzi Section of Jiangyou area,Sichuan range from -54.12‰to-20‰,with an average of-41.12‰.The sulfur isotope composition of pyrite framboids is heavily depleted in ³⁴S compared with the isotope value of seawater（up to 75‰）which records the maximum fractionation degree of sulfate reduction in seawater.This phenomenon can only appear in the process of sulfate reduction by sulfate-reducing bacteria according to the simulation experiment of modern sulfate-reducing bacteria,and there must be microbial disproportionation reaction in this process.The existence of disproportionation reaction indicates that there is a small amount of oxygen in the microbialite which is consistent with the statistical results of the particle size of pyrite framboids indicates the microbial rock is formed in a dysoxia environment.The multiple disproportionation in the reduction of sulfate process by sulfate-reducing bacteria will consume residual oxygen resulting the local anoxic environment in microbialite is conducive to the formation of pyrite framboids.Micro-Raman analysis showed that there was obvious organic matter residue around the calcite sparry calcareous microspheres which was considered that the geological activity of sulfate reducing bacteria decomposes organic matter and promotes the precipitation of calcium carbonate.Significant difference in sulfur isotope composition between strawberry pyrite and whole rock pyrite because sulfur isotopic composition of whole-rock pyrite is considered to be a signal mixed with the late diagenesis stage.The sulfur isotope composition of pyrite framboids suggests that there may be a larger seawater sulfate pool in the Early Triassic.The results of multi-sulfur isotope Δ³³S analysis showed that pyrite framboids were formed inside the microbialite,without anoxic seawater upwelling.