| About 252 million years ago,the Permian-Triassic mass extinction event occurred,which was one of the largest mass extinctions since Phanerozoic,accompanied by environmental and climate disasters.It has been one of the focuses of earth science.This event greatly changed the marine and terrestrial ecosystems,and the cause of this mass extinction has been widely debated.In recent years,scholars have put forward a lot of new hypotheses and found some relevant evidence,but still have not got a conclusive conclusion.Moreover,after this extinction event,ecosystems experienced an unusually slow recovery during the Early Triassic.Such extreme conditions occurred several times during the Early Triassic,and it took about 5 million years for the ecosystem to become stable.However,scholars have not made a conclusion for the reasons about the unusual recovery,and have put forward a variety of hypotheses,such as high temperature,anoxia,sulfate,ocean acidification and so on.Ocean acidification has drawn more attention by scholars in recent years,and calcium isotopes have been accepted by more and more scholars as evidence of ocean acidification.However,the research on calcium isotopes is developing,and with developing,many factors affecting calcium isotopes have been discovered.According to the latest research,ocean acidification is one of the main reasons for the extinction event across the Permian-Triassic boundary.However,the relationship between calcium isotopes and ocean acidification is still unidentified.For the reason that calcium isotopes are affected by many factors,it is impossible to simply attribute isotopic drift to ocean acidification.Therefore,quantitative use of model is necessary and it’s also an indispensable part in the study of calcium isotopes.In this paper,two marine carbonate sections,Chaohu in Anhui Province and Guryul Ravine in Kashmir,India,are selected as research objects.The Chaohu section,located in South China,is a candidate section for Global Standard stratotype section and point of Indian-Olenekian boundary.The Guryul Ravine section is located in the middle–high latitudes of the southern hemisphere,which is far away from the Siberian Large Igneous Province.Studying this section can better explain the scale and mechanism of the Early Triassic extinction event.By the cathodoluminescence analysis of the two sections,we consider that the Chaohu section has been affected by the diagenetic alteration,while the Guryul Ravine section preserved the original information of the seawater.Diagenetic alteration has always been a difficult problem of calcium isotope analysis in carbonate rocks.According to previous studies,contemporaneous sedimentation and post-sedimentation can change the calcium isotope value in carbonate rocks.Recrystallization,dolomitization and neomorphism can change the initial calcium isotope values in carbonate rocks.The resident time of calcium in seawater is about 1Ma,and the mixing time of seawater is about 1ky.Therefore,the global calcium isotope value in the ocean can be considered as uniform during the same period.However,due to the regional diagenetic alteration,the calcium isotope values of different sections can be different.By comparing with other sections and modeling the diagenetic alteration of Chaohu section in Anhui province,we can get the following understandings:(1)If the carbonate rocks have been affected by post-depositional diagenesis,the calcium isotope value and the major and trace elements will be changed.For example,Mg/Ca can show the dolomitization extent,and Mn/Sr can provide evidence on whether the samples have been affected by atmospheric and fluid alteration.Therefore,it is necessary to judge the diagenetic alteration of the samples before making calcium isotope of carbonate rocks to exclude the samples which has been influenced by dolomitization or fluid alteration.(2)The calcium isotope value of carbonate is affected by the ratio of aragonite to calcite.The distribution coefficient of aragonite is 0.6‰ higher than that of calcite,so the mineral phase transition of aragonite and calcite is also one of the reasons for the calcium isotope change.It can be judged by Sr/Ca ratios.(3)In addition to post-depositional diagenesis,the initial aragonite to calcite ratio and the recrystallization also resulted in different calcium isotope values in different sections during the same period.The initial ratio of aragonite to calcite can be modeled by the water-rock interaction model of carbonate rocks to explain the calcium isotope values and evaluate the degree of diagenesis.As there are many factors affecting calcium isotope,the traditional analysis by observing calcium isotope change through the trend and drift is not enough to provide an objective and comprehensive explanation.Therefore,it is an essential step for the calcium isotope analysis to model the calcium flux and calcium isotope to conduct quantitative simulation analysis,which can find the appropriate factors that may affect the calcium isotope values.On the basis of many previously published data(such as accurate biostratigraphic information to establish a high-resolution time-stratigraphic framework),we performed a high-resolution calcium isotope analysis of the Guryul Ravine section and obtained the following understanding:(1)Calcium isotope has obvious drift in Permian-Triassic Boundary(PTB),IndianOlenekian Boundary(IOB)and Smithian-Spathian(SSB),and the drift range decreases as the time went by.(2)The calcium isotope drift in the PTB period of the Guryul Ravine section is much larger than the calcium isotope records in South China.At the same time,the carbon isotope and calcium isotope decoupled.According to the modeling result,the continental weathering change is the best explanation of the calcium isotope drift in this period.It is basically consistent with the previous research conclusions in South China.(3)The calcium isotope in the IOB period of this section has a negative shift of about0.45‰,which corresponds to the strong positive shift of the carbon isotope and is consistent with the carbon and calcium isotope type of the Dajiang section in South China.However,there were no reports of mass extinction events in this period.According to the modeling result,it is considered that changes of the sedimentary flux of carbonate rocks is the main reason for the calcium isotopes changes in this period.Besides,there was a possibility of geological historical events in this time.(4)The calcium isotope in the SSB period has a negative shift of about 0.2‰,which is basically consistent with the carbon isotope and calcium isotope characteristics in the South China sections during this period.Through modeling,we believe that ocean acidification events have also been experienced at the SSB boundary,resulting in environmental changes.This event is directly related to the changes in size and shape of conodonts during this period. |
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