Early Triassic Carbon-Sulfur Cycles And Paleo-Marine Environmental Changes In South China And Kashmir

The mass extinction at the end of Permian is the most destructive biological environmental crisis in Phanerozoic.Siberian volcanism is considered to be the main cause of the extinction.In the Early Triassic after the mass extinction,the marine ecosystem was still very fragile.The adverse factors such as high temperature and lack of oxygen occurred repeatedly in this period,which led to the heavy damage of some marine organisms in the Early Triassic.It was not until the Early Triassic spas period that the marine environment began to improve,which accelerated the recovery process of the marine ecosystem and laid the foundation for the comprehensive recovery of the Middle Triassic.In view of the slow recovery process of the marine environment in the Early Triassic,a series of global profiles have been studied to varying degrees.Based on previous studies,it is suggested that there may be significant heterogeneity of marine chemical conditions in the Early Triassic,which may be due to the differences of climate and paleo marine environment in different regions.Therefore,a global comparative study of the paleo marine environment in the Early Triassic can help us understand the regularity and difference of the paleo marine chemical composition and evolution characteristics between different regions in this period,and provide important information for further exploring the slow recovery process of the Early Triassic ecosystem.The anomaly of marine carbon cycle is the most typical feature of Early Triassic marine environment.The study of chemical composition of marine sediments shows that the evolution curve of carbon isotope of sediments in this period showed many large amplitude oscillations.As the most basic chemical component of life,carbon is an important medium for biological photosynthesis and respiration.The fluctuation of its isotopic composition indicates that the earth’s ecosystem is still unstable during this period.Accompanied by the abnormal carbon cycle,the global climate,marine productivity and redox state of seawater all show a certain degree of regular changes.In addition,the abnormal evolution of marine sulfur cycle in the Early Triassic is closely related to the carbon cycle,which may be caused by a series of chemical changes（such as bacterial sulfate reduction and sulfate driven anaerobic oxidation of methane）.Compared with carbon isotope,sulfur isotope is more sensitive to paleoclimate and paleo marine environment changes.Land weathering,marine productivity and seawater hypoxia directly affect the long-term and short-term changes of marine sulfur isotope.The results show that the concentrations of dissolved inorganic carbon and sulfate in the Early Triassic ocean are very low,so they may be more vulnerable to regional paleo marine environmental changes,but so far there are few studies in this area.Zuodeng and Qiyueshan sections in South China and Guryul Ravine section in Kashmir are selected for this study.The exposed strata of Zuodeng and Guryul Ravine include upper Permian Changxing stage to lower Triassic Spasian stage,while the exposed strata of Qiyueshan section only expose Lower Triassic Smithian stage.Inorganic carbon isotope(δ¹³C_carb),sulfur isotope of carbonate associated sulfate(δ³⁴S_CAS),and whole rock major and trace element analysis have been carried out in all three sections.After discriminating and analyzing the preservation ofδ¹³C_carb andδ³⁴S_CAS signals in the process of diagenesis,theδ¹³C_carb-δ³⁴S_CAS is used as a global index to carry out a comparative analysis within the Tethyan range,and the regularity and difference characteristics of carbon sulfur isotope evolution curves in the Early Triassic are pointed out.At the same time,we use a series of element geochemical indexes,such as chemical weathering index CIA,Al content,Mn-U-Mo enrichment factor and P/Al and BA/Al ratios,to discuss the variation characteristics of regional weathering intensity,marine terrigenous debris input,seawater redox state and marine productivity.Finally,we discuss the evolution characteristics of paleo marine environment reflected by the carbon sulfur isotope evolution curve of Early Triassic,and the influence of regional paleo marine chemical conditions on the carbon sulfur isotope evolution curve.The main achievements are as follows:1.Theδ¹³C_carbandδ³⁴S_CAS signals of the Zuodeng,Qiyueshan and Guryul Ravine sections were not significantly altered by diagenesis,which recorded the original marine geochemical information.2.Theδ¹³C_carbevolution curves of the Zuodeng,Qiyueshan and Guryul Ravine sections show multiple positive and negative migration in the Early Triassic,showing multipleδ¹³C_carb maxima（P1,P2,P3）and minima（N1,N2,N3）,which can be compared globally.The Early Triassicδ³⁴S_CASof the Zuodeng and Guryul Ravine sections have a positive trend,which is similar to the previous results in Spiti Valley,southern Tethys,but slightly different from the trend of multiple large fluctuations in South China.During the Smithian,theδ³⁴S_CAS evolution curves of the Zuodeng,Qiyueshan and Guryul Ravine sections are quite different,reflecting that the marine heterogeneity in this period is very significant.3.Compared with the Guryul Ravine section,the chemical weathering of the Zuodeng section is stronger,but the composition of clastic materials is lower,which may be due to the fact that the former is located in low latitude and far away from the continent.The increase of the input of clastic materials in the Early Triassic may be related to the increase of weathering induced by global climate change.The redox state of the Early Triassic seawater in the study area is relatively stable,mainly oxygen poor oxidation,occasionally accompanied by anoxia.There is no anoxia sulfurization phenomenon reported by predecessors,which may be related to the distance from OMZ in the study area.The rapid decline of marine productivity near the Permian Triassic boundary in the Zuodeng section and the slow rise of productivity at a lower level in the Early Triassic are consistent with the overall outline of the global biological environment changes during this period,while the changes of productivity in the Guryul Ravine section in the Early Triassic are mainly regional.4.In the Zuodeng and Guryul Ravine sections,the evolution curves ofδ¹³C_carb andδ³⁴S_CAS in the Griesbachian-Dienerian period generally fluctuate synchronously,mainly due to the synchronous burial of organic matter and pyrite,which is controlled by the change of marine productivity.The large fluctuation ofδ³⁴S_CAS in Guryul Ravine section during the Griesbachian-Dienerian may be caused by the superposition of lower marine sulfate concentration in the Early Triassic and regional environmental changes.5.In the Zuodeng,Qiyueshan and Guryul Ravine sections,the evolution curves ofδ³⁴S_CAS during the Smithian are different,and cannot be compared with the previous results.The low concentration of marine sulfate in Early Triassic and the difference of paleo-ocean chemical conditions in different areas during Smithian Thermal Maximum probably be the reason for the different sulfur cycle process in different areas.