Stablecarbon And Oxygenisotopecomposition Of Marine Carbonate Rocks Of The Lower Triassic In Sichuan And Chongqing Areas

The origin,the development and the evolution of earth and life are always the hot topics in earth sciences research.The early Triassic is widespread concerned by most researchers.On the one hand,because it is the geological history transition from the Paleozoic to the Mesozoic,on the other hand,because it has experienced two event,the mass extinction of the late Permian and the ecological recovery of the early Triassic.Based on the geochemical characteristics of carbonate rocks of the early Triassic of 6 sections in Sichuan basin,combined with abnormal geological events and sedimentary record,we discussed the driving menchanism of abnormal carbon cycle of carbonate and the difference between dolomites and coexisting calcites in hand specian.The petrology of carbonate samples in study areas have been researched on detail,depending on thin section observation,cathodoluminescence analysis,scanning electron microscopy?sem?and energy spectrum analysis.Meanwhile we choose Mn/Sr and ?18O values toassessed samples representative of the seawater geochemical information.The results shows that: The limestone better represents the primary seawater geochemical information.Many of grain limestone,transitional rocks,dolostone represent the information of paleo-seawaterin a great degree.However,recrystallization limestone worse represents the primary seawater geochemical information.Evaporite-solution breccia-secondary limestone represent geochemical information of the near-surface environment,that of geochemical information be deviated the feature of paleo-seawater.Based on evaluation the diagenetic alteration of carbonate samples,we have established 3 carbon isotope curves of the early Triassic and compared with global and reginal areas carbon curves.This indicates the carbon curves we establishedhave good consistency with other areas,which show that it has experienced more than once floats from the late Permian to the early Triassic.These floats include the nagetive excursion of P/T boundary and the positive excursion of Denerian-Smithian,Smithian-Spathian and Spathian-Anisian boundary,thathas formed the early Triassiccarbon cycling evolution pattern during 5 Myr,with high frequency and wide ranges.There are ?¹³C minimum value-2‰ and ?¹³C maximum value +6‰,and the float range could reach +8‰?It can infer that the carbon isotope curves of the Great bank of Guizhou and this study area can compare with each other,but also exist difference.The curve peak of Great bank of Guizhou lie in the Denerian-Smithian boundary,while the curve peak of our study located in the Smithian-Spathian boundary and the peak value is less than the former.So the abnormal excursion of the early Triassic has explained the unsteadiness and complicacy of the environment in this period.We compared the lithostratigraphyunit with chronostratigraphy unit by means of carbon isotope curves in study areas.It shows that the Feixianguan Formation is in accordance with the Induan and the first Member of Jialingjiang Formation is corresponding to the Smithian substage,the second Members and the part of the fourth Member of Jialingjiang Formation would be correlated to Spathian substage.We have discussed the probable mechanism of the abnormal carbon isotope values of carbonate.We have proposed that single geological event could not give rise to the symmetric excursion.The light carbon from Siberian dark rock eruption to atmosphere may cause 1‰ negative carbon isotope shift,this is not in accordance with that we observed.The negative carbon excursion of P/T boundary may be related to methane hydrate,which was trigger by volcanic eruption and other related geological events.But the early Triassic seawater carbon isotope curve is almost positive and negative shift,this phenomenon can not be interpreted with methane hydrate.These fluctuations type may be related to the rate of organic carbon and inorganic carbon burial with cyclical change,positive seawater carbon isotope may be related to increased organic carbon burial rate.?¹³C values of dolomite are higher than?¹³C valuesof the adjacent formation limestonein the early Triassic.We quantitatively studied in hand specimen scales isotope difference between dolomites and coexisting calcites under deposition conditions.We thought that ?¹³C values of dolomite is about 2².6‰ higher thancoexisting calcites.This results are consistent with results of the carbon isotope fractionation in the process of dolomitization experiment and theoretical calculation.Carbonate oxygen isotope was sensitive to diagenetic alteration,so studying quantitative diffenencebetween dolomite and calcite in hand specianhave meaningless,but we can confirm that dolomite have higher ?18O values than coexisting calcite under deposition conditions.