Carbon And Oxygen Isotope Characteristics Of Permian-Triassic Boundary In Majiangba Area,Longmenshan Region,Sichuan And Their Geological Significance

The origin,development and evolution of the earth and life have always been a hot topic in earth science research.At the intersection of Permian and Triassic,the largest and most far-reaching mass extinction event occurred in the phanerozoic eon.Carbon is an important element in living organisms,and its isotopic composition is a direct response to environmental changes and life activity and carbonate carbon and oxygen isotopes in global climate change,sea-level fluctuation change,extinction and volcanic events,etc widely used,has become people to study the ancient one of the important means of environmental and geological events.Previous studies have revealed that the permian-triassic extinction event was accompanied by negative migration of carbon isotopes in many sedimentary profiles.Under this article above Permian series-Triassic carbonate as the research object,selection of longmen mountain horse Angle on the dam area in sichuan-Triassic under section of Permian system of carbon oxygen isotope analysis and geochemical analysis,stable isotope and element geochemistry in the upper Permian series-Triassic transition under the interface of geochemical anomaly information and the abnormal geological events of late Permian background may be the cause of the mechanism.Through the research,the following results are obtained:(1)Through the Angle of longmen mountain horse dam in sichuan area in Permian system-in the Triassic boundary under the measured field outcrop section and sedimentary geology research shows that the study area in Permian series-Triassic stratigraphic boundary clear,lithology is mainly carbonate,Permian system formation lithology study area section micrite,containing chert TB microcrystalline limestone and micrite;The lower Triassic stratigraphic lithology consists of mudstone and marl interbedded,"round"limestone,micritic limestone,vermicular limestone,lamellar limestone,oolitic limestone and micritic limestone.The flint tuberculosis and stripe is to identify one of the main sign of Permian series of strata in southern China,in the Permian system and Triassic near interface under development for mudstone formation boundary and layer,on the Permian series Wu Jiaping formation sedimentary facies evolution characteristics for the platform and the platform edge,the Triassic of feixianguan sedimentary facies evolution characteristics of restricted platform-platform-margin shoal.(2)The upper Permian-lower Triassic profile was systematically sampled and analyzed by carbon and oxygen isotopes and elemental geochemistry,and the feasibility of the data was tested.The representative geochemical information of the carbonate samples to seawater in the study area was evaluated by using the Mn/Sr ratio,¹⁸O content and the correlation of carbon and oxygen isotopes.The results show that the values of Mn/Sr of three samples are between 3 and 5,and the values of Mn/Sr of other samples are all less than 2 and 3,but the values of Mn/Sr are all less than 10.¹⁸O values are between-5‰?-10‰and greater than-10‰.There was no significant correlation between carbon and oxygen isotopes.It indicates that the carbonate rocks in the study area are not subjected to diagenetic alteration and have a good indication to the seawater information in the sedimentary period.(3)The evolution curves of carbon and oxygen isotopes from upper Permian to lower Triassic were established,and the variation range of carbon isotope was-4.54‰?3.88‰,showing an oscillating saw-saw pattern along the stratum.The carbon isotopes are negatively shifted near the upper permian-lower Triassic boundary,ranging from-0.33‰to-3.11‰,with a range of 2.78‰.The variation range of oxygen isotope is-9.53‰?-4.61‰,which fluctuates along the stratum.The fluctuation of carbon and oxygen isotopes in the stratigraphic profile of the study area reflects the instability of the sedimentary environment at that time.(4)Through the comprehensive analysis of carbon and oxygen isotope geochemical information,the paleomarine environment and climate were discussed by using the paleosalinity and paleotemperature environmental substitution index(Z value,T value).The results show that the Z value in the study area varies from 114.79to 133.53.Among them,the Z value of upper Permian strata ranges from 120.02 to133.53.,which represents the carbonate open platform facies environment.The z-value of the lower segment of the early Triassic feixianguan formation ranged from114.79 to 129.89,reflecting the sedimentary environment of brackish water to fresh water seawater.The z-value near the upper permian-lower Triassic boundary changed greatly(124.75?117.54).,and the decrease of salinity may cause the death of narrow haline organisms,which cannot utilize the ¹²C in the air after death,resulting in the negative ¹³C value of seawater precipitation.The range of T value in the study area was 12.81??37.42?,the range of T value in the study area near the upper permian-lower Triassic boundary was 20.14??33.61?,The range of change was13.47?,the temperature rising in line may be associated with the late Permian volcanic activities,the temperature will affect the life of biological metabolic activity,killing creatures buried,the content of ¹²CO₂in the air,the water in the precipitation of diagenetic ¹³C relatively negative bias.Isotopic temperature estimation shows that most of the temperature in the study area ranges from 20.3?to 37.42?.Combining with the regional geological background,lithologic characteristics and stratigraphic sequence characteristics of the study area,it belongs to the warm-hot humid climate.(5)The isotopes of the upper permian-lower Triassic boundary were compared globally and regionally.The carbon isotope evolution curve has a good similarity in the region near the upper permian-lower Triassic transition interface and the south China region.The negative migration of carbon isotope has a good response in the upper permian-lower Triassic boundary of south China and even the whole world,the cause of the carbon isotopes of the same trend of fluctuations may and the time of the emeishan volcanic activity,at the end of the Permian extinction events and sea level changes.Large quantities of greenhouse gases and toxic and harmful gases emitted by volcanic activities as well as the resulting acid rain will acidify the oceans and cause the death of living things.Organisms preferentially absorb¹²C,and a large number of dead organisms will increase the¹²C content in the atmosphere.The¹³C of carbonate rock precipitated by seawater is relatively negative.With the decrease of sea level,the expansion of land area,and the increase of weathering of land crust,acid rain will aggravate the rate of weathering of land and cause the re-weathering of existing sedimentary organic matter into the ocean,resulting in the increase of ¹²C content in seawater and the relatively negative ¹³C content.(6)The negative migration of carbon isotopes at the upper permian-lower Triassic boundary in the study area may be related to the volcanic activity of mount emei and its associated secondary geological events.Emeishan large igneous province is China's only one recognized by the international large igneous province,emei mantle plume volcanic activity on the time and the Permian mass extinctions consistent(?252 ma),emeishan volcanic activity is likely to be caused by the carbon isotope curve evolution in the study area present volatility and changes in the formation and the Permian-Triassic boundary,the main drive mechanism of the negative deviation of carbon isotope;The rise of mantle plume intrusion in emei mountain leads to the rise of crust,which leads to the decrease of sea level,the increase of crust weathering,and the loss of shallow Marine habitats,which leads to the biological death.The negative migration of carbon isotopes corresponds to the decrease of sea level,which is manifested as the local range regression under the background of late Permian transgression in northwest Sichuan,which corresponds to the global wide range regression of late Permian.