Carbon Isotopic Records And Its Paleoenvironmental Significance During The Triassic-jurassic Transition. In Qiangtang Area,Tibet

The global creature has experienced five major mass extinction events since Phanerozoic.For the past few years,the change of the palaeoenvironment has been a field that highly valued by the geologists and the biotics researchers.The mass extinction of the end-Triassic was the least studied one,which was characterized by the drowning of the marine carbonate plateform coincided with organic and inorganic carbon isotopic anomaly.On the basis of the previous biostratigraphic data,this thesis studied the early Jurassic marine strara of Qiangtang area in Tibet,and carried out carbon and oxygen isotope,rock-eval,elemental geochemical analysis,established the carbon isotope curve across Triassic-Jurassic in Tibet area of the eastern Tethys,and the global correlation was also discussed.We rebuilt the palaeoenvironment across Triassic-Jurassic transition and explained the possible mechanism of the kerogen and carbonate carbon isotope anomaly.The representative of the carbon and oxygen isotope of the paleo-ocean information was evaluated by Mn/Sr,Mg/Ca atomic ratio.We also evaluated the thermal degeneration by rock-eval and H/C ratio of the kerogen.The result showed that,?¹³C_orgwas not affected by thermal alteratiom,?¹⁸O was affected by meteoric diagenesis,but?¹³C_carb still revealed the information of the paleo-ocean.We use equation of linear regression and residual analysis to eliminated the abnormal value of carbon and oxygen isotope data.On the basis of detailed evaluation of diagenetic alteration,we established the long trend of organic and inorganic carbon isotope curve across Triassic-Jurassic transition,and discussed the global correlation.The results showed that the range of?¹³C_orgwas between-26.3‰?-24.1‰,-25.0‰on average.The trend of carbon isotope across Triassic-Jurassic transition was a long trend negative drift with a anomaly about 2.0‰.¹³C_carbwas between-4.7‰?3.3‰,0.88‰on average.The drift of ¹³C_carb was a little bit delayed behind?¹³C_org,?¹³C_carbcurve include two negative drift and one positive drift,the maximum negative drift was 7.2‰.The evolution model of the organic carbon isotope was comparable to New York Canyon in Nevada,USA.The inorganic carbon isotope was comparable to Iseo Section in Italy,Loruns in Austria,which prove the carbon isotope anomaly was a global event.The controlling factors of carbon isotope anomaly are studied by mass balance equation and carbon isotope fractionation formula.The inorganic carbon isotopes are mainly affected by temperature,productivity and organic carbon burial.The organic carbon isotope is mainly affected by the concentration of carbon dioxide.A method for calculating the isotopic records of sea water is established,we use?¹³C_org,?¹³C_carband?¹⁸O to reconstructed the palaeoenvironment across the Triassic-Jurassic by qualitative and quantitative methods,the equations of mass balance and carbon isotope fractionation are are used too.We calculate the global mean sea surface temperature,atmospheric CO₂concentrations,p H value of the paleo-ocean,and the curve of?¹³DIC,TA,[HCO₃^-],?¹³CO₂(aq),?¹³CO₂(g).The trace and rare elements were also analyzed to reveal the environment of paleo-sea bottom water.The results indicate that the change of the palaeoenvironment could be divided into three long term phases.Phase?,the stable stage of the temperature and CO₂ concentratio in late Triassic,p CO₂ was 530ppm on average,the global mean sea surface temperature was17.8?.Phase?,p CO₂and the global mean sea surface temperature became gradualy rising and reached a high value platform.p CO₂ was 600.7 ppm on average and peaked at 615.7ppm in Phase?,which was 106ppm higher than that in late Triassic,and it's3.42 times that of Quaternary glaciation,1.53 times that of modern CO₂concentration.The global mean sea surface temperature was 18.42?,the highest temperature was 0.8?higher than late Triassic,it was 4.9?and 3.8?higher than that of Quaternary glaciation and modern time.Phase?,p CO₂and global mean sea surface temperature synchronously decreased to the level of late Triassic.The distribution of bulk elements,trace elements,rare-earth element reveal that the enviorment of the marine bottom water was oxic-suboxic across Triassic-Jurassic transition,and the distribution characters are similar to coastal upwelling areas.Comprehensive analysis of the palaeoenvironment parameters and carbon isotope curve,we discussed the mechanism of the organic and inorganic carbon isotope anomaly in this special transition.We present that the unsynchronized changes of?¹³C_organd?¹³C_carb could not be explained by single factor.?¹³C_organomaly was driven by synchronous changes of p CO₂,temperature and primary production.?¹³C_carbanomaly was driven by organic carbon buried fraction conversion which was caused by the global mass transgression across Triassic-Jurassic.Single geological events are not sufficient to influence the paleoenvironment in Triassic-Jurassic,and the mechanism of the carbon isotope anomaly was a combination of palaeoclimate,sea level,tectonic variation and so on.