Abnormal Records Of Paleo-Temperature In The Permian-Triassic Terrestrial Ecosystem On The Evidence Of Clumped Isotopes In Carbonate

The largest mass extinction of the Phanerozoic occurred at the end of the Permian,which not only destroyed the marine ecosystem,leading to the disappearance of more than 80%of marine life,but also ruined the terrestrial ecosystem,causing the vanishment of about 70%of terrestrial vertebrates and most of plants in land,associated with coal gap,changes in fluvial types,soil collapse,enhanced continental weathering,widespread wildfire and other environmental abnormities.Most studies have been conducted on the process,mechanism,and rate of the marine ecosystem collapse,and a lot of achievements have been made in the past.However,studies on the terrestrial ecosystem collapse were relatively rare due to the lack of biostratigraphical correlation criteria between terrestrial and marine realm,marks of the event horizon,and the precise chronological frame.Besides,suitable techniques in high resolution are particularly lacked.Among various mechanisms possibly contributing to the environmental anomalies and biological extinction at the end of the Permian,temperature may be a key factor that led to the collapse of both marine and terrestrial ecosystems.In the marine,conodont and brachiopod oxygen isotope thermometry could be used to reconstruct paleotemperature,but suitable media and methods are hard to find in the continent.Carbonate clumped isotope thermometry emerging in recent years could provide possible solutions to this problem.The Permian-Triassic boundary strata in the Dalongkou terrestrial section of Jimsar,Junggar Basin,Xinjiang is well-developed,with continuous deposits and detailed paleontological data,which has laid a good foundation for us to carry out further research.This thesis focuses on the sedimentary environment,diagenesis and consolidation mode of carbonate nodules in the Dalongkou section and attempt to use the clumped isotope thermometry to recover the climatic temperature for the first time.Results show that there were frequent fluctuations of paleotemperature in the Permian-Triassic transition.The temperature ranges from 23? to 51?.Temperature changes were coupled with biota variation.When the climate became cold,the sporopollen flora began to show the appearance of the Early Triassic,while the first occurrence of Lystrosaurus and the variation of ostracod fauna occurred in the increasingly hot environments.Two negative excursions of organic carbon isotope corresponded to the low temperature period during the Permian-Triassic transition,while increasing temperature leaded to the final collapse of terrestrial ecosystem.Change of continental weathering intensity was also an important manifestation of the terrestrial environment during the Permian-Triassic transition.Using several weathering indicators to mudstones from Dalongkou terrestrial section,we found that chemical weathering kept weakening persistently in the latest Permian.Under the overall warm climate,the mid-latitude of the northeastern Pangea became increasingly arid.After the strong fluctuations of temperature,the study area shortly entered the humid environment in the early Triassic,followed by low intensity and the prevalence of fluvial activity which were corresponding to the violent oscillations of organic carbon isotopes.Intermittent strong rainfall could occur,which was possibly influenced by intensification of megamonsoon.During the entire Permian-Triassic transition,the climate state was largely unstable in the continental system,manifested in multiple episodes of rise and fall in temperatures,which also had controls on weathering.Furthermore,terrestrial mass extinction shows stepwise multistage characteristic.The records of the Dalongkou section show that there was at least one cooling and one warming event before the extinction,followed by a drop in temperature,and then staying in a relatively high temperature range with fluctuations for a long time,which is different from the previous interpretations.Two organic carbon isotopic anomalies,mercury isotopic anomalies,the emergence of anomalous multi-sacci pollen,and the intensification of wildfire activities were all evidence of the stepwise collapse of the terrestrial ecosystem.Mass extinctions both in marine and on land were controlled by the temperature.Temperature could be the most direct factor leading to the differences of collapse model between marine and land.