In-situ Shrimp Oxygen Isotopic Study On Conodonts From The Permian/Triassic Boundary Of Wenbudangsang Section, Tibet

Geologists have been dedicated into the research of the global climate changingmodel since Quaternary, in order to learn the current stage of the global climatechanging and predict the trends of the climate changing and associated effect on thebiosphere in the near future. However, the present study could not provide strongevidence to prove whether the current climate changing could lead to mass extinctionsor not. To answer this question scientifically, we need to carefully study the climatechanging in the earth history when mass extinctions occurred and their relationship.It has been well known that five periods of mass extinctions have occurred sincePhanerozoic, among which the mass extinction at the transition of Permian andTriassic, also of Paleozoic and Mesozoic, was the largest one when more than90%ofthe marine and75%terrestrial species became extinct. This mass extinction attractsgeneral interests and has been widely studied, leading to various interpretations on thereasons for this mass extinction, including domestic and exotic events. However, theresulted changing in the media （marine water and atmosphere） where the organismlived was the direct reason for the mass extinction, which has been preserved in thestrata during the transition of P/T. Thus the study at the transition of Permian toTriassic provides the best breakthrough for understanding the relationship between thepattern of global climate changing and mass extinction.Temperature change is one of the most significant research aspects in the climatechanging study. The thermometer based on oxygen isotopes has been applied to studythe changing of the oceanic water for long. Especially, oxygen isotopes of in calciumphosphate of the organic fossils, extremely sensitive to the temperature changing. Conodont is one of the most important fossils which widely occur in the strata fromPaleozoic to Triassic and conodont is also mainly composed of calcium phosphate,therefore, some geologists have analyzed conodont Oxygen isotopes to study thetemperature of the surface water in the ancient ocean.In this study, we established a systematic method of in-situ analysis on theOxygen isotopes in conodont by using the Sensitive High Resolution Ion Micro Probe（SHRIMP） IIe-Mc at Australian National University, and the conodonts collectedfrom the strata across the Permian/Triassic boudary in the Wenbudangsang section inTibet were chosen to be the test materials. The Oxygen isotopes mounts with thediameter of35mm were used rather than traditional mounts （diameter=25mm） whichcould lead to the mass fractioning due to edge effect. Further more, the conodontsamples were placed at the center within5mm to effectively lower the derivation ofthe isotopes fractioning caused by geometric effect. Continuous analyses on thestandard of Durango apatite (δ¹⁸O_apatite) for seven days have yielded253Oxygenisotopic data with very small variation (δ¹⁸O_apatite=9.78±0.29‰), which is inaccordance with the reference (δ¹⁸O_apatite=9.81±0.25‰, Ian Williams, unpublished).The results suggest that the established method of in-situ analysis on the Oxygenisotopes is reliable with the accuracy meeting with the requirement of the study ontemperature changing and could be regarded as the standard method of in-situ analysison the Oxygen isotopes in conodonts.Based on the established method, we analyzed237conodont samples collectedfrom49horizons across the P/T boundary in the Wenbudangsang section, and we got914Oxygen isotopic data. By applying the available temperature calculation method（Kolodny et al.,1983）,49temperature data were obtained. The results show thetemperature of the paleo-oceanic water at the end of Late Permian were between17.4-19.2℃(δ¹⁸O=20.90‰,080826-25) whereas the temperature increased by8.2℃to25.6℃(δ¹⁸O=19.03‰,080826-32) when crossing the geological event boundary.However, no significant changing in the paleo-oceanic water has been recorded acrossthe biostratigraphic boundary of P/T （between080826-33and080826-34） when thetemperature droped from26.0℃（080826-33） to24.7℃（080826-34）. The temperature at the beginning of early Triassic increased gradually increased from24.7℃（080826-34） to31.3℃（080826-112）. Our study further favored the results that thetemperature at the end of Permian increased by8℃based on the conocont oxygenisotopic study at the Meishan section in South China （Joachimaski et al.,2012）. Ourstudy also proves that the temperature increase event at the end of Permian was globalrather than regional.Besides the oxygen isotopes analyses, we also examined the carbon isotopicgeochemistry on the conodonts from the same section. A significant negativederivation of the carbon isotopic curve is observed, which clearly reveals the massextinction near the stratigraphic boundary at the end of Permian. This stratigraphycorresponds to the abrupt increase in the temperature of the surface water of theancient ocean. This observation indicate a close relationship between the O and Cisotopes in the conodonts.Based on the analysis and comparison in biostratigraphy, event stratigraphy andCarbon-Oxygen isotopic studies between Wenbudangsang section and Meishansection, the period with fast increasing temperature corresponds to the fast decrease inC isotopes and the fastest extinct rate. Thus the global warming event at the end ofPermian is obviously in positive correlation with the mass extinction.