The Application In Paleoclimate Research Of Fish Tooth Fossils From Southern Tibet In The Mid-Cretaceous Priod

The Cretaceous period is a typical greenhouse climate period in geological history.The study of the paleoclimate of this period has a great significance for understanding the impact of current climate warming and future trends on the global environment and studying the causes of major geological events during this period.According to the statistics of paleoclimate-related research results in the Cretaceous period in various regions of the world in recent decades,paleoclimate studies in the Pacific,Atlantic,and Western Tethys gradually tend to be systematic and complete,but the Cretaceous Eastern Tethys area Quantitative palaeoclimate restoration is still lacking in research.In order to fully understand the global Cretaceous palaeoclimate state,there is an urgent need to fill in the data gaps in the Eastern Ocean.However,because the area is located at the collision site of the Eurasian plate and the India-Australian plate,it has undergone a strong late-stage diagenetic transformation,which makes it difficult to obtain quantitative paleoclimate data in this area.Fish tooth fossils are composed of phosphate minerals,which can preserve the original isotope signals better than other materials,so it could provide new ideas for quantitative paleoenvironmental research in the Cretaceous pe-riod in this area.In this study,we discovered 12 fish tooth fossils in the middle of the Cretaceous at the Din-gri-Nirang section and the Gamba Qiangdong section in Tibet.In this study,we conducted ca-thodoluminescence observation,scanning electron microscopy analysis,element geochemistry and isotope geochemistry studies on fish tooth fossils,trying to determine whether it retains orig-inal paleoenvironmental information,and conducted a global comparison and analysis of pale-otemperature and ocean current parameters.The main results are as follows:The outcropping horizons of the fish tooth fossils are in the Late Albian Chaqiela Formation and the Turonian Period Gambacunkou Formation.Cathodoluminescence results showed that the preservation of tooth enamel was basically intact,except for a few cracks that were filled with calcite strips,and almost all of the dentin was replaced by calcite.According to the results under the electron microscope,apatite columnar crystals can be seen.According to the results of energy spectrum analysis,the main elements contained in the tooth enamel in this study are phosphorus and oxygen,and the dentin mainly contains carbon and oxygen.In addition,the rare earth ele-ment MREE/MREE*value of fish tooth fossils ranges from 1.88 to 4.08,and only one sample has a MREE/MREE*value slightly greater than 4,and the correlation between Th and log?REE is extremely low,and Y/HO is moderately correlated with?REE.Based on the above analysis results,this article preliminarily believes that the enamel of the Cretaceous fish tooth fossils in the study area preserves the original isotope information,and the material has the potential to carry out quantitative paleoenvironmental research in the Eastern Tethys Cretaceous.Using the ?¹⁸O data of the fish tooth fossils,we have recovered the paleotemperature values of the middle Cretaceous in the southern margin of the Eastern Tethys Ocean:the temperature from 27 to 28?in the Late Albian and in the Turonian from 25 to 30?.The preliminary results are obtained in the Albian of the Eastern Tethys Ocean.The temperature range of the late period is similar to that of the Turonian period;Based on the?Nd value of the Eastern Tethys Ocean in the Cretaceous,combined with the?Nd values of other regions at the same time,we analyzed the composition of ocean currents,sources of water masses and influencing factors on the northern margin of the Indian continent of the Eastern Tethys Ocean in the middle of the Cretaceous.We speculate that the late Albian period in the study area was mainly affected by the water masses from the Indian Ocean and the Tethys circulation.The bottom water of the Indian Ocean was transported northward to the East Tethys Ocean,forming an upward current at the northern edge of the Indian continent.The encounter of the counterclockwise and counterclockwise backflow of the Sri Lankan Circulation in the African continent blocked the formation of the African con-tinent and jointly affected the study area;The value of?Nd in the Turonian period ranges from-8.7 to-9.9,and the average is about-9.3.Compared with the Albian period,there is a negative deviation of two units,which is significantly lower than the?Nd value of water bodies such as West Tethys and the Pacific in the same period.It is speculated that as the Indian continent drifts northward,continental weathering will increase,which may lead to a decrease in the?Nd value of the East Tethys Ocean.