Paleoclimte Of Reptiles In Gansu Province Beishan Area During The Early Cretaceous Inferred From Oxygen And Carbon Isotopes

During the late early cretaceous, the continent has a very unique compounding geographical characteristics in east Asian, and the terrestrial environments were populated by rich and diverse vertebrate faunas, which is a cradle for the origination of advanced dinosaur taxa. The latitudinal gradient of temperature, the existence of geographic barriers and different local environmental conditions likely accounted for some aspects of heterogeneous distribution of faunas. Therefore, research on biological species and distribution characteristics can restore part of the survival environment, provide new perspectives and convincing proof of the reconstruction of paleoclimate as well as paleogeographic. Reptiles(dinosaurs, crocodiles, turtles) apatite stable isotope has been used to analyze and explain the ancient ecology and environment.In this study, the research area of Beishan is in the north of Jiuquan city Gansu province, lies in southwest Tianshan-Xingmeng tectonic belt which owns intense and complicated tectonic deformation. This area is under a background of stretch during the late period of early cretaceous. Fossil buried formation is character with red continental clastic rock and lacustrine deposition. We collected samples such as dinosaur teeth, crocodile teeth and tortoise shell from the Xinminbao formation of lower cretaceous. Because tooth enamel layer and shell parts are hard and dense, they have strong resistance to weathering and avoid being affected by diagenesis. It is available to extract apatite phosphate and get the oxygen isotopic fractionation between apatite and atmospheric water. Using the linear equations of atmospheric water oxygen isotopes and the mean annual atmospheric temperature(MAAT), we calculated out the MAAT of the area, it’s 19±3 ℃. Besides, the results proved that phosphate is able to protect the original oxygen isotopic signal from the later geological influence. On the other hand, the fractionation between carbonate carbon isotope of apatite and plant leaf can be used to identify local plant type, they are C3 plants. According to a least-squares regression equations for carbon isotope of plants and precipitation, we get the annual average precipitation(MAP) of this area, the result is calculated for 500±200 mm/yr. Figure in the climate zoning triangle, this area belongs to the semi-arid forest, grassland climate type in late early cretaceous, in accordance with former palynology and sedimentology research results.Terrestrial reptiles oxygen isotopic values higher than the semiaquatic- aquatic animals, and the temperature fluctuation range of the water environment is smaller. Herbivorous dinosaurs(iguanodon, ceratposian) have different carbon isotopic composition, shows that they have a difference source of food. Thus, they can probably avoid the competition and live together peacefully. Crocodiles are cold-blooded animals and they have strict living temperature range. Paleolatitude of the study area(35°N) is the critical lowest point of its living temperature. And turtles’ survival condition range are more broader, they are still live in more north area.