Cenozoic Organic Carbon Isotope Of The Xining Basin And Its Palaeoenvironmental Evolution

Deep sea carbon and oxygen isotope records have already revealed the pattern of long-term ecological environment change in the Cenozoic,characterized by three remarkable stepwise coolings and initiation and development of polar ice-sheets.Such a change from a "greenhouse" to a "icehouse" earth has also led to the important transition of the global carbon reservoir and ecosysterms.How terrestrial ecosystems respond to these changes is becoming one of the heated topics with the need for understanding of the modern global warming and its ecologic effects.Organic carbon isotope(?13C)has been regarded as a basic sensitive indicator to ecosystem evolution,the is one of the most important role in global carbon cycle,and across the earth surface layer of all biological systems,is the most direct record of the earth's biological process,can explain in detail the surface vegetation ecological and environmental conditions.However,the studies on the terrestrial ecosystem are mainly focused on the Quaternary,especially the last glacial cycle.However,the researches about the Cenozoic long-term ?13C and paleoenvironmental evolution on the continent are rare probably due to lack of continuous sediment records with precise age controls.For long time scales since the Cenozoic organic carbon isotopic records for sea-land contrast of climate and environment change,discusses the evolution of the ancient land ecological environment and its mechanism and so on are helpful.On such a long time scale,however,so far also can't take out a continuous high quality can be compared with deep-sea records of terrestrial organic carbon isotope curve,is imminent.In Asia,besides the global cooling,other three aspects must be taken into account for the ecological environment change,that is,the uplift of the Tibetan Plateau,the development of the East Asian monsoon,and the retreat of the Paratethys Sea.Under such a unique paleoenvironmental setting,how the continental ecology evolves in response to these factors and their processes is the important scientific question.However,no such researches have been carried out.The Xining Basin in the northeastern Tibetan Plateau is filled with continuous fine fluvial-lacustrine sediments from the early Eocene to the early Miocene,completely records uplift history of the plateau and climate changes of the northeastern part of the plateau.Here we firstly present a long-term ?13C record(38-17 Ma),together with pollen record and C-O isotopic records,of a continuous fluvial-lacustrine sediment sequence from the well-known Xiejia section in the Xining Basin to detect the trend and variation of paleoecologic environment and their possible mechanisms.Detailedly,the main results and conlusions are listed as follows:(1)We firstly obtained a late Eocene to early Miocene continental sedimentary ?13Corg record in Asia from the Xining Basin,NW China.(2)The ?13Corg values of the measured Xiejia section range from-20.0‰ to-23.5‰,with an average of?-22.0‰.We regard the ?13Corg values of the Xiejia section to mainly reflect changes in terrigenous C3 plants.(3)The ?13Corg values of Xiejia section show a long-term decreasing trend and short pulsed.These variations generally correlate with changes of gymnosperm content or the ratio of gymnosperm to angiosperm(G/A);i.e.,heavier 813Corg values correspond with higher gymnosperm content or higher G/A,showing that the gymnosperms content has a controlling role in the carbon isotope values before the appearance of C4 plants.(4)The ?13Corg values of Xiejia section in Xining Basin from late Eocene to early Miocene is well correlated with the deep-sea oxygen curve.Significant low values of 813Corg occurred at 32.5 Ma,23 Ma,21.5 Ma and 18 Ma,corresponding with global cold events Oi-1?Mi-1.1?Mi-1a and Mi-lb respectively,which illustrates the global alternating warm and cold climate has already spreaded to the northeastern part of the Qinghai-Tibet Plateau at that time and the terrestrial vegetation and its carbon isotopic composition are ultimately controlled by the global temperature.(5)During the 38-17 Ma of Xining Basin palaeoenvironment evolution:38-32.5 Ma gradually cool?32.5 Ma fast cooling and 32.5-25 Ma Stable at low temperature?25-22 Ma temperature recovery(23 Ma fast cooling)?22-17 Ma relatively low temperature(21.5 and 18 Ma fast cooling,18-17 Ma temperature recovery).