The Paleoenvironmental Evolution During The Pliocene-Pleistocene Transition Recorded By The Gaolanshan Red Clay-Loess Core In The Lanzhou Basin

The Pliocene-Pleistocene transition represents the critical stage from the Pliocene warm climate to the Quaternary glacial-interglacial cycle.The Northern Hemisphere has experienced the onset of ice sheet expansion during this period.Meanwhile,the intensive tectonic uplift of the Tibetan Plateau has not only changed the pattern of the Asian monsoon circulation and resulted in the interior Asian aridification,but also profoundly shaped the geomorphologic pattern and ecological environment of the Tibetan Plateau and its surrounding areas.For example,thick conglomerates have been accumulated around the Tibetan Plateau due to the dramatic uplift of the Qingzang Movement,while the Chinese Loess Plateau began to deposit the continuous Pliocene red clay-Quaternary loess-paleosol sequence.Therefore,the study on the environment evolution during this transition can not only further understand the environmental effects of Tibet uplift,but also provide an important reference for discussing the coupling relationships between tectonic activity and global cooling.The Lanzhou basin,located at the junction of three natural geographical divisions in China and in the northeastern margin of the Tibetan Plateau,is the ideal region to illustrate the interactions between the tectonic activity and climate change.In this study,the continuous red clay-loess sequence?265²30 m?of the Gaolanshan core in the Lanzhou basin were well constrained based on the previous loess stratigraphy and high-resolution paleomagnetic chronology,and detailed analyzed via the integrated biomarker compound?n-alkanes and microbial membrane lipids?and isotope geochemistry?total organic carbon and compound-specific carbon isotopes?methods.So the paleotemperature,paleohydrology and paleoecological evolution during the Pliocene-Pleistocene transition?3².2 Ma?in Lanzhou basin were firstly reconstructed.Furthermore,the possible driving mechanism was discussed by comparing the global marine and continental records with regional tectonic activity history.The main conclusions are as follows:1.The n-alkanes in the eolian sequence from Gaolanshan during the Pliocene-Pleistocene transitional period showed a unimodal pattern with C₃₁ as the main peak,which indicated the dominance of grassland vegetation.In the typical loess layers of L32 and L33,n-alkanes were characterized by double bulge,indicating the influence of microbial activities.Combined with the changes in organic carbon isotope,grain size and magnetic susceptibility,we suggested that n-alkanes were affected by exogenous input.2.By the comprehensive comparison of organic carbon isotope and compound specific carbon isotope,we found that organic carbon isotope could be affected by exogenous input.Thus,we point out that caution should be taken in the application of total organic carbon isotopes in reconstructing the paleoecology during the Pliocene-Pleistocene in Lanzhou Basin.Based on the conventional indexes and compound specific carbon isotope of n-alkanes,the paleoecology during the Pliocene to Pleistocene transition in Gaolanshan was dominated by C₃ vegetation,which did not show obvious fluctuations on glacial-interglacial scale.It should be noticed that there was a sharp decrease of C₄ vegetation during 3 Ma.3.The paleotemperature and paleohydrological evolution during the late Pliocene-Pleistocene transitional period in Gaolanshan were reconstructed by microbial membrane lipid.Integrated paleoecology,paleotemperature,paleohydrological changes,two major events in Gaolanshan were proposed.The first event happened in 2.8 Ma when the sedimentary facies changed from red clay to loess,the C₄ vegetation declined due to the decrease of temperature.Meanwhile,the precipitation hardly changed.The second transition occurred from 2.7 to 2.6 Ma,during which the temperature and precipitation decreased synchronously.The vegetation was dominated by C₃,which closely linked with the rapid uplift of the Tibet plateau and the expansion of ice volume in the Northern Hemisphere.