Carbon And Oxygen Isotopic Record From A Late Cenozoic Sedimentary Sequence In Lanzhou Area And Its Palaeoenvironmental Implications

Late Cenozoic is an important period of aridification in Asian Inland and the Qinghai-Tibet Plateau uplift, thus reliable geological record is important means of researching the palaeoenvironmental evolution. Lanzhou area is located in the northeastern margin of the Qinghai-Tibet Plateau and the western margin of the Loess Plateau, developed a late Cenozoic sedimentary sequence composed of an aeolian red clay, alluvial-lacustrine section and a loess-paleosol section. The stable carbon and oxygen isotope from the carbonate of the sequence has been carried out and it is found that the late Cenozoic Lanzhou change and carbon isotopes in contrast to the central region of Loess Plateau, reflecting the global climate change information; and oxygen isotope changes in the northeastern margin of Tibetan Plateau The same basin, reflecting the Tibetan Plateau and its effects is provided in the late Cenozoic. Through detailed rock magnetic study found that magnetic susceptibility, remanent magnetic susceptibility without hysteresis, saturation isothermal remanent magnetic parameters such as number of different sedimentary facies have different values, its changes and hydrodynamic size and the amount of related substances in dust Can be used as indicators of drought; hard magnetic, remanent coercive force of some magnetic parameters reflects the hematite content, these parameters lead to increased sediment redness value of the main reasons, it can be used with the same degree of red Temperature indicators. Chronological framework based on magnetic stratigraphy, lithology changes, compared to carbon and oxygen isotopes, rock magnetism and existing environmental changes such as color and size proxies and found that the late Cenozoic environmental change in Lanzhou three important events are about 7.2 Ma began to deposition of aeolian Red Clay,5.2Ma strong aridity and cooling and 3.5-1.4Ma before and after the hiatus and the large-scale environmental changes.～8Ma the uplift of the Tibetan Plateau may have contributed to the elevation of oxygen isotope effects.5.4-5.2Ma of the Western Drought drought, so that the oxygen isotope values in Lanzhou rapid shift to the positive. A movement began in the Tibetan Plateau 3.5Ma screen to Lanzhou, the collapse of the Tertiary sedimentary basins, and loess deposits in 1.4Ma begin to become a part of the western margin of the Loess Plateau, and the more arid climate to the area to increase the proportion of C4 plants and Increase the ratio of evaporation and water.