过去8Ma兰州盆地古温度、古降水和古高度的半定量估算

QUANTITATIVE RECONSTRUCTION OF LATE CENOZOIC PALEOCLIMATE AND PALEOELEVATION OF LANZHOU BASIN IN NORTH EAST TIBETAN PLATEAU

兰州盆地位于青藏高原与黄土高原过渡地带,晚新生代全球降温和青藏高原的构造隆升对这一地区的温度、降水和高程产生了显著的影响。利用红度-温度和磁化率-降水转换函数并结合古生物、地层地貌资料对兰州盆地过去8Ma的温度、降水和高度的半定量重建,结果表明:8.3～3.5Ma兰州盆地气候较为暖湿,期间经历了3个阶段两次显著的降温和变干过程,年均温度从16.7±2.9℃降低到11.9±3.2℃,年均降水从1150±350mm减小至500±100mm,这一时期盆地的冷干化可能受到全球降温和高原隆升的双重影响,但盆地海拔可能一直维持在1000m以下;3.5～1.7Ma盆地经历了青藏运动的A,B和C三幕,形成了一级山麓剥蚀面,代表了兰州盆地演化的新高度,海拔可能达到了1400～1600m,年均温度变为6.0±2.5℃,年均降水为240±40mm;1.70～0.45Ma山麓剥蚀面海拔达到1791～1991m,年均温度和年均降水分别为4.7±3.8℃和145±135mm;0.45～0Ma期间青藏高原再次隆起使兰州盆地继续抬升,达到现今的高度,并最终形成了目前的地貌格局,年均温度为5.2±4.8℃,年均降水为363±237mm,现今的气候特征也由此形成。

Lanzhou Basin located between Tibetan plateau and Loess Plateau,where the Cenozoic uplift of Tibetan Plateau and the global cooling have significant impact on its paleoelevation and paleoclimate. Quantitative reconstruction of Late Cenozoic paleoclimate and paleoelevation of Lanzhou Basin is not only helpful to understand the contribution of uplift of Tibetan Plateau and the global cooling during Late Cenozoic to regional climate evolution, respectively, but also necessary to discuss the coupled relationship between the tectonic and climate. Abundant evidence proved that the change of magnetic susceptibility and redness of the Chinese loess are controlled by pedogenesis,which is the product of climate change. Based on the research about the quantitative relationship between surface soil and climatic parameter, many authors have established magnetic susceptibility-annual rainfall transfer function and redess-annual temperature transfer function. In this study,we chose the representative Jiuzhoutai loess profile and Nanshan red-clay profile and calculated the paleo-rainfall and paleo-temperature of Lanzhou Basin during the past 8Ma by using these transfer function. In addition,the paleo-vegetation of Lanzhou Basin was rebuild by making use of the pollen results from Nanshan red-clay profile and Yongdeng loess profile, which also could be regard as an indicator of paleo-rainfall and paleo-temperature and the paleo-ahitude was estimated by distribution of Yellow River terrace in Lanzhou Basin. Finally, Combining the magnetic susceptibility-paleorainfall transfer function and redesspaleotemperature transfer function with plenty of paleobiological and geological evidence,the results of quantitative reconstruction of paleoclimate and paleoelevation for the past 8Ma in Lanzhou Basin indicate that the evolution of three paleo-parameters can be divided into four stages. As follows： stage 1 （8.3 -3.5Ma） ,the climate relatively get cooler and drier including two significantly rapid events, correspondingly, the mean annual temperature （MAT） decreased from 16.7±2.9℃ to 11.9±3.2℃ and mean annual precipitation（MAP） from 1150±350mm to 500± 100mm. The elevation of basin during this period is lower than 1000m; stage 2（3.5 ± 1.7Ma） ,during this stage, the basin may be elevated to 1400 - 1600m resulting from the intensively uplift of Tibetan Plateau and MAT and MAP continuously decline to 6.0±2.5℃ and 240±40mm, stage 3 （ 1.70 - 0.45Ma） , as the altitude is further elevated and increase to 1791 ± 1991m driving by uplift of Tibetan Plateau,the MAT and MAP reach the minimum corresponding to 4.7±3.8 ℃ and 145± 135 mm, stage 4 （0.45 Ma - 0） , the modern climate state and landform have ultimately formed,when the MAT and MAP increased to 5.2±4. 8℃ and 363±237mm respectively.