| The Middle Pleistocene is an important period of global climatic and environmental evolution during the Quaternary.In this study,based on the Optically Stimulated Luminescence（OSL）and paleomagnetic dating,We conducted pollen analyses of 144 fossil samples and 1436 modern surface samples to characterize the processes of vegetation succession and climate change in the Nihewan Basin during the Middle Pleistocene,especially the Mid-Brunhes Event.The WA-PLS method was used to reconstruct the average annual temperature（Tann）and average annual precipitation（Pann）in the study period.The results showed that the pollen assemblages during 0.8-0.3 Ma were dominated by herbaceous pollen（>60%）,indicating that the vegetation in the study area was mainly forest steppe.The quantitative climatic reconstruction indicates that annual average temperature was generally>7℃,and annual average precipitation was generally>500 mm.The warm climatic conditions persisted for a long interval,and the interval of cold conditions was shorter.Compared with the warm period,during the interval of cold climate Tann decreased by～5℃,and Pann decreased by～100 mm.In the interval of 0.478-0.424 Ma,Picea dominated the pollen assemblages（>80%）,Tann was generally<2℃,and Pann was generally<400 mm,which may be a manifestation of the Mid-Brunhes event in the Nihewan Basin,which is corresponded with MIS 12.During this period,Northern Hemisphere insolation and the atmospheric CO2concentration were low,the Eurasian ice sheet expanded,the winter monsoon strengthened,and the regional climate was cold and dry.The intervals of 0.742-0.468 Ma and 0.424-0.266 Ma were two pronounced warm periods in the Nihewan Basin,which correspond well with MIS 13 and MIS 11,respectively.MIS 13 was the warmest and wettest period during the studied interval,and it may have been related to the unusually strong summer monsoon caused by the climatic asymmetry between the Northern and Southern Hemispheres. |
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