Research Of The Climatic And Environmental Changes Of The Pliocene-Pleistocene Transitional Period In The Xiashagou Area Of The Nihewan Basin

The study of the stratigraphic profile of the continental facies of the lower quaternary boundary and the characteristics of climatic and environmental change in the PliocenePleistocene transitional period are new hotspots in the international quaternary study.In Nihewan Basin,located in northern China,it deposited huge layers of the fluvio-lacustrine sediments(i.e.,Nihewan formation)from the Pliocene to the Late Pleistocene.The Nihewan Basin is an ideal area for studying the evolution of paleoclimate and paleoenvironment.In this study,the characteristics of paleoclimate and environmental evolution in the PliocenePleistocene transitional period of the Nihewan basin were studied and revealed,by analyzing the pollen,charcoal,grain size and loss on ignition of 107 strata samples obtained by continuous sampling in the Xiashagou profile of the Nihewan Basin.According to the existing paleomagnetic age and stratigraphic data in this area,the age of the sampled strata is 2.63-2.32 Ma BP.The results of the study on the Xiashagou profile by using each surrogate indicator show that: in the Pliocene-Pleistocene transition period(2.63-2.32 Ma BP),the Paleoclimatic characteristics of the Nihewan Basin were changed from warm-humid to cold-dry,and mainly influenced by river hydrodynamic forces,the sediment characteristics in the Xiashagou were large and changed frequently.According to the changing characteristics of each surrogate indicator,the climatic and environmental changes in the study area can be divided into five stages: warm-humid(2.63-2.58 Ma BP),cold-dry(2.58-2.51 Ma BP),mild-dry(2.51-2.46 ma BP),mild-humid(2.46-2.41MaBP)and cold-dry(2.41-2.32 Ma BP).In the Late Pliocene(2.63-2.58 Ma BP),the climate characteristics of Nihewan Basin is warm-humid.Woody plants,including the thermal trees,such as Anacardiaceae,Meliaceae and Tsuga,were largely distributed.The deluvial materials formed by aeolian red clay of the Pliocene and other detrital materials from surrounding mountains were deposited in the Xiashagou area.2.58-2.51 Ma BP,Nihewan Basin is characterized by cold-dry climate.Woody plants have been greatly reduced,and the thermal tree species have almost disappeared.The study area has formed a grassland ecological environment,dominated by Chenopodiaceae,Lamiaceae and Artemisia.Under cold-dry climate,the river hydrodynamic force was enhanced and the grain sizes of the sediments were larger in the Xiashagou area.2.51-2.41 Ma BP,The climate in Nihewan Basin turned warmer,but the degree of warmth is less than that in the Late Pliocene.Woody plants,represented by Betula,Ulmus and Quercus has increased significantly,and vegetation in the study area was more prosperous.The river hydrodynamic force in Xiashagou area decreased and the grain size of the sediments became smaller.In this period,the climate in Nihewan Basin is mild-dry during 2.51-2.46 Ma BP.It formed a patch-shaped forest-grassland pattern with Pinus,Betula,Ulmus,Quercus and other trees.After that,the climate of 2.46-2.41 Ma BP changed from mild-dry to mild-humid.Woody plants grew rapidly and the forest area expanded.2.41-2.32 Ma BP Nihewan Basin is characterized by cold-dry climate.The Xiashagou area was significantly influenced by the rivers,and the river hydrodynamic force enhanced and fluctuated frequently.The characteristics of sediments shows a great variance.In this paper,the climatic characteristics of the Pliocene-Pleistocene transitional period in the Nihewan Basin,from warm-humid to cold-dry,are similar to the results of previous studies on the Nihewan Basin and its neighboring areas.The climate of 2.51-2.41 Ma BP in the study area was relatively warmer,after 2.41 Ma BP it transformed into cold-dry,this result coincides well with the previous simulation reconstruction of the Northwest Pacific Ocean surface temperature(SST)change during this period.