Climate And Environmental Characteristics During The Early Life (2.2-2.14 Ma) Of The Xiashagou Fauna In The Nihewan Basin,North China

The Xiashagou fauna in the Nihewan Basin(the narrowly defined Nihewan fauna)is the most abundant and representative fauna in China,which can be directly compared with the fauna of the Villafranchina age in Europe.It is of great significance in the study of Quaternary fauna in China.On the basis of the magnetic polarity sequence of the Xiashagou profile,the pollen,particle size and loss on ignition of 112 stratigraphic samples were analyzed,and the characteristics of climate change and environmental evolution in early life(2.2-2.14 Ma,early Pleistocene)of the Xiashagou fauna in Nihewan area were revealed at relatively high-resolution.The results show that:(1)During the occurrence period of the Xiashagou fauna(2.188-2.15Ma),the mean particle size of sediments is the finest in the whole profile,and the hydrodynamic force of river is small;the pollen type,content of organic matter and woody plants such as Pinus,Ulmus and Quercus are the highest in the whole profile,while those of herbaceous plants such as Artemisia and Chenopodiaceae are the lowest in the whole profile.The results showed that the climate in Nihewan area was warm and humid during this period,and the vegetation was mixed coniferous broad leaved forest with higher openness.Higher biodiversity,relatively calm water and abundant aquatic plants attract a large number of herbivores to forage,drink and inhabit.(2)The environment of Nihewan area oscillated during the occurrence period of the Xiashagou fauna: At 2.188-2.175 Ma,the silt and clay content in the whole profile is the highest,which is the river-lake delta sedimentary facies;the pollen type and concentration in the pollen assemblages are relatively higher,the tree pollen contents of Pinus,Ulmus and Quercus increased significantly;the organic matter content in the whole profile is the highest.It indicates that the climate was the warmest and wettest in this period,corresponding to the marine isotope warm stage 83.At 2.175-2.15 Ma,the sediment composition changed greatly,and the median particle size become coarser,indicating that the hydrodynamic force is larger;the pollen content of tree in pollen assemblages increased to the highest in the whole profile,especially in the for the low-temperature Betula and the warm-drying Ulmus;the content of liking for warmth Quercus and organic matter decreased significantly.It indicates that the climate was relatively warm-cool-slightly damp in this period,and changed frequently compared with the previous stage,corresponding to the early period of the marine isotope cold stage 82.(3)Before the emergence of the Xiashagou fauna(early Pleistocene),the median size of sediments and the hydrodynamic force of river is larger;the content of herbal pollen in the pollen assemblages is the highest in the whole profile,the content of woody pollen in the whole profile is the lowest and the coniferous Pinus is the dominant species;the content of organic matter is relatively low.It indicates that the climate was relatively warm-cool-slightly dry in this period,corresponding to the marine isotope cold stage 84.The grassland dominated by Poaceae-Artemisia-Chenopodiaceae and the coniferous forest dominated by Pinus showed patchy distribution.The warm-cool-slightly dry climate can not provide warm environment,adequate water and grass resources and suitable habitat for mammals,so there were no large number of mammals living in Xiashagou area.(4)At 2.15-2.14 Ma,the clay content is the lowest and the median particle size is the thickest in the whole profile,and the river hydrodynamics is large;the pollen type of pollen assemblage is the lowest in the whole profile,and Chenopodiaceae,representing arid environment,rises to the highest in the whole profile;the content of organic matter and tree pollen,represented by Ulmus,decreases to the lowest in the whole profile.The results showed that the climate in Nihewan area became colder and drier during this period,forming a staggered distribution pattern of grassland dominated by Chenopodiaceae and Artemisia and coniferous forest dominated by Pinus,corresponding to the occurrence of the positive polarity event in Reunion.The environmental characteristics of the event are characterized by cold and dry in the early stage and warm and humid in the later stage.