Vegetation Succession And Climate Change During The Early Pleistocene (2.2-1.8 Ma) In Nihewan Basin

The Nihewan Formation is considered as the standard stratum of the early Quaternary in North China and an ideal area for studying environmental changes during the early Pleistocene.Among them,2.2-1.8 Ma was an important period of the early Pleistocene,during which two paleomagnetic inversion events occurred.Based on accurate palaeomagnetic dating,sporopollen,grain size and geochemical analyses were performed on120 samples from²4 m?12980-10600 cm?of NHA core of Haojiatai districtin in Nihewan Basin,and the vegetation succession and climate change during 2.2-1.8 Ma of the early Pleistocene in the study area were reconstructed.The results showed that,the pollen assemblages in the study area were mainly composed of arbor pollen,in which Pinus was dominanted,and the percentage content was higher than 60%,indicating that the vegetation was primarily pine forest and the climate was relatively warm and wet,while some variations occurred in different periods:From 2.22-2.14 Ma,in the pollen assemblages,apart from the warmer Pinus pollen.Cold-loving Picea pollen reached more than 10%?up to 80%?in most samples,reflecting that the main vegetation in this period was pine forest and spruce forest in the mountains,which expanded in individual periods?approximately 2.19-2.18 Ma and 2.15-2.14 Ma?,with a short-term cold period.2.14-1.97 Ma was the warmest and wettest period in the study section.Pinus pollen was the main pollen in the pollen combination,and its content was more than 50%.Spruce pollen was mostly less than 10%.Broad-leaved tree pollen increased significantly,with an average content of about 10%,mainly Quercus,Betula and Ulmus.After 2.05 Ma,broad-leaved tree pollen content decreased,and xerophytic herbs pollen such as Artemisia and Chenopodiaceae increased.The results showed that the temperature and humidity decreased after 2.05 Ma in the study section.1.97-1.83 Ma was the coldest and drought period in the study section,which corresponded roughly to the Olduvai paleomagnetic inversion period.During this period,the pollen content of Picea increased significantly,the pollen content of Pinus?mostly less than50%?and broad-leaved Tree?mostly less than 5%?decreased significantly,and the drought-tolerant herbaceous pollen increased significantly which was more than 15%,mainly Artemisia and Chenopodiaceae,and the grain size of sediment was obviously coarsened;CIA and the content of Ti decreased,and the drought index values,CaCO₃ content and SiO₂/Al₂O₃ratio increased,the area of pine and broad-leaved forest decreased,spruce forest expanded to low altitude,the openness of the forest increased in the study area,herbaceous plant content increased,the climate was generally dry,and the temperature gradually decreased.Further,the climate change in the early Pleistocene revealed by NHA core in the Nihewan Basin was well correlated with marine oxygen isotope stages:MIS 84 and MIS 82were characterized by increasing pollen content of the Picea and the cold and wet climate;MIS 74,MIS 72 and MIS 70 were characterized by increasing pollen contents of Picea,Artemisia and Chenopodiaceae and the cold and dry climate.In the study region,the vegetation change suggested the cold and dry climate of the Olduvai period in the Nihewan Basin,which might be related to global cooling,the strengthening of the winter monsoon and the uplift of the Tibetan Plateau.