The Paleoenvironment Change Of Early Pleistocene In Nihewan Basin

Nihewan basin is located in the northeastern margin of the Loess Plateau,which contains well exposed Quaternary standard strata of northern China,is known as the "China's Olduvai Gorge" due to the richness of its Paleolithic and abundant fossils of mammalian fauna.Therefore,it was one of important site of the Quatrenary study in China.Previous studies have showed that Nihewan basin was occupied by lake during the Quaternary period and fruitful achievement in this area.Those studies were focused on the paleontology,stratigraphic chronology,paleolithic and paleoanthropo-logy.However,the evolution of ancient lake and environmental background is still at a weak stage relatively.The relationship between palaeoenvironment and paleo-anthropology is lack of futher study.Majuangou section lies in the eastern margin of the Nihewan basin,which is most abundant of ancient human activities and cultural strata in this basin.In this thesis,we collected samples with high-resolution and carried magnetic susceptibility,thermal magnetic,grain-size and XRD mineral analysis,combined with pollen and microfossils studied by former researchers,set up the paleoenvironment sequence of 1.72?0.92 Ma B.P.in this basin.The results show that variations paleoenvironment index curves of the basin represent five distinct phases.Phase I(1.72?1.66 Ma B.P.)and phase III(1.40?1.20 Ma B.P.)is characterized by lacustrine sedimentary environment,with a predominance of gray-green fine silt and clay silt.The sedimentary facies presents horizontal bedding and mollusc fossils rich.During these two phases,the magnetic susceptibility relatively low and the magnetite is the main magnetic mineral,the size frequency curves are frequently single peak,detrital mineral content is low and carbonate mineral content is abundant relatively.The ostracod type showed freshwater assemblage and the number of Picea were less by former researchers.Revealed warm and humid paleoenvironment and the ancient lake was expanded in these two phases.Phase II(1.66?1.40 Ma B.P.)and phase V(1.09?0.92 Ma B.P.)is characterized by lake-delta facies environment,with a predominance of yellow and brown silt.The sedimentary facies is characterized by cross bedding and sand lenses.During these two phases,the magnetic susceptibility relatively high and the main magnetic mineral is hematite,the detrital mineral content is high and carbonate mineral content is low.The ostracod type displayed salt tolerance assemblage and the number of Artemisia and Chenopodiaceae decreased and Pinus increased by former researchers.Indicated warm and dry environment and the shrinked of the ancient lake in these two phases.The study indicate that the palaeoenvironment evolution of the basin well responds to Middle Pleistocene climatic transition(MPT): IV(1.20?1.09 Ma B.P.).During this phase,the lithology of sediments and the palaeoenvironment curves changed obviously.The sedimentary facies is characterized by clay silt and brown clay,with a predominance of calcium tuberculosis and poor stratification.The curves of palaeoenvironment presents low frequency and high amplitude oscillation and has many extreme values.The magnetic susceptibility value,sand and carbonate content all increased suddenly and the changed unstable in this phase.The grain frequency curves ranges from single peak to multiple peak,present a combination of multiple sedimentary facies.The pollen and mammal fossil assemblages shows palaeo-environment changed significantly about 1.20 Ma B.P.in the basin,indicats MPT is an important role influenced the palaeoenvironment in the basin.According to the distribution ages of archaeological sites around the profile of Majuangou section,most of the sites ages were between 1.7-1.2 Ma B.P.,while archaeological sites were rarely found after 1.2 Ma B.P.,which shows MPT has an important impacted on paleoanthropology activities.The integrated effect of various factors on the ecological environment in north China during MPT may have provided the impetus for the paleoanthropology evolution,enhanced the abilities to adapted paleoenvironment.Meanwhile,the cold and variable paleoenvironment during the MPT was probably the main factor for the paleoanthropology migrated,lived in caves and began to use fire(0.8 Ma B.P.).Achievements in this study includes: The multi-proxy used to reconstructed the paleoenvironmental evolution sequence of Nihewan basin in early Pleistocene(1.72-0.92 Ma B.P.).Preliminarily discussed the relationship between paleoanthropogenic activities and paleoenvironment in this basin.The Middle Pleistocene climate transformation occured 1.2Ma B.P.is particularly obvious in this area,and the changed of basin paleoenvironment under the MPT may be the main factor leaded to the paleoanthropology migrated or extincted in this basin.