Palaeomagnetic Chronology And Paleoenvironmental Records Of Sediments From Wuqi Paleolake In Northern Chinese Loess Plateau

Aeolian red clay and loess deposited in northern China since late Neogene is an ideal palaeoclimate recorder. The application of palaeomagnetic chronology and multi-proxy enable us to understand the process of climate change in a better way. A profile consisting of loess, lacustrine deposits and red clay was found at Wuqi County in northern Chinese Loess Plateau. Previous research showed that there was a paleolake (named Wuqi paleolake) at Wuqi County in northern Chinese Loess Plateau (CLP) during late Neogene/Quaternary period, but the chronological results are unclear yet. So a precise age constraints is essential and it will provide not only new data for palaeoclimate and palaeoenvironment research, but also a reliable time scale in comparing with other coetaneous paleolakes.Primary investigations, including rock magnetism and palaeomagnetic chronology combining with multi-proxy method, were carried out on the typical Tufosi section at Wuqi County which consisted of overlying loess, lacustrine deposit and underlying red clay. Rock magnetic results demonstrate that the magnetic carriers of the characteristic remanent magnetization of the sediments are mainly magnetite and hematite. The correlation between magnetic polarity sequence and Geomagnetic Polarity Time Scale (GPTS) is very well. Totally eleven magnetozones are recognized, including 6 normal magnetozones and 5 reverse magnetozones. The age of the bottom of the section is 4.28 Ma B. P. approximately and Wuqi paleolake formed at about 3.0 Ma B. P. The age of the top of the lacustrine deposit is presumed as about 1.2 Ma B. P. because of the plane of unconformity between the lacustrine deposit and overlying loess. Palaeoenvironment of this region became colder and drier during this period as implied by the variations of lithology, magnetic susceptibility, grain size and sedimentation rates. The evolution history of Wuqi paleolake can be divided into three stages: the formation of the paleolake was during the period of 3.0-2.5 Ma B. P. and the water body was relatively deep. Although there were some precipitation and surface runoff in this region during this period, the overall trend of paleoclimate which became semiarid/arid led to the reduction of the paleolake afterwards. During 2.50-2.05 Ma B. P. the paleolake became shallow markedly and the sediments may even experience exposure above the water surface. During 2.05～1.20 Ma B. P. there were still some water body in the paleolake (relatively deeper than previous stage). The age of the top of lacustrine deposit is about 1.2 Ma B. P. estimated at the average sedimentation rate of the whole section. Climatic deterioration and the headward erosion of North Luo River caused by regional tectonism in this region around 1.2 Ma B. P. lead to the end of Wuqi paleolake finally.