Chaofeng Fu

Magnetostratigraphic determination of the age of ancient Lake Qinghai, and record of the East Asian monsoon since 4.63 Ma

Lake Qinghai, in North China, is the largest interior plateau lake in Central Asia, and is sensitive to climate change and the environmental effects of Tibetan Plateau uplift. We have obtained an almost continuous 626 m long sediment core from an in-filled part of the southern lake basin, which documents both the age of the origin of the lake and the evolution of the East Asian monsoon during the Late Cenozoic. High-resolution magnetostratigraphy provides a chronology back to ca. 5.1 Ma. Analysis of lithofacies and depositional environments reveal that the change from eolian to lacustrine facies occurred at ca. 4.63 Ma, corresponding to a shift from an arid or semi-arid to a more humid climate, which resulted in the origin of Lake Qinghai. Changes in sediment lithology and mean grain size indicate that the lake level fluctuated considerably, superimposed on a long-term trend from higher to lower levels in response to variations in the East Asian Monsoon. This archive is a significant additional source of information on regional and global environmental change, complementing the existing records from north China, which are mainly based on analysis of loess deposits.

Occurrence of greigite in the Pliocene sediments of Lake Qinghai, China, and its paleoenvironmental and paleomagnetic implications

Lake Qinghai in North China is the largest interior plateau lake in Central Asia and is climatically sensitive. An almost continuous 626 m long sediment core was drilled in an infilled part of the southern lake basin of Lake Qinghai. The magnetic susceptibility record reveals the presence of two distinct peaks within an interval of fine-grained lacustrine sediments of Lower Pliocene age. We selected a depth interval of approximately 40 m spanning the magnetic susceptibility peaks for detailed rock magnetic and geochemical analyses in order to identify the magnetic mineralogy responsible and to assess its possible paleoenvironmental and paleomagnetic implications. Rock magnetic, X-ray diffraction analysis, scanning electron microscopy (SEM), and energy dispersive analysis of X-ray (EDAX) analyses reveal that the main magnetic carrier is greigite (Fe3S4). The greigite is of early diagenetic origin and formed in an inerval of high lake level and inferred relatively warm, humid climate. The greigite-enriched zones are separated by an interval of relatively high total sulfur and organic carbon content, and we infer that in the adjacent greigite-bearing zones, the lower concentrations of sulfur and organic carbon, and high levels of reactive iron, arrested the process of pyritization resulting in the preservation of the greigite on a time scale of several million years. The greigite zones contain narrow intervals of normally magnetized sediments which may be previously unrecognized cryptochrons within the Gilbert Chron, or alternatively they may reflect the continued formation of greigite long after the age of deposition of the surrounding sediment matrix.

Geochronological and stratigraphical evidences for the uplift of the Liupan Shan, Northeastern boundary of the Tibetan Plateau

The Liupan Shan (Mountains) is an important tectonic boundary between the Tibetan Plateau and North China Platform. According to basin-moutain coupling model, the deposits in basin is sensitive to the tectonic and environmental events information related to the adjacent mountains. Geo- chronological of paleomangetism of late Cenozoic six red clay/bed sequences from the east foreland basin of the Liupan Shan revealed that the red clay/bed began to accumulate as early as 8.1 Ma. The initiation of red clay/bed deposition indicated the uplift of Liupan Shan at that time, which cause the disaggregating of Ordos planation surface. Stratigraphical and sedimentary rate imply the Liupan Shan experienced several tectonic uplifts, especially at late Pliocene. The undulating nature of the planation surface may be responsible for the discrepant and controversial ages and origins of the red-clay. The uplift events were mainly associated with tectonic thrusting along the northeastern boundary of the Tibetan Plateau.

Formation of the Yazi Spring Stream and its significance on tectonics-climate on the northern slope of Kunlun Mountains

The sequences of fluvial terraces in the Yazi Spring Stream are signs of the stepwise uplift of the Kunlun Mountains in the northern part of the Tibetan Plateau since the Late Pleistocene. Geomorphic and sedimentary features of the terraces reveal that they have resulted from the phased tectonic uplift and the consequent river incision in the northern plateau. Using the method of Single-aliquot Regenerative-dose (SAR) Protocol and Radiocarbon 14C dating, the deposit ages of three-grade terraces were obtained, which are 57.5, 12.8 and 5.7 kaBP, respectively. The features and ages of terraces reveal that the incision rate of the stream accelerated at the beginning of the Holocene. The incision rate changed suddenly at 12.8 KaBP, from 0.43±0.07 mm/a to 1.59±0.55 mm/a. This implicates that uplift of the Kunlun Mountains is intensive at the first onset of the Holocene, corresponding to the obvious change of slip-rate on the Altyn Tagh Fault. But its uplift rate is much lower than that of the latter, which suggests that growth of the northern margin of the Tibetan Plateau is stronger than its interior.