Shangfa Xiong

Pedostratigraphy and paleomagnetism of a ∼7.0 Ma eolian loess–red clay sequence at Lingtai, Loess Plateau, north-central China and the implications for paleomonsoon evolution

Abstract A 305-m-thick loess–red clay sequence was discovered recently at Lingtai, which is located in the middle part of the Chinese Loess Plateau. It consists of a complete Pleistocene loess–soil sequence with a thickness of about 175 m and 130 m of Tertiary red clay deposits. The red clay sequence at Lingtai is the thickest one presently known in the Loess Plateau. Field observations show that the Lingtai red clay contains over 110 couplets of pedogenic B horizons and horizontal carbonate nodule horizons, and that it can be regarded as an extremely thick soil complex. Paleomagnetic studies of 680 samples suggest that the basal age of the Lingtai loess–red clay sequence is about 7.05 Ma. Grain size analysis of samples taken at 3.3 cm intervals indicates that the red clay has the same sedimentological characteristics as those observed in the loess, thus suggesting a wind-blown origin of the Tertiary red clay and continuous atmospheric dust deposition in the Loess Plateau during the last 7.05 Ma. The pedogenic characteristics of the paleosols within the Pleistocene loess and the B horizons in the red clay suggest that the East-Asia summer monsoon in the latest Miocene may have already been slightly stronger than that during the Holocene. This implies that at about 7.5 Ma BP, the Tibetan Plateau could have been uplifted to a critical height in maintaining the East-Asia summer monsoon system. Observations of the loess–red clay sequence also suggest that the long-term changes in the East-Asia summer monsoon strength have been nonlinear since the latest Miocene.

Coeval changes in grain size and sedimentation rate of eolian loess, the Chinese Loess Plateau

To determine the temporal-spatial relationship between strength of the winds transporting the Chinese loess and aridity conditions of the loess source area, six loess sections located respectively in the northern, middle, and southern Loess Plateau were studied. Grain size distributions were analyzed on loess-paleosol samples that are closely spaced in the sections. Time scales were established by tuning each of the grain size records to theoretical changes in the Earth‧s orbital parameters. Results obtained show that during the last two glacial cycles, particle size of the loess co-varies with apparent sedimentation rates, suggesting that variations in winter monsoon intensity and the aridity of the loess source area are tightly coupled. This proximal relationship is in striking contrast with the observation of distal eolian clays in North Pacific sediments. We propose that this phenomenon may result from the proximity of the Loess Plateau to the source area and from the significant control of the winter monsoon on the expansion-retreat changes of the deserts in the northern and northwestern China.

Glacial environments on the Tibetan Plateau and global cooling

Abstract The glacial environments on the Tibetan Plateau and the mechanisms for glacier and snow accumulation are discussed on the basis of new evidence of global temperature fluctuations and regional biome type changes. The biome types show that extensive snow and glacier fields could develop on the Tibetan Plateau with a temperature lowering of 7–9°C and precipitation decrease by 30–70%. Considering the cooling effect due to an increase of albedo resulting from increased snow and ice coverage, it is possible that the valley glaciers became enlarged to form large glaciers and snow fields during the Last Glacial Maximum. This new environmental modeling shows that the environmental conditions changed considerably during past glaciations. Such changes might have notable impacts on the monsoons and possibly on global climate.

Comparisons of grain size characteristics of red earth from southern China with that of loess and dune sand from Bering region

The comparisons of grain size distributions among eolian deposits along the monsoon wind trajectory are a fundamental task for the reconstruction of the past wind regimes. Here the authors sampled the dune sands in Hunsandak desert, loess deposits in Beijing region and red earth in southern China. Grain size analyses show that the distribution models of the loess and dune sand samples are unimodal whereas the red earth and paleosol are polymodal. The red earth, paleosol, loess and dune samples are distinguished and closely related to one another in grain size parameter diagrams. Moreover, the grain size distributions and parameters of the red earth and paleosol are similar, implying that the red earth in southern China and the paleosol in northern China had the same origin.

Pedogenic carbonate isotope record of vegetational evolution since late Miocene in Loess Plateau

Isotopic analyses for paleovegetational evolution have been carried out on samples of the pedogenic carbonate nodules in the Red Clay-Loess sequence at Lingtai (35°N), the Loess Plateau. Stable carbon isotopic composition indicates that (i) C4 plants might be present at least by 7.0 Ma; (ii) C4 plants expanded gradually between ∼4.0 and ∼3.2 Ma, and their biomass fraction was up to 50%; and (iii) the biomass of C4 vegetation since ∼ 2.0 Ma seems to have been decreased to the level (about 20%) before 4.0 Ma. C4 plant expansion at Lingtai cannot be fully understood with the “global C4, expansion” model because it occurred much later up to 3.0 Ma than in Pakistan, which indicates that some changes in the regional climatic system may also contribute to C3/C4 shift except changes in atmospheric CO2, concentrations and temperature. The latitudinal zone for C3,/C4 transition seems to move southwards slightly in East Asia, compared with the case in North America where 37°N is the ideal boundary for the C4, expansion.

The worm-shaped veins in the red earth of South China—Pedological evidence for root traces of past forest

Field observation of the macromorphology and analyses of micromorphology and geochemistry were conducted to the worm-shaped veins (WSVs) in the red earth of South China. The WSVs exhibit outward features of root traces and contain the traces of stele that can be used as the definite evidence of root traces. Surrounding the worm-shaped veins there are some micro-features pressed by past root growth. Geochemical analyses indicate that the white part in the WSV was depleted in Fe content. These lines of evidence suggest the WSVs in the red earth of South China are the root traces of past forest.

Rainfall thresholds for the precipitation of carbonate and evaporite minerals in modern lakes in northern China

Authigenic carbonate and evaporite minerals in lake sediments are widely used to qualitatively reconstruct climate. However, uncertainties still remain about their quantitative relationship to climate. Here we investigate 86 modern lakes in northern China to examine the relationships between mineral formation, lake water chemistry, and climate. The results show that from east to west, with increasing salinity and ionic concentration, calcite, dolomite, and evaporite minerals (gypsum and halite) occur in sequence. Their eastern boundaries approximate modern isohyets, and we define for the first time rainfall thresholds of 600 mm, 400 mm, and 350 mm for the formation of calcite, dolomite, and evaporite minerals, respectively. Since the 400 mm and 600 mm isohyets also coincide with vegetation boundaries, our findings enable a new approach for the quantitative reconstruction of paleoprecipitation and paleovegetation based on mineral analysis.

A climatic reversal at the last interglacial-last glacial transition recorded in the loess section at Zhaitang, Beijing

A weakly developed paleosol was identified at S1-L1 transition of the loess section of Zhaitang, Beijing. This paleosol may represent a climatic reversal which is associated with the strengthening in summer monsoon and correlated well with the climatic regression event documented in the paleoclimate records from Greenland ice, marine sediments and European pollen at the last interglacial-last glacial transition.