Guo Zhengtang

The magnetic susceptibility of modern soils in China and its use for paleoclimate reconstruction

SummaryMagnetic susceptibility of more than 160 modern silty soil samples in China was measured to assess the relationship between the magnetic susceptibility and modern climatic parameters. Correlation between magnetic susceptibility and mean annual temperature (MAT) or mean annual precipitation (MAP), shows a complex picture and no single function can be found to fit all the data on the national scale. In East China, where East Asian monsoon plays an important role for the climate conditions, magnetic susceptibility increases with the increase of MAT or MAP in temperate semi-arid regions of the Loess Plateau and surrounding areas. This can be attributed to increasing intensity of pedogenesis which would favor the formation of strongly magnetic minerals and/or reduce depositional rate of eolian dust. Magnetic susceptibility tends to decrease with the increase of temperature and precipitation in the tropical and subtropical warm and humid regions of the vast areas south of the Yangtze River. This may be explained by pedogenic transformation of iron-bearing minerals to weakly magnetic minerals. Between these two different correlation patterns, 15°C of MAT and/or 1200 mm of MAP seem to be the thresholds. In West China, the correlation becomes quite complex in the great mountains and vast sedimentary basins in the north-west. This may be due to the prevailing continental climate in this region and topographic contrast within short distance. The correlation for the Qinghai-Xizang (Tibetan) Plateau is not clear because very few samples were collected. Fluctuations of paleo-temperature and paleo-precipitation at Luochuan for the last 130 ka were estimated using the climofunction obtained from this study.

Multiple expansions of C4 plant biomass in East Asia since 7 Ma coupled with strengthened monsoon circulation

The expansion of plants using the C4 photosynthetic pathway is one of the most prominent changes in the global ecosystem during the Cenozoic Era. A significant late Miocene expansion is well documented. However, the existence and cause of subsequent expansions are still not clear, owing in part to the lack of long, continuous climate-vegetation records. Here we present two high-resolution carbon isotope time series, covering the past 7 m.y., derived from eolian deposits on the Chinese Loess Plateau. The data indicate three intervals of significant C4 plant expansions within the semiarid monsoonal region of East Asia (ca. 2.9–2.7 Ma, 1.3–0.9 Ma, and 0.6 Ma–present). These expansions covary with strengthened East Asian summer monsoon circulation. We conclude that in East Asia, large-scale late Miocene C4/C3 vegetation changes in semiarid areas have been primarily driven by warm seasonal precipitation and temperature variations associated with changes in monsoon circulation.

Prehistoric vegetation on the Loess Plateau: steppe or forest?

and Xian) was covered by an open meadow steppe with the presence of some coniferous species, while the middle part of the plateau (Luochuan) was covered by steppe. Our results do not support the notion that dense forest cover had developed on the Loess Plateau during the Holocene. Copyright 0 1996 Elsevier Science Ltd

Evolution of the monsoon and dry climate in East Asia during late Cenozoic: A review

Climate in Eastern Asia is composed of monsoon climate in the east, arid and semi-arid climate in the north and west, and the cold and dry climate of Qinghai-Tibetan Plateau in the southwest. The underlying causes for the evolution of East Asian climate during late Cenozoic have long been investigated and debated, particularly with regards to the role played by the Qinghai-Tibetan Plateau uplift and the global cooling. In this paper, we reviewed major research developments in this area, and summarized the important results. Based on a synthesis of data, we propose that the Qinghai-Tibetan Plateau uplift alone cannot fully explain the formation of monsoon and arid climates in Eastern Asia during the past 22–25 Ma. Other factors such as the global ice volume and high-latitude temperature changes have also played a vital role. Moreover, atmospheric CO2 changes may have modulated the monsoon and dry climate changes by affecting the location of the inter-tropical convergence zone (ITCZ), which controls the monsoon precipitation zone and the track of the East Asian winter monsoon during late Cenozoic. The integration of high-resolution geological record and numerical paleoclimate modeling could make new contributions to understanding the climate evolution and variation in eastern Asia in future studies. It could facilitate the investigation of the regional differences in East Asian environmental changes and the asynchronous nature between the uplift of Qinghai-Tibetan Plateau and their climatic effects. These would be the keys to understanding underlying driving forces for the evolution of the East Asian climate.

Thick Miocene eolian deposits on the Huajialing Mountains: The geomorphic evolution of the western Loess Plateau

The geomorphic evolution of northwestern China during the Cenozoic has been a subject of much geological interest because of its link with the uplift of the Himalayan-Tibetan complex. Much information about these changes is recoverable from the sedimentary sequences of the region. We report here on the thick eolian deposits mantling the Huajialing Mountains, a relatively flat mountain range in the western Loess Plateau. Correlation of magnetic susceptibility stratigraphy with the QA-I Miocene eolian sequence dates a 134.7 m section (NL-VII) for the interval from 18.7 to 11.8 Ma, as confirmed by micromammalian fossils. These eolian deposits demonstrate a much wider distribution of the Miocene eolian deposits, and also indicate that the topography contrasts in the western Loess Plateau, including the uplifts of the Huajialing Mountains and the bedrock highlands in the Qinan region, were formed by the early Miocene. The near-continuous Miocene eolian sequence from 18.7 to 11.8 Ma indicates that the substratum of Huajialing had not experienced any intense tectonic changes during this time interval, which suggests further, the relative tectonic stability of the nearby Tibetan Plateau.

Impact of topography and land-sea distribution on east asian paleoenvironmental patterns

Much geological research has illustrated the transition of paleoenvironmental patterns during the Cenozoic from a planetary-wind-dominant type to a monsoon-dominant type, indicating the initiation of the East Asian monsoon and inland-type aridity. However, there is a dispute about the causes and mechanisms of the transition, especially about the impact of the Himalayan/Tibetan Plateau uplift and the Paratethys Sea retreat. Thirty numerical sensitivity experiments under different land-sea distributions and Himalayan/Tibetan Plateau topography conditions are performed here to simulate the evolution of climate belts with emphasis on changes in the rain band, and these are compared with the changes in the paleoenvironmental patterns during the Cenozoic recovered by geological records. The consistency between simulations and the geological evidence indicates that both the Tibetan Plateau uplift and the Paratethys Sea retreat play important roles in the formation of the monsoon-dominant environmental pattern. Furthermore, the simulations show the monsoon-dominant environmental pattern comes into being when the Himalayan/Tibetan Plateau reaches 1000–2000 m high and the Paratethys Sea retreats to the Turan Plate.

Natural and anthropogenic impacts on the Asian monsoon precipitation during the 20th century

The increase in the global average temperature during the last century is considered an integrated result of anthropogenic and natural forcing, but different views remain about the anthropogenic impacts on the Asian monsoon precipitation. Based on the ∼2000-year records of stalagmite δ18O from the Dongge Cave (Guizhou Province) and Wanxiang Cave (Gansu Province), we address the possible anthropogenic impacts on the southwest and southeast Asian monsoon from 1900–2000 AD, using the method of Singular Spectrum Analysis (SSA). The results show that the monsoon precipitation trends in the last 100 years at both sites can be obtained through SSA prediction using the data prior to 1900 AD. These suggest that human activity has not significantly affected the trends of monsoon precipitation despite of its impact on the global temperature.

Climate changes in China since the Last Glacial Maximum and Holocene human adaptation

Based on various geological records of high-resolutions, this project aims at addressing the climate and environmental histories in China since the Last Glacial Maximum (LGM) and human impacts on the Holocene environmental changes. Significant progresses have been obtained with regards to the impacts of past temperature changes to Asian monsoon and to the arid/semi-arid ecosystems in China. The results also suggest that changes in the concentrations of greenhouse gases in the pre-industrial era were mainly attributable to natural driven forces although early agriculture had significantly affected the landscapes.

Sedimentary and geochemical evidence of Eocene climate change in the Xining Basin, northeastern Tibetan Plateau

The northeastern Tibetan Plateau began to grow during the Eocene and it is important to understand the climatic history of Asia during this period of so-called ‘doubthouse’ conditions. However, despite major advances in the last few decades, the evolutionary history and possible mechanisms of Eocene climate change in the northeastern Tibetan Plateau remain unclear. The Xining Basin in the northeastern Tibetan Plateau contains a continuous sequence of Early to Late Eocene non-marine sediments which provides the opportunity to resolve long-term climate changes during this period. In this study, we report the results of analyses of lithofacies, sediment color and geochemistry of bulk samples collected from the Xijigou section of the Xining Basin. An abrupt lithofacies change between the Early (∼52–40 Ma) and Late Eocene (∼40–34 Ma) indicates a change in the depositional environment from a shallow lake to a playa lake in response to a significant climatic shift. During ∼52–40 Ma, higher values of sediment redness (a*), redness/lightness (a*/L*) and higher modified Chemical Index of Weathering (CIW’) indicate a relatively warm and humid climate, while from ∼40–34 Ma the lower values of a*, a*/L* and lower CIW’ imply sub-humid to semi-arid climatic conditions. The paleoclimatic records indicate a long-term (∼52–34 Ma) trend of decreasing chemical weathering, consistent with global climate change. An abrupt sharp excursion of the proxy records during ∼42–40 Ma suggests a relatively brief warm interval, corresponding to the Middle Eocene Climatic Optimum (MECO). We suggest that global cooling substantially reduced humidity in inner Asia, resulting in sub-humid to semi-arid climatic conditions after 40 Ma in the Xining Basin, which may have been responsible for the long-term trend of decreasing chemical weathering during the Eocene.