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    Holocene humidity changes in inland China inferred from lake sediments
    GUO Chao, MAYuzhen, HU Caili, WU Yongqiu, LU Ruijie
    PROGRESS IN GEOGRAPHY    2014, 33 (6): 786-798.   DOI: 10.11820/dlkxjz.2014.06.007
    Abstract1681)      PDF (7648KB)(1857)      
    Environmental changes recorded by lake sediments have been one of the major tools for reconstructing the palaeoenvironment and palaeoclimate in many parts of the world. Spatial and temporal patterns of humidity changes during Holocene have been reconstructed based on the compilation of recently published paleoclimate records from the inland areas of China. The climate of this region is sensitive to large-scale climate forcing. We divided China's inland areas into the arid northwestern region, the East Asian monsoon-margin region and the Qinghai-Tibet Plateau according to the amount of precipitation and natural zones. Sediment records from 30 lakes with reliable chronologies and robust proxies were selected to reconstruct dry-wet conditions based on a three-class ordinal wetness index (dry, sub-humid, humid) with assigned scores from dry to wet periods at individual sites for 500-year time slices. Then we formulated the regional dry-wet index, which may represent the average change of the regional dry-wet conditions. The proxies used in these records include pollen assemblages, oxygen isotope ratio (δ 18O), organic matter and carbonate content (mainly pollen assemblages) data. The results of our synthesis show that the moisture conditions have experienced diverse changes over the Holocene in different regions of china's inland areas. Arid climate prevailed during the early Holocene, and relatively wet climate characterized the middle and late Holocene in arid northwestern China. But the climate change differed from place to place—the further into the west, the drier the climate during the early and middle Holocene and wetter in the late Holocene. In the East Asian monsoon-margin region, drier climate also prevailed during the early Holocene, and a wetter period may had occurred in the mid-Holocene, then the climate underwent another transition—it was driest in the late Holocene. In the Qinghai-Tibet Plateau, the wettest period may had occurred in the early and mid-Holocene, then the climate became dry gradually, but there was a tendency of wetness in late Holocene. The wet periods in eastern Qinghai-Tibet Plateau were longer than in the central and western regions. The comparative analysis reveals that the climate may have been mainly controlled by westerly circulation in arid northwestern China. But there were differences among the lake records in the timing of the beginning and end of the dry or humid periods as well as the intensity of the dry or wet conditions, which may be related to the strength of the Asian monsoon in the early Holocene and the influence of westerly circulation in the late Holocene. Furthermore, the insolation, global ice-sheets and topography of the Tibetan Plateau may have played important roles in controlling climate change in this area. The climate change in the East Asian monsoonmargin region is mainly influenced by the East Asian monsoon, which has also been recorded by the loess and desert deposits. The moisture conditions in the Qinghai-Tibet Plateau may have been controlled by the Indian monsoon, and the wetness in the early Holocene may be related to the strengthening of the Indian monsoon. Around 4 ka BP, a significant dry event commonly occurred in China's inland areas, which may be a global dry event that may have resulted in the decline of some ancient civilizations in the whole world.
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    Cited: CSCD(11)
    Spatial variation of palaeoflood deposits at Yanjiapeng site in the upper reach of the Han River
    ZHENG Shuwei, PANG Jiangli, HUANG Chunchang, ZHA Xiaochun, LI Xin
    PROGRESS IN GEOGRAPHY    2014, 33 (6): 799-806.   DOI: 10.11820/dlkxjz.2014.06.008
    Abstract865)      PDF (11011KB)(1030)      
    Through the stratigraphy and geomorphology investigation along the upper reach of the Han River, a loess-soil profile recorded palaeoflood slack-water deposits (SWD) was found at the riverbank near the Yanjiapeng site. This profile (YJP) was examined in great details in the field. The palaeoflood slack-water deposits samples were collected systematically and the grain size of the samples was analyzed in the laboratory. Results indicate that the grain size of the palaeoflood slack-water deposits shows a significant change in space. In the horizontal direction, grain size distribution, cumulative probability distribution and average grain size are the same in the same layer. In the top samples, the content of clay (<2 μm) is between 2.46%-2.97%, the content of silt (2-63 μm) is between 33.80%-44.50%, and the content of sand-sized particles (>63 μm) is between 52.66%-63.64%. The grain size distribution curves of these deposits are consistent in the same layer. In the vertical direction, however, grain size distribution, cumulative probability distribution and average grain size vary, and the grain size becomes smaller down from the top—the grain size distribution curves shift to the left gradually. The particle size of the peak position shows the following feature: top samples > middle samples > bottom samples. Some previous studies indicate that in the vertical direction the grain size of palaeoflood slack-water deposits becomes larger down from the top in the Wei River and Jing River. The results of this study at the YJP site in the upper reach of the Han River proved that the spatial variation of palaeoflood slack-water deposits has different forms. Similar grain size in the same layer indicates that the hydrodynamic force was basically the same in the horizontal direction and this result is good for sample collection in the field. The change of grain size in the vertical direction indicates that the hydrodynamic force was different and the reasons for this change may be related to the temporary river damming during the floods or the gradual built-up and quick withdrawal cycles of the palaeofloods. Samples should be collected layer by layer in the field study of grain size of palaeoflood slack-water deposits. If the palaeoflood slack-water deposits pinch out point cannot be identified in the field, grain size of palaeoflood slack-water deposits pinch out point of the same layer at close proximity can be used instead.
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    Cited: CSCD(3)
    Reconstruction and simulations for precipitation and atmospheric circulation over the past 30000 years in Asia
    LIAO Mengna, YU Ge
    PROGRESS IN GEOGRAPHY    2014, 33 (6): 807-814.   DOI: 10.11820/dlkxjz.2014.06.009
    Abstract965)      PDF (3805KB)(1514)      
    Lake-level changes respond to variations in regional water balance and are sensitive to climate changes. Thus it has been one of the most important indicators applied to reconstruct precipitation and water budget. Tracing long-term lake-level changes can provide references to scientific prediction of catchment floods and droughts, rational utilization of lake water resources, and protection of lake ecological environments in Asia. As there was no observed data from pre-industrial time, almost all long-term precipitation and water budget reconstructions rely on geomorphologic, sedimentologic, biogeologic and archaeological records and so on. In addition, the construction of systematic lake level databases has facilitated regional climate change research of Asia for the late Quaternary. This paper summarizes and analyzes lake-level changes in Asia over the past 30,000 years and is composed of 4 sections. Section 1 introduces the history of Asian lake level studies, construction of indicator system and lake status database, and deep-lake drilling in Asia. Section 2 analyzes time sequences of four subregions of Asia (Middle East, Central Asia, Tibetan Plateau, and East Asia) lake-level changes and infers climate conditions from the time sequences. Section 3 focuses on the spatial patterns and climate mechanisms of two key climate periods (mid-Holocene, LGM). General circulation models (GCM) and regional climate models (RegCM2) were used to reveal spatial distribution patterns of mean annual temperature, precipitation and water budget (P-E). (1) 6-ka BP (mid-Holocene): the increase in the Northern Hemisphere summer insolation led to a temperature increase in the mid-and high latitudes in Asia. Significantly enhanced Asian monsoon induced the increase of precipitation in Arabian, Indian, and Tibetan plateaus. The reduction in precipitation over southern China was linked with adjustments in the position and strength of the Pacific Subtropical High. Annual convergences over Mongolia, north-northeast China, Tibet, and India indicated more moisture and frequently wet conditions. Conversely, divergences over southern China contributed to dry conditions. (2) 21-ka BP: Annual temperatures were generally lower than today. Less precipitation over most areas of Asia and negative P-E anomalies in the Southeast Asian tropical area and East Asian lowlands were due to a weakened Asian summer monsoon and a persistent winter Mongolian High. A southward and eastward shift in the position of the Westerlies plus a decrease in evaporation in the cool conditions that prevailed across Asia led to increases in annual precipitation and P-E in west China. Section 4 provides a brief account of the importance of Asia lake-level research to the understanding of environmental changes in the future and the uncertainties about the past lake-level changes.
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    Cited: CSCD(3)