关键词: 全新世, 洪水, 干旱, 土壤退化, 黄土高原

Abstract: More and more intensified dust storms and dust deposition currently over the Loess Plateau of China provideanalog for understanding the monsoonal climatic change and reconstruction of the Holocene environment in thissemiarid zone. T'hick profiles of the Holocene loess palacosol sequence were found on the Zhouyuan loess table-land in the western Guanzhong Basin, Shaanxi Province. Detailed stratigraphic and pedological observationswere made in the profile during the filed work. The fragments of the Neolithic cultural remains identified in theprofile provide chronological control in the sequence. Multi-disciplinary analyses, including magnetic susceptibili-ty, total Fe, grain-size and total organic carbon, carried out in laboratory show that Holocene climate has beenhighly variable in this environmental sensitive region. Several important climatic and hydrological events arerecorded in this Holocene profile. A flood phase was identified between 9 000~8 800 a BP (cal. ) during the dli-matic amelioration in the beginning of the Holocene. The flood was from the uplifted pediment of the BeishanMountains on the north. T'he Holocene Megathermal (climatic optimum) was identified between 8 500~3 100 aBP. during which policyclic cinnamon soils were developed on the loess tableland.An abrupt climatic shift fromhumid to dry conditions took place at about 6 000 a BP, therefore, a remarkable climatic aridity phase was foundbetween 6 000~5000 a BP. The Holocene Megathermal (climatic optimum) was thus split into two main peri-ods (8 500～6 000 a BP and 5 000~3 100 BP) with warm wet climate. T'he earlier period was more intensivein its climatic warmness and humidity. This means the Holocene Megathermal was not constant and stable asthought in the past. Another climatic shift was identified at about 3 100 a BP. The increase in climatic ariditysince then has caused intensified dust storms and acceleration of dust accumulation- Because of the largely decreased precipitation, the deficit soil moisture and intense dust deposit has thus caused soil deterioration on theloess tableland. At the present, a layer of about 0.5~～O. 8 m thick loess has blanketed the landscape on the loesstableland. The present ploughed topsoil is only the top part of this recently accumulated eolian loess sediment. This topsoil is rather poorly developed compared with the Holocene palaeosol So in the middle part of the profile. These results are very important for underst anding of the regional response to global change in the semi humid tosemi-arid zone within the middle reaches of the Yellow River drainage basin.

Key words: Holocene, floods, climatic aridity, soil degradation, Loess Plateau