关中盆地西部黄土台塬全新世气候事件研究

HOLOCENE CLIMATIC EVENTS ON THE LOESS TABLELAND IN THE WESTERN GUANZHONG BASIN,CHINA

通过对陕西歧山黄土剖面地层学和土壤学研究 ,磁化率、全铁、粒度、CaCO3 和TOC测定分析 ,发现在全新世的全球性“大暖期”即将到来之前 ,黄土台塬面有洪水发生。在全新世大暖期当中 ,季风气候有明显的波动变化。尤其是在 6 0 0 0aBP前后季风气候转折 ,形成了 6 0 0 0～ 5 0 0 0aBP显著的干旱阶段。这使得全新世大暖期分裂成为两个主要的温暖湿润阶段。各项气候代用指标显示 85 0 0～ 6 0 0 0aBP是大暖期中最为温暖湿润的时期。在大约 310 0aBP季风格局发生变化 ,气候向着干旱化发展 ,全新世的成壤期为黄土堆积期所取代。这些事实对于我们客观地评价半湿润 -半干旱地区的季风气候对于全球变化的响应规律具有重要的科学意义。

More and more intensified dust storms and dust deposition currently over the Loess Plateau of China provide analog for understanding the monsoonal climatic change and reconstruction of the Holocene environment in this semi-arid zone. Thick profiles of the Holocene loess-palaeosol sequence were found on the Zhouyuan loess tableland in the western Guanzhong Basin, Shaanxi Province. Detailed stratigraphic and pedological observations were made in the profile during the filed work. The fragments of the Neolithic cultural remains identified in the profile provide chronological control in the sequence. Multi-disciplinary analyses, including magnetic susceptibility, total Fe, grain-size and total organic carbon, carried out in laboratory show that Holocene climate has been highly variable in this environmental sensitive region. Several important climatic and hydrological events are recorded in this Holocene profile. A flood phase was identified between 9 000～8 800a BP (cal.) during the climatic amelioration in the beginning of the Holocene. The flood was from the uplifted pediment of the Beishan Mountains on the north. The Holocene Megathermal (climatic optimum) was identified between 8 500～3 100a BP,during which policyclic cinnamon soils were developed on the loess tableland. An abrupt climatic shift from humid to dry conditions took place at about 6 000a BP, therefore, a remarkable climatic aridity phase was found between 6 000～5 000a BP. The Holocene Megathermal (climatic optimum) was thus split into two main periods (8 500～6 000a BP and 5 000～3 100a BP) with warm-wet climate. The earlier period was more intensive in its climatic warmness and humidity. This means the Holocene Megathermal was not constant and stable as thought in the past. Another climatic shift was identified at about 3 100a BP. The increase in climatic aridity since 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 the loess tableland. At the present, a layer of about 0.5～0.8m thick loess has blanketed the landscape on the loess tableland. 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 S 0 in the middle part of the profile. These results are very important for understanding of the regional response to global change in the semi-humid to semi-arid zone within the middle reaches of the Yellow River drainage basin.