河北平原南部曲周地区早、中全新世冲积物的分析及古环境状况的推测

在河北平原南部的曲周县四町乡河间洼地一深为250cm的探坑中,取得50～250cm深度范围的沉积物样品。对这些冲积物样品做了AMS14C测年、粒度分析、矿物磁性测量、碳酸盐中氧、碳稳定同位素分析、全有机碳和碳氮比值分析、孢粉分析。结果表明,全有机碳极低,孢粉甚少;矿物磁性参数和稳定同位素含量随粒度的变化与黄土高原的黄土-古土壤序列中的情形十分相似。由此推测这些变化在一定程度上反映了成土作用对冲积物的影响和气候状况的变化,并对这一地区的早、中全新世古环境划分为6个变化阶段:在约10210～10000aB.P.,气候温暖湿润;在10000～8500aB.P.,气候凉干;在8500～8000aB.P.,气候又转为温暖湿润;在8000～7200aB.P.,气候再趋凉干;在7200～约6500aB.P.,气候再度变为温暖湿润;在约6500aB.P.以后,气候渐趋凉干。

ALLUVIUMS OF THE EARLY AND MIDDLE HOLOCENE IN THE QUZHOU AREA, THE SOUTHERN HEBEI PLAIN AND PALAEOENVIRONMENT INFERENCES

Sediments were sampled from the depth range of 50～250 cm in a 250cm-deep pit that is located in an inter channel depression in Sitan Township, Quzhou County, southern Hebei Province. These samples were studied by several methods: AMS 14C dating, grain size analysis, mineral magnetism measurements, analysis of oxygen and carbon stable isotopes in carbonates, analysis of the total organic carbon content and carbon/nitrogen ratio, and pollen analysis. The results indicate that the total organic carbon content is very low, pollen grains are very sparse, and the variations of mineral magnetism and stable isotopes with grain size are very similar to those in the loess/palaeosol sequences of the Chinese Loess Plateau. They may to some extent reflect different effects of pedogenic modification on the alluvium and the changes of climatic conditions. Thus it is inferred that the climate in this area was warm and wet in the period of about 10 210～ 10 000 a B.P., cool and dry in the period of 10 000～8 500 a B.P., warm and wet in the period of 8 500～8 000 a B.P., cool and dry in the period of 8 000～7 200 a B.P., and warm and wet in the period of 7 200～about 6 500 a B.P. It has gradually been getting cool and dry again after about 6 500 a B.P. These inferences are generally in agreement with what was previously concluded by other researchers.