中国北方近二万年来环境变化的若干地质证据

SOME GEOLOGICAL EVIDENCES OF ENVIRONMENTAL CHANGE IN THE LAST 10,000 YEARS IN NORTH CHINA

中国北方晚冰期以来的连续堆积主要是湖泊沼泽相粘土、淤泥和风积黄土,这两套堆积地层保存有丰富的环境变化历史的信息。沉积相的分析和年代学、孢粉学的研究结果表明,近一万年来曾出现过三度凉暖和干湿更替,从而可划分出早、中、晚三个不同的自然环境历史阶段,年龄界线大约是7,500aB.P.和2,500aB.P.。全新世早期湿润温凉,全新世中期湿润温暖,5,000aB.P.是植被最繁盛时期。北部地区的植被在全新世早、中期没有明显的变化界线。约5,000aB.P.后,气候有变干变凉趋势。约2,300aB.P.为全新世以来气候最干凉时期。此后,温度和湿度都有逐渐回升的趋势。在中国北方地层中尚未找到小冰期气候的充分的地质证据。

Continuous deposition sections of the last 10,000 years in North China are mainly two stratigrahdic units: one is the lacustrine-swamp clay or muds and another is aeolian loess. Generally, both of them contain rich records of the environmental changes. With the help of analysis of sedimentary facies and studies on chronology and palynology, the upper partof the Dishaogou section of the Salawusu Formation indicates that three climatic episodes in the last 10,000 years can be distinguished, and Holocene period can be subdivided into three stages——Early, Middle and Late Holocene with boundary ages of ca, 7,500 and 2,500 a B.P.respectively. Early Holocene is a moister and colder stage, and the Middle Holocene is a moister and warmer one. A lush growth of trees and grass existed in about 5,000 a B. P. No sharp boundary between Early and Middle Holocene have been found. After the lush growth of vegetation (ca. 5,000 a B. P.), there was a tendency getting drier or colder and the coldest and driest period laid in about 2,300 a B. P. Afterwards, the temperature and humidity show a tendency of increasing. Three layers of black loam developed in Holocene aeolian loess section, their ages show a longitudinal change in general. The samples from south area with an older age and those from north area with younger age, which suggested the development of the black loam started from southern part then northward. Whereas, overlying loess superimposed on black loam was deposited from northern part then southward. Middle Holocene was under the warmest climate condition, this period was the optimum stage for the btack-laom development. So the black loam of Middle Holocene became widespread in an extensive area in middle reaches of the Huanghe River. Holocene lacustrine section in the Aibi Lake in North Xinjiang contains 3 burned layers with ages of 6,600±700, 1,600±130 a B. P. and 1962 A. D. respectively. According to stratigraphic levels, they occurred in the end or middle period of lower hydrographic level stage and desiccation stage. This kind of geological records should be further noticed.