甘肃天水全新世黄土粒度、磁化率特征及其古气候意义

Characteristics of grain-size and magnetic susceptibility and their paleoclimatic significance in Tianshui City, Gansu Province

结合光释光测年和文化遗迹的断代技术,在系统测试甘肃天水师家崖黄土-古土壤剖面的磁化率和粒度基础上,分析天水黄土-古土壤的沉积学特征和成壤演化特征,揭示天水地区全新世以来的气候变化规律.结果表明:师家崖剖面是典型的风成堆积的黄土剖面,全新世剖面中粒度组成主要以粗粉砂(16~63μm)和细粉砂(2~16μm)为主,平均质量分数分别为54.9%、20.3%,其次为黏粒和砂粒;粒度参数显示,粒度组成较均一,分选性较好,峰态呈中等,偏度属正偏型.黏粒、细粉砂、黏粒/粗粉砂和磁化率在古土壤层出现高值,在黄土层为低值;而粗粉砂变化趋势相反.天水地区全新世时期气候变化可以分为3个阶段:全新世早期(11 500~8 500 aBP)气候温凉;全新世中期(8 500~3 100 aBP),气候总体上温暖湿润,但并不持续稳定,其中7 000~6 000 aBP出现气候波动;全新世晚期(3 100 aBP以来),气候又进入一个相对温凉少雨的时期.

Based on an analysis of the magnetic susceptibility and grain size of a representative loess-paleosoil profile in Tianshui, Gansu Province, and combined with the optically stimulated luminescence technique and dating techniques of cultural remains, we discussed the sedimentary and pedogenic characteristics since the Holocene. The results showed that the grain-size distribution was mainly composed of coarse silt(16-63 μm) and fine silt(2-16 μm), which accounted for 54.9% and 20.3%, respectively;the composition of loess particles had the characteristics of high homogeneity. The frequency curve was multimodal and positive and the sorting of the aeolian loess was medium and better. According to the results,the content of clay, fine silt, clay/coarse silt rate and magnetic susceptibility showed a similar curve trend,with a relatively high concentration in the paleosol layer while being lower in the loess. The variation tendency of coarse silt was different from that of clay, fine silt and clay/coarse silt rate. Based on the data,climate change could be divided into 3 stages: the warming up during the Early Holocene(115 00-8 500 aBP), but of still cool and relatively rich precipitation;the generally warm and humid during the Mid Holocene(8 500-3 100 a BP) with the climatic fluctuation during 7 000-6 000 a BP;and the relative cool and less rainfall during the Late Holocene(3 100 aBP-at present). More detailed studies could promote our understandings of the global climates changes and the mechanisms behind these changes.