柳夼剖面末次冰期层段Rb/Sr的不稳定变化及其揭示的气候特征

Climatic Characteristics Indicated by the Variations of Rb/Sr in the Liukuang Section during the Last Glacial Period

位于山东半岛北部成山头附近的末次冰期地层,被称为"柳夼红层"。在野外调查的基础上,选择柳夼剖面进行工作,其末次冰期地层主要由沙丘砂与古土壤或湖积层叠覆堆积组成,地质时代为13.1―78.9 ka BP。根据铷、锶、Rb/Sr和矿物组成的分析结果,得出结论如下:1)不稳定矿物尤其是钾长石和斜长石等分解释放出铷、锶,释放量随着风化作用加强而增加。地球化学行为的差异使铷在风化过程中被吸附而锶淋失,导致铷的质量分数在沉积相中表现为沙丘砂<砂质古土壤<湖积层<粉砂质古土壤,锶则表现为沙丘砂>湖积层>砂质古土壤>粉砂质古土壤。2)铷、锶和Rb/Sr在层段上呈现出明显的峰谷变化,将末次冰期的气候波动特征呈现为LKS4、LKS2的寒冷期和LKS3的温暖期,LKS3又可划分为LKS3b的寒冷期和LKS3c、LKS3a的温暖期,体现了万年尺度的气候波动特征。这些暖阶段同步于中国季风区葫芦洞石笋记录的夏季风强盛期和65°N 7月份天文辐射总量的高峰,冷阶段对应于夏季风减弱期和天文辐射总量低谷,表明柳夼剖面末次冰期的气候变化是太阳辐射量和东亚夏季风格局共同影响下的结果。

Liukuang Red bedding of the Last Glacial is located near Chenshantou in the north of Shandong Peninsula. Based on the field investigation, we found the Liukuang section mainly consists of the alternating deposition of dune sands and paleosol or lacustrine, and its geological age is 13.1-78.9 ka BP. According to the analysis of the contents ofRb, Sr, Rb/Sr ratio and other mineral composition, we got the conclusion as follows： 1） Rb and Sr are released by the decomposition of instable minerals, especially by the potash feldspar and plagioclase, and their amount is increasing as the weathering strengthens. Rb is absorbed and Sr is leaching because of their different geo-chemical properties, which led to the different Rb contents in different sedimentary facies, that in dune sand〈 sandy paleosol 〈 lacustrine 〈 silty paleosol, and the Sr contents also varying in different sedimentary facies, that in dune sand 〉 lacustrine 〉 sand paleosol 〉silty paleosol. 2） The value of Rb, Sr and Rb/Sr ratios shows remarkable variations in alternation of peaks and valleys, of which LKS4 and LKS2 represent cold periods and LKS3 represents warm period which includes LKS3b cold period and LKS3a and LKS3c warm periods. It suggests the climate variations on ten-millennium scale. These warm periods mainly correspond to strong summer monsoon indicated by stalagmite in Hulu Cave in China, as well as the peak of total amount of astronomical insolation at 65~N in July, and cold periods correspond to weak summer monsoon and low value of total amount of insolation, which reveals that the climate changes in the Liukuang section during the Last Glacial period probably results from the mutual impact of total amount of insolation and East Asian summer monsoon.