黄土高原35万年来冬、夏季风变化周期的差异——陆生蜗牛化石的证据

针对我国北方风积黄土广泛分布、不同地点黄土-古土壤序列可准确对比和黄土沉积剩磁稳定等特征,通过对不同地壳块体上相同层位的黄土样品沉积剩磁方向的测定,进一步探讨了应用"虚磁极法"研究鄂尔多斯块体与相邻块体的相对运动的可能性.初步的研究结果显示:该方法有可能应用于块体相对运动的定量研究,并初步得到,在第四纪时期鄂尔多斯块体相对于邻近块体作逆时针转动,自2.5Ma B.P.以来相对秦岭块体的转动约为31.5°±11.4°,自1.17Ma B.P.以来相对西宁块体的转动约为29.8°±14.7°,自0.46Ma B.P.以来相对太行山块体的转动约为8.2°±5.8°,相对于西宁块体的转动约为10.5°±6.4°.本文还根据这些块体间相对转动的角度初步估算了鄂尔多斯块体与相邻块体之间的活动断裂带不同时期的水平位移总量.

ORBITAL FORCING OF EAST ASIAN SUMMER AND WINTER MONSOONVARIATIONS IN THE PAST 350 000 YEARS

A high-resolution study of terrestrial mollusk from the Luochuan and Changwu loess sequences in the Loess Plateau, China provides a continuous record of paleoenvironmental and paleoclimate changes in the past 0.35Ma. 251 samples for the Luochuan section and 254 samples for the Changwu section, collected at an interval of 10 cm, were analyzed for fossil mollusks. Generally, 100～800 individuals were counted in each sample. Three typical mollusk species were selected to be the proxy indexes of East Asian summer and winter monsoon variations. The changes in the absolute abundance of thermo-humidiphilous mollusk species (Punctum orphana) indicate that the strength of the East Asia summer monsoon is sensitive to orbital forcing at the obliquity period (41 000a) and at the precession period (23 000a). On the contrary, the variations in the absolute abundance of coldaridiphilous mollusk species (Vallonia tenera and Pupilla aeoli) display a different pattern, and the strongest powers were centered at two periods: about 0.1Ma and 41 000a, which imply the major periods of winter monsoon variations in the Loess Plateau of China. This different response suggests that these two monsoon systems are tightly coupled in a 41 000a cycle, but not consistently correlated to the precession cycle (19 000～23 000a) and the eccentricity cycle (0.1Ma). The relationship between the winter monsoon and the summer monsoon at the orbital timescale is not a simple model of one falling after another rising.