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

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

通过对黄土高原洛川、长武剖面最近 3 5万年来陆生蜗牛化石的高分辨率研究 ,揭示出喜温湿和喜冷干蜗牛种类分别存在不同的变化周期 :喜温湿类蜗牛化石所揭示的东亚夏季风的变化存在明显的 41 0 0 0a和 2 3 0 0 0a周期 ,与地轴倾角和岁差 (低纬度太阳辐射 )的变化周期有关 ;而反映冬季风变化的喜冷干蜗牛种类则表现出不同的变化特点 ,其主要为 0 .1Ma和 41 0 0 0a的周期。冬、夏季风的变化与地轴倾角 41 0 0 0a周期都存在很好的相关性 ,但与地球轨道的岁差 (1 90 0 0～ 2 3 0 0 0a)及偏心率 (0 .1Ma)的周期没有一致的相关关系。在轨道尺度上冬、夏季风之间不存在此消彼长的简单变化模式。

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.