黄土高原黄土粒度的空间变化及其古环境意义

SPATIAL CHANGES IN GRAIN SIZE OF LOESS DEPOSITS IN THE CHINESE LOESS PLATEAU AND IMPLICATIONS FOR PALAEOENVIRONMENT

20世纪50～60年代，刘东生先生组织了黄河中游黄土区十条大断面（六纵四横）的野外考察和室内分析，从而证实了从西北到东南黄土粒度逐渐变细的现象，并划分出砂黄土、黄土和粘黄土带。从此，粒度成为黄土研究最基本、最重要的物理指标之一。时至今日，黄土粒度的古气候意义依然有待进一步明确。在前辈研究的基础上，我们对黄土高原57个s2以上剖面进行了详细调查和粒度分析，构建了典型冷暖时期粒度等值线。结果显示，无论是黄土还是古土壤，其粒度均从北向南变细，粒度等值线整体上呈现近东西向展布，表明粒度空间分异以南北向为主。地质记录综合对比显示，沉积区距物源区的距离变化对黄土粒度的影响是第一位的，冬季风风力变化的影响是第二位的。据此，我们构建了“黄土中值粒径一沉积区距源区最小距离”的模型。模型显示：距源区最小距离在400km以内为“快速分异区”，粉尘沉积的粒度随搬运距离增加迅速变细，黄土高原黄土即在此区域；在400～2000km间为“缓慢分异区”，粉尘粒度随搬运距离的增加缓缓变细，黄土高原新近纪红粘土即为典型代表；距源区2000km以上为粉尘沉积与搬运风力的“平衡区”，粉尘粒度随搬运距离增加变化很小，深海粉尘沉积为典型代表。依据低空搬运的黄土“粒度一距离”模型推测的远距离（〉2000km）搬运后的粉尘粒度为1～3μm，同高空搬运的北太平洋现代降尘和深海粉尘沉积粒度（2～4μm）非常吻合，表明该模型可能揭示了风力这样一种地质营力的内在动力学特征，这一点需要在今后研究中予以关注。

In 1950- 1960s, Liu Tungsheng organized field and laboratory investigations on ten loess transects （six meridional and four transverse directions） across the Chinese Loess Plateau, confirmed the southeastward fining pattern of loess grain size in the Loess Plateau, and identified zones of sandy loess, loess and clayey loess from northwest to southeast. Since then, loess grain size has become a basic palaeoclimatic proxy. However, the palaeoclimatic significance of loess grain size remains debated. In this study, 57 loess sections （above soil unit S2） in the Loess Plateau were sampled and analyzed in the main body of the Chinese Loess Plateau. The grain-size records from the Loess Plateau clearly display an overall southward decrease in particle size in the case of both loess and soil horizons. The grain-size contour maps were constructed for the Last Glacial Maximum and the last interglacial. All the grain-size isolines show a near north-south gradient for both the Last Glacial Maximum and the last interglacial. By combining the glacial-interglacial changes in the extent of deserts in Northern China and corresponding loess grain size variations, we suggest that the source-to-sink distance plays a primary role in the downwind loess grain size variations, and the effect of the winter monsoon intensity is secondary. In this context, a model of median grain size versus the minimum source-to-sink distance was developed, on the basis of loess grain size data from the Hongde-Yangling transect. According to this model, the dust deposits show a rapid differentiation in grain size within a transport distance of 400km, in which the Chinese Loess Plateau is right located. From 400km to 2000km, the dust deposits show a gradual differentiation in grain size, as represented by the Neogene red clay in the Loess Plateau. After transport of over 2000km, the grain size of the eolian grains is in equilibrium with the transporting winds and becomes essentially constant, as exampled by the eolian deposition in the deep sea. The grain size-distance model, based on loess deposits transported by low-level winds, predicts median grain sizes of 1-3μm for the dust deposits transported over 2000km, similar to those （3- 4μm） of both the high-level transported modern dust collected over the North Pacific Ocean and the eolian dust preserved in deep-sea sediments. This may reveal an inherent dynamic characteristic of wind regime on Earth, which requires investigation in future studies.