风尘堆积常见的同沉积和沉积后改造特征及其环境意义

SYN-AND POST-DEPOSITIONAL MODIFICATION OF LOESS DEPOSITS AND THEIR PALEOENVIRONMENTAL SIGNIFICANCE

黄土堆积作为气下沉积,任何一个深度都曾经暴露于地表,因而必然受到相关地表过程的作用。由于这些过程均发生于特定的环境条件下,形成的特征多数具有明确的环境意义;而黄土在沉积后也可能受到各种地质过程的改造,从而对研究中常用的气候代用指标有一定影响。文章基于野外、微形态等分析,结合前人成果,对我国北方新近纪风尘堆积中常见的同沉积和沉积后改造特征、形成过程及环境意义进行研究。由于一些特征在黄土堆积中具有普遍性,可作为识别风成堆积的标志和环境事件研究的指标,并有助于全面理解常用的替代指标的环境意义。

Loess-paleosol sequences are widely used for paleoenvironmental reconstructions. To be sub-aerial deposit, loess or soil was exposed to earth surface, and was subject to syn-depositional and post depositional modifications by various processes. In this paper, some commonly found syn-depositional and post-depositional features of loess deposits are studied mainly based on field and micromorphological investigations on the eolian deposits in Northern China. Since most of these features are formed under specific environmental conditions, they may be used as indicators for paleoenvironmental reconstruction. Meanwhile, such modifications may affect the significance of some climate proxies that are frequently used in loess studies. Consequently, their influence should be taken into considerations in the climate interpretations. Firstly, we show that the lowest parts of some loess-soil sections usually contain water-reworking features. This is because the substratum of loess deposits was never flat enough so that eolian dust deposited in the earliest stage was more or less affected by surface water. These processes may lead to a progressive flatting of the land surface, resulting in the formation of typical loess-soil sequences. Vegetation degradations under extremely dry conditions may cause eolian and sheet-water erosions of loess deposits. These features are highly common in the loess deposits in Northern China. Wind and sheet-water erosion features are most frequently observed across the boundaries between loess and soil layers, suggesting that climate transitions at orbital scales are usually marked by a severe dry event, leading to vegetation degradation. Freeze-thaw features are much less common in the loess deposits in China than in the loess of Europe and Americas,probably due to the much drier conditions. However, some weak frost features are observed in the northern and northwestern Loess Plateau while they are absent in the southern Loess Plateau. These are consistent with the present-day climate pattern in the region. A number of post-depositional modifications are also described as they may influence the significance of some climate proxies. Also, because some features of the loess-soil sequences are easy to be confused with the modifications described above. Their basic morphological properties ,formation processes and their significance are also discussed.