花海湖泊古风成砂的粒度特征及其环境意义

The Grain Size Characteristics of Aeolian Sand and Its Environmental Significance

通过野外调查和室内分析 ,发现在花海剖面全新世出现了三次特有的风成砂 ,并对其原因进行了初步的分析。前两次风沙形成期分别与第一、第二新冰期相对应 ,而且分析表明第一风沙形成期的冬季风强度要比全新世其它时期强 ,持续时间长。这为研究本区土地退化、荒漠化等问题提供了自然背景 。

The grain size analyses of the whole section in Huahai Lake reveal that several segments consist of grains with larger mean diameter, which are different from the field observation. On the basis of laboratory analyses, we found that the characteristics of paleo-eolian sand from sand wedges are so similar to those of modern eolian sand that they coincide with previous studies. Based on the above, detailed grain size distribution comparisons were conducted in order to make it clear that which segments are eolian. The results show that sand from layers of 4.75～5.33m,3.85～3.81m,0.37～0m shares good similarity with compared eolian samples,implying that the sand of these layers originate from aeolian. And the time during which the earlier two layers of sand were formed may correspond with the first and the second New Ice Age,respectively. Moreover, we found that samples at the boundary of Huahai Lake encompass more coarse grains,less fine components than those sampled from the center of the lake. That is to say, compared eolian sand in Huahai Lake has especially spatial discrepancy, which is different from fine grain distribution originated from Westerly in loess. The researches have proved that coarse silt of 4.32～6.64Φ is easier to suspend in the air than other grain size. However, for grains of <4.32Φ, the larger the grain diameter becomes, the more difficult its suspension gets. Therefore, we preliminarily assume that the fine grains was from lacustrine nearby, while the coarser from desert, and that the duration of lake recession positively correlates with grain size, which bears special spatio temporal relationship in arid lacustrine area inland. Further analyses show that median diameter is better correlated with sand,silt and clay. So we may use median diameter as an effective indicator of winter monsoon history reconstruction. The grain size curves show that the first sand layer has the largest median diameter, the fewest fine grain and the maximum thickness, suggesting that the intensity of winter monsoon with longer duration at that time, may have been stronger than other two in Holocene. These provide natural background of the soil regeneration and desertification, and has significant implication to the preservation and construction of ecological environments in west China .