碳酸盐岩差异性风化成土特征及其对石漠化形成的影响

DIFFERENTIAL WEATHERING AND PEDOGENETIC CHARACTERISTICS OF CARBONATE ROCKSAND THEIR EFFECT ON THE DEVELOPMENT OF ROCK DESERTIFICATION IN KARST REGIONS

碳酸盐岩极低的酸不溶物含量使岩石风化产生的残积土壤物质数量极少 ,并产生巨大的体积缩小 ,促使早期形成的残积土在重力作用下不断向下塌陷。显著的差异性风化使基岩面强烈起伏 ,甚至形成大量的岩溶洼地、裂隙、地下管道、洞穴系统等。在重力和水的作用下 ,土粒沿垂直和水平方向上经微距离或短距离搬运到上述低洼或地下空间中 ,甚至由地下河带到更远的地方 ,这是碳酸盐岩地区土壤丢失的重要方式 ,也是形成石漠化最主要的地质因素。土壤在地表分布高度不均匀 ,是碳酸盐岩地区地表少土的重要原因。对于酸不溶物含量相当的石灰岩和白云岩而言 ,由于白云岩的差异性风化明显弱于石灰岩 ,以及受各种应力作用后 ,白云岩形成的节理及裂隙密集而均匀 ,从而提高了近地表白云岩的含水能力 ,延长了风化过程中的水 岩反应时间 ,使风化作用可以相对集中于地表或近地表进行 ,有利于岩石的整体风化作用的进行 ,同时使白云岩风化壳基岩面起伏相对较小 ,风化残积形成的土壤分布也相对均匀。因此 ,白云岩区地表土层较厚 ,石漠化程度也稍弱于灰岩区。

The rock desertification is prone to form in karst areas because of their eco-environmental fragibility. However, up to now, the essential factors that control the development of rock desertification cannot be well understood. In this paper, non-isometric and differential weathering is considered and the main natural factors that cause rock desertification in karst areas are dealt with. The remarkable volume decrement will happen during the weathering and pedogenetic processes in virtue of the low content of insoluble residues of carbonate rocks, so it is a non-isometric weathering processes. Meanwhile, the differential weathering of carbonate rocks is very distinct because of easy dissolution and non-uniform distribution of fissures and fractures. Uneven land surface and underground caves or gaps can be formed due to the non-isometric and differential weathering or corrosion, and soil particles can be transported into the depressions on the land surface and the underground spaces. So the soil particles can disappear from the land surface by dropping into gaps and/or caves without long-distance transport. Sequentially, soil loss will occur by short-distance displacement without human activity, and soil will become more uneven in distribution on the land surface. In this way, some places become more and more barren, and other places can receive much more soil material and soil-layers become thicker and more thicker, but the former is predominant in karst areas and rock desertification begin to occur and form during the natural weathering processes. Compared to limestone, dolostone takes on the characteristics of whole weathering due to the uniform and densely spaced fissures, fractures and the original interspaces, and these characteristics enhance the validity of weathering and minish the difference of weathering on or near the land surface. So the soil distribution and thickness of dolostone area are more even and greater than those of limestone area. This is why rock desertification in the dolostone area is not so severe as in the limestone area.