策勒绿洲-沙漠过渡带风成沉积物粒度的空间变化

Spatial change of the grain-size of aeolian sediments in Qira oasis-desert ecotone,Northwest China

为了探讨绿洲-沙漠过渡带对绿洲可持续发展的环境效应,本研究对策勒绿洲西缘绿洲-沙漠过渡带、防护林带和绿洲内部的各种风成沉积物及策勒河谷沉积物粒度特征进行比较.结果表明:绿洲-沙漠过渡带风成沉积物粒度特征(包括粒度分布特征、粒度参数、各粒级含量等)在空间上基本一致,其粒度范围为0.3~200μm,分选性良好,主峰众数为67μm,主要为悬浮组分和变性跃移组分,未出现>200μm的跃移和蠕移组分,表现出典型的大气沙尘近距离传输沉积的特点.与外围(上风向)的策勒河谷(粒度范围0.3~800μm)和沙漠地区的沉积物比较,过渡带粒度明显偏细,但与过渡带、防护林带和绿洲内部现代尘暴降尘(下风向)各种粒度特征基本一致.策勒绿洲-沙漠过渡带由于存在较高植被覆盖度,能够有效地抑制沙漠地区较粗颗粒组分(>200μm)通过变性跃移或蠕移方式输送进入过渡带和绿洲内部.绿洲-沙漠过渡带与防护林体系一样,可有效抑制沙漠入侵绿洲,二者形成一个整体,共同维护策勒绿洲生态环境的稳定.

In order to understand the environmental influence of oasis-desert ecotone to oasis ecological system, we comparatively analyzed the grain size characteristics of various aeolian sediments, including the sediments in oasis-desert ecotone, shelterbelt and the inside oasis and in Qira River valley. The results showed that the grain size characteristics （including grain-size distribution curve, grain size parameters, and content of different size classes） of sediments in the oasis-desert ecotone were consistent along the prevailing wind direction with a grain-size range of 0.3-200 I^m and modal size of 67 txm. All of the sediments were good sorting and mainly composed of suspension components and saltation components, but not denatured sahation and creeping components （〉200 Ixm）. They were typically aeolian deposits being short-range transported. The grain sizes of sedi- ments in oasis-desert ecotone were smaller than that in the material sources of Qira River valley and desert （0.3-800 Ixm） , but very similar to those of the modem aeolian deposits in oasis-desert ecotone, shelterbelt and the inside oasis. The denatured saltation and creep components （ 〉 200 txm） were suppressed to transport into oasis-desert ecotone because of the high vegetation cover in oasisdesert ecotone. Therefore, like the shelterbelts, the oasis-desert ecotone could also block the invasion of desert. They safeguarded the oasis ecological environment together.