Dust transport and the question of desert loess formation
Dust transport and the question of desert loess formation
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DOI：
10.1111/j.1365-3091.1987.tb00566.x
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发表时间：
1987-02
期刊：
影响因子：
3.5
通讯作者：
H. Tsoar;K. Pye
中科院分区：
文献类型：
--
作者：
H. Tsoar;K. Pye
虽然当代干旱半干旱地区沙尘暴频发，但沙漠黄土沉积不发达。世界上大部分黄土分布在更新世期间经历过冰川作用的中纬度地区。海洋岩心证据表明，更新世寒冷时期，亚热带沙漠的沙尘输送增加，但黄土只在某些沙漠边缘形成，其原因尚未得到充分解释。本文重新探讨了沙尘的输运和沉积机制，以及导致厚黄土堆积的环境。典型的黄土主要由中等泥沙颗粒组成，这些颗粒在离地面几米的地方以短期悬浮的形式运移。只有当灰尘被困在一个有限的区域内，通常相对靠近源头时，才会形成显著的黄土厚度。粒径大于20 μm的粉尘主要以长期悬浮形式在较大的高度范围内运移，并可能广泛分散。泥沙的可得性和吹沙风的频率、大小和方向是决定黄土形成可能性的重要因素，但是否有合适的捕尘器尤为重要。圈闭可能由地形障碍、潮湿地面或植被表面形成。更新世中纬度地区冰川外冲平原附近的植被和辫状融水流捕获了尘埃。冰川时期从某些沙漠吹来的灰尘，特别是在西奈半岛、苏联中亚和中国，在邻近的半干旱地区堆积成黄土。在其他沙漠边缘，由于缺乏植被圈闭，黄土形成受到一定程度的抑制。在全新世的大部分时间里，所有沙漠边缘地区的净降尘率都过低，没有明显的黄土堆积。这主要是由于可利用泥沙的减少和景观稳定的趋势。由于人类活动的影响，过去50年报告的沙尘暴频率高估了全新世较长期的沙尘通量。许多现代灰尘的起源是半干旱地区耕地土壤的侵蚀，比典型的黄土更细。
Although contemporary dust storms are frequent in arid and semi-arid areas, desert loess deposits are poorly developed. Much of the World's loess occurs in mid-latitude areas which experienced glaciation during the Pleistocene. Ocean core evidence indicates that dust transport from sub-tropical deserts increased during cold stages of the Pleistocene, but loess formed only on certain desert margins, for reasons which have not been fully explained. This paper re-examines the mechanisms of dust transport and deposition, and the circumstances leading to the accumulation of thick loess. Typical loess is composed mainly of medium silt grains which are transported in short-term suspension a few metres above the ground. Significant thicknesses of loess form only when dust is trapped within a limited area, often relatively close to the source. Dust particles finer than 20 μm are transported mainly in long-term suspension over a greater height range and may be widely dispersed. The availability of silt and the frequency, magnitude and direction of dust-transporting winds are important factors governing the potential for loess formation, but the existence of a suitable dust trap is particularly important. Traps may be formed by topographic obstacles, areas of moist ground, or vegetated surfaces. Vegetation adjacent to glacial outwash plains and braided meltwater streams trapped dust in mid-latitudes during the Pleistocene. Dust blown during glacial periods from certain deserts, notably in Sinai, Soviet Central Asia and China, accumulated as loess in neighbouring semi-arid regions. On the margins of other deserts loess formation was inhibited partly by the absence of vegetation traps. During most of the Holocene net dust deposition rates in all desert-marginal areas have been too low for significant loess accumulation. This is mainly due to a reduction in silt availability and a tendency towards landscape stability. Reported dust storm frequencies during the past 50 years over-estimate the longer-term Holocene dust flux due to the effects of human activities. Much modern dust owes its origin to erosion of cultivated soils in semi-arid areas and is finer than typical loess.