Geomorphic processes and palaeoclimatology in deserts

摘要

Drylands occupy one-third of the Earth's land surface, and are mostly characterized by desert landscapes. Depending on the selection of indicators, various systems have been applied to classify the deserts. Geologically and geomorphologically, one can distinguish between shield deserts and mountain-basin deserts, and can divide them further into sand deserts, stony deserts, clay plains, and riverine deserts (Goudie, 2002). Although deserts are often associated with sand dunes, the proportions of sand dunes in various deserts vary considerably. For example, active sand seas cover between 15% and 30% of the arid areas in the Sahara, Arabian Peninsula, Australia, and Southern Africa (Goudie, 2002). Active and stable dunes occupy as much as 45% of deserts in China (Zhu et al, 1980). By contrast, aeolian sand covers only less than 1 % of the arid zone in Americas (Lancaster, 1995). Regardless of the forms of landscapes, all major geomorphic processes, in particular erosion and deposition caused by water and/or wind agents, are active in most major deserts of the world and crucial to their landscape evolution. In addition, salt accumulation and removal as well as tectonic movements have strong imprints on the landscapes of some deserts. Since these processes are controlled largely by global and local climates, they respond sensitively to global changes, reflecting the interactions between atmosphere, lithosphere, hydrosphere, and biosphere in the Earth system. Consequently, landforms and sediments are significant archives of palaeoenvironmental information in deserts, and they provide important clues for understanding the Earth system, in particular its surface processes.