INITIAL TIMING OF FORMATION OF SAND SEAS AND FORMATIVE PROCESSES IN NORTHERN CHINA - A CRITICAL REVIEW (Invited Presentation)

Compared with other desert regions on Earth, one distinct character of China’s deserts is the large proportion of the sand dunes which occupy as much as 45% of the drylands in northern China. The aeolian sequences in these deserts and marginal deposits are major archives for deciphering variability of the eastern Asian monsoon systems, tectonic activities in the region and the uplift of the Tibetan Plateau. Climatic aridity is often considered by palaeoclimatologists as the sole factor for determining the desert landscapes in northern China and elsewhere. Thus, the Miocene loess in the downwind sites was interpreted as evidence of the occurrence of sand seas in northern China and potential changes of the landscape since then are often ignored. In the community of aeolian geomorphology, it is, however, well-accepted that at least three aspects, i.e., sand supplies, erodibility of the land surface and wind power, are collectively responsible for the forming of the dune landscape. The sediment sequences reported from the desert regions in northern China and their chronologies mostly indicate a much young age for the sand seas. For the case of the Hunshandake Sandy Land in the eastern portion of Asian mid-latitude’s desert belt, the earlier stratigraphical correlation suggested early Tertiary as the initial formation of this desert. The abundant outcrops of lacustrine sequences in this desert and their OSL chronologies indicated, however, large lakes and wetlands occurred in the region about 5000 years ago. The current dune landscape was established only after rapid drop in the groundwater level and drying up of these lakes. There might be aeolian landforms prior to the occurrence of these lakes and the Tertiary age assignment could be a maximum limit age on aridification for one phase events earlier but not directly related to the current landscape. For a full understanding of the sand seas in the drylands of Asian middle latitudes it is imperative to give consideration to all potential factors contributing to the formation of the sand seas, not just the initial climatic aridity.