ISOTOPIC CONSTRAINTS ON INTENSIFIED ARIDITY IN CENTRAL ASIA AROUND 12 MA

The relation between the central Asia aridification and the evolution of the Himalayan-Tibetan orogen remains elusive. New isotopic data from pedogenic and lacustrine carbonates, sampled from the well-dated Neogene strata (15.7-1.8 Ma) in the northeastern Qaidam basin of the northern Tibetan Plateau, identify a positive shift of ~2.5 ‰ in δ¹⁸O values from -9.9‰ before 12 Ma to -7.4‰ after 10.7 Ma. High values were maintained until ~3.3 Ma when δ¹⁸O values shift back to -8.4 ‰. The timing of the positive shift in δ¹⁸O values is remarkably consistent with isotopic records from a huge region along and within the northern Tibetan Plateau, ranging from the Tarim, to Qaidam, and to Xunhua and Linxia basins. Isotopic, mineralogical, petrologic, and facies analyses suggest that diagenetic effects, if any, are minimal. This positive shift in δ¹⁸O values, as well as high carbon isotope values from pedogenic carbonates (which are suggestive of low soil respiration rates), is interpreted to indicate intensified aridity in central Asia ca. 12 Ma. The recognition of intensified aridity is critical to understanding of the climatic effects of the development of the Himalayan-Tibetan orogen. We attribute this climatic change in the central Asia to: (1) retreat of Paratethys from central Asia, strengthening the seasonal contrast and resulting in the loss of a nearby moisture source; (2) attainment of high elevations in the Himalayas and south-central Tibet, blocking moisture-bearing air masses from the Indian and Pacific oceans; and (3) enhanced isolation and outward growth of the northern Tibetan Plateau. The negative shift around 3.3 Ma might be related to the intensified East Asia summer monsoon, bringing ¹⁸O-depleted precipitation, and/or attainment of critical elevations in the Qilian Shan, or global cooling related to northern hemisphere glaciation.