Isotopic constraints on intensified aridity in Central Asia around 12Ma

The relationship between central Asian aridification and the evolution of the Himalayan-Tibetan orogen remains elusive. New isotopic data from pedogenic and lacustrine carbonates sampled from well-dated Neogene strata (15.7-1.8Ma) in the northeastern Qaidam basin of the northern Tibetan Plateau identify a positive shift of ~2.5ppt in delta 18O values from 12Ma to 10.7Ma. High values were maintained until ~3.3Ma when delta 18O values drop by 1ppt. The timing of the positive shift in delta 18O values is remarkably consistent with isotopic records from a vast region along and within the northern Tibetan Plateau. Isotopic, mineralogical, petrologic, and facies analyses suggest that diagenetic effects and detrital contamination are minimal. The positive shift in delta 18O values, as well as high delta 13C values from pedogenic carbonates (which are suggestive of low soil respiration rates), is interpreted to indicate intensified aridity in central Asia ca. 12Ma. The recognition of intensified aridity is critical to understanding 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 3.3Ma might be related to the onset of Northern Hemisphere glaciation, intensification of the East Asia monsoon, and/or effects of orographic rain-out.