Uplift-driven climate change at 12 Ma: a long δ18O record from the NE margin of the Tibetan plateau

Carbonates from fluvial and lacustrine sediments were sampled from multiple measured sections in the Linxia basin of western China. Based on textural and mineralogical evidence, lacustrine carbonates are primary precipitates from lake water. A 29 million year record of the oxygen isotope composition of meteoric water is inferred from the δ18O values of these carbonates. This inference is based on the most negative δ18O values in the lake carbonates, which represent lake waters that have experienced the least evaporative enrichment. Carbonate δ18O values, a proxy for rainfall δ18O, are ∼−10.5‰ throughout the interval of 29–12 Ma. At 12 Ma there is a shift to −9‰, a value that remains into the Pliocene. This implies a major reorganization of atmospheric circulation patterns and a shift to more arid conditions at the NE margin of the Tibetan plateau with the post-12 Ma system similar to that of today. The 12 Ma event may represent the time at which the Tibetan plateau achieves sufficient elevation to block the penetration of moisture from the Indian Ocean or south Pacific into western China. The period of greatest aridity is from 9.6 to 8.2 Ma, a time interval which agrees well with other climate records.