Intensified aridity in inland Asia preserved in lacustrine carbonates of the northeastern Tibetan Plateau.

It is generally accepted that extensive and intensified aridity in inland Asia began in the mid-Miocene as a result of global cooling. We present new data from mid-Miocene drill cores in the Qaidam Basin that suggest a new view about the timing, depositional characteristics and driving force of this change in climate. To understand the intensified aridity in the Qaidam Basin, we investigated the carbon and oxygen isotopes and magnetic susceptibility of 400 m of successive drill cores and analyzed 2600 m of gamma ray logging data from the whole borehole. By comparing the gamma ray logging data and global temperatures, we easily identified the Middle Miocene Climatic Optimum and Middle Miocene Climate Cooling in this borehole. The positively biased oxygen isotopes and the negative shift in magnetic susceptibility from the fine-grained mixed sediments and carbonate rocks showed a climate drying within the Middle Miocene Climatic Optimum. A decrease in the interlayers of algal/oolitic limestone and an increase in the interlayers of dolostone marked this drying event as slightly earlier than 15.3 Ma. The mountain-building spanning the northern margin of the Tibetan Plateau changed the local precipitation patterns and probably led to the intensified aridity in inland Asia as early as the Middle Miocene Climatic Optimum. [ABSTRACT FROM AUTHOR]