Middle to late Miocene cooling and drying in the northern Tibetan Plateau based on evidence from plant-insect interactions

The Qaidam Basin in the northern Tibetan Plateau is currently arid, but may have had a semi-humid to semi-arid climate in Miocene times, as suggested by pollen and isotopic data; however, the lack of macroscopic fossils hinders more precise paleoclimatic calibration. In this paper, we report fossil leaves with insect damage from a late Miocene layer (HT-5) in the Huaitoutala section dated ∼11.4 Ma, and compare them with records from a middle Miocene layer (HT-1) dated ∼12.7 Ma. Results show that damage diversity dropped from 36 types in HT-1 to 24 types in HT-5, suggesting a fall in mean annual temperature. Damage frequency decreased from 70 % in HT-1 to 32 % in HT-5, pointing to a drop in the coldest month temperature. Moreover, a slight fall in the diversity of mining damage, from 5 types in HT-1 to 4 types in HT-5, suggests a shift towards a slightly more arid climate from the middle to late Miocene. The flora from HT-5 is composed mainly of Betulaceae, Populus, Ulmus, and shrubs. Based on the distribution of these taxa in modern vegetation, the climate was probably semi-humid, not entirely arid during the late Miocene. These results are corroborated by fossil mammal data from the same section. Therefore, despite a cooling and drying trend from the middle to late Miocene, the climate of the Qaidam Basin was not as extremely arid as today.