A palynological insight into the Miocene aridification in the Eurasian interior

Monsoonal systems have gained dominance in East Eurasia since the latest Oligocene, evidenced by eolian deposits in North China. Consequently, the central Eurasia became a middle-latitude inland arid area, and in the Late Miocene finally formed a pattern like that of modern day. But the timing, extent, and forcing of the aridification remain poorly understood. Here we gathered palynological sequences from the continental interior and divide them into four chronological parts to reconstruct the development of the central Eurasian aridity during the Miocene. The oldest known aridification record shows the rise of xerophytes observed in the fluviolacustrine Jingou River section at the south margin of the Junggar Basin, suggesting a latest Oligocene drying. Compared with the well-dated records from the inland, the available palynological records reveal two stepwise expansions of arid conditions from the Junggar Basin to its adjacent regions, which occurred in late Middle Miocene and Late Miocene, respectively. The two major expansions are roughly synchronous with the development of North American steppe and call for a global forcing explanation. We discuss various possible explanations for the inland aridity expansions, including regional uplift, land-sea redistribution, global changes in CO2 concentration, and conclude that none of these are fully consistent with the available data. The most likely explanation for the northern mid-latitude drying is global cooling during the Miocene. For eastern Eurasia, cooling weakens monsoon circulation, and consequently drying conditions expand following retreat of the monsoonal rain belt, while in the west, cooling reduces water vapor pressure and therefore reduces the moisture mass transported into the continental interior.