Pollen assemblages reflect a Mid-Miocene precipitation gradient in the northeastern Tibetan Plateau

The onset of a modern-type paleoclimate in Asia has been traced back to the Late Oligocene-Early Miocene, and is characterized by a monsoonal climate in eastern Asia, and an arid to semiarid climate in mid-latitude Asia. However, the climatic characteristics at the transitional boundaries of these two regions remain poorly constrained. In this study, we compiled a series of pollen assemblages from four sites spanning a width of 900 km in the northeastern Tibetan Plateau to analyze precipitation patterns during the Mid-Miocene (18-14 Ma). The pollen assemblages were dominated by desert-steppe (DS) taxa in the basin, while coniferous and broadleaved trees dominated in the mountains, with DS taxa decreasing eastwards. According to quantitative relationships between the DS taxa percentages of 2059 surface pollen samples and precipitation, the mean annual precipitation was estimated to be 305 mm in the westernmost portion of the study area, 380-440 mm in the central portion, and 632 mm in the easternmost portion. This indicates a clear precipitation gradient, with absolute values that strongly contrast with those of the present day (50, 90-180, and 480 mm, respectively). We argue that moderntype paleoclimate gradients in arid mid-latitude Asia might have formed as early as the Mid-Miocene, with precipitation being five times greater than at present in the westernmost part of the region. Our Mid-Miocene reconstruction shows a stronger monsoonal climate system than that of the present day, while the boundary between the monsoonal and arid/semi-arid regions was over 10 degrees west of its present-day position.