Multiple forcing on Late Miocene East Asian Summer Monsoon Precipitation Variability in NE Tibetan Plateau.

• A high resolution, quantitative late Miocene EASM rainfall record was obtained. • The EASM declined from ∼ 10.8 Ma to 7.6 Ma and strengthened from ∼ 7.6 Ma to 6.3 Ma. • Global cooling and paleotopography play key roles in long-term late Miocene EASM evolution. • Late Miocene EASM evolution was dominated by eccentricity forcing in NE TP. Understanding how the East Asian Summer Monsoon (EASM) evolves at different time scales affords a valuable opportunity to reveal the interactions between the hydrosphere, land, oceans and atmosphere. However, the pre-Quaternary evolutionary history and the driving forces that controlled its variability in response to different boundary conditions remain enigmatic. Here, we focus on the late Miocene (∼10.8 to 6.3 Ma) – a period of profound climatic and topographic changes in Asia – and present a quantitative reconstruction of EASM precipitation using the probabilistic CREST (Climate Reconstruction Software) method with a high temporal resolution pollen record from the Tianshui Basin in NE Tibetan Plateau (TP). Our new EASM precipitation record shows a slowly decreasing long-term trend during the period of ∼ 10.8–7.6 Ma, which was followed by a strengthening period from ∼ 7.6 to 6.3 Ma with a large amplitude of precipitation variability. We argue the decrease and increase periods of EASM precipitation were primarily response to late Miocene global cooling and TP uplift after ∼ 8 Ma, respectively. These results are supported by existing climate model simulations, wherein both global climate and paleotopography play key roles in regulating the long-term evolution of late Miocene EASM. On orbital time scales, the precipitation time series exhibit a dominant ∼ 410 kyr eccentricity periodicity, with lower (higher) values intervals corresponding to eccentricity minima (maximum). The synchronous phase of the precipitation and eccentricity records indicate that the eccentricity exerts a dominant influence on the EASM precipitation cycles via its modulation of the precessional amplitude, and the period expansion and contraction of Antarctic ice sheet (AIS) also probably play an important role during that time. Our quantitative late Miocene EASM precipitation records provide new insight into late Miocene EASM precipitation evolution and its relation with global climate, paleotopography, and cryosphere. [ABSTRACT FROM AUTHOR]