Miocene Paleoclimate Evolution Revealed From The Deposits In The Tianshui Basin

There is increasing evidence suggesting that the Miocene is a critical period for the formation and evolution of the East Asian monsoon and Asian aridification. However we have little knowledge of how these evolved.The Tianshui basin is located in the interactional region of three natural zones (Eastern monsoon zone, Northwestern arid zone and Qinghai-Tibet plateau zone), thus sensitive to the evolution of East Asia monsoon and aridification. The basin received thick Cenzoic lacustrine sediments in the south and distal fan/mudflat sediments in the north of the Tianshui basin during the Miocene.Detailed Rb/Sr, CaCO3 content and redness a* have been carried out in the Xiashan section for climatic change. They indicate that the Tianshui basin has experienced four stages of climate change during the Miocene, i.e., (1) the climate was relatively dry in the late Middle Miocene in 16.7-14Ma; (2) gradually changed to wet in the late Middle Miocene (14-10.7Ma); (3) much wetter in the early Late Miocene (10.7-8.3Ma); and (4) tuned to dry in the late Late Miocene (8.03-6Ma).Comparison of the evolution of the Miocene climate recorded in the Tianshui basin with global and other East Asian climate records suggests that the Mid-Miocene global cooling at ca.14 Ma may have exerted a great influence on the East Asia climate.The Tianshui climate records reveal that the East summer monsoon was weak in 16.7-14Ma and strong in 14-8.3Ma. The aridification of Northwest China became apparently in the Late Miocene after 8.3Ma, suggesting that the uplift of the Tibet Plateau may have played a key role in the Late Miocene climate change in East Asia.