Abstract: Background, aim, and scope The Middle Miocene Climatic Optimum is the most recent climatic optimum, when the climate was characterized as warm and humid. Northwestern China is an important part of the largest arid region in the mid-latitude of Northern Hemisphere, and how did its climate respond to the global climatic change during Middle Miocene Climatic Optimum is of great guiding significance, based on which we can predict the climatic change in Northwestern China on the background of the global warming and build ecological construction. Materials and methods We summarize 19 sedimentary records including sporopollen, isotope and susceptibility from 15 sections with age control in Northwestern China, from which we select the climatic proxies with clearly indicative meanings for integration and analysis. Results The majority of the selected sections presented humid climatic characteristic at different stages of Middle Miocene Climatic Optimum, and the ending time of wet episode generally becomes later from west to east, in other word, the occurrence time of the aridification enhancement that after Middle Miocene Climatic Optimum becomes later from west to east. Discussion Humid climatic stage can be identified in almost all of these Middle Miocene sections in the study area, which shows great similarity with global climate and implying its evolution was influenced by global climate change during Middle Miocene. However, the environmental gradient from west to east during Middle Miocene Climatic Optimum in the study area is possibly caused by climatic system differences and the gradually weakening of the East Asian summer monsoon. In addition, the temporal and spatial differences of the aridification occurrence or enhancement that after Middle Miocene Climatic Optimum may be result from regional tectonic activities, varied intensity of monsoon induced by global cooling and Paratethys sea retreat. Conclusions The majority of these sections presented humid climatic feature in Middle Miocene Climatic Optimum with different occurrence and ending time. The ending time of wet episode or the occurrence time of aridification enhancement that after Middle Miocene Climatic Optimum becomes later from west to east. The climatic change of Northwestern China during Middle Miocene Climatic Optimum is mainly influenced by global climate change, while the ending of the wet episode or enhancement of aridification that after Middle Miocene Climatic Optimum and its temporal and spatial differences are controlled by the regional tectonic activities and climate systems.Recommendations and perspectives Although this paper provides a review of the climatic evolution during the Middle Miocene Climatic Optimum in Northwestern China, it is still having some uncertainties that is urgent to continue to carry out relevant research with precise age control and indicative climatic proxies.
Keywords: Middle Miocene Climatic Optimum; Northwestern China; climatic proxies; global climate change; tectonic activities