中中新世气候适宜期西北内陆干旱区气候演化特征

中中新世气候适宜期是距离现今最近的一个未受人类活动影响的气候适宜阶段,现有记录大多显示全球在中中新世气候适宜期期间气候以暖湿为特征。我国西北内陆干旱区作为北半球中纬度面积最大的干旱带的重要组成部分,其气候如何响应全球中中新世气候适宜期的气候变化对预测全球变暖背景下西北内陆干旱区气候变化趋势及实现生态建设具有重要的指导意义。本文总结了西北内陆干旱区具有年代控制的15个剖面的孢粉、同位素、磁化率等19项记录,并选取各记录中指示意义明确的气候代用指标进行集成、分析,发现多数剖面在中中新世气候适宜期的不同时段出现了湿润化特征,但湿润化的出现、结束时间不同,自西向东,湿润事件结束(或适宜期后干旱化增强事件出现)的时间变晚。推测西北内陆干旱区中中新世气候适宜期的气候演化主要受全球气候变化影响,但湿润事件结束或适宜期后干旱化增强事件出现及其呈现出的时空差异则受区域构造活动及气候系统等因素控制。

Climate evolution of the Northwest China during Middle Miocene Climatic Optimum

Background, aim, and scope The Middle Miocene Climatic Optimum is the most recent climatic optimum unaffected by human activity, when the climate was characterized as warm and humid. Northwest 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 climate change during Middle Miocene Climatic Optimum is of great guiding significance, based on which we can predict the climatic change in Northwest China on the background of the global warming and build ecological construction. In this paper, we integrated the existing Middle Miocene sedimentary records in Northwest China, and we aimed to get the regional climate evolution characteristics of Northwest China and discuss how its climate responded to the global climate change and tectonic activity during Middle Miocene Climatic Optimum. Materials and methods We summarized 19 sedimentary records including sporopollen, isotope and susceptibility from 15 sections with age control in Northwest China, from which we selected 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 became later from west to east, in other word, the occurrence time of the aridification enhancement that after Middle Miocene Climatic Optimum became 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 was 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 might 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 became later from west to east. The climatic change of Northwest China during Middle Miocene Climatic Optimum was 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 were 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 Northwest 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.