始新世—渐新世(E-O)气候转变(34Ma前后)是新生代气候演化过程中最显著的变冷事件之一,标志着地球气候由“温室”进入“冰室”.这一转变伴随着地球环境的一系列重大变化,对于研究新生代气候变冷的驱动机制、区域气候对全球重大气候事件的响应方式、重大气候事件对生态环境及生物演替的影响等具有重要意义.近年来地质记录和气候模拟在E-O转变研究上取得了一系列重要进展:①不同纬度区的地质记录揭示出这一转变伴随着全球性的显著降温,指示其触发因素是全球性的；②气候模拟研究揭示出大气CO2浓度降低及与之联系的全球碳循环变化是导致这一转变的主因,否定了传统认为的环南极流形成导致E-O气候变冷的假说；③深海沉积记录揭示出这一转变过程持续400～500ka,表现为全球降温和南极冰盖形成先后两阶段变化；④海—陆气候记录的对比初步揭示出陆地区域的干旱化可能主要与全球降温(对应于E-O转变的第一阶段)相关.当前对E-O气候转变的研究还存在地质记录分辨率不够高、模拟结果与地质记录不完全吻合、陆地记录相对较少等方面的不足.更精确的大气CO2浓度和温度的重建、更多高分辨率海—陆气候记录的研究以及古气候数值模拟的改进有望进一步揭示出E-O转变过程中气候系统各要素的变化特征和相互关系,为更深入认识这一转变的驱动机制提供依据.%The Eocene-Oligocene climate transition (EOCT) was a period of rapid global cooling at abou 34 Ma ago, representing a major step from early Cenozoic greenhouse toward modern icehouse conditions. It was accompanied by a series of dramatic changes in Earths environmental system, including an expansion of ice sheets on the Antarctic continent, a global sea-level drop, a declining atmospheric C02 concentration, a deepening of the calcite compensation depth, and a biotic reorganization, thus has great significance in understanding the mechanisms behind the long-term Cenozoic cooling, the behavior of regional climate in response to rapid cooling and the effects of major climatic changes on the biotic reorganization. This review summarizes recent progress in studying EOCT. (1) Significant cooling during this transition inferred from geological records of different latitudes indicates this phenomenon was triggered by global rather than regional factors. ( 2 ) The declining of atmospheric CO, and the associated global carbon cycle changes have been proved as the primary causes of EOCT by modeling studies, rather than the previous hypothesis that the Antarctica was thermally isolated by the initiation of Antarctic Circumpolar Current. (3) Marine records revealed that this transition is marked by a two-step increase of benthic foraminiferal oxygen isotope values occurred within 400 ~ 500 ka, reflecting the drop of global temperature and establishment of Antarctic ice sheets, respectively. (4) A comparison of the EOCT interval in Asian terrestrial records with marine records suggests that the regional aridification mainly corresponds to atmospheric cooling (i. e. , the first step of EOCT) rather than ice volume growth. However, there are still many questions remained unsolved. Future application of more detailed and accurate proxies of atmospheric C02 and paleotemperature, recovery of high-resolution sediment archives from the oceans and continents and more complex models across the EOCT will further reveal the changes of each environmental factor during this transition and their relationship with each other, and obtain a thorough understanding of the mechanisms behind this critical climate transition.
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