古新世-始新世极热事件研究进展

古新世-始新世极热事件(PETM事件)是发生在早新生代的一次极端碳循环扰动和全球变暖事件，主要表现为大气CO2浓度快速增加和全球增温。研究显示该时期全球地表温度增加了5～6℃，高低纬度间温度梯度减小，同时伴随有水循环加快及大规模生物灭绝、演替和迁徙现象。据估算PETM时期从地层释放到表生系统的CO2总量同预期人类过去与未来燃烧化石燃料所产生的总量可资对比，因此国际学术界将其视作预估今后可能发生的增温效应、环境效应和生态效应的重要基础。本文根据最新研究成果，对该事件的触发机制、碳排放量、气候及生态效应等方面作一综述。

A REVIEW ON THE PALEOCENE-EOCENE THERMAL MAXIMUM

The Paleocene-Eocene Thermal Maximum (PETM) is an extreme carbon perturbation and global warming event, which was superimposed on the long-term Early Cenozoic warming trend. The PETM is characterized by an abrupt and large negative excursion (-2 ‰～-6 ‰) of carbon isotope compositions of marine and terrestrial sedimentary materials. Paleotemperature proxies suggest an increase in global surface temperature of approximately 5～6 ℃， and a coincident decrease in temperature gradient between high-latitudes and low-latitudes. This event is correlative with a global accelerated hydrological cycle and large-scale biotic turnover and vertebrate mammalian migrations. Based on carbon isotope records and computer simulations, the amount of carbon totally released to the exogenic carbon cycle system is estimated to be comparable with the anticipative carbon amount that would be released to the atmosphere by burns of fossil fuels. Therefore, a comprehensive study of this event is potentially helpful to the projection of the environmental and ecological impacts induced by human activity in the future. In this paper, we overviewed the recent progresses of studies on the event, particularly focusing on the triggering mechanisms, estimation of released total carbon, and the climatic and ecological impacts.Of all mechanisms put forward,large-scale dissociation of methane hydrate is the reliable hypothesis. It is likely that increased temperature before PETM event led to hydrate instability and thus to an enhanced emission of methane,imposing a strong positive feedback that would made dissociation of more methane hydrate along continental slope,and then caused the PETM event. Because of pervasive dissolution of deep sea sedimentary carbonate and severe acidification of ocean,the real magnitude of carbon isotope excursion may be embodied in terrestrial carbon isotope records( ca. -6 ‰)rather than the ocean archives(ca.-2. 5 ‰).The -6 ‰ of CIE(carbon isotope excursion)indicates a release of approximately 5075 Ct C methane into the ocean-atmosphere system. Based on our calculation,atmospheric CO2 concentration would have increased about 1000 ppmv during the PETM event. CO2-induced greenhouse effect changed the pattern of precipitation,that is,less precipitation at low latitude and more precipitation at high latitude. The PETM event caused turnover of ecosystem,including an extinction of mass benthic foraminifera and vertebrate mammalian migrations.