Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01002, USA.
Nature. 2012 Apr 4;484(7392):87-91. doi: 10.1038/nature10929.
Between about 55.5 and 52 million years ago, Earth experienced a series of sudden and extreme global warming events (hyperthermals) superimposed on a long-term warming trend. The first and largest of these events, the Palaeocene-Eocene Thermal Maximum (PETM), is characterized by a massive input of carbon, ocean acidification and an increase in global temperature of about 5 °C within a few thousand years. Although various explanations for the PETM have been proposed, a satisfactory model that accounts for the source, magnitude and timing of carbon release at the PETM and successive hyperthermals remains elusive. Here we use a new astronomically calibrated cyclostratigraphic record from central Italy to show that the Early Eocene hyperthermals occurred during orbits with a combination of high eccentricity and high obliquity. Corresponding climate-ecosystem-soil simulations accounting for rising concentrations of background greenhouse gases and orbital forcing show that the magnitude and timing of the PETM and subsequent hyperthermals can be explained by the orbitally triggered decomposition of soil organic carbon in circum-Arctic and Antarctic terrestrial permafrost. This massive carbon reservoir had the potential to repeatedly release thousands of petagrams (10(15) grams) of carbon to the atmosphere-ocean system, once a long-term warming threshold had been reached just before the PETM. Replenishment of permafrost soil carbon stocks following peak warming probably contributed to the rapid recovery from each event, while providing a sensitive carbon reservoir for the next hyperthermal. As background temperatures continued to rise following the PETM, the areal extent of permafrost steadily declined, resulting in an incrementally smaller available carbon pool and smaller hyperthermals at each successive orbital forcing maximum. A mechanism linking Earth's orbital properties with release of soil carbon from permafrost provides a unifying model accounting for the salient features of the hyperthermals.
大约在 5550 万至 5200 万年前，地球经历了一系列突然而极端的全球变暖事件（超暖期），这些事件叠加在长期变暖的趋势之上。这些事件中的第一个也是最大的一个是古新世-始新世极热事件（PETM），其特征是大量的碳输入、海洋酸化以及全球温度在几千年内上升了约 5°C。尽管已经提出了各种关于 PETM 的解释，但一个能够解释 PETM 以及随后的超暖期碳释放的来源、规模和时间的令人满意的模型仍然难以捉摸。在这里，我们使用来自意大利中部的一个新的具有天文校准的旋回地层记录来表明，早始新世的超暖期发生在具有高偏心率和高倾角组合的轨道上。相应的气候-生态系统-土壤模拟考虑了背景温室气体浓度的升高和轨道强迫作用，表明 PETM 及其随后的超暖期的幅度和时间可以通过轨道触发的北极和南极陆地永冻层土壤有机碳的分解来解释。这个巨大的碳库有可能在 PETM 之前，一旦达到长期变暖的阈值，就向大气-海洋系统中释放数千太克（10^15 克）的碳。在达到峰值变暖之后，永冻土土壤碳储量的补充可能有助于从每次事件中快速恢复，同时为下一次超暖期提供了一个敏感的碳库。随着 PETM 之后背景温度的持续升高，永冻土的面积稳步下降，导致可用的碳库逐渐减少，每次连续的轨道强迫最大值的超暖期规模也越来越小。将地球轨道特性与永冻土土壤碳释放联系起来的机制为解释超暖期的显著特征提供了一个统一的模型。
