Edgar Kirsty M, Wilson Paul A, Sexton Philip F, Suganuma Yusuke
National Oceanography Centre, School of Ocean and Earth Science, European Way, Southampton, SO14 3ZH, UK.
Nature. 2007 Aug 23;448(7156):908-11. doi: 10.1038/nature06053.
Major ice sheets were permanently established on Antarctica approximately 34 million years ago, close to the Eocene/Oligocene boundary, at the same time as a permanent deepening of the calcite compensation depth in the world's oceans. Until recently, it was thought that Northern Hemisphere glaciation began much later, between 11 and 5 million years ago. This view has been challenged, however, by records of ice rafting at high northern latitudes during the Eocene epoch and by estimates of global ice volume that exceed the storage capacity of Antarctica at the same time as a temporary deepening of the calcite compensation depth approximately 41.6 million years ago. Here we test the hypothesis that large ice sheets were present in both hemispheres approximately 41.6 million years ago using marine sediment records of oxygen and carbon isotope values and of calcium carbonate content from the equatorial Atlantic Ocean. These records allow, at most, an ice budget that can easily be accommodated on Antarctica, indicating that large ice sheets were not present in the Northern Hemisphere. The records also reveal a brief interval shortly before the temporary deepening of the calcite compensation depth during which the calcite compensation depth shoaled, ocean temperatures increased and carbon isotope values decreased in the equatorial Atlantic. The nature of these changes around 41.6 million years ago implies common links, in terms of carbon cycling, with events at the Eocene/Oligocene boundary and with the 'hyperthermals' of the Early Eocene climate optimum. Our findings help to resolve the apparent discrepancy between the geological records of Northern Hemisphere glaciation and model results that indicate that the threshold for continental glaciation was crossed earlier in the Southern Hemisphere than in the Northern Hemisphere.
大约3400万年前，接近始新世/渐新世边界时，南极大陆永久性地形成了大型冰盖，与此同时，全球海洋中的方解石补偿深度也永久性地加深。直到最近，人们一直认为北半球的冰川作用开始得要晚得多，在1100万至500万年前之间。然而，这一观点受到了挑战，因为有始新世时期高北纬地区冰筏漂流的记录，以及全球冰量的估计，这些估计表明，在大约4160万年前方解石补偿深度暂时加深的同时，全球冰量超过了南极洲的储存能力。在这里，我们利用赤道大西洋海洋沉积物中氧和碳同位素值以及碳酸钙含量的记录，来检验大约4160万年前两个半球都存在大型冰盖这一假说。这些记录最多只能显示出一个很容易在南极洲容纳的冰量预算，这表明北半球当时不存在大型冰盖。这些记录还揭示，在方解石补偿深度暂时加深之前不久，有一个短暂的时期，在此期间，赤道大西洋的方解石补偿深度变浅、海洋温度上升，碳同位素值下降。4160万年前左右这些变化的性质意味着，在碳循环方面，与始新世/渐新世边界的事件以及始新世早期气候适宜期的“高温事件”存在共同联系。我们的研究结果有助于解决北半球冰川作用地质记录与模型结果之间明显的差异，模型结果表明，大陆冰川作用的阈值在南半球比在北半球更早被突破。
