FIGURE 1. From greenhouse to icehouse, across the Eocene–Oligocene boundary.

From the following article:

Global change: A world in transition...

Harry Elderfield

Nature 407, 851-852(19 October 2000)

doi:10.1038/35038196

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Remains of tropical plants such as those on the left are found in many lower Eocene deposits from high latitudes; in the Oligocene, Antarctic glaciation developed and the seas fell to their lowest levels of the past 100 million years. A possible course of events is that tectonic movements led to widening of the oceanic passages, isolating Antarctica and allowing development of the circumpolar current. This provided a moisture source for snow precipitation and led to the initiation of a large-scale Antarctic ice sheet. At the same time, a gradual long-term lowering of atmospheric CO2, perhaps driven by changes in the intensity of weathering (which uses up CO2 in the formation of carbonic acid, the weathering agent of rocks), diminished the greenhouse effect. As the temperature gradient steepened between the poles and the tropics, oceanic and atmospheric circulation intensified. The system flipped from greenhouse to icehouse. The changes in seasonality identified by Ivany et al.1 may have resulted from the oceanic perturbation and in turn have led to the marine extinctions.

LEN WEBB B. & C. ALEXANDER/ANN HAWTHORNE

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