FIGURE 6.5 Estimated seawater temperature changes during different Cenozoic eras (Table 6.1) based on the oxygen isotope ratios in the calcium carbonate shells of planktonic (closed circles) and benthic (open circles) foraminifera from marine sediment cores around Antarctica. A 1%c shift in the 518O value equates with a temperature change of nearly 4°C. Note the major cooling events at the Eocene-Oligocene and Pliocene-Pleistocene boundaries. The long-term cooling trend has been ongoing throughout the past 55 million years (solid line) with relatively short-term fluctuations that presumably will continue into the future. Modified from Kennett (1977).

Northern Hemisphere glaciation during the Pliocene, which occurred nearly 10 million years after the middle Miocene climate threshold, was influenced by the closure of the Isthmus of Panama between North and South America. With the Middle American Seaway shut down, changes in the oceanic current systems and atmospheric precipitation patterns allowed more moisture to be transported into the Arctic region (Chapter 8: Breathing Planet). Subsequent shifts in the <518O values in the ocean, by more than 1%c, reflect the glacial-interglacial cycles that have dominated Earth's climate ever since (Chapter 7: Flowing Planet).

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