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NEOGENE ICE AGE IN THE NORTH ATLANTIC REGION: CLIMATIC CHANGES, BIOTIC EFFECTS, AND FORCING 118 FACTORS 7 Neogene Ice Age in the North Atlantic Region: Climatic Changes, Biotic Effects, and Forcing Factors Steven M. Stanley Johns Hopkins University William F. Ruddiman University of Virginia ABSTRACT Long-term climatic trends culminated in the recent ice age of the Northern Hemisphere. As late as mid-Pliocene time, however, many sectors of the North Atlantic region remained substantially warmer than today. Oxygen isotope ratios for marine microfossils indicate that a pulse of cooling occurred relatively suddenly at high and middle latitudes at ~3.2 to 3.1 million years ago (Ma) and that large ice sheets formed ~2.5 Ma, when more severe cooling and regional drying of climates occurred. Cycles of glacial expansion and contraction reflected orbital forcing at periodicities of ~41,000 yr until about 0.9 Ma and ~100,000 yr thereafter. Aridification in Africa at ~2.5 Ma resulted in the extinction of many forest-dwelling species of mammals and, soon thereafter, in the origins of numerous species adapted to savannas. Mammalian extinction intensified closer to 2 Ma in North America and was weaker in Europe, where forests changed in floral composition but remained widespread. Beginning at ~2.5 Ma and continuing into mid-Pleistocene time, life occupying shallow seafloors in the North Atlantic region suffered heavy extinction from climatic cooling, leaving an impoverished, eurythermal Recent fauna. Long- term climatic trends in the North Atlantic region during Neogene time probably resulted primarily from tectonic events, notably closure of the Straits of Panama and uplift of the Tibetan Plateau and other regions. A decrease in atmospheric CO2 and consequent weakening of a greenhouse effect also appears to be required, perhaps due to increased weathering that accompanied the uplifting of plateaus.
