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mation (Hazel, 1971; Stanley and Campbell, 1981). This pulse of warming has been taken to reflect a strengthening of the Gulf Stream with the closure of the Isthmus of Panama. The temporary disappearance of the planktonic foraminiferan Pulleniatina from the Caribbean at 3.1 Ma suggests that this was the time of complete closure (Keigwin, 1978), as does evidence of strongly increased winnowing along the Yucatan Channel (Brunner, 1984). The interval of warming in the southeastern United States was short-lived. As discussed below, ice age cooling affected the region profoundly within a few hundred thousand years. Thermal conditions in the Caribbean have not been assessed for the first half of the Pliocene, but in the Bahamas, reefs flourished to a greater extent than today, and the Bahama Banks comprised atolls (Beach and Ginsburg, 1980). The molluscan fauna of southern Florida differed markedly from that of the Caribbean, in sharp contrast to the situation today, when Florida shares nearly all of its strictly tropical shallow water species with the Caribbean. The distinctive Caribbean Pliocene fauna even characterized the Bahamas, which are separated from the Florida peninsula only by the narrow Straits of Florida (McNeill et al., 1988). Possibly during the highstand of sea-level of mid-Pliocene time, upwelling around Florida formed a biogeographic barrier of relatively cool water (Stanley, 1986). This may explain why, whereas hermatypic corals and many molluscan species of tropical affinities thrived in a lagoonal setting as far north as Sarasota, Florida, there was very little carbonate mud production by calcareous algae here even in the near absence of terrigineous clays. It appears that this region resembled the relatively cool but thermally stable tropical region of the modern Eastern Pacific, where upwelling also prevails (Stanley, 1986).

While the Panamanian straits connecting the Atlantic and Pacific became obstructed during mid-Pliocene time, the Bering Strait opened, to connect the two large oceans across the Arctic. This new connection apparently resulted largely from the global elevation of sea-level that ultimately inundated the eastern United States along the Orangeburg scarp. Sediments of mid-Pliocene age have been recognized at elevations in Alaska as high as 35 m above present sea-level (Carter et al., 1986). The breaching of the Bering land bridge left its mark in the stratigraphic record of northeastern Iceland, where a host of Pacific mollusk species appear abruptly in mid-Pliocene sediments (Einarsson et al., 1967).

Fossil occurrences and biogeographic distributions of molluscan species in the modern world indicate that the exchange of species between the Atlantic and Pacific was asymmetric. At least 125 species migrated from the Pacific to the Atlantic, whereas only 16 species are known to have moved in the opposite direction (Hopkins, 1967). The reason for this disparity is unclear (Vermeij, 1989), but it is significant that many groups that participated in the exchange are unable to live in most areas of the Arctic Ocean today (Carter et al., 1986). This fact, and the occurrence of a fossil sea otter on the north slope of Alaska, indicate that temperatures in the Arctic were warmer in mid-Pliocene time than they are now. At least some fringes of the Arctic were apparently ice free during at least part of the year.

The First Pulse, 3.2 to 3.1 Ma

The first events presaging the onset of the Plio-Pleistocene ice age occurred about 3.2 to 3.1 Ma, but they did not include the buildup of major ice sheets. At this time, in many low latitude areas fossil planktonic foraminifera exhibit a shift to heavier d18O values that was sustained for only about 100,000 yr. In contrast, a similar increase for deep-sea benthic foraminifera persisted, with fluctuations, to the present day (Prell, 1984). This divergence of values is taken to indicate a sustained cooling of the deep-sea where, in fact, an interval of scouring by descending cold water is also recorded (Ledbetter et al., 1978). This implies climatic cooling at high latitudes.

The temporary nature of the isotopic perturbation for planktonic fossils indicates that there was no permanent buildup of large ice sheets. Nonetheless, the oldest Pliocene tillites in northeastern Iceland occur just above a basaltic unit dated at 3.1 ± 0.1 Ma by the potassium-argon method (McDougall and Wensink, 1966) and just below the base of the Mammoth interval of reversed magnetism, dated at 3.15 Ma (Mankinen and Dalrymple, 1979). Climatic changes extended at least as far equatorward as the northwestern Mediterranean region. Here the coastal forest, dominated by cypresses, gave way to oaks and other forms of vegetation adapted to relatively drier, more seasonal climates (Suc, 1984). Statistical analysis of changes in the composition of the planktonic foraminiferal fauna of the Mediterranean reveal that a pulse of cooling between about 3.2 and 3.0 Ma reduced mean annual temperature by 2 to 4°C (Keigwin and Thunell, 1979). In northwestern Europe, several subtropical species of land plants, including palms, disappeared at the end of the Branussumian interval, about 3.2 Ma (van der Hammen et al., 1971).

Onset of the Ice Age at 2.5 to 2.4 Ma

Warmer temperatures returned temporarily to at least some regions between 3 and 2.5 Ma. Fluctuations in the composition of the planktonic foraminiferal fauna of the Mediterranean indicate oscillating temperatures for this interval (Thunell, 1979). Fossils in transgressive shallow marine strata on the north slope of Alaska reveal that sea otters were again present along the margin of the Arctic-



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