1950). Saharan dust fluxes also increased abruptly near 4.2 Ma (Tiedemann et al., 1989). In general, climates of the Northern Hemisphere prior to about 3.2 Ma were warmer than those of the past 2.5 m.y., including climates during glacial minima. To set the scene for discussion of the ice age, we review climatic indicators of the widespread warmth that preceded it.
The terrestrial floras that have thus far been used most effectively to document continent-wide climatic changes during the Pliocene Epoch are those of Europe (Figure 7.1). Early in Pliocene time, dense coastal forests fringed the northwestern coast of the Mediterranean Sea. Dominance here of species belonging to the cypress family indicates that moist and relatively warm conditions persisted throughout the year (Suc and Zagwin, 1983).
The Mediterranean Sea itself was characterized by a marginally tropical, or at least warm subtropical, thermal regime during Early Pliocene time. This is indicated, for example, by fossil occurrences of numerous species of the generally tropical gastropod genus Conus (Marasti and Raffi, 1979). Furthermore, 5% of the polysyringian bivalve mollusks of the Mediterranean that survive from the Early Pliocene are restricted to tropical seas along the west coast of Africa today, apparently being unable to tolerate subtropical conditions (Raffi et al., 1985). Quantitative assessment of the history of extant lineages of planktonic foraminifera reveals that surface waters of the Early Pliocene Mediterranean were characterized by equable thermal conditions, with winter temperatures usually higher than those of the present (Thunell, 1979).
The North Sea was also warmer than today: 20% of the extant polysyringian bivalve species from the lower Pliocene of the North Sea region today live only south of the North Sea, and 10% occur only in waters at least as warm as subtropical. The reproductive requirements of the warm-adapted survivors suggest that for 3 or 4 months, mean temperatures reached 20°C or more (Raffi et al., 1985).
Marine biotas reveal that in the Western Atlantic region, as well, climates prior to about 3 Ma were warmer than today. Fossiliferous strata representing a highstand of sea-level between about 3.5 and 3.0 Ma are exposed along the Atlantic Coastal Plain from Virginia to Florida. They were deposited seaward of the Orangeburg scarp, a conspicuous topographic feature from North Carolina to Florida cut by wave erosion during a stillstand coinciding with the maximum advance of the shoreline, when sea-level stood 35 ±18 m above its present level (Dowsett and Cronin, 1990). At the border between North and South Carolina, the scarp lies about 150 km inland from the present shoreline. Fossil ostracods and microplankton from the Duplin Formation, which extends eastward from the scarp in this region, indicate an age of about 3.5 to 3.0 Ma, and the thermal tolerances of surviving ostracods suggest that nearshore bottom-water temperatures ranged from 26°C in August to 18°C in February (Dowsett and Cronin, 1990). By way of comparison, bottom temperatures along the continental shelf of this region today at a depth of 30 m drop to about 12°C in February. Rich fossil molluscan faunas represent themid-Pliocene highstand from Virginia to southern Florida. In southern Florida they are associated with coral reefs at latitudes about 150 km north of the limit of reef growth today (Meeder, 1979).
Although climates in the southeastern United States were generally warmer than today throughout the 3.5 to 3.0 Ma interval, they became especially warm toward the end of the interval. Ostracod occurrences also suggest a rise of temperatures during the mid-Pliocene transgression. In Virginia, changing ostracod and molluscan faunas indicate a shift from warm temperate to subtropical conditions during deposition of the uppermost Yorktown For-