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Abrupt Climate Change: Inevitable Surprises
pared data from 25 additional sites with limited sampling resolution and 14C dating. Similarly, evidence of dry conditions during the Younger Dryas is summarized by Gasse (2000) for equatorial regions, subequatorial West Africa, and the Sahel. In South Africa, however, no strong terrestrial evidence of changes in temperature or moisture during the Younger Dryas was observed (Scott et al., 1995).
Glacial-Geological Evidence of the Younger Dryas
Glaciers are highly responsive to rapid climate change. Notable Younger Dryas advances of Norwegian and Finnish outlet glaciers and those in the Scottish mountains have been documented (Mangerud, 1991; Sissons, 1967). In the Americas, potential glacial evidence of the Younger Dryas event was observed near the Crowfoot glacier in Canada (Osborne et al., 1995; Lowell, 2000), the Titcomb Lakes moraine in the Wind River range in Wyoming (Gosse et al., 1995), and the Reschreiter glacier in Ecuador. More recent research suggests that the Younger Dryas in Peru was marked by retreating ice fronts, probably driven by a reduction in precipitation (Rodbell and Seltzer, 2000). In New Zealand, the Franz Joseph glacier began advancing early in the Younger Dryas (Denton and Hendy, 1994).
Marine Evidence of Younger Dryas Oscillation
The first evidence of Younger Dryas cooling in marine sediment cores was the observation of a return to increased abundance of the polar planktonic foraminiferal species Neogloboquadrina pachyderma in the North Atlantic (Ruddiman and McIntyre, 1981). This change suggested that reduction in formation of North Atlantic deep water was responsible for the Younger Dryas cooling observed on land (Oeschger et al., 1984; Broecker et al., 1985; Boyle and Keigwin, 1987). Later work documented North Atlantic ice-rafting events that correlate with rapid climate oscillations in Greenland, not only during the glacial period but also throughout the Holocene (Bond and Lotti, 1995). Deep-water corals from Orphan Knoll in the North Atlantic show large changes in intermediate-water circulation during the Younger Dryas (Smith et al., 1997). Cadmium:calcium ratios in shells from the North Atlantic subtropical gyre indicate increased nutrient concentrations during the Younger Dryas and the glacial period, and suggest millennial-scale oscillations affecting climate (Marchitto et al., l998). Sediment color and other data from the Cariaco Basin in the Caribbean