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Abrupt Climate Change: Inevitable Surprises
al., l994). For example, mean July temperatures in northern Spain might have been as much as 8°C lower than today (Beaulieu et al., l994).
For many years, the Younger Dryas was thought to be a solely European event (Mercer, l969; Davis et al., 1983). It was the high-resolution reexamination of pollen stratigraphy, the identification of plant macrofossils, and the new technique of accelerator mass spectrometry 14C dating of these macrofossils that enabled documentation of the event in the southern New England region of the United States (Peteet et al., l990, 1993) and in the eastern maritime provinces of Canada (Mott, 1994; Mayle et al., l993). The climate signal in southern New England was a 3-4°C July cooling; in eastern Canada, a cooling of 6-7°C is estimated (from pollen). Midge fly fossils in lake sediments from the White Mountains of New Hampshire indicate about 5°C Younger Dryas cooling of maximum summertime lake temperatures, a somewhat smaller change than suggested for a coastal transect from Maine to New Brunswick (Cwynar and Spear, 2001). In the central Appalachians, a warm, wet interval coincident with the Younger Dryas event suggests a sharp climatic gradient that might have forced the northward movement of storm-track moisture (Kneller and Peteet, l999). Later North American studies have identified the Younger Dryas event in other regions, such as the US Midwest (Shane and Anderson, l993), coastal British Columbia (Mathewes, l993) and coastal Alaska (Peteet and Mann, l994). The documentation of the Younger Dryas event over much of North America demonstrated that it was not limited to the circum-Atlantic region (Peteet et al., l997).
Central America and the Caribbean
Marine evidence of the Younger Dryas event is recorded as an interval of increased upwelling or decreased riverine runoff from adjacent South American land in a core from the Cariaco Basin in the Caribbean (Hughen et al., 1996, 2000a,b; Peterson et al., 2000) (Figure 2.4). Terrestrial evidence is primarily from three sites (Leyden, 1995). Evidence indicates a temperature decline of 1.5-2.5°C during deglaciation, probably correlated with the Younger Dryas, registered at high and low elevations about 13,100-12,300 years ago as far south as Costa Rica, and just before 12,000 years ago in Guatemala (Hooghiemstra et al., l992; Leyden et al., l994). The