Biology Reference
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8200 BP. (Dean et al. 2002; Ellison et al. 2006). Warming periods include
the PGCO (Postglacial Climatic Optimum, hypsithermal interval, or xero-
thermic period) between 8000 and 6000 BP (
2°C-3°C warmer than at
present), and a Medieval Warm Period at 800-1200 CE (
+
1°C warmer) in
the North Atlantic region. As recounted in chapter 2, the Medieval Warm
Period was when the Vikings made their extended sea voyages into the New
World. It was also the last time large mobile sand dunes formed in the Ne-
braska Sand Hills (Sridhar et al. 2006). The warm period was followed by
the Little Ice Age between 1300 and 1850 CE, when the Vikings abandoned
their sites. An early sign of near-glacial or impending glacial climates is the
formation of sea ice along the shore (fi g. 8.1), which leaves telltale evidence
in coastal sediment composition. Short-term glacial fl uctuations include
Heinrich events of a few to several thousand years' duration, representing
the culmination of cold periods when armadas of icebergs are calved from
the edge of the continents into the ocean (fi g. 8.2).
The physical evidence for Heinrich events is layers of gravel on the ocean
fl oor released from icebergs as they melt. The latest H1 event, of a possible
thirteen (Hiscott et al. 2001), occurred about 15,000 BP. These short-term
+
Figure 8.1 Sea ice, Scoresby Sound, East Greenland. Photograph by Shirley A. Graham.
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