Parallel changes in lake-level and pollen data show that the rapid decline
of the Laurentide Ice Sheet (LIS) between 10,000 and 8000 cal yr BP triggered
a step-like change in North American climates: from an ice-sheet-and-insolation-dominated
climate to a climate primarily controlled by insolation. Maps of the lake-level
data from across eastern North America show a reorganization of climate patterns
that the pollen data independently match. Raised lake-levels and expanded populations
of moist-tolerant southern pines (Pinus) document that summer monsoons
intensified in the southeastern United States between 9000 and 8000 cal yr BP.
Simultaneously, low lake-levels and an eastward expansion of the prairie illustrate
an increase in mid-continental aridity. After the Hudson Bay ice dome collapsed
around 8200 cal yr BP, lake-levels rose in New England, as populations of mesic
plant taxa, such as beech (Fagus) and hemlock (Tsuga), replaced
those of dry-tolerant northern pines (Pinus). Available moisture increased
there after a related century-scale period of colder-than-previous conditions
around 8200 cal yr BP, which is also recorded in the pollen data. The comparison
between pollen and lake-level data confirms that vegetations dynamics reflect
climatic patterns on the millennial-scale.
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Figure 1. Modern pollen percentages for northern pines (Pinus) and beech (Fagus) from modern sediment samples collected in eastern North America (north of 39o Lat., and east of 110o Long.). The maximum abundances of the two taxa occur within the same temperature range, but occur within different precipitation ranges. |
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Figure 2. Maps contrasting 10,000 and 9000 cal yr BP against 8000 and 7000 cal yr BP illustrate changing moisture-balance patterns and vegetation distributions as the LIS collapsed. The uppermost panel shows three types of lake-level data: multi-core, multi-proxy studies like Digerfeldt et al. (1992), qualitative assessments of lake-level indicators following Harrison (1989), and hiatuses in published pollen stratigraphies. The second panel shows the general trends in moisture-balance according to a locally weighted interpolation of the lake-level data. Two lower panels show parallel changes in the extent of regions with >25% pine (Pinus), 5% beech (Fagus), 15% prairie forb (Asteraceae, Chenopodiaceae/Amaranthanceae, and Artemisa), 10% ragweed (Ambrosia), and 10% hemlock (Tsuga) pollen. Four levels of elm (Ulmus) pollen percentages are mapped in the lowest panel. |
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Figure 3. Fossil pollen percentages of hemlock (Tsuga) and beech (Fagus) at North Pond, Massachusetts (Whitehead and Crisman, 1978), plotted versus calendar years along with the oxygen isotope record from GISP2 (Stuiver et al., 1995) and the area of the LIS over time, estimated from Dyke and Prest (1987) and Barber et al. (1999). |
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Figure 4. Fossil pollen percentages of spruce (Picea), pine (Pinus), alder (Alnus), birch (Betula), heaths (Ericaceae), oak (Quercus), hemlock (Tsuga), and beech (Fagus) at Crooked Pond (Shuman et al., in press) and Makepeace Cedar Swamp, Massachusetts (Newby et al., 2000), with lithostratigraphy and calibrated radiocarbon ages (Stuiver et al., 1998) for each core. Ages given for Crooked Pond, core H, are based on dates from cores D and K (italics; Shuman et al., 2001). Grey bands mark the Younger Dryas chronozone (YD, 12,900–11,600 cal yr BP) and the century-scale event around 8200 cal yr BP (~8.2). The estimated low lake-levels at both sites between these two periods are based on data from transects of cores (Newby et al., 2000; Shuman et al., 2001), including shallow-water lithostratigraphic facies (sand and peat) shown here. |
