The anatomy of a climatic oscillation:
Vegetation change in eastern North
America during the Younger Dryas chronozone.
Bryan Shuman, Thompson Webb III, Patrick Bartlein, John W. Williams (2002).
Quaternary Science Reviews 21:1777-1791.
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Figure1: Spatial patterns of climate change and vegetation response during the YDC. Maps show the difference between YDC and modern surface temperatures during January (A) and July (B), as simulated by the GISS GCM (Rind et al., 1986: Rind, 1994). Colder-than-modern temperatures over the North Atlantic are shown in shades of blue, and warmer-than-modern temperatures over the mid-continent are shown in red. Changes in mean January and July temperatures (C), simulated by the GISS model (Rind, 1994), are used to represent the possible differences between four locations along an east¯west transect (B). Red symbols (C) represent simulated conditions before the YDC and blue symbols represent simulated conditions during the YDC. Red arrows illustrate the simulated magnitude and direction of temperature change. The modern percentages of spruce (Picea), northeastern pine (Pinus), and elm (Ulmus) pollen (C) are also plotted as large grey and black symbols, which represent two levels of abundance for each taxon. Small grey symbols represent the climatic position of all modern pollen surface samples from eastern North America. The modern percentages of each taxon is also shown with respect to a single climate variable, mean July temperatures (D), with responses to simulated conditions at locations 1 and 2 before (red) and during the YDC (blue) also shown (D). Smaller black symbols (D) highlight a selection of modern pollen samples within a specific range of moisture conditions, in order to show that moisture explains much of the variance at a given temperature. To avoid the ranges of southern and western pine species, which have different climatic tolerances (Thompson et al., 1999), the pine data shown are from the region north of 39° N latitude and east of 110° W longitude. |
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Figure 2: Late glacial pollen stratigraphies from nine sites in eastern North America (from left to right: King, 1981; Williams, 1974; Shane, 1987; Shane and Anderson, 1993; Spear and Miller, 1976; Davis, 1969; Suter, 1987; Davis et al., 1975; Mayle and Cwynar, 1995). Light grey bars mark the European Bölling/Alleröd chronozone (B/A), from ca 15,000 to 12,900 cal yr B.P. Dark grey bars indicate the Younger Dryas chronozone (YDC), from ca 12,900 to 11,600 cal yr B.P. (see Appendix A with regard to these ages). Sedge (Cyperaceae) and birch (Betula) pollen percentages are shown in grey. Oak (Quercus) pollen percentages are also shown in grey, but are shown only for Chatsworth Bog, Illinois, where no significant pine (Pinus) pollen (black) was recorded (King, 1981). Triangles indicate the position of calibrated radiocarbon dates. Grey triangles denote dates that have been adjusted for old carbon. |
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Figure 3: Pine and spruce pollen percentages track abrupt range shifts. Records of pine (Pinus) (A) and spruce (Picea) (B) pollen percentages are plotted with time on a vertical axis to show changes in their geographic distributions. Arrows indicate the east¯west range shifts of pine (A) and the north¯south range shifts of spruce (B). Each site is labeled by state as in Fig.2. The Stotzel-Leis site in Ohio (Shane, 1987) is shown in grey in inset A, and superimposed upon the stratigraphy from Pretty Lake in Indiana (Williams, 1974). |
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Figure 4: The abundance of pine pollen across eastern North America before, during, and after the YDC. Histograms represent the percent pine (Pinus) pollen at each of nine sites (Fig.2) at 13,000, 12,000 (YDC), and 11,000 cal yr B.P. The percentages are plotted with respect to longitude to show changes in abundance across the range of pine. Each site is labeled by state, as in Fig.2. Horizontal grey bars mark the range of sites where the presence of pine populations is indicated by pollen percentages of >10%. For comparison to modern, see Fig.1D. |
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Figure 5: Maps of plant associations and vegetation change at 1000-cal-yr intervals between 14,000 and 10,000 cal yr B.P. Different colors illustrate different vegetation assemblages. Individual plant taxa are mapped as red, yellow, or blue with overlapping ranges represented by the combinations of primary colors: orange, purple, green, or gold. Grey represents the absence of the mapped taxa, and white represents regions with no data. Two combinations of three taxa are mapped: (A) regions with greater than 20% spruce (Picea; red), 5% sedge (Cyperaceae; yellow), and 20% pine (Pinus; blue) pollen, and (B) regions with greater than 5% ash (Fraxinus; red), 15% oak (Quercus; yellow), and 6% elm (Ulmus; blue) pollen. Maps of square-chord distances (SCD) represent the dissimilarity between fossil and modern pollen assemblages (C). SCD values greater than 0.15 (black) represent pollen assemblages that have no modern equivalent. Offset maps (D) use SCDs to show the amount of change between 1000-yr intervals. High SCDs document large changes. |
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Figure 6: Lake-level elevations and pollen percentages in Wisconsin and Minnesota from 14,000 to 7000 cal yr B.P. Lake-level elevations at Lake Mendota, Wisconsin (A), are inferred from the elevations of radiocarbon-dated sediments (Winkler et al., 1986). Aquatic macrofossil assemblages and sediment characteristics from Almora Lake, Minnesota (Digerfeldt et al, 1992), constrain an envelope of possible water levels there (B). The lake-level data are compared to changes in the relative abundance of spruce (Picea), pine (Pinus), and oak (Quercus) pollen at near-by sites: (A) Devil's Lake, Wisconsin (Maher, 1982) and (B) Reidel Lake, Minnesota (Almquist-Jacobson et al., 1992). |