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Quantitative Interpretation of Fossil Pollen Spectra: Dissimilarity Coefficients and the Method of Modern Analogs
- J. T. Overpeck, T. Webb III, I. C. Prentice
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- Journal:
- Quaternary Research / Volume 23 / Issue 1 / January 1985
- Published online by Cambridge University Press:
- 20 January 2017, pp. 87-108
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Dissimilarity coefficients measure the difference between multivariate samples and provide a quantitative aid to the identification of modern analogs for fossil pollen samples. How eight coefficients responded to differences among modern pollen samples from eastern North America was tested. These coefficients represent three different classes: (1) unweighted coefficients that are most strongly influenced by large-valued pollen types, (2) equal-weight coefficients that weight all pollen types equally but can be too sensitive to variations among rare types, and (3) signal-to-noise coefficients that are intermediate in their weighting of pollen types. The studies with modern pollen allowed definition of critical values for each coefficient, which, when not exceeded, indicate that two pollen samples originate from the same vegetation region. Dissimilarity coefficients were used to compare modern and fossil pollen samples, and modern samples so similar to fossil samples were found that most of three late Quaternary pollen diagrams could be “reconstructed” by substituting modern samples for fossil samples. When the coefficients indicated that the fossil spectra had no modern analogs, then the reconstructed diagrams did not match all aspects of the originals. No modern analogs existed for samples from before 9300 yr B.P. at Kirchner Marsh, Minnesota, and from before 11,000 yr B.P. at Wintergreen Lake, Michigan, but modern analogs existed for almost all Holocene samples from these two sites and Brandreth Bog, New York.
The Continental Record of Environmental Conditions at 18,000 yr B.P.: An Initial Evaluation
- G.M. Peterson, T. Webb III, J. E. Kutzbach, T. van der Hammen, T. A. Wijmstra, F. A. Street
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- Journal:
- Quaternary Research / Volume 12 / Issue 1 / July 1979
- Published online by Cambridge University Press:
- 20 January 2017, pp. 47-82
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The development of reliable paleoclimatic maps at a global scale requires data at the following three levels of analysis: (1) well-recorded observations of evenly positioned, well-dated geological evidence (Level I), (2) paleoclimatic estimates derived from this evidence by well-defined quantitative repeatable methods (Level II), and (3) maps synthesizing the estimates from several independent sources of geological evidence (Level III). Our paper describes much of the currently available paleoclimatic data from unglaciated terrestrial areas at ca. 18,000 yr B.P. and illustrates the quantity and quality of the data at both the Level I and the Level II stages of analysis. Although the scarcity of well-dated evidence for this time period precluded any major Level III syntheses of the information, comparisons were drawn where possible between the geological evidence and the climatic conditions simulated by general-circulation model experiments of Gates (1976a, b) and Manabe and Hahn (1977). Of the more than 320 sites with data from 18,000 yr B.P., only 65 are well-dated with bracketing dates within the interval of 23,000 to 13,000 yr B.P., whereas about 100 are undated or poorly dated. We concentrated our survey on palynological and paleobotanical evidence and also thoroughly reviewed the evidence for water levels in lakes at 18,000 yr B.P. In areas with few of these sources of evidence, data on former snowlines, periglacial features, and eolian deposits were included, but the survey of these data is far from complete. Maps of the assembled data reveal the consistency of the paleoclimatic estimates in “data-rich” areas and also show which areas required additional information. The maps show that conditions were colder than present at 18,000 yr B.P. for all sites with temperature estimates. Estimated temperature depressions varied from ca. 1° to 12°C or more, depending on the location of the sample, the type of geological evidence, and the method of temperature estimation. Interpreted hydrological conditions were more variable spatially than the temperature estimates. The southwestern U.S. was moister than present, whereas the southeast may have been drier. Europe and the northern Mediterranean across to Afghanistan were drier than present, but northwest Africa was wetter. Australia was mainly drier than present, but several sites there as well as in Africa show significant climatic changes between 21,000 year period and 16,000 yr B.P. This latter evidence suggests that considerable variability may have occurred during the several thousand-centered on 18,000 yr B.P. Accurate time control is therefore required for the geological data used to study the climate dynamics of 18,000 yr ago. Large portions of South America and Asia as well as significant portions of the other continents lack the data base, or at least the well-dated base, required to define the 18,000 yr B.P. climate. In the few areas where comparisons were made with the Ice Age climates simulated by general-circulation models, general agreement existed between the geological evidence and the model simulations. Many critical comparisons were thwarted, however, by the lack of model simulations for all seasons at 18,000 yr B.P. Difficulty in validating precipitation anomalies in the tropics also arose because surface-albedo values, which are a vital input to the general circulation models, are estimated from the same evidence that is used to validate the results of the models.
Holocene Climatic Change in the Northern Midwest: Pollen-Derived Estimates
- P. J. Bartlein, T. Webb III, E. Fleri
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- Journal:
- Quaternary Research / Volume 22 / Issue 3 / November 1984
- Published online by Cambridge University Press:
- 20 January 2017, pp. 361-374
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Mapping of Holocene pollen data in the midwestern United States has revealed several broadscale vegetational changes that can be interpreted in climatic terms. These changes include (1) the early Holocene northward movement of the spruce-dominated forest and its later southward movement after 3000 yr B.P. and (2) the eastward movement of the prairie/forest border into southwestern Wisconsin by 8000 yr B.P. and its subsequent westward retreat after 6000 yr B.P. When certain basic assumptions are met, multiple regression models can be derived from modern pollen and climate data and used to transform the pollen record of these vegetational changes into quantitative estimates of temperature or precipitation. To maximize the reliability of the regression equations, we followed a sequence of procedures that minimize violations of the assumptions that underlie regression analysis. Reconstructions of precipitation during the Holocene indicated that from 9000 to 6000 yr B.P. precipitation decreased by 10 to 25% over much of the Midwest, while mean July temperature increased by 0.5° to 2.0°C. At 6000 yr B.P. precipitation was less than 80% of its modern values over parts of Wisconsin and Minnesota. After 6000 yr B.P. precipitation generally increased, while mean July temperature decreased in the north, and increased in the south. The time of the maximum temperature varies within the Midwest and is earlier in the north and later in the south.