HR: 0800h
AN: PP21C-1593    [Abstracts]
TI: A Comparison of 20th Century and Holocene Historical Lake Areas and Implications for Reconstruction of Holocene Moisture Balance.
AU: * Plank, C P
EM: plan0093@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury drive SE, Minneapolis, MN 55455 United States
AU: Ferreira, M
EM: marcia.ferreira.813@csun.edu
AF: Department of Geological Sciences, California State University, Northridge, 18111 Nordhoff St., Northridge, CA 91330 United States
AU: Shuman, B
EM: bshuman@umn.edu
AF: Department of Geography, University of Minnesota, 414 social sciences building 267-19th ave. South, Minneapolis, Mn 55455 United States
AU: Ito, E
EM: eito@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury drive SE, Minneapolis, MN 55455 United States
AB: Low lake stands during the Holocene document a period of drier than modern conditions in the upper Midwest after 8000 cal yr B.P. Here, we seek to compare the magnitudes and spatial extent of these mid-Holocene water-level changes with the changes that occurred during the 1930s Dust Bowl drought. The comparison allows us to consider the potential range of hydrologic conditions in the mid-continent, and to detect similarities or differences between the two periods as a way to learn more about the climatic controls on the mid-Holocene moisture-balance. (How good an analog is the 1930s drought for the mid-Holocene aridity?) Each lake record, however, is unique and comparisons among lakes have been hindered by local hydrologic influences and sedimentary processes. We therefore test the simple methodological assumption that the stratigraphic records of small lakes may be used in concert to produce valid and meaningful maps of Holocene moisture balance. First we confirm that broad regional trends in lake area reflect long-term moisture balance patterns by studying the 1930s aerial photography. Second, we confirm that regionally coherent patterns of sedimentary change related to past water levels can be detected for the mid-Holocene via GPR profiles. Changes in open water surface area in response to the 1930's drought are quantified and mapped for over 300 small (1km sq. or less) lakes of northern and central Minnesota in order to determine if small basins responded to drought with explainable regional coherency. A map of percent area change (between AD 1930 and 2003) reveals that coherent trends in surface area response reflect the gradient of moisture balance change over the same period. We found a trend in percent surface area change from east to west with western lakes completely drying out (>100% change), a wider variability of responses in central Minnesota, and eastern lakes undergoing almost no change (<10%). GPR profiles were collected on two east-west transects for a subset of the lakes included in the historical aerial photo analysis. Interpretation of the GPR profiles reveals that the apparent frequency of erosional events increased towards the west, mimicking the behavior of modern drought response. However, unlike the modern drought response, we detect some large magnitude lake area changes throughout Minnesota during the Holocene. The preservation of well defined packages of lake strata appears to increase eastward, with thicker units between unconformities, but we find low elevation unconformities throughout the state. Taken together, we observe that an east-west gradient of lake area variability has remained in place throughout the Holocene, even if the magnitude of some water level changes was much larger in the past than during the 20th century. The influence of basin morphometry on the clarity of the lake level signal is, however, significant and records are commonly complicated by the evolution of the sedimentary basins from more complex, in some cases multi-basinal forms, to less complex flat-bottom forms.
DE: 1812 Drought
DE: 1830 Groundwater/surface water interaction
DE: 1843 Land/atmosphere interactions (1218, 1631, 3322)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005