VARVED LAKE-SEDIMENT EVIDENCE FOR CYCLIC AND ABRUPT CLIMATE EVENTS IN THE MIDWESTERN UNITED STATES 8200 BP.


Dirk Slawinski*, Kerry Kelts, and Feng Sheng Hu
Limnological Research Center,
Pillsbury Hall, University of Minnesota, Minneapolis, MN 55455.
dirkman@geolab.geo.umn.edu
NOTE: the above details are out of date, use info from main page

Annually laminated (varved) sediments from Deep Lake (47° 45' N, 95° 23' W), north-central Minnesota, provide a sensitive monitor for aeolian and hydrologic processes along the eastern edge of the vast Lake Agassiz plain. Varve couplets are thick (0.2 - 1.5 cm) during the middle Holocene, likely reflecting dry climatic conditions and enhanced aeolian activity in the region. Microscopic examinations of these varve couplets show that varve thickness increases as a result of increased terrigenous influx and diatom frustules, rather than increased calcite precipitation during the summer. Varve-thickness increases markedly at 8700 cal. yr BP and reaches the maximum value of ~1.5 cm at 8200 cal. yr BP. The maximum varve thickness at 8200 cal. yr BP at Deep Lake coincides with decreases in the methane concentration and del18O of the GISP II ice cores. This coincidence suggests a causal link between the climatic systems of the midwestern United States and the North Atlantic and offers support for a widespread climatic event ca. 8200 cal. yr BP. Spectral analysis of the varve sequence shows periodicities of 47 and 18 years. The 47 year rhythm, which has been found in other paleorecords from the midwestern United States (e.g., Elk Lake, MN), is possibly related to the half-Gleissberg solar magnetic cycle.



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