Holocene climatic and limnologic history of the north-central United States as recorded in the varved sediments of Elk Lake, Minnesota: A synthesis

Integration of the results and interpretations of geochemical, paleoecological, and sedimentological analyses of a varved sediment record provides a detailed chronicle of limnological and climatic changes for the past 10 ka at Elk Lake, west-central Minnesota. The early Holocene record at Elk Lake was controlled by circumstances of glacial history (e.g., basin morphometry and surrounding till lithology) in combination with global warming at the end of the Pleistocene. Later, the interplay of climate change and a disintegrating ice sheet determined the character of local environments that were affected by reduction of precipitation and increased windiness during the middle Holocene. Elk Lake became more productive and clastic sediment increased to dominate the record as the forests thinned and prairie vegetation characterized the region. During this prairie period, winters may have been cold because disintegrating northern ice sheets ceased to block winter outbreaks of Arctic air. Correlations of wind-deposited materials throughout much of the eastern two-thirds of the United States suggest that drought conditions and strong winds were widespread between 8 and 4 ka. The mid-Holocene was climatically variable, however, with strong fluctuations in varve thickness at decadal, centennial, and millennial scales testifying to rapid climatic changes. Although the cause of such climatic cycles is not yet clear, correlations between ¹⁴C anomalies and varve thickness suggest that variations in solar flux and resulting magnetic storms and zonal winds may have induced strong climatic changes. A particularly strong 600 yr fluctuation to cool and wet climates that may document neoglacial conditions around 5 ka interrupted the prairie period. After 4 ka, the climate at Elk Lake was dominated by a tropical airstream during the summer, and dry arctic and Pacific airstreams during the winter. Large-scale variations ceased, although decadal and multi-decadal variations in varve thickness chronicle changes similar, but not clearly correlative, to historically documented climatic episodes such as the Medieval Warm Period and the Little Ice Age.