A continuous record of Holocene eolian activity and vegetation change at Lake Ann, east-central Minnesota
Wind-blown silt and sand in the lacustrine sediments of Lake Ann in east-central Minnesota provide a continuous record of Holocene eolian activity of the adjacent Lake Ann dune field. Results of grain size, loss-on-ignition, and especially magnetic susceptibility determinations indicate the variable quantities of eolian material present in the lacustrine sediments. Comparison of this eolian component with the vegetation history determined by pollen analysis indicates that dunes were apparently not present during Late Wisconsin (late glacial) and early Holocene times when trees were common on the Anoka Sand Plain. Dune development began about 8000 yr B.P. when arboreal vegetation was seriously reduced by dry conditions. The middle Holocene history is subdivided into three distinct episodes, each with a similar sequence of events: (1) drought and reduced vegetation cover, (2) increased eolian flux, (3) increased precipitation and tree cover, and (4) slow decline in eolian erosion. Maximum eolian flux during these episodes occurs at ca. 7400, 5800, and 4900 yr B.P. The mid-Holocene climate at Lake Ann is similar to that inferred for other Midwest sites, such as Elk Lake in northwestern Minnesota, where intervals of dryness or strong winds occur when eolian activity is high at Lake Ann. Dune activity in east-central Minnesota appears to have been primarily initiated by severe droughts of relatively brief duration during the general aridity of the mid-Holocene interval.