Modern sedimentation in Elk Lake, Clearwater County, Minnesota
The varved sediments of Elk Lake, Clearwater County, Minnesota, contain a 10,000 year record of climatic and limnologic events. Sediment traps deployed in the lake’s water column from 1979 to 1981 and from 1983 to 1984 collected samples that permitted us to identify materials, to see the timing of sedimentation events, and to deduce processes that form the microlaminae within varves. Fall and spring microlaminae consist mainly of sequential accumulations of biogenic silica and resuspended calcific and siliceous materials. Precipitates of iron, manganese, and organic detritus dominate the thin winter microlaminae. Calcific microlaminae are deposited in summer. Concentrated iron and manganese precipitates form when the onset of seasonal circulation (especially in autumn) oxygenates the lower water column, but precipitation of these metals also continues throughout periods of seasonal stratification, when these dissolved elements migrate upward and are converted to particles that rain back to the bottom. Mineraloids dominate the sediment; minerals compose only a minor part and include quartz, calcite, rhodochrosite, and rockbridgeite (iron phosphate). The bulk of the bottom accumulation occurs during the longer, calmer summer and winter periods, but important contributions are also made during spring and autumn overturn events. Sediment resuspended from the shallows accumulates together with newly formed endogenic sediment, and can even briefly dominate the seston in autumn and spring. Vigor and duration of seasonal circulations in the upper water column dictate the amount of resuspended sediment contributed annually to a varve. When abrupt warming within days after ice-out stratifies the lake, sedimentation in that year is diminished by resultant suppression of plankton blooms and lack of vernal resuspension that would normally move sediment from the littoral areas into the deep parts of Elk Lake. Thin sections of varves confirm that resuspension during autumn and spring is a varve-forming process that has probably varied in importance as a function of climate and changing morphometry due to infilling. Through an entire year, sediment traps catch a greater proportion of material from spring and autumn overturns than accumulates on the bottom. Lake morphometry is the most important factor governing sediment resuspension and associated annual accumulation rates in traps.