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A 900-yr diatom and chrysophyte record of spring mixing and summer stratification from varved Lake Mina, west-central Minnesota, USA

Prairie Adaptation Research Collaborative (P.A.R.C.), 120 - 2 Research Drive, University of Regina, Regina SK S4S 7H9, Canada, jmcheval{at}sympatico.ca, Paleoecological Environmental Assessment and Research Lab (P.E.A.R.L.), Department of Biology, Queen's University, Kingston Ont. K7L 3N6, Canada

Brian F. Cumming

Paleoecological Environmental Assessment and Research Lab (P.E.A.R.L.), Department of Biology, Queen's University, Kingston Ont. K7L 3N6, Canada

John P. Smol

Paleoecological Environmental Assessment and Research Lab (P.E.A.R.L.), Department of Biology, Queen's University, Kingston Ont. K7L 3N6, Canada

A high-resolution, independent pollen-inferred paleoclimate record and direct algal seasonality data from the actual time of sediment deposition are used to interpret the high-resolution diatom and chrysophyte record of varved Lake Mina, west-central Minnesota, USA during AD 1116—2002. This direct algal seasonality information was obtained by splitting varves into constituent winter-spring and summer lamina, and separately analyzing the siliceous algae in each layer. Analyses of integrated, continuous four-year diatom and chrysophyte samples from a sedimentary sequence show that the time period AD 1116—1478 (ie, the Atlantic-centered `Medieval Climate Anomaly' (MCA)) was characterized by periods of vigorous and prolonged spring mixing, suggesting that ice-out occurred early. However, the warm summer temperatures in the MCA, particularly in a massive drought spanning AD 1300—1400, frequently caused the lake to stratify strongly, leading to nutrient depletion. During AD 1478—1870 (ie, the Atlantic-centered `Little Ice Age' (LIA)), Lake Mina was characterized by weak spring circulation and increasing nutrient depletion, suggesting late ice-out conditions. Strong summer stratification and/or nutrient depletion in both time periods is shown by the occurrence of the nutrient-poor oligotrophic taxon Cyclotella pseudostelligera. The diatom and chrysophyte assemblages of the period of Euro-American settlement AD 1870—2002 show higher nutrient availability and increased spring mixing intensity, as a result of forest clearance and increasingly earlier ice-out (documented in regional historical records).

Key Words: Diatoms • chrysophytes • Minnesota • seasonality • spring mixing • stratification • varves.

The Holocene, Vol. 19, No. 4, 537-547 (2009)
DOI: 10.1177/0959683609104030


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