Our recent studies of sediment-magnetic properties of three small glacial lakes in Minnesota and Illinois result in a magnetic record of glacial and interglacial paleoenvironments. The magnetic response to certain paleoenvironmental changes is similar in all three sites and allows us to formulate a first-order model that links paleoclimatic change to sediment-magnetic variations.
Postglacial conditions are characterized by high erosion rates and low organic productivity, and the magnetic properties of the corresponding sediments are similar to the source material. The magnetic fraction consists of coarse grained (MD and PSD) (titano-)magnetite which occur in high concentrations. During interglacial periods, when large areas of the watershed are covered by deciduous forest, erosion rates decrease while organic productivity is high. Overall concentration of magnetic minerals in these organic sediments is very low, and the sediment magnetic properties can be dominated by fine grained (SD and SP) authigenic magnetite which is likely of biogenic origin. Interglacial dry periods can result in the erosion of magnetically enhanced soil material. Lake sediments are then rich in fine grained (SD and SP) maghemite. Low- and fluctuating water levels result in an increase in erosion rates. Repeated cycling through anoxic-oxic conditions, however, can lead to reductive dissolution during phases of anoxia and precipitation of magnetically hard minerals such as hematite during periods of low water tables and subaerial weathering. This magnetically hard component causes S-ratios as low as 0.8 and is still detectable after several 10,000 years. These dissolution and reprecipitation processes can affect older sediments to considerable depth (up to several meters) and erase part of the sediment-magnetic signal. The deposition of eolian dust can lead to a similar magnetic signal, and it may be possible to determine the onset and magnitude of loess deposition using sediment magnetic techniques.
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