We present sediment magnetic data from Pittsburg Basin, a small kettle lake in south-central Illinois. The lake developed in glacio-fluvial sediments of Illinoian age and is approximately 60 km south of the maximum extent of the Wisconsinan ice sheets. Drainage of the lake resulted in a fluctuating water table and caused severe oxidation of the upper two meters of sediment but the magnetic record spans the last glacial-interglacial cycle. Sediment-magnetic properties are not a priori proxies of paleoclimate. They are mainly influenced by geological processes and respond only indirectly to climatic change. It is therefore necessary to identify the processes that link paleoclimatic change to variations in the sediment magnetic record in order to interpret them in terms of paleoclimate. In Pittsburg Basin the main magnetic minerals are (titano) magnetite and maghemite with only minor contributions of iron-sulfides in some horizons. Glacial sediments are characterized by relatively coarse grained magnetic minerals, while interglacial sediments show an additional fine grained component. A comparison of our sediment-magnetic results with non-magnetic analyses, such as pollen, ostracode or diatom studies, reveals similarities in all proxies, but also significant time lags between vegetational responses and the magnetic signal. During the initial part of interglacial sediment deposition overall erosion rates were very low and an authigenic component of biogenically produced magnetite dominates the magnetic signal. During the following prairie period erosion rates increased and pedogenetically produced maghemite was deposited in the lake. This interpretation is confirmed by studies of the Sangamon geosol, exposed within the watershed of Pittsburg Basin. The end of the interglacial and the following glacial sediments are characterized by coarse grained (multi-domain) maghemite. Our study shows that magnetic investigations that rely on the interpretation of a single parameter (such as magnetic susceptibility) are unlikely to detect the complex processes that cause the observed magnetic signal. They can lead to simplified and probably incorrect interpretations in terms of paleoclimatic change.
GSA Abstracts with Programs, V.30, p.A-260, 1998
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