Ch.E. Geiss, S.K. Banerjee (Institute for Rock Magnetism and Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, USA)
In order to obtain a high resolution comparison of oceanic and continental records during the last interglacial, we have studied Sangamonian sediments (130 - 80 ka B.P.) from two sites near Vandalia, south central Illinois. A 9 m core, taken from a small kettle lake, shows magnetic variations that coorelate well with pollen data from the same site. The second site lies within the watershed of Pittsburg Basin and shows a good exposure of the Sangamon paleosol. The paleosol developed in on older Illinoian gravel and extends into the overlying loess. Grain-size dependent parameters, such as ARM/SIRM or Hcr/Hc show a decrease in average grain-size in the Sangamon paleosol horizon, which is unaffected by the lithological gravel-loess boundary. Measure- ments of partial ARM reveal that the main magnetic carrier throughout the section is coarse grained (larger PSD), but pedogenetically altered horizons show an additional fine component. Furthermore, samples that are strongly affected by pedogenesis during the Sangamon tend to show higher Verwey temperatures, indicating a higher degree of stoichiometry in soil-magnetite compared to magnetite of the parent material.
The corresponding section of the lake sediments shows a similar magnetic behaviour, but higher variability. This indicates that the paleosol signal is still present in the lake sediments, and can be used to recognize stages of intense soil formation during the Sangamon. In connection with other non-magnetic studies we plan to use this core to yield a high resolution paleoclimate reconstruction during the last interglacial for the central United States.
Annales Geophysicae Supplement I, V.14, p. C129, 1996
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