We propose a model that explains variations in magnetic parameters of lake sediments as a record of Holocene climate change. Our model is based on records from four lakes and incorporates the effects of erosion, dust deposition, and the authigenesis and diagenesis of the magnetic component of the sediment. Once checked against high resolution multi proxy climate records, which are currently being established for some of our study sites, it will allow us to use magnetic proxies to establish high-resolution climate reconstructions on a regional scale. Our model utilizes a combination of concentration-dependent parameters (magnetic susceptibility, IRM) and grain-size-dependent parameters (ARM/IRM, hysteresis parameters). Magnetic mineralogy is characterized by a combination of low-temperature measurements and S-ratios, and our magnetic measurements are complemented by XRD, LOI and smearslide analyses. During periods of forest growth within the watershed, deposition of terrigenous material is low and the sediment magnetic properties are characterized by low concentrations of mainly authigenic minerals (low values of IRM, high ratios of ARM/IRM). During the early to mid-Holocene dry period, deposition of terrigenous material increased due to intensified dust deposition and the erosion of lake margins caused by lowered water levels. Concentration of magnetic minerals increases (high IRM, χ) and so does the grain-size of the magnetic fraction (low ARM/IRM). During the late Holocene, sediment magnetic properties depend on the varied position of the site with respect to the prairie-forest ecotone.
Bibliographical noteFunding Information:
We are grateful to Barb Reichel, Fred Runquist, Carol Schuelke and Margaret Sharkey for granting us lake access, Herb E. Wright Jr. and Mike Jackson for great discussions and support, Kristina Houser and Leah Dvorak for measuring biogenic silica and LOI in record time, as well as numerous field assistants for their coring help. Our work was sponsored by NSF/ ATM grant 9909523. John W. King and an anonymous reviewer provided valuable comments. The magnetic analyses were performed at the Institute for Rock Magnetism (IRM) which is funded by the W.M. Keck foundation, the National Science foundation’s Earth Science Division’s Instrumentation and Facilities Program and the University of Minnesota. This is IRM publication 0107.
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