Christoph E. Geiss, Trinity College, 300 Summit St., Hartford, CT 06106, christoph.geiss@trincoll.edu
Charles E. Umbanhowar, St. Olaf College, ceumb@stolaf.edu
Phil Camill, Carleton College, pcamill@carleton.edu

Kimble Pond (44E13'15" N, 93E50'24" W) and Sharkey Lake (44E35'39" N, 93E24'49" W) are two small glacial lakes in SW Minnesota, located near the present prairie - forest ecotone. Sediments from these lakes yield a climate record for the past 12 ka B.P. at sub decadal resolution throughout most of the cores. We used a combination of magnetic parameters, pollen analyses, biogenic silica and LOI measurements to reconstruct paleoclimatic changes throughout the Holocene. We show that the utilization of magnetic parameters, with supporting data from other climate proxies, allows us extract high resolution records of paleoclimate change from lake sediment archives.

Magnetic analyses are based on a combination of concentration (mag. susceptibility, IRM) and grain size dependent (ARM / IRM) parameters that were measured at very high resolution. At both sites the Holocene hypsithermal period lasts from 9.0 ka B.P. to 4.7 ka B.P., based on rock-magnetic measurements, which are confirmed by low resolution pollen counts. An increase in clastic material, as seen through LOI and biogenic silica analyses, is reflected in increased concentrations of coarse grained magnetic material, which leads to higher values of mag. susceptibility and IRM and low ratios of ARM/IRM. Several marked peaks in IRM suggest periods of extreme drought that occurred on a century scale throughout the main hypsithermal period.

Magnetic susceptibility increases during this period as well, however, its dependence on authigenic minerals, such as siderite make it less straightforward to interpret and a poor stand- alone proxy of paleoenvironmental change.