MAGNETIC PROPERTIES OF VARVED LAKE SEDIMENTS AS INDICATORS OF PALEOCLIMATE CHANGE

CHRISTOPH E. GEISS, Subir K. Banerjee (University of Minnesota, Newton Horace Winchell School of Earth Sciences, Minneapolis, MN 55455, USA, Email: chgeiss@umn.edu). 

Varved lakes offer the potential of well-dated high-resolution records of paleoclimate. In principle, rock magnetic measurements can offer a rapid way to obtain records from such lakes. The highly anoxic environments encountered in varved lakes, however, can lead to the postdepositional dissolution of iron oxides and the alteration of the sediment magnetic climate signal.

Our study of varved sediments from three small glacial lakes in Minnesota, U.S.A. shows that dissolution processes can significantly reduce the concentration of magnetic minerals in these sediments. Concentration dependent parameters, such as magnetic susceptibility, saturation magnetization or isothermal magnetic remanence are therefore not always the best magnetic proxy for quantifying the input of terrigenous material into the lake system.

In the case of the studied lakes, the terrigenous component consists of highly magnetic, multi-domain sized material, while authigenically produced magnetic minerals span the SD - SP particle size range. This change in magnetic grain size, as expressed by the ratio of ARM/IRM, appears to be a more reliable proxy of terrigenous input than the concentration dependent parameters mentioned above. It can be used to reconstruct episodes of drought in the NE Great Plains during the Holocene.