HR: 09:00h
AN: GP51A-05    [Abstracts]
TI: Magnetic Properties of Surface Sediments in Small Temperate Lakes: Modern Analogues for Paleolimnologic Research
AU: * Lascu, I
EM: lascu003@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455, United States
AU: Plank, C
EM: plan0093@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455, United States
AB: Magnetic properties of lake sediments are routinely measured as part of paleolimnological and paleoclimatic research. Basic parameters such as magnetic susceptibility (MS), anhysteretic remanent magnetization (ARM), and isothermal remanent magnetization (IRM) are used for correlating cores from different sites in the same basin, tracking erosion history and lake level changes, or investigating eutrophication and microbial processes. However, a detailed investigation of the syn-depositional processes that control the distribution of magnetic minerals across lake basins is lacking for most types of lake systems. In order to understand the main controls on environmental magnetic records, we systematically investigated the magnetic properties of surface sediments collected along transects in nine Minnesota lakes. The lakes are small (<1 sq. km), have simple morphologies, are hydrologically closed, and are distributed across an east-west moisture gradient, as well as a north-south temperature gradient. The structure of lake water columns was investigated by measuring temperature, specific conductivity, dissolved oxygen, and pH. Sediment composition was determined via loss on ignition (LOI). The magnetic properties of the sediments reflect the change in depositional environments from shallow to deep water, as defined in sedimentological context by LOI and sediment granulometry. All lake basins exhibit a characteristic pattern in terms of concentration (MS and IRM) and grain size (ARM/IRM) of magnetic minerals. Sediments above wave base (0.5 m) have high concentrations of coarse grained magnetic minerals. Below wave base, but in the thermally mixed layer, magnetic particles are finer-grained and present in lower concentrations. Profundal slope sediments are characterized by variable magnetic and compositional parameters, indicative of a dynamic sedimentological and geochemical environment. In the deep, anoxic regions, magnetic concentration increases again, associated with a coarsening in grain size, most likely an effect of sediment focusing. The generalized model of lacustrine magnetic response to both external and internal forcing factors provided by our modern analogue approach will provide an improved basis for paleolimnologists to interpret records of lacustrine environmental magnetism.
DE: 0458 Limnology (1845, 4239, 4942)
DE: 1512 Environmental magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting