Rock-magnetic Records of Arctic Lake Sediments Reveal Complex Links Between Lakes and Environmental Change

As part of a multi-parameter study of subarctic watersheds we analyzed the rock-magnetic properties of six lake sediment cores from northern Manitoba, Canada. These cores are all located within 75 km of Nejanilini Lake (59.7°N, -97.8°W) and straddle the present northern tree line. The studied sediment cores are between 2 and 3.5 m in length and they record environmental changes over the past 7500 - 8000 years. To characterize the magnetic properties of the cores we measured magnetic susceptibility (κ), anhysteretic remanent magnetization (ARM), isothermal remanent magnetization (IRM) for all samples, and magnetic coercivity distributions for a selected subset of samples. Basal layers are characterized by strongly magnetic, coarse grained sediments whose magnetic properties are dominated by the surrounding till and outwash. More recent organic rich sediments are weakly magnetized and characterized by varying abundances of magnetotactic bacteria, which supply an abundance of small single-domain particles. In some sites, changes in magnetic parameters correlate to variations in non-magnetic parameters, such as macroscopic charcoal concentrations, pollen percentages or geochemical parameters (biSi, C:N ratios, pollen, bulk grain-size, Fe:Ti ratios). However, the observed changes are site-dependent underscoring the complex nature of lacustrine sedimentary records.