2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 149-10
Presentation Time: 3:45 PM

CLIMATIC AND LOCAL DRIVERS OF SEDIMENT-MAGNETIC PROPERTIES FOR SUB-ARCTIC LAKES


GEISS, Christoph, Environmental Science Program, Trinity College, 300 Summit St, Hartford, CT 06106, CAMILL, Phil, Earth and Oceanographic Science, Bowdoin College, 6800 College Station, Brunswick, ME 04011, EDLUND, Mark B., Saint Croix Watershed Research Station, Science Museum of Minnesota, 16910 152nd St. North, Marine on St. Croix, MN 55047 and UMBANHOWAR Jr, Charles E., Department of Biology and Environmental Studies, Saint Olaf College, 1520 Saint Olaf Ave, Northfield, MN 55057

We analyzed the magnetic properties of eight subarctic lakes near the present-day tundra-forest ecotone in northern Manitoba and compared them to a set of non-magnetic properties to investigate the drivers of the sediment-magnetic signal in these lakes. The magnetic characterization included measurements of magnetic susceptibility, anhysteretic remanent magnetization (ARM) and isothermal remanent magnetization (IRM), which were performed on U-channels at 2-cm resolution. Hysteresis loops and magnetic coercivity distributions were measured for a subset of discrete samples. In all lakes the transition from postglacial to interglacial conditions is easily identified between 7.5 and 6.5 ka B.P. Early postglacial sediments are characterized by high concentrations of coarse-grained ferrimagnetic minerals, while interglacial sediments contain much lower abundances of finer-grained ferromagnetic minerals. The drop in abundance of magnetic minerals is likely caused by reductive dissolution of iron-oxide minerals as the abundance of clastic sediment is still high and cannot explain a decrease in concentration-dependent magnetic parameters by more than an order of magnitude. Increased Fe:Ti ratios suggest the input of dissolved iron during the Holocene, and corresponding increases in ARM/IRM ratios as well as changes in coercivity distributions indicate that biogenically produced single-domain magnetite contributes significantly to Holocene sediments. Soil formation within the watershed has no significant impact on the magnetic grain-size of the lake sediments. Lakes in the southern part of our field area, where 40-100% of the watershed are currently forested contain an abundance of magnetic minerals, while lakes that are located in tundra, with 0 – 20% forest cover, are extremely weakly magnetized, either due to a lesser degree of secondary magnetite formation or increased rates of reductive dissolution. Neoglacial cooling is inconsistently reflected in sediment-magnetic properties as all lakes respond to a combination of local (e.g., peatland development) or regional (e.g., climatic) factors.

Session No. 149

T34. Modern and Ancient Continental Depositional Environments: Linking Landscape Evolution with Fluvial Systems and Lacustrine Basins through the Cenozoic
Monday, 20 October 2014: 1:00 PM-5:00 PM
211 (Vancouver Convention Centre-West)
Geological Society of America Abstracts with Programs. Vol. 46, No. 6, p.379
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