HR: 1340h
AN: GP53B-1218    [Abstracts]
TI: Environmental Magnetism of mid-Pleistocene Lacustrine Sediments of the Valles Caldera, New Mexico
AU: * Hurley, L L
EM: ldonohoo@unm.edu
AF: University of New Mexico, 1 University of New Mexico, Albuquerque, NM 87131, United States
AU: Geissman, J W
EM: jgeiss@unm.edu
AF: University of New Mexico, 1 University of New Mexico, Albuquerque, NM 87131, United States
AU: Fawcett, P
EM: fawcett@unm.edu
AF: University of New Mexico, 1 University of New Mexico, Albuquerque, NM 87131, United States
AU: Tim, W
EM: tfw@unm.edu
AF: University of New Mexico, 1 University of New Mexico, Albuquerque, NM 87131, United States
AU: Goff, F
EM: candf@swcp.com
AF: University of New Mexico, 1 University of New Mexico, Albuquerque, NM 87131, United States
AB: Rock magnetic measurements are applied to almost 80 m of lacustrine sediment (VC-3) to augment independent means of interpreting the environmental conditions of a mid-Pleistocene lake in the Valle Grande of the Valles Caldera, northern New Mexico. An age model for the core is pinned to an Ar/Ar age determination of 552 ± 3 ka for a sanidine bearing ash layer at 78 m depth and major transitions in the organic carbon record at 53 m, 40 m, 27 m, and 17 m; these data suggest that deposition of VC-3 spans MIS 14 to MIS 10, including glacial terminations VI (531 ka) and V (426 ka). AF demagnetization resolves positive inclination magnetizations from most of the core, consistent with Brunhes normal polarity. Three thin (< 22 cm) intervals of shallow to negative inclination magnetizations may indicate poorly-recorded geomagnetic polarity events at ~406 ka (11α), ~536 ka (14α) and the Big Lost excursion (~580 ± 8 ka). Data from an array of rock magnetic investigations indicate magnetite, titanomagnetite, and pyrhotite characterize VC-3 sediments. Susceptibility and frequency dependent susceptibility experiments at low temperature reveal an abundance of paramagnetic and super-paramagnetic material. Scanning electron microscopy of magnetic separates show an array of Fe-oxide and Fe-sulfide grains, including titanomagnetites with trellis ilmenite intergrowths. NRM intensities of sediment deposited during glacial periods typically range from 0.04 mA/m to values as high as 1.6 mA/m; interglacial sediment NRM intensities range from 0.05 mA/m to 0.2 mA/m. NRM values increase to 3.3 mA/m, between 48 m to 43 m, where the sediment exhibits shallow mud crack, bioturbation, and oxidation. Overall, trends in susceptibility, ARM, and SIRM are similar to those in NRM intensity. Bivariate plot of susceptibility/ARM shows little variation in concentration of low coercivity minerals (i.e. magnetite) in VC-3 sediments. Also, the concentration of high coercivity minerals (i.e. hematite) indicated by the IRM/susceptibility curve show similar variation between sediments deposited during glacial and interglacial times as the remanence data sets but the relative intensities and exact shape of the curve indicate these materials are strongly influenced by local oxidation and reduction conditions within the lake environment.
DE: 1512 Environmental magnetism
DE: 1513 Geomagnetic excursions
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
DE: 1845 Limnology (0458, 4239, 4942)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting