AN INVESTIGATION OF ENVIRONMENTAL MAGNETISM OF THE VALLE GRANDE PLEISTOCENE LACUSTRINE SEDIMENTS, NEW MEXICO

Patterns in magnetic mineralogy of lacustrine sediments can be used as a proxy for climatically controlled changes in lithology. Sediment core VC304, drilled in 2004, represents 80m of lacustrine sediments from the Valles Caldera, northern New Mexico. A tephra at 76m depth was used to obtain an Ar/Ar date of 552 +/- 3 ka. Carbon isotopes, percent organic carbon, and sedimentology from the core have been used independently to suggest that VC304 spans two mid-Pleistocene glacial cycles.

AF demagnetiztion of NRM yields well-defined inclination vectors (specimens taken at a minimum of 20 cm) consistent with Brunhes normal polarity, but three intervals of negative inclinations values may reflect partial recording of geomagnetic polarity events. Rock magnetic data (e.g. intensity and AF demagnetization of ARM, intensity and AF demagnetization of SIRM) show that an assemblage of fine multidomain and pseudo-single domain ferromagnetic particles characterizes the sediment. NRM intensities of uppermost half of VC304 sediments typically range between 0.1mAm to 1.5mAm. However, for the interval of ~40m-~50m intensities are typically ~10.0mAm. Bulk magnetic susceptibility ranges from 8.1E-6 to 1.4E-4 and increase to 1.6E-4 to 5.6E-4 over 40m-50m depth. Typical values of bulk susceptibility/ARM for the sediments range from 1.0E-05 to 3.8E-05. Three samples (~44m-~45m) show higher ratios, between 5.5E-5 and 1.6E-04, which suggests a greater abundance of magnetite.

Three intervals of anomalous remanence magnetizations of negative inclination have been isolated, although not necessarily well-defined. Isotopic age constrains and the inferred depth of glacial cycles suggest the following: the negative inclination recorded at17m may compare with the ~410 + 10 ka polarity feature (Langereis et al., 1997) and/or feature11? (Lund et al., 2001) and the negative inclination intervals at 67.6m and ~78m may correlate with feature 14? (Lund et al., 2001) and the Big Lost excursion (560 ka-570 ka) respectively. The latter is stratigraphically below the tephra horizon and therefore older than 552 ka. Paleomagnetic remanence data corroborate the age estimate for VC304 suggested by carbon isotope, percent organic carbon, and sedimentology observations.