ENVIRONMENTAL MAGNETISM OF MID-PLEISTOCENE LACUSTRINE SEDIMENTS, VALLES CALDERA, NEW MEXICO

We use remanence, susceptibility, and other rock magnetic data to interpret environmental conditions from sediments of a mid-Pleistocene lake, Valles Caldera of northern New Mexico. Lake sediments, sampled by core VC3-04, were deposited during an interval of ~200 ky spanning three glacial and two interglacial events and coincides with marine isotope stages (MIS) 14 through 10. Within the core, we identify two possible geomagnetic excursions. The younger event occurs at 17 m below the surface within interglacial MIS 11, and may correspond to excursion 11α. The older event occurs at 67 m within glacial MIS 14 and may correspond to excursion 14 α. Detrital magnetic phases include iron oxides with variable amounts of titanium likely sourced from surrounding Quaternary volcanic rocks. Secondary phases, including iron-bearing sulfides and carbonates, are present in several depth intervals. Remanence ratios, between 0.15 and 0.5 are consistent with a mixture of magnetite grain sizes and coercivity ratios, between 1.5 and 5.5, are shifted to higher values than expected for linear mixing theory. Both high and low temperature magnetic ordering points for magnetite are shifted or suppressed. The combined observations of increased coercivity ratios and subdued ordering points suggest detrital phases have cation substitution, or have experienced oxidation during lake conditions of high organic productivity. Several climate proxies, including organic carbon and pollen variations examined independently of this study, identify intervals where temperature, moisture, and biologic productivity changed from cold/wet during glacial to warm/dry during interglacial conditions. These shifts are reflected in intensity values for natural remanence magnetization (RM), between 0.5 and 1 mA/m, anhysteretic RM, between 0.5 and 10 mA/m, and isothermal RM, between 0.2 to 0.7 A/m. The climate proxy and environmental magnetic data together suggest that preservation of detrital magnetic phases varies with depth and secondary phase formation increases during interglacial conditions. An interval near ~43 m depth shows an anomalously high magnetic remanence and susceptibility and is related to a m-long mudcrack suggesting prolonged subaerial exposure and magnetic enhancement of the sediment.