MID-PLEISTOCENE (MIS 14 TO MIS 10) VEGETATION AND FIRE HISTORY FROM THE VALLES CALDERA SITE, NORTH-CENTRAL NEW MEXICO

The Valle Grande (35^o 52’ N, 106^o 28’ W, 2553 m asl), a large meadow in the Valles Caldera National Preserve of north-central New Mexico, contains at least 82 m of lacustrine sediments, obtained from a new drill core in 2004. An earlier core taken in the late 1940’s yielded a pollen record thought by Paul Sears and Katherine Clisby in 1952 to span the most recent two glacial cycles. Subsequent refinement of the chronology confirms at least two glacial cycles are represented, but instead MIS 14 to MIS 10, from about 552 kyr to 368 kyr. We present here an initial assessment of the vegetation, fire and climate history of the mid-Pleistocene of this region. Our present efforts document millennial-scale variability in Core VC-3, with an average 20-cm, 360-year pollen and charcoal sampling resolution. Samples from the bottom 5 m of the core (MIS 14) are dominated by conifers, primarily spruce (Picea) and pine (Pinus). Such high percentages (up to 40% for each) are presently unknown in the modern pollen rain of the region, and suggest dense boreal conifer forest vegetation. Late MIS 14 sediments transition to include, not only pine and spruce, but also sagebrush (Artemisia) and other shrubs (sunflower [Asteraceae] and goosefoot [Chenopodiaceae-Amaranthus] families), and grasses (Poaceae), among others. These pollen samples are most similar to latest Pleistocene spectra from higher elevations in the southern Rocky Mountains. In contrast, pollen samples from MIS 13 show reductions in these types with an increase in pine, perhaps both ponderosa (P. ponderosa) and piñon (P. edulis), oak (Quercus), juniper (Juniperus), and warm desert shrubs (i.e. ragweed (Ambrosia) and greasewood (Sarcobatus)). These pollen spectra are consistent with modern pollen assemblages within and adjacent to the caldera, suggesting interglacial conditions. Higher in the core, terrestrial pollen spectra from MIS 12 are quite similar to glacial spectra from MIS 14, whereas pollen spectra from MIS 11 approximate those from interglacial MIS 13. A final reversal to more glacial-like pollen spectra occurs at ca. 17 m depth at the MIS 11-10 transition. Higher charcoal concentrations, indicative of landscape burning, occur during interglacials than during glacials, paralleling the more recent MIS 2-1 transitions of the Southwest.