2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 10
Presentation Time: 4:15 PM


BRIGHT, Jordon, Department of Geosciences, University of Arizona, Tucson, AZ 85721, REHEIS, Marith, U.S. Geol Survey, Box 25046, MS 980, Denver Federal Center, Denver, CO 80225 and MILLER, David M., U.S. Geological Survey, Menlo Park, CA 94025, jbright1@email.arizona.edu

A 45-m-long core has been recovered from the Manix basin, approximately 30 km northeast of Barstow, CA. The core contains the entirety of the Manix Formation found in the vicinity of Manix Wash. Preliminary chronologic investigations suggest that the sediments in the core were deposited between 30 and 450 ka, but the majority of the ostracode-bearing sediments are older than about 185 ka.

Sediment samples for ostracode faunal and stable isotope analyses were taken from the lower 35 m of core (n = 123). The basal part of the core (40.5-43 meters below surface (mbs)) contains Limnocythere bradburyi and L. ceriotuberosa valves. The ostracode fauna from 30 to 38 mbs is comprised of L. platyforma and L. ceriotuberosa, with low numbers of L. robusta. L. ceriotuberosa is essentially the only species present from 22-30 mbs, although L. robusta occurs again briefly at 25 mbs. Above 20 mbs ostracodes are relatively rare, and exclusively L. ceriotuberosa. Soils separate the L. bradburyi and L. platyforma sections and are also present within the L. ceriotuberosa-dominated section.

The L. bradburyi in the basal part of the core implies that those sediments were deposited in shallow, summer precipitation-supported lakes. Winters were likely warm. The lack of L. bradburyi in the rest of the core suggests a shift to colder winter temperatures and snow melt-supported lakes. The L. platyforma section of core likely represents relatively fresh-water phases of Lake Manix. The remaining L. ceriotuberosa-dominated sediments were deposited in hydrologically variable, higher salinity settings.

Stable isotope analyses (O and C) on ostracode calcite from 27 to 41 mbs are complete. Stable isotope analyses on the remainder of the ostracode samples are pending. Variation within a sample (3 valves per analysis, 3 analyses per sample) is often as high as between samples. Oxygen and carbon isotope values typically fluctuate between -1 and +2 per mil, and -2.5 and -3.5 per mil, respectively. Oxygen and carbon isotope values at ~ 33 mbs decrease several per mil in conjunction with an increase in silt and clay. Although the overall stable isotope variability is high, large fluctuations in stable isotope values that co-occur with prominent changes in core lithology likely represent fluctuations in the influx of Mojave River water into the basin.