Paper No. 5
Presentation Time: 1:30 PM-5:30 PM
THE VIABILITY OF OSTRACODA AS PROXIES FOR PLEISTOCENE-HOLOCENE CLIMATE CHANGE AT MONO LAKE, CALIFORNIA
Mono Lake is a hydrologically closed lake located at the foot of the Sierra Nevada in Eastern California. Associated Pleistocene lake terrace deposits contain rich ostracode fauna with visible secondary calcite overgrowths on the Pleistocene material. The prominent ostracode genera are Limnocythere and Candona. Samples were analyzed for standard micropaleontologic census counts and elemental compositions using an Oxford EDS system housed on a Hitachi variable pressure scanning electron microscope. The two stratigraphic sections that were sampled include the lower part of the type section at Wilson Creek (0.0-6.5 m) and Mill Creek (~8.0-~10.2 m), both located on the northwestern shore of modern Mono Lake. Ash beds from the Wilson Creek section yield age ranges between 66.2 (± 1.2) to 23.0 (± 0.9) ka, indicating that the section falls entirely within the Pleistocene. Ash beds at the bottom of the Mill Creek section date to c.16.0 ka, and this section potentially includes the Pleistocene-Holocene transition. Visual clarity indices were recorded for the ostracode carapaces such that on a scale of 1 to 5, one represents translucent, pristine skeletal calcite. Clarity indices for the ostracoda span this entire range, with an average 2.9. Qualitative elemental analyses revealed a suite of elements not usually found in pristine ostracode carapaces that include copper, iron, potassium, and aluminum. Ostracodes with relatively deteriorated visual clarities and relatively high frequency of major elements characterize the older Pleistocene deposits at Wilson Creek. The presences of secondary calcite and/or clay minerals on the surface of the ostracoda require that great care be taken to select pristine material for geochemical analyses. Samples from the younger Pleistocene-Holocene deposits at Mill Creek revealed pristine calcite with respect to both visual clarity and elemental compositions. Samples from both localities and ages will be analyzed for both oxygen and carbon isotopes and quantitative trace element geochemistry to test the viability of the ostracodes as primary environmental indicators.