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Developing U-Series Dating Capabilities for Quaternary Hydrogenic Deposits 
        Using the Ion-Microprobe: Katharine Maher

Project Title: Developing U-Series Dating Capabilities for Quaternary Hydrogenic Deposits Using the Ion-Microprobe
Mendenhall Fellow: Katharine Maher, (650) 329-4978,
Duty Station: Menlo Park
Start Date: February 22, 2005
Education: Ph.D. (Geochemistry), University of California, Berkeley, 2005
Research Advisor: Dave Miller, (650)329-4923,; Jim Paces, (303)236-0533,; Joe Wooden, (650)725-9237,
      Katharine Maher

Project Description: The magnitude and timing of Pleistocene climate change in the southwestern United States has significant consequence given the uncertainty of future climate change. Proxies used to assess climate change, such as δ13C and δ18O in soil or hydrogenic deposits require accurate estimates of the age of the deposits. In the arid southwestern United States, climate change events are often recorded in pedogenic materials such as secondary opal and carbonate. These authigenic minerals provide a record of the prevailing environmental and hydrologic conditions at the time of deposition. If such material can be accurately dated, a chronological framework can be constructed to allow comparisons with other climate records.

Silica- and carbonate-rich hydrogenic deposits present throughout the western United States possess several unique properties that make their use as Quaternary geochronometers and climate proxies very attractive. Hydrogenic deposits often contain discrete layers of varying thickness, reflecting growth histories controlled by the prevailing hydrology -- high infiltration fluxes typically result in fast growth rates, while arid periods may create hiatuses in the depositional record. A variety of Quaternary dating methods exist, such as luminescence, cosmogenic nuclide, electron-spin resonance, amino acid racemization, and radiocarbon. However, most require significant assumptions, have large uncertainties, or are not applicable over longer Pleistocene timescales. In contrast, U/Th dating can provide precise Quaternary geochronological data, especially if discrete zones within a sample can be analyzed. However, when larger samples are analyzed, the inter-layer averaging can yield ages that are tens to hundreds of thousands of years younger than the true age. A promising new method for dating these layered deposits, which combines micro-extraction procedures with high-precision isotopic measurements, utilizes the SHRIMP RG instrument (sensitive high-resolution ion microprobe reverse geometry) present at the joint Stanford/USGS facility in Palo Alto, CA. Analyses using the ion microprobe have the added benefit of introducing the solid sample directly to the instrument source, therefore circumventing the chemistry required for conventional isotope mass spectrometry. The “spot” of the beam is approximately 25 to 45 microns, thus providing the potential to resolve the microstratigraphy of the material beyond what is typically feasible by mechanical/chemical means.

This project seeks to expand the analytical techniques of the existing SHRIMP RG to include U/Th dating of a variety of Quaternary hydrogenic materials. The ability to extend this method to pedogenic materials, silicretes, opals and possibly high U carbonate deposits will comprise a large portion of the research. A second goal of this project is to combine the age constraints with geochemical considerations in order to reconstruct paleo-fluid flow and infiltration rates in the Mojave desert.

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