New Mexico Geological Society Annual Spring Meeting — Abstracts

Sediments deposited on the floors of lakes are frequently sensitive indicators of climatic conditions. Studies of cores from saline lakes have provided valuable paleoclimatic records, although, until recently dating of the sediments has been difficult. A newly developed technique using ³⁶Cl has provided a means by which to date saline deposits such as those found in Searles and Panamint Valleys, CA. Chlorine-36 (half-life = 301,000 years) is produced in the atmosphere and at the earth's surface by cosmic-ray interactions, and in the subsurface by nuclear reactions arising from U, Th, and daughter species decay. Each of these sources releases chloride with a distinctive ³⁶Cl/Cl ratio into the hydrologic system. Since chloride is hydrophilic, most of it will be transported to the terminal playa or lake in a closed drainage basin. Much of the chloride
released into the Owens River system during Pleistocene pluvial stages was deposited in Searles Lake and other nearby hydrologically-connected basins. The initial ³⁶Cl/Cl ratio of the chloride salts at the time of deposition was about 55 x 10^-15 during the last 800,000 years. Decrease in this ratio with the time due to radioactive decay allows dating of the deposition time of the salts. Ages calculated by this method are concordant with those determined by other methods. Dating of the evaporites in these successive, hydrologically-connected basins will provide a detailed reconstruction of pluvial fluctuations during the mid to late Pleistocene.