HR: 1340h
AN: PP43A-0597    [Abstracts]
TI: A Modified Method for Saline Lake Calcite Isotope Analysis: Application to a Study of Climate Change over 200,000 Years in Death Valley, California.
AU: * Yang, W
EM: wenbo@berkeley.edu
AF: University of California, Dept. of Earth & Planetary Science, Berkeley, CA 94720-4767 United States
AU: Lowenstein, T K
EM: lowenst@binghamton.edu
AF: State University of New York, Dept. of Geological Sciences, Binghamton, NY 13901 United States
AU: Krouse, R H
EM: roy@earth.geo.ucalgary.ca
AF: The University of Calgary, Dept. of Physics & Astrnomy, Calgary, AB T2N 1N4 Canada
AU: Spencer, R J
EM: spencer@geo.ucalgary.ca
AF: The University of Calgary, Dept. of Geology & Geophysics, Calgary, AB T2N 1N4 Canada
AU: Ku, T
EM: rku@usc.edu
AF: University of Southern California, Dept. of Earth Sciences, Los Angeles, CA 90089 United States
AB: The standard method of oxygen and carbon isotope analyses for carbonate minerals was first reported by McCrea (1950). Carbonates are converted to CO2 by the reaction of carbonates with 100% phosphoric acid at temperatures between 25 and 95$\deg$C for C- and O-isotope analyses: 3CaCO3 + 2H3PO4 = 3CO2 + 3H2O + Ca3(PO4)2 The reaction time for this method can vary depending on different minerals and temperature. For example, at room temperature, the reaction time could be an hour or less for calcite and aragonite, three days for dolomite, two weeks for magnesite, and several months for siderite. This method is very reliable for almost every carbonate-dominated sample or even trace carbonates in silicate rocks. However, Death Valley saline core sediments showed that this standard method could be problematic for chloride-rich or soluble sulfate-rich carbonate samples because of the production of SO2 and/or HCl gas by partial reaction of the chloride or sulfate minerals with 100% H3PO4. The SO2 and HCl gases can affect the $\delta$-values significantly in two ways: (1) The contaminating gases may react with the CO2 in the mass spectrometer source region, isotopically fractionating the CO2 and/or generating background peaks in the CO2 + spectrum; and (2) The SO2 and HCl may react with interior parts of the mass spectrometer reducing its stability and/or sensitivity. In this study, we choose 85% H3PO4 to react with the lacustrine calcite at room temperature by off-line "Y" tube preparation for 2 to 3 minutes. This modification to the traditional method has resulted in negligible SO2 and HCl production. The CO2 gas generated from each bulk lacustrine sediment sample was manually introduced into a VG 609 mass spectrometer for C and O isotope analyses. The analytical precision is better than $\pm$0.2$\permil$ for both $\delta$13C and $\delta$18O. This modification of the method of McCrea (1950) was applied to determining carbon and oxygen isotopic compositions of lacustrine calcite in bulk saline lake sediments. For a continuous 200,000-year $\delta$18O record of lacustrine calcite from a 186-meter sediment core from Badwater Basin, Death Valley, California, a two-level climatic fluctuation model is suggested.. This record provides new insight to the debate on the timing and driving forces of late Quaternary paleoclimatic changes. Excursions in calcite $\delta$18O are similar to those of $\delta$18O in sulfate in the Death Valley core, as well as to those in marine carbonate (SPECMAP) and polar ice in the Summit ice core (GRIP), Greenland. The Death Valley record shows periodicities of 96000, 39000, 21000, 14000 and 8000 years. The longer-term (96000, 39000 & 21000 years) fluctuations match Milankovitch orbital forcing, and are thus likely to be global in origin; the shorter-term (14000 and 8000 years) fluctuations probably reflect regional climatic and/or hydrologic forcing.
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1094 Instruments and techniques
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting