HR: 0800h
AN: PP11A-0554    [Abstracts]
TI: Ratio of multi substituted isotopologues in natural carbonates, a novel tool to study palaeo temperature.
AU: * Ghosh, P
EM: pghosh@gps.caltech.edu
AF: Division of Geological Sciences, California Institute of Technology, Pasadena, CA 91125 United States
AU: Eiler, J
EM: eiler@gps.caltech.edu
AF: Division of Geological Sciences, California Institute of Technology, Pasadena, CA 91125 United States
AU: Adkins, J F
AF: Division of Geological Sciences, California Institute of Technology, Pasadena, CA 91125 United States
AU: Balta, B
AF: Division of Geological Sciences, California Institute of Technology, Pasadena, CA 91125 United States
AU: Schauble, E
AF: Division of Geological Sciences, California Institute of Technology, Pasadena, CA 91125 United States
AU: Wanamaker, A D
AF: Climate Change Institute, University of Maine, Orono, ME 04469-5790 United States
AU: Introne, C
AF: Climate Change Institute, University of Maine, Orono, ME 04469-5790 United States
AU: Kreutz, K
AF: Climate Change Institute, University of Maine, Orono, ME 04469-5790 United States
AB: It has recently been shown that gaseous molecules containing more than one rare stable isotope (e.g., 18O18O, 13C18O16O, etc.) can be precisely analyzed at their natural, ppm-level abundances. Application of such measurements to the geologic record will require development of analogous measurements of `clumped' rare stable isotopes in condensed phases. This study demonstrates that 13C-18O `clumps' can be measured in calcite and that their concentration reflects the temperature of carbonate growth (perhaps among other factors not yet recognized). Accompanying abstracts by Eiler et al. and Schauble et al. discuss relevant mass spectrometric methods and theoretical models of isotopic `clumping' in carbonate minerals. The concentration of 13C-18O bonds in carbonates has potential as a thermometer because they form as a result of an order/disorder reaction, such as: 40Ca13C16O3 + 40Ca12C18O16O2 = 40Ca13C18O16O2 + 40Ca12C16O3 and this reaction has a temperature-dependent equilibrium constant. The most important feature of this reaction is that it involves a homogeneous equilibrium (that is, a reaction among components of one phase, rather than between two or more phases), and therefore rigorously constrains temperature without knowing the 18O of waters from which carbonates precipitated. We analyzed the concentration of 13C18O16O in CO2 extracted from various calcite samples by phosphoric acid extraction at 25 to 50 ˚C (McCrea, 1950), with the aim of determining whether it is related to the concentration of 13C18O16O2= ionic groups in the reactant carbonate, which theoretical models presented by Schauble et al. (this volume) show should have distinctive and temperature-sensitive concentrations in calcite grown at thermodynamic equilibrium. In reporting these data, we define the variable ∆47 as the difference in permil between the measured value of R47sample (=[mass 47 isotopologues]/[mass 44 isotopologue] in the sample) and the value of R47 sample expected in that sample if its stable C and O isotopes are randomly distributed among all isotopologues- a case we refer to as the stochastic distribution. First, we analyzed a sample of MZ carbonate that had been re-crystallized at 1000 ˚C, 800˚C, 600˚C and 450˚C, yielding CO2 with ∆47 of 0.022 %, 0.1%, 0.32% and 0.45% respectively. Second, analyses of a variously purified Red Sea coral imply an uncontaminated ∆47 value of 0.598ñ0.002 (n=4); this coral grew at ~27.5 ˚C. We also analyzed a deep-sea coral, 85080 (which grew at ~4 ˚C), yielding a ∆47 value of 0.764. Finally, we analyzed a modern surface coral collected just west of Sumatra, yielding a ∆47 value of 0.637 (needing no correction after purification). The growth temperature of this coral is unknown but was likely 28-30 ˚C based on average sea surface temperatures from this area. Taken together, these data suggest that the abundance of 13C-18O bonds in carbonate has a temperature sensitivity broadly resembling, but stronger than, predicted from lattice vibration models. Potential usage of such isotopic discrimination to constrain the past global temperature history require calibration of the temperature scale with laboratory precipitated carbonates and natural carbonates cultured at a controlled temperature settings, and careful study of the effects of re-setting over geological timescales.
DE: 1040 Isotopic composition/chemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting