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Calcium carbonate has three polymorphs and calcite is the most abundant, followed by aragonite and vaterite. Measurement of the isotopic compositions of carbonate minerals have been used to obtain information about ocean temperatures for more than 50 years.  A new type of carbonate clumped isotope paleothermometer was proposed by a group of scientists at Caltech about five years ago.  This carbonate ‘clumped’ isotope thermometer requires only the carbonate mineral to estimate its formation temperature, in contrast to the conventional carbonate-water paleothermometer which requires the oxygen isotope compositions of both the carbonate and parent water.  They used inorganic calcite for the calibration of the new paleothermometer. 

The goal of our ACS-PRF funded research is to provide scientists with a high-quality calibration of an inorganic aragonite clumped isotope paleothermometer.  The new clumped isotope paleothermometer based on the abundance of the doubly-substituted CO₃            isotopologue,

¹³C¹⁸O¹⁶O₂ in the aragonite mineral lattice.  It provides a calibration that can be used in situations where inorganic aragonite rather than calcite forms.  It also will allow for determination of the formation temperature of the carbonate.  With the temperature constrained, the isotopic composition of oxygen can then be used to constrain the isotopic composition of past ocean water which in turn can be used to place constraints on the mass of water in the oceans compared to the mass of water in ice sheets. 

We  prepared over 30 aragonite samples under well-controlled laboratory conditions between 2 and 70 ºC for the proposed study.  Dr. Sang-Tae Kim visited the stable isotope laboratory at Caltech and determined the abundances of the clumped CO₂ (¹³C¹⁸O¹⁶O) liberated from the acidification of synthetic aragonite samples.  The proportion of ¹³C-¹⁸O bonds in aragonite was estimated quantitatively from the ¹³C¹⁸O¹⁶O abundance of the acid-liberated CO₂ gas from the aragonite.  The results of these experiments indicate a temperature dependence of ¹³C-¹⁸O clumping in synthetic aragonite that agrees with that of Ghosh et al. [1] at 25 °C, but exhibits lower temperature sensitivity and departs significantly from that calibration at higher and lower

temperatures. The results of the proposed research were presented at the Goldschmidt conference held in Knoxville, U.S.A. in July 2010 and are being prepared as a manuscript for peer review.  The outcome of the research is expected to provide a powerful geochemical tool for climate reconstruction of value to the paleoclimate research communities.

Work to follow up on this work is underway by Sang-Tae Kim and Weifu Guo to calibrate the level of ¹³C-¹⁸O bonds in dissolved inorganic carbon as a way to constrain information about the transfer of the ¹³C-¹⁸O signals from dissolved inorganic carbon to aragonite and calcite during the mineral precipitation process.

References Cited:

 Ghosh et al. (2006) GCA 70, 1439–1456. [2] Schauble et al. (2006) Geochimica et Cosmochimica Acta  70, 2510–2529. [3] Kim et al. (2007) GCA 71, 4704–4715.