James Farquhar , University of Maryland (College Park)
One goal of our ACS-PRF funded research is to provide scientists with a high-quality calibration of an inorganic aragonite clumped isotope paleothermometer. Another goal has been to study the role of clumped isotopes in dissolved inorganic carbon. A third, more recent goal, is to calibrate isotope effects associated with the aqueous phase oxidation of S(IV). Each of these will be discussed below.
The clumped isotope thermometer is based on the abundance of the doubly-substituted CO3 isotopologue, 13C18O16O2 in carbonate mineral latices. The abundance of this moiety is determined by the isotopic abundances and partitioning of the carbon isotopes and oxygen isotopes between sites in the mineral latice. It has been recognized that it is possible to determine the abundance of the associated 13C18O in carbonate which made it possible to develop an isotopic thermometer that was independent of the isotopic composition of other phases in the system (e.g., water) and that was not subject to the same vital effects (effects introduced by biological processes) that limit the accuracy of temperatures determined using more traditional isotopic techniques. This work has been largely pioneered at Caltech by John Eiler and people working with him. The work that we have been conducting has been led by Sang-Tae Kim and Weifu Guo (in collaboration with John Eiler) and has focused on experiments with the carbonate mineral aragonite, and with dissolved inorganic carbon.
In the first year of the grant Sang-Tae Kim and Weifu Guo 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 previously published data 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.
In the second year of the grant, Weifu Guo, working with Irene Kadel-Harder, an undergraduate assistant, developed techniques to calibrate the level of 13C-18O bonds in dissolved inorganic carbon. In this work, sodium bicarbonate-carbonate solutions of different pH values (pH=~8.3, ~10.1 and ~10.7) were prepared with a total DIC concentration of 5mmol, and equilibrated at 5, 25, 40, 50 and 70°C (± 0.01°C). The DIC species inside each solution were then precipitated as barium carbonate by injecting excess amount of BaCl2 and NaOH solutions. Series of tests have been conducted to assess the best protocol to achieve quantitative 100% yield of precipitates. Two sets of precipitate samples derived from 25°C and 50°C precipitation experiments respectively, both with yields >95%, were analyzed for their clumped isotope compositions at Dr. Ben Passey's clumped isotope laboratory (JHU) during Weifu Guo's two visits there in March and November 2011. Based on clumped isotope analyses of these precipitates, the relative abundances 13C-18O bonds were determined for in the dissolved carbonate and bicarbonate ions. In contrast to the large difference in their equilibrium oxygen isotope compositions, the equilibrium clumped isotope compositions of dissolved carbonate and bicarbonate ions are very similar. These experimentally determined values are consistent with estimations made using ab initio quantum mechanical calculations of the equilibrium clumped isotope fractionations among different DIC species, and could potentially explain the general absence of clumped isotope vital effects in most biogenic carbonates. An abstract describing this work that is led by Weifu Guo has been submitted to the 2012 Goldschmidt Conference in Montreal, Quebec Canada.
The third task supported by this grant is an experimental investigation of another type of aqueous phase isotope effect, one associated with the aqueous phase oxidation of dissolved S(IV) (sulfite and bisulfite). To date work has been undertaken to develop an experimental protocol for studying this reaction. This has involved developing techniques for introducing reactants, running controlled reactions, and then later separating the products from the reactions in pH controlled solutions. Much of this work has been developed by Daniel Eldridge (a Ph.D. student) with early help of Marc Peters. The first experiments undertaken by Daniel Eldridge indicate that the isotope effect associated with oxidation of sulfite is markedly different than for bisulfite solutions. Ab initio calculations suggest that this is not due to an equilibrium isotope effect convolved with an effect related to speciation of S(IV) species in solution. An abstract describing this work authored by Daniel Eldridge has been submitted to the 2012 Goldschmidt Conference in Montreal, Quebec Canada. Work is planned to characterize the speciation of different bisulfite isomers (H bound to S compared to H bound to O) that includes using both ab initio methods and NMR. Further experimental work is planned to continue calibrations of these reactions.