Clumped isotope thermometry applied to bivalve mollusks
Final Report Abstract
We have successfully installed the technique for measuring the clumped isotopic composition of CO2, carbonates and bioapatites in our laboratory at Goethe University. Detailed methodological investigations imply that carbonates should be digested at 90°C using a common acid bath. At lower temperatures, partial re-equilibration of the evolved CO2 or reaction intermediates with free molecules of water may occur inside the phosphoric acid. Time-consuming analyses of heated gases that are necessary to correct for a mass spectrometric bias caused by electrons can be circumvented if the slit width of the m/z = 44 collector is wider than that of the m/z = 47 Faraday cup. We have empirically determined the temperature dependence of Δ47 for calcites. Our calibration is identical with another empirical and theoretical calibration. The consistency of these calibrations strongly implies that i) kinetic (vital) effects might be of minor importance only in controlling the extent of 13C-18O clumping inside the investigated biogenic calcites and ii) consistent data can be generated by different laboratories if samples are reacted at 90°C and data is directly reported in the absolute reference frame. Further investigations reveal that aragonite seems to exhibit a temperature sensitivity of Δ47 that is very much comparable to that of calcite. However, more analyses on aragonite are necessary to assert with more confidence. Clumped isotope analysis of Silurian brachiopods from Gotland, Sweden, reveals that the clumped isotopic composition of the shells was partly reset by recrystallization. Recrystallization most likely occurred at very low water-rock ratios, thereby not affecting the bulk oxygen isotopic composition of the shells. In this respect, oxygen within the Brachiopod shells might still record the conditions of pristine crystallization.
Publications
- (2013) Background effects on Faraday collectors in gas-source mass spectrometry and implications for clumped isotope measurements. Rapid Commun. Mass Spectrom. 27, 603- 612
Bernasconi, S.M., Hu, B., Wacker, U., Fiebig, J., Breitenbach, S.F.M., and Rutz, T.
(See online at https://doi.org/10.1002/rcm.6490) - (2013) Clumped isotope analysis of carbonates: comparison of two different acid digestion techniques. Rapid Commun. Mass Spectrom. 27, 1631-1642
Wacker, U., Fiebig, J., and Schöne, B.R.
(See online at https://doi.org/10.1002/rcm.6609) - (2014) Clumped isotope analysis of carbonates: analytical aspects, calibration and application to Silurian brachiopod shells and diagenetic phases from Gotland/Sweden. PhD thesis, Goethe-Universität Frankfurt am Main, 194pp.
Wacker, U.
- (2014) Empirical calibration of the clumped isotope paleothermometer using calcites of various origins. Geochim. Cosmochim. Acta 141, 127-144
Wacker, U., Fiebig, J., Tödter, J., Schöne, B.R., Bahr, A., Friedrich, O., Tütken, T., Gischler, E., and Joachimski, M.M.
(See online at https://doi.org/10.1016/j.gca.2014.06.004)
