Development of Carbonate Clumped-Isotope Paleothermometry for Application to Paleozoic Fossils
Making precise measurements of clumped-isotopes in CO2 (reported as Δ47) is resource-intensive and time-consuming. Instrument and time-related variability of measured Δ47 values have hindered inter-laboratory calibration efforts. Additionally, clumped-isotopes measured in Paleozoic fossils suggest ancient marine temperatures and/or ocean isotopic compositions that are difficult to accept at face value.
This dissertation describes a strategy for measuring Δ47 on a conventional mass spectrometer, with the usual CO2 set of three Faraday collectors by "multicollector peak hopping" (MPH). The MPH method involves directing m/z = 46 and m/z = 47 ion beams into the Faraday cups used for m/z = 44 and m/z = 45, and then calculating Δ47 from sample δ13C, δ18O, and measured 47/46CO2+. This method includes a protocol for correcting ion-beam intensities for secondary electrons in order to remove Δ47 dependence on δ47CO2.
Measurements of Δ47 in CO2 liberated from modern bivalved mollusc shells demonstrate that electron scatter has a significant influence on Δ47 values. As this effect is similar in magnitude to the pairwise differences between carbonate temperature calibrations that were made using different instruments, it is possible that electron scatter is responsible for these discrepancies. The observed relationship between Δ47 and growth temperature preserved in modern mollusc shell is very close to theoretical predictions for Δ47 in calcite.
Measurements of Δ47 in CO2 liberated from shells of the Permian bivalved mollusc Eurydesma cordatum yield paleotemperatures that are too high to conform with geologic indicators for near-freezing water temperatures, yet too low to represent maximum burial temperatures. Cycles in δ18O that are in-phase with growth bands suggest that this fossil did not exchange oxygen isotopes with water after the precipitation of the carbonate, meaning that 13C-18O bonds were re-ordered at low temperature in the absence of significant amounts of fluid. It is suggested that original seawater was present in trace amount, enough to catalyze 13C-18O bond re-ordering, but without affecting δ18O of the shell. Such subtle alteration eludes detection with current screening methods. Unless the role of geologic processes in altering Δ47 is better understood, successful application of clumped-isotope paleothermometry to Paleozoic fossils may be limited.