UCLA

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.