| Summary: | The "clumped-isotope" geochemistry of CO2 (tracking enrichment of 16O13C18O, reported as Delta47 values, on CO2 derived from carbonate minerals or the atmosphere) are becoming central to a wide range of geochemical investigations. Previously published analytical methods suffer from instrumental drifts, we have determined that the source of most of the drift results from changes over time in the shift in the baseline signal on sensitive collectors (m/z 47, 48, and 49). We introduced a method for monitoring the pressure baseline and integrating a routine correction to ion beam intensity measurements during every analysis. This correction dramatically improves long-term precision of Delta47 measurements to approach instrumental analytical uncertainty.
Owing to the fact that it is challenging to measure much smaller ion beam associated with multiply substituted isotopologues of CO2 (e.g., 18O13C16O, 12C18O2), typically 7-12 mg of carbonate (or the equivalent amount of CO2 gas) is required for one measurement. As a result, mixing introduced during sampling is ubiquitous among nearly all applications. We present a quantitative mass conservation approach to examine the effect of mixing.
We conducted CO2-H2O experiments to investigate the temperature dependence of the reaction exchange rates. In addition, by evaluating the reaction at controllable surface area, our experiments achieved partial separation of the two processes (gas exchange and chemical reactions in solution). We have observed that delta values (delta 18O, delta45, delta46, and delta 47) approach equilibrium states monotonically at three temperatures (4ºC, 30ºC, and 60ºC). Owing to its self-referenced nature, the time evolution of Delta47 does not follow a traditional, Arrhenius-type kinetic function. Instead, large Delta47 departures were observed from a smooth transition between initial and final equilibrium states. We developed a computational framework to examine the time evolution of Delta 47 values, and the results have relevance to understanding kinetic isotope effects related to speleothem formation and vital effects.
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