Carbonate clumped isotope geochemistry of marine mollusk and brachiopod shells and its application to deep-time paleoclimatology
Henkes, Gregory Arthur
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The main objective of this dissertation research was to study the carbonate clumped isotope compositions of marine mollusk and brachiopod shells, both modern and ancient, and to investigate critical problems with the preservation of isotope clumping in shell carbonate over the Phanerozoic Eon. This was accomplished through a series of empirical and laboratory studies detailed in each of the three chapters of this dissertation. This approach was aided by the knowledge gained from decades of previous research on the bulk carbon and oxygen isotope compositions of marine shells, but was novel in its application of modern methods for measuring carbonate clumped isotopes. Unlike conventional carbonate-water oxygen isotope thermometry, carbonate clumped isotope thermometry is independent of the isotopic composition of the precipitating fluid, which for shells growing in ancient seawater is largely unknown and the long-time subject of debate. Therefore, clumped isotopes, combined with bulk isotopic measurements of ancient shells, can provide an independent estimate of paleotemperature and the isotopic composition of paleoseawater. The first chapter of this dissertation describes a comprehensive calibration of the carbonate clumped isotope thermometer using modern mollusk and brachiopod shells collected worldwide from waters of known temperature and isotopic composition. The results are in accord with the temperature dependence of the thermometer predicted from theory, but are different from earlier empirical calibration attempts by other laboratories. The second chapter explores the phenomenon of carbonate clumped isotope reordering—that is closed-system alteration of 13C-18O ‘clumped’ bonds—in Paleozoic brachiopod shells, finding that sedimentary burial temperatures above 100°C can alter primary clumped isotope compositions over geologic timescales. The third chapter presents the emerging Phanerozoic clumped isotope record of seawater temperature and δ18O from well-preserved marine mollusk and brachiopod fossils, and evaluates it with respect to clumped isotope bond reordering described in chapter two. Altogether, this research lays the groundwork for continued study of the clumped isotope geochemistry of marine shells, with future studies expected to shed insight into temperature calibration discrepancies between materials and laboratories, the limits and mechanism(s) of 13C-18O bond reordering, and paleoclimatology of Paleozoic and Mesozoic worlds.