Graduate Thesis Or Dissertation

Hydrogen isotopic composition of C₃₇ alkenones and the ecology and physiology of coccolithophores

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  • A three-part study was conducted into the impact of physiological and ecological variables on the net isotopic fractionation of hydrogen, α[subscript K₃₇], expressed in C₃₇ alkenones. First, alkenone-producer production, abundance, and export were characterized in the summertime Gulf of California and Eastern Tropical North Pacific using compound-specific, labeled in situ incubations. It was found that these organisms routinely exhibited maximum production rates at depths associated with subsurface chlorophyll features and the nitracline, as opposed to the N-depleted surface, and that the relative contribution of coccolithophore productivity to overall productivity was decoupled from nutrient conditions. Our observations suggest that, in sufficiently well-stratified settings, a coccolithophore-favorable 'mid-to-low nutrient' niche may be absent. Second, results from a culture study were compared to samples from the initial field campaign in order to more firmly establish the physiological controls on the δD of alkenones. Nutrient-limitation experiments in culture, combined with previously published data, show that net fractionation between the growth medium and alkenones (α[subscript K₃₇]) increases rapidly with increasing cellular alkenone content and production rate, and, by extension, growth phase. To explain these results, a mechanism is proposed in which changing NADPH sources result in isotopically-depleted lipids at both high growth rates and in stationary phase growth. Comparison of these results to the field samples suggests that, in the water column, this dynamic (a relationship α[subscript K₃₇] and growth rate) drives correlations between α[subscript K₃₇] and both cell abundance and the carbon-fractionation term εp. Lastly, the chemical and isotopic composition of alkenones was measured in sediment samples along a transect of the North-American Pacific margin from ~42°N to the tip of Baja California. It was found that both core-top and LGM intervals expressed a strong relationship between the temperature proxy U₃₇[superscript K'] and estimated α[subscript K₃₇]. If covariation between U₃₇[superscript K'] and estimated α[subscript K₃₇] is robust in settings such as these, then paired analysis of U₃₇[superscript K'] and alkenone δD may enable paleoceanographic estimates water δD. In light of the results of the second study, it is proposed that this relationship derives from a control of temperature on cellular division rate on sedimentary time scales. Deviations from this relationship were tentatively interpreted as indicative of export from locations where alkenone producers are/are not subjected to nutrient deprivation before sedimentation. Overall, the results of this three-part study suggest A) that coccolithophore-derived inorganic carbon export may serve as a weaker positive feedback on atmospheric CO₂, in the future, than previously suggested; B) that α[subscript K₃₇] may lend valuable context to studies of alkenone-producer ecology, and C) that alkenone δD may prove a better than expected hydrologic proxy in marine settings and/or a coccolithophore-specific growth rate proxy.
  • Keywords: deuterium, Baja, coccolithophore, LGM, lipids, fractionation, stratification, alkenone
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