I. Rhenium and Iridium in Natural Waters. II. Methyl Bromide: Ocean Sources, Ocean Sinks, and Climate Sensitivity. III. CO₂ Stability and Heterogeneous Chemistry in the Atmosphere of Mars

Citation

Anbar, Ariel David (1996) I. Rhenium and Iridium in Natural Waters. II. Methyl Bromide: Ocean Sources, Ocean Sinks, and Climate Sensitivity. III. CO₂ Stability and Heterogeneous Chemistry in the Atmosphere of Mars. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/rrjq-k179. https://resolver.caltech.edu/CaltechTHESIS:01202022-231109666

Abstract

Part I: Rhenium and iridium were measured in natural waters by isotope dilution and negative thermal ionization mass spectrometry, following clean chemical separation from 200 mL (Re) and 4 L (Ir) samples. In the Pacific Ocean, Re is well-mixed in the water column, confirming predictions of conservative behavior. The Re concentration is 7.42 ± 0.04 ng kg⁻¹. The concentration of Ir in the oceans is fairly uniform with depth and location, ranging from 2.9 to 5.7 x 10⁸ atoms kg⁻¹. Pristine river water contains ≈ 20 x 10⁸ atoms kg⁻¹ while polluted rivers have 50 - 100 x 10⁸ atoms kg⁻¹. Concentrations in the Baltic Sea are much lower than expected from conservative estuarine mixing, indicating rapid removal of ≈75% of riverine Ir. Under oxidizing conditions, Ir is scavenged by Fe-Mn oxyhydroxides. Ir is enriched in anoxic waters relative to overlying oxic waters, indicating that anoxic sediments are not a major Ir sink. The residence time of dissolved Ir in the oceans is 10³ - 10⁴ years, based on these and other observations. The amount of Ir in Ktr boundary sediments is ≈10³ times the total quantity in the oceans.

Part II: The biogeochemistry of methyl bromide (CH₃Br) in the oceans was studied using a steady-state mass-balance model. CH₃Br concentrations are sensitive to temperature and the rate of CH₃Br production. Model production rates correlate strongly with chlorophyll concentrations, indicating CH₃Br biogenesis. This correlation explains discrepancies between two observational studies, and supports suggestions that the ocean is a net sink for atmospheric CH₃Br. The Southern Ocean may be a CH₃Br source.

Part III: High resolution, temperature-dependent CO₂ cross sections were incorporated into a 1-D photochemical model of the Martian atmosphere. The calculated CO₂ photodissociation rate decreased by as much as 33% at some altitudes, and the photodissociation rates of H₂O and O₂ increased by as much as 950% and 80%, respectively. These results minimize or even reverse the sense of the CO₂ chemical stability problem due to increased production of HOₓ species which catalyze CO oxidation. The effect of heterogeneous chemistry on the abundance and distribution of HOₓ was assessed using observations of dust and ice aerosols and laboratory adsorption data. Adsorption of HO₂ can deplete OH in the lower atmosphere enough to significantly reduce the CO/CO₂ ratio.

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