Date of Award
Doctor of Philosophy (PhD)
Chemistry and Biochemistry
This dissertation explores processes affecting the composition of dissolved organic matter (DOM) and how DOM composition changes in sunlit surface waters and in the dark interior ocean. Simulated solar irradiations were used to investigate the impact of photochemistry on terrestrial waters and deep ocean DOM. The photochemically mediated processes observed in Dismal Swamp samples included (i) light induced flocculation of up to 12% of the organic matter and 84% of the dissolved iron originally present; (ii) 74-88% mineralization of dissolved organic carbon (DOC) and 95-99% bleaching of chromophoric DOM (CDOM) during 110 days of irradiation; and (iii) nearly complete loss of the biochemical markers for terrestrial DOM: lignin phenols, CDOM absorption and fluorescence, and aromaticity determined by nuclear magnetic resonance (NMR) spectroscopy. Extensively photo-degraded terrestrial DOM exhibited spectroscopic signatures similar to DOM isolated from ocean water (except that it lacked protein-like fluorescence and appeared to contain excess carboxyl carbon), and photo-degraded deep ocean DOM exhibited optical properties similar to surface ocean DOM.
The heretofore-unexamined DOM removal process of light induced flocculation was further investigated using solid-state 13C NMR and infrared spectroscopy.
Photochemical decarboxylation and production of alkyl functionality drives the initial phase of photochemical flocculation, while adsorption to iron flocculates is important during later phases of the process. Carboxyl amides appeared to resist mineralization, but were susceptible to photochemical flocculation. A fraction of the photodegraded DOM is more susceptible to mineral adsorption, which may be an important pathway for DOM export from surface waters to the sediments and subsequent preservation.
Advanced solid-state 13C NMR characterization of DOM isolated by reverse osmosis — electrodialysis (RO/ED) from marine environments with varying biogeochemistries revealed new insights into the biodegradation of carbohydrates as well as preservation of carboxyl groups and condensed aromatic structures in the ocean's interior. Quaternary anomeric carbons were identified as a potentially important structural component of the poorly characterized pool of bio-refractory carbohydrates.
The present biogeochemical paradigm for ocean DOC cycling, the "three-pool" model, is re-examined along with the "three-pool photoreactivity" classification system. A new conceptual model is proposed, which incorporates both biological and photochemical reactivity of dissolved organic matter.
Helms, John R..
"Spectroscopic Characterization of Dissolved Organic Matter: Insights into Composition, Photochemical Transformation and Carbon Cycling"
(2012). Doctor of Philosophy (PhD), Dissertation, Chemistry and Biochemistry, Old Dominion University, DOI: 10.25777/1y3p-e444