Molecular characterization and photochemical transformation of dissolved organic matter (DOM) in both rivers and the ocean is the main research focus of this dissertation. Chemical characterization of DOM is hampered by the limited application of advanced techniques to desalt, concentrate, isolate and then molecularly characterize DOM. An affordable, commercially available mini-electrodialysis (mini-ED) system has been evaluated and recommended for the efficient desalting of small volume samples of seawater prior to analysis by electrospray Fourier transform ion cyclotron resonance mass spectrometry (ESI FTICR-MS).
A high-recovery technique of DOM isolation – reverse osmosis coupled with electrodialysis (RO/ED) – was used to isolate DOM from various major oceanic water masses, prior to ESI FTICR-MS analysis. RO/ED isolated DOM samples share a significant number of common molecular formulas, accounting for 54-79% of formulas in each sample. MS peaks enriched in surface samples have higher H/C values than peaks enriched in deep samples. This enrichment pattern is likely due to the selective photo-degradation of aromatic compounds and the bio-production of aliphatic and carbohydrate-like compounds in surface waters, and the selective bio-degradation of aliphatic and carbohydrate-like compounds with increasing depth. MS peaks enriched in the North Pacific intermediate and deep DOM have significantly higher 0/C values than the North Atlantic oxygen minimum layer and deep DOM. This suggests oxidation of DOM, possibly via microbial activity during the ageing of DOM or the preferential remineralization of DOM from sinking particles at depth in the Pacific.
Our studies show that terrestrial DOM exposed to simulated sunlight is altered to produce POM with a markedly different molecular composition enriched in newly-formed aliphatic and condensed aromatic molecules. This process is closely tied to the chemistry of iron, which primarily exists as dissolved Fe(II) and Fe(III)-organic complexes in initial DOM and photochemically matures to Fe(III) oxyhydroxides before co-precipitating out with POM. The newly formed condensed aromatic compounds resemble black carbon, which until now was thought to be produced only by combustion. These new molecules contribute a novel pool of Fe-rich, aliphatic and black carbon organic matter to sediments as the terrestrial DOM is transported through rivers
