Abstract
As one of the Earth’s largest active carbon pools, accounting for around 90% of the organic carbon in the oceans, dissolved organic matter (DOM) plays a fundamental role in carbon storage and other biogeochemical processes. It exists as a highly functionalized and complex mixture of organic compounds which are diverse in their source, reactivity, and history, with about 95% of aquatic DOM mixture remaining un-identified on a molecular level. This thesis uses ultra-high resolution mass spectrometry to characterize the origin, composition, and lability of DOM in the aquatic system. In this thesis, water and sediment samples were collected from sampling sites in the Arctic Ocean, as well as the northern and southern slope of the Gulf of Mexico, to discern the compositional differences in DOM that occur geographically. The DOM of waters sampled across the different ecosystems show a homogenized composition with little variability in their compound class distribution, consisting primarily of NOx, N2Ox, N3Ox, and Ox classes, with a mass range between m/z 170–960. Along the water column, surface water DOM samples show the most variation in their relative intensity and abundance of multi-oxygenated species, owing to their susceptibility to photo-oxidation, in contrast to the more refractory and homogenized DOM in the bathypelagic ocean. Unlike the homogenized water DOM composition near the ocean floor, the water extractable organics from sediments underneath (top 15 cm of sediment) show significant variability in both the relative intensity and abundance of compound classes, geographically. Juxtaposed to the oxygen rich water DOM, the sediment water extractable organic matter (WEOM) are enriched in nitrogen containing species (N1-6O1-17) with smaller carbon number values. The changes in DOM from the water surface towards the sediment are attributed to the differences in solar irradiation exposure, availability of oxygen, and resident microbes. The nitrogen containing classes in sediment WEOM show compositional trends similar to melanoidins, suggesting the preservation of organics via melanoidin or melanoidin-like compounds in the sediments.
Beyond the primary productivity from phyto- and zoo-plankton, marine DOM is fed by terrigenous input (atmospheric deposition or runoff) and anthropogenic sources (oil spills), which were both independently characterized. The WEOM from plant biomass (terrigenous input) comprise of species with heteroatom distributions of NOx, N2Ox, and Ox, with a high concentration of multi-oxygenated species, similar to that observed in the sampled marine water DOM. The WEOM from thermally oxidizing terrigenous plant biomass produces homogenized spectra, with an increased abundance of highly unsaturated moieties of decreasing bio-availability and lability, that closely resemble marine water DOM, suggesting that pyrogenesis yields a more refractory DOM whose characteristics are not greatly variable. The anthropogenic input from oil spills comprise of species with heteroatom distributions of NOx, Ox, OxS, and OxS2, adding a high concentration of sulfur containing compounds to the water phase. The partitioning behavior of organics from oil to water phase during a submarine oil spill were found to depend greatly on the dissolved gas pressure. The dissolved gas pressure in the oil gushing from the wellhead, causes higher concentrations of organics to partition into the water phase as the plume approaches the water surface. The increased water temperature closer to surface further enhances this effect. The study showed that the addition of dispersants to the water phase, a commonly used spill response, increase the extent of organics partitioning from the oil into the water phase. The extent of partitioning was also found to increase twofold with the advent of in situ burning at the water surface, introducing highly condensed and oxidized aromatics into the water phase. It remains a herculean task to identify DOM at the molecular level in the various aquatic systems worldwide. This thesis has taken steps to identify both commonalities and variabilities in the DOM pool at various locations and depths, and increased the understanding of how various inputs are incorporated into this ever-evolving DOM make-up. This new information will help provide some of the pieces of one of the greatest remaining puzzles on earth, our oceans.
