Congratulations to Collin Ward who successfully defended his dissertation on April 22, 2015.
Advisor: Rose Cory
Abstract: In northern latitudes, carbon fluxes from freshwaters may account for up to 40% of the net land surface exchange of carbon dioxide (CO₂) with the atmosphere. These large fluxes of CO₂ to the atmosphere are supported by the movement of dissolved organic matter (DOM) from land and its subsequent complete oxidation to CO₂ in freshwaters. The remaining DOM may be unprocessed or partially degraded and exported in rivers to oceans. Photochemical processes dominate the degradation of DOM in arctic freshwaters, but the molecular controls are too poorly characterized to predict the conversion of DOM to CO₂ vs. partially oxidized or degraded compounds that are exported to the ocean. To address this knowledge gap, I investigated the effect of the chemical composition of DOM on its photochemical conversion to CO₂ (photo-mineralization) vs. partially oxidized or degraded compounds (partial photo-oxidation). First, I studied the photo-degradation of dissolved black carbon, an aromatic-rich fraction of DOM produced by tundra fires. The expectation was that sunlight degrades black carbon to CO₂, but I found that most black carbon was partially oxidized or degraded, and thus likely would be exported to the ocean before being completely mineralized to CO₂. Second, I studied the chemical composition and photo-degradation of DOM leached from the shallow organic mat and deeper permafrost layer of soils in the Alaskan Arctic. Despite large differences in chemical composition between organic mat and permafrost DOM (e.g., degree of saturation and oxidation), there was no significant difference in the photo-mineralization of organic mat vs. permafrost DOM. Molecular evidence suggested that in both organic mat and permafrost DOM, acidic functional groups (i.e., carboxyl C) were preferentially degraded to CO₂ by sunlight, and that the degradation of carboxyl moieties could account for up to 90% of the CO₂produced as DOM is completely oxidized by sunlight. This finding suggested that carboxyl C content may be an important control on the photo-mineralization of DOM in arctic freshwaters, now and in the future as permafrost soils continue to thaw. Permafrost DOM was significantly more susceptible to partial photo-oxidation compared to organic mat DOM, possibly because it was depleted in phenolic C, a class of aromatics that may inhibit partial photo-oxidation by quenching oxidants during exposure to sunlight. Together, these results suggested that increased tundra fires and permafrost thaw in a warming Arctic may not change the magnitude of CO₂ produced by the photo-mineralization of DOM, but may increase the partial oxidation and degradation of DOM by sunlight as it is transported through rivers and lakes to the Arctic Ocean.