Probing the photochemical reactivity of oceanic dissolved organic carbon
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After 20 years of research in aquatic photochemistry, there remains a need to identify and constrain many photochemical reactions involving colored dissolved organic matter (CDOM). This thesis uses laboratory and field studies in diverse marine systems to examine the photochemical efficiency and reaction rates for carbon monoxide (CO), carbon dioxide (CO2), superoxide (O2-) and hydrogen peroxide (H2O2), all important photoproducts, and improves quantitative understanding of photochemistry’s significance in biogeochemical cycles. Input of these results to ocean color-based models allowed estimation of photochemical rates on regional and global scales. Direct photo-oxidation of dissolved organic carbon (DOC) to CO2 and CO is a significant, albeit poorly constrained, DOC removal mechanism. Working in the Northern Gulf of Mexico, we amassed the largest cohesive CO apparent quantum yield (AQY) data set for any marine region (n=99; 18 paired with CO2), defining distinct inshore and offshore AQY spectra for improved regional photochemical rate models. Analytical limitations for determining CO2 AQY spectra forced the use of ill-defined coastal CO2:CO ratios (~6 − 66) for blue water CO2 calculations, highlighting the need for new direct methods or better proxies to quantify CO2 photochemistry in marine waters. Significant photochemical removal of biologically refractory DOC (RDOC), well-mixed in the ocean but isolated at depth, is not compatible with its 14C age. Reevaluation of RDOC photochemical reactivity using paired O2- and H2O2 photoproduction studies for abyssal Gulf of Alaska samples showed declining O2- steady-state concentrations during long-term exposure, with little or no H2O2 accumulation past ~6 − 12 hours. This is consistent with a loss of O2- source, a shift to oxidative pathways for O2 decay, and a lack of long-term photochemical reactivity for RDOC. Because H2O2 formation is a thermal reaction involving O¬2-, blending remotely sensed sea surface temperature and color allowed correction of H2O2 AQY spectra and production of the first global H¬2O2 and O2 photoproduction rate maps from remotely sensed data. Further analysis of paired H2O2 and CO2 photoproduction experiments indicated that H2O2 is a far better proxy for CO2 photoproduction than CO (CO2:H2O2 ratio = 6.89 ± 1.64), and will lead to constrained estimates for global CO2 photochemical fluxes.