Photochemical degradation of dissolved organic matter in arctic surface waters

[摘要] A critical component in the global carbon cycle is the fate of dissolved organic matter (DOM) drained from soil into inland waters. However, we currently lack the information needed to predict the conversion of DOM to carbon dioxide (CO₂) versus partially oxidized or degraded compounds that are exported to the ocean. In arctic freshwaters, complete and partial photo-oxidation dominate DOM processing in the water column. However, the relative importance of these sunlight-driven pathways is likely governed by the chemical composition of DOM, which is expected to change with increasing ecological disturbances in the Arctic (e.g., tundra fires and permafrost thaw). Here, the molecular level controls on the photo-degradation of DOM in arctic freshwaters were investigated. The dominant sink of dissolved black carbon, a byproduct of tundra fires incorporated into the DOM pool, was partial photo-oxidation to compounds that are likely exported to the ocean. DOM draining the shallow organic mat of arctic soils contained higher molecular weight, more oxidized, and more unsaturated aromatic species compared to DOM draining the deeper permafrost layer. Despite differences in chemical composition (i.e., degree of saturation and oxidation), organic mat and permafrost DOM exhibited similar susceptibilities to complete photo-oxidation. Photo-decarboxylation of tannin-like compounds was likely an important mineralization pathway of organic mat and permafrost DOM, suggesting that these compounds may control the photo-mineralization of DOM in arctic surface waters. Permafrost DOM was significantly more susceptible to partial photo-oxidation compared to DOM leached from the organic mat, potentially due to the depletion of phenolic moieties with antioxidant properties in permafrost DOM that may quench the oxidation of DOM by sunlight. Thus, increased permafrost thaw and increased inputs of permafrost DOM to arctic surface waters may increase the rates of photochemical DOM processing of DOM and shift the dominant DOM degradation pathway from complete to partial photo-oxidation. Measurements of apparent quantum yield spectra for the photo-mineralization of a wide range of DOM sources indicated that yields were largely controlled by the initial DOM composition. This dissertation has demonstrated that the chemical composition of DOM strongly controls its susceptibility to photochemical degradation in arctic surface waters.

[发布机构] University of Michigan

[效力级别] Arctic

[关键词] Aquatic carbon cycling;Arctic;Aquatic photochemistry;Dissolved organic matter;Permafrost;Surface waters;Geology and Earth Sciences;Science;Earth and Environmental Sciences