Combustion-Derived Dissolved Organic Matter in Aquatic and Marine Environments

Fire-derived compounds have received considerable attention as a refractory form of dissolved organic carbon (DOC), the largest carbon pool in the ocean. Due to its recalcitrant nature, pyrogenic or black carbon, which is produced by the incomplete combustion of biomass and fossil fuel on land, is an important compound for potential long-term carbon sequestration. The major objective of this dissertation was to test the hypothesis that dissolved pyrogenic organic carbon accounts for a significant fraction of DOC in different environmental systems, and that the distribution and transport of pyrogenic DOC may be an important key to understanding of how terrestrial and marine DOC are linked. To test this hypothesis, solid phase extraction of DOC was coupled with the benzene polycarboxylic acid (BPCA) method for accurate analysis of combustion-derived compounds in the Southern Ocean, rivers and estuaries in southeastern Brazil, Minnesota's peatlands, and the groundwater and coastal ocean of the Gulf of Mexico. The homogenous distribution of the thermogenic signatures including pyrogenic and non-pyrogenic sources found in marine DOC across whole water masses in the deep ocean indicated that thermogenic DOC can act as a long-term sink. Approximately 2% of the DOM in the deep ocean was determined to be of thermogenic origin, derived from ancient biomass burning, and, possibly, the geothermal flux in the deep sea. Dissolved pyrogenic carbon was found to account for up to 9 ± 2% of riverine and estuarine DOC. Pyrogenic DOC entering the watersheds in Brazil appeared to be derived mainly from former forest fires rather than current agricultural uses, in particular sugarcane burning. Dissolved pyrogenic carbon flux was affected by seasonal variability in runoff and water management in reservoirs. Inputs of pyrogenic DOC to the ocean via groundwater were identified, revealing groundwater discharge as a newly-discovered source of marine dissolved pyrogenic carbon. Large amounts of pyrogenic DOC were found in the peatlands of northern Minnesota. Such fire-derived materials are likely derived from old peat soil, thus, the export of aged condensed aromatic compounds from peatlands has implications for our understanding of the oceanic carbon cycle. Results of lignin phenols analysis indicated that the pyrogenic DOC in the peatlands might have originated from non-vascular plant-derived materials with highly altered lignin signatures. A multi-proxy approach, including both molecular tracers (BPCA and lignin oxidation products) and the stable carbon isotopic composition of bulk DOC, was adopted to investigate the sources and cycling of terrestrial and pyrogenic organic matter in a fire-impacted coastal watershed in the northern Gulf of Mexico. The distinct trends in the temporal and spatial variations of pyrogenic DOC in groundwater reflected the coupling of groundwater discharge and estuarine processes in creating the conditions for the transport of terrestrial DOC to the ocean. The calculated pyrogenic DOC flux transport by groundwater was similar to the flux discharged by the Apalachicola River in the Gulf of Mexico. The results of this study demonstrated that the loading of terrestrial and pyrogenic DOC from groundwater-fed estuaries in the ocean could have significance on a regional scale.