Black Carbon: Sources, Mobility and Fate in Freshwater Systems
- Publication Year:
- black carbon; dissolved organic matter; fourier transform ion cyclotron resonance mass spectrometry; molecular characterization; benzenepolycarboxylic acid analysis; size distribution; land use; wildfire; Biogeochemistry; Environmental Chemistry
thesis / dissertation description
Black carbon (BC) is a complex mixture of polycondensed aromatic compounds produced by the incomplete combustion of biomass during events such as wildfires and the burning of fossil fuels. Black carbon was initially considered to be a refractory form of organic matter. However, recent studies have shown that BC can be quite mobile and reactive in the terrestrial environment. Black carbon can be translocated from soils and sediments in the form of dissolved BC (DBC). A global correlation between DBC and bulk dissolved organic carbon (DOC) has been established for fluvial systems where DBC comprises approximately 10% of the total DOC pool, which suggests that DBC may be a significant contributor to the global carbon cycle. The primary objective of this thesis was to further characterize DBC and elucidate some of the specific physical and chemical processes that promote its transfer to the aqueous phase and drive the DBC-DOC relationship. The molecular composition and qualitative distribution of DBC was assessed using Fourier transform ion cyclotron resonance mass spectrometry. Black carbon in both dissolved and particulate (PBC) phases was quantified by the benzenepolycarboxylic acid method. Dissolved BC was found to contain considerable amounts of nitrogen and the export of this dissolved black nitrogen was linked to watershed land use in global rivers. The riverine flux of PBC, a previously unstudied BC removal mechanism, was significantly increased by local wildfire activity. However in-stream DBC did not appear to be affected by short-term fire events. Once translocated to surface waters, DBC is susceptible to photodegradative processes. Dissolved BC in high molecular weight DOC fractions was more photoreactive than DBC associated with lower molecular weight fractions. In the coming decades, wildfire frequency is expected to increase with climate change and natural lands will continue to be altered for anthropogenic use. These processes have already been shown to significantly impact the composition of DOC and associated DBC exported to inland waters. The quality of DBC influences its stability in soil and resistance to degradation. Therefore, it is essential that we aim to fully understand DBC dynamics in natural systems in order to assess its contribution to global carbon cycling.