Black Carbon Dynamics in Tropical River Catchments and Implications for its Global Cycle

Black carbon (BC) is a major constituent of the charcoal and soot (aerosol) produced by biomass and fossil fuel combustion and is distinguished from other naturally occurring forms of organic carbon (OC) by its aromatic chemical structure. The aromaticity of BC makes it inherently stable with implications for its storage and dynamics in Earth’s terrestrial and aquatic environments. The residence time of BC in soils is on the order of centuries, while in oceanic pools it is thought to be on the order of millennia. Changes to the magnitude of global BC stocks, driven by misbalances in its production and decomposition, thus influence the atmospheric stock of carbon. As these processes are not routinely considered in Earth System Models, the cycling of BC may constitute a “missing” net sink for atmospheric carbon. A review conducted herein finds that 100-436 Pg of BC is stored globally in terrestrial and marine environments (12-53% of the atmospheric stock of CO2) and that contemporary changes in the magnitude of global BC stocks may cause changes to the atmospheric carbon stocks on the order of -100 to +300 Tg C year-1. Nonetheless, it is not currently clear whether global stocks of BC are in a state of equilibrium, accumulation or depletion and thus the influence of BC cycling on atmospheric carbon is poorly constrained. The use of process-based models, including Earth System Models, will be pivotal to determining the effects of the BC cycle on atmospheric carbon concentrations in past, present and future climates. In order to integrate the BC cycle into global process-based models, it will be necessary to constrain uncertainty in the processes driving its environmental dynamics, including production, transport and mineralisation. The longer residence time displayed by BC in oceanic than terrestrial environments means that processes responsible for its transfer to oceans are particularly important determinants of its decomposition rate. This makes the export of BC by rivers a critical element of the global BC cycle. In the past, the global riverine export of dissolved BC (DBC) has been understood through a simple linear relationship with dissolved organic carbon (DOC), suggesting that the dynamics of BC and OC in river catchments are spatially uniform. This thesis presents two lines of evidence that challenge the simplicity of this universal relationship: first, evidence is presented for the systematic spatial variability in DBC/DOC ratios both within South American tropical river systems and between tropical, temperate and high-latitude systems; second, an analysis of the environmental drivers of large-scale variability in DOC and DBC concentrations in Brazilian Rivers reveals that some environmental factors are capable of driving divergent responses in the fluxes of DBC and DOC. Overall, this work highlights that the predictive models currently used to understand BC dynamics in river catchments are likely to be oversimplified and provides some numerical constraints conducive to its representation in process-based models. An additional element of the BC cycle considered in this thesis is the post-depositional fate of BC aerosol in terrestrial and marine environments. The contribution of BC aerosol to riverine BC fluxes has hitherto been understudied on the basis of its relatively small contribution to total BC production. Nonetheless, models of riverine DBC export, presented herein, suggest that a nontrivial fraction of export derives from aerosol BC and that this signal can be registered at the continental scale, specifically in South America. On the basis of these results, and with the support of other recent empirical studies, global estimates were made for the contribution of aerosol BC to the global riverine export of DBC. In this thesis it was found that the combined fluxes of aerosol BC to oceans through riverine export and direct deposition contribute 23-45% of the total modern influx of BC to the oceanic DBC pool. Therefore, the post-depositional dynamics of aerosol BC must be considered an important element of the BC cycle in terrestrial and marine environments and greater unity between the fields of atmospheric research and biogeochemistry will be required in future studies of this cycle