Fire and Water: Deciphering the Black Carbon River-to-Ocean Continuum in the western Arctic

The recalcitrant nature of black carbon (BC) plays an important role in modulating carbon cycling on both global and regional scales. This is especially the case in the Arctic, which is strongly affected by climate change, and where BC production is amplified. BC results from the incomplete combustion of organic matter, particularly vegetation fires, where it enters the soil carbon pool. BC is subsequently mobilized from soils in both dissolved (DBC) and particulate (PBC) phases, and transported via rivers to the ocean. The balance between remineralization and sequestration of this BC influences the extent to which ecosystems act as a carbon source or sink. Arctic rivers, including the Mackenzie River in northern Canada, transport substantial amounts of DBC and PBC to the Beaufort Sea (Arctic Ocean). Permafrost soils within Arctic watersheds hold vast stores of long-sequestered organic material, including BC. Because of its condensed aromatic structure, BC is resistant to biological degradation, resulting in its accumulation in different carbon pools, with radiocarbon measurements indicating that it comprises amongst the slowest cycling and most refractory forms of naturally-formed organic carbon. Currently, however, our understanding of BC turnover and sinks within different carbon reservoirs – particularly in the vulnerable Arctic region - remains limited, leading to poor constraints on the dynamics of BC in terrestrial and aquatic systems and potential feedbacks with respect to climate change. The aim of this doctoral thesis is to increase our understanding of the controls of BC cycling across the river-to-ocean continuum over different seasons and years in the Arctic region. In particular, the goal is to shed light on seasonal variations in BC supply, especially DBC, and its subsequent offshore fate, which remain poorly understood. Ocean water samples were collected during late summer (September) and early winter (November) in proximity of the Mackenzie River Delta and along the Beaufort shelf to determine concentrations of DBC in context to DOC concentrations and the molecular composition of DOM. We then focused on radiocarbon signatures of DBC as these are a crucial tool to decipher the fate of riverine DBC in the ocean. Since DBC radiocarbon values are scarce, we undertook a detailed investigation of DBC radiocarbon signatures in the Beaufort Sea and compared these signatures with those of DBC exported by the Mackenzie River. Furthermore, in order to gain further insights into DBC radiocarbon patterns in the global ocean, we combined our Arctic data set with previously published data from different ocean basins. River water samples from the Mackenzie River and its tributaries were further analyzed to investigate the provenance and mobilization pathways of exported BC. Radiocarbon signatures of PBC during high river water discharge (freshet) collected over four years were determined, while radiocarbon signatures of DBC and DOC during freshet and fall were used to decipher interannual and seasonal variation in BC sources and export. Although Mackenzie River water discharge rates were dramatically lower in early winter, the Mackenzie River plume was evident from elevated concentrations of DOC (118 4 µmol L­1) and DBC (3.3 1 µmol L­1) and by the abundance of molecular formulae characteristic of terrestrial organic matter. In the Beaufort Sea, two offshore water masses were characterized by characteristic marine molecular formulae: meteoric and sea-ice melt water (MWM) in September and remnant winter water (RWW) in November. While DOC concentrations of MWM and RWW were similar (92 13 µmol L­1 vs. 90 28 µmol L­1), MWM was substantially influenced by sea-ice melt, as illustrated by an inverse trend between DBC concentration and salinity. In early winter the DBC Δ14C values in the Beaufort Sea did not exhibit a strong trend with depth or salinity. Further the Δ14C values of DBC exported from the Mackenzie River were higher (i.e. younger 14C ages) during freshet (­181‰) and lower (i.e. older 14C ages) in fall (-431‰), when discharge is low. The early winter DBC Δ14C values in the Beaufort Sea were younger (ave. ­363‰) than the Mackenzie River, emphasizing the persisting influence of high riverine DBC fluxes during freshet. When comparing DBC radiocarbon signatures in the Beaufort Sea to global DBC signatures, we find that DBC Δ14C values in the deep ocean and in the Pacific Ocean were significantly lower (older) compared to North Atlantic and Arctic Ocean basin waters. We also observe that DBC radiocarbon ages consistently exceed corresponding DOC radiocarbon ages, typically by several millennia. The Mackenzie River transports most of the annual PBC and DBC flux during freshet when high river discharge and high concentrations of POC and DOC prevail. Permafrost soils were identified as the main source of aged PBC and DBC, however the mobilization pathways for PBC and DBC were decoupled. The most pre-aged PBC (11,000-17,000 years) was mobilized at high water discharge during freshet, and is attributed to soil erosion and landslides of thawed permafrost soils. In contrast, aged DBC became more dominant in late summer/fall (3,600-8,000 years), likely as a result of porewater release and mobilization from thawed permafrost soils. This investigation of DBC and PBC dynamics in the western Arctic highlights the seasonal contrast in sources and mobilization pathways of BC, with implications for region and global-scale carbon cycling. Amplified warming in the Arctic is anticipated to result in both greater fire activity and enhanced erosion of permafrost soils, with both processes resulting in increased fluxes of BC to the Arctic Ocean. The efficiency by which this BC is exported to ocean sediments and deep waters or is remineralized along the land-ocean aquatic continuum plays a crucial role in determining the extent and timescales over which the Arctic system represents a net sink or source of carbon to the atmosphere. Further, in-depth investigations of DBC and PBC in river systems and in ocean waters is indispensable to understand the overall fate of BC, and to elucidate the dynamics of this recalcitrant component of OC. Moreover, investigation of BC fluxes and signatures in terrestrial watersheds under different climate regimes and during variable seasons are crucial to understand BC dynamics. Overall, this study highlights the need for more extensive analysis of BC in ocean waters and along the river-to-ocean continuum in order to improve our understanding of this important but enigmatic component of the global carbon cycle and its feedbacks with respect to climate change.

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