Fate of Fire altered Organic Carbon in the arctic river-to-ocean continuum: Resolving Mackenzie River Black Carbon in the Beaufort Sea

Keywords: Black carbon, Dissolved organic carbon, BPCAs, Mackenzie River, Beaufort Sea, Climate change

Climate change is amplified in the arctic and boreal regions. This causes higher average temperatures and less precipitation in the summer months and is resulting in longer wildfire seasons, severity, frequency and extent. This increases the relies of carbon into the atmosphere as greenhouse gases and aerosols, amplifying climate change even further. Black carbon (BC) is a fraction of organic carbon, resulting from the incomplete combustion of biomass and fossil fuels. BC may be inaccessible for biodegradation, because of its highly condensed aromatic molecular structure and therefore stores carbon on long timescales on land and in the ocean. BC is produced on land, but is transported as dissolved BC (DBC) by the rivers to the oceans, where it cycles on millennial timescales, sequestering BC. Thus, it is important to understand the significance of BC in the context of increased fires in this vulnerable region in the face of climate change.

The Mackenzie River is a major source of terrestrial dissolved organic carbon (DOC) and the largest source of sediments to the Arctic Ocean. Here, we resolve the cycling of riverine DBC from the Mackenzie River to its fate in the Beaufort Sea, and the influence of mixing with Pacific water masses entering from the Chukchi Sea. We present DBC concentration data in ocean water, which was collected on two cruises in the Beaufort Sea in 2021 and 2022 covering the outflow of the Mackenzie River.

For DBC concentrations, we digested solid phase extracts of DOC with nitric acid to oxidize BC molecules into benzenepolycarboxylic acids (BPCAs), which were then quantified on High Performance Liquid Chromatography (HPLC). We compare the concentrations of the DBC and DOC to trace the mixing of DBC river outflow with the ocean water. Since DBC originates on land and is relatively stable to biodegradation we can resolve the pathways of DBC from the Mackenzie River to the Arctic Ocean.