Radiocarbon signatures of dissolved black carbon in early winter water masses from the Beaufort Sea

Black carbon (BC) is a fraction of organic carbon resulting from the incomplete combustion of biomass and fossil fuels. The production and fate of BC is a topic of great interest in the context of ongoing climate change, as the intensity and severity of fires is increasing. The recalcitrant nature enables BC to buffer these changes by removing biomass-derived carbon into longer cycling pools. BC is mainly produced on land and a portion is transported in both particulate and dissolved form by the rivers to the oceans. Dissolved BC (DBC) cycles on millennial timescales, thereby storing BC as fraction of Dissolved Organic Carbon (DOC) in the marine DOC pool before deposition to sediments or complete degradation. However, there is currently limited information on the cycling, transport and evolution of modern riverine DBC, and how it contributes to the deep ocean DOC pool.The arctic and boreal regions are well recognized as a nexus for climate change, given amplified rates of change in average temperatures and summer precipitation, which exacerbate carbon cycle feedbacks, including enhanced BC production by intensified wildfire seasons. The Beaufort Sea in the Arctic Ocean is composed of different water masses, with Pacific water masses entering from the Chukchi Sea, and arctic rivers - in particular the Mackenzie River - being the major source of freshwater that delivers both terrestrial DOC and DBC. Presently, information on the sources and fate of BC in the Arctic Ocean remains sparse.Here, we report DBC concentrations and Δ14C values in the Beaufort Sea during early winter conditions. Distinct water masses were sampled, including the outflow of the Mackenzie River and the Pacific water jet on the shelf break, during two cruises in 2021 and 2022 that spanned the coast of north Alaska to the Amundsen Gulf. Preliminary radiocarbon results show that DBC on the shelf break is up to five millennia old. We discuss our findings in the context of regional hydrography and carbon cycle processes.