Fluvial versus coastal input of permafrost organic carbon - insights from the Canadian Beaufort Sea

Abstract

Around 65% of the Arctic coastline consists of permafrost. Rising global air temperatures cause these permafrost grounds to thaw which leads to the release of organic matter and sediments into the coastal ocean. This influences coastal ecosystem functioning and may further enhance atmospheric warming due to greenhouse gas emissions when the released carbon decomposes. Permafrost organic carbon enters the coastal ocean either through coastal erosion or through fluvial discharge and both fluxes are expected to increase in the future. The Canadian Beaufort Sea receives material from both sources, the region has some of the highest erosion rates in the Arctic and receives additional input through the Mackenzie River, the largest sediment supplier to the Arctic Ocean. To reliably estimate the current and future impacts of permafrost carbon on the coastal ocean and its potential climate feedback, we need to distinguish between these two sources whose fluxes may respond differently to ongoing Arctic change. However, we still lack reliable methods to do so. Here we propose a multiproxy approach to distinguish between sources of permafrost organic carbon by combining organic with inorganic geochemical tracers, grain size and grain shape data on a land-coast-ocean transect in the Mackenzie River Delta. The combined data pinpoints to differences in sediment source, composition, degradation, and transport pathways of both fluvially-discharged and coastally-eroded carbon. Degradation processes of organic and inorganic matter are tightly coupled, but do change within different environments (salinity, energy regimes). By combining degradation state (stable isotopes) and transport indicators (such as grain roundness) with source region tracers (XRF, radiogenic isotopes) we aim to gain insights into the interaction, transition, and origin of this different kind of matter. If successful, this approach can be applied and compared to other Arctic delta environments to fully understand the impacts of increased permafrost thaw and changing river discharge patterns on the coastal Arctic Ocean.