Paving the way for improved representation of the carbon cycle in Arctic River plumes

Processes affecting the transformation of riverine dissolved organic carbon (DOC) across the land-to-ocean aquatic continuum (LOAC) are still poorly constrained in Arctic models, leading to large uncertainties in simulated air-sea CO2 fluxes along the coastal periphery of the Arctic Ocean. Here we use the ECCO-Darwin ocean-ice-biogeochemistry model in a regional configuration of the Southeastern Beaufort Sea (SBS) to analyze the sensitivity of simulated carbon cycling to 1) the vertical discretization of the model and 2) different parameterizations of Mackenzie River freshwater and carbon discharge. We show that riverine DOC chemical composition and lifetime rather than the quantity of tDOC exported from the river largely modulate Mackenzie River plume air-sea CO2 fluxes. Experiments testing the range of tDOC bioavailability reported for the Mackenzie River result in strong variability of net CO2 fluxes, leading to the SBS being either a source (0.03 TgCyr-1) or sink (-0.20 TgCyr-1) of atmospheric carbon. We show that estuarine processes, such as flocculation, also play an important role and can dampen CO2 outgassing by up to 0.07 TgCyr-1. In terms of model physics, increasing the vertical grid resolution improves the model's agreement with the observed plume structure without altering the simulated concentrations of biogeochemical tracers. However, the greater amount of tDIC forced into a smaller plume volume increases local pCO2 and promotes elevated outgassing in the vicinity of the Mackenzie River Delta. Our work demonstrates that future Arctic land-ocean models must consider the intricate details of river plume systems to realistically simulate coastal-ocean physics and biogeochemistry.