Drivers of inorganic carbon dynamics in first-year sea ice: a model study

Sea ice is an active source or a sink for carbon dioxide (CO₂), although to what extent is not clear. Here, we analyze CO₂ dynamics within sea ice using a one-dimensional halothermodynamic sea ice model including gas physics and carbon biogeochemistry. The ice-ocean fluxes, and vertical transport, of total dissolved inorganic carbon (DIC) and total alkalinity (TA) are represented using fluid transport equations. Carbonate chemistry, the consumption, and release of CO₂ by primary production and respiration, the precipitation and dissolution of ikaite (CaCO₃·6H₂O) and ice-air CO₂ fluxes, are also included. The model is evaluated using observations from a 6 month field study at Point Barrow, Alaska, and an ice-tank experiment. At Barrow, results show that the DIC budget is mainly driven by physical processes, wheras brine-air CO₂ fluxes, ikaite formation, and net primary production, are secondary factors. In terms of ice-atmosphere CO₂ exchanges, sea ice is a net CO₂ source and sink in winter and summer, respectively. The formulation of the ice-atmosphere CO₂ flux impacts the simulated near-surface CO₂ partial pressure (pCO₂), but not the DIC budget. Because the simulated ice-atmosphere CO₂ fluxes are limited by DIC stocks, and therefore <2 mmol m⁻² d⁻¹, we argue that the observed much larger CO₂ fluxes from eddy covariance retrievals cannot be explained by a sea ice direct source and must involve other processes or other sources of CO₂. Finally, the simulations suggest that near-surface TA/DIC ratios of ∼2, sometimes used as an indicator of calcification, would rather suggest outgassing.