Sea ice melt drives vertical pCO₂ variability modulating air-sea gas exchange

Abstract. Strong spatial and temporal gradients in salinity, temperature, and carbonate chemistry in Arctic coastal surface waters complicate the estimation of air-sea CO₂ exchange, particularly during sea ice breakup. This study evaluates the applicability of the widely used bulk flux model under such conditions. The bulk approach assumes homogeneous surface conditions and linear vertical pCO₂ gradients. However, our observations in a stratified Arctic fjord reveal pronounced vertical variability in pCO₂ within the upper water column, including non-linear gradients near the air-sea interface. Micrometeorological measurements captured episodic upward CO₂ fluxes even when waters 1 m and below were CO₂-undersaturated. We hypothesize that transient, high-pCO₂ layers at ~0.1 m depth intermittently decouple the atmospheric exchange from subsurface waters, reversing the expected flux direction. These findings highlight the importance of resolving near-surface variability during the transition from ice-covered to open water conditions. We recommend incorporating micrometeorological techniques and high-resolution vertical profiling in Arctic fjords to improve flux estimates of CO₂ in this rapidly changing region.