Low pCO₂ under sea-ice melt in the Canada Basin of the western Arctic Ocean

Abstract. In September 2013, we observed an expanse of surface water with low CO₂ partial pressure (pCO₂^sea) (< 200 µatm) in the Chukchi Sea of the western Arctic Ocean. The large undersaturation of CO₂ in this region was the result of massive primary production after the sea-ice retreat in June and July. In the surface of the Canada Basin, salinity was low (< 27) and pCO₂^sea was closer to the air–sea CO₂ equilibrium (∼  360 µatm). From the relationships between salinity and total alkalinity, we confirmed that the low salinity in the Canada Basin was due to the larger fraction of meltwater input (∼  0.16) rather than the riverine discharge (∼  0.1). Such an increase in pCO₂^sea was not so clear in the coastal region near Point Barrow, where the fraction of riverine discharge was larger than that of sea-ice melt. We also identified low pCO₂^sea (< 250 µatm) in the depth of 30–50 m under the halocline of the Canada Basin. This subsurface low pCO₂^sea was attributed to the advection of Pacific-origin water, in which dissolved inorganic carbon is relatively low, through the Chukchi Sea where net primary production is high. Oxygen supersaturation (> 20 µmol kg⁻¹) in the subsurface low pCO₂^sea layer in the Canada Basin indicated significant net primary production undersea and/or in preformed condition. If these low pCO₂^sea layers surface by wind mixing, they will act as additional CO₂ sinks; however, this is unlikely because intensification of stratification by sea-ice melt inhibits mixing across the halocline.