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

In September 2013, we observed an expanse of surface water with low CO2 partial pressure (pCO(2)(sea)) (< 200 mu atm) 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 pCO(2)(sea) was closer to the air-sea CO2 equilibrium (similar to 360 mu atm). 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 (similar to 0.16) rather than the riverine discharge (similar to 0.1). Such an increase in pCO(2)(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 pCO(2)(sea) (< 250 mu atm) in the depth of 30-50 m under the halocline of the Canada Basin. This subsurface low pCO(2)(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 mu mol kg(-1)) in the subsurface low pCO(2)(sea) layer in the Canada Basin indicated significant net primary production undersea and/or in preformed condition. If these low pCO(2)(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.