Early Wintertime CO₂ Uptake in the Western Arctic Ocean

To investigate CO₂ uptake in the western Arctic Ocean (north of 65°N), we conducted underway, ship-based observations of the partial pressures of CO₂ (pCO₂) and total dissolved inorganic carbon (TCO₂) in surface seawater in early winter (November 2018). From these two properties of the seawater inorganic carbon system, we calculated total alkalinity (TA). In the early winter, surface seawater pCO₂ in most places was lower than atmospheric pCO₂. The weighted mean of the air-sea influxes of CO₂ were calculated to be 7.5 ± 1.6 mmol m⁻² d⁻¹. The calculated influxes imply that the area acted as a moderate sink for atmospheric CO₂ in early winter, and its rate of CO₂ uptake was comparable to that (8.0 ± 1.7 mmol m⁻² d⁻¹) in summer (late August−September 2017). Spatial variations of surface seawater pCO₂ in the early winter could be attributed mostly to conservative mixing changes of TCO₂ and TA, which together accounted for more than 70% of the pCO₂ variations. In the marginal ice zone, however, there was a decrease of surface seawater pCO₂ by 70–90 μatm because of horizontal advection of water with an anomalously high temperature from the Pacific Ocean and its subsequent cooling. We concluded that mixing of water masses was as important as biological processes in causing spatial variations of pCO₂ and CO₂ uptake in the western Arctic Ocean, especially during seasons and in areas associated with little biological activity.