Influence of the biological pump on carbon uptake over the annual cycle across the North Pacific Ocean

The North Pacific features band of strong atmospheric CO₂ uptake between 30°N and 45°N, making it a region of particular interest for quantifying the mechanisms of ocean carbon uptake. We measured the partial pressure of carbon dioxide to determine air-sea CO₂ flux and oxygen/argon dissolved gas ratios as a geochemical tracer of net community production (NCP) on sixteen container ship transects across the North Pacific from 2008-2012. Using these data, we construct mixed layer budgets to evaluate the relative roles of biological carbon export, physical circulation, and temperature-driven solubility changes in driving air-sea CO₂ flux over seasonal and annual time scales across the North Pacific basin (35°N – 50°N, 142°E – 125°W).

The dynamic western basin experiences a stronger seasonal cycle in air-sea CO₂ flux and NCP than the more quiescent eastern basin. The annual ocean carbon sink is stronger in the western than the eastern basin (air-sea CO₂ flux of -2.1 to -2.7 mol C m^-2 yr^-1 west of 160°W, as compared to -1.4 mol C m^-2 yr^-1 east of 160°W). During the productive season from spring through fall, NCP in the mixed layer is also stronger in the western basin; however deep winter mixed layers (to 210 m) in the western basin ventilate >70% of this seasonally exported carbon while only ~20% of seasonally exported carbon in the eastern basin is ventilated in winter where mixed layers are <120 m. As a result the biological pump more effectively sequesters CO₂ from the atmosphere on an annual basis in the east than the west (annual NCP of 1.5 ± 0.2 mol C m^-2 yr^-1 east of 160°W, decreasing to 1.0 ± 0.5 mol C m^-2 yr^-1 between 170°E and 160°W and 0.6 ± 1.5 mol C m^-2 yr^-1 west of 170°E). This indicates that carbon export via the biological pump can fully account for annual CO₂ uptake in the eastern basin, but can only account for one-third of annual CO₂ uptake in western basin, requiring a significant contribution from physical processes in driving the strong CO₂ sink in the west.