Enhancement of the North Atlantic CO2 sink by Arctic Waters.

The North Atlantic north of 50° N is one of the most intense ocean sink areas for atmospheric CO₂ considering the flux per unit area, 0.27 Pg-C yr⁻¹, equivalent to −2.5 mol C m⁻² yr⁻¹. The Northwest Atlantic Ocean is a region with high anthropogenic carbon inventories. This is on account of processes which sustain CO₂ air-sea fluxes, in particular strong seasonal winds, ocean heat loss, deep convective mixing and CO₂ drawdown by primary production. The region is in the northern limb of the Global Thermohaline Circulation, a path for the long term deep sea sequestration of carbon dioxide. The surface water masses in the North Atlantic are of contrasting origins and character, on the one hand the northward flowing North Atlantic Drift, a Gulf Stream offspring, on the other hand southward moving cold low salinity Polar and Arctic Waters with signatures from Arctic freshwater sources. We have studied by observations, the CO₂ air-sea flux of the relevant water masses in the vicinity of Iceland in all seasons and in different years. Here we show that the highest ocean CO₂ influx is to the Arctic and Polar waters, respectively, −3.8 mol C m⁻² yr⁻¹ and −4.4 mol C m⁻² yr⁻¹. These waters are CO₂ undersaturated in all seasons. The Atlantic Water is a weak or neutral sink, near CO₂ saturation, after poleward drift from subtropical latitudes. These characteristics of the three water masses are confirmed by data from observations covering 30 years. We relate the Polar and Arctic Water persistent undersaturation and CO₂ influx to the excess alkalinity derived from Arctic sources, particularly the Arctic rivers. Carbonate chemistry equilibrium calculations indicate clearly that the excess alkalinity may support a significant portion of the North Atlantic CO₂ sink. The Arctic contribution to the North Atlantic CO₂ sink which we reveal is previously unrecognized. However, we point out that there are gaps and conflicts in the knowledge about the Arctic alkalinity budget and that future trends in the North Atlantic CO₂ sink are connected to developments in the rapidly warming Arctic. The results we present need to be taken into consideration for the question: Will the North Atlantic continue to absorb CO₂ in the future as it has in the past? [ABSTRACT FROM AUTHOR]