Abstract
Abstract. 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, with the northward-flowing North Atlantic Drift, a Gulf Stream offspring, on the one hand and on the other hand the cold southward-moving low-salinity Polar and Arctic waters with signatures from Arctic freshwater sources. We have studied by observation 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±0.4 and -4.4±0.3 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 Water and Arctic Water persistent undersaturation and CO2 influx to the excess alkalinity derived from Arctic sources. Carbonate chemistry equilibrium calculations clearly indicate that the excess alkalinity may support at least 0.058 Pg-C yr−1, a significant portion of the North Atlantic CO2 sink. The Arctic contribution to the North Atlantic CO2 sink which we reveal was previously unrecognized. However, we point out that there are gaps and conflicts in the knowledge about the Arctic alkalinity and carbonate budgets and that future trends in the North Atlantic CO2 sink are connected to developments in the rapidly warming and changing Arctic. The results we present need to be taken into consideration for the following question: will the North Atlantic continue to absorb CO2 in the future as it has in the past?
Highlights
The oceans take up about a quarter of the annual anthropogenic CO2 emissions (Friedlingstein et al, 2019)
We describe long-term carbon chemistry characteristics of water masses in mid-winter when physical forces prevail over biological processes
The North Atlantic region we describe has Atlantic Waters advected from southern temperate latitudes and cold lowersalinity Arctic and Polar waters carried with the East Greenland Current from the Arctic
Summary
The oceans take up about a quarter of the annual anthropogenic CO2 emissions (Friedlingstein et al, 2019). This may even be an underestimate (Watson et al, 2020). The reasons are strong winds and large natural partial pressure differences, pCO2 = (pCO2sw − pCO2a), between the atmosphere and the surface ocean. The pCO2 is proportional to the concentration of undissociated CO2 molecules, [CO2]aq, which constitutes about 1 % of the total CO2 dissolved in seawater (the remainder being about 90 %–95 % as [HCO−3 ] and 4 %–9 % as [CO23−]). The seawater pCO2 depends sensitively on temperature and the TCO2 / Alk ratio, the relative concentrations of total CO2 species dissolved in seawa-
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