The subpolar North Atlantic (SPNA) is one of the few regions where the deep ocean is in direct contact with the atmosphere, making it a key location for interior ocean ventilation through gas exchange. We use observational data to analyze large-scale patterns of mean annual air-sea flux, biological production and consumption, and physical transport of oxygen for the subpolar North Atlantic Ocean (45N-65N), finding a net annual flux of 48.1±14.6 Tmol (1Tmol = 1012 mol) of oxygen from the atmosphere into the ocean, largely balanced by a removal of oxygen through physical transport. Wintertime increases in oxygen content in isopycnal layers match the location and magnitude of net oxygen uptake from the atmosphere, supporting the connection between air-sea gas exchange at the surface and ventilation of deeper layers. Integrated over the whole SPNA, 90% of the net oxygen influx and 80% of the seasonal oxygen content increase occur at densities of σ0 < 27.6 kg m-3, in the upper branch of the Atlantic Meridional Overturning Circulation (AMOC). The subpolar gyre (SPG) is ventilated with oxygen largely at these lower densities, accumulating oxygen along its cyclonic pathway from the North Atlantic Current towards the Labrador Sea. Our results thus suggest that the subpolar gyre is oxygenated cumulatively throughout the SPNA, as mode waters formed each winter become progressively denser and more oxygenated along the SPG’s path, culminating in the oxygen-rich Labrador Sea Water which is ultimately exported to the rest of the ocean in the lower branch of the AMOC.

Profiles of oxygen measurements from Argo profiling floats now vastly outnumber shipboard profiles. Air calibration of a float’s oxygen optode upon surfacing enables accurate measurements in the upper ocean but does not necessarily provide similar accuracy at depth. In this study we use a quality controlled shipboard dataset to show that, on average, the entire Argo oxygen dataset is offset relative to shipboard measurements (float minus ship) at pressures of 1450 to 2000 db by -1.9 ± 4.7 µmol kg-1 (95% confidence interval around the mean of {-2.3, -1.5}) and air calibrated floats are offset by -3.1 ± 5.3 µmol kg-1 (95% CI: {-3.7, -2.4}). The difference between float and shipboard oxygen is most likely due to offsets in the float oxygen data and not due to oxygen changes at these depths or biases in the shipboard dataset. In addition to posing problems for the calculation of long-term ocean oxygen changes, these float oxygen offsets impact the adjustment of float nitrate and pH measurements and therefore bias important derived quantities such as the partial pressure of CO2 (pCO2) and dissolved inorganic carbon. Correcting floats with air-calibrated oxygen for the float-ship oxygen offsets changes float pH by 3.2 ± 3.8 mpH and float-derived surface pCO2 by -3.3 ± 4.1 µatm. This adjustment to float pCO2 represents half, or more, of the bias in float-derived pCO2 reported in studies comparing float pCO2 to shipboard pCO2 measurements.

Increased oceanic uptake of CO­2 due to rising anthropogenic emissions has caused lowered pH levels (ocean acidification) that are hypothesized to inhibit biotic calcification and reduce the export of total alkalinity (AT) as carbonate minerals from the surface ocean. This “CO2-biotic calcification feedback” is a negative feedback on atmospheric CO2, as elevated levels of surface AT increase the ocean’s capacity to uptake CO2. We detect signatures of this feedback in the global ocean for the first time using repeat hydrographic measurements and seawater property prediction algorithms. Over the course of the past 30 years, we find an increase in global surface AT of 0.072 ± 0.023 µmol kg-1 yr-1, which would have caused approximately 20 Tmol of additional AT to accumulate in the surface ocean. This finding suggests that anthropogenic CO2 emissions are measurably perturbing the cycling of carbon on a planetary scale by disrupting biological patterns. More observations of AT would be required to understand the effects of this feedback on a regional basis and to fully characterize its potential to reduce the efficiency of marine carbon dioxide removal technology.