Offset between profiling float and shipboard oxygen observations at depth imparts bias on float pH and derived pCO2

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.