FY 2025

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

Bushinsky, S.M., Z. Nachod, A.J. Fassbender, V. Tamsitt, Y. Takeshita, and N. Williams

Global Biogeochem. Cycles, 39(5), e2024GB008185, doi: 10.1029/2024GB008185, View article online at Wiley/AGU (2025)

Profiles of oxygen measurements from Argo profiling floats now vastly outnumber shipboard profiles. To correct for drift, float oxygen data are often initially adjusted to deployment casts, ship-based climatologies, or, recently, measurements of atmospheric oxygen for in situ calibration. Air calibration enables accurate measurements in the upper ocean but may not provide similar accuracy at depth. Using a quality controlled shipboard data set, we find that the entire Argo oxygen data set is offset relative to shipboard measurements (float minus ship) at pressures of 1,450–2,000 db by a median of −1.9 μmol kg⁻¹ (mean ± SD of −1.9 ± 3.9, 95% confidence interval around the mean of {−2.2, −1.6}) and air-calibrated floats are offset by −2.7 μmol kg⁻¹ (−3.0 ± 3.4 (CI95%{−3.7, −2.4}). The difference between float and shipboard oxygen is likely due to offsets in the float oxygen data and not oxygen changes at depth or biases in the shipboard data set. In addition to complicating the calculation of long-term ocean oxygen changes, these float oxygen offsets impact the adjustment of float nitrate and pH measurements, therefore biasing important derived quantities such as the partial pressure of CO₂ (pCO₂) and dissolved inorganic carbon. Correcting floats with air-calibrated oxygen sensors for the float-ship oxygen offsets alters float pH by a median of 3.0 mpH (3.1 ± 3.7) and float-derived surface pCO₂ by −3.2 μatm (−3.2 ± 3.9). 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.

Plain Language Summary. Oxygen has historically been measured using chemical titrations on water collected by ships at sea. Over the past 20 years, sensors that measure oxygen have been deployed on robotic profiling floats. Measurements by oxygen sensors on profiling floats greatly exceed those collected by ships. Here, we compare all available float oxygen data to shipboard measurements in deep waters (1,450–2,000 m depth) where we do not expect oxygen to change in the ocean. We find a difference between float and shipboard data. If left uncorrected, this difference would give the false impression of a long-term oxygen change. This difference also impacts float-measured pH and float-estimated carbon dioxide, both of which rely on float oxygen measurements. Correcting oxygen, and therefore float pH and carbon dioxide, would largely address a widely studied bias in float measurements of these parameters.