Increasing the Low Oxygen Accuracy of Commercial Optodes

Using standard calibration schemes, commercial oxygen optode sensors typically yield oxygen concentrations in the range of 2–4 μ mol kg −1 under anoxic conditions. They are thus unable to detect the roughly 0.1 μ mol kg −1 levels of oceanic functional anoxia. Here, an empirically modified Stern-Volmer equation is used to characterize and calibrate 26 optodes deployed on 16 autonomous floats in the Eastern Tropical Pacific oxygen deficient zone (ODZ) using a combination of manufacturers’, laboratory and in situ data. Laboratory calibrations lasting several months and conducted over 2 years show that optodes kept under anoxic conditions drift at rates of order ±0.2 μ mol kg −1 yr −1 , with much higher drifts in the first month. High accuracy, ship-based STOX profiles show that the anoxic cores of the ETNP ODZ have oxygen concentrations less than 0.05 μ mol kg −1 . An algorithm is developed to detect these cores in the float profiles and used to estimate and remove the optode drift. A section across the ETNP using this data shows the southward thinning of the functionally anoxic core from 600 m to 100 m thick, demonstrating an order of magnitude improvement in the low oxygen performance of the optodes. Similar recalibration could be implemented on the existing database of Argo oxygen floats. A more detailed understanding and modeling of the drift rates facilitated by a combination of laboratory studies and routine STOX profiles could further improve the low-oxygen optode calibration.