[causi]

The team reasoned that, if sensors showed a constant, unchanging value of oxygen in a continuous, vertical section of the ocean, regardless of the true value, then it would likely be a sign that oxygen had bottomed out, and that the section was part of an oxygen-deficient zone.

Did they bother testing this idea at all? It's a hypothesis absolutely essential to their study. There must be some method of accurately measuring oxygen at various depths, even if it isn't practical to deploy it across the ocean. You could for example spectrographically analyze a set of sample bottles for oxygen content, take a series of measurements of the water column, and then analyze the bottles afterward to determine how much oxygen leached out of them.

[taneq]

If I saw data like that my first thought would be a comms or sensor problem of some kind. Funky sensor readings sometimes mean funky physical values but it's the exception, not the rule.