• oceanography;
  • biogeochemistry

[1] Changes in dissolved O2 observed across the world oceans in recent decades have been interpreted as a response of marine biogeochemistry to climate change. Little is known however about the spectrum of oceanic O2 variability. Using an idealized model, we illustrate how fluctuations in ocean circulation and biological respiration lead to low-frequency variability of thermocline oxygen. Because the ventilation of the thermocline naturally integrates the effects of anomalous respiration and advection over decadal timescales, short-lived O2 perturbations are strongly damped, producing a red spectrum, even in a randomly varying oceanic environment. This background red spectrum of O2 suggests a new interpretation of the ubiquitous strength of decadal oxygen variability and provides a null hypothesis for the detection of climate change influence on oceanic oxygen. We find a statistically significant spectral peak at a 15–20 year timescale in the subpolar North Pacific, but the mechanisms connecting to climate variability remain uncertain.