This nice paper from 2020 by Marcantonio et al shows – perhaps paradoxically – from
meticulous sediment isotope data over nearly 200k years, that higher atmospheric CO2 results in higher deep ocean oxygenation. They found that deep ocean anoxia, indicated by the presence of a chemical form of uranium, was associated with glacial periods of low atmospheric CO2 less than 200ppm.
This suggests that healthy levels of CO2 in the atmosphere enhance planktonic primary production which among other things (like supporting nearly all life in the ocean) result in more oxygen being carried down to the ocean floor.
That’s interesting since it suggests that cold alone is not sufficient to oxygenate (ventilate) the ocean floor. Robust atmospheric CO2 is also needed.
All further evidence of course of how simplistic and wrong it is to regard CO2 in the atmosphere as something bad. It’s not. We actually live in a CO2 starved period in history and so adding a bit more to the air is a good thing – making plants grow better, greening the Sahara and oxygenating the deep ocean.