Halting the production of marine biota C and maintaining the same fluxes of C out of marine biota, the marine biota C would be used up in about 3 weeks, at which point a 0.0022 % / year ** drawdown of O2 could persist for some longer
time (** the actual removal of O2 from the atmosphere
via oxidation in the deeper ocean may
occur over a timescale of 1000 years, but the oxydation of organic C in the ocean might be completed
over a significantly shorter
time, so the actual removal of O2 from the atmosphere may be slower than 0.0022 % / year)-- but it still wouldn't have much total effect on the amount of atmospheric O2.