I suspect that a better explanation would be that the ice age induced temperature variation served to suppress the general robustness of the biome itself. When that ended recently, human meta agriculture and the effects of newly released glacial till served to keep it all suppressed and trending downward.
That can now change radically as human terra forming truly takes hold and serves to make both the land and ocean fully productive with teeming life.
We are actually at a bottom here of a million year trend line.
Earth's Oxygen Levels Are Declining And Scientists Don't Know Why
A new study shows Earth's oxygen levels continue to decline, a phenomena that has puzzled scientists. The study, published in Science by Princeton University professor Daniel Stolper, presents data measured from small air bubbles trapped in ice on Greenland and Antarctica.
Air bubbles trapped within ice provides clues to the atmospheric composition at the time of "deposition" and can be analyzed for paleo-oxygen levels. The study finds that over the past 800,000 years the amount of oxygen found in the atmosphere has decreased by 0.7% and continues to decline. Fortunately, the 0.7% decline is not something that will or has caused significant problems for life on Earth. An equivalent decrease in oxygen content would accompany a change in elevation from sea level to 100 meters above sea level.
Why Are Oxygen Levels Declining?