During the Carboniferous Period, there were widespread coal swamps. When there is rapid burial of plant matter (in the absence of decomposers in an anoxic swamp), what happens to the relative concentrations of CO2 and O2 in the atmosphere?

The Carboniferous era had large deposits of coal matter in the soil. However there was absence of decomposers which gave rise to increased accumulation of organic matter in the soil.

Decomposers make use of oxygen to breathe in the soil due to their existence as animals and not plants. They inhale oxygen and give out Carbon dioxide.

Absence of these decomposers mean increased level of oxygen due to its non usage and reduction in CO2 which is the end product of respiration for the decomposers.