A new study has found that:
In oxygen-starved places such as marshes and in floodplains, microorganisms do not equally break down all of the available organic matter. Instead, carbon compounds that do not provide enough energy to be worthwhile for microorganisms to degrade end up accumulating. This passed-over carbon, however, does not necessarily stay locked away below ground in the long run. Being water soluble, the carbon can seep into nearby oxygen-rich waterways, where microbes readily consume it.
Tests found that, in contrast to the layers where oxygen was available, leftover carbon compounds in the sediment samples where sulfur had been used for respiration instead of oxygen were mostly of the sort that requires more energy to degrade than would be liberated through the degradation itself. Making these carbon compounds of no use, then, to growing microbes, and had remained within the deeper sediment layers.