Effect of oxygen on degradation rate of refractory and labile organic matter in continental margin sediments

Abstract

In order to study the effect of oxygen on degradation rate of bulk sedimentary organic matter, we developed a new incubation method. In contrast to most previous experiments, (1) all of the sediment undergoing decomposition was maintained under oxygenated or oxygen free conditions and (2) organic matter of varying lability was studied. The production of ΣCO 2 during incubations of sediment in glass test tubes, corrected for dissolution/precipitation of calcium carbonates, was used as a measure of degradation rates. The laboratory experiments, using surficial and buried continental shelf and slope sediment from the open Skagerrak (northeastern North Sea), demonstrated that the effect of oxygen on the degradation rate of sedimentary organic matter is a function of the lability of the decomposing material. Fresh material was degraded with little difference in rates in the presence or absence of oxygen, whereas old material was decomposed significantly (up to 3.6 times) faster with oxygen than without oxygen. An implication of these findings is that bioturbation, by exposing old buried material to oxygen, may enhance integrated organic carbon oxidation in marine sediments. This constitutes a previously unexplored mechanism by which faunal reworking may stimulate carbon degradation. The anoxic decomposition rates of organic material buried at 20 cm depth in sediment were the lowest measured. We found, however, that the extent of oxidation of this buried old sediment was considerably larger than that of surficial sediment under oxygenated conditions, which indicated that the oxic-anoxic-oxic redox transitions (deposition under oxic conditions, burial under anoxia and reexposure to oxygen) promoted degradation. Our results, therefore, also suggest that the extent of long-term decomposition of sedimentary organic material is smaller under oxygenated or anoxic conditions alone, than when the material is exposed to the repeated activities of both oxic and anoxic microorganisms.