EFFECT OF OXYGEN ON DEGRADATION RATE OF REFRACTORY AND LABILE ORGANIC-MATTER IN CONTINENTAL-MARGIN SEDIMENTS

In order to study the effect of oxygen on degradation rate of bulk sed imentary organic matter, we developed a new incubation method. In cont rast to most previous experiments, (1) all of the sediment undergoing decomposition was maintained under oxygenated or oxygen free condition s and (2) organic matter of varying lability was studied. The producti on of Sigma CO2 during incubations of sediment in glass test tubes, co rrected for dissolution/precipitation of calcium carbonates, was used as a measure of degradation rates. The laboratory experiments, using s urficial and buried continental shelf and slope sediment from the open Skagerrak (northeastern North Sea), demonstrated that the effect of o xygen on the degradation rate of sedimentary organic matter is a funct ion of the lability of the decomposing material. Fresh material was de graded with little difference in rates in the presence or absence of o xygen, whereas old material was decomposed significantly (up to 3.6 ti mes) faster with oxygen than without oxygen. An implication of these f indings is that bioturbation, by exposing old buried material to oxyge n, may enhance integrated organic carbon oxidation in marine sediments . This constitutes a previously unexplored mechanism by which faunal r eworking may stimulate carbon degradation. The anoxic decomposition ra tes of organic material buried at 20 cm depth in sediment were the low est measured. We found, however, that the extent of oxidation of this buried old sediment was considerably larger than that of surficial sed iment under oxygenated conditions, which indicated that the oxic-anoxi c-oxic redox transitions (deposition under oxic conditions, burial und er 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 co nditions alone, than when the material is exposed to the repeated acti vities of both oxic and anoxic microorganisms. Copyright (C) 1998 Else vier Science Ltd.