Although organic matter (OM) settling on the seafloor is generally rapidly recycled, a key ecological process, large scale burial events manifest itself in the marine sedimentary record as organic carbon (Corg)-rich layers. Presently, this prevails under certain oceanic settings such as the oxygen minimum zones (OMZ) where OM accumulates in underlying sediments. A basic question that remains is as to what extent this Corg accumulation reflects ecological “malfunctioning” or a shunting of ecological processes? Experimenting with eastern Arabian Sea OMZ sediment we found no evidence that Corg accumulation here is not due to trophic satiation or to low tolerance of biota to severe oxygen depletion. However, we found direct evidence that suggests that the OMZ sediment Corg has very low bioavailability that probably impairs biological transformation.
In the first set of experiments, the impact of oxygenation on the benthic ecological functioning was examined by following the fate of fresh, highly degradable OM (13C-labelled diatoms) in intact sediment cores incubated for 7 days under normoxic versus suboxic bottom water conditions. Tracer organic matter assimilation (by bacteria and fauna) and respiration was evident and similar under both treatments and demonstrates that the benthic response was not hindered by severe oxygen depletion. Furthermore, relatively low biomass standing stock of fauna and bacteria, in spite of sediment high Corg content, together with this clear uptake of fresh tracer OM suggest that the benthic community was not food saturated.
In a second set of experiments, the bioavailability of in situ OMZ organic matter was determined directly through CO2 production rate measurements in bottle sediment–water slurry incubations. In sharp contrast to fresh tracer algal carbon which had a half-life 0.07 years, the OMZ surficial sediment OM half-life was ~ 67 years already in very early diagenesis. Clearly, a distinct difference in functioning and indicates that a large fraction of OMZ sediment organic matter is evidently excluded from immediate first-hand biotic transformation but on its own represents a link between the “fast (biological)” and the “slow (geological)” carbon cycle along the continuum of OM recycling. Furthermore, while this rapid shift out of the “fast” biological cycle may be common or characteristic of large scale OM accumulation events, a comparison with an ancient Corg-deposit suggest that the trigger mechanisms may not be uniform.