Anoxic microniches in marine sediments induced by aggregate settlement: biogeochemical dynamics and implications

Spherical (~2 mm diameter) diatom (Skeletonema sp.) aggregates, representing analogues of “marine snow”, were placed at the sediment–water interface of an experimental sediment system. Optode measurements showed that, after an initial lag period, oxygen concentrations within the aggregates decreased and then were gradually replenished, resulting in a temporary anoxic microniche. A multi-species, 3-dimensional, reactive transport model was used to simulate the oxygen dynamics and the associated biogeochemical consequences. Temporal and spatial changes in oxygen were replicated assuming an exponential increase in the mineralisation rate constant and a gradual exhaustion of reactive organic material. The peak value of the time-dependent reaction rate constant of organic matter mineralisation (k _OMM) was seven to sixty times greater than analogous values measured previously in water column experiments. The validated model was used to investigate how the size and reactivity of parcels of organic matter influence the formation of anoxic microniches at the sediment–water interface of typical deep-sea environments. As well as k _OMM, the concentration of reactive organic matter in the aggregate, its size and porosity were also critical in determining the likelihood of anoxic microniche formation. For the optimum fitted parameters describing k _OMM and the concentration of reactive organic matter, the minimum diameter of the parcel to induce anoxia was 1.8 mm, whereas it was 2.8 mm to make a significant contribution to the denitrification occurring in a typical deep-sea sediment. This work suggests that anoxic microniches resulting from the settlement of marine aggregates may play an overlooked role for denitrification activities in deep-sea sediments.