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Abstract

Irrigation and bioturbation lead to transport of dissolved oxygen into anoxic sediments. The depth-distribution of local oxygen input and the total time of oxygen presence was measured at randomly sampled locations within anoxic sediments, originating from the North Sea.
In the laboratory, continuous redox potential records, displaying a transient increase and decrease when in contact with oxygen for a limited time, were used to record 'oxidation events'. Oxygen microelectrodes were used to confirm the presence of oxygen. Measurements were performed for 180 d at 230 locations in the top 6 cm of sediment cores containing the natural macrofauna community.
4783 oxidation events . m(-2). d(-1) were recorded within the upper 6 cm of the sediment. The number and duration of oxidation events declined with sediment depth below the oxic zone. Oxygen was present in the anoxic sediment, as determined from redox potential measurements, greater than or equal to 6 h per day at 10% of the randomly chosen locations within the upper 3 cm. The overall distribution of oxidation events and their duration suggest that local, pulsed additions of oxygen by irrigation may be sufficient to maintain an oxidised sediment layer (sensu JORGENSEN & REVSBECH, 1989). Oxic environments along burrow walls. rapidly fluctuate between oxic and anoxic conditions. Using oxygen microelectrodes the presence of oxygen toxic conditions) in these halos was found to range from 2 to 12 h per day. Continuous redox measurements show that oxidised conditions fluctuate with the oxygen pulses and display the same durations, although these may range up to 21 h per day. Oxic and oxidised sediment volumes are estimated to represent < 1% and 3.7%, respectively, of the anoxic sediment to 6 cm depth. Recognition of temporal variability and spatial heterogeneity of sedimentary conditions may prove valuable for future conclusions drawn in other research fields.