Spatial and Temporal Distribution of Oxidation Events Occurring Below the Sediment-Water Interface

Abstract. Irrigation and bioturbation lead to transport of dissolved oxygen into anoxic sediments. The depthdistribution 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 confm 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, ≥ 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 J orgensen amp; R evsbech , 1989). Oxic environments along burrow walls rapidly fluctuate between oxic and anoxic conditions. Using oxygen microelectrodes the presence of oxygen (oxic 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.     

Dissolved Fe2+ and ∑H2S Behavior in Sediments Seasonally Overlain by Hypoxic-to-anoxic Waters as Determined by CSV Microelectrodes

Variability of dissolved Fe²⁺ and ∑H₂S concentrations in porewaters were studied, using cathodic stripping voltammetry (CSV) microelectrodes, in sediments overlain by hypoxic waters in the summer from the southeastern region of Corpus Christi Bay, Texas (CCB) and the Mississippi River Bight (MRB), Louisiana. These measurements were complimented by sediment microcosm studies where oxygen concentrations in the overlying water were manipulated. Sulfate reduction rates, benthic oxygen demand, total reduced sulfide, porewater sulfate, and total organic carbon were also determined. Fe²⁺ and ∑H₂S were the major dissolved redox-reactive dissolved species in these sediments. During hypoxic conditions, an upward migration of porewater Fe²⁺ and ∑H₂S occurred, with Fe²⁺ reaching much higher maximum concentrations than ∑H₂S. Statistically significant (p < 0.05) differences in both Fe²⁺ and ∑H₂S occurred between sediments at the CCB and MRB study sites. Although both sites were Fe-dominated, reactive and dissolved iron were higher while ∑H₂S was lower at the MRB site. However, there were no statistically significant (p > 0.05) difference in regard to ∑H₂S between microcosm and field monitoring studies. Results indicated that, for Fe²⁺ and ∑H₂S, relatively large and rapid changes occurred in both the concentrations and distributions of these important porewater constituents in response to relatively short-term changes in overlying water oxygen content. Model calculations indicated that conditions in the sediments can be responsible for the induction of hypoxic conditions in the formation of hypoxic overlying waters in <6 days at CCB and ~20 days at MRB.