SULFIDE RELEASE FROM ESTUARINE SEDIMENTS UNDERLYING ANOXIC BOTTOM WATER

Independent measurements of dissolved sulfide (DS) production in and release from mesohaline Chesapeake Bay sediments underlying anoxic bottom water were made during summer. DS accumulated under benthic chambers at a rate of 27.3+/-8.2 mmol m-2 d-1. Rates of DS release ranged from 60 to 80% of depth-integrated (0-12 cm) sulfate reduction (SR) rates (average 39.0+/-9.0 nmol m-2 d-1) determined with (SO42-)-S-35. The balance between DS production, accumulation in pore waters, and sediment-water exchange was examined by steady state and transient state transport-reaction modeling, with a DS sediment diffusion coefficient derived from that determined for sulfate by the instantaneous source technique. The results indicate that DS transport is influenced by processes other than vertical molecular diffusion, most likely gas bubble ebullition driven by methane production beneath the SR zone. Although such processes may significantly reduce pore-water DS accumulation (i.e. promote DS release), the models show that most (65-95%) DS formation during summer is not stored in the DS pool regardless of the transport processes occurring. The observed rates of DS production and release correspond to a potential oxygen demand of 1-2 g O2 m-2 d-1, equal to or greater than estimated rates of eddy-diffusive bottom-water reaeration during summer stratification. Thus, sulfur cycling alone can maintain anoxia in these subpycnocline waters during summer under quiescent water conditions.