BENTHIC SULFUR CYCLING IN THE SEDIMENTS FROM THE OXYGEN MINIMUM ZONE OFF BAJA CALIFORNIA

One of the most pronounced oceanographic features in the northeast Pacific Ocean is the oxygen minimum zone (OMZ) located at water depth ranging from 500 m to 900 m. In the sediment underlying the low oxygenated water in the OMZ, respiration of organic matter is sufficient to cause sulfate reduction. We present here pore water concentration depth profiles of sulfide from a suite of multi-cores collected at 23.5 ^oN on the open margin off Baja California and nearby Soledad Basin (200 m sill) to compare sulfide production under a range of bottom water oxygen conditions. The sulfide concentrations, measured on board of R.V. Melville between Nov. and Dec. 1999 by cathodic square wave stripping voltammetry and by methylene blue colorimetry ranged from a few nM to millions of nM.

At several sites with laminated sediment within the OMZ (oxygen 1 to 5 mM) on the open margin, sulfide concentrations rose to a maximum of 1 mM at 10 cm depth. In contrast, three sites with laminated sediments within the Soledad Basin with similar bottom water dissolved oxygen concentration, sulfide concentrations rose to a maximum of 1000 mM at 10 cm depth. We postulate that such a contrast of sulfide concentration profiles may be explained on the first order by the difference of sedimentation rates between the two regions. An improved understanding of benthic sulfur cycling will shed light on the mechanisms of enrichment of many elements in the sediments that interact with sulfide, such as Cd, Mo, As, etc.