SULFUR, IRON AND ORGANIC-CARBON FLUXES IN THE BLACK-SEA - SULFUR ISOTOPIC EVIDENCE FOR ORIGIN OF SULFUR FLUXES

In the southern and central Black Sea, reduced sulfur comprises nearly 1% of the total particulate flux in the anoxic water column, as measured by time-series sediment traps. Sulfur isotopic composition of particulate sulfide fluxes indicates an origin near the oxic-anoxic interface, where dissolved sulfide is isotopically heavier than dissolved sulfide below 175 m due to chemical oxidation. A strong seasonal correlation exists between sulfur and organic carbon fluxes; this, together with particulate sulfide precipitation at the oxic-anoxic interface, suggests that sulfides are scavenged from the interface during maximum production of settling organic aggregates, particularly during the late summer coccolithophorid blooms. Sulfide precipitation deeper in the water column and in sediments may be limited by availability of iron and/or polysulfides, since there are no particulate sulfide fluxes which have the same isotopic composition as ambient dissolved sulfide at trap depths. Since sulfides in surface sediments are often isotopically similar to dissolved sulfide at the top of the sulfide zone and to sediment trap fluxes, sedimentary sulfides could originate from metal sulfide fluxes forming at the oxic-anoxic interface and would reflect sulfur-cycling processes at the oxic-anoxic interface. Sedimentary sulfate reduction rates estimated from assuming 65-80% utilization of particulate organic carbon fluxes agree reasonably well with other estimates of sulfate reduction and sulfide production rates. A sulfur isotope-based box-model for sulfur cycling between dissolved and particulate phases in the water column and sediments is presented.