Dissolved sulfide distributions in the water column and sediment pore waters of the Santa Barbara Basin

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Geochimica Et Cosmochimica Acta
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Dissolved sulfide concentrations in the water column and in sediment pore waters were measured by square-wave voltammetry (nanomolar detection limit) during three cruises to the Santa Barbara Basin in February 1995, November-December 1995, and April 1997. In the water column, sulfide concentrations measured outside the basin averaged 3 +/- 1 nM (n = 28) in the 0 to 600 m depth range. Inside the basin, dissolved sulfides increased to reach values of up to 15 nM at depths >400 m. A suite of box cores and multicores collected at four sites along the northeastern flank of the basin showed considerable range in surficial (<0.5 cm) pore-water sulfide concentrations: <0.008, 0.01, 0.02, to as much as 0.4 mu M at the 340, 430, 550, and 590 m sites, respectively. At a core depth of 10 cm, however, pore-water sulfides exhibited an even wider range: 0.005, 0.05, 0.1, and 100 mu M at the same sites, respectively. The sulfide flux into the deep basin, estimated from water-column profiles during three cruises, suggests a fairly consistent input of 100-300 nmole m(-2) h(-1). In contrast, sulfide fluxes estimated from pore-water sulfide gradients at the sediment water interface were much more variable (-4 to 13,000 nmole m(-2) h(-1)). Dissolved silicate profiles show clear indications of irrigation at shallow sites (340 and 430 m) in comparison to deeper basin sites (550 and 590 m) with low (<10 mu M) bottom-water dissolved-oxygen concentrations. Pore-water profiles indicate ammonia generation at all sites, but particularly at the deep-basin 590 m site with concentrations increasing with sediment depth to >400 mu M at 10 cm. Decreases in water-column nitrate below the sill depth indicate nitrate consumption (-55 to - 137 mu mole m(-2) h(-1)) similar to nearby Santa Monica Basin. Peaks in pore-water iron concentrations were generally observed between 2 and 5 cm depth with shallowest peaks at the 590 m site. These observations, including observations of the benthic microfauna, suggest that the extent to which the sulfide flux, sustained by elevated pore-water concentrations, reaches the water column may be modulated by the abundance of sulfide-oxidizing bacteria in addition to iron redox and precipitation reactions. Copyright (C) 1999 Elsevier Science Ltd.


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