Variations and Controls of Sulfate Reduction in the Continental Slope and Rise of the Ulleung Basin off the Southeast Korean Upwelling System in the East Sea

Abstract

Despite its importance in carbon mineralization pathways, relatively little is known about sulfate reduction along a continental slope and rise. We investigated the temporal and spatial variations and controls of sulfate reduction rates (SRRs) in the continental margin sediment of the Ulleung basin (UB), off the southeast Korean upwelling system in the East Sea. SRRs ranged from 1.22 to 8.07 mmol m-2 d-1 at the slope sites and from 0.69 to 3.18 mmol m-2 d-1 at the basin sites. These values were exceptionally higher than those observed within other marginal seas and were comparable to SRR values at the same depth range in the Peruvian and Chilean upwelling systems. The high sulfate reduction in the UB was attributable to enhanced primary production in the water column associated with coastal upwelling and a high export flux of large organic particles into the basin via the Ulleung warm eddy. The depth integrated SRR was approximately 4 times higher in the highly productive spring (4.91 +/- 2.55 mmol m-2 d-1) than during the summer, which exhibited a stratified water column (1.28 +/- 0.48 mmol m-2 d-1). In the meantime, despite the high organic carbon content ( 2.5% dry wt.) in the UB, the SRR was consistently suppressed in the Mn oxide-enriched (174 mol cm-3) surface sediments of the continental rise. Overall, these results indicate that the production of organic carbon in the water column and its subsequent deposition on the seafloor is the primary source controlling the temporal variability of sulfate reduction, whereas the Mn oxides that were enriched in the basin are responsible for the spatial variability of the SRR in the UB. Carbon oxidation in the UB accounted for approximately 30% of primary production and approximately 60% of the export flux, indicating that the continental slope and rise of the UB is a region of rapid organic carbon turnover and nutrient regeneration.