Sulfur diagenesis and partitioning in Holocene Peru shelf and upper slope sediments

Sediments of the Peru margin were sampled in two transects across the shelf and upper slope (75 to 1000 m) in October of 1992. This region is characterized by high surface-water productivity and consequent high organic carbon fluxes, low terrigenous inputs, as well as an intense oxygen-minimum zone (OMZ) impinging on the slope at depths between 75 and 550 m. Sediments analyzed in this study were collected from one ship-deployed box core and five submersible-deployed push cores. Organic carbon (TOC) concentrations in surface sediments range from 1 to 18% on a dry weight basis. Formation of pyrite begins at the sediment/water interface (SWI). Maximum concentrations of pyrite sulfur in the cores from this study range from 0.5 to 1.8 wt.%. Concentrations of pyrite are influenced by availability of reactive iron and an oxidant capable of converting AVS to pyrite. Highly reactive iron is stripped from terrigenous detritus in the core of the OMZ. Some of this iron may contribute to glauconite formation in sediments near the base of the OMZ, where it is concentrated into a glauconitic green sand as a result of reworking and winnowing. In a shallow-water site (109 m) at the upper edge of the OMZ, high concentrations of pyrite, total iron and iron/aluminum ratios indicate that easily reduced iron species are available for sulfidation at the SWI. Pyrite formation at this site is nearly complete in the upper 4 cm of the sediment. In contrast, pyrite concentrations are slightly lower at the surface of two of the deeper-water sites (at 309 m, in the center of the OMZ, and to a lesser extent at 671 m, at the bottom edge of the OMZ), and appear to be influenced, in part, by availability of reactive iron. Concentrations of acid-volatile sulfides (AVS) are low in all sediments (≤ 0.04 wt.%), but are slightly elevated in sediments from the center of the OMZ. The ratio of organic sulfur (S-org)to organic carbon (TOC)at the sediment surface of the cores examined here is remarkably constant (0.012 ± 0.002) and within the range observed in marine biota (0.01 to 0.02), suggesting that organic sulfur at the SWI is dominantly biogenic. Although S-org/TOC ratios increase downcore, there is no clear correlation between concentrations of organic sulfur and organic carbon below the SWI. There is also no clear relation between concentrations of organic sulfur and total iron. Organic sulfur concentrations vary from site to site (0.06 to 0.33 wt.%), but are relatively constant downcore at any given site. Maximum concentrations are in cores from the center of the OMZ.