Stable Sulfur Isotopic Compositions Research Articles

Stable sulfur isotopes indicate net sulfate reduction in near-surface sediments of the deep Arabian Sea

Depth profiles of the stable sulfur isotopic composition of dissolved sulfate in near-surface sediments were measured at five stations of the deep Arabian Sea between 1918 and 4426 m water depth (WAST, WAST-Top, CAST, SAST, NAST), sampled in April 1997. The results clearly indicate that net microbial sulfate reduction took place in the sediments at stations WAST and NAST below about 12 cm depth. Sulfate reduction at WAST was more pronounced compared to station NAST, most likely due to higher organic carbon content in turbiditic sediments. No net sulfate reduction took place within the upper 10 cm of the surface sediments at all stations, and no significant isotopic indication for sulfate reduction was found down to 30 cm bsf at station SAST. Results are in accordance with accumulation of reduced isotopically light sulfur species below about 6 cm bsf at station WAST. It is concluded that the sulfur isotopic composition of remaining sulfate is more sensitive to net sulfate reduction than the [SO 4]/[Cl] ratio. The sulfur isotopic composition of a vertical profile for dissolved sulfate through the water column at station WAST was essentially constant (250–4047 m: δ 34 S=+20.49±0.08 ‰ vs. V-CDT n=8). A similar constancy (20–4565 m water depth δ 34 S=+20.57±0.06 ‰ vs. V-CDT n=15) was found for the station BIOTRANS in the northeastern Atlantic (47°11′ 19°33W), indicating that the oxygen minimum zone in the Arabian Sea has no influence on the sulfur isotopic composition of dissolved sulfate.

Early diagenetic alteration of organic matter by sulfate reduction in Quaternary sediments from the northeastern Arabian Sea

Hemipelagic sediments from the northeastern Arabian Sea off Pakistan were analyzed by geochemical and microscopical methods to study the importance of bacterial degradation processes on the quantity and composition of sedimentary organic matter (OM). On the basis of elemental analysis of total organic carbon (TOC) and total sulfur (TS), the decay of OM by sulfate-reducing bacteria were calculated. Concentrations of total iron, reactive iron, total nitrogen (TN), total phosphorus (TP) and hydrogen indices (HI) were used to assess organic matter reactivity and redox conditions. Stable sulfur isotopic compositions provided further information about pathway, timing and mechanisms of sulfate reduction. Microscopical observations indicate that throughout the study area, OM is amorphous (75–95%) in almost all samples with only a small contribution of terrigenous OM. Bacterially mediated sulfate reduction leads to continuously decreasing TOC/TS ratios from 0.4 m to a burial depth of 2–3 m. At this depth, the anaerobic oxidation of particulate organic matter ceases. Sulfate reduction in this narrow interval accounts for the decay of up to 70% of the OM which has primarily entered the anoxic domain. During the bacterially mediated diagenesis preferential consumption of nitrogen and phosphorus containing organic compounds can be observed and quantified. The regeneration of up to 60% of TN and more than 50% of TP can lead to a feedback of nutrients to the seawater. This recycling process may contribute to the high productivity in the northeastern Arabian Sea.

Stable carbon and sulfur isotope distributions of crude oil and source rock constituents from Burgan and Raudhatain oil fields (Kuwait)

Seven crude oils and ten possible source rocks from Kuwait have been investigated for the stable carbon and sulfur isotopic compositions of chromatographic oil and extract fractions, asphaltenes, kerogen, carbonates and S-bearing minerals. The crudes are isotopically very similar and suggest a single organic source. They also show characteristics intermolecular isotopic distribution patterns.On the basis of the criteria: organic richness [extractable organic matter-total organic carbon (EOM-TOC) relationships], carbon and sulfur isotopic similarities of oils, extracts, pyrite and kerogen, samples from the Ratawi Formation (Raudhatain oil field) and the Minagish Formation (Burgan oil field) showed indications of source potential. The excellent isotopic correlations with the sample from the Mauddud Formation (Raudhatain oil field) are explained by impregnation of rock samples by reservoir oil. In the core samples, an increase in the organic matter content is correlated with13C enrichment in the carbonates.The results demonstrate the potential of stable carbon and sulfur isotope correlation methods in geochemical hydrocarbon exploration.

Stable sulfur isotopic distributions and sulfate reduction in lake sediments of the Adirondack Mountains, New York

Cores from five lakes of the Adirondack Mountains, New York, were analyzed for sedimentary sulfur concentrations and stable sulfur isotopic compositions. Isotopic values of total sedimentary sulfur were as much as 6 to 8‰ lower than isotopic values of sulfur sources (soils, tree leaves and lake water sulfate) which showed little isotopic variation in the Adirondacks. The low isotopic values of recent sediments indicate increased sulfate reduction, probably in response to increased sulfate loading and acid deposition. Detailed historical reconstruction of sulfate loading histories from sedimentary sulfur profiles is premature, however, since model calculations indicate that sulfur can be added to deeper older sediments by ongoing, contemporary sulfate reduction.

Stable sulfur and nitrogen isotopic compositions of crude oil fractions from Southern Germany

Eleven samples of crude oil from the Molasse Basin of Southern Germany were fractionated and their contents of sulfur and nitrogen as well as the stable isotope compositions of these elements ( 34S 32S and 15N 14N , resp.) investigated. According to the δ 34 S determinations, all crude oils from the Tertiary base of the Western and Eastern Molasse belong to one oil family and differ significantly from the Triassic and Liassic oils in the Western Molasse. An enrichment of 34S was observed with increasing polarity of crude oil fractions. The isotope distributions of sulfur in the polar constituents of the biodegraded oils from the sandstones of Ampfing, however, approach a homogeneous distribution. The nitrogen isotope distribution is rather uniform in Southern German oils. A regional differentiation can be recognized, although the overall isotopic variation is small. The δ 15 N values of the crudes and asphaltenes do not correlate.