Alon Amrani

Compound-Specific δ34S Analysis of Volatile Organics by Coupled GC/Multicollector-ICPMS

We have developed a highly sensitive and robust method for the analysis of delta(34)S in individual organic compounds by coupled gas chromatography (GC) and multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). The system requires minimal alteration of commercial hardware and is amenable to virtually all sample introduction methods. Isobaric interference from O(2)(+) is minimized by employing dry plasma conditions and is cleanly resolved at all masses using medium resolution on the Thermo Neptune MC-ICPMS. Correction for mass bias is accomplished using standard-sample bracketing with peaks of SF(6) reference gas. The precision of measured delta(34)S values approaches 0.1 per thousand for analytes containing >40 pmol S and is better than 0.5 per thousand for those containing as little as 6 pmol S. This is within a factor of 2 of theoretical shot-noise limits. External accuracy is better than 0.3 per thousand. Integrating only the center of chromatographic peaks, rather than the entire peak, offers significant gain in precision and chromatographic resolution with minimal effect on accuracy but requires further study for verification as a routine method. Coelution of organic compounds that do not contain S can cause degraded analytical precision. Analyses of crude oil samples show wide variability in delta(34)S and demonstrate the robustness and precision of the method in complex environmental samples.

Sulfur isotope homogeneity of oceanic DMSP and DMS

Significance Oceanic emissions of volatile dimethyl sulfide (DMS) represent the largest natural source of biogenic sulfur to the global atmosphere, where it mediates aerosol dynamics and may affect climate. Sulfur isotope ratios (34S/32S) offer a way to estimate oceanic DMS contribution to aerosols. We used a unique method for the analysis of 34S/32S of DMS and its precursor, dimethylsulfoniopropionate (DMSP), in a range of marine environments. Surface water collected from six different ocean provinces revealed a remarkable consistency in 34S/32S ratios of DMS and DMSP ranging between +18.9 and +20.3‰. The 34S/32S of oceanic DMS flux to the atmosphere is thus relatively constant and distinct from anthropogenic sources of atmospheric sulfate, thereby enabling estimation of the DMS contribution to aerosols. Oceanic emissions of volatile dimethyl sulfide (DMS) represent the largest natural source of biogenic sulfur to the global atmosphere, where it mediates aerosol dynamics. To constrain the contribution of oceanic DMS to aerosols we established the sulfur isotope ratios (34S/32S ratio, δ34S) of DMS and its precursor, dimethylsulfoniopropionate (DMSP), in a range of marine environments. In view of the low oceanic concentrations of DMS/P, we applied a unique method for the analysis of δ34S at the picomole level in individual compounds. Surface water DMSP collected from six different ocean provinces revealed a remarkable consistency in δ34S values ranging between +18.9 and +20.3‰. Sulfur isotope composition of DMS analyzed in freshly collected seawater was similar to δ34S of DMSP, showing that the in situ fractionation between these species is small (<+1‰). Based on volatilization experiments, emission of DMS to the atmosphere results in a relatively small fractionation (−0.5 ± 0.2‰) compared with the seawater DMS pool. Because δ34S values of oceanic DMS closely reflect that of DMSP, we conclude that the homogenous δ34S of DMSP at the ocean surface represents the δ34S of DMS emitted to the atmosphere, within +1‰. The δ34S of oceanic DMS flux to the atmosphere is thus relatively constant and distinct from anthropogenic sources of atmospheric sulfate, thereby enabling estimation of the DMS contribution to aerosols.

A sensitive method for the sulfur isotope analysis of dimethyl sulfide and dimethylsulfoniopropionate in seawater.

RATIONALE Dimethyl sulfide (DMS) is the major volatile sulfur species emitted to the atmosphere from the oceans. The sulfur isotope ratio ((34)S/(32)S) of DMS may offer a way to calculate the contribution of marine DMS to global sulfur cycling. The S-isotopic analysis of DMS is difficult due to its low concentrations in natural seawater and high chemical reactivity. Here we present a sensitive, precise and accurate method for determining the S-isotopic composition of natural DMS and its precursor, dimethylsulfoniopropionate (DMSP), in seawater. METHODS The method was based on a purge of DMS from aqueous solutions or natural seawater to a cryogenic trap and subsequent separation of DMS by gas chromatography. The separated DMS was then transferred from the gas chromatograph to a multicollector inductively coupled plasma mass spectrometer (GC/MC-ICPMS system) for measurement of (34)S/(32)S ratios. Correction for mass bias was accomplished using standard-sample bracketing with peaks of SF6 as a reference gas. RESULTS Results obtained from synthetic DMS and DMSP dissolved in artificial seawater show >98% recovery of DMS and very good precision (0.1 to 0.3‰), accuracy and linearity (0.2‰) for the 26-179 picomoles (pmol) of DMS or DMSP injected. The system was tested with natural seawater from Eilat (Red Sea, Israel) and similar precision and accuracy for both DMS and DMSP were obtained. The δ(34)S values of DMS and DMSP from Eilat seawater were 19.2 ± 0.2‰ and 19.7 ± 0.2‰, respectively. CONCLUSIONS The coupling of a purge-and-trap system with a GC/MC-ICPMS system was shown to be a sensitive, accurate and robust approach for the S-isotope analysis of nanomolar (nM) concentrations of DMS and DMSP from aqueous solutions and natural seawater.

Significance of δ34S and evaluation of its imprint on sedimentary organic matter: I. The role of reduced sulfur species in the diagenetic stage: A conceptual review

Both carbon and sulfur cycles in the geosphere are biogenically and chemically interwoven. Sulfate is the main source for sulfur in marine sediments. The incorporation of sulfur into biogenic organic matter (OM) via the assimilatory process has very little isotopic discrimination. The pioneering work by Kaplan and Rittenberg showed that sulfate-reducing bacteria (SRB) oxidize organic carbon to CO2 while producing H2S depleted in the 34S isotope. The use of sulfate as an electron acceptor during bacterial dissimilatory processes produces H2S that can be up to 72% depleted in 34S relative to the sulfate. Carbon source, SRB species and hence rate of sulfate reduction may influence the overall isotopic fractionation ΔSO42- → S2-. In addition, the supply of sulfate (open versus closed system) is important for determining isotopic fractionation. The H2S formed by the SRB quickly reacts with available iron to form pyrite (FeS2) via the precursor FeS. Sulfide-oxidizing bacteria may form elemental sulfur, that at pH ~7–9, reacts with sulfide to form polysulfides. Polysulfides were found to be chemically the most reactive species of sulfur with OM. Isotopically, polysulfides carry the dissimilatory δ34S value. Hence, if the secondary sulfur enrichment in sedimentary organic matter (SOM) is by chemical reaction with the polysulfides, then the δ34S values for the OM, rich in sulfur, will gradually be imprinted by the dissimilatory process. Most of the information on δ34S values of the reduced sulfur in sediments derives from both acid “volatile” sulfide (FeS) and pyrite (FeS2). In a few cases, both sulfate and other sulfur species were isotopically compared. Due to analytical difficulties, organic sulfur and elemental sulfur isotope (δ34S) ratios were studied only in cases where the secondary enrichment led to OM rich in sulfur. This secondary enrichment forms type II-S kerogens. Three such natural cases of secondary enrichment of OM will be discussed: (a) Solar lake—young cyanobacterial mat (Sinai, Egypt); (b) Dead Sea—immature asphalts and bituminous rocks (Senonian Ghareb Formation, Israel); (c) Monterey Formation selected samples (Miocene Formation, California, USA). These three case studies are typified by low-medium maturity. The Solar Lake mats are at the early stages of diagenesis, the Monterey and the Ghareb Formations have already formed type II-S kerogens. In most cases, the pyrite records the most 34S-depleted sulfur in the sediments and sedimentary rocks, whereas sulfate is the most 34S enriched. The organically bonded sulfur has a wider range of isotopic compositions probably due to its dependence on timing and multiple step reactions discussed in this review. It is our intention in this review to offer a feasible mechanistic approach to connect δ34S ratios recorded with depositional environment and diagenetic processes.

Olive oil storage during the fifth and sixth millennia BC at Ein Zippori, Northern Israel

Several occupation levels dating to the sixth to fifth millennia BC (the Wadi Rabah and pre-Ghassulian Late Neolithic and Chalcolithic cultures as well as the Early Bronze Age IB–II) were found in a salvage excavation conducted at Ein Zippori in the lower Galilee. Pottery vessels from the different periods were sampled for organic residue analysis study and were analyzed using gas chromatography (GC) coupled with mass spectrometry (GC-MS). Olive oil was one of the most common organic residues detected in the vessels, from the levels of the Wadi Rabah occupation and onwards (sixth to fifth millennia BC). This find throws new light on the exploitation of olives in the southern Levant as well as on the large-scale production and consumption of olive oil in the Late Pottery Neolithic and pre-Ghassulian Chalcolithic times.

Significance of δ34S and evaluation of its imprint on sedimentary sulfur rich organic matter II: Thermal changes of kerogens type II-S catagenetic stage controlled mechanisms. A study and conceptual overview

Abstract Kerogens of type II-S are rich in sulfur, containing up to 10–12% organically bound sulfur. Most of this sulfur is thermally unstable due to the presence of catanated poly-S-S linkages. The δ 34 S values for these kerogens carry the imprint of the pore water polysulfides introduced into the organic matter at the diagenetic stage as described in the previous review (Part I). The catagenetic stage, covered in Part II, is mostly driven by the increase of temperature, leading to rearrangement of both carbon and sulfur bonds that are reformed thermally to stabilized alicyclic and aromatic sulfur-containing structures. The controlling factors for δ 13 C changes during these modifications are the release of CO 2 and C 1 -C 5 hydrocarbons, mostly CH 4 . The sulfur stabilization releases H 2 S and S° during the forming of the C-S-C moieties and their aromatization. The present report and review examines the influence of the above geochemical changes and the mechanisms controlling them on the stable isotope distribution of the thermally derived products. The understanding of these changes can lead to a better correlation between potential source rocks (PSR) and petroleum generated from them. The released carboncontaining molecules, i.e. CO 2 and CH 4 are chemically stable and not reactive (non-reversible reactions); in contrast at elevated temperature the sulfur released (H 2 S, S°) can re-react with the organic matter if not removed. It is therefore very important to examine the sulfur functionality changes during catagenesis that are thermally controlled through the mechanisms leading to sulfur isotope ratios variation. In addition, whether the system is open or closed influences the free radical restructuring of the organic matter and hence will influence the isotopic distribution of sulfur. The thermal cleavage and restructuring of kerogen to produce oil has an impact on the δ 13 C of the asphalts and petroleum of 2%. Moreover, the various fractions such as gas (CH 4 ), saturates, aromatics, resins and asphaltenes show different δ 13 C ranges. The most depleted in 12 C is methane. Despite the recognized impact of maturity on carbon isotopes ratios, it has been previously suggested that the decrease in concentration of sulfur from kerogens of type II-S during maturation to generate oil does not cause δ 34 S changes. While many hydrous pyrolysis and “dry” pyrolysis thermal simulation experiments were carried out and the thermal behavior of kerogens (type II-S) was studied, very few of these experiments were monitored for δ 34 S changes. However, in the last 15 years some studies showed that the loss of sulfur and associated thermal stabilization is reflected in 32 S enrichment in H 2 S and concurrently, 34 S enrichment of the petroleum produced. Based on these experiments we will offer mechanisms for the observed trend. Some new laboratory experiments performed by us in both closed and open systems are reported. Only very rough examination of the various organic sulfur-containing fractions was carried out in these studies. Some natural (geological) sites such as the Monterey Formation (Miocene, California, USA) and Senonian Formation (Dead Sea Area, Israel) are presented for comparison. In the general scheme, the δ 34 S signature recorded in the kerogen changes during catagenesis to form petroleum depleted in sulfur and isotopically heavier. This enrichment in 34 S could amount to + 4 to + 8% relative to the kerogen of the PSR in thermally controlled experiments. In a field-based source rock to oil generated comparison, the isotope discrimination could be even higher, leading also to secondary metal sulfides (including relatively heavy pyrite). The chemically controlled thermal sulfate reduction ( ≧ 200°C) is discussed only briefly.

Isotopic evidence for the origin of dimethylsulfide and dimethylsulfoniopropionate-like compounds in a warm, monomictic freshwater lake

Environmental context The volatile sulfur compound, dimethylsulfide (DMS), plays a major role in the global sulfur cycle by transferring sulfur from aquatic environments to the atmosphere. Compared to marine environments, freshwater environments are under studied with respect to DMS cycling. The goal of this study was to assess the formation pathways of DMS in a freshwater lake using natural stable isotopes of sulfur. Our results provide unique sulfur isotopic evidence for the multiple DMS sources and dynamics that are linked to the various biogeochemical processes that occur in freshwater lake water columns and sediments. Abstract The volatile methylated sulfur compound, dimethylsulfide (DMS), plays a major role in the global sulfur cycle by transferring sulfur from aquatic environments to the atmosphere. The main precursor of DMS in saline environments is dimethylsulfoniopropionate (DMSP), a common osmolyte in algae. The goal of this study was to assess the formation pathways of DMS in the water column and sediments of a monomictic freshwater lake based on seasonal profiles of the concentrations and isotopic signatures of DMS and DMSP. Profiles of DMS in the epilimnion during March and June 2014 in Lake Kinneret showed sulfur isotope (δ34S) values of +15.8±2.0 per mille (‰), which were enriched by up to 4.8 ‰ compared with DMSP δ34S values in the epilimnion at that time. During the stratified period, the δ34S values of DMS in the hypolimnion decreased to –7.0 ‰, close to the δ34S values of coexisting H2S derived from dissimilatory sulfate reduction in the reduced bottom water and sediments. This suggests that H2S was methylated by unknown microbial processes to form DMS. In the hypolimnion during the stratified period DMSP was significantly 34S enriched relative to DMS reflecting its different S source, which was mostly from sulfate assimilation. In the sediments, δ34S values of DMS were depleted by 2–4 ‰ relative to porewater (HCl-extracted) DMSP and enriched relative to H2S. This observation suggests two main formation pathways for DMS in the sediment, one from the degradation of DMSP and one from methylation of H2S. The present study provides isotopic evidence for multiple sources of DMS in stratified water bodies and complex DMSP–DMS dynamics that are linked to the various biogeochemical processes within the sulfur cycle.

Compound-Specific Sulfur Isotope Analysis of Petroleum Gases

We describe a simple, sensitive, and robust method for sulfur isotope ratio (34S/32S) analysis of ppm-level organic sulfur compounds (OSCs) in the presence of percent-level H2S. The method uses a gas chromatograph (GC) coupled with a multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). The GC, equipped with a gas inlet and a valve that transfers the H2S to a thermal conductivity detector (TCD), enables a precise heart cut and prevents the saturation of the MC-ICPMS. The sensitivity and accuracy of the method are better than 0.3‰ for OSCs at a concentration of 25 pmol or 1.4 ppm, and better than 0.5‰ for concentrations ≥0.7 ppm of OSCs. An order of magnitude increase in sensitivity, with no effect on accuracy, can be achieved if the loop volume (0.5 mL) is changed to 5 mL. High concentrations of methane (95% v/v) and/or H2S (20% v/v) had no effect (within 0.5‰) on the precision and accuracy of the gas sample containing 2 ppm of OSCs after heart cut. The applicability and robustness of this method are demonstrated on a gas sample (10% v/v H2S) that was produced by pyrolysis of sulfur-rich kerogen. The results show good precision and reveal sulfur isotope variability between individual OSCs that may represent key processes during formation and degradation of OSCs.