Sulphur cycling in organic-rich marine sediments from a Scottish fjord

In this study, the biogeochemical transformations of sulphur in organic‐rich marine sediments in a Scottish fjord are investigated by a combination of pore water and sediment geochemistry with sulphide diffusive gradient thin‐film probes and sulphate isotopic data (δ³⁴S and δ¹⁸O). Particular attention is paid to sulphur cycling in the upper sediment profile where sulphate reduction occurs but free sulphide is below the detection limits of conventional pore water geochemical analysis but quantifiable by sulphide diffusive gradient thin film. In the uppermost part of the sediment core, δ¹⁸O sulphate decreased from near‐sea water values to +7‰, indicating that anoxic sulphide oxidation dominated during this interval. Sulphate δ³⁴S remained unchanged as there was no net sulphate reduction (i.e. reduction was balanced by re‐oxidation). Below 4 cm depth, there was a slight increase in sulphate δ³⁴S from 20‰ to 23‰ associated with minor accumulation of iron sulphide. The δ¹⁸O of the sulphate also increased, to around +10‰ at 10 cm depth, as a result of the isotopic exchange of sulphate–oxygen with pore water and/or sulphur disproportionation reactions mediated during sulphur cycling. These processes continued to increase the δ¹⁸O of the sulphate to 14‰ at 20 cm depth with no further change in the δ³⁴S of the sulphate. Below 20 cm depth, free sulphide is detectable in pore waters and both the δ³⁴S of the sulphate and sulphide increase with depth with an offset controlled by kinetic fractionation during bacterial sulphate reduction. The δ³⁴S of the sedimentary organic fraction shifted towards lower, more bacteriogenic, values with depth in the profile, without any increase in the size of this sulphur pool. Thus, the organic sulphur fraction was open to interaction with bacteriogenic sulphide without the occurrence of net addition. Therefore, caution should be exercised when using sulphur isotopic compositions to infer simple net addition of bacteriogenic sulphide to the organic sulphur fraction.     

Combined organic and inorganic geochemical reconstruction of paleodepositional conditions of a Pliocene sapropel from the eastern Mediterranean Sea

Layers of organic-carbon-rich sapropels in the sediment record of the Mediterranean Sea give evidence of repetitive changes in regional Plio-Pleistocene climate. Results from biomarker molecule and major and trace element analyses of closely spaced samples are used to reconstruct the conditions leading to deposition of a Pliocene sapropel at Ocean Drilling Program (ODP) Site 969 on the Mediterranean Ridge. Organic carbon concentrations increase from 0.2% outside the sapropel and peak to more than 30% within it. Major and trace elemental composition and biomarker-derived parameters indicate elevated productivity, depletion of water-column dissolved-oxygen content, and changes in sediment provenance in response to climatic changes. Budgets of rhenium, thallium, and other trace metals indicate that deep-water exchange between the Mediterranean subbasins and the Atlantic Ocean was not completely interrupted during sapropel formation. Enrichment factors of redox-sensitive and sulfide-forming trace metals as well as the presence of isorenieratene derivatives and high stanol/sterol ratios point to an extended zone of anoxic water masses. Depth profiles of biomarker compositions (sterols, long-chain alkenones, alkandiols and -ketols, fatty acids) indicate great floral diversity during deposition of a single sapropel and highlight the sensitive response of the marine community to variable environmental conditions. Changes in water mass circulation and eolian transport can be reconstructed by use of both lithogenic elements and average chain lengths of n-alkanes (ACL index).     

Sulphur isotopic investigation of lower carboniferous vein deposits of the British Isles

Similar characteristics and age data suggest that the Pb + Zn vein deposits hosted by Lower Palaeozoic and Vendian strata of the British Isles form a genetically related group, coeval with the Lower Carboniferous carbonatehosted base-metal deposits of Ireland. Sulphur isotopic data demonstrate that there were separate sulphide and sulphate sulphur sources for the vein minerals. ³⁴S values from minerals in individual vein systems are consistent but there is a large variation between deposits, revealing distinctive local sulphur sources. The data suggests that sulphide sulphur (total range of values of ³⁴S_H2S of -6.5 to 18.62) was derived from the underlying strata while the sulphate (baryte) sulphur source (³⁴S_baryte mainly in the range 12 to 24) was groundwater or surface water that mixed with rising hydrothermal fluids in the upper reaches of the veins.     

Extensive degradation and fractionation of organic matter during subsurface weathering

Organic carbon, sulphur, ¹³C_org, iron, manganese and calcium have been measured across a subsurface-weathering front in Pliocene sediments in southern Sicily. The results show an almost quantitative removal of C_org and sulphur and an increase in iron and manganese oxides over the weathering front, accompanied with a significant shift of the ¹³C_org to lower values. These data are among the first to support the rapid, extensive weathering of sedimentary organic matter and sulphur, a basic assumption made in global biogeochemical models on a Phanerozoic timescale.     

Geochemistry of Diagenesis of Organogenic Sediments: An Example of Small Lakes in Southern West Siberia and Western Baikal Area

Organogenic sediments (sapropels) in lakes are characterized by a reduced type of diagenesis, during which organic compounds are decomposed, the chemical composition of the pore waters is modified, and authigenic minerals (first of all, pyrite) are formed. Pyrolysis data indicate that organic matter undergoes radical transformatons already in the uppermost sapropel layers, and the composition of this organic matter is principally different from the composition of the organic matter of the its producers. The sapropels contain kerogen, whose macromolecular structure starts to develop during the very early stages of diagenesis, in the horizon of unconsolidated sediment (0–5 cm). The main role in the diagenetic transformations of organic matter in sediments is played by various physiological groups of microorganisms, first of all, heterotrophic, which amonifying, and sulfate-reducing bacteria. SO₄^2? and Fe²⁺ concentrations in the pore waters of the sediments are determined to decrease (because of bacterial sulfate reduction), while concentrations of reduced Fe and S species (pyrite) in the solid phase of the sediment, conversely, increase. Comparative analysis shows that, unlike sapropels in lakes in the Baikal area, sapropels in southern West Siberia are affected by more active sulfate reduction, which can depend on both the composition of the organic matter and the SO₄^2? concentration in the pore waters.     

Evolution of organic matter degradation in Cretaceous black shales inferred from authigenic barite: A reaction-transport model

A reaction-transport model was used to infer the long-term evolution of anaerobic organic matter degradation in Cretaceous black shales from the distribution of authigenic barite in sediments drilled at Demerara Rise (ODP Leg 207, Site 1258). In these sediments, sulfate-reduction and methanogenesis are the major pathways of organic matter decomposition and the depth-distribution of authigenic barite serves as an indicator for the temporal evolution of the sulfate-methane transition zone (SMTZ), the strength of the biogenic methane flux and, ultimately, the organic matter reactivity in the black shales over geological timescales. Organic matter degradation is described according to the reactive continuum model approach and parameters values are determined by inverse modeling, based on present-day porewater and authigenic barite profiles. Fully transient simulations were performed over a period of 100 Myrs and indicate that important features of the biogeochemical dynamics are associated to changes in the boundary forcing. Hiatuses in sediment accumulation rate result in quasi-steady-state conditions and lead to distinct accumulations of authigenic barites in the SMTZ. The inversely determined parameters reveal that the reactivity of the organic matter was already low (apparent first order rate constant ) at the time of its deposition in the Cretaceous. The geochemical characteristics of sediments drilled at Demerara Rise, as well as the presence of specific biomarkers, suggest that this low reactivity is most likely due to the euxinic palaeo-conditions which favored the sulfurization of the organic matter. Simulation results predict average initial organic carbon contents between 8.1 and 9.5 wt%, implying a high preservation efficiency of the organic matter (between 79% and 89%). Calculated mass accumulation rates (between 0.43 and 0.5 ) compare well with estimations for the western basin of the Cretaceous southern North Atlantic. Simulation results thus indicate that the enhanced preservation of organic matter under euxinic conditions may have been the main cause for the formation of organic-rich Cretaceous black shales at Demerara Rise.     

Sulphur isotopes in Lower Proterozoic iron and sulphide ores in northern Sweden

The present investigation deals with sulphur isotope distribution in Lower Proterozoic iron and sulphide mineralizations in northern Sweden. The contrasting sulphur isotope patterns are indicative of different genesis. Some 267 sulphur isotope analyses of pyrite, pyrrhotite, chalcopyrite, sphalerite, galena and bornite from 23 occurrences have been performed. Some deposits exhibit uniform compositions, although the mean ³⁴S values are clearly different, while other mineralizations have widely fluctuating values.The ³⁴S values in syngenetic, exhalative sedimentary skarn iron ores, quartz-banded iron ores and sulphide mineralizations of the 2.0–2.5 Ga old (Lapponian) Greenstone group show a large spread, supporting the existence of bacteriogenic sulphate reduction processes. The spread of the sulphur isotope values ( ³⁴S = -8 to +25), and the non-equilibrium conditions, point to a biogenic rather than to an inorganic reduction of seawater sulphate.The isotopic composition of the sulphides in the epigenetic Lannavaara iron ores which were formed by a hydrothermal scapolite-tourmalme-related process, indicates a sulphur source similar to that of the Greenstone group. The ³⁴S values of Cu-(Au) sulphide mineralizations in the Malmberget region (e.g. Aitik), which were formed by a similar process and hosted by the volcanics-volcanoclastics of the 1.9 Ga old Porphyry group, are slightly below zero , indicating a magmatic origin. The existence of different sulphur compositions for these mineralization types formed by a similar hydrothermal process, probably reflects the influence of the host rock, the solutions leaching pre-existing sulphides.In southern Norrbotten, epigenetic, Cu-Zn-Pb veintype mineralizations in metavolcanics and metasediments have ³⁴S values close to zero indicating a magmatic origin. The sulphur isotope data of the volcanogenic, massive sulphide ores of the Skellefte district, in particular the ores of the Adak dome, are close to zero .The lead and sulphur isotopic features of the sulphides in northern Sweden show that the ore-forming processes were of a different nature on both sides of the Archean-Proterozoic border, implying differences in the crustal development. Lead isotopes show that lead was mobilized from specific sources on each side of the border. The sulphur of the sulphides in the Greenstone group in NE Sweden and Finland was introduced by sedimentary processes, whereas the sulphur of the sulphide occurrences towards the SW, mainly in the Porphyry group, is dominated by a magmatic sulphur component.     

Sources and preservation of organic matter in the Cretaceous Toolebuc Formation, eastern Australia

The organic matter-rich Toolebuc Formation of eastern Australia was deposited in a Lower Cretaceous epicontinental sea. Parameters from biological marker studies indicate that the organic matter is immature to marginally mature for hydrocarbon generation. The occurrence of abundant coccoliths and the distribution of alkane biomarkers suggest that the organic matter (Type II) is largely of planktonic origin and only in the southeastern part of the depositional area can a terrestrial influence be discerned. Variations in kerogen composition can be attributed to the extent of the oxidation of the source materials and the degree of incorporation of sulphur. The atomic H/C ratios (c. 1.1) are remarkably constant for most of the Toolebuc Formation. Atomic O/C ratios vary from 0.1 and 0.4 and can be related both to depth and paleogeographic position. Kerogen sulphur contents range up to 7%, and the highest values occur in the most carbonate-rich sediments. Total sulphur (inorganic + organic) to carbon ratios in the sediments vary from 1 to <0.2 and are a function of paleogeographic position and lithology. Most of the sulphur in the sediments is in the form of pyrite, but the proportion of sulphur in organic form increases as the total sulphur content decreases. The evidence for oxidation of the organic matter and incorporation of sulphur into it during deposition suggests that bituminite, which is the dominant organic maceral in the Toolebuc Formation, was formed from an organic gel derived by decay of predominantly algal material. These data support a modified gyttja model (Kauffman, 1981) for the deposition of organic matter in the Toolebuc Formation.     

噻吩系列化合物的形成机理模拟

通过模拟实验发现硫能和各种链状化合物反应形成噻吩系列化合物。其反应机理可能是：硫先作用于链状化合物将其改造成共轭烯中间体,其他硫再从这个部位进入碳骨架从而形成该类化合物。硫还可进一步和噻吩系列反应形成更复杂的含硫芳烃,而温度升高会促进这种反应,所以噻吩系列的含量可以作为成熟度指标,在沉积条件相同的前提下,噻吩系列含量高指示成熟度低,反之则指示成熟度高。     

Contrasting sulphide contents of the Bushveld and Sudbury Igneous Complexes

The Sudbury Igneous Complex (SIC) contains abundant sulphides, especially near the base, and hosts one of the worlds largest nickel and copper deposits. The Bushveld Complex (BC) contains relatively little sulphide, but hosts the worlds largest platinum-group element deposits. The most recent calculations of the sulphur solubility in magmas that produced the BC are based on the sulphur solubility of mid-ocean ridge basalts that have less SiO₂ than Bushveld magmas. Such a difference may lead to an overestimation of sulphur solubility by as much as 25%. The revised sulphur solubility curve presented here for Bushveld magmas may also have relevance to the SIC in view of its siliceous nature. Sulphur solubility curves can be used to determine the proportion of sulphide expected in cumulate rocks once sulphur saturation is attained. These models are tested using observed sulphide contents in both intrusions. The observed decreasing sulphur contents (>0.3–0.05% S) from the base of the SIC upward are broadly consistent with these sulphur solubility curves, and are consistent with sulphide saturation through the entire mafic portion. In contrast, the lower half of the BC contains extremely little sulphur (generally <0.02% S), except for two thin layers, which is not consistent with sustained sulphide saturation at any level. Previous interpretations of the sulphur content of Bushveld rocks have suggested that the Lower and Critical Zones were sulphide saturated, but that they had then lost some of the sulphide due to various processes. The present sulphide content of the cumulates of the BC is so low that, if they had once been saturated, over 90% of all the sulphide must have been removed. Mass balance calculations indicate that these large amounts of displaced sulphur remain unaccounted for in such models. Instead, the observed sulphur contents are in reasonable agreement with that expected in a cumulate sequence forming from a sulphur-undersaturated magma. Whereas the Merensky Reef and Bastard pyroxenite contain minor sulphides, the compositions of the immediate hanging wall rocks indicate sulphide undersaturation. Such an abrupt return to sulphide undersaturation is not consistent with models involving sulphide formation from large volumes of magma. One possible explanation for these two observations is that intermittent sulphur degassing occurred through a fractured roof of the BC, so that the magma was never continuously sulphur-saturated with respect to an immiscible sulphide liquid.     

Experimental and Thermodynamic Constraints on the Sulphur Yield of Peralkaline and Metaluminous Silicic Flood Eruptions

Many basaltic flood provinces are characterized by the existenceof voluminous amounts of silicic magmas, yet the role of thesilicic component in sulphur emissions associated with trapactivity remains poorly known. We have performed experimentsand theoretical calculations to address this issue. The meltsulphur content and fluid/melt partitioning at saturation witheither sulphide or sulphate or both have been experimentallydetermined in three peralkaline rhyolites, which are a majorcomponent of some flood provinces. Experiments were performedat 150 MPa, 800–900°C, fO₂ in the range NNO –2 to NNO + 3 and under water-rich conditions. The sulphur contentis strongly dependent on the peralkalinity of the melt, in additionto fO₂, and reaches 1000 ppm at NNO + 1 in the most stronglyperalkaline composition at 800°C. At all values of fO₂,peralkaline melts can carry 5–20 times more sulphur thantheir metaluminous equivalents. Mildly peralkaline compositionsshow little variation in fluid/melt sulphur partitioning withchanging fO₂ (D_S 270). In the most peralkaline melt, D_S risessharply at fO₂ > NNO + 1 to values of >500. The partitioncoefficient increases steadily for S_bulk between 1 and 6 wt% but remains about constant for S_bulk between 0·5 and1 wt %. At bulk sulphur contents lower than 4 wt %, a temperatureincrease from 800 to 900°C decreases D_S by 10%. These results,along with (1) thermodynamic calculations on the behaviour ofsulphur during the crystallization of basalt and partial meltingof the crust and (2) recent experimental constraints on sulphursolubility in metaluminous rhyolites, show that basalt fractionationcan produce rhyolitic magmas having much more sulphur than rhyolitesderived from crustal anatexis. In particular, hot and dry metaluminoussilicic magmas produced by melting of dehydrated lower crustare virtually devoid of sulphur. In contrast, peralkaline rhyolitesformed by crystal fractionation of alkali basalt can concentrateup to 90% of the original sulphur content of the parental magmas,especially when the basalt is CO₂-rich. On this basis, we estimatethe amounts of sulphur potentially released to the atmosphereby the silicic component of flood eruptive sequences. The peralkalineEthiopian and Deccan rhyolites could have produced 10¹⁷ and10¹⁸ g of S, respectively, which are comparable amounts to publishedestimates for the basaltic activity of each province. In contrast,despite similar erupted volumes, the metaluminous Paraná–Etendekasilicic eruptives could have injected only 4·6 x 10¹⁵g of S in the atmosphere. Peralkaline flood sequences may thushave greater environmental effects than those of metaluminousaffinity, in agreement with evidence available from mass extinctionsand oceanic anoxic events. KEY WORDS: silicic flood eruptions; sulphur; experiment; Ethiopia; Deccan     

Images of concentrations of dissolved sulphide in the sediment of a lake and implications for internal sulphur cycling

The high-resolution mapping of dissolved sulphide in modern sediment of a permanently stratified (meromictic) lake is possible using a novel sulphide imaging technique (called a 'sulphur print'). The technique is simple, easy to use and can map a relatively large area (up to several dm²). In situ sulphur prints from anoxic fine-grained sediments are able to reveal internal structures in apparently homogeneous materials. Sulphur prints have been used to examine the formation and distribution of dissolved sulphide in the sediment and water column of the meromictic Lake Cadagno (southern Switzerland). The prints show clearly that a previously unknown laminar convective flow of sulphide-free porewater occurs across the sediment–water interface. Such convective flow out of the sediment must be accompanied by convective flow of sulphide- and sulphate-rich lake water into the sediment, and may be an important mechanism for the accumulation of sulphur in the sediment.     

A quantitative reconstruction of organic matter and nutrient diagenesis in Mediterranean Sea sediments over the Holocene

A multicomponent diagenetic model was developed and applied to reconstruct the conditions under which the most recent sapropel, S1, was deposited in the eastern Mediterranean Sea. Simulations demonstrate that bottom waters must have been anoxic and sulphidic during the formation of S1 and that organic matter deposition was approximately three times higher than at present. Nevertheless, most present day sediment and pore water profiles — with the exception of pyrite, iron oxyhydroxides, iron-bound phosphorus and phosphate — can be reproduced under a wide range of redox conditions during formation of S1 by varying the depositional flux of organic carbon. As a result, paleoredox indicators (e.g., C_org:S ratio, C_org:P_org ratio, trace metals) are needed when assessing the contribution of oxygen-depletion and enhanced primary production to the formation of organic-rich layers in the geological record. Furthermore, simulations show that the organic carbon concentration in sediments is a direct proxy for export production under anoxic bottom waters.The model is also used to examine the post-depositional alteration of the organic-rich layer focussing on nitrogen, phosphorus, and organic carbon dynamics. After sapropel formation, remineralisation is dominated by aerobic respiration at a rate that is inversely proportional to the time since bottom waters became oxic once again. A sensitivity analysis was undertaken to identify the most pertinent parameters in regulating the oxidation of sapropels, demonstrating that variations in sedimentation rate, depositional flux of organic carbon during sapropel formation, bottom water oxygen concentration, and porosity have the largest impact. Simulations reveal that sedimentary nutrient cycling was markedly different during the formation of S1, as well as after reoxygenation of bottom waters. Accumulation of organic nitrogen in sediments doubled during sapropel deposition, representing a significant nitrogen sink. Following reventilation of deep waters, N₂ production by denitrification was almost 12 times greater than present day values. Phosphorus cycling also exhibits a strong redox sensitivity. The benthic efflux of phosphate was up to 3.5 times higher during the formation of S1 than at present due to elevated depositional fluxes of organic matter coupled with enhanced remineralisation of organic phosphorus. Reoxygenation of bottom waters leads to a large phosphate pulse to the water column that declines rapidly with time due to rapid oxidation of organic material. The oxidation of pyrite at the redox front forms iron oxyhydroxides that bind phosphorus and, thus, attenuate the benthic phosphate efflux. These results underscore the contrasting effects of oxygen-depletion on sedimentary nitrogen and phosphorus cycling. The simulations also confirm that the current conceptual paradigm of sapropel formation and oxidation is valid and quantitatively coherent.     

Paleoceanographic Setting and Preservation of Buried Manganese Deposits in DSDP/ODP Cores

Abstract. The occurrence, lithology, and stratigraphic setting of buried manganese deposits and associated host sediments in cores obtained on Legs 123–210 of the Ocean Drilling Program (ODP) are examined in order to establish the formative environment and conditions of preservation. Fossil manganese nodule and crusts are found to have formed or deposited throughout the period from 100 Ma to the present, with an additional example of formation near 137 Ma, suggesting that the deep-sea environment has been oxic and suitable for the formation of manganese nodules and crusts since the Cretaceous. Many manganese nodules and crusts occur on horizons corresponding to hiatuses in sedimentation or periods of slow sedimentation, consistent with the environment in which modern nodules form (sedimentation rate less than 10 m/m.y.). Sediments overlying the fossil nodules and crusts are oozes or biogenic sediments with sedimentation rates of 1–18 m/m.y. Low total organic carbon (<0.1 wt%) in the overlying sediments and high sulfate content (>25 mM) in interstitial water around the manganese horizon suggest that no strong reduction occurred within the overlying sediments. Coverage by biogenic sediments containing only small amounts of organic matter is therefore considered important for the preservation of manganese nodules and crusts. Manganese carbonate occurs sporadically as nodules, concretions or thin layers in various host sediments, including clay, calcareous ooze and siliceous ooze with sedimentation rates of 6–125 m/m.y. Hiatuses are rare around the host sediments of manganese carbonate. Higher total organic carbon (0.2–1.8 wt%) in the host sediments and lower sulfate content (0–25 mM) in interstitial water around the manganese carbonate horizon suggest that reduction in association with decomposition of organic matter would have proceeded in the host sediments.     

Sulphur isotopic fractionation between sulphate and sulphide in hydrothermal ore deposits: disequilibrium vs equilibrium processes

Correlative fractionation relationships of sulphur isotope data for coexisting sulphate and sulphide pairs from hydrothermal ore deposits on δ³⁸S versus Δ³⁴S diagrams are deciphered theoretically. Taking into account dissolved H₂S and SO₄^2- in hydrothermal fluids during precipitation of both sulphate and sulphide minerals, a 4-species closed system is suggested for describing the conservation of mass among all sulphur-bearing species on the δ-Δ diagrams. The covariation in the δ³⁴S values of both sulphate and sulphide is ascribed to isotopic exchange between oxidized and reduced sulphur species during mineral precipitation. The isotopic exchange could be a thermodynamic process due to simple cooling of high temperature fluids, which results in an equilibrium fractionation, or a kinetic process due to mixing of two sulphur reservoirs, which leads to a disequilibrium fractionation. The δ³⁴S value of total sulphur in a hydrothermal system could change due to the precipitation of minerals, or due to the escape of H₂S and/or SO₄^2-. Sulphur isotope data for anhydrite and pyrite pairs from the Luohe porphyrite iron deposit in the Yangtze River Valley is used to illustrate the mixing responsible for the disequilibrium fractionation.     

Sulphur and sulphate-oxygen isotopes and the origin of the silvermines deposits,Ireland

New sulphur and sulphate-oxygen isotope measurements for the main discordant and stratiform lead-zinc-barite orebodies at Silvermines Co. Tipperary, allow reappraisal of previously offered differing interpretations (Graham, 1970; Greig et al., 1971) of the bearing of sulphur isotopes on the genesis of this important Irish deposit. The following aspects of the data are confirmed: barite ³⁴ S-values range from 17–21, similar to lower Carboniferous seawater sulphate: stratiform sulphide lens pyrites have ³⁴ S-values ranging from –13 to –36; vein sulphide ³⁴ S-values range from –8 to 4; sulphide ³⁴ S-values increase upwards and outwards respectively in the related discordant and stratiform G orebodies; galena-sphalerite isotope palaeotemperatures are not too consistent, ranging from 40 to 430°C (using the calibration of Czamanske and Rye (1974). New facts are as follows: barite ¹⁸O-values range from –13 to –17, stratiform barites ranging from 13 to 14.5; sulphides separated from a single stratiform ore lens hand specimen usually have ³⁴ S_sl > ³⁴ S_ga > ³⁴ S_py; the outward decrease in ³⁴ S-values in the stratiform G orebody is confined to the first few hundred feet only; pyrite ³⁴ S-values progressively increase downwards through one stratiform sulphide orebody; yet variations of 13 occur within a single colloform pyrite structure from another stratiform orebody. It is concluded that there were at least two sources of sulphur, seawater sulphate and deep-seated sulphur. The former was the dominant source of all sulphate and, via biogenic reduction, of the sulphur in the bulk of the stratiform sulphide. The latter was the source of the sulphur in the vein sulphides. There was minimal isotopic interaction between the cool seawater sulphate and the warm unwelling ore fluid sulphur species, even though the latter precipitated under near isotopic equilibrium conditions when the temperature dropped and/or the pH and Eh increased. The lack of isotopic equilibrium between pyrite and ore sulphides in the stratiform ore lenses may result from the latter having precipitated slightly later than the former because of solubility relationships. Overall the present isotopic evidence supports considerable geological evidence favoring a syngenetic origin for the stratiform Silvermines orebodies.     

新疆吉木萨尔南油页岩成因分析

吉木萨尔南油页岩矿床属近海型.通过对该油页岩物理性质、化学成分分析,可知吉木萨尔油页有机碳含量为8.65％～16.52％,氯仿沥青“A”含量为0.07％～0.25％,生烃潜量(S1+S2)在20 mg/g以上,达到好一最好标准生油岩,但有机碳含量偏低,有机质划为Ⅱ1型-腐植腐泥型.油页岩的形成经历3个阶段:一是生物的繁盛和有机质的沉积;二是有机质的成岩转化;三是油页岩形成后的自然演化.     

Sulphur and carbon isotopic composition of power supply coals in the Pannonian Basin, Hungary

The present work is an attempt to establish the stable isotope database for Mesozoic to Tertiary coals from the Pannonian Basin, Hungary. Maceral composition, proximate analysis, sulphur form, sulphur isotopes (organic and pyritic), and carbon isotopes were determined. This database supports the assessment of the environmental risks associated with energy generation, the characterization of the formation and the distribution of sulphur in the coals used.The maceral composition, the sulphur composition, the C, S isotopic signatures, and some of the geological evidences published earlier show that the majority of these coals were deposited in freshwater and brackish water environments, despite the relatively high average sulphur content. However, the Upper Cretaceous, Eocene, and Lower Miocene formations also contain coal seams of marine origin, as indicated by their maceral composition and sulphur and carbon chemistry.The majority of the sulphur in these coals occurs in the organic form. All studied sulphur phases are relatively rich in ³⁴S isotopes (δ³⁴S_organic = + 12.74‰, δ³⁴S_pyrite = + 10.06‰, on average). This indicates that marine bacterial sulphate reduction played a minor role in their formation, in the sense that isotopic fractionation was limited. It seems that the interstitial spaces of the peat closed rapidly during early diagenesis due to a regime of high depositional rate, leading to a relative enrichment of the heavy sulphur isotopes.     

Sulphur solubility in andesitic to basaltic melts: implications for Hekla volcano

The solubility of sulphur in sulphide-saturated, H₂O-bearing basaltic–andesitic and basaltic melts from Hekla volcano (Iceland) has been determined experimentally at 1,050°C, 300 and 200 MPa, and redox conditions with oxygen fugacity (logfO₂) between QFM−1.2 and QFM+1.1 (QFM is a quartz–fayalite–magnetite oxygen buffer) in the systems containing various amounts of S and H₂O. The S content of the H₂O-rich glasses saturated with pyrrhotite decreases from 2,500 ppm in basalt to 1,500 ppm in basaltic andesite at the investigated conditions. Furthermore, the reduction of water content in the melt at pyrrhotite saturation and fixed T, P and redox conditions leads to a decrease in S concentration from 2,500 to 1,400 ppm for basaltic experiments (for H₂O decrease from 7.8 to 1.4 wt%) and from 1,500 to 900 ppm (for H₂O decrease from 6.7 to 1.7 wt%) for basaltic andesitic experiments. Our experimental data, combined with silicate melt inclusion investigations and the available models on sulphide saturation in mafic magmas, indicate that the parental basaltic melts of Hekla were not saturated with respect to sulphide. During magmatic differentiation, the S content in the residual melts increased and might have reached sulphide saturation with 2,500 ppm dissolved S. With further magma crystallization, the S concentration in the melt was controlled by the sulphide saturation of the magma, decreasing from ~2,500 to 900 ppm S.     

Formation of pyrite in freshwater sediments: Implications for C/S ratios

The vertical distribution of pyrite, acid volatile sulphide (AVS), carbon, and total S (St) were determined directly in the sediments of three lakes of different trophic status. The results showed that freshwater pyrite formation reflects the redox status of the sediment or overlying waters. It appears to form chiefly in reducing sediments which are subject to oxidizing influences, by either a low turnover of organic carbon or periodic incursions of oxygen. Although there are high concentrations of AVS in the near-surface sediments of productive lakes, very little is diagenetically converted to pyrite.The feasibility of using sulphur ratios to diagnose whether rocks were formed in marine or freshwater environments is assessed. New values for FeS₂/FeS of 0.5-5 show that this ratio does not provide a reliable test. Values of C/Sp, where Sp represents pyrite sulphur, lie within the range of 160–700 and are much higher than previously measured ratios of C/S_t of 1–50. These new determinations show that, if pyrite sulphur is unequivocally measured, C/S ratios may be a more sensitive indicator of salinity than had been previously thought.