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Fluid inclusions and stable isotopes of giant transparent gypsum crystals in a huge geode recently discovered in Pulpi (SE Spain) are useful tools for explaining its geological formation. Fluid inclusions suggest a mixture of fluids, starting with freshwater and including seawater in more advanced stages of the gypsum crystal growth. The isotopic composition of the hydration water in the gypsum crystals agrees with a meteoric fluid. The gypsum is enriched in [sup.34]S, which denotes genetic links with marine sulphates via freshwater dissolution-recrystallization of earlier marine evaporites. Keywords: gypsum, geodes, fluid inclusions, sulphur isotopes.

Researchers have tested sulfur isotope enrichment during sulfate reduction in 32 sulfate-reducing prokaryotes, over the total microbial temperature range, a variety of substrates, and differing salinities and pH values. Under these conditions, isotope fractionation factors ranged from 2% to 42%.

Research was conducted to determine the sources of sulfur for marine bacteria in the synthesis of sulfur containing proteins, amino acids, and other biomolecules. The results show the baceria obtain sulfur through methanethiol generated in the dimethylsulfoniopropionate degradation process.

Sulfur isotope ratios for carbonate-hosted sulfate show a rapid increase (+6 [per thousand] to +9 [per thousand]/m.y.) across the Livello Bonarelli black shale (Marche-Umbrian Apennines of Italy), which was deposited at the Cenomanian-Turonian boundary in the middle Cretaceous (93.5 Ma). The increase suggests that rates of pyrite burial increased substantially over the event, resulting in a decrease of 9%-27% in the oceanic reservoir of [Mathematical Expression Omitted]. The enhanced pyrite burial may have significantly increased the flux of oxygen to the atmosphere (9-27 x [10.sup.18] mol/m.y.). Furthermore, removal of the large amount of iron associated with this process might have had a profound effect on iron cycle.

Sulfur isotopes are potent indicators of the way in which sulfur behaves chemically during diagenesis. We have studied sulfur isotope ratios (34S/32S) from a number of minerals and compounds across the Permian-Triassic boundary in the Khuff Formation, Abu Dhabi. The [[Delta].sup.34]S in dissolved marine sulfate increased by 10% from the Late Permian to the Early Triassic. Despite precipitation of gypsum from Permian and Triassic seawater and its subsequent dehydration to anhydrite at depths greater than about 1000 m, the primary marine stratigraphic sulfur isotope variation has been preserved in anhydrite in the Khuff Formation. A combination of biostratigraphic and absolute age data show that this 10% shift occurred over

Pyrite within and below sapropels in the eastern Mediterranean is a result of microbial S[O.sub.[4.sup.2-]] reduction within the sapropel, and the subsequent reaction of sulfide (H[S.sup.-]) with detrital Fe and [Fe.sup.2+] diffusing upward from underlying sediments. Below the youngest Mediterranean sapropel, S1, pyrite (as much as 281 [[micro]mol] pyritic S/g) is mostly present as euhedral crystals, whereas within the sapropel only framboidal pyrite (as much as 360 [[micro]mol] pyritic S/g) has been detected. Framboidal microtextures indicate pyrite formation at the site of H[S.sup.-] production within the sapropel. Euhedral pyrite, below the sapropel, forms when sulfate reduction in the sapropel outcompetes iron liberation and supply, and H[S.sup.-] diffuses out of the sapropel. Sulfur isotope values of pyrite are extremely light in the sapropel (-37.3[per thousands] to -38.2[per thousands]) as well as below the sapropel (-45.6[per thousands] and -49.6[per thousands]), indicating that H[S.sup.-] has formed in a system with abundant S[O.sub.[4.sup.2-]] and in the presence of oxidants.

Pyrite formation is associated with fossils where the organic material decays by sulphate reduction, or where the carbonate skeleton acts either as a nucleation substrate or induces iron sulphide precipitation by dissolution. These requirements define specific combinations of porewater chemistry and saturation state with respect to carbonates and iron sulphides. A review of modern marine sediments suggests that near-surface porewaters are always over-saturated, or close to saturation, with iron sulphides but are alternately rich in either dissolved iron or dissolved sulphide. However, significant variations in carbonate saturation state may occur, with porewaters being undersaturated in the early stages of sulphate reduction, but subsequently becoming oversaturated until the later stages of methanogenesis. Here undersaturation may return if there is a substantial input of carbon dioxide, and if little alkalinity has arisen from iron reduction. Organic matter pyritization requires that soft-tissue material decays by sulphate reduction to release dissolved sulphide, with dissolved iron supplied by the surrounding porewaters. The modern sediment studies therefore suggest that organic matter pyritization can occur only during the earliest stages of sulphate reduction, where there are iron-rich porewaters which are saturated with iron sulphides. Pyrite replacement of carbonate shell material requires porewater undersaturation with respect to carbonates and analogy with modern sediments suggests that these conditions occur either during the earliest stages of sulphate reduction, or when methanogenesis produces undersaturation. Between these periods porewaters are saturated with respect to carbonates and pyrite of different morphologies can precipitate on carbonate shells. Sulphur isotope data provide support for these associations of porewater chemistry and style of fossil pyritization. The best-described examples are for soft tissue pyritization in Beecher's Trilobite Bed (Ordovician) and the Hunsruck Slate (Devonian), where the isotopic data are consistent with rapid, early pyritization in iron-rich porewaters. More data are needed on shell pyritization with the range of isotopic compositions indicating formation through all stages of diagenesis. Keywords: pyritization, fossils, sulphur, isotopes.

The timing of the evolution of non-photosynthetic sulphur (S)-oxidizing bacteria and changes in stable S isotopes indicates that these events were caused by a large increase in the atmospheric oxygen during late Proterozoic. The oxygen levels increased to more than 5 to 18% of the present atmospheric level and this permitted the development of an oxidative S cycle and bacterial evolution. Analysis of the 16S rRNA of gamma-proteobacteria shows that they evolved almost 0.76 billion years (Gyr) ago. A significant change in the S isotope composition took place between 0.64 to 1.05 Gyr ago.

The pyritization of fossils in the Lower Devonian Hunsruck Slate, Germany, occurred in the later stages of authigenic mineralization due to the absence of organic matter and the presence of sulfur (S) and iron. An analysis of S isotope and C-S-Fe elemental data reveals that the slates containing fossils possess low amounts of pyrite S and organic substances but high levels of total and HCl-soluble Fe. The presence of highly enriched 34S in the adjacent slates indicates continued fossil pyritization due to exceptional diagenetic conditions.

Epithermal veins in early Triassic turbidites on Hurd Peninsula are isotopically homogeneous over an area of 13 km x 2 km (mean [[Delta].sup.34]S = +2.2[per thousand], 1[Sigma] = 1.7, n =65) suggesting derivation from a deep circulating, neutral-chloride hydrothermal plume containing magmatic sulphur. The sense of fractionation (pyrite = +3.5, arsenopyrite = +3.2, sphalerite = +2.9, chalcopyrite = +2.5, galena = +0.9%o) and isotopic temperatures between 250 and 345 [degrees] C suggest partial isotopic equilibration. Sulphides in massive dark carbonate/peperitic dyke breccias, with abundant magnetite, hematite and titanite and traces of barite, garnet, zircon and monazite are isotopically heavy (+7 to +14.9[per thousand], n = 12) due either to dyke intrusion into the hydrothermal system resulting in degassing of [H.sub.2]S, or to a second. isotopically heavy and sulphate-bearing solution in fault zones along which dykes were subsequently intruded. Vein sulphides in nearby Cretaceous volcanic rocks are similar to the epithermal system (mean = + 1.8%o, 1[Sigma] = 0.9, n = 6, pyrite = +2.0, chalcopyrite = +1.5), suggesting sulphur-derivation by degassing of sub-volcanic magma, or remobilization of disseminated sulphides during plutonism. Sulphide in an Eocene tonalire pluton is slightly 32S-enriched (mean = 0[per thousand], 1[Sigma] = 1.9, n = 7, molybdenite = +0.7, pyrite = + 1.5, chalcopyrite = -2.6) and was exsolved directly from the cooling tonalitic magma. Keywords: Andean Orogeny, South Shetland Islands, veins, sulphur, isotopes.

An on-line method for S-isotope analysis is described. Samples are combusted in an elemental analyzer. S[O.sub.2] is separated from other combustion gases by gas chromatography, and the gases enter the ion source of the mass spectrometer through a split interface. Integrated peak areas for 32S[[O.sub.2].sup.+] and 34S[[O.sub.2].sup.+] are compared to the response for a standard gas sample to determine the [Delta]34S value. [Delta]34S values of samples analyzed using the on-line method correspond linearly with those achieved from the same sample prepared off-line, where Kiba reduction followed by oxidation of the sulfur to S[O.sub.2] is carried out prior to S-isotope analysis against a known standard. With the described on-line method, the amount of sulfur necessary for S-isotope analysis is reduced to about 10[[micro]gram] of S per analysis. The time needed for on-line preparation and measurement is less than one-third of the off-line procedure.

Iron monosulfides and pyrite (total Cr-reducible S-TRS) in the bottom sediments of the exceptionally sulfide-rich euxinic fjord, Framvaren, south Norway, are unusually enriched in 34S. Most of these TRSs probably form in bottom waters, where reactive detrital iron phases encounter isotopically heavy dissolved sulfide. Enrichment in 34S in Framvaren is a result of intense bacterial reduction of a sulfate reservoir whose exchange with the open sea is limited by shallow sills and a low-salinity surface-water layer. 34S-enriched TRS in the deposits of ancient euxinic environments is commonly interpreted as evidence for early diagenetic sulfide formation where residual, less reactive iron phases were in contact with isotopically heavy dissolved sulfide (sulfate-depleted pore waters) for prolonged periods during burial. However, the Framvaren data indicate that isotopically heavy pyrite may form in relatively shallow (80-180 m), super-anoxic bottom-water pools. In contrast, the presence of isotopically light pyrite does not indicate an absence of restricted conditions, but only that most iron sulfide formation was completed before 34S-enriched sulfide could be encountered in the water column.

In a continuing effort to understand crust-mantle dynamics, we have determined the S and Pb isotopic compositions of mantle sulfides encapsulated within diamonds from under the Siberian craton and compared these results to those of previously investigated African counterparts. Because diamond inclusions are isolated from exchange with surrounding mantle, they may preserve the history of diamond growth and act as direct tracers of the origins of mantle materials. Study of these inclusions may thus offer the best chance of recognizing global-scale interaction between Earth's crust and mantle. Although delta(sup 34)S values of the Siberian sulfides do not deviate significantly from the mantle value of 0 0/00 plus or minus 3 0/00, Pb isotopic compositions are highly variable. Pb isotopic compositions of sulfides from peridotitic suite diamonds generally plot near the terrestrial Pb growth curve, with model ages ranging between 0 and 2 Ga, whereas sulfides from eclogitic suite diamonds have radiogenic compositions, plotting beyond the growth curve. These results, which are similar to those for sulfides in African diamonds, suggest that the sulfides from eclogitic suite diamonds were derived from a source with an unusually high U/Pb ratio and may indicate a common process (such as subduction of crustal materials into the mantle) operating beneath Africa and Siberia. The absence of extremely radiogenic Pb in sulfides from eclogite xenoliths suggests that the radiogenic material from which eclogitic suite diamonds grew was a transient feature of the mantle, associated with diamond growth. The ultimate origin of this high U/Pb signature, however, remains enigmatic. Large variations in Pb isotopic composition of sulfides from different zones in a single peridotitic suite diamond document (1) crystallization of the diamond's core near 2.0 Ga, (2) growth of its outer zone in an environment with a high U/Pb ratio similar to the growth environment of eclogitic suite diamonds, and (3) growth of the diamond rim near the time of kimberlite emplacement.

The origin and dynamics of sulfate salts in selected saline soils derived from marine and continental sediments are determined by using the isotopes of S and O. Results show that pyrite oxidation and hydrolysis of natrojarosite in marine and continental sediments are the dominant sources of sulfate salts. Lastly, oxygen isotope analysis is very useful and provides more information than sulfur isotope analysis.

A diverse assemblage of sulfide morphologies and a wide range of sulfur-isotope compositions indicate that precipitation of sulfide minerals was an ongoing process during both early and later stages of diagenesis in the Silurian Waldron Shale in Indiana. Five distinct forms of sulfide minerals have been identified in samples from seven locations in southern Indiana: (1) pyritized brachiopod shells, (2) pyritized burrows, (3) pyrite cubes, (4) prismatic marcasite, and (5) cockscomb marcasite. Petrographic evidence is used to determine a probable sequence of sulfide precipitation. Isotope compositions range from about -40[unkeyable] to +40[unkeyable], and each sulfide morphology exhibits a distinctive range of [delta.sup.34.S] values. These results are consistent with a model in which sulfate reduction and subsequent precipitation of sulfide minerals occurred in a system that became progressively more closed with respect to sulfate. The sequence of sulfide mineralization reflects the chemical evolution of pore waters during diagenesis of the Waldron Shale. Redox conditions in the sediment were initially aerobic to dysaerobic, sulfate reduction occurring only in reducing microniches where brachiopod shells and then organic-walled burrows were pyritized. Sediments became more fully reducing with burial, when euhedral sulfides were formed. The transition from pyrite to marcasite precipitation suggests a decrease in pore-water pH during later diagenesis and may also indicate fluctuations in redox conditions.

Coexisting dissolved sulfide and sulfate from hypersulfidic interstitial waters of a 380m-long sediment core show a large isotopic difference of up to 72 [salinity] caused by in situ microbial sulfate reduction. This is considerably larger than the assumed biological maximum of 46 [salinity] derived from laboratory studies with pure cultures of sulfate-reducing bacteria. Similar high fractionations inferred from sedimentary metal sulfides have been previously explained by a multistage process, involving sulfide reoxidation and disproportionation of sulfur intermediates. Our data show that extreme isotopic differences between sulfate and the reduced sulfur species can also be generated during microbial single-step fractionation. This result indicates that the sulfate-reducing communities and/or their cellular metabolic activities in the deep biosphere may differ from those observed in near-surface sediments or the water column. Keywords: isotope geochemistry, biogeochemistry, bacteria, continental margin, ore-forming fluids, sulfur isotopes.

A continuous flow method (CF-IRMS) for the rapid determination of the sulfur isotope composition of sulfide and sulfate minerals has significant advantages over the classic extraction method in terms of the reduced sample quantity and a rapid analytical cycle of less than 8 min/ analysis. For optimum performance, the technique is sensitive to a number of operating parameters, including sample weight and the 02 saturation of the Cu-reduction reactor. Raw data are corrected using a calibration based on five international and internal standards ranging from -17.3 to +20.3%, which requires monitoring in order to correct the effect of changing [Delta][sup.18]O of the sample gas on the measured mass 66 values. Measured sulfur contents are within 1-1.5% of expected values and the reproducibility of [Delta][sup.34]S values is [+ or -] 0.1% (1[Sigma]). The technique has been used successfully for more than 1000 analyses of geological samples with a wide range of [Delta][.sup.34]S from -20 to +20%0.

An area of active fluid discharge characterized by authigenic barite formation and populated by vestimentiferan tubeworms has been discovered by remotely operated vehicle exploration in Monterey Canyon. Slumping and mass wasting, presumably triggered by seismic activity, have exposed barium-rich pore fluids to sulfate-rich bottom water, leading to barite precipitation on the sea floor. Age estimates based on [sup.210]Pb/[sup.226]Ra dating of the barite samples and growth rates of vestimentiferan tubeworms indicate that the seep site might be as young as 100 yr. Systematic variations in sulfur and strontium isotope ratios, the lack of visible fluid flow, and the abundance of dead tubeworms indicate decreasing flow rates. This newly discovered site shows many similarities to a previously described barite deposit in the California Borderland, possibly misinterpreted as a hydrothermal vent site. The discovery of a cold seep associated with barite deposits on the seismically active, transpressional California margin raises the possibility that numerous unexplored areas along the continental margins might have similar low-temperature deposits. Keywords: cold seep, barite, sulfur isotopes, strontium isotopes, [sup.210]Pb/[sup.226]Ra ages.

High sulphate concentrations lead to sulphides with highly depleted sulphur isotope (34S) in modern microbial mats. This indicates that increase in seawater sulphate concentrations occurred only after the initial accumulation of oxygen into the atmosphere in post-Archean times. This increase is represented by the high negative isotope shift in shale-hosted sedimentary sulphides at between 2.2 to 2.3 Gyr. The timing of this event coincides with the beginning of a high early Proterozoic burial pulse of organic carbon which released oxidants to the Earth surface.

Atmospheric oxygen enrichment during Neoproterozoic era may have caused the diversification of multicellular animals. According to the carbon isotope hypothesis, increase in the organic carbon burial rate can lead to atmospheric oxygen enrichment. The sulphur metabolism and oxidation in bacteria imply the underlying function of organic carbon burial in oxygen enrichment. Sulphur isotope data on the pre- and post-Neoproterozoic era indicate a gradual increment in organic carbon burial rate.

Several criteria should determine which radioisotope is used to label nucleic acids; this labelling is a necessary step in many molecular biology research techniques including filter hybridization and DNA sequencing. Three radioisotopes are now available: phosphorus 32 (32P), sulphur 35 (35S) and the recently introduced phosphorus 33 (33P). Detection sensitivity and the desired degree of spatial resolution are the most important factors to consider in choosing which of these radioisotopes to use. Although 32P and 35S will continue to be standard, new uses for 33P will probably soon be developed.

The top 20 cm of sediments at active cold seeps in Monterey Bay, coastal California, contain framboidal pyrite that occurs as infillings and pseudomorphs of the chambers of the tests of foraminifera and rarely as irregularly shaped grains. Sulfur isotope compositions obtained with the ion microprobe show depletions in 34S ([Delta]34S = -41 to -5%, CDT), and large variations both within and among these pyrite grains. Intergranular differences in [Delta]34S values in the same sediment are as large as 35%, and intragranular zoning reaches 15%. Zoning is regular in some grains, with systematic isotope changes from core to rim or from one foraminiferal chamber to another, but irregular in others. The regular zoning is consistent with an increase in 34S through time. Backscattered-electron imaging reveals three types of pyrite: isolated framboids in a porous aggregation ('PF-pyrite'), agglomerated framboids with cementing interstitial pyrite ('F+I-pyrite'), and recrystallized pyrite with isolated relicts of framboids ('RF-pyrite'). In individual grains, RF-pyrite cores grade into F+I-pyrite toward grain rims, and F+I-pyrite grades into PF-pyrite at the grain edges. These textures are consistent with a paragenetic sequence whereby framboids first agglomerate (PF-pyrite), then cement (F+I-pyrite), and finally recrystallize (RF-pyrite). The [Delta]34S values of RF-pyrite are generally lower than that of F+I-pyrite; if the paragenetic sequence is correct, then this trend parallels the regular core-rim isotopic zoning observed in some grains. The implied increase in [Delta]34S with time is consistent with Rayleigh fractionation of sulfur in a closed system. Bacteria are intimately involved in the production of pyrite from our samples, and heterogeneous colonization by bacteria provides a simple explanation for the sulfur isotope heterogeneity among and within grains: The foraminifera provide open space for colonization and local nutrients for bacterial growth, whereas the cell walls of the bacteria may provide a local nucleation site for sulfides. If so, then initial colonization is reflected in lower [Delta]34S values, whereas later bacterial emigration to other foraminifera chambers is indicated by higher [Delta]34S values.