H. Roy Krouse

Products and distinguishing criteria of bacterial and thermochemical sulfate reduction

Bacterial and thermochemical sulfate reduction apparently occur in two mutually exclusive thermal regimes, i.e., low-temperature diagenetic environments with0 < T < 60–80°C and high-temperature diagenetic environments with80–100 < T < 150–200°C, respectively. The major reaction products and by-products are identical in both thermal regimes and include altered and oxidized hydrocarbons (originally mainly crude oil, gas condensate, and/or methane), hydrogen sulfide, base and transition metal sulfides, elemental sulfur, and carbonates (mainly calcite and dolomite). The mere presence of the above reaction products and by-products does not discriminate between the low- and high-temperature diagenetic environments. However, petrographic, isotopic and compositional data of these products and by-products may permit identification of a bacterial versus a thermochemical origin. Regarding the inorganic phases, the carbon isotope ratios of the carbonates, sulfur isotope ratios of elemental sulfur and sulfides, and fluid inclusion data appear to yield the best discriminating geochemical criteria. Among the organic phases, reservoir bitumen and gas condensates display a number of useful isotopic and compositional (chromatographic) criteria. The most reliable approach for discriminating bacterial versus thermochemical sulfate reduction is to combine as many of these criteria as possible. These criteria can be used in exploration or deposits of hydrocarbons, sour gas, elemental sulfur, and certain metal sulfides.

Shifts in carbon isotope ratios of two C3 halophytes under natural and artificial conditions

SummaryThe total carbon δ13C values of two C3 halophytes,Salicornia europaea L. ssp.rubra (Nels.) Breitung andPuccinellia muttalliana (Schultes) Hitch., native to inland saline areas of Alberta, Canada, were determined for plants grown under controlled conditions of supplied NaCl in the nutrient solution, and for plants found growing in the field. Field specimens were collected along line transects which ran from areas of high salinity to areas of low salinity across the pattern of species zonation. The δ13C value of the two species seemed to reflect the water potential of the soil (ψwsoil) as measured arbitrarily at a depth of 10 cm, becoming less negative as the ψwsoil decreased. Over a linear distance of 5.55 m,S. europaea spp.rubra showed a shift of +5.3‰ as the ψwsoil went from-25x102 kPa to a minimum of-73x102 kPa. ForP. nuttalliana, the δ13C values differed by 3.4‰ over a distance of 7.45 m where the maximum difference in ψwsoil was 12.7x102 kPa. However, δ13C values ofP. nuttalliana only roughly reflected the spatial trends in ψwsoil at the time of collection. In the growth chamber, the δ13C value ofS. europaea ssp.rubra changed by a maximum of +8.0‰ when the solute potential of the nutrient solution (ψwsoil) was dropped from-0.25x102 kPa to-64.25x102 kPa; while the δ13C value ofP. nuttalliana changed by a maximum of +10.8‰ when the ψwsoil was dropped from-0.25x102 kPa to-40.25x102 kPa. Linear regression analyses indicated that the δ13C values of both species were strongly correlated (P<0.2%) with ψwsoil. The observed shifts in δ12C may represent changes in the mode of photosynthetic CO2 fixation. However, a number of other explanations, some of which are discussed in the text, are also possible. A proper ecophysiological interpretation of such shifts in δ13C values of C3 plants awaits a better understanding of the isotope fractionation mechanisms involved.

Sulphur and Oxygen Isotopes in Sulphate

The origin and the fate of sulphate in groundwater has concerned hydrogeologists for many decades. With increasing industrialisation throughout this century, the differentiation between natural and ‘man-made’sulphate became important. A particular challenge is evaluating potential and existing additions of the latter to groundwater and drinking water supplies. On regional and even global scales, sulphate as a component of ‘acid rain’captured scientific and public awareness in the 1970s and 1980s. Subsequently, intensive research programs on the effects of atmospheric sulphate deposition on terrestrial and aquatic ecosystems were initiated. Today, more than two-thirds of the sulphate in atmospheric deposition in heavily industrialised regions of the northern hemisphere is of anthropogenic origin (Section 7.3.2.4). The rate and the degree to which these increased sulphate inputs have led to acidification of surface water and groundwater has been a matter of considerable scientific debate in the past two decades.

Sulphur isotope data consistency improved

The standard used for reporting relative sulphur isotope-abundance data has, historically, been troilite (FeS) derived from the Cañon Diablo meteorite, CDT. However, the isotopic inhomogeneity of this material — the variability in its sulphur-34/sulphur-32 isotope ratio is at least 0.4‰ — greatly exceeds the achievable analytical uncertainty of 0.05‰ (ref. 1). Accordingly, the International Union of Pure and Applied Chemistry (IUPAC) has recommended the setting up of a VCDT scale, which is expected to improve agreement between laboratories in measuring this ratio, δ34S.

Neoproterozoic strata of the southern Canadian Cordillera and the isotopic evolution of seawater sulfate

Abstract Neoproterozoic strata of the southern Canadian Cordillera comprise metasedimentary rocks that are impressive in their thickness (6–9 km), areal extent (35,000 km2 minimum) and predominance of sedimentary rocks of deep-water affinity. The stratigraphy can be subdivided into two broad successions that record the response to rifting and subsequent formation of a passive margin during the breakup of western Laurentia. Syn-rift strata include glaciogenic rocks that likely correlate with earlier Neoproterozoic glaciations elsewhere (e.g. Sturtian). Dating of associated granitic and volcanic rocks in Canada suggests that these units were deposited between 762 Ma and 728 Ma. Post-rift strata are interpreted as the deposits of an elongate terrigenous-sourced turbidite system that formed at the base of the continental slope and shoaled upward through slope into platformal facies. Evidence of a second Neoproterozoic glaciation (Varanger equivalent) is recognized in the post-rift turbidite succession as a regionally mappable condensed interval formed during post-glacial sea-level rise. Deep-water carbonate deposits punctuate the record of dominantly siliciclastic slope sedimentation and hold promise for future C Sr isotope chemostratigraphy. Coeval shallow-water facies are rarely preserved in the southern Canadian Cordillera owing to erosional removal during uplift associated with a younger rift event that preceded Cambro-Ordovician passive margin formation. The Kaza Group (sand-rich basinal turbidites) and the overlying Isaac Formation (mud-rich slope facies) are post-rift strata in the Cariboo Mountains that contain abundant pyrite as disseminated, large (up to 5 cm) crystals and finely crystalline stratabound layers. Isotopic analysis of pyrite separates from these two units demonstrates a broad range in δ34S values (49 and 53‰, respectively), typical of sulfides formed by the bacterial reduction of seawater sulfate. There is a distinct shift of ca. +8‰ in median δ34S values and +15‰ in the maximum-minimum values from pyrites in the Kaza Group into the overlying Isaac Formation. The magnitude of the shift, in addition to its stratigraphic position above Sturtian age glacial deposits, suggests that this excursion in the sulfide record overlaps with the large sulfate sulfur excursion observed in late Neoproterozoic to early Cambrian evaporites. We suggest that the Kaza Group and Isaac Formation pyrites are the deep-water, 32S-enriched, complement to the sulfate record. Burial of 32S-enriched pyrite in continental margin strata, such as exemplified by the Windermere Supergroup, likely occurred on a global scale in response to widespread passive margin formation during the breakup of western Laurentia with resultant enrichment of residual seawater sulfate in 34S. Thus open marine sulfate reduction and pyrite burial is interpreted as the driving mechanism by which Neoproterozoic sulfate became enriched in 34S and requires microbial reduction of a substantial volume of the global sulfate reservoir (ca. 25%). The coupled process of sulfate reduction and organic matter oxidation implies that the process of sulfate reduction on this scale would have had a substantial impact on the evolution of the oxygen reservoir in the Neoproterozoic.

Isotopic and elemental hydrogeochemistry of a major river system: Fraser River, British Columbia, Canada

The Fraser drains one quarter of British Columbia, but within its watershed there occurs a disproportionately large share of the provinces economic activity. Agriculture, fishing, forestry, mining and tourism are all important, the Fraser being the worlds largest source of salmon. To preserve this range of activities, the quality of its waters must be ensured. This study documents the chemistry of the waters. Samples from seventeen sites along the Fraser and its principal tributaries were analyzed for major anions and cations, for 28 dissolved trace and ultratrace elements, and for the isotope ratios of O, H, S, C and Sr. The data show that the primary control on chemistry is the diverse geological terranes that are drained. For example, S04 in the headwaters comes from dissolution of sedimentary sulphate minerals. Towards the coast, weathering of sulphide minerals become progressively more important as a source of SO4, including igneous sulphides that contribute a substantial flux of elements such as Se and Sb. Superimposed on the natural fluxes are anthropogenic contributions. Some, such as dissolved organic carbon and NaCl from pulp mills, can be readily identified. The sources of others are less certain. Molybdenum is much higher in the Fraser than in the Ottawa River. But it is not clear if this high level of Mo is natural or is the result of mining for this metal. During the period of peak flow in July, 1993, Fraser waters had an unusually high level of CO2. The CO2 is derived from the decay of vegetation on land and may have been influenced by timber operations.

Nitrogen isotope geochemistry of organic matter and minerals during diagenesis and hydrocarbon migration

The magnitude of isotopic variations between organic and inorganic nitrogen was examined in samples from three stacked hydrocarbon reservoirs in the Fordoche Field (Louisiana Gulf Coast Basin, USA). Measurements were made of δ 15N in kerogen, bitumen, oil, formation water, and fixed-NH4 extracted from mudstones, nonproductive sandstones, and productive sandstones. Nitrogen isotope fractionation occurs because 14N is released preferentially to 15N from organic molecules during thermal maturation. Released 14N goes into solution, or may be adsorbed by minerals, leaving crude oil enriched in 15N. Diagenetic clay minerals (e.g., illite) commonly form in the temperature range of hydrocarbon generation, and NH4+ may be fixed in clay interlayers with an isotopic ratio similar to that of the migrating fluids. Results indicate that the influence of organic matter on mineral δ 15N depends on the timing of authigenic mineral formation relative to fluid migration. The average δ 15N of kerogen (3.2 ± 0.3‰) and fixed-NH4 from mudstones (3.0 ± 1.4) is similar, while bitumen increases from +3.5 to +5.1‰ with depth. In deep reservoir sandstones (>100°C), the δ 15N of crude oil averages +5.2 ± 0.4‰, similar to the δ 15N of bitumen in the proposed source rocks. Formation waters are 14N-enriched with an average δ 15N of −2.2 ± 2.6‰. Fixed-NH4 δ 15N values lie between that of the oil and water. The average δ 15N of fixed-NH4 is 3.0 ± 1.2‰ in productive sandstones, and 0.2 ± 2.4‰ innonproductive sandstones. In the shallower reservoir sandstones (<90°C) fixed-NH4 is apparently not influenced by the presently associated fluids. Productive and nonproductive sandstones have distinctly low average δ 15N values (−1.2 ± 0.8‰), yet crude oil (+11.1 ± 0.3‰) and water (+3.8 ± 0.1‰) have been 15N-enriched by ∼6‰ relative to the deeper reservoirs. This suggests that the present fluids migrated into the reservoir after authigenic illite had formed. Fluids become enriched in 15N during migration and the amount of enrichment may be a function of the amount of interaction with argillaceous sediments.

Crystal structure, stable isotopes (δ18O), and development of sea ice in the Ross, Amundsen, and Bellingshausen seas, Antarctica

The crystal structure and oxygen isotopic composition of ice cores obtained from floes at the end of summer in the eastern Ross Sea, the Amundsen Sea, and the western Bellingshausen Sea were investigated to determine the ice growth processes and conditions that contribute to sea ice development in the eastern Pacific sector of the southern ocean. The isotope data indicate that a moderate amount of snow contributes to the development of the sea ice. However, even the combined use of isotopes and crystal structure analysis does not unambiguously explain the means by which all of the snow is entrained in the ice. Nevertheless, it seems clear that much of the snow is contained in granular snow-ice that results from seawater flooding of floes and the base of the snow cover. The snow cover in the Ross-Amundsen region was as much as 2 m deep and supported by 7- to 8-m-thick floes primarily composed of frazil ice. In the Bellingshausen region the snow cover and the floes were thinner than in the Ross-Amundsen region. The Bellingshausen cores were composed primarily of multiple layers of frazil and congelation ice. In addition, in both regions there were numerous tipped or inclined blocks of congelation ice and layers of rafted nilas in the cores. The data indicate that the sea ice develops by multiple mechanisms in a turbulent environment.

A 10-yr record of stable isotope ratios of hydrogen and oxygen in precipitation at Calgary, Alberta, Canada

Short-term (0.5–3 d) precipitation samples were collected from January 1992 to December 2001 in Calgary, Alberta, Canada, and the stable isotope ratios of hydrogen (2H/1H) and oxygen (18O/16O) for these samples were determined. The 10-yr amount-weighted average δ2H and δ18O values of precipitation were −136.1° and −17.9°, respectively. Consistent with International Atomic Energy Agency (IAEA) established practice, the following local meteoric water line (LMWL) for Calgary was derived using amount-weighted monthly average δ2H and δ18O values: δ2H = 7.68 δ18O −0.21 (r2= 0.96, n= 104). The correlation equation between δ2H and δ18O values from individual samples was found to be δ2H = 7.10 δ18O −13.64 (r2= 0.95, n= 839), which is different from the LMWL, exhibiting lower slope and intercept values. A comparison of δ2H and δ18O correlation equations with temperature during precipitation events showed a trend of decreasing slopes and intercepts with increasing temperature. Our data suggest that this is caused by incorporation of moisture derived from evaporation from water bodies and soils along the storm paths and by secondary evaporation between the cloud base and the ground during precipitation events. These processes compromise the usefulness of d-excess values as an indicator for the meteorological conditions in the maritime source regions. The δ18O temperature dependence at Calgary was found to be ⊼ 0.44°¼C−1. The study shows that short-term sampling of individual precipitation events yields valuable information, which is not obtainable by the widely used monthly collection programs.

Application of Stable Isotope Variation in Human Tissues to Problems in Identification

ABSTRACTVariation in the stable isotope composition of human tissues and fluids suggests the use of such data as an aid in identification of unknown human remains. Carbon isotope values vary depending specifically on the photosynthetic pathways utilized by plants consumed as feed by domesticated animals, and by humans directly. Sulphur isotopes vary depending on environmental sulphur in the atmosphere and lithosphere, and stable isotopes of oxygen, nitrogen and hydrogen also vary in predictable manners in different geographic locations.The use of a multi-element approach is demonstrated as a means of narrowing down the possible place of residence of unidentified human remains. Carbon and sulphur isotope ratios were determined on samples of human hair from five countries. Results show that while values for one element may be similar for samples from diverse regions, values for a second element differ such that samples are distinguishable. Hair samples from three regions of Canada were analysed for carbon i...