Chlorine Stable Isotope Research Articles

Reply to comment on: “Chlorine stable isotopes and halogen concentrations in convergent margins with implications for the Cl isotopes cycle in the ocean”

Reply to comment on: “Chlorine stable isotopes and halogen concentrations in convergent margins with implications for the Cl isotopes cycle in the ocean”

Microbial perchlorate reduction: A precise laboratory determination of the chlorine isotope fractionation and its possible biochemical basis

Perchlorate-reducing bacteria fractionate chlorine stable isotopes giving a powerful approach to monitor the extent of microbial consumption of perchlorate in contaminated sites undergoing remediation or natural perchlorate containing sites. This study reports the full experimental data and methodology used to re-evaluate the chlorine isotope fractionation of perchlorate reduction in duplicate culture experiments of Azospira suillum strain PS at 37 °C (Δ 37Cl Cl −–ClO 4 − ) previously reported, without a supporting data set by Coleman et al. [Coleman, M.L., Ader, M., Chaudhuri, S., Coates, J.D., 2003. Microbial Isotopic Fractionation of Perchlorate Chlorine. Appl. Environ. Microbiol. 69, 4997–5000] in a reconnaissance study, with the goal of increasing the accuracy and precision of the isotopic fractionation determination. The method fully described here for the first time, allows the determination of a higher precision Δ 37Cl Cl −–ClO 4 − value, either from accumulated chloride content and isotopic composition or from the residual perchlorate content and isotopic composition. The result sets agree perfectly, within error, giving average Δ 37Cl Cl −–ClO 4 − = − 14.94 ± 0.15‰. Complementary use of chloride and perchlorate data allowed the identification and rejection of poor quality data by applying mass and isotopic balance checks. This precise Δ 37Cl Cl −–ClO 4 − value can serve as a reference point for comparison with future in situ or microcosm studies but we also note its similarity to the theoretical equilibrium isotopic fractionation between a hypothetical chlorine species of redox state + 6 and perchlorate at 37 °C and suggest that the first electron transfer during perchlorate reduction may occur at isotopic equilibrium between an enzyme-bound chlorine and perchlorate.

The Chlorine Isotope Composition of Earth's Mantle

Chlorine stable isotope compositions (delta37Cl) of 22 mid-ocean ridge basalts (MORBs) correlate with Cl content. The high-delta37Cl, Cl-rich basalts are highly contaminated by Cl-rich materials (seawater, brines, or altered rocks). The low-delta37Cl, Cl-poor basalts approach the composition of uncontaminated, mantle-derived magmas. Thus, most or all oceanic lavas are contaminated to some extent during their emplacement. MORB-source mantle has delta37Cl </= -1.6 per mil (per thousand), which is significantly lower than that of surface reservoirs (approximately 0 per thousand). This isotopic difference between the surface and deep Earth results from net Cl isotopic fractionation (associated with removal of Cl from the mantle and its return by subduction over Earth history) and/or the addition (to external reservoirs) of a late volatile supply that is 37Cl-enriched.

A simplified procedure for the determination of stable chlorine isotope ratios (δ37Cl) using LA-MC-ICP-MS

Stable chlorine isotopes (37Cl, 35Cl) are considered as important tracers of geochemical processes, especially in subduction zone systems. However, high-quality chlorine isotope data are scarcely available. Here we present a comparatively simple procedure for the precise and accurate determination of stable chlorine isotope ratios (δ37Cl) using LA-MC-ICP-MS. Chlorine was extracted from solid samples by pyrohydrolysis. After quantitative precipitation as AgCl the dried precipitates where analysed in a sample-standard bracketing approach using a weak laser ablation (0.3 J/cm2) for sample evaporation. Atlantic Ocean sea salt and the sea water standard IAPSO were used as SMOC (standard mean ocean chloride) for normalisation (δ37Cl = 0 ‰). The precision and accuracy of the presented method was validated analysing the reference materials JB-1a and JB-2. The chlorine isotope ratios of these standards were determined as δ37ClJB-1a = (−0.99 ± 0.06) ‰ and δ37ClJB-2 = (−0.60 ± 0.03) ‰ (errors 2SE), respectively, in accordance with published data. Applying the presented method a total amount of less than 1 μg of chlorine was consumed during a typical measurement including 10 ablation periods on the sample.

Chlorine stable isotopes and halogen concentrations in convergent margins with implications for the Cl isotopes cycle in the ocean

Chlorine stable isotopes ( δ 37Cl) and halogen concentrations ( e.g. Br/Cl) in 168 pore Fluids and 23 serpentines and other solids from three subduction zones, the Nankai Trough, Costa Rica, and Mariana Forearc, provide critical information on fluid sources, flow paths, and reaction conditions. The δ 37Cl values of pore fluids at the Nankai and Costa Rica subduction zones, are significantly more negative (minimum − 7.8‰, 2σ ± 0.3‰) than seawater value (0‰). At Nankai Trough, the minimum δ 37Cl value is situated below the décollement and evolves laterally from − 7.8‰ at the most arcward ODP Site 808, to − 7.1‰ at Site 1174, ∼ 2 km seaward from Site 808, and to − 5.8‰ at the reference Site 1173. At Costa Rica, along the décollement the minimum δ 37Cl value evolves from − 5.5‰ at the most arcward ODP Site 1040/1254, to − 3.2‰ at Site 1043/1255, ∼ 1 km seaward, and to 0‰ at the reference Site 1039/1253. At both subduction zones, the Br/Cl ratios are higher than the seawater value (1.5 × 10 − 3 ) and also show seaward evolutions. These pore fluids originate from greater depth arcward, at ≥ 250 °C, from hydrous mineral formation that preferentially incorporates 37Cl and excludes Br. In contrast, the δ 37Cl values in the pore fluids at the Mariana serpentine mud volcanoes are higher than the seawater value (+ 0.3‰ to + 1.8‰); and the Br/Cl ratios are lower. These pore fluid values and the high Cl concentrations with positive δ 37Cl values (+ 1.2 to + 6.0‰) in the serpentines, support that the upwelling pore fluid originates from dehydration of the subducting slab that releases water enriched in 37Cl, into the fluid phase. The constancy of the ocean δ 37Cl over the past 200 Ma suggests that the isotopically fractionated chlorine in serpentinites and the Cl exchanged in subduction zones are efficiently recycled back into seawater. If the efficiency is < 100%, the residual would be transferred to the mantle, with a maximum Cl flux between 2 to 3 × 10 17 moles/Ma that would lead to an isotopic difference between the mantle and seawater over the age of the earth on the order of a few per mil.

Monitoring and evaluation of dechlorination processes using compound‐specific chlorine isotope analysis

A simple, quick and sensitive method for the compound-specific stable chlorine isotope analysis of chlorinated solvents by conventional quadrupole gas chromatography/mass spectrometry (GC/MS) is presented. With this method, compound-specific stable chlorine isotope ratios of typical chlorinated solvents like tetrachloroethene (PCE) and trichloroethene (TCE) can be determined quantitatively within 30 min by direct injection. The chlorine isotope ratios of target substances are calculated from the peak areas of several selected molecular ions and fragment ions of the substances, using a set of unique mathematical equations. The precision of the method was demonstrated through reproducibility tests. An internal precision of +/-0.4 per thousand to +/-1.1 per thousand was obtained when analyzing PCE and TCE in the 10-1000 pmol range. The validity of the method was further demonstrated by determining the chlorine isotopic fractionation factor during the reductive dechlorination of TCE in a batch experiment using zero-valent iron. The chlorine isotopic fractionation factor was calculated as 0.9976 +/- 0.0011 with a correlation coefficient of 0.9469 (n = 38). The high correlation coefficient indicates that compound-specific stable chlorine isotope analysis can be performed with sufficient accuracy using conventional quadrupole GC/MS when significant fractionation takes place during a reaction. For the first time, the chlorine isotope fractionation factor of TCE during an abiotic anaerobic dechlorination process was determined using quadrupole GC/MS, without offline sample preparation.

Chlorine stable isotopic composition of basement fluids of the eastern flank of the Juan de Fuca Ridge (ODP Leg 168)

This paper presents chlorine stable isotope compositions (δ 37Cl) of sediment pore waters collected by squeezing sediment cores from the sediment–basement interface along an East–West transect through the eastern flank of the Juan de Fuca Ridge (ODP Leg 168). These “near basement fluids” (NBF) are generally thought to be representative of low-temperature fluids circulating in the off-axis basaltic crust. The δ 37Cl value of the fluid directly sampled from a flow at the base of Site 1026 (WSTP1026) is also reported. NBF display δ 37Cl values between − 2.09‰ and − 0.12‰ relative to the Standard Mean Ocean Chloride (SMOC defined as 0‰) and small variations in chlorinity (∼ 4%). These data contrast with the homogeneity of δ 37Cl values associated with highly variable chlorinities observed in high-temperature on-axis fluids [M. Bonifacie, J.L. Charlou, N. Jendrzejewski, P. Agrinier, J.P. Donval, Chlorine isotopic compositions of ridge axis high temperature hydrothermal vent fluids, Chem. Geol. 221(2005) 279–288.]. The NBF δ 37Cl values show a general decreasing trend with distance from the ridge-axis except for two fluids. When plotted against δ 18O values, the δ 37Cl of the NBF show two different trends. This paper discusses the possible contributions on NBF δ 37Cl values of fluid-mixing, water–rock interactions and transport processes (diffusion, ion membrane filtration) that can occur in the igneous basement. However, as none of these processes can fully explain the observed δ 37Cl variations, the potential effect of the sediment cover is also investigated. At site 1026, the interstitial pore fluid displays a δ 37Cl signature significantly lower than that of the fluid discharge sample (− 1.90‰ and − 0.28‰, respectively). This difference, demonstrated here cannot be an artifact of the sampling method, rather indicates the influence of the sediment cover on NBF δ 37Cl values. The potential contributions of physical processes associated with transport/compaction (e.g., diffusion, ion membrane filtration, adsorption, ion exchange) on NBF δ 37Cl values are qualitatively discussed here but require additional studies for further insights. However, this study indicates that “near basement fluids” (NBF) are not, at least for Cl isotopic compositions, necessarily as representative of fluids circulating in the basaltic crust as initially thought. These results add new constraints on Cl geodynamics and show that Cl-isotopes fractionate during low-temperature circulation of fluids in off-axis and off-margin flow contexts, but not to the extent observed for active margins. Fluids circulating at low-temperature in the magmatic and/or the sedimentary part of the oceanic crust might have played a major role on the δ 37Cl evolution of seawater over geologic time.

37Cl, 15N, 13C isotopic analysis of common agro-chemicals for identifying non-point source agricultural contaminants

The isotopic compositions of commercially available herbicides were analyzed to determine their respective 15N, 13C and 37Cl signatures for the purposes of developing a discrete tool for tracing and identifying non-point source contaminants in agricultural watersheds. Findings demonstrate that of the agrochemicals evaluated, chlorine stable isotopes signatures range between δ 37Cl = −4.55‰ and +3.40‰, whereas most naturally occurring chlorine stable isotopes signatures, including those of road salt, sewage sludge and fertilizers, vary in a narrow range about the Standard Mean Ocean Chloride (SMOC) between −2.00‰ and +1.00‰. Nitrogen stable isotope values varied widely from δ 15N = −10.86‰ to +1.44‰ and carbon stable isotope analysis gave an observed range between δ 13C = −37.13‰ and −21.35‰ for the entire suite of agro-chemicals analyzed. When nitrogen, carbon and chlorine stable isotope analyses were compared in a cross-correlation analysis, statistically independent isotopic signatures exist suggesting a new potential tracer tool for identifying herbicides in the environment.

Chlorine isotope homogeneity of the mantle, crust and carbonaceous chondrites

Chlorine in the Earth is highly depleted relative to carbonaceous chondrites and solar abundances. Knowledge of the Cl concentrations and distribution on Earth is essential for understanding the origin of these depletions. Large differences in the stable chlorine isotope ratios of meteoritic, mantle and crustal materials have been used as evidence for distinct reservoirs in the solar nebula and to calculate the relative proportions of Cl in the mantle and crust. Here we report that large isotopic differences do not exist, and that carbonaceous chondrites, mantle and crust all have the same 37Cl/35Cl ratios. We have further analysed crustal sediments from the early Archaean era to the Recent epoch and find no systematic isotopic variations with age, demonstrating that the mantle and crust have always had the same delta37Cl value. The similarity of mantle, crust and carbonaceous chondrites establishes that there were no nebular reservoirs with distinct isotopic compositions, no isotopic fractionation during differentiation of the Earth and no late (post-core formation) Cl-bearing volatile additions to the crustal veneer with a unique isotopic composition.

Pyrohydrolysis-IRMS determination of silicate chlorine stable isotope compositions. Application to oceanic crust and meteorite samples

This contribution describes the optimization of chlorine extraction from silicate samples by pyrohydrolysis prior to the precise determination of Cl stable-isotope compositions ( δ 37Cl) by gas source, dual inlet Isotope Ratio Mass Spectrometry (IRMS) on CH 3Clg. The complete method was checked on three international reference materials for Cl-content and two laboratory glass standards. Whole procedure blanks are lower than 0.5 μmol, corresponding to less than 10 wt.% of most of the sample chloride analysed. In the absence of international chlorine isotope rock, we report here Cl extracted compared to accepted Cl contents and reproducibilities on Cl and δ 37Cl measurements for the standard rocks. After extraction, the Cl contents of the three international references compared within error with the accepted values (mean yield = 94 ± 10%) with reproducibilities better than 12% (1 σ). The laboratory glass standards – andesite SO100DS92 and phonolite S9(2) – were used specifically to test the effect of chloride amount on the measurements. They gave Cl extraction yields of 100 ± 6% (1 σ; n = 15) and 105 ± 8% (1 σ; n = 7), respectively, with δ 37Cl values of − 0.51 ± 0.14‰ and − 0.39 ± 0.17‰ (1 σ). In summary, for silicate samples with Cl contents between 39 and 9042 ppm, the Pyrohydrolysis/HPLC method leads to overall Cl extraction yields of 100 ± 8%, reproducibilities on Cl contents of 7% and on δ 37Cl measurements of ± 0.12‰ (all 1 σ). The method was further applied to ten silicate rocks of various mineralogy and chemistry (meteorite, fresh MORB glasses, altered basalts and serpentinized peridotites) chosen for their large range of Cl contents (70–2156 ppm) and their geological significance. δ 37Cl values range between − 2.33 and − 0.50‰. These strictly negative values contrast with the large range and mainly positive values previously reported for comparable silicate samples and shown here to be affected by analytical problems. Thus we propose a preliminary, revised terrestrial Cl cycle, mainly dominated by negative and zero δ 37Cl values.

Chlorine stable isotope studies of old groundwater, southwestern Great Artesian Basin, Australia

Stable Cl isotope ratios ( 37Cl/ 35Cl) were measured in groundwater samples from the southwestern flow system of the Great Artesian Basin, Australia to gain a better understanding of the Cl − sources and transport mechanisms. δ 37Cl values range from 0‰ to −2.5‰ (SMOC), and are inversely correlated with Cl − concentration along the inferred flow direction. The Cl isotopic compositions, in conjunction with other geochemical parameters, suggest that Cl − in groundwaters is not derived from salt dissolution. Mixing of the recharge water with saline groundwater cannot explain the relationship between δ 37Cl and Cl − concentration measured. Marine aerosols deposited via rainfall and subsequent evapotranspiration appear to be responsible for the Cl − concentrations observed in wells that are close to the recharge area, and in groundwaters sampled along the southern transect. δ 37Cl values measured in the leachate of the Bulldog shale suggest that the aquitard is the subsurface source of Cl − for the majority of groundwater samples studied. Diffusion is likely the mechanism through which Cl − is transported from the pore water of the Bulldog shale to the aquifer. However, a more detailed study of the aquitard rocks is required to verify this hypothesis.

Chlorine isotope vapor–liquid fractionation during experimental fluid-phase separation at 400 °C/23 MPa to 450 °C/42 MPa

We measured experimentally the chlorine isotope fractionation between coexisting vapor and liquid in the system H 2O–NaCl. Experiments were performed between 23 and 28 MPa along the 400 °C isotherm and between 38 and 42 MPa along the 450 °C isotherm. Calculated chlorine isotope fractionation Δ 37Cl vapor–liquid = δ 37Cl vapor − δ 37Cl liquid (1 σ = ± 0.17‰) between coexisting vapor and liquid are generally within 0.2‰ of 0, although there is a trend at both 400 °C and 450 °C in which the Δ 37Cl vapor–liquid values change with pressure beyond statistical uncertainty as follows: P critical − P [MPa] Δ 37Cl vapor–liquid 0 0.0 1.0 to 1.5 + 0.2 2.5 to 3.0 − 0.2 3.5 to 4.0 + 0.2 > 4.5 0.0 This trend is supported by earlier work of Magenheim [A.J. Magenheim, 1995. Oceanic borehole fluid chemistry and analysis of chlorine stable isotopes in silicate rocks. PhD Thesis, University of California, San Diego, 184]. While the fractionation trend is interesting from a theoretical viewpoint, the near-zero Δ 37Cl vapor–liquid values suggest that evolved vapor–liquid samples, such as volcanic fumarolic gases and degassed glasses should faithfully preserve the δ 37Cl value of the fluid responsible for magma production at depth. Even extensive Rayleigh fractionation could not change the chlorine isotope composition of a fluid by more than 0.5‰ from its original degassed value.

Stable and Other Isotope Techniques for Perchlorate Source Identification

Stable isotopes of chlorine, oxygen, nitrogen, strontium, and hydrogen can be used as forensic indicators to fingerprint and possibly differentiate perchlorate plumes. Direct analytical methods involve isotope ratio mass spectrometry (IRMS) of δ37Cl and δ18O, which comprise the perchlorate anion (ClO4 −). Plots of δ37Clperchlorate versus δ 18Operchlorate have been used to differentiate between anthropogenic and geogenic (natural) perchlorate sources. Indirect analytical methods involve the stable isotopic analysis of substances associated with perchlorate. These can be used as surrogates and include δ15N and δ 18O in dissolved nitrate, and 87Sr/86Sr in solid and/or dissolved strontium nitrate associated with potassium perchlorate in safety flares. Strontium compounds may also exist as impurities in ammonium and other perchlorate compounds. Stable isotopes of δ18O and δ2H in water can be used to differentiate different groundwater sources such as those from river recharge or precipitation. Other indirect methods include analysis of radiogenic isotopes such as tritium (T) and helium-3 (3He), which can be used to approximate the mean residence time (“age”) of relatively young groundwater.

Chlorine stable isotope ratios as tracer for pore‐water advection rates in a submarine gas‐hydrate field: implication for hydrate concentration

AbstractAt Ocean Drilling Program Site 997 in Blake Ridge gas‐hydrate field in West Atlantic, pore‐water studies revealed a pronounced downward depletion of the heavy chlorine isotope to nearly −4‰δ37Cl at approximately 750 m below sea floor (mbsf) associated with a 10% downward chlorinity decrease. This is one of the stronger 37Cl depletions hitherto reported for marine pore waters. Chlorinity reductions in hydrate‐bearing sediments commonly result from fresh‐water release by hydrate melting. However, in situ measurements at Site 997 suggest that &gt;50% of the chlorinity reduction occurred prior to hydrate dissociation. Modeling the chlorinity profile shows that advection of a strongly 37Cl‐depleted, low‐chlorinity water (506 mm) from below the drilled sequence can explain the reduction prior to sampling. Fitting the model to the δ37Cl curve yielded an advection rate of 0.18 mm year−1. Diffusive mixing with near‐0‰‐δ37Cl paleo‐seawater with maximum chlorinity at shallow subsurface depths (561 mm at approximately 20 mbsf) produced the smooth, steady trend. Separating that part of the freshening caused by advection and diffusion from that due to hydrate dissociation allowed estimation of average hydrate concentrations of 3.8% of the pore space (up to 24.5% in hydrate‐rich layers; near‐100% in rare massive hydrate layers). The deep‐seated source of the 37Cl‐depleted, low‐chlorinity water remains unknown and might be located below the sedimentary section in the oceanic basement.

A chlorine isotope study of DSDP/ODP serpentinized ultramafic rocks: Insights into the serpentinization process

Eight DSDP/ODP cores were analyzed for major ion concentrations and δ 37Cl values of water-soluble chloride ( δ 37Cl WSC) and structurally bound chloride ( δ 37Cl SBC) in serpentinized ultramafic rocks. This diverse set of cores spans a wide range in age, temperature of serpentinization, tectonic setting, and geographic location of drilled serpentinized oceanic crust. Three of the cores were sampled at closely spaced intervals to investigate downhole variation in Cl concentration and chlorine isotope composition. The average total Cl content of all 86 samples is 0.26 ± 0.16 wt.% (0.19 ± 0.10 wt.% as water-soluble Cl ( X WSC) and 0.09 ± 0.09 wt.% as structurally bound Cl ( X SBC)). Structurally bound Cl concentration nearly doubles with depth in all cores; there is no consistent trend in water-soluble Cl content among the cores. Chlorine isotope fractionation between the structurally bound Cl − site and the water-soluble Cl − site varies from − 1.08‰ to + 1.16‰, averaging to + 0.21‰. Samples with negative fractionations may be related to reequilibration of the water-soluble chloride with seawater post-serpentinite formation. Six of the cores have positive bulk δ 37Cl values (+ 0.05‰ to + 0.36‰); the other two cores (173-1068A (Leg-Hole) and 84-570) have negative bulk δ 37Cl values (− 1.26‰ and − 0.54‰). The cores with negative δ 37Cl values also have variable Cl − / SO 4 2 ratios, in contrast to all other cores. The isotopically positive cores (153-920D and 147-895E) show no isotopic variation with depth; the isotopically negative core (173-1068A) decreases by ∼1‰ with depth for both the water-soluble and structurally bound Cl fractions. Non-zero bulk δ 37Cl values indicate Cl in serpentinites was incorporated during original hydration and is not an artifact of seawater infiltration during drilling. Cores with positive δ 37Cl values are most likely explained by open system fractionation during hydrothermal alteration, with preferential incorporation of 37Cl from seawater into the serpentinite and loss of residual light Cl back to the ocean. Fluid / rock ratios were probably low as evidenced by the presence of water-soluble salts. The two isotopically negative cores are characterized by a thick overlying sedimentary package that was in place prior to serpentinization. We believe the low δ 37Cl values of these cores are a result of hydration of ultramafic rock by infiltrating aqueous pore fluids from the overlying sediments. The resulting serpentinites inherit the characteristic negative δ 37Cl values of the pore waters. Chlorine stable isotopes can be used to identify the source of the serpentinizing fluid and ultimately discern chemical and tectonic processes involved in serpentinization.

Chlorine isotopic compositions of high temperature hydrothermal vent fluids over ridge axes

We present chlorine stable isotope compositions of high temperature hydrothermal vent fluids from two mid-oceanic ridges. The six hydrothermal fluids come from sites along the slow-spreading Mid-Atlantic Ridge (Menez Gwen at 37°50′ N, Lucky Strike at 37°17′ N and Rainbow at 36°14′ N) and the ultrafast-spreading East Pacific Rise (from three contiguous segments centered, respectively, at 17°25′ S, 18°15′ S and 18°26′ S). These samples exhibit a large range in temperature, pH, chlorinity and Mg content reflecting various geological settings and substrata nature (basaltic or ultramafic). Although showing a large range of chlorinity from 251 to 821 mmol/kg, the fluids display homogeneous chlorine isotope signatures: δ 37Cl SMOC = − 0.03 ± 0.06‰, indistinguishable from that of seawater. This homogeneity contrasts with the large range of δ 37Cl previously observed in continental or off-axis oceanic fluids. This study explores possible processes that may change Cl concentration during fluid genesis and discusses their theoretical isotopic effects. The δ 37Cl homogeneity implies that, at the P– T conditions of the ridge axis, the process of phase separation (at sub- or super-critical conditions) that explains most of chlorinity variations, does not significantly fractionate chlorine isotopes. This study shows that the physical and chemical processes that likely alter Cl concentrations in ridge axis systems (i.e. transport processes, mixing or water–rock interaction) contribute to minimize the range of δ 37Cl around the seawater δ 37Cl value.

Application of Multi-stable Isotope (13C, 15N, 34S, 37Cl) Assays to Assess Spatial Separation of Fish (Longnose Sucker Catostomus catostomus) in an Area Receiving Complex Effluents

Abstract Incorporation of stable isotope analysis (SIA) into routine environmental effects monitoring programs of receiving waters may enable determination of the spatial extent of biotic exposure and discrimination among sources of complex effluents. To evaluate this hypothesis, longnose sucker (Catostomus catostomus) were collected from four sites along the Athabasca River, Alberta (upstream reference site, two sites downstream of effluents from two pulp and paper mills, and a site downstream of effluent from a municipal sewage treatment plant). Stable isotopes of carbon, nitrogen, sulfur and chlorine were analyzed in bone, gonad, liver and white muscle tissues of the fish. In general, most sites and tissues differed according to δ13C, δ15N and δ34S values. Also, an interaction between site and tissue was observed for δ15N values. A better insight into the usefulness of stable isotopes was obtained through the use of multivariate discriminant function analysis. δ15N and δ34S signatures of gonad and liver tissues of males were most effective at classifying fish according to site (~70% for both tissues). For all tissues except bone, fish from the upstream reference site were most separable from all others, especially females. δ37Cl values for female gonads and male livers were related to sites downstream of the pulp and paper mills. Future research should routinely include SIA of fish tissues, but also of effluents, receiving waters and food web components to better resolve links between specific effluents and fish exposure.

Cosmogenic, radiogenic, and stable isotopic constraints on groundwater residence time in the Nubian Aquifer, Western Desert of Egypt

Measurements of radiochlorine (36Cl), radiogenic noble gases (4He and 40Ar), and stable chlorine isotope ratios were obtained to assess the residence time of groundwater in the Nubian Aquifer of the Western Desert of Egypt. Measured 36Cl/Cl ratios yield apparent residence times from ∼0.2 to 1.2 × 106 years in the deep (600–1200 m) groundwater (assuming constant Cl) and ≤0.16 × 106 years in the shallow (&lt;600 m) groundwater. Values of δ37Cl in the groundwater strengthen the application of the 36Cl dating method by constraining Cl sources and identifying groundwater mixing. Dissolved gases were measured in some of the deep groundwater samples. Measured 4He concentrations indicate accumulation of radiogenic 4He that is qualitatively consistent with the age progression indicated by the 36Cl/Cl ratios, but the flux of external 4He from the underlying crust has not been quantified and is not constant throughout the aquifer. Concentrations of 40Ar range from 3.3 to 6.7 × 10−4 ccSTP/g and indicate excess air incorporation at recharge. Measured 40Ar/36Ar ratios do not exceed the atmospheric ratio. A two‐dimensional numerical hydrodynamic transect of the aquifer was modeled from the area of the Uweinat Uplift to the northern Bahariya Oasis. Predicted groundwater velocities in the deep portion of the aquifer are 0.5–3.5 m/yr with groundwater residence times up to 9 × 105 years; residence times up to 1.3 × 106 years are predicted in the confining shale. Aquifer properties are estimated by using the model to fit the measured 36Cl/Cl ratios. Under these conditions, hydrodynamic residence times are within about 30% of those calculated from 36Cl when mixing of Cl− is accounted for in the highest‐Cl− deep groundwaters. By mutually calibrating multiple methods (hydrodynamic, 36Cl, and 4He), a consistent picture of the Nubian Aquifer has emerged in which lateral flow from a southern recharge area dominates the deep horizons, while shallow horizons contain younger, autochthonous recharge.

Determination of inorganic chlorine stable isotopes by continuous flow isotope ratio mass spectrometry

Chlorine stable isotope analyses of inorganic samples were conducted using continuous flow isotope ratio mass spectrometry (CF-IRMS) coupled with gas chromatography (GC). Inorganic chloride was precipitated in the form of silver chloride (AgCl) by using silver nitrate in a standard methodology. Chlorine stable isotope analysis was carried out on methyl chloride (CH3Cl) after converting AgCl into CH3Cl by reacting it with methyl iodide (CH3I). The reaction between AgCl and CH3I took place in 20 mL size vials. Addition of CH3I was performed in a glove bag under helium flow. An Agilent 6890 gas chromatograph equipped with a CTC Analytics CombiPAL autosampler and a DB-5MS 60 m column was used to separate CH3Cl from CH3I. This new technique uses samples as small as 0.2 mg of AgCl (1.4 micromol of Cl-). The chlorine stable isotope analysis using continuous flow technology showed excellent precision and accuracy. The internal precision using pure CH3Cl gas is better than +/-0.04 per thousand (+/-STDV). The external precision using seawater standard is better than +/-0.07 per thousand (+/-STDV) for n=12. Moreover, the sample analysis time is much shorter and many more samples can be analyzed in one day than by using the conventional off-line techniques.

Secondary ion mass spectrometry for the determination of δ37Cl : Part II. Intercalibration of SIMS and IRMS for aluminosilicate glasses

Secondary ion mass spectrometry (SIMS) offers unparalleled capability for the in situ determination of chlorine stable isotope ratios ( δ 37Cl) in small objects, such as igneous melt inclusions. However, SIMS analyses must be carefully calibrated for instrumental mass fractionation (IMF) of 37Cl/ 35Cl, which is highly dependent on the major-element composition of the sample. For the purpose of IMF calibration, eight synthetic aluminosilicate glasses, ranging from mafic to felsic composition, were fused from natural and synthetic rock powders with an internally heated pressure vessel (IHPV). This collection of standard materials was supplemented with one natural rhyolite glass. We used alkali fusion, followed by gaseous dual inlet isotope ratio mass spectrometry (AF-IRMS) as the reference technique for δ 37Cl determination of these glasses. Major-element and volatile contents of the glasses were measured by electron microprobe (EMPA) and infrared spectroscopy (FTIR). The absolute difference in IMF between basalt and rhyolite composition glasses can exceed 10‰. However, a strong linear correlation ( R 2=0.99) exists between IMF and expressions of the form (aSiO 2−bAl 2O 3−cCaO+dFeO), where oxides, based on EMPA analysis, are expressed in weight percent. Thus, calibrations based on a small set of glass standards allow SIMS determinations of δ 37Cl over a large range of silicate glass compositions, with an overall reproducibility of single spots better than ±1.0‰ (2 σ). Moreover, δ 37Cl data can be obtained even if the sample does not exactly match the composition of an individual reference glass.