Delta Chromium-53/52 isotopic composition of native and contaminated groundwater,Mojave Desert,USA

Chromium(VI) concentrations in groundwater sampled from three contaminant plumes in aquifers in the Mojave Desert near Hinkley, Topock and El Mirage, California, USA, were as high as 2600, 5800 and 330 μg/L, respectively. δ^53/52Cr compositions from more than 50 samples collected within these plumes ranged from near 0‰ to almost 4‰ near the plume margins. Assuming only reductive fractionation of Cr(VI) to Cr(III) within the plume, apparent fractionation factors for δ^53/52Cr isotopes ranged from ε_app = 0.3 to 0.4 within the Hinkley and Topock plumes, respectively, and only the El Mirage plume had a fractionation factor similar to the laboratory derived value of ε = 3.5. One possible explanation for the difference between field and laboratory fractionation factors at the Hinkley and Topock sites is localized reductive fractionation of Cr(VI) to Cr(III), with subsequent advective mixing of native and contaminated water near the plume margin. Chromium(VI) concentrations and δ^53/52Cr isotopic compositions did not uniquely define the source of Cr near the plume margin, or the extent of reductive fractionation within the plume. However, Cr(VI) and δ^53/52Cr data contribute to understanding of the interaction between reductive and mixing processes that occur within and near the margins of Cr contamination plumes. Reductive fractionation of Cr(VI) predominates in plumes having higher ε_app, these plumes may be suitable for monitored natural attenuation. In contrast, advective mixing predominates in plumes having lower ε_app, the highly dispersed margins of these plumes may be difficult to define and manage.     

A chromate-contaminated site in southern Switzerland – Part 1: Site characterization and the use of Cr isotopes to delineate fate and transport

The risk of groundwater contamination by chromate at a former chromite ore processing industrial site in Rivera (Switzerland) was assessed by determining subsoil Cr(VI) concentrations and tracking naturally occurring Cr(VI) reduction with Cr isotopes. Using a hot alkaline extraction procedure, a total Cr(VI) contamination of several 1000 kg was estimated. Jarosite, KFe₃((SO₄)_x(CrO₄)_1−x)₂(OH)₆, and chromatite (CaCrO₄) were identified as Cr(VI) bearing mineral phases using XRD, both limiting groundwater Cr(VI) concentrations. To track assumed Cr(VI) reduction at field scale δ⁵³Cr values of contaminated subsoil samples in addition to groundwater δ⁵³Cr data are used for the first time. The measurements showed a fractionation of groundwater δ⁵³Cr values towards positive values and subsoil δ⁵³Cr towards negative values confirming reduction of soluble Cr(VI) to insoluble Cr(III). Using a Rayleigh fractionation model, a current Cr(VI) reduction efficiency of approximately 31% along a 120 m long flow path was estimated at an average linear groundwater velocity of 3.3 m/d. Groundwater and subsoil δ⁵³Cr values were compared with a site specific Rayleigh fractionation model proposing that subsoil δ⁵³Cr values can possibly be used to track previous higher Cr(VI) reduction efficiency during the period of industrial activity. The findings strongly favor monitored natural attenuation to be part of the required site remediation measures.     

Occurrence of hexavalent chromium in ground water in the western Mojave Desert,California

About 200 samples from selected public supply, domestic, and observation wells completed in alluvial aquifers underlying the western Mojave Desert were analyzed for total dissolved Cr and Cr(VI). Because Cr(VI) is difficult to preserve, samples were analyzed by 3 methods. Chromium(VI) was determined in the field using both a direct colorimetric method and EPA method 218.6, and samples were speciated in the field for later analysis in the laboratory using a cation-exchange method developed for the study described in this paper. Comparison of the direct colorimetric method and EPA method 218.6 with the new cation-exchange method yielded r² values of 0.9991 and 0.9992, respectively. Total dissolved Cr concentrations ranged from less than the 0.1 μg/l detection limit to 60 μg/l, and almost all the Cr present was Cr(VI). Near recharge areas along the mountain front pH values were near neutral, dissolved O₂ concentrations were near saturation, and Cr(VI) concentrations were less than the 0.1 μg/l detection limit. Chromium(VI) concentrations and pH values increased downgradient as long as dissolved O₂ was present. However, low Cr(VI) concentrations were associated with low dissolved O₂ concentrations near ground-water discharge areas along dry lakes. Chromium(VI) concentrations as high as 60 μg/l occurred in ground water from the Sheep Creek fan alluvial deposits weathered from mafic rock derived from the San Gabriel Mountains, and Cr(VI) concentrations as high as about 36 μg/l were present in ground water from alluvial deposits weathered from less mafic granitic, metamorphic, and volcanic rocks. Chromium(III) was the predominant form of Cr only in areas where dissolved O₂ concentrations were less than 1 mg/l and was detected at a median concentration of 0.1 μg/l, owing to its low solubility in water of near-neutral pH. Depending on local hydrogeologic conditions and the distribution of dissolved O₂, Cr(VI) concentrations may vary considerably with depth. Samples collected under pumping conditions from different depths within wells show that Cr(VI) concentrations can range from less than the 0.1 μg/l detection limit to 36 μg/l in a single well and that dissolved O₂ concentrations likely control the concentration and redox speciation of Cr in ground water.     

Paleoclimate record in the Nubian Sandstone Aquifer,Sinai Peninsula,Egypt

Sixteen groundwater samples collected from production wells tapping Lower Cretaceous Nubian Sandstone and fractured basement aquifers in Sinai were analyzed for their stable isotopic compositions, dissolved noble gas concentrations (recharge temperatures), tritium activities, and ¹⁴C abundances. Results define two groups of samples: Group I has older ages, lower recharge temperatures, and depleted isotopic compositions (adjusted ¹⁴C model age: 24,000–31,000 yr BP; δ¹⁸O: − 9.59‰ to − 6.53‰; δ²H: − 72.9‰ to − 42.9‰; < 1 TU; and recharge T: 17.5–22.0°C) compared to Group II (adjusted ¹⁴C model age: 700–4700 yr BP; δ¹⁸O: − 5.89‰ to − 4.84‰; δ²H: − 34.5‰ to − 24.1‰; < 1 to 2.78 TU; and recharge T: 20.6–26.2°C). Group II samples have isotopic compositions similar to those of average modern rainfall, with larger d-excess values than Group I waters, and locally measurable tritium activity (up to 2.8 TU). These observations are consistent with (1) the Nubian Aquifer being largely recharged prior to and/or during the Last Glacial Maximum (represented by Group I), possibly through the intensification of paleowesterlies; and (2) continued sporadic recharge during the relatively dry and warmer interglacial period (represented by Group II) under conditions similar to those of the present.     

Contrasting isotopic signatures between anthropogenic and geogenic Zn and evidence for post-depositional fractionation processes in smelter-impacted soils from Northern France

Zinc isotopes have been studied along two smelter-impacted soil profiles sampled near one of the largest Pb and Zn processing plants in Europe located in northern France, about 50 km south of Lille. Mean δ⁶⁶Zn values along these two soil profiles range from +0.22 ± 0.17‰ (2σ) to +0.34 ± 0.17‰ (2σ) at the lowest horizons and from +0.38 ± 0.45‰ (2σ) to +0.76 ± 0.14‰ (2σ) near the surface. The δ⁶⁶Zn values in the lowest horizons of the soils are interpreted as being representative of the local geochemical background (mean value +0.31 ± 0.38‰), whereas heavier δ⁶⁶Zn values near the surface of the two soils are related to anthropogenic Zn. This anthropogenic Zn occurs in the form of franklinite (ZnFe₂O₄)-bearing slag grains originating from processing wastes at the smelter site and exhibiting δ⁶⁶Zn values of +0.81 ± 0.20‰ (2σ). The presence of franklinite is indicated by EXAFS analysis of the topsoil samples from both soil profiles as well as by micro-XANES analysis of the surface horizon of a third smelter-impacted soil from a distant site. These results indicate that naturally occurring Zn and smelter-derived Zn exhibit significantly different δ⁶⁶Zn values, which suggests that zinc isotopes can be used to distinguish between geogenic and anthropogenic sources of Zn in smelter-impacted soils. In addition to a possible influence of additional past sources of light Zn (likely Zn-sulfides and Zn-sulfates directly emitted by the smelter), the light δ⁶⁶Zn values in the surface horizons compared to smelter-derived slag materials are interpreted as resulting mainly from fractionation processes associated with biotic and/or abiotic pedological processes (Zn-bearing mineral precipitation, Zn complexation by organic matter, and plant uptake of Zn). This conclusion emphasizes the need for additional Zn isotopic studies before being able to use Zn isotopes to trace sources and pathways of this element in surface environments.     

Geochemistry and Cr stable isotopes of Cr-contaminated groundwater in León valley,Guanajuato, México

León valley, located in the Mexican state of Guanajuato, has a long history (35+ a) of Cr contamination of groundwater and surface water. Here data are presented for Cr, major ion and trace element concentrations and Cr stable isotope measurements of groundwater in a heavily contaminated aquifer in Buenavista, where Cr ore processing residue piles (COPRPs) located in a chromate production factory are the main source of Cr. The aquifer directly beneath the factory still retains very high Cr(VI) concentrations (∼121 mg/L). Ongoing pump and treat remediation is keeping the high concentration plume confined to the factory area and immediate vicinity, though Cr is also detected at some distance away. Chromium isotope data of the aquifer directly under the factory show only a small increase in δ⁵³Cr (+0.33‰ to +0.81‰) and indicates minimal reduction of Cr(VI) to Cr(III). Very high Cr(VI) concentrations have possibly overwhelmed natural reductants and furthermore fresh Cr(VI) was being leached into the groundwater from the COPRP. From just one year of Cr isotope data it is clear that more aggressive remediation techniques will be necessary to reduce or eliminate the contamination. The fringes of the Cr plume have substantially lower concentrations and can be partially explained by transport of the main plume or mixing between waters from the nearby landfill and highly contaminated waters from QC. While the source of Cr at the fringes of the plume could be attributed to a source from the nearby landfill instead of the main plume from the factory, the Cr stable isotope data show enrichment in the heavier isotopes and point to varying amounts of reduction. Isotopic enrichment seen between 2007 and 2008 along the fringe may indicate either reduction or less unreacted Cr(VI) is being transported to the fringes. In either case some potential for natural attenuation of Cr(VI) exists at the western margin of the plume with the landfill playing a role.     

Stable isotope and petrologic evidence for open-system degassing during the climactic and pre-climactic eruptions of Mt. Mazama, Crater Lake, Oregon

Evaluation of the extent of volatile element recycling in convergent margin volcanism requires delineating likely source(s) of magmatic volatiles through stable isotopic characterization of sulfur, hydrogen and oxygen in erupted tephra with appropriate assessment of modification by degassing. The climactic eruption of Mt. Mazama ejected approximately 50 km³ of rhyodacitic magma into the atmosphere and resulted in formation of a 10-km diameter caldera now occupied by Crater Lake, Oregon (lat. 43°N, long. 122°W). Isotopic compositions of whole-rocks, matrix glasses and minerals from Mt. Mazama climactic, pre-climactic and postcaldera tephra were determined to identify the likely source(s) of H₂O and S. Integration of stable isotopic data with petrologic data from melt inclusions has allowed for estimation of pre-eruptive dissolved volatile concentrations and placed constraints on the extent, conditions and style of degassing.Sulfur isotope analyses of climactic rhyodacitic whole rocks yield δ³⁴S values of 2.8-14.8‰ with corresponding matrix glass values of 2.4-13.2‰. δ³⁴S tends to increase with stratigraphic height through climactic eruptive units, consistent with open-system degassing. Dissolved sulfur concentrations in melt inclusions (MIs) from pre-climactic and climactic rhyodacitic pumices varies from 80 to 330 ppm, with highest concentrations in inclusions with 4.8-5.2 wt% H₂O (by FTIR). Up to 50% of the initial S may have been lost through pre-eruptive degassing at depths of 4-5 km. Ion microprobe analyses of pyrrhotite in climactic rhyodacitic tephra and andesitic scoria indicate a range in δ³⁴S from −0.4‰ to 5.8‰ and from −0.1‰ to 3.5‰, respectively. Initial δ³⁴S values of rhyodacitic and andesitic magmas were likely near the mantle value of 0‰. Hydrogen isotope (δD) and total H₂O analyses of rhyodacitic obsidian (and vitrophyre) from the climactic fall deposit yielded values οf −103 to −53‰ and 0.23-1.74 wt%, respectively. Values of δD and wt% H₂O of obsidian decrease towards the top of the fall deposit. Samples with depleted δD, and mantle δ¹⁸O values, have elevated δ³⁴S values consistent with open-system degassing. These results imply that more mantle-derived sulfur is degassed to the Earth’s atmosphere/hydrosphere through convergent margin volcanism than previously attributed. Magmatic degassing can modify initial isotopic compositions of sulfur by >14‰ (to δ³⁴S values of 14‰ or more here) and hydrogen isotopic compositions by 90‰ (to δD values of −127‰ in this case).     

Nitrogen isotope fractionations during progressive metamorphism: A case study from the Paleozoic Cooma metasedimentary complex, southeastern Australia

The well-studied Paleozoic Cooma metamorphic complex in southeastern Australia is characterized by a uniform siliciclastic protolith, of uniform age, with a continuous range of metamorphic grade from subgreenschist- to upper amphibolite-facies, and migmatite-grade in an annular pattern around the Cooma granodiorite. Those conditions are optimal for investigating variations of N concentrations and δ¹⁵N values during progressive metamorphism. Nitrogen concentrations decrease and δ¹⁵N increases with increasing metamorphic grade (sub-chlorite zone: 120 ppm N, δ¹⁵N = 2.3‰; chlorite zone: 110 ppm N, δ¹⁵N = 3.0‰; biotite and andalusite zone: 85 ppm N, δ¹⁵N = 3.8 ‰; sillimanite and migmatite zones: 40 ppm N, δ¹⁵N = 10.7‰). Covariation of K and N contents is consistent with N substituting for K as NH₄⁺ in micas. Observed trends of increasing δ¹⁵N values with decreasing nitrogen concentrations can be explained by a continuous release of nitrogen depleted in ¹⁵N with progressive metamorphism, which causes an enrichment of ¹⁵N in the residual nitrogen of the rock. Equilibrium models for Rayleigh distillation and batch volatilisation for data of the greenschist and amphibolite facies metasedimentary rocks can be explained by N₂-NH₄⁺ exchange at temperatures of 300-600 °C, whereas observed large fractionations for the upper amphibolite-facies and melt products in the migmatite-grade samples may be interpreted as NH₃-NH₄⁺ exchanges at temperature of 650-730 °C. Lower values in the highest grade zones may also stem in part from input of ¹⁵N-depleted fluids from the granodiorite.The magnitude of isotope fractionation of nitrogen is about 1-2‰ during progressive metamorphism of metasedimentary rocks from sub-chlorite zone to biotite-andalusite zone, which is consistent with previous studies. Consequently, the large spread of δ¹⁵N values in Archean greenschist-facies metasedimentary rocks of −6‰ to 30‰ can be accounted for by variable mixtures of mantle plume-dominated volatiles with a δ¹⁵N of −5‰, and a ¹⁵N-enriched marine sedimentary kerogen component inherited from a CI chondrite veneer having δ¹⁵N of 30‰ to 42‰.     

Tracing dissolved O2 and dissolved inorganic carbon stable isotope dynamics in the Nyack aquifer: Middle Fork Flathead River, Montana, USA

The geochemistry and microbiology of shallow groundwater aquifers is greatly influenced by the concentration of dissolved oxygen gas (DO); however, the mechanisms that consume DO in groundwater (e.g., biotic or abiotic) are often ambiguous. The use of stable isotopes of molecular O₂ (δ¹⁸O-DO), in conjunction with stable isotopes of dissolved inorganic carbon (δ¹³C-DIC), has potential to discriminate between the various mechanisms causing DO depletion in subsurface waters.Here we report the results of spatial and seasonal changes in δ¹⁸O-DO and δ¹³C-DIC at the Nyack floodplain aquifer along the Middle Fork of the Flathead River near West Glacier, Montana, USA. Over a short, well constrained flow path (∼100 m) near a main recharge zone of the floodplain, the δ¹⁸O-DO consistently increased as DO concentrations decreased with distance from the recharge source. Concurrently, DIC concentrations increased and δ¹³C-DIC values decreased. These observations are explained by community respiration coupled with dissolution of calcite from cobbles in the aquifer matrix. When these results are compared to data from wells distributed over the entire floodplain (several km) a much less predictable relationship was observed between DO concentration and δ¹⁸O-DO. Many wells with low DO concentrations (e.g., <125 μmol L⁻¹ or 4 mg L⁻¹) had anomalously low δ¹⁸O-DO values (e.g., <20‰). Mass balance calculations show that approximately equal amounts of O₂ may be contributed to the aquifer by diffusion from the vadose zone and by advection from the river recharge. Calculations presented here suggest that diffusion across a narrow air-water interface can contribute isotopically light δ¹⁸O-DO to the saturated zone. Possible contributions of light δ¹⁸O-DO from other processes, such as isotopic exchange and radial oxygen loss from plant roots in or near the water table, are compared and evaluated.     

Stable sulfur and oxygen isotope ratios of the Świętokrzyski National Park spring waters generated by natural and anthropogenic factors (south-central Poland)

This paper presents the results of an isotopic study of spring waters in ?wi?tokrzyski (Holy Cross Mountain) National Park (?NP), south-central Poland. The δ³⁴S_V-CDT and δ¹⁸O_V-SMOW of soluble sulfates (n = 40) varied from 0.5‰ to 18.1‰ and from 3.5‰ to 12.2‰, respectively. The average δ³⁴S values are closely similar to those of rainwater, soils and rocks (comprising scattered pyrite). This suggests that soluble sulfates in the springs originated from mixing of recent and historic deposition, sulfates derived from pyrite oxidation, and CS-mineralization in soils and debris. An additional anthropogenic sulfur input (inorganic fertilizer) occurs in the water of spring S-61 located in the ?wi?tokrzyski National Park buffer zone. The δ¹⁸O_V-SMOW of spring waters (n = 4) were in the range of −10.6‰ to −10.2‰ indicating that they are derived from vadose groundwater in ?NP. This was the first isotope study of spring waters in the national parks of Poland. It enabled the determination of sulfur pathways and discrimination between natural and anthropogenic sources of this element in a relatively pristine area.     

Chromium oxidation by manganese (hydr)oxides in a California aquifer

There are increasing concerns with elevated levels of Cr(VI) in the environment because it is a strong oxidant, corrosive, and carcinogenic. The concerns extend to the presence of Cr(VI) in many aquifers in California and elsewhere, where relatively high levels have been attributed to both industrial pollution and natural processes. The authors have, therefore, determined if natural redox processes contribute to the presence of high Cr(VI) concentrations (6–36 μg L⁻¹) in an aquifer in central California relative to non-detectable concentrations (<0.1 μg L⁻¹) in an adjacent aquifer. Specifically, the distribution and the redox speciation of dissolved (<0.45 μm) Cr have been compared with those of particulate Mn and Fe oxy-hydroxides in sediments, using X-ray absorption spectroscopy at the Mn and Fe L-edges. The analyses show a correlation between the presence of dissolved Cr(VI) and Mn (hydr)oxide minerals, which are the only common, naturally occurring minerals known to oxidize Cr(III) in laboratory experiments. This covariance substantiates the results of those experiments and previous field studies that indicate natural oxidation mechanisms might account for the relatively high levels of Cr(VI) in the study site, as well as for elevated concentrations in other aquifers with similar biogeochemical conditions.     

Geochemical features and origin of natural gas in heavy oil area of the Western Slope,Songliao Basin,China

The Western Slope of the Songliao Basin is rich in heavy oil resources (>70 × 10⁸ bbl), around which there are shallow gas reservoirs (∼1.0 × 10¹² m³). The gas is dominated by methane with a dryness over 0.99, and the non-hydrocarbon component being overwelmingly nitrogen. Carbon isotope composition of methane and its homologs is depleted in ¹³C, with δ¹³C₁ values being in the range of −55‰ to −75‰, δ¹³C₂ being in the range of −40‰ to −53‰ and δ¹³C₃ being in the range of −30‰ to −42‰, respectively. These values differ significantly from those solution gases source in the Daqing oilfield. This study concludes that heavy oils along the Western Slope were derived from mature source rocks in the Qijia-Gulong Depression, that were biodegraded. The low reservoir temperature (30–50 °C) and low salinity of formation water with neutral to alkaline pH (NaHCO₃) appeared ideal for microbial activity and thus biodegradation. Natural gas along the Western Slope appears mainly to have originated from biodegradation and the formation of heavy oil. This origin is suggested by the heavy δ¹³C of CO₂ (−18.78‰ to 0.95‰) which suggests that the methane was produced via fermentation as the terminal decomposition stage of the oil.     

Geochemistry of natural chromium occurrence in a sandstone aquifer in Bauru Basin, São Paulo State, Brazil

Anomalous concentrations of Cr(VI) occur in groundwaters of the Adamantina Aquifer, in a large region in the western state of São Paulo, sometimes exceeding the potability limit (0.05 mg L⁻¹). To identify the possible geochemical reactions responsible for the occurrence of Cr in groundwater in Urânia, borehole rock samples were collected in order to carry out mineralogical and chemical analyses. In addition, multilevel monitoring wells were installed and groundwater samples were analyzed. Analyses of the borehole rock samples show the occurrence of a geochemical anomaly of Cr in the quartzose sandstones (average concentrations of 221 ppm). Chrome-diopside is one of the main minerals contributing to this anomaly, having an average Cr content of 1505 ppm. Sequential extraction experiments indicated weakly adsorbed Cr in the order of 0.54 ppm, and this quantity is enough to provide the Cr concentrations observed in groundwater. Groundwaters from the monitoring wells proved to be stratified, with the highest concentrations of Cr(VI) (0.13 mg L⁻¹) being associated with high redox and pH values (over 10) and high concentrations of Na. Geochemical reactions that may explain the release of Cr from the solid phase to groundwater involve the release of Cr(III) from minerals (like chrome-diopside and Cr-Fe hydroxide), followed by oxidation of Cr(III) to Cr(VI), probably related to the reduction of Mn oxides present in the aquifer. Then cation exchange occurs and dissolution of carbonates which increases the pH of groundwater, resulting in the desorption and mobilization of Cr(VI) into groundwater.     

Boron isotopic composition of atmospheric precipitations and liquid-vapour fractionations

Boron isotope compositions (δ¹¹B) and B concentrations of rains and snows were studied in order to characterize the sources and fractionation processes during the boron atmospheric cycle. The ¹¹B/¹⁰B ratios of instantaneous and cumulative rains and snows from coastal and continental sites show a large range of variations, from −1.5 ± 0.4 to +26.0 ± 0.5‰ and from −10.2 ± 0.5 to +34.4 ± 0.2‰, respectively. Boron concentrations in rains and snows vary between 0.1 and 3.0 ppb. All these precipitation samples are enriched in ¹⁰B compared to the ocean value (δ¹¹B = +39.5‰). An empirical rain-vapour isotopic fractionation of +31‰ is estimated from three largely independent methods. The deduced seawater-vapour fractionation is +25.5‰, with the difference between the rain and seawater fractionations principally reflecting changes in the speciation of boron in the liquid with ∼100% B(OH)₃ present in precipitations. A boron meteoric water line, δD = 2.6δ¹¹B − 133, is proposed which describes the relationship between δD and δ¹¹B in many, but not all, precipitations. Boron isotopic compositions of precipitations can be related to that of the seawater reservoir by the seawater-vapour fractionation and one or more of (1) the rain-vapour isotopic fractionation, (2) evolution of the δ¹¹B value of the atmospheric vapour reservoir via condensation-precipitation processes (Rayleigh distillation process), (3) any contribution of vapour from the evaporation of seawater aerosols, and (4) any contribution from particulate matter, principally sea salt, continental dust and, perhaps more regionally, anthropogenic sources (burning of biomass and fossil fuels). From the δ¹¹B values of continental precipitations, a sea salt contribution cannot be more than a percent or so of the total B in precipitation over these areas.     

Application of sulphur isotope ratios to examine weaning patterns and freshwater fish consumption in Roman Oxfordshire, UK

This study investigates the application of sulphur isotope ratios (δ³⁴S) in combination with carbon (δ¹³C) and nitrogen (δ¹⁵N) ratios to understand the influence of environmental sulphur on the isotopic composition of archaeological human and faunal remains from Roman era sites in Oxfordshire, UK. Humans (n = 83), terrestrial animals (n = 11), and freshwater fish (n = 5) were analysed for their isotope values from four locations in the Thames River Valley, and a broad range of δ³⁴S values were found. The δ³⁴S values from the terrestrial animals were highly variable (−13.6‰ to +0.5‰), but the δ³⁴S values of the fish were clustered and ³⁴S-depleted (−20.9‰ to −17.3‰). The results of the faunal remains suggest that riverine sulphur influenced the terrestrial sulphur isotopic signatures. Terrestrial animals were possibly raised on the floodplains of the River Thames, where highly ³⁴S-depleted sulphur influenced the soil. The humans show the largest range of δ³⁴S values (−18.8‰ to +9.6‰) from any archaeological context to date. No differences in δ³⁴S values were found between the males (−7.8 ± 6.0‰) and females (−5.3 ± 6.8‰), but the females had a linear correlation (R² = 0.71; p < 0.0001) between their δ¹⁵N and δ³⁴S compositions. These δ³⁴S results suggest a greater dietary variability for the inhabitants of Roman Oxfordshire than previously thought, with some individuals eating solely terrestrial protein resources and others showing a diet almost exclusively based on freshwater protein such as fish. Such large dietary variability was not visible by analysing only the carbon and nitrogen isotope ratios, and this research represents the largest and most detailed application of δ³⁴S analysis to examine dietary practices (including breastfeeding and weaning patterns) during the Romano-British Period.     

Stable Cu isotope fractionation in soils during oxic weathering and podzolization

Copper stable isotope ratios are fractionated during various biogeochemical processes and may trace the fate of Cu during long-term pedogenetic processes. We assessed the effects of oxic weathering (formation of Cambisols) and podzolization on Cu isotope ratios (δ⁶⁵Cu). Two Cambisols (oxic weathered soils without strong vertical translocations of soil constituents) and two Podzols (soils showing vertical translocation of organic matter, Fe and Al) were analyzed for Cu concentrations, partitioning of Cu in seven fractions of a sequential extraction and δ⁶⁵Cu values in bulk soil. Cu concentrations in the studied soils were low (1.4-27.6 μg g⁻¹) and Cu was mainly associated with strongly bound Fe oxide- and silicate-associated forms. Bulk δ⁶⁵Cu values varied between −0.57‰ and 0.44‰ in all studied horizons. The O horizons had on average significantly lighter Cu isotope compositions (−0.21‰) than the A horizons (0.13‰) which can either be explained by Cu isotope fractionation during cycling through the plants or deposition of isotopically light Cu from the atmosphere. Oxic weathering without pronounced podzolization in both Cambisols and a weakly developed Podzol (Haplic Podzol 2) caused no significant isotope fractionation in the single profiles, while a slight tendency to lower δ⁶⁵Cu values with depth was visible in all four profiles. This is the opposite depth distribution of δ⁶⁵Cu values to that we observed in hydromorphic soils (soils which show indication of redox changes because of the influence of water saturation) in a previous study. In a more pronounced Podzol (Haplic Podzol 1), δ⁶⁵Cu values and Cu concentrations decreased from Ah to E horizons and increased again deeper in the soil. Humus-rich sections of the Bhs horizon had higher Cu concentrations (2.8 μg g⁻¹) and a higher δ⁶⁵Cu value (−0.18‰) than oxide-rich sections (1.9 μg g⁻¹, −0.35‰) suggesting Cu translocation between E and B horizons as organo-Cu complexes. The different depth distributions in oxic weathered and hydromorphic soils and the pronounced vertical differences in δ⁶⁵Cu values in Haplic Podzol 1 indicate a promising potential of δ⁶⁵Cu values to improve our knowledge of the fate of Cu during long-term pedogenetic processes.     

Cl,N, C 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 δ³⁷Cl = −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 δ¹⁵N = −10.86‰ to +1.44‰ and carbon stable isotope analysis gave an observed range between δ¹³C = −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.     

Non-biological fractionation of stable Ca isotopes in soils of the Atacama Desert, Chile

We measured Ca stable isotope ratios (δ^44/40Ca) in an ancient (2 My), hyperarid soil where the primary source of mobile Ca is atmospheric deposition. Most of the Ca in the upper meter of this soil (3.5 kmol m⁻²) is present as sulfates (2.5 kmol m⁻²), and to a lesser extent carbonates (0.4 kmol m⁻²). In aqueous extracts of variably hydrated calcium sulfate minerals, δ^44/40Ca_E values (vs. bulk Earth) increase with depth (1.4 m) from a minimum of −1.91‰ to a maximum of +0.59‰. The trend in carbonate-δ^44/40Ca in the top six horizons resembles that of sulfate-δ^44/40Ca, but with values 0.1-0.6‰ higher. The range of observed Ca isotope values in this soil is about half that of δ^44/40Ca values observed on Earth. Linear correlation among δ^44/40Ca, δ³⁴S and δ¹⁸O values indicates either (a) a simultaneous change in atmospheric input values for all three elements over time, or (b) isotopic fractionation of all three elements during downward transport. We present evidence that the latter is the primary cause of the isotopic variation that we observe. Sulfate-δ³⁴S values are positively correlated with sulfate-δ¹⁸O values (R² = 0.78) and negatively correlated with sulfate δ^44/40Ca_E values (R² = 0.70). If constant fractionation and conservation of mass with downward transport are assumed, these relationships indicate a δ^44/40Ca fractionation factor of −0.4‰ in CaSO₄. The overall depth trend in Ca isotopes is reproduced by a model of isotopic fractionation during downward Ca transport that considers small and infrequent but regularly recurring rainfall events. Near surface low Ca isotope values are reproduced by a Rayleigh model derived from measured Ca concentrations and the Ca fractionation factor predicted by the relationship with S isotopes. This indicates that the primary mechanism of stable isotope fractionation in CaSO₄ is incremental and effectively irreversible removal of an isotopically enriched dissolved phase by downward transport during small rainfall events.     

Multi-isotopic composition (δLi–δB–δD–δO) of rainwaters in France: Origin and spatio-temporal characterization

In the present work, the first results are reported for both Li and B isotope ratios in rainwater samples collected over a long time period (i.e. monthly rainfall events over 1 a) at a national scale (from coastal and inland locations). In addition, the stable isotopes of the water molecule (δD and δ¹⁸O) are also reported here for the same locations so that the Li and B isotope data can be discussed in the same context. The range of Li and B isotopic variations in these rainwaters were measured to enable the determination of the origin of these elements in rainwaters and the characterization of both the seasonal and spatio-temporal effects for δ⁷Li and δ¹¹B signatures in rainwaters. Lithium and B concentrations are low in rainwater samples, ranging from 0.004 to 0.292 μmol/L and from 0.029 to 6.184 μmol/L, respectively. δ⁷Li and δ¹¹B values in rainwaters also show a great range of variation between +3.2‰ and +95.6‰ and between −3.3‰ and +40.6‰ over a period of 1 a, respectively, clearly different from the signature of seawater. Seasonal effects (i.e. rainfall amount and month) are not the main factors controlling element concentrations and isotopic variations. δ⁷Li and δ¹¹B values in rainwaters are clearly different from one site to another, indicating the variable contribution of sea salts in the rainwater depending on the sampling site (coastal vs. inland: also called the distance-from-the-coast-effect). This is well illustrated when wind direction data (origin of air masses) is included. It was found that seawater is not the main supplier of dissolved atmospheric Li and B, and non-sea-salt sources (i.e. crustal, anthropogenic, biogenic) should also be taken into account when Li and B isotopes are considered in hydrogeochemistry as an input to surface waters and groundwater bodies as recharge. In parallel, the isotopic variations of the water molecule, vector of the dissolved B and Li, are also investigated and reported as a contour map for δ¹⁸O values based on compiled data including more than 400 δ¹⁸O values from throughout France. This δ¹⁸O map could be used as a reference for future studies dealing with δ¹⁸O recharge signature in relation to the characterization of surface waters and/or groundwater bodies.