H41J-01
Mass Dependent and Mass Independent Fractionation of Hg Isotopes and Estimation of Photochemical Loss of Hg in Aquatic Systems
Mercury is a globally distributed and highly toxic pollutant, the mobility and bioaccumulation of which is dependent on its redox cycling. Hg isotope analysis is an important new tool for identifying Hg sources and tracking Hg transformations in the environment. Most natural samples analyzed for Hg isotopes display mass-dependent isotope fractionation (MDF), but a small body of data suggests that some natural samples also display mass- independent isotope fractionation (MIF) of the odd Hg isotopes. Here we document MIF of Hg isotopes during an important natural process, constrain the potential mechanism of isotope fractionation, and apply the MIF observed in natural samples to quantify the photochemical reduction of Hg species in the environment. Reduction of Hg species to Hg0 vapor is an important pathway for removal of Hg from aqueous systems into the atmosphere and occurs by abiotic and biotic mechanisms. In laboratory experiments, we find that photochemical reduction Hg species by natural sunlight leads to large MIF of the odd isotopes. Also, the relationship between MIF for the two odd isotopes of Hg is significantly different for different photo-reduction pathways. In contrast, both biological reduction (Kritee et al., 2006) and dark abiotic organically-mediated reduction follow MDF. Natural samples from aquatic ecosystems preserve both MDF and MIF. In fish, MDF increases with the size and Hg concentration of fish suggesting MDF may be useful in understanding Hg bioaccumulation. Fish also display a large range in MIF (4‰), and the relationship between the MIF of the two odd isotopes in fish has a similar slope to the slope found for photo-reduction of CH3Hg+. Since fish bioaccumulate CH3Hg+, fish may be recording the extent to which CH3Hg+ is lost via photochemical reduction in an aquatic ecosystem. Fish populations from different locations have different MIF values, but mostly display similar MIF within a given locale. This suggests that MIF is preserved in the food web and could be used to quantify photo-reduction of CH3Hg+ in ecosystems. Both MDF and MIF of Hg isotopes will be useful for quantifying and understanding Hg biogeochemical cycling in the environment.
H41J-02
Using Zn isotopes to investigate trace metal partitions between dissolved and suspended loads in the Seine River, France
Recent improvements in MC-ICP-MS allow measuring isotope compositions of Zn, a particular interesting element due to its five isotopes, bio-vital nature, essential use and global presence, this makes Zn isotopes a potential device to investigate metal comportments in geochemical and biochemical system, but only few studies have focused on Zn isotopes in aqueous environment, leaving an important gap in research of Zn hydrological and geochemical cycle. Our research focusing on the Zn isotope geochemistry in the largely anthropized Seine River completes the gap of recent works on Zn isotopes, moreover it is the first in depth study using Zn isotopes to assess the metal partition between dissolved and particulate phases since it plays an important role in the transport and the fate of trace metals in water system. We have developed a new protocol of two-column separation of Zn from dilute aqueous solution, the protocol is proven to be reproducible by tests on standard-doped distilled and column-purified Seine river waters, the mean δ66Zn value is -0.006‰ for all purification tests with an average yield of about 100%, permitting accurate measurement of Zn isotopes ratios in samples of the Seine watershed. Repeated measurements of Zn AAS standard (Alfa Aesar) on MC-ICP-MS (Neptune) during two years led to a precision of 0.04‰ (2s). Preliminary results show a total variation of 0.65‰ for δ66Zn in dissolved phases of the Seine basin, and a light isotope enrichment in anthropogenic sources compared to other water samples. The determined conservative behaviour of Zn in river water makes Zn isotopes an effective probe of anthropogenic contamination. Suspended particular matters (SPM) display different Zn isotope compositions compared to dissolved loads. Zn concentrations and its isotope compositions in SPM reveal inverse relationships as function of the distance from the headwater and the SPM content for geographical and temporal samples, respectively. Isotopic offsets between particulate and dissolved phases show variable relationships according to the major element ratio, the SPM content and other parameters, providing us more clues about mechanisms controlling trace metals partitions in river water system.
H41J-03
Isotopes of Dissolved Copper and Zinc in Stream Waters
A variety of biological and geochemical processes control the cycling and availability of Zn and Cu in stream waters. It is likely that these mechanisms result in distinctive mass dependent fractionations of the stable isotopes of dissolved Zn and Cu. The relative abundances of these isotopes may elucidate biogeochemical reactions impacting stream water chemistry and/or fingerprint the sources of these metals in natural waters. Stream waters from six historic mining districts located in the United States and Europe were examined for their relative abundances of dissolved Zn and Cu isotopes. Isotopic signatures were measured as a function of time to determine whether changes in biogeochemistry were recorded over well-defined diel (24-hour) metal cycles. Average δ66Zn and δ65Cu values for the streams varied from +0.02 to +0.46 ‰ and -0.7 to +1.4 ‰, respectively, demonstrating that Zn and Cu isotopic signatures are heterogeneous in the measured streams. There appears to be no correlation between the isotopic signatures for Zn and stream water parameters (pH, Zn-speciation, organic content, flow rate, etc.). Hence, we suspect that differences in the Zn isotopic composition among the metal source materials strongly influenced the measured Zn isotopic signatures of the stream waters. Cu isotopic data was limited to just three streams, and the source(s) of Cu isotopic variation remain unclear. In most of the streams isotopic changes were not determined for Zn or Cu within the resolution of our measurements over diel cycles. However, diel changes in Zn isotopes were recorded in a single dataset where the fluctuation of dissolved Zn was the greatest. We calculate a separation factor of 0.35 ‰ between the dissolved and solid Zn reservoirs in this stream with the solid Zn reservoir preferring the lighter Zn isotope. Although several explanations for the diel variation in Zn isotopes are possible, we speculate that the preference for the lighter isotope may reflect metabolic uptake by microorganisms.
H41J-04
The Impact of Different Acids on the Fractionation of Cu and Zn Isotopes During Leaching in Open Systems.
Despite great promise, our ability to apply transition metal isotopic measurements to hydrologic systems remains limited by our knowledge of how metal isotopes are impacted by weathering under different chemical and mechanical pathways. With this in mind, we designed experiments to examine the impact of different anions (NO3-, Cl-, and SO42-) on the fractionation of Cu and Zn during leaching of a sulfide- rich rock by acidic (pH ~ 2.0) waters. Experiments consisted of five 6 hour cycles, where after each cycle the solid was separated from the solution and then allowed to react with a fresh batch of acidic solution. In this way, the experiments simulated an open system in which pulses of water leach the rock and are then removed. This is a significantly different approach than close-system leach experiments that have investigated metal isotopes. Zinc and Copper isotopic data shows that the magnitude and direction of isotope effects during cyclic leaching is partly dependent on the anion used during the leaching experiment. Moreover, the isotopic signatures of aqueous Cu and Zn in an open system fluctuate as leaching cycles progress. Leaching of zinc by HNO3 results in aqueous zinc that shows a constant enrichment of δ66Zn ~ 0.2‰ relative to the isotopic composition of Zn in the rock. In contrast, zinc leached by H2SO4 shows no resolvable fractionation relative to the zinc in the rock, and zinc leached by HCl is increasingly light with each progressive leaching cycle. Isotopic measurements of copper from the experiments in which HNO3 and HCl were used demonstrate that Cu in leach fluids gets progressively lighter with each cycle. And, in the case of leaching by H2SO4, δ65Cu values decrease in the first 3 cycles and then level to a value ~ 0.8 ‰ lighter than the isotopic composition of Cu in the rock. Our results suggest that the relationship between the isotopes of Cu and Zn source rocks and the isotopes of Cu and Zn in hydrologic systems is not straightforward, and isotopic fractionations of Cu and Zn during weathering are impacted both by the chemistry and degree of leaching.
H41J-05 INVITED
Chromium isotope variation along a contaminated groundwater plume: a coupled Cr(VI)- reduction, advective mixing perspective
Chromium (Cr) is a common contaminant in groundwater, used in electroplating, leather tanning, wood preservation, and as an anti-corrosion agent. Cr occurs in two oxidation states in groundwater: Cr(VI) is highly soluble and mobile, and is a carcinogen; Cr(III) is generally insoluble, immobile and less toxic than Cr(VI). Reduction of Cr(VI) to Cr(III) is thus a central issue in approaches to Cr(VI) contaminant remediation in aquifers. Aqueous Cr(VI) occurs mainly as the chromate (CrO22-) and bichromate (HCrO2-) oxyanions, while Cr(III) is mainly "hexaquo" Cr(H2O)63+. Cr has four naturally-occurring stable isotopes: 50Cr, 52Cr, 53Cr and 54Cr. When Cr(VI) is reduced to Cr(III), the strong Cr-O bond must be broken, resulting in isotopic selection. Ellis et al. (2002) demonstrated that for reduction of Cr(VI) on magnetite and in natural sediment slurries, the change of isotopic composition of the remnant Cr(VI) pool was described by a Rayleigh fractionation model having fractionation factor εCr(VI)-Cr(III) = 3.4‰. We attempted to use Cr isotopes as a monitor of Cr(VI) reduction at a field site in Hinkley, California (USA) where groundwater contaminated with Cr(VI) has been under assessment for remediation. Groundwater containing up to 5 ppm Cr(VI) has migrated down-gradient from the contamination source through the fluvial to alluvial sediments to form a well-defined plume. Uncontaminated groundwater in the aquifer immediately adjacent to the plume has naturally-occurring Cr(VI) of 4 ppb or less (CH2M-Hill). In early 2006, colleagues from CH2M-Hill collected 17 samples of groundwater from within and adjacent to the plume. On a plot of δ53Cr vs. log Cr(VI), the data array is strikingly linear and differs markedly from the trend predicted for reduction of Cr(VI) in the contaminated water. There appear to be two groups of data: four samples with δ53Cr >+2‰ and Cr(VI) <4 ppb, and 13 samples with δ53Cr <+2‰ and Cr(VI) >15 ppb. Simple mixing lines between the groundwater samples having <4 ppb Cr(VI), taken to be representative of regional groundwater, and the contaminated water do not pass through the remainder of the data, discounting a simple advective mixing scenario. We hypothesize a more likely scenario that involves both Cr(VI) reduction and advective mixing. As the plume initially expands downgradient, Cr(VI) in water at the leading edge encounters reductant in the aquifer resulting in limited Cr(VI) reduction. As a result of reduction, δ53Cr of Cr(VI) remaining in solution at the leading edge increases along the "reduction" trend from 0 to ~+2‰. Inevitable mixing of this water at the leading edge with regional groundwater results in a suitable mixing end-member to combine with Cr(VI) within the plume in order to explain the bulk of the remaining data. Neither Cr(VI) reduction nor advective mixing of plume and regional groundwaters can explain the data on their own, implying an interplay of at least these two processes during plume evolution. Ellis, A.S., Johnson, T.M. and Bullen, T.D. 2002, Science, 295, 2060-2062.
H41J-06
Selenium enrichment in weathering zones: Se isotopes at Yutangba, China
Selenium-rich carbonaceous chert and carbonaceous shale occur in the Permian Maokou Fm. of Hubei Province, China; humans and livestock have been adversely affected by the Se. In Yutangba, 81km SE of Enshi city, rock and soil can exceed 1000 mg/kg. We used Se isotope ratio data to reveal the mechanism of Se enrichment. Mass-dependent fractionation of Se isotopes is caused mainly by reduction of Se oxyanions and thus serves as an indicator these reactions. 82Se/76Se ratios were measured by double spike MC-ICP-MS, with 0.20 per mil precision. Unweathered drill core samples from >40 m depth, averaging 108 ±54 mg/kg Se (n=25), have a narrow range of δ 82/76Se, from -2.54 to +1.74 per mil (vs. SRM3149). The mean is 0.08 per mil, similar to published values for shales and the bulk earth (i.e., meteorites and mafic igneous rocks). Outcrop samples from a horizontal transect across the nearly vertical, partially weathered beds are extremely Se- rich (2327 ±4484mg/kg Se; n=45), and show very wide Se isotope variation, from -13.19 to +11.37 per mil. This indicates most of the Se has been subjected to at least one cycle of oxidation, transport, re-reduction, and precipitation in these organic-rich rocks. Se isotope values vary sharply on a small scale (e.g., 30 cm) among the interbedded chert and shale layers, with a tendency toward enrichment in heavier isotopes in chert. The expected pattern of isotopic variation in such a system is heavy isotope depletion in accessible zones that Se-rich infiltrating waters encounter early, and heavy isotope enrichment in less accessible zones that receive Se only after it has been strongly reduced. The observed pattern fits this model, though the fracture-dominated system is complex and other factors may be involved. Overall, the data are consistent with a scenario of extreme Se enrichment in a rolling redox front that has existed for a long time as the near-vertical strata have weathered and eroded.
H41J-07
A Multi-isotope (B, Sr, O, H) and Age-Dating (3H-3He, 14C) Study of Saline- Water Intrusion and Cross-Formational Flow in the Southern High Plains Aquifer
Identification of aquifer source waters can be difficult with traditional geochemical tracers such as solute concentrations because of the variability of rock-water interactions and complex recharge pathways. The growing use of traditional and non-traditional stable isotopes for identification of hydrologic processes allows the coupling of multiple stable isotopes and ion concentrations for cross-validation and better constraint of influences within an aquifer. A multi-isotope and age-dating study of ground water along the Western Caprock Escarpment of the Southern High Plains was implemented to identify saline-water intrusion and cross-formational flow in the Southern High Plains aquifer. This study coupled major ion and trace element concentrations with the stable isotopes of boron, strontium, hydrogen, and oxygen along with tritium-helium and carbon-14 age dating to identify potential source waters through differences in rock-water interactions and recharge pathways. Ground-water samples were collected from 16 wells, 13 of which were completed in the Ogallala Formation (primary formation of the Southern High Plains aquifer) and three that were completed in a minor aquifer of the underlying Dockum Group. Within the study area, a separate, local flow system originates at a topographic high and flows into a regional flow system defined by a large paleochannel. At a recharge area atop the topographic high and within the regional flow system, ground water is composed of a mixed cation-bicarbonate water, but a sodium-chloride type is present in parts of the local flow system and in ground water from the underlying aquifer in the Dockum Group. Major ion and trace element concentrations and the stable isotope composition of water (δ2H of -43.29 to -71.74 ‰ and δ18O of -5.85 to -9.95 ‰) do not indicate a simple two member mixing scenario but likely multiple sources and cross-formational flow between three formation deposits-- Permian salts, Dockum shales, and Ogallala alluvium. Boron (δ11B of 6.0 to 46.0 ‰) and strontium (0.70845 to 0.70906) stable isotope results helped to constrain the influence of rock-water interactions through different mineral signatures likely associated with the three formation deposits. The combining of various ion and isotope tracers allows the discrimination of source waters and the mixing strengths of the waters within the primary aquifer. Preliminary analysis suggests dissolution of Permian salts and migration of salt-laden water through fractures in the Dockum shales to a productive sand lens in the upper Dockum Group. Mixing within the Ogallala Formation and Dockum Group is likely occurring through upward and downward leakage between the two formations. Changes in the aquifers' potentiometric surfaces and available pathways control spatial mixing that produces a greater saline water contribution to parts of the local flow system that is diluted and (or) suppressed in the regional flow system of the Southern High Plains aquifer.
H41J-08 INVITED
Calcium Isotopes in Marine Sediments and Soils: Paleoceanography, Diagenesis, and Soil Processes
Calcium has 6 stable isotopes covering a wide mass range from 40 to 48. Because it is a critical component of the carbon cycle and a major constituent of common minerals like calcite, apatite, and gypsum, there is growing interest in understanding its stable isotope fractionation patterns in nature. The first work on Ca isotopes emphasized fractionation in food chains and in the formation of biogenic mineral matter in vertebrates. More recent studies have emphasized the fractionations observed in marine carbonate deposits, plants, and hydrological systems. Laboratory experiments show that Ca isotopes are fractionated during precipitation of calcite and aragonite from aqueous solution. Comparison with natural carbonate indicates that the fractionations are probably kinetic in origin although the exact mechanisms are not understood. There is little difference between the fractionation observed in inorganic and organic systems; precipitation rate (or solution oversaturation) seems to be the controlling factor, rather than temperature. One promising aspect of Ca isotopes is in their application to marine carbonate diagenesis. Especially when combined with studies of Sr, C, S, U, and O isotopes in deep sea sediments, Ca isotopes can give unique information on calcite dissolution and recrystallization rates. In a pure carbonate section from DSDP site 807A, Ca isotopes in pore fluids are found to differ greatly from seawater values and require that the equilibrium fractionation factor for 44Ca/40Ca between calcite and dissolved carbonate is almost exactly 1.0000. This fractionation corresponds to very slow calcite deposition rates at near-equilibrium conditions, implying that this fractionation factor may be generally applicable to diagenetic calcite precipitation, distinguishing it from biogenic precipitation, for which the factor is about 0.9985. This property can also be used to establish the dissolution rates of young carbonate sediments in the uppermost few meters of the sediment column. A deep-sea section where calcite is present in subequal proportions with clay, and in which there is also organic matter and rapid sulfate reduction, has a different pattern. Pore water 44Ca/40Ca values are closer to those of seawater, suggesting that there is virtually no calcite dissolution below a few meters depth. Analysis of the available data on fractionation during calcite precipitation suggests that the fractionation is controlled by attachment kinetics at the mineral surface. The observations also require that the presence of clay and/or organic material change the calcite dissolution rates by a large factor. Studies of Ca isotopes in hyper-arid soils and in alkaline lakes also show evidence of inorganic Ca isotope fractionation, probably controlled by relatively rapid, non-equilibrium precipitation of calcite, gypsum, or anhydrite during dessication events.