Iron Stable Isotopes in Bulk Soil and Sequential Extracted Fractions Trace Fe Redox Cycling in Paddy Soils.

In paddy soils, iron (Fe) forms are highly influenced by the seasonal redox changes and leave detectable isotope signals because of fractionation between different Fe forms. Here, we present Fe concentrations and Fe isotope compositions (expressed as δ56Fe values) in a paddy soil profile from Suzhou, China. Light Fe isotopes were enriched in two iron-accumulation layers (Br3 and G1) with high Fe concentrations. In particular, large shifts in both Fe concentrations and δ56Fe values were found at the Br2 and Br3 boundaries, showing fast and efficient transformation between these horizons. With sequential extraction, we show that Fe isotopes in the short-range-ordered Fe minerals and crystalline Fe oxides were lighter than those in the residual silicate minerals. Iron enriched in light isotopes was leached from the Ap horizon and subsequently moved to Br horizon, quickly precipitating there as Fe oxides.

Microbial U Isotope Fractionation Depends on the U(VI) Reduction Rate.

U isotope fractionation may serve as an accurate proxy for U(VI) reduction in both modern and ancient environments, if the systematic controls on the magnitude of fractionation (ε) are known. We model the effect of U(VI) reduction kinetics on U isotopic fractionation during U(VI) reduction by a novel Shewanella isolate, Shewanella sp. (NR), in batch incubations. The measured ε values range from 0.96 ± 0.16 to 0.36 ± 0.07‰ and are strongly dependent on the U(VI) reduction rate. The ε decreases with increasing reduction rate constants normalized by cell density and initial U(VI). Reactive transport simulations suggest that the rate dependence of ε is due to a two-step process, where diffusive transport of U(VI) from the bulk solution across a boundary layer is followed by enzymatic reduction. Our results imply that the spatial decoupling of bulk U(VI) solution and enzymatic reduction should be taken into account for interpreting U isotope data from the environment.

Forensic isoscapes based on intra-individual temporal variation of δ 18O and 206Pb/207Pb in human teeth.

Isotopic signatures used in the georeferencing of human remains are largely fixed by spatially distinct geologic and environmental processes. However, location-dependent temporal changes in these isotope ratios should also be considered when determining an individual's provenance and/or trajectory. Distributions of the relevant isotopes can be impacted by predictable external factors such as climate change, delocalisation of food and water sources and changes in sources and uses of metals. Using Multi-Collector Inductively-Coupled Plasma Mass Spectrometer (MC-ICP-MS) analyses of 206Pb/207Pb in tooth enamel and dentin from a population of 21 ± 1-year-old individuals born circa 1984 and isotope ratio mass spectrometry (IRMS) of δ 18O in their enamel, we examined the expected influence of some of these factors. The resulting adjustments to the geographic distribution of isotope ratios (isoscapes) found in tooth enamel and dentin may contain additional useful information for forensic identification, but the shifts in values can also impact the uncertainty and usefulness of identifications if they are not taken into account.KEY POINTSIsoscapes of 206Pb/207Pb and δ 18O used for geolocation are not static.Within a few years, the enamel and dentin of a person may exhibit measurable differences in 206Pb/207Pb even without changing locations.Changes in climatic patterns tied to rising temperatures are more significant than the direct effect of increasing temperature on δ 18O fixed in tooth bioapatite.Third molar (M3) enamel mineralisation includes material incorporated from before formal amelogenesis takes place.

Field Application of 238U/235U Measurements To Detect Reoxidation and Mobilization of U(IV).

Biostimulation to induce reduction of soluble U(VI) to relatively immobile U(IV) is an effective strategy for decreasing aqueous U(VI) concentrations in contaminated groundwater systems. If oxidation of U(IV) occurs following the biostimulation phase, U(VI) concentrations increase, challenging the long-term effectiveness of this technique. However, detecting U(IV) oxidation through dissolved U concentrations alone can prove difficult in locations with few groundwater wells to track the addition of U to a mass of groundwater. We propose the 238U/235U ratio of aqueous U as an independent, reliable tracer of U(IV) remobilization via oxidation or mobilization of colloids. Reduction of U(VI) produces 238U-enriched U(IV), whereas remobilization of solid U(IV) should not induce isotopic fractionation. The incorporation of remobilized U(IV) with a high 238U/235U ratio into the aqueous U(VI) pool produces an increase in 238U/235U of aqueous U(VI). During several injections of nitrate to induce U(IV) oxidation, 238U/235U consistently increased, suggesting 238U/235U is broadly applicable for detecting mobilization of U(IV).

Evaluation of the efficacy of spatiotemporal Pb isoscapes for provenancing of human remains.

Geospatially distributed isotopes (isoscapes) from biogeochemically fractionated processes have been applied in many forensic investigations, such as authentication of food and sourcing of drugs. Provenancing of human remains using isotopes has been hindered by a lack of appropriate isoscapes, by changes in these isoscapes over time, and by various homogenization processes. In this study we create spatiotemporal isoscapes for anthropogenic lead (Pb) for the contiguous United States and Europe using literature data from dated sediments, soils and biological tissues. We compare (206)Pb/(207)Pb isoscapes with isoscapes of δ(13)C, δ(18)O and (87)Sr/(86)Sr to determine their relative efficacy for the forensic identification of human remains. We do this comparison using third molar enamel data from 22 United States Air Force Academy cadets with known life trajectories born between 1983 and 1985. We use these spatiotemporal isoscapes with osteologic analyses, hospital records and isotopic analyses of tooth enamel carbonate from permanent teeth to help identify 32 individuals from unmarked graves found in a forgotten 19th century mental asylum cemetery.

No measurable changes in (238)U/(235)U due to desorption-adsorption of U(VI) from groundwater at the Rifle, Colorado, integrated field research challenge site.

Groundwater samples were collected from the Integrated Field Research Challenge field site in Rifle, Colorado, over the course of a bicarbonate-induced U desorption-adsorption experiment. Uranium concentrations and high precision U isotopic compositions ((238)U/(235)U) of these groundwater samples were determined and used to assess the impact of bicarbonate-induced U(VI) desorption from contaminated sediments on the (238)U/(235)U of groundwater. The (238)U/(235)U of groundwater was not significantly impacted by bicarbonate-induced desorption of U(VI) from mineral surfaces or by adsorption of advecting U(VI) from upgradient locations onto those surfaces after the treatment. Assuming this absence of a significant shift in U isotopic composition associated with desorption-adsorption applies to other systems, reduction of U(VI) to U(IV) is expected to be the dominant source of U isotopic fractionation associated with removal of U(VI) from pore water as a result of natural and stimulated reductive pathways. Thus, changes in the (238)U/(235)U composition of uranium-bearing fluids should be useful in quantifying the extent of reduction.

Mobilization of arsenic in aquifers from the Datong Basin, China: evidence from geochemical and iron isotopic data.

Iron isotope compositions of various Fe pools in aquifer sediments were measured at a known As-contaminated site in the Datong Basin, China. The δ(56)Fe values of HCl-extracted poor-crystalline Fe(III) range widely from -0.41‰ to 0.36‰. We interpret the low Fe(II)/Fe(Extractable) ratios (<50%) and the negative correlation between Fe(II)/Fe(Extractable) and δ(56)Fe values in HCl-extracted poor-crystalline Fe to be best explained by redox cycling of Fe induced by microbial Fe(III) reduction. However, the high Fe(II)/Fe(Extractable) ratios (~/>70%) and positive correlation between Fe(II)/Fe(Extractable) and δ(56)Fe values for HCl-extracted poor-crystalline Fe indicates production of sulfides (FeSs). The δ(56)Fe values of crystalline Fe(III) extracted by reductant appears to be comparatively small varying from -0.01‰ to 0.24‰, which is consistent with the δ(56)Fe values for ferric oxides/hydroxides having undergone microbial Fe(III) reduction. The Fe isotope composition of various Fe pools shows the transformation between crystalline Fe(III) and poor-crystalline crystalline Fe(III) and the secondary Fe(II) phases has already occurred or is occurring in aquifer sediments. More importantly, there is a significant difference in the As concentrations in crystalline Fe(III) oxides/hydroxides and HCl-extracted Fe phases. The concentrations of As range from 1.6 to 29.9 mg kg(-1) and from 0.6 to 3.0 mg kg(-1), for crystalline Fe(III) and HCl-extracted Fe phases respectively. Accordingly, the transformation of Fe minerals induced by microbial Fe(III) reduction can contribute to the mobilization of As. This study is the first to examine the Fe isotope compositions in high As aquifer sediments; the results show that the Fe isotope would be an important tool in demonstrating the enrichment of As in groundwater.

Anatomically modern human in Southeast Asia (Laos) by 46 ka.

Uncertainties surround the timing of modern human emergence and occupation in East and Southeast Asia. Although genetic and archeological data indicate a rapid migration out of Africa and into Southeast Asia by at least 60 ka, mainland Southeast Asia is notable for its absence of fossil evidence for early modern human occupation. Here we report on a modern human cranium from Tam Pa Ling, Laos, which was recovered from a secure stratigraphic context. Radiocarbon and luminescence dating of the surrounding sediments provide a minimum age of 51-46 ka, and direct U-dating of the bone indicates a maximum age of ~63 ka. The cranium has a derived modern human morphology in features of the frontal, occipital, maxillae, and dentition. It is also differentiated from western Eurasian archaic humans in aspects of its temporal, occipital, and dental morphology. In the context of an increasingly documented archaic-modern morphological mosaic among the earliest modern humans in western Eurasia, Tam Pa Ling establishes a definitively modern population in Southeast Asia at ~50 ka cal BP. As such, it provides the earliest skeletal evidence for fully modern humans in mainland Southeast Asia.

Isotope fractionation by thermal diffusion in silicate melts.

Isotopes fractionate in thermal gradients, but there is little quantitative understanding of this effect in complex fluids. Here we present results of experiments and molecular dynamics simulations on silicate melts. We show that isotope fractionation arises from classical mechanical effects, and that a scaling relation based on Chapman-Enskog theory predicts the behavior seen in complex fluids without arbitrary fitting parameters. The scaling analysis reveals that network forming elements (Si and O) fractionate significantly less than network modifiers (e.g., Mg, Ca, Fe, Sr, Hf, and U).

Uranium 238U/235U isotope ratios as indicators of reduction: results from an in situ biostimulation experiment at Rifle, Colorado, U.S.A.

The attenuation of groundwater contamination via chemical reaction is traditionally evaluated by monitoring contaminant concentration through time. However, this method can be confounded by common transport processes (e.g., dilution, sorption). Isotopic techniques bypass the limits of concentration methods, and so may provide improved accuracy in determining the extent of reaction. We apply measurements of 238U/235U to a U bioremediation field experiment at the Rifle Integrated Field Research Challenge Site in Rifle, Colorado. An array of monitoring and injection wells was installed on a 100 m2 plot where U(VI) contamination was present in the groundwater. Acetate-amended groundwater was injected along an up-gradient gallery to encourage the growth of dissimilatory metal reducing bacteria (e.g., Geobacter species). During amendment, U concentration dropped by an order of magnitude in the experiment plot. We measured 238U/235U in samples from one monitoring well by MC-ICP-MS using a double isotope tracer method. A significant approximately 1.00 per thousand decrease in 238U/235U occurred in the groundwater as U(VI) concentration decreased. The relationship between 238U/235U and concentration corresponds approximately to a Rayleigh distillation curve with an effective fractionation factor (alpha) of 1.00046. We attribute the observed U isotope fractionation to a nuclear field shift effect during enzymatic reduction of U(VI)(aq) to U(IV)(s).

Experimentally determined uranium isotope fractionation during reduction of hexavalent U by bacteria and zero valent iron.

Variations in stable isotope ratios of redox sensitive elements are often used to understand redox processes occurring near the Earth's surface. Presented here are measurements of mass-dependent U isotope fractionation induced by U(VI) reduction by zerovalent iron (Fe0) and bacteria under controlled pH and HCO3- conditions. In abiotic experiments, Fe0 reduced U(VI), but the reaction failed to induce an analytically significant isotopic fractionation. Bacterial reduction experiments using Geobacter sulfurreducens and Anaeromyxobacter dehalogenans reduced dissolved U(VI) and caused enrichment of 238U relative to 235U in the remaining U(VI). Enrichmentfactors (epsilon) calculated using a Rayleigh distillation model are -0.31% per hundred and -0.34% per hundred for G. sulfurreducens and A. dehalogenans, respectively, under identical experimental conditions. Further studies are required to determine the range of possible values for 238U/235U fractionation factors under a variety of experimental conditions before broad application of these results is possible. However, the measurable variations in delta(5238)U show promise as indicators of reduction for future studies of groundwater contamination, geochronology, U ore deposit formation, and U biogeochemical cycling.