The redox-sensitive stable isotope geochemistry of chromium bears the potential to monitor the attenuation of chromate pollution and to investigate changes in environmental conditions in the present ...and the past. The use of stable Cr isotope data as a geo-environmental tracer, however, necessitates an understanding of the reaction kinetics and Cr fractionation behaviour during redox transition and isotope exchange. Here, we report stable chromium isotope fractionation data for Cr(VI) reduction, Cr(III) oxidation and isotopic exchange between soluble Cr(III) and Cr(VI) in aqueous media. The reduction of Cr(VI) to Cr(III) with H
2O
2 under strongly acidic conditions shows a near-equilibrium isotope fractionation of Δ
53/52Cr
(Cr(III)–Cr(VI)) of −3.54
±
0.35‰. At pH neutrality, however, the reduction experiments show a kinetic isotope fractionation Δ
53/52Cr
(Cr(III)–Cr(VI)) of −5‰ for the extent of reduction of up to 85% of the chromium. The oxidation of Cr(III) to Cr(VI) in alkaline media, using H
2O
2 as the oxidant, cannot be explained by a single, unidirectional reaction. Our experiments indicate that the involvement of the unstable intermediates Cr(IV) and Cr(V) and their disproportionation during redox reactions between Cr(III) and Cr(VI) influence the overall fractionation factor, depending on the prevailing pH conditions and the reaction rates. No detectable isotope exchange between soluble Cr(VI) and Cr(III) species at pH values of 5.5 and 7 was revealed over a timescale of days to weeks. This means that, at least within such a time frame, the isotopic composition of Cr(VI) in a natural system will not be influenced by equilibration with any Cr(III) and thus reveal the true extent of reduction, given that the Cr isotope composition of the source Cr(VI) and the fractionation factor for the prevailing conditions are known.
Iron (Fe) fluxes from reducing sediments and subglacial environments are potential sources of bioavailable Fe into the Southern Ocean. Stable Fe isotopes (δ56Fe) are considered a proxy for Fe sources ...and reaction pathways, but respective data are scarce and Fe cycling in complex natural environments is not understood sufficiently to constrain respective δ56Fe “endmembers” for different types of sediments, environmental conditions, and biogeochemical processes.
We present δ56Fe data from pore waters and sequentially extracted sedimentary Fe phases of two contrasting sites in Potter Cove (King George Island, Antarctic Peninsula), a bay that is affected by fast glacier retreat. Sediments close to the glacier front contain more easily reducible Fe oxides and pyrite and show a broader ferruginous zone, compared to sediments close to the ice-free coast, where surficial oxic meltwater streams discharge into the bay. Pyrite in sediments close to the glacier front predominantly derives from eroded bedrock. For the high amount of easily reducible Fe oxides proximal to the glacier we suggest mainly subglacial sources, where Fe liberation from comminuted material beneath the glacier is coupled to biogeochemical weathering processes (likely pyrite oxidation or dissimilatory iron reduction, DIR). Our strongest argument for a subglacial source of the highly reactive Fe pool in sediments close to the glacier front is its predominantly negative δ56Fe signature that remains constant over the whole ferruginous zone. This implies in-situ DIR does not significantly alter the stable Fe isotope composition of the accumulated Fe oxides. The nonetheless overall light δ56Fe signature of easily reducible Fe oxides suggests pre-depositional microbial cycling as it occurs in potentially anoxic subglacial environments. The strongest 56Fe-depletion in pore water and most reactive Fe oxides was observed in sediments influenced by oxic meltwater discharge. The respective site showed a condensed redox zonation and a pore water δ56Fe profile typical for in-situ Fe cycling.
We demonstrate that the potential of pore water δ56Fe as a proxy for benthic Fe fluxes is not straight-forward due to its large variability in marine shelf sediments at small spatial scales (−2.4‰ at the site proximal to oxic meltwater discharge vs. −0.9‰ at the site proximal to the marine glacier terminus, both at 2 cm sediment depth). The controlling factors are multifold and include the amount and reactivity of reducible Fe oxides and organic matter, the isotopic composition of the primary and secondary ferric substrates, sedimentation rates, and physical reworking (bioturbation, ice scraping). The application of δ56Fe geochemistry may prove valuable in investigating biogeochemical weathering and Fe cycling in subglacial environments. This requires, however (similarly to the use of δ56Fe for the quantification of benthic fluxes), that the spatial and temporal variability of the isotopic endmember is known and accounted for. Since geochemical data from subglacial environments are very limited, further studies are needed in order to sufficiently assess Fe cycling and fractionation at glacier beds and the composition of discharges from those areas.
Lithium (Li) is a scarce and technologically important element; the demand for which has recently increased due to its extensive consumption, particularly in manufacturing of Li-ion batteries, ...renewable energy, and electronics. Li is extracted from brines, pegmatite, and clay minerals; though extraction from brines is economically preferred. More than 200 salt plugs are in the Zagros Mountains which represent potential sources for Li exploration. This preliminary study collected first data on the abundance of Li in the salt plugs in southern Iran, and investigated Li distribution during evaporation of halite-producing brine ponds. The XRD analysis of powdered samples showed that gypsum and halite are the dominant solid phases in the ponds in which Li is concentrated in gypsum, while halite is depleted of Li. ICP-MS and ICP-OES analyses showed that Li in brines is concentrated during the evaporation by factors up to 28 with total contents up to 40 mg kg
. The Mg/Li ratio was higher than 70, which makes the brine unsuitable for conventional evaporation extraction techniques which require Mg/Li ratios of less than 6. Considering that 25 mg kg
is a suitable concentration of Li for exploration purposes, the results of this study suggest that with the advancement of extraction techniques, the depletion of presently used high-grade Li reserves, the increasing demand for lithium, the need for extraction from diverse sources, and the exploration of new resources, the salt plug brines have an exploratory potential for Li in the future.
Reactive iron (oxyhydr)oxide minerals preferentially undergo early diagenetic redox cycling which can result in the production of dissolved Fe(II), the adsorption of Fe(II) onto particle surfaces, ...and the formation of authigenic Fe minerals. The partitioning of iron in sediments has traditionally been studied by applying sequential extractions that target operationally-defined iron phases. Here, we complement an existing sequential leaching method by developing a sample processing protocol for δ56Fe analysis, which we subsequently use to study Fe phase-specific fractionation related to dissimilatory iron reduction in a modern marine sediment. Carbonate-Fe was extracted by acetate, easily reducible oxides (e.g. ferrihydrite and lepidocrocite) by hydroxylamine–HCl, reducible oxides (e.g. goethite and hematite) by dithionite–citrate, and magnetite by ammonium oxalate. Subsequently, the samples were repeatedly oxidized, heated and purified via Fe precipitation and column chromatography. The method was applied to surface sediments collected from the North Sea, south of the island of Helgoland. The acetate-soluble fraction (targeting siderite and ankerite) showed a pronounced downcore δ56Fe trend. This iron pool was most depleted in 56Fe close to the sediment–water interface, similar to trends observed for pore-water Fe(II). We interpret this pool as surface-reduced Fe(II), rather than siderite or ankerite, that was open to electron and atom exchange with the oxide surface. Common extractions using 0.5M HCl or Na-dithionite alone may not resolve such trends, as they dissolve iron from isotopically distinct pools leading to a mixed signal. Na-dithionite leaching alone, for example, targets the sum of reducible Fe oxides that potentially differ in their isotopic fingerprint. Hence, the development of a sequential extraction Fe isotope protocol provides a new opportunity for detailed study of the behavior of iron in a wide range of environmental settings.
•The full extent of Fe isotope fractionation in sediments may be determined by selective analysis of reactive components.•We expanded an existing sequential extraction method targeting reactive Fe minerals for stable Fe isotope analysis.•Matrix removal by repetitive oxidation, Fe precipitation and column chromatography did not induce Fe isotope fractionation.•The new method was applied to shallow North Sea sediments characterized by intense dissimilatory Fe reduction.•A downcore Fe isotopic trend was observed only for the acetate-leached fraction likely comprising surface-reduced Fe(II).
Dissimilatory iron reduction (DIR) is suggested to be one of the earliest forms of microbial respiration. It plays an important role in the biogeochemical cycling of iron in modern and ancient ...sediments. Since microbial iron cycling is typically accompanied by iron isotope fractionation, stable iron isotopes are used as tracer for biological activity. Here we present iron isotope data for dissolved and sequentially extracted sedimentary iron pools from deep and hot subseafloor sediments retrieved in the Nankai Trough off Japan. Dissolved iron (Fe(II)
) is isotopically light throughout the ferruginous sediment interval but some samples have exceptionally light isotope values. Such light values have never been reported in natural marine environments and cannot be solely attributed to DIR. We show that the light isotope values are best explained by a Rayleigh distillation model where Fe(II)
is continuously removed from the pore water by adsorption onto iron (oxyhydr)oxide surfaces. While the microbially mediated Fe(II)
release has ceased due to an increase in temperature beyond the threshold of mesophilic microorganisms, the abiotic adsorptive Fe(II)
removal continued, leading to uniquely light isotope values. These findings have important implications for the interpretation of dissolved iron isotope data especially in deep subseafloor sediments.
The isotope composition of iron in soils can display the environmental conditions that formed this soil. But plants extract only the mobile iron from soil, which is a small fraction of the soils' ...total iron. Yet this fraction is notoriously difficult to extract experimentally. Here we provide evidence that this signature is provided readily in the form of strategy II plants (grasses). We determined the stable Fe isotope signature of iron pools in two agronomic soils with two different sequential extraction methods. The Fe isotopic composition of the following soil mineral pools was measured: exchangeable iron, iron of poorly-crystalline (oxyhydr)oxides, iron in organic matter, iron of crystalline oxides and silicate bound iron. We found variations of about 1 per mil in δ
56Fe (δ
56Fe/‰
=
(
56/54Fe
sample/
56/54Fe
IRMM-014)
−
1·10
3) in the iron isotopic composition between the different soil mineral pools. The pools that contribute most to plant nutrition are water-extractable- and exchangeable iron, iron in organic matter and iron of poorly-crystalline (oxyhydr)oxides. These fractions are about 0.3 per mil lighter than the bulk soils. Silicates in our soils have a δ
56Fe of up to 0.4‰, suggesting preferential loss of light Fe during weathering. We compared the isotope composition of the plant-available Fe to that of typical strategy I and strategy II plants, grown on the soils. While redox and other transformation processes in the rhizosphere enrich strategy I plants to varying degrees in light Fe isotopes, strategy II plants exhibit a uniform Fe isotopic composition and are only slightly enriched in the heavier iron isotopes by about 0.3‰. Therefore these plants may record the Fe isotope composition of plant-available iron in soils, to which the composition of strategy I plants can be compared to.
► Extraction procedure for the Fe isotopic composition of different soil iron pools. ► Determination of the stable Fe isotope composition of plant-available iron in soil. ► Strategy II plants record the Fe isotope composition of plant-available soil iron.
Li/Ca and Li isotope ratios in marine and freshwater calcium carbonates are potential tracers of past environmental variables such as weathering intensity and temperature. To make use of their ...potential requires full understanding of the fractionation of Li/Ca and
7Li/
6Li during growth of calcium carbonate. This study improves such understanding with two sets of new measurements: on inorganic calcites and aragonites precipitated over a range of salinity and on the test of benthic foraminifera formed at a range of temperatures.
Li/Ca in inorganic calcite increases by a factor of four as salinity increases from 10‰ to 50‰. Inorganic aragonite demonstrates no significant change in Li/Ca over the same salinity range. The difference in behaviour is probably due to the incorporation mechanism of Li. Substitution of Li
+ in the Ca
2+ site in aragonite makes growth-solution Li/Ca the key variable, while interstitial incorporation of Li
+ in calcite means that the growth-solution Li concentration is more important. Compared to the growth solution,
7Li/
6Li is ≈3‰ lower in calcite and ≈11‰ lower in aragonite, with no relationship to salinity for either mineral. Similar isotope fractionation to these inorganic experiments are observed for biogenically produced calcite (foraminifera) and aragonite (corals). This suggests little biological control during Li incorporation into biogenic calcium carbonates. The difference in fractionation between calcite and aragonite is consistent with an equilibrium isotope fractionation reflecting different incorporation mechanisms for Li into the two minerals.
Four samples of the benthic foraminifera
Uvigerina, precipitated over a temperature range of 14 °C, have Li/Ca ranging from 18.5 to 13.1 μmol/mol. The ratio is inversely related to temperature with a sensitivity of 2.5±0.8% per °C. This sensitivity is similar to that observed in a previous study of foraminiferal Li/Ca, but about half that observed during inorganic calcite growth. The large variability in both Li/Ca and temperature sensitivity exhibited between various foraminifera species suggests an additional control on calcite Li/Ca apart from those of salinity and temperature observed in this study.
The central Baltic Sea is a marginal brackish basin which comprises anoxic bottom waters and is surrounded by geological source terrains with a wide variety of compositions and ages. This allows the ...investigation of water mass mixing using radiogenic isotope compositions of Nd and Hf as well as their geochemical cycling across varying redox conditions in the water column. In this study, we present the distribution of Nd and Hf concentrations and their isotopic compositions for 6 depth profiles and 3 surface water sites obtained during a cruise in the central Baltic Sea onboard the RV Oceania as a part of the international GEOTRACES program.
The results obtained indicate that Nd isotopes effectively trace the mixing between more radiogenic saline waters from the south and unradiogenic fresh waters from the north, which helps to understand the reliability of Nd isotopes as water mass tracer in the open ocean. In surface waters, Nd shows higher concentrations and less radiogenic isotope compositions at the northern stations, which are progressively diluted and become more radiogenic to the south, consistent with the counterclockwise circulation pattern of central Baltic Sea surface waters. In contrast to the variable Nd concentrations, Hf shows much less variability. At the Gotland Deep station, the Nd concentrations of the euxinic waters are higher by a factor >10 than those of the overlying oxygen-depleted waters, whereas Hf only shows small concentration variations. This indicates faster removal of Hf from the water column than Nd. Moreover, the dissolved Hf isotope signatures document great variability but no consistent mixing trends. Our explanation is that Hf has a lower residence time than Nd, and also that the Hf isotope signatures of the sources are highly heterogeneous, which is attributed to their differing magmatic and tectonic histories as well as incongruent post-glacial weathering around the central Baltic Sea.
We systematically investigated the spatial distribution of sulfates, chlorides, and nitrates in Atacama Desert soils in order to identify their relationship to long-term aridity gradients and to ...secondary redistribution and phase transformation. Thin surface crusts, powdery surface material and subsurface concretions from up to 40 cm depth were sampled along several latitudinal transects between 19.5–25°S and 68.5–70.5°W. The samples were characterized by total soil chemical analysis (ICP-OES and spectrophotometric analysis) complemented by XRD and thermogravimetric analysis to determine contents of chloride, nitrate, and major elements along with gypsum and anhydrite abundance in Atacama Desert soils.
Our results demonstrate that the spatial distribution of gypsum, anhydrite, halite, and nitrates in Atacama Desert soils is indeed linked to aridity gradients, but also sources, and secondary dissolution processes. Nitrates and chlorides are best preserved between 19 and 22°S, which thus may constitute the long-term hyperarid core of the Atacama Desert. Remobilization within the soil is ubiquitous south of 22°S, but generally also occurs on the debris fans prograding from the Precordillera into the Central Depression. A near-constant concentration ratio of Na/Cl = 0.83 – very similar to the sea water ratio – throughout the desert and concentration maxima within the reaches of coastal fog penetration below the altitude of 1200 m reveals that sea spray is the primary source of halite in Atacama Desert soils. Calcium sulfates dominate Atacama Desert soils. Deposition is primarily as gypsum, but anhydrite is abundant in the northern Central Depression between 19 and 22°S. The apparent association of anhydrite with high concentrations of nitrate and chloride may point to a formation by dissolution and secondary reprecipitation from salt-concentrated fluids. The predominance of anhydrite in the Central Depression suggest geomorphology and water availability as additional factors remaining to be determined.
•The spatial distribution of soluble salts reflects long-term aridity gradients.•Long-term aridity is most severe in the Central Depression between 19 and 22°S.•Chlorides are dominated by a marine source.•Anhydrite mainly forms by dissolution-reprecipitation in the hyperarid core of the Atacama.
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