Sr Source Research Articles

Correlating surface stoichiometry and termination in SrTiO3 films grown by hybrid molecular beam epitaxy

Hybrid oxide molecular beam epitaxy (hMBE), a thin-film deposition technique in which transition metal cations are delivered using a metal-organic precursor, has emerged as the state-of-the-art approach to the synthesis of electronic-grade complex oxide films with a stoichiometric growth window. However, numerous questions remain regarding the chemical mechanisms of the growth process and the surface properties of the resulting films. To examine these properties, thin film SrTiO3 (STO) was prepared by hMBE using a titanium tetraisopropoxide (TTIP) precursor for Ti delivery and an elemental Sr source on annealed STO and Nb-doped STO substrates with varying TTIP:Sr flux ratios to examine the conditions for the reported stoichiometric growth window. The films were transferred in vacuo to an x-ray photoelectron spectroscopy system to study the surface elemental composition. Samples were examined using x-ray diffraction to compare our surface sensitive results with previously reported measurements of the bulk of the films in the literature. Ex situ studies by atomic force microscopy, scanning tunneling microscopy, and low-energy electron microscopy confirmed the presence of surface reconstructions and an Ehrlich–Schwoebel barrier consistent with A-site SrO termination. We find that a surface exhibiting a mixture of SrO and TiO2 termination or a full SrO termination is necessary to obtain stoichiometric adsorption-controlled growth. These results indicate that surface Sr is necessary to maintain the chemical equilibrium for stoichiometric growth during the hMBE process, which is important for the design of future interfacial systems using this technique.

The geographic distribution of bioavailable strontium isotopes in Greece – A base for provenance studies in archaeology

Sr isotopes are a powerful tool used to reconstruct human mobility in archaeology. This requires extensive bioavailable 87Sr/86Sr baselines used as reference for deciphering potential areas of origin. We define the first extensive bioavailable Sr isotope baselines for the different geographical regions and surface lithologies of Greece by combining new Sr data with previously published bioavailable 87Sr/86Sr data. We present 82 new Sr concentrations and 87Sr/86Sr signatures of plants, soil leachates, surface waters and spring waters from Central Greece and combine these with published baseline values from all over Greece. We define individual baselines for ten of the thirteen geographical regions of Greece. We also provide soil leachate 87Sr/86Sr ratios from the two archaeological Bronze Age sites of Kirrha and Ayios Vasileios in Central and Southern Greece and demonstrate the validity and applicability of the new baselines for these sites. The bioavailable 87Sr/86Sr compositions of Central Greece define a narrow range of 87Sr/86Sr values between 0.70768 — 0.71021, with the widest range observed for the soil leachates. Sr derived from carbonate weathering appears to be the most important Sr source sampled by the proxies. There is an overall larger variability in baseline ranges of the different geographical regions, the narrowest is that for West Greece and the widest that for West Macedonia. In addition, we computed statistical Sr isotope ranges for the five main surface lithological groups characterising the sampling sites of the various proxies. Narrowly ranged, unradiogenic bioavailable Sr isotope signatures are typical of areas characterised by igneous outcrops as well as by Cenozoic and Mesozoic sediments. Areas, where Palaeozoic and Precambrian bedrock outcrops dominate, produce significantly wider ranges. Our study promotes the usefulness of multi-proxy baselines for geographical reference purposes and thus their promising applicability for future human mobility studies.

Flow-sediment regulation regime influencing mobilization of trace metals in shallow aquifer and surface water in the Yellow River Delta, China

The Yellow River Delta (YRD) is a typical case of frequent interaction between surface water and groundwater under the seasonal flow-sediment regulation regime. A total of 40 samples (20 surface water samples, 19 groundwater samples, and 1 seawater sample), were collected during the high flow (October, 2019). Trace metals, hydrochemical indicators (EC, pH, major ion of Na+, K+, Mg2+, Ca2+, Cl−, SO4 2−, and HCO3 −) and stable isotopes (δ18O and δD) were determined. The impact of water diversion on the hydrochemistry of groundwater was significant, accounting up to a 90% of river discharge in some sites calculated by 2-component mixing model. Factor analysis indicated that Fe, Mn, Al, and B in surface water were closely correlated and were present in the dissolution phase due to sediment remobilization controlled by flow-sediment regulation; Sr was influenced by the dissolution of evaporate salt, while Ba suffered precipitation in the oxidation processes. However, the source of Sr, Mn, B, and Fe in groundwater was accompanied by hydrochemical compositions and per cent surface water, implying that the trace metals in surface water contributed to that of accumulation in the shallow aquifer; Al was influenced by plagioclase weathering, while Ba precipitated too.

Testing the Recycled Gabbro Hypothesis for the Origin of “Ghost Plagioclase” Melt Signatures Using 87Sr/86Sr of Individual Olivine‐Hosted Melt Inclusions From Hawai'i

AbstractMelt inclusions with large, positive Sr anomalies have been described in multiple tectonic settings, and the origins of this unusual geochemical feature are debated. Three origins have been proposed, all involving plagioclase as the source of the elevated Sr: (i) direct assimilation of plagioclase‐rich lithologies, (ii) recycled lower oceanic gabbro in the mantle source, and (iii) shallow‐level diffusive interaction between present day lower oceanic crust (i.e., plagioclase‐bearing lithologies) and the percolating melt. A “ghost plagioclase” signature (i.e., a large, positive Sr anomaly without associated high Al2O3) is present in melt inclusions from Mauna Loa. We present new 87Sr/86Sr measurements of individual olivine‐hosted melt inclusions from three Hawaiian volcanoes, Mauna Loa, Loihi, and Koolau. The data set includes a Mauna Loa melt inclusion with the highest reported Sr anomaly (or highest (Sr/Ce)N, which is 7.2) for Hawai'i. All melt inclusions have 87Sr/86Sr values within the range reported previously for the lavas from each volcano. Critically, the 87Sr/86Sr of the high (Sr/Ce)N melt inclusion lies within the narrow range of 87Sr/86Sr for Mauna Loa melts that lack high (Sr/Ce)N signatures. Therefore, to explain the high (Sr/Ce)N ratio of the ghost plagioclase signature using an ancient recycled gabbro, the gabbro‐infused mantle source would have had to evolve, by chance, to have the same 87Sr/86Sr as the source of the Mauna Loa melts that lack a recycled gabbro (ghost plagioclase) signature. Alternatively, shallow‐level diffusive interactions between Mauna Loa plagioclase‐rich cumulates and a percolating mantle‐derived melt provides a simpler explanation for the presence of the high (Sr/Ce)N Mauna Loa melts.

Provenance, and environment context of pedogenic carbonates formation from MIS 3 to MIS 1 in the Teotihuacan Valley, Mexico

Pedogenic carbonates are considered important indicators of past climatic changes. Their formation is due to calcite accumulation through evaporation processes in arid or semi-arid regions. Over time, secondary carbonate accumulation results in the development of calcic and petrocalcic horizons in soil profiles. In this study, we conducted an analysis of pedogenic carbonates as records of paleoenvironmental characteristics in the Teotihuacan Valley, Basin of Mexico. Micromorphological, mineralogical, and geochemical characteristics of secondary carbonates from three paleosol profiles (Cerro Gordo 1, Cerro Gordo 2, and Maseca 1) were analyzed. Pedogenic carbonates were characterized by irregular and variable carbonate crystal sizes, inclusions of soil particles, and dark or light micrite and microsparite laminations. Micromorphology and scanning electron microscopy revealed weathered plagioclase minerals as a potential source of calcium for carbonate precipitation. The 87Sr/86Sr ratio of carbonate laminar crusts and nodules from the analyses pedons in the study area also indicated that rocks of the Transmexican Volcanic Belt were the principal source of Ca and Sr for carbonates, while contribution of a far-distance eolian input was minimal. The δ13C isotopic composition evidenced a mixture of C3 and C4 plants during the Pleistocene and a transition to a higher proportion of C4 vegetal cover during the Holocene. This agreed with an increase in modeled paleo-temperature from 10.9 °C to 13.7 °C based on 18O-temperature equations. Our results support those determined for the regional paleoenvironment during the Pleistocene, the Pleistocene-Holocene transition and Mid-Late Holocene.

Changing inputs of continental and submarine weathering sources of Sr to the oceans during OAE 2

Ocean anoxic events (OAE) are characterized by increased organic content of marine sediment on a global scale with accompanying positive excursions in sedimentary organic and inorganic δ 13C values. To sustain the increased C exports and burial required to explain the C isotope excursion, increased supplies of nutrients to the oceans are often invoked during ocean anoxic events. The potential source of nutrients in these events is investigated in this study for Oceanic Anoxic Event 2, which spans the Cenomanian-Turonian boundary. Massive eruptions of one or more Large Igneous Provinces (LIPs) are the proposed trigger for OAE 2. The global warming associated with volcanogenic loading of carbon dioxide to the atmosphere has been associated with increased continental weathering rates during OAE 2, and by extension, enhanced nutrient supplies to the oceans. Seawater interactions with hot basalts at LIP eruption sites can further deliver ferrous iron and other reduced metals to seawater that can stimulate increased productivity in surface waters and increased oxygen demand in deep waters. The relative importance of continental and submarine weathering drivers of expanding ocean anoxia during OAE 2 are difficult to disentangle. In this paper, a box model of the marine Sr cycle is used to constrain the timing and relative magnitudes of changes in the continental weathering and hydrothermal Sr fluxes to the oceans during OAE 2 using a new high-resolution record of seawater 87Sr/86Sr ratios preserved in a marl-limestone succession from the Iona-1 core collected from the Eagle Ford Formation in Texas. The results show that seawater 87Sr/86Sr ratios change synchronously with Os isotope evidence for the onset of massive LIP volcanism 60 kyr before the positive C isotope excursion that traditionally marks the onset of OAE 2. The higher temporal resolution of the seawater Sr isotope record presented in this study warrants a detailed quantitative analysis of the changes in continental weathering and hydrothermal Sr inputs to the oceans during OAE 2. Using an ocean Sr box model, it is found that increasing the continental weathering Sr flux by ∼1.8-times captures the change in seawater 87Sr/86Sr recorded in the Iona-1 core. The increase in the continental weathering flux is smaller than the threefold increase estimated by studies of seawater Ca isotope changes during OAE 2, suggesting that hydrothermal forcing may have played a larger role in the development of ocean anoxic events than previously considered

An Experimental Study on the Sources of Strontium in Mineral Water and General Rules of Its Dissolution—A Case Study of Chengde, Hebei

Chengde City boasts a wealth of high-quality mineral water resources characterized by a high level of strontium (Sr), a low level of sodium, and low alkalinity. In order to study the mechanism of formation of Sr-bearing mineral water in Chengde and to scientifically guide future mineral water exploration, taking three typical mineral water exploration areas in Chengde as examples, this paper studies the sources of Sr in mineral water and the general rules of its dissolution via a laboratory static leaching experiment and impact experiments, and it provides an analysis of the characteristics of typical rock samples. The research results indicate that the content of Sr in surrounding rock and the characteristics of minerals existing in surrounding rock jointly control the dissolution of Sr in water; that CO2 can promote the formation of mineral water containing Sr; and that temperature increases may boost the dissolution of Sr from carbonate minerals but also inhibit the dissolution of Sr from silicate minerals.

Isotopic range of bioavailable strontium on the Peloponnese peninsula, Greece: A multi-proxy approach

Sr isotopes are a powerful tool used for provenancing in many disciplines, but their successful application requires the availability of robust Sr baselines of potential target areas. This study presents 87Sr/86Sr signatures and Sr concentrations of water, plants and soil leachates from the Peloponnese peninsula, Greece, to establish the first comprehensive bioavailable Sr isotope baseline for this region. Additionally, this study aims to evaluate which proxy is most suitable to characterise bioavailable Sr in a geologically complex area also exposed to foreign aeolian Sr sources. Our recorded bioavailable Sr isotope signatures correspond well with the surface lithologies characteristic of the Peloponnese. Unradiogenic 87Sr/86Sr ratios and a narrow isotope range (0.70779–0.70955) characterise the bioavailable Sr signatures of the sedimentary deposits and more radiogenic and isotopically variable values (0.70791–0.72370) were measured for metamorphic and igneous rock outcrops. The differences in 87Sr/86Sr values measured between proxies of one site are comparatively low for samples from the sedimentary and igneous deposits, while the overall spread in 87Sr/86Sr values is wider for samples from metamorphic deposits. We propose to define bioavailable 87Sr/86Sr baseline ranges as the average bioavailable 87Sr/86Sr ratio of all proxies of each lithology ± its double standard deviation (x̅ ± 2σ). This results in narrow baselines for the sedimentary outcrops of 0.70832 ± 0.00053 (n = 58) for clastic sediments and 0.70835 ± 0.00089 (n = 29) for chemical sediments. The metamorphic deposits are characterised by wider bioavailable 87Sr/86Sr baselines of 0.70906 ± 0.00116 (n = 4) and 0.71429 ± 0.01133 (n = 13) for marble and schist, respectively. The bioavailable Sr baseline for igneous rock outcrops is also characterised by a comparatively wide range with 0.70950 ± 0.00259 (n = 7). The wide range in inter- and intra-site specific bioavailable 87Sr/86Sr variation observed in this study emphasise the need for comprehensive multi-proxy sampling strategies within geologically-complex areas.

The isotopic geochemistry of CaCO3 encrustations in Taylor Valley, Antarctica: Implications for their origin

Calcium carbonate (CaCO<sub>3</sub>) encrustations occur in most desert soils, including polar ones, and such encrustations preserve records of geochemical, hydrological, and atmosphere processes affecting these soils. We have collected a series of CaCO3 encrustations found underneath surface rocks in the soils and tills of Taylor Valley, McMurdo Dry Valleys (~78°S lat.), Antarctica. These encrustations were analyzed for 87Sr/86S and δ18O and δ13C to determine what relation they have with the underlying soils, and the material in which they are in contact, and to identify the processes that control their formation. In all but one case, the isotopic data indicate that the source of Sr to these encrustations is not from the rock on which it is associated. The primary source of Sr (and by analogy Ca) is either from dust that has been deposited through aeolian processes or from the aggregate of till material within the soils. The δ13C values for Taylor Valley encrustations ranged from 5.7 to 11.0‰, and are consistent with a carbon source from atmospheric CO<sub>2</sub>. The δ18O values range from –8.1 to –11.2‰ and are heavier than expected for equilibrium calcite precipitation from Taylor Valley meteoric water. Taken together these results indicate that the CaCO<sub>3</sub> was formed by rapid evaporation of films beneath clasts that had become supersaturated with respect to CaCO<sub>3</sub>.

The occurrence and distribution of strontium in U.S. groundwater

Groundwater samples from 32 principal aquifers across the United States (U.S.) provide a broad spatial scope of the occurrence and distribution of strontium (Sr) and are used to assess environments and factors that influence Sr concentration. Strontium is a common trace element in soils, rocks, and water and is ubiquitous in groundwater with detectable concentrations in 99.8% of samples (n = 4,824; median = 225 μg/L). Concentrations in 2.3% of samples exceeded the 4,000 μg/L health-based screening level. The relative importance of controlling factors on Sr concentration are spatially variable and partly dependent on the type of groundwater well. Three case settings illustrate controls on Sr concentration. For drinking-water supply wells, most high concentrations (>4,000 μg/L) were measured in samples from carbonate aquifers that resulted from water-rock interaction with Sr-bearing rocks and minerals. High Sr concentrations from monitoring wells were more common in unconsolidated sand and gravel aquifers in arid or semi-arid setting where shallow groundwater is affected by irrigation and evaporative concentration of dissolved constituents in combination with lithologic or applied Sr sources. Upwelling saline groundwater is also a source of Sr in some locations. Total dissolved solids concentration is an indicator of high Sr in all settings. An estimated 2.2 million people in the conterminous U.S. are potentially supplied water from public-supply wells with high Sr concentration, ~86% of whom use carbonate aquifers (with more than half supplied by the Floridan aquifer system). An additional 120,000 people are potentially supplied high-Sr-concentration water from domestic wells, more than half of whom (~58%) are in Texas. This study markedly expands the coverage of previous surveys of Sr in groundwater and is of interest given potential adverse human-health effects related to elevated concentrations of Sr and consideration of Sr for drinking-water regulation. Case settings with elevated Sr described for U.S. groundwater are likely indicative of settings and processes affecting Sr concentration in groundwater globally.

Strontium Uptake and Intra-Population 87Sr/86Sr Variability of Bones and Teeth—Controlled Feeding Experiments With Rodents (Rattus norvegicus, Cavia porcellus)

Strontium isotopes in biogenic apatite, especially enamel, are widely employed to determine provenance and track migration in palaeontology and archaeology. Body tissues record the 87Sr/86Sr of bioavailable Sr of ingested food and water. To identify non-local individuals, knowledge of the 87Sr/86Sr of a non-migratory population is required. However, varying factors such as tissue turnover rates, feeding selectivity, Sr content, digestibility of food, and the ingestion of mineral dust can influence body tissue 87Sr/86Sr. To evaluate the Sr contribution of diet and water to mammalian hard tissues 87Sr/86Sr, controlled feeding studies are necessary. Here we present 87Sr/86Sr from controlled feeding experiments with two rodent species (Rattus norvegicus, Cavia porcellus). Due to the continuous and fast incremental growth of rat and guinea pig incisors (~0.1 – 0.5 mm/day), their enamel is expected to record isotopic dietary changes. For Experiment-1: Diet Switch, animals were switched from their respective supplier food to a pelleted experimental diet containing either insect-, plant-, or meat-meal and a staggered-sampling approach was used to monitor the 87Sr/86Sr changes in rat incisor enamel and bone over the course of the experiment. In Experiment-2: Basic Diets, separated cohorts (n = 6) of rats and guinea pigs were fed one of the three pelleted diets and received tap water for 54 days. While the rat incisors showed a complete tissue turnover, the slower-growing guinea pig incisors partially retained supplier diet-related isotopic compositions. In addition, one group of rats fed plant-meal pellets received Sr-rich mineral water, demonstrating that drinking water can be an important Sr source in addition to diet. Additionally, a leaching experiment showed that only a small fraction of diet-related Sr is bioavailable. Finally, in Experiment-3: Dust Addition, guinea pigs were fed pellets with and without addition of 4% of isotopically distinct dust (loess or kaolin). Animals that received kaolin-containing pellets displayed increased enamel 87Sr/86Sr. Intra-population 87Sr/86Sr variability within each feeding group was small and thus we conclude that it should not affect interpretations of 87Sr/86Sr in provenance studies. However, the differences between bulk food and leachate 87Sr/86Sr highlight the importance of Sr bioavailability for provenance studies and Sr isoscapes.

Source of strontium in archaeological mobility studies\u2014marine diet contribution to the isotopic composition

The strontium isotope composition of human tissues is widely used in archaeological mobility studies. However, little attention is paid to the relative contributions of terrestrial versus marine sources of strontium in these studies. There is some debate over the role of a solid diet versus drinking water as the most important source of strontium for the human body, with related possibilities of misinterpretation of the archaeological record if only strontium isotope compositions of the biosphere are studied. However, there is a third component, marine strontium, which is commonly not assumed to contribute towards the strontium isotope composition of archaeological skeletal remains, especially in locations that are not directly coastal. To illustrate the potentially obfuscating effects of mixed Sr sources in a human population, we present a case study of twelve individuals from the medieval Finnish site Iin Hamina with a known dietary history. Our study shows that marine consumption is a significant factor explaining the strontium isotope composition of the Iin Hamina human remains, with implication of erroneous conclusions about immigration without prior knowledge of diet composition. Thus, future studies should always incorporate a rigorous analysis of dietary history, with special regard to potential consumption of aquatic resources, when strontium isotope analysis is used as a method in the study of palaeomobility.

Trace Elements and Their Variation with pH in Rain Water in Arid Environment.

Climate change in Kuwait has resulted in temperature fluctuations, frequent dust storms, and noticeable changes in the amount of precipitation. Pollutants released into the atmosphere from various sources affect the chemical composition of rainwater and impact its usability. The present study on rainwater focused on the determination of trace elements, sources, and their variation with respect to change in temperature and pH. The samples were collected from 12 different locations in both industrial and urban regions during significant rain events (n = 31) from November 2018 to March 2019and samples were analyzed for trace elements in ICP-OES using standard USEPA 200.7 method. The mean concentration of the 16 elements analyzed followed the trend: Co < Cd < Cr < Mo < V<Ni < Pb < As < Se < Fe < Cu < Mn < Zn < Al < Ba < Sr and were inferred to be within the WHO permissible limits of drinking water. The analytical results revealed that Strontium (Sr) had the highest mean concentration (188μg/L) followed by barium (Ba), aluminum (Al), and zinc (Zn) with mean concentrations of 95.2μg/L, 30.4μg/L, and, 16.6μg/L respectively. The sources of contamination in rainwater were identified by calculating the enrichment factor (EF) using element concentration reported in Kuwait dust and from continental crustal values. EF for Fe, V, Ni, and Cr were below 10, indicating purely crustal sources. Ni, Zn, Cu, and Mn exhibited values between 10 and 100, reflecting industrial sources of contamination. EF for Sr was greater than 100, due to inputs from anthropogenic sources. A strong association between Al and pH along with correlation between Sr, Cr, Cu, Ni, Mo, V, and meteorological parameters was revealed from statistical analysis. Furthermore, pH Redox Equilibrium C programming (PHREEQC) was used to simulate changes in pH and temperature in rainwater to predict the resultant variations in trace element concentrations. There was no significant change observed in pH with rise in temperature, but the concentration of trace elements varied with change in pH. The concentration of V, Cr, and Al were most sensitive to pH variations. The results indicated that industrial emissions, fuel combustion, and dust in Kuwait are the primary sources of Al, Sr, Mn, Zn, and Ba in the rainwater samples. Since, the concentrations of these elements are relatively low, rainwater in Kuwait could be harvested for drinking and domestic purposes and used for recharging aquifers.

Impact of salinity and carbonate saturation on stable Sr isotopes (δ88/86Sr) in a lagoon-estuarine system

Local carbonate cycling in lagoon-estuarine systems, involving processes such as inorganic and biogenic carbonate precipitation/dissolution, represents an important but poorly constrained component of the coastal carbon budget. This study investigates the sensitivity of stable Sr isotope tracer (δ88/86Sr) with respect to carbonate saturation and salinity of local waters in the Coorong, Lower Lakes and Murray Mouth (CLLMM) estuary in South Australia. The CLLMM has an extensive range of salinity from fresh to hypersaline (from ∼0 to over 100 PSU), with corresponding variations in water chemistry and major ion composition that in turn controls mineral saturation states, and thus CaCO3 precipitation/dissolution in local waters. Here we use the novel δ88/86Sr tracer in tandem with the more established radiogenic Sr isotope ratio (87Sr/86Sr), where the latter is a robust proxy for Sr sources and thus water provenance. We also produced a geochemical (PHREEQC) model of calcium carbonate (CaCO3) saturation changes across this unique lagoon-estuarine system.The results indicate a systematically increasing trend of δ88/86Sr (from ∼0.25‰ to ∼0.45‰) with increasing salinity and CaCO3 (aragonite, calcite) saturation indices of the coastal waters, which in turn suggest an overall control of carbonate dissolution/precipitation processes on the stable Sr isotope composition in the CLLMM system. This was further corroborated by Ca isotope data (δ44/40Ca) published previously on the same samples from the Coorong, as well as a quantitative simulation of local carbonate removal in the lagoon based on Rayleigh modelling and Sr isotope data.Overall, our results confirm that a coupled Sr isotope approach (combining 87Sr/86Sr and δ88/86Sr) can be used to constrain not only the main water sources (continental versus marine Sr) but also local CaCO3 dissolution/precipitation processes, and thus inorganic carbon and coastal carbonate cycling in the CLLMM system. Finally, this coupled δ88/86Sr and 87Sr/86Sr approach can be potentially applied to fossil carbonate archives to reconstruct paleo-hydrology and salinity changes in the CLLMM and/or other carbonate-producing coastal systems.

Strontium isotopes in high- and low-Ge coals from the Shengli Coalfield, Inner Mongolia, northern China: New indicators for Ge source

The coal-hosted Ge deposit (Early Cretaceous) at Wulantuga, Inner Mongolia, northern China, is one of the largest Ge deposits in the world. Although the origin, modes of occurrence, and spatial distribution of Ge in this deposit have been investigated by several studies, the provenance and pathways of Ge from the source to the deposit are largely unknown. In this study, we investigated the 87Sr/86Sr values in the high-Ge coal of the Wulantuga deposit and the stratigraphically-equivalent low-Ge coal of the No. 1 Pit, both from the Shengli Coalfield, Inner Mongolia, northern China. In addition to an organic association, Sr and Ca in the high-Ge and low-Ge coals also occur in gypsum. The water-soluble Sr (i.e., in gypsum) in the high-Ge coal (Ge concentrations: 45–1170 ppm, 273 ppm on average, whole coal basis) has 87Sr/86Sr values in the range of 0.710411 to 0.710633 (with an average of 0.710478). In contrast, the low-Ge coal (Ge concentrations: up to 4.23 ppm, 0.45 ppm on average, whole coal basis) has lower 87Sr/86Sr values in two sub-groups, a low range of 0.709896–0.71005 (with an average of 0.709736) and a high range of 0.710173–0.710547 (with an average of 0.710329 and dominantly in the range of 0.710173–0.710406). The higher 87Sr/86Sr values in the high-Ge coals indicate that the highly-elevated concentration of Ge is associated with a local source of radiogenic Sr. The enrichment of Ge in the coal is due to several consecutive processes. Surface waters penetrated deeply into granite and dissolved at least partially more soluble phases of the granite. The Ge-rich solution flowed over ground or underground into the Shengli Coalfield and Ge precipitated in the peat, the precursor of the coal investigated in the coalfield. The 87Sr/86Sr values of the water-soluble fraction, the elemental compositions of high-Ge and low-Ge coals, and the geomorphological setting of the coalfield point to the Hercynian granodiorite intrusion on the nearby SE margin of the Shengli Coalfield as the primary source of Ge in the Wulantuga coal-hosted Ge deposit. In addition to serving as an indicator of the mode of formation of the coal-hosted Ge deposit, the 87Sr/86Sr values and their spatial distribution can provide an indication for the hydrological pathways of the mineralising fluid.

Heavy metals in different moss species in alpine ecosystems of Mountain Gongga, China: Geochemical characteristics and controlling factors.

Terrestrial mosses are promising tracers for research concerning metal atmospheric deposition and pollution. Concentrations of Cr, Co, Ni, Zn, Sr, Cd, Ba, and Pb in different moss species from Mountain Gongga, China were analyzed to investigate the effects of growth substrates, geographic elevation, and type of moss species on the accumulation characteristics of heavy metals, as well as to identify heavy metal sources. The ability of heavy metals to accumulate in moss varied significantly, with low concentrations of Cd and Co; medium concentrations of Cr, Ni, and Pb; and high concentrations of Zn, Sr, and Ba. Elevation significantly influenced the accumulation characteristics of heavy metals, with high concentrations found at lower elevations due to proximal pollution. Growth substrate and moss species were found to have certain influence on the bioconcentration capacities of heavy metals in moss in this study. Correlation analysis showed similar sources for Sr, Zn, and Ba, as well as for Ni, Co, and Cr. The positive matrix factorization (PMF) model was consistent with atmospheric deposition of Pb and Cd; substrate sources of Cr, Co, and Ni; and anthropogenic sources of Ba, Sr, and Zn. This research characterized the accumulation characteristics of heavy metals and their influence factors in different mosses found in alpine ecosystems and provides a reference for future studies in similar areas.

Trace and Rare Earth Elements, and Sr Isotopic Compositions of Fluorite from the Shihuiyao Rare Metal Deposit, Inner Mongolia: Implication for Its Origin

Abundant fluorites occur in the Shihuiyao rare metal (Nb-Ta-Rb) deposit in Inner Mongolia of NE China, and they can be classified by their occurrence into three types. Type I occurs disseminated in greisen pockets of albitized granite. Type II occurs in the skarn zone between granite and carbonate host rocks, and it can be subdivided into different subtypes according to color, namely dark purple (II-D), magenta (II-M), green (II-G), light purple (II-P), and white (II-W). Type III are the fluorite-bearing veins in the silty mudstones. On the basis of petrography of the fluorites and their high contents of HFSEs (high field strength elements) and LILEs (large ion lithophile elements), strong negative Eu anomalies, and tetrad effects, we suggest that Type I fluorites crystallized in a late-magmatic stage with all the components derived from the granite. The high Y/Ho ratios suggest that the Type II fluorites crystallized in the early- or late-hydrothermal stage. The rare earth elements (REEs) characterized by various Eu anomalies of the Type II fluorites indicate a mixed origin for ore-forming metals from granite-related fluids and limestones, and the oxygen fugacity increased during fluid migration and cooling. Compared to the Type II fluorites, the similar trace element contents of the Type III suggest a similar origin, and remarkable positive Eu anomalies represent a more oxidizing environment. The Sr isotopic composition (87Sr/86Sr)i = 0.710861) of the Type I fluorites may represent that of the granite-derived fluids, whereas the (87Sr/86Sr)i ratios of the Type II (0.710168–0.710380) and Type III (0.709018) fluorites are lower than that of the Type I fluorites but higher than those of the Late Permian-Early Triassic seawater, suggesting a binary mixed Sr source, i.e., granite-derived fluids and marine limestones. Nevertheless, the proportion of limestone-derived Sr in the mixture forming the Type III fluorites is much higher than that of Type II. The rare metal Nb and Ta get into the granite-derived F-rich fluids by complexing with F and precipitate in the form of columbite-group minerals after the Type I fluorites crystallize. Most of Nb and Ta may have deposited as columbite-group minerals during the magmatic stage, resulting in no Nb-Ta mineralization in the hydrothermal stage when the Type II and III fluorites formed. Hence, the Type I fluorites in the Shihuiyao mining area can be used as an important exploration tool for the Nb-Ta mineralization.

Seawater 87Sr/86Sr ratios along continental margins: Patterns and processes in open and restricted shelf domains

To better constrain the Sr isotope budget in marginal domains without any fluvial inputs, we analyzed the chemical composition and 87Sr/86Sr ratio of waters and shells from four locations: two coastal lagoons, one hemipelagic platform and one open marine shelf. Our results highlight homogeneous 87Sr/86Sr ratios typical of oligotrophic oceanic waters (OOW) (i.e., 0.709172 ± 0.000023) in the Pacific Tatakoto atoll and along a Mediterranean shore to offshore transect (~25 km off Banyuls-sur-Mer, BSM). This attests that oceanic inputs from oligotrophic areas remain the main Sr source in open shelf areas compared with submarine groundwater discharges (SGD) or particulate dissolution influences. In BSM, only foreshore data are more radiogenic, possibly due to rainwater mixing, local groundwater springs or more efficient particle dissolution in the intertidal zone. In restricted areas, we report variable 87Sr/86Sr ratios between the Salses-Leucate (France) and Oualidia (Morocco) lagoons. The first one has homogeneous 87Sr/86Sr ratio typical of OOW except close to SGD. In Oualidia, 87Sr/86Sr ratios decrease by 1.2 × 10−3 from OOW values close to the Atlantic inlet to progressively less radiogenic ones upstream within the interior of the lagoon. These differences depend on several factors including the leaky, restricted or choked morphology of lagoons modulating the oceanic Sr inputs, but also SGD fluxes whose 87Sr/86Sr ratios and Sr concentrations are highly variable according to the nature of rocks leached in karstic aquifer. In Oualidia, the low 87Sr/86Sr ratios correspond to high Sr concentrations (up to 150 μmol·l−1) issued from the dissolution of Mesozoic evaporites, leading to SGD fluxes accounting for 60% of the local Sr budget. Through data compilation, we show that similar 87Sr/86Sr gradients and processes prevail at the whole Mediterranean scale. Finally, we postulate that high coastal water retention times can also account for anomalous coastal 87Sr/86Sr ratios and that the combination of water mass restriction, SGD, bioadsorption and early diagenetic processes could decrease seawater Sr concentrations in some marginal areas.

Strontium isotopes trace biological activity in the Critical Zone along a climate and vegetation gradient

Weathering and ecosystem nutrition are intimately linked through the supply of fresh mineral nutrients from regolith and bedrock (the “geogenic nutrient pathway”). However, the prominence of this link is dependent on the efficiency of nutrient recycling from plant litter (the “organic nutrient cycle”). Isotope ratios of strontium (Sr), an element that behaves similarly to Ca in ecosystems, confer two types of information: radiogenic Sr isotopes inform as to the sources of Sr and the degree of weathering, while stable Sr isotopes constrain partitioning between compartments of the Critical Zone (bedrock, water, secondary solids, and plants). To date, however, neither the reactions nor the mass balance between compartments that fractionate Sr isotopes, nor the fractionation factors involved, are well understood. Here, we present geochemical budgets of Sr (using radiogenic and stable Sr isotopes, and Ca/Sr ratios) at four sites along a substantial climate and primary production gradient in the coastal mountains of Chile. We found that Sr release through weathering is isotopically congruent, and released Sr is not strongly isotopically fractionated either during secondary mineral formation or transfer into the exchangeable pool. Despite this, the 88Sr/86Sr ratio of bio-available Sr, which should reflect the ratio of dissolved Sr, is higher than that of rock and regolith. We propose that this offset is caused by plants: while 88Sr/86Sr in plant organs at the four study sites systematically increased from roots towards their leaves, whole-plant Sr isotope compositions indicate preferential uptake of light Sr into plants (with a fractionation of up to −0.3‰ relative to the bio-available pool). Despite this strong biological fractionation, 88Sr/86Sr ratios in bio-available Sr do not covary with biomass production across our study sites, because with greater plant growth Sr is recycled more times after release by weathering – an isotope-neutral process. Rather, the loss of Sr from the ecosystem in solid organic material sets the isotope ratio of dissolved or bio-available Sr. Organic solids thus appear to constitute a significant export path of elements released during weathering, with the removal of solid plant debris reducing the recycling factor of Sr, and possibly that of other mineral nutrients too.

Concentration, likely sources, and ecological risk assessment of potentially toxic elements in urban soils of Shiraz City, SW Iran: a preliminary assessment

Potentially toxic elements (PTE) are considered as hazardous pollutants that could have adverse effects on the environment. In recent decades, Shiraz, one of the largest cities of Iran, has been faced with extensive development and its environmental consequences. In this paper, urban soils of Shiraz were collected for preliminary evaluation of potential ecological risk as well as likely source identification of PTE (As, Pb, Cu, Co, Cr, Ni, Sr, and Zn). The concentrations of PTE were analyzed by inductively coupled plasma-optical emission spectrometry (ICP-OES). Geochemical indices (geo-accumulation index (Igeo) and enrichment factor (EF)) and potential ecological risk (PER) index were used to measure the contamination level. Correlation coefficient (CA) and principal component analyses (PCA) as statistical models were applied to recognize the probable sources of PTE. The study shows that the average levels of As, Pb, Cu, Co, Cr, Ni, Sr, and Zn in urban topsoil are 3.96, 11.5, 50.0, 11.5, 334.5, 102.0, 339.4, and 49.0 mg/kg, respectively. For both Igeo and EF, the values are Cu > Sr > Pb > Cr > Zn > Ni > Co > Cd > As. Geochemical indices indicate that the studied soils are mostly un-polluted to moderately polluted. Also, potential ecological risk assessment values of PTE are < 150, illustrating a low ecological risk level. Correlation coefficient and PCA suggest likely anthropogenic origins for Zn, Pb, Co, Ni, Cu, and As and geogenic source for Sr, while Cr appears to have a mixed anthropogenic origins.