Stable Sr Isotope Research Articles

Growth Responses of Holcus lanatus L. (Velvet Grass) in Soils Contaminated with Cesium or Strontium

Radiocesium (r-Cs) and radiostrontium (r-Sr) released from nuclear accidents (e.g., Chornobyl, Fukushima) and routine operations (reactors, reprocessing) pose environmental and health concerns. Their primary pathway to humans is through plant uptake and subsequent bioaccumulation within the food chain. While soil amendments with potassium (K) and calcium (Ca) are known to mitigate r-Cs and r-Sr uptake, respectively, the impact on plant growth remains unclear. This study investigates the effects of Cs and Sr on the growth of Holcus lanatus L. seedlings under hydroponic and soil conditions with varying Cs and Sr concentrations. Stable isotopes of Cs and Sr served as non-radioactive analogs. Seedling growth was assessed across a range of Cs and Sr concentrations (≤1 and ≥4 mg L−1). The impact of the addition of K and Ca on Cs/Sr uptake in amended soils was also evaluated. Additionally, this study examined how Cs and Sr amendments affected the influx rates of other nutrients in H. lanatus. Higher Cs and Sr concentrations (≥4 mg L−1) significantly inhibited seedling growth, while lower concentrations had no effect. Notably, H. lanatus exhibited moderate Cs tolerance and strong Sr tolerance. Furthermore, K and Ca supplementation in Cs/Sr-amended soils demonstrably reduced plant uptake of these elements. This study also observed alterations in the uptake rates of other nutrients within H. lanatus due to Cs/Sr addition. This study suggests that H. lanatus exhibits moderate tolerance to Cs and Sr contamination, potentially making it suitable for revegetation efforts in contaminated grasslands. Additionally, K and Ca amendments show promise as a strategy to mitigate plant uptake of these radioisotopes further. These findings contribute to the development of safer revitalization strategies for areas impacted by nuclear accidents.

Fractionation Mechanism and Flux Estimation of Strontium Isotopes During Basalt Weathering

AbstractThe fluxes of metal cations and isotopes released by weathering of silicate rocks are crucial and a prerequisite for constraining geochemical fluxes to rivers and oceans. This study presents mineral and elemental compositions along with 87Sr/86Sr and δ88Sr data from a basaltic weathering regolith on Hainan Island, South China to elucidate Sr isotope fractionation and weathering fluxes. The 87Sr/86Sr ratios vary from 0.703936 to 0.706338 as a result of differential weathering of the minerals. δ88Sr values in the weathering regolith range from −0.29 to 0.37‰, with the majority of the weathering regolith having lower δ88Sr values than the parent rock. Sr is leached into the soil solution during plagioclase decomposition, while 86Sr is preferentially adsorbed on the surface of secondary minerals. As weathering progresses, smectite decomposes and kaolinite desorbs under weakly acidic conditions, releasing the previously adsorbed 86Sr into the soil solution. The differential weathering of kaolinite and smectite controls the δ88Sr values of the weathering regolith, with pH being an important determinant of isotope fractionation. Furthermore, Sr elemental fluxes (SrFlux) and Sr isotopic fluxes (δ88SrFlux) of this weathering regolith were calculated using a mass balance model, yielding mean values of 0.20 (mg cm−3 Myr−1) and 0.052 (‰ (mg cm−3 Myr−1)), respectively. The δ88SrFlux exhibits a nonlinear positive correlation with the Chemical Index of Alteration (CIA), indicating that enhanced weathering leads to significant stable Sr isotope fractionation at CIA values below 95%. Our research promotes the understanding of Sr recycling and the fractionation behavior of stable Sr isotopes during consecutive weathering.

Equilibrium Fractionation of Stable Strontium Isotopes During Adsorption to Mn Oxides

AbstractUnderstanding the stable strontium isotope fractionation between seawater and ferromanganese crusts/nodules is crucial for reconstructing seawater Sr isotopic history. This study explores stable Sr isotope fractionation during adsorption to manganese oxides, as preliminary experiments indicated their higher Sr adsorption capacity compared to iron oxides. We observed that equilibrium in concentration and isotopic composition can be achieved within a day. The lighter isotope (86Sr) is preferentially adsorbed, with the fractionation extent showing no significant change with respect to reaction time, pH, or Sr concentration under low ionic strength conditions (−0.14‰). Equilibrium fractionation processes, rather than Rayleigh fractionation, more accurately represent the isotopic composition trends of dissolved and adsorbed Sr. Experiments in synthetic seawater demonstrated greater Sr isotope fractionation (−0.20‰) compared to low ionic‐strength solutions. This fractionation during adsorption by manganese oxides in synthetic seawater is consistent with the isotopic discrepancy between seawater and ferromanganese nodules, suggesting that Mn oxide adsorption is the primary mechanism for Sr accumulation in ferromanganese deposits. These results imply that ferromanganese deposit Sr isotopic compositions could be proxies for reconstructing seawater Sr isotopic history, which is influenced by carbonate burial and continental weathering, reflecting the global carbon cycle changes.

Effect of terrestrial weathering on stable Sr and Ba isotope compositions of eucrites

Stable strontium and barium isotopes are potential tracers for understanding planetary differentiation and the nature of the building blocks of terrestrial planets. Strontium and barium are fluid-mobile elements, but it remains unclear how terrestrial weathering affects the Sr-Ba isotopes compositions in achondrites, thus hampering the utility of Sr-Ba isotopes in cosmochemistry. In this study, we conducted acetic acid leaching on three eucrites with varying weathering degrees (fall: Qiquanhu, hot desert find: Northwest Africa (NWA) 13583, and Antarctic find: Grove Mountains (GRV) 13001). Combined with detailed petrography observations and major and trace element analyses, we investigated the variations in Sr-Ba isotopes during terrestrial weathering. The degree of weathering follows an order of: NWA 13583 > GRV 13001 > Qiquanhu, evaluated based on several alteration signs, including: the presence of secondary carbonate, the enrichment of large ion lithophile elements (e.g., Sr, Ba, and U), and the Ce and Eu anomalies. The concentrations of Sr and Ba in the leachates of NWA 13583 show a good correlation with Ca, suggesting that the soluble Sr and Ba are derived from secondary carbonate. Differently, the concentrations of Sr and Ba in the leachates of Qiquanhu correlate with Al and Na, suggesting that the soluble Sr and Ba in Qiquanhu are derived from primary plagioclases. This also indicates that silicates dissolution may be inevitable in an acid leaching experiment for achondrites, even when using weak acetic acid. GRV 13001 shows no variation in Sr and Ba isotopes during leaching experiments. The δ138/134Ba in the leachate (0.26 ± 0.02 ‰) of Qiquanhu is higher than that of the residue (0.04 ± 0.03 ‰), reflecting that aqueous fluids preferentially uptake heavy Ba isotopes during plagioclase dissolution. Conversely, the leachate of NWA 13583 shows lower δ138/134Ba (-0.19 ± 0.05 ‰) than that of residue (-0.10 ± 0.03 ‰), reflecting the lighter Ba isotope composition in carbonate. Notably, the residue of NWA 13583 has δ138/134Ba ∼ 0.1 ‰ lower than those of Qiquanhu and GRV 13001. This discrepancy may reflect plagioclase dissolution during hot-desert weathering rather than magmatism on the parent body. Different from Ba isotopes, the δ88/86Sr of Qiquanhu shows tiny variation in the leaching experiment, suggesting that the dissolution of plagioclase may cause negligible Sr isotope fractionation. For NWA 13583, the δ88/86Sr of leachate is slightly heavier than that of leaching residue and bulk rock, reflecting the high δ88/86Sr in terrestrial fluids. Our results suggest that Ba and Sr isotopes of eucrites show different behaviors during terrestrial weathering. Sr isotopes show a smaller fractionation scale and may have greater resistance for terrestrial weathering than Ba isotopes.

Identification of alkaline amendment sources (slash and burn versus marling) for cereal crops grown in the North of France: A multiple isotope approach (87Sr/86Sr, δ44/40Ca, δ88/86Sr)

Early farmers used at least two types of agrarian amendments that could raise pH and base saturation levels to allow the cultivation of cereals: marling and plant ash. Ash can be input in many different ways: felling and burning in place, transferring plant material from wastelands and spreading the ash, charring sod or peat blocks, burning stubble after harvest. Marling includes all the practices of amending limestone, from marl to chalky limestone composed of 99% CaCO3. In order to understand the evolution of these agricultural practices, it is important to identify which of the two techniques was used to amend cereal crops in the past. In order to test the potential of δ44/40Ca-δ88/86Sr-87Sr/86Sr multiple isotope approach for archaeological samples, we first applied the technique to currently grown crops, amended either with marl or with ash from freshly-cut and burned trees. We found that this approach makes it possible to discriminate cereal grains amended either by marling (less radiogenic Sr) or with tree ash (more radiogenic Sr). We also identified a positive correlation between stable Ca and Sr isotope values, suggesting that the Ca and Sr came from similar sources and had undergone similar mass-dependent isotopic fractionation mechanisms. Consequently, we later on mainly focused on stable and radiogenic Sr isotopes. Stable Sr isotope fractionation was also observed between different locations, different organs of a given cereal species and between different cereal types, but also within the same cereal species or the number of grains studied for a given locality, pointing to biological fractionation combined with source variation.

Experimental and theoretical investigations of stable Sr isotope fractionation during its incorporation in aragonite

Strontium partitioning and isotope fractionation between aragonite and fluid have been determined experimentally at low values of the fluid saturation state (Ω) with respect to this mineral (1.1 ≤ Ωaragonite ≤ 2.2), and the measured isotope fractionation has been compared with the results of first-principles simulations. For the latter, Density Functional Theory (DFT) was used for estimation of the equilibrium Sr isotope fractionation between aragonite and Sr2+(aq). The obtained results suggest that, for values of Ωaragonite close to unity, the apparent distribution coefficient of Sr in aragonite (DSr,aragonite=XSrCO3XCaCO3(CaSr)fluid) exhibits values higher than one that rapidly decrease at increasing aragonite growth rate. Under equilibrium conditions (i.e. Ωaragonite=1) a DSr,aragoinite value of 2.7 can be extrapolated. Additionally, for aragonite growth rates rp ≤ 10−8.0±0.2 (mol/m2/s) the Sr isotope fractionation between aragonite and the fluid (i.e. Δ88/86Sraragonite-fluid) shows a constant value of −0.1±0.05‰, whereas it decreases to −0.40‰ when the growth rate increases to 10−7.7(mol/m2/s). The surface reaction kinetic model (SRKM) developed by DePaolo (2011) has been used to describe the dependence of DSr,aragonite and Δ88/86Sraragonite-fluid on mineral growth rate. In this model the best fit for DSr,aragonite and Δ88/86Sraragonite-fluid were 4 and −0.01‰, respectively, whereas the kinetic isotope fractionation factor for Δ88/86Sraragonite-fluid was −0.6‰. The results of first-principles calculations yield an equilibrium Sr isotope fractionation factor of −0.04‰ which is in excellent agreement with the experimental value of the present study. These results are the first experimental measurements of Sr isotope fractionation during inorganic aragonite precipitation as a function of growth rate and the first DFT calculations of Sr equilibrium fractionation in the aragonite-fluid system. The results of this study provide new insight into the mechanisms controlling stable Sr isotope composition in aragonite, which has implications for using Sr isotopes for paleo-reconstructions of natural archives, particularly those of abiogenic origin.

An investigation of factors affecting high-precision Sr isotope analyses (87Sr/86Sr and δ88/86Sr) by MC-ICP-MS

The stable strontium (Sr) isotope system is being increasingly utilized to move forward our understanding of environmental, geological, and cosmological processes. Two analytical techniques are commonly used to measure stable Sr isotopes: 1) double-spike thermal ionization mass spectrometry (DS-TIMS) and 2) Zr-doped sample-standard bracketing multi-collector inductively coupled plasma mass spectrometry (Zr-doped SSB via MC-ICP-MS). Relative to DS-TIMS, Zr-doped SSB via MC-ICP-MS allows simultaneous determination of both 87Sr/86Sr and 88Sr/86Sr ratios, increasing measurement efficiency and sample throughput. However, several factors limit the analytical precision and accuracy of this technique, including matrix effects and potential intra- and inter-session offsets of measured isotope ratios. In this study, we evaluate potential causes for the observed offsets. We performed a series of experiments to assess the effect of different solution matrices and sources of contamination on Sr isotope measurements and to explore potential underlying mechanisms. Our tests show that incomplete Sr recovery during chromatographic separation, mismatches of Sr and Zr concentrations and acid molarity between samples and bracketing standards, and cation contamination all affect the precision and accuracy of Sr isotope measurements. We present evidence that analyses using our recommended updated preparation procedure for Zr-doped SSB via MC-ICP-MS can achieve a long-term reproducibility (2σSD: 87Sr/86Sr = ±0.000015 and δ88/86Sr = ±0.03 ‰) comparable to that of DS-TIMS.

Stable strontium isotope fractionation in hydroponically grown mung and soy bean sprouts

This study investigated whether stable strontium isotope (88Sr/86Sr) fractionation occurs during nutrient uptake by bean plants. Mung and soy beans were cultivated hydroponically using two nutrient solutions with different strontium (Sr) isotope compositions. The Sr and calcium (Ca) concentrations, 87Sr/86Sr ratios and δ88/86Sr values of the seeds, organs, initial nutrient solutions, and residual nutrient solutions were measured. The triple Sr isotope plot illustrates that bean sprout organs deviated considerably from the binary end-member mixing lines between intact bean seeds and the initial nutrient solutions, indicating fractionation of δ88/86Sr. In addition, the residual nutrient solutions had heavier δ88/86Sr values than the initial nutrient solutions, further supporting preferential uptake of lighter Sr isotopes by bean sprouts. This study indicates that stable Sr isotope fractionation occurs during nutrient uptake by bean plants, and that controlled hydroponic experiments are highly effective for observing this process.

The Origin of the Black Shale Series and Bentonites from the Wufeng Formation in the Southwestern Upper Yangtze: Implications for the Convergence of the Yangtze and Cathaysia Blocks in the Late Ordovician

AbstractEarly Paleozoic black organic sediments and bentonites occur widely in the craton basin within the Yangtze block and are generally believed to be genetically related to a specific tectonic setting on the cratonic boundary. However, the intimate relationship between their origins and the dynamic mechanisms are unclear, as exemplified by the genesis of the black shale series and bentonites from the Wufeng Formation during the Ordovician–Silurian transition (OST). In order to reveal the relationship between the Wufeng Formation and the convergence of the Yangtze and Cathaysia blocks (i.e., the intracontinental Kwangsian Orogeny), two stratigraphic sections respectively in Zhaotong area (Northeast Yunnan) and Puyi area (Northwestern Guizhou) that were located in the semi‐restricted inner Yangtze Sea during the OST were systematically studied, on the basis of whole‐rock geochemical composition, pyrite δ34S (δ34Spy), total organic carbon (TOC), stable Sr isotope, pyrite framboid size distribution and zircon U‐Pb age, trace elements. The evidence shows that the paleo‐oceanic environment changed significantly at the turn of the early–late Katian and formed the black shale series in the Wufeng Formation. These acritarch assemblages were formed in the transition process of the Upper Yangtze Basin from passive continental margin basin to foreland basin during this interval. Based on previous research on the genetic relationship between black shale series and plate tectonic movement, a basin‐mountain evolution model suitable for South China in the Late Ordovician is presented. The two bentonites in the Wufeng Formation with U‐Pb ages of 445.5 ± 0.8 Ma and 441.9 ± 2.4 Ma primarily originated from the intermediate–acid volcanic eruption during the collision and convergence between the Yangtze and Cathaysia blocks in the Late Ordovician, the provenance region probably being located in the Jiangnan orogenic belt. Thus, we believe that the appearance of the black shale series and bentonite in the Wufeng Formation at the turn of the early–late Katian may represent the initiation of basin‐mountain transformation and the Kwangsian Orogeny in South China, which provides important evidence for the collision and convergence of the Yangtze and Cathaysia blocks in the Late Ordovician.

Stable Sr isotope (88Sr/86Sr) fractionation in calcite precious corals

Stable isotope ratios of Sr (88Sr/86Sr) are a novel indicator for identifying terrestrial and oceanic Sr cycling and biological processes. Here we evaluate the temperature dependence of stable Sr isotope ratios (δ88Sr) in the calcite skeletons of precious corals, known as octocorals. High-precision Sr isotope measurements by double spike thermal ionization mass spectrometry were applied to different precious corals (Corallium japonicum, Pleurocorallium elatius, P. konojoi, Corallium sp., Keratoisis sp., and Coralliidae sp.) collected from water depths of 30–1500 m in the Pacific Ocean, corresponding to annual mean water temperatures of 2.5 to 19.5°C. The precious corals showed mean δ88Sr values of 0.101 ± 0.023‰. The magnitude of Sr isotope fractionation between the skeleton and ambient seawater (Δ88Sr) is −0.291 and is about −0.1‰ lower than the value predicted from the precipitation of inorganic calcite. This fractionation is not affected by changes in skeletal Sr/Ca ratio and associated biological effects on δ88Sr are negligible. δ88Sr of precious coral skeleton is an excellent recorder of ambient seawater.

Geochemical behavior of stable strontium isotopes during continental weathering process: A review

Continental weathering, an important geological process that affects the formation and evolution of the earth, involves the interaction of matter and energy among the lithosphere, atmosphere, hydrosphere, and biosphere. Stable Sr isotope (δ88/86Sr) is a novel non-traditional stable isotope. It has been widely discussed in tracing continental weathering, studying marine Sr cycle, and analyzing biogeochemical behavior in recent years. Its combination with 87Sr/86Sr can indicate the source of Sr and the mass fractionation of geological processes, which has incomparable advantages over other isotopic systems, such as Li, Mg, and Ba. However, owing to the small mass difference (2%) between 86Sr and 88Sr, fractionation is not significant, which makes the measurement of the ratio δ88/86Sr difficult and limits its application to trace continental weathering.In this study, major developments and challenges in using stable Sr isotopes to trace continental weathering are reviewed, when considering high-precision testing methods, characteristics of δ88/86Sr in natural reservoirs, and geochemical behaviors. The high-precision measurement methods currently used for stable Sr isotopes mainly include multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) and thermal ionization mass spectrometry (TIMS), both of which involve chemical purification by a cation exchange column or Sr specific resin, coupled with various methods for quality discrimination correction. The present analytical precision for the determination of δ88/86Sr in geological samples is 0.02‰ ∼ 0.08‰. The δ88/86Sr varies widely in natural reservoirs. In particular, the range of δ88/86Sr values in extraterrestrial reservoirs, the biosphere, the lithosphere, and the hydrosphere are -1.73‰ to +0.66‰, -0.87‰ to +0.85‰, -0.20‰ to +0.53‰ and -0.45‰ to +0.66‰, respectively. Stable Sr isotope fractionation occurs in the processes of differential dissolution of primary minerals (Δ88/86Sr = -0.13‰), formation and adsorption of secondary minerals (Δ88/86Sr = -0.15‰), precipitation of carbonate (Δ88/86Sr = -0.20‰), and biological processes (Δ88/86Sr = -0.15‰), which causes the δ88/86Sr in fluid (solid) phase to δ88/86Sr increases (decrease). Previous research on the fractionation mechanisms and constraint conditions of stable Sr isotopes during continental weathering is insufficient. Thus, promoting a wide application of stable Sr isotopes to trace continental weathering through the combination of field profiles, laboratory experiments and theoretical calculation is urgently warranted.

Sr isotope variations in Oligocene–Miocene and modern biogenic carbonate formations of Koko Guyot (Emperor Seamount Chain, Pacific Ocean)

The Hawaiian–Emperor Seamount Chain, a major topographic feature of the Pacific Ocean floor, is composed of seamounts capped with fossil coral reef deposits that originally formed close to sea level but are now covered by hundreds of meters of water owing to prolonged subsidence. These fossil reef deposits are important archives of paleoenvironmental change and yield information on the subsidence history of the seamounts. We studied the Sr isotope compositions of Oligocene–Miocene coral reef limestone from Koko Guyot in the southern Emperor Seamount Chain to assess the dynamics of the subsidence. The ages of the studied samples containing coral fragments established by Sr isotope stratigraphy vary from 26.3 to 20.1 Ma. In contrast, the youngest samples (15.3 Ma), which were deposited in water depths of >120 m, are barren of corals and are composed exclusively of bryozoans and coralline algae. The subsidence rate of the Koko Guyot volcanic structure was not constant over time. Integration of our new data with the results of previous studies reveals that the subsidence rate was 0.046 ± 0.005 mm/yr during the first 25–30 Myr (from 49–44 to 20 Ma). During this period, Koko Guyot was in a bathymetric interval favorable for coral reef development, and its subsidence was compensated by rapid vertical growth of the reef. Subsequently, the subsidence rate decreased to an average value of 0.019 ± 0.003 mm/yr from 20 to 15 Ma. The decrease in the rate of bottom subsidence coincided with unfavorable environmental conditions for coral reef development, leading to the disappearance of corals. The average subsidence rate has been 0.015 ± 0.002 mm/yr since 15 Ma, comparable to the present-day subsidence rate. We also analyzed the stable Sr isotope ratios (δ88/86Sr) of warm-water coral samples formed at 25–20 Ma (0.32‰ ± 0.1‰), as well as carbonate of large benthic foraminifera, coralline algae, and other non-coral species for the period 20–15 Ma (0.10‰ ± 0.09‰). We suggest that the large difference in carbonate δ88/86Sr between 25–20 and 20–15 Ma corresponds to a difference in the fractionation factor caused by environmental and benthic community change.

Simultaneous analysis of stable and radiogenic strontium isotopes in reference materials, plants and modern tooth enamel

Radiogenic strontium isotopes (87Sr/86Sr) are a useful tool in forensics, ecology, bioarcheology and paleoanthropology allowing investigation of present and past migration and landscape use. The measurement of the 87Sr/86Sr ratio traditionally assumes a constant stable (88Sr/86Sr) isotope ratio. However, some studies indicate that these stable Sr isotopes may display mass-dependent fractionation, suggesting that the 88Sr/86Sr ratio may fingerprint previously unknown dietary and physiological information. Here we present a survey of the variability of δ88Sr values, along with the 87Sr/86Sr ratios, in fourteen reference materials of geological and biological origin using MC-ICPMS. The measurements employ a simple sample-standard bracketing method and zirconium external correction. Comparisons with double-spiked δ88Sr TIMS analyses show a very good agreement (0.014 ‰; n = 10). We then applied this method to explore the fractionation of the 88Sr/86Sr ratio in tooth enamel of mammals from two modern food-chains (Kruger National Park and Western Cape, South Africa), and from modern South African chacma baboon populations. Clear differences in the δ88Sr values are observed between plants and teeth of herbivores (~ −0.26 ‰; n = 5), but the distinction between herbivores and carnivores requires further investigation. Variations between tooth enamel of young and adult baboons suggests that the δ88Sr is a promising indicator of weaning behaviors. Our method implementation and preliminary results highlight the importance of coupled radiogenic and stable Sr isotope determination in extant and extinct vertebrates.

Precise measurements of δ88/86Sr for twenty geological reference materials by double-spike MC-ICP-MS

Analytical method using double-spike and MC-ICP-MS can provide high efficiency of measurement for stable Sr isotopic compositions (δ88/86Sr). Here we report a method for high precision measurement of δ88/86Sr using MC-ICP-MS. A84Sr–87Sr double-spike was used to correct instrumental mass bias of stable Sr isotopes. Strontium was purified by a cation exchange resin (AG50W-X12, 200–400 mesh), and for samples with high Er, Yb, Lu, and Hf contents, an additional chromatographic purification procedure was applied. The potential effects of doubly charged ion isobaric inferences and different measurement conditions were rigorously evaluated to improve the analytical precision. Data quality was monitored by measuring three USGS reference materials (BHVO-2, AGV-2, and G-2) and two in-house reference materials, giving the long-term precision of 0.03‰ (2SD) for δ88/86Sr. Twenty reference materials, including nine igneous rocks, four sedimentary rocks, two sediments, and five soils, have δ88/86Sr values range from −0.06‰ to 0.35‰, showing resolvable isotope fractionation during magmatic and supergene processes. This study provides a useful standard dataset for inter-laboratory comparison of stable Sr isotope studies.

Two-End-Member Mixing in the Fluids Emitted From Mud Volcano Lei-Gong-Huo, Eastern Taiwan: Evidence From Sr Isotopes

Mud volcano is one of the most important conduits for deep seated materials to migrate upward in sedimentary basins, convergent margins, and subduction zones. Understanding their temporal and spatial characteristics and variations provides us the important information on fluid sources and chemical compositions at depth. Mud volcano Lei-Gong-Huo (MV LGH) is a unique mud volcano, which is located on the mélange formation lying on the andesitic volcanic arc. Fluids emitted from 46 mud pools in MV LGH in eastern Taiwan were sampled and their major trace constitutes as well as H, O, and Sr isotopes (87Sr/86Sr and δ88Sr) were measured. Major constitutes of the fluids are Cl−, Na, and Ca. Compared with seawater, LGH fluids have lower Cl−, δD, δ18O, Na/Cl, K/Cl, and Mg/Cl but higher Ca/Cl ratios, indicating water–rock interaction of igneous rock and the ancient seawater at the source region. This interpretation is further supported by Sr isotopes, which show low value of 87Sr/86Sr ratio down to 0.70708. The result of spatial distribution showing strong negative correlation between Na and Ca concentration as well as Ca and 87Sr/86Sr ratios indicates that two end-member mixing is the major chemical characteristic. The fluids interacting with igneous rock carry high Ca, high δ88Sr, low Na, and low 87Sr/86Sr ratio, while those interacting with sedimentary rock carry low Ca, low δ88Sr, high Na, and high 87Sr/86Sr ratio. The source from the igneous region dominates the eastern and southeastern parts of the mud pools while sedimentary source dominates the western and northwestern parts. Most mud pools show mixing behavior between the two sources. Some of the sedimentary-dominated mud pools reveal existence of residual ancient water as indicated by 87Sr/86Sr. The major factor to fractionate the stable Sr isotopes in LGH waters is the source lithology. In summary, fluids emitted by mud pools in LGH originate from two sources, which are water–rock interactions of igneous rock with the ancient seawater from the east and sedimentary rock from the west at depth, resulting from the complex geologic background of mélange formation.

Determining 88Sr/86Sr of barite using the Na2CO3 exchange method

This study developed a method using Na2CO3 exchange reaction and MC-ICP-MS to measure the stable Sr isotope composition of barites. Using excessive Na2CO3 can avoid Sr isotope fractionation.

Radiogenic and stable Sr isotopes constrain weathering processes in rapidly eroding Taiwan catchments

Small mountainous rivers (SMRs) in subtropical Taiwan supply a disproportionately large quantities of suspended and dissolved flux to the ocean due to extraordinarily high denudation rates in the world. We present major elements, 87Sr/86Sr, and δ88/86Sr data of river water and sediment from two typical Taiwan SMRs, Zhuoshui (Choshui) and Liwu Rivers. High contents of dissolved Ca and Mg are generally accompanied by high 87Sr/86Sr ratios in waters of the Zhuoshui and Liwu Rivers (0.710721–0.715551 and 0.710461–0.711618). Sequential extraction experiments of river sediments reveal that Sr mainly resides in the residual silicate and acid-soluble fractions. High 87Sr/86Sr ratios of 0.709908–0.714168 in the acid-soluble fraction of the Zhuoshui River sediments indicate that disseminated calcite is present in silicate rocks. Rapid weathering of disseminated calcite in response to high denudation rate determines the high 87Sr/86Sr ratios in the upper Zhuoshui River water. Its downstream decreasing of 87Sr/86Sr ratios across major lithology could be explained by isotopic exchange between river water and sediment leaching fractions. However, all the 87Sr/86Sr ratios of the sequential leaching sediments from the Liwu River largely overlap along its course. The relatively low 87Sr/86Sr ratios of 0.708747–0.711070 in the acid-soluble fraction of the Liwu River sediments could be sourced from disseminated calcite and/or Tailuko marble, thus supplying low 87Sr/86Sr ratios to river waters.Spatial variations in river water δ88/86Sr values mainly occur between the mainstram and the tributary. The relation between δ88/86Sr and 87Sr/86Sr in both rivers conservatively traces water-mass mixing, while the stable Sr isotope fractionation processes (e.g., biological uptake, clay reactions, and calcite precipitation) during transport in streams is negligible. Therefore, it is suggested that combining radiogenic and stable Sr isotopes in river water and/or sequential extracted sediment is an effective means to constrain chemical weathering as well as fractionation processes in a multi-lithological mountainous catchment with high denudation rate and wet climate.

Origin of volatile species and aqueous fluids in the Los Humeros Geothermal Field, Mexico

The Los Humeros Geothermal Field, discovered in the late 1970s, is the last geothermal field in Mexico to be significantly developed in recent years. Located in a nested caldera system of the Central Mexican Volcanic Belt, the field is within an andesitic reservoir, with low porosity and permeability, limiting the recharge of the system. Here, a new dataset of noble gas and stable isotopes (δ2H and δ 18O), tritium (3H), CO2 contents, halogens (Cl content and δ37Cl, δ81Br), and isotopes of Sr (87Sr/86Sr) were analyzed and interpreted along with previously published noble gas data. The goal was to identify the fluid sources present in the Los Humeros reservoir. This study highlights a complex hydrogeological scenario, with multiple generations of meteoric waters. Furthermore, some volatile species of mantle origin (He, Ar, C) appear to be partially decoupled from the aqueous phase, providing only partial information on the origin and evolution of the waters present in the system. The northern sector of the field contains meteoric waters having interacted with the basalt-andesitic reservoir or deeper melts (87Sr/86Sr = 0.7040) and containing mantle He and Ar. Together with this fluid, a meteoric component containing tritium confirms the presence of local and rapid meteoric recharge in this sector. The central and southern sectors of the field contain meteoric waters having interacted with carbonates from the basement. However, the relationship between 87Sr/86Sr and CO2/3He suggests a possible decoupling between mantle C and He and the aqueous phase, transporting Sr. Of particular interest, limestone C seems to be sourced from subducting slab components from the northern sector of the field, while the often-cited C contribution from local basement metacarbonates could be negligible. Finally, chlorine is sourced by magmatic HCl and partitioned in the steam phase, causing acidic waters.

Isotope shift and hyperfine structure measurements on triple resonance excitation to the autoionizing Rydberg state of atomic strontium

Triple resonance excitation to the autoionizing Rydberg state of atomic strontium was studied for the isotope-selective photoionization of strontium-90 (90Sr). We investigated a new resonance ionization scheme, namely, 689.4 nm–472.4 nm–404.2 nm that is less affected by the multiphoton ionization induced by the first (689.4 nm) laser compared to the scheme in our previous study, namely, 689.4 nm–487.4 nm–393.8 nm. The isotope shifts and hyperfine structure constants of stable Sr isotopes were measured for the second and third transitions of these two schemes. Thus, the 90Sr isotope shift for each of the four transitions was evaluated using the King plot analysis. Under the current laser conditions, no significant difference was observed in the measured 90Sr optical isotopic selectivity between these two schemes, but the new scheme could show better selectivity if the first (689.4 nm) laser output is increased to improve the ionization efficiency. Our spectroscopic data will be useful for future analysis of 90Sr in real samples and may also help promote studies in other fields, such as precision measurements using cold atoms in optical traps.

A 35-million-year record of seawater stable Sr isotopes reveals a fluctuating global carbon cycle

Changes in the concentration and isotopic composition of the major constituents in seawater reflect changes in their sources and sinks. Because many of the processes controlling these sources and sinks are tied to the cycling of carbon, such records can provide insights into what drives past changes in atmospheric carbon dioxide and climate. Here, we present a stable strontium (Sr) isotope record derived from pelagic marine barite. Our δ88/86Sr record exhibits a complex pattern, first declining between 35 and 15 million years ago (Ma), then increasing from 15 to 5 Ma, before declining again from ~5 Ma to the present. Numerical modeling reveals that the associated fluctuations in seawater Sr concentrations are about ±25% relative to present-day seawater. We interpret the δ88/86Sr data as reflecting changes in the mineralogy and burial location of biogenic carbonates.