Exchangeable Sr Research Articles

Cation exchange parameters for Opalinus Clay and its confining units

The knowledge of cation exchange parameters is important to understand and model the porewater chemistry in clayrocks. The Opalinus Clay (OPA) is a well-studied formation in view of its importance as host rock for radioactive waste repositories, but uncertainties exist regarding its cation exchange parameters, in particular its selectivity coefficients (Kc). In fact, porewater chemistry models for OPA have so far been based on generic selectivity coefficients.Thanks to a recent extensive siting programme, involving eight vertical deep boreholes in northern Switzerland, a large number of porewater and cation exchange data from the OPA and its clay-rich confining units could be analysed. Thus, from the combination of porewater compositions (obtained from high-pressure squeezing and advective displacement), and exchangeable cation data (obtained from Ni ethylenediamine extraction), reliable selectivity coefficients could be derived. The values for KcNa/K, KcNa/Ca, KcNa/Mg are consistent with those obtained in parallel from the Mont Terri underground rock laboratory although the scatter is somewhat larger in the latter dataset. The derived selectivity coefficients indicate no dependency on ionic strength or in-situ temperature. Minor differences in Kc values between OPA and confining units can be attributed to differences in clay mineralogy. The validity of a two-site cation exchange model including illite and smectite for estimating exchangeable Na, Ca and Mg from measured porewater compositions could be shown, while this simple model underpredicts the share of exchangeable K. The exchangeable cation extraction procedure induces the dissolution of Sr-bearing minerals whose amount is in the same range as exchangeable Sr, thus not enabling the derivation of selectivity coefficients for Sr.Finally, derived selectivity coefficients confirm results of previous studies, thus providing confidence in the applied experimental and modelling protocols on the one hand and in the outcome of those studies on the other hand.

Potassium applications reduced cesium uptake and altered strontium translocation in soybean plants

ABSTRACT After the Tokyo Electric Power Company’s Fukushima Dai-ichi Nuclear Power Plant accident in 2011, radioactive cesium (RCs) was released in greater concentrations than radioactive strontium (RSr) in the surrounding environment. Most of the countermeasures were developed to mitigate the RCs transfer from the soil to plants. However, to avoid what has happened after the Chernobyl and Mayak accidents, preventing the transfer of RSr from soil to plants should be a priority. Although the application of potassium (K) fertilizers is the most effective method for preventing agricultural crops from absorbing RCs in contaminated fields, this implementation increases the cost and labor requirements. Considering the preparedness for nuclear accidents, it remains unclear how this countermeasure will be affected if RCs and RSr are released simultaneously. We aimed to explore the effect of K applications on cesium (Cs) and strontium (Sr) uptake and their interaction with and correlation to other elements in the soybean plants and soil. The field experiments were conducted in Fukushima Prefecture, Japan, using different K applications (i.e., no, normal, and high K applications). The dry weight and mineral concentrations of K, Cs, Sr, calcium (Ca), magnesium (Mg), and nitrogen (N) concentration in plants and exchangeable K (ExK), exchangeable Cs (ExCs), exchangeable Sr (ExSr), exchangeable Ca (ExCa), exchangeable Mg (ExMg), NH4+ (ammonium), and NO3- (nitrate) concentrations in the soils were evaluated. This study revealed that K application reduced Cs, Ca, and Mg uptake but did not affect the ExSr, ExCa, and ExMg concentrations in the soil and did not change the uptake of Sr. On the other hand, K concentration of the plant especially at later growth stage, which indicates re-translocation of Sr was negatively regulated by K concentration.

Tracing changes in base cation sources for Arctic tundra vegetation upon permafrost thaw

Upon permafrost thaw, the volume of soil accessible to plant roots increases which modifies the acquisition of plant-available resources. Tundra vegetation is actively responding to the changing environment with two major directions for vegetation shift across the Arctic: the expansion of deep-rooted sedges and the widespread increase in shallow-rooted shrubs. Changes in vegetation composition, density and distribution have large implications on the Arctic warming and permafrost stability by influencing the albedo, the snow accumulation and the litter decomposition rate. A better understanding of these cumulated effects of changing vegetation on warming and permafrost requires assessing the changes in plant nutrient sources upon permafrost thaw, nutrient access being a limiting factor for the Arctic tundra vegetation development. In this study, we determined the influence of permafrost degradation on the base cation sources for plant uptake by using the radiogenic Sr isotope ratio as a tracer of source, along a permafrost thaw gradient at Eight Mile Lake in Interior Alaska (USA). As plants take up Sr from the exchangeable soil fraction with no measurable fractionation, we determined the differences in 87Sr/86Sr ratio of the exchangeable Sr between shallow and deeper soil horizons, and we compared the 87Sr/86Sr ratio of foliar samples for three Arctic tundra species with contrasted rooting depths (Betula nana, Vaccinium vitis-idaea, and Eriophorum vaginatum) upon different permafrost thaw conditions. The higher foliar 87Sr/86Sr ratios of shallow-rooted Arctic tundra shrubs (B. nana, V. vitis-idaea) was consistent with a shallow source of soil exchangeable Sr from surface soil horizons, whereas the lower foliar 87Sr/86Sr ratios of deep-rooted Arctic tundra sedges (E. vaginatum) reflected a source of Sr from deeper soil horizons. There is a shift between poorly and highly thawed soil profiles towards lower foliar 87Sr/86Sr ratios in both deep- and shallow-rooted plant species. This shift supports that micro-landscape variability in the exchangeable base cation reserve with soil depth represents a key source of readily available nutrients for both shallow- and deep-rooted plant species upon permafrost thaw. This study highlights a key change in plant nutrient source to consider upon thaw. This finding lies beyond the common view that nutrient release at the permafrost thaw front preferentially benefits deep-rooted plant species.

Role of desorption-adsorption and ion exchange in isotopic and chemical (Li, B, and Sr) evolution of water following water–rock interaction

In a complex water–rock system that contains clay (or/and organic matter) and carbonates, multiple factors can affect specific isotopic compositions or ratios of fluids, such as the adsorption–desorption, ion exchange, and dissolution-precipitation of the related minerals. In this situation, the Rayleigh model is inappropriate for simulating the pore fluid isotopic data, and the ultimate mechanism causing isotopic fractionation has not yet been resolved. Besides, the endmembers (including concentration and isotopic composition/ratio) related to geochemical processes have not been well identified. Shale is enriched in clay and organic matter and usually contains carbonates. Comprehensive adsorption–desorption and ion exchange occurs within a short time. Accordingly, the flowback water following hydraulic fracturing of shale gas reservoirs is a unique material for studying the mechanism affecting the specific isotopic composition/ratio (lithium, boron, and strontium). This study aims to investigate the mechanism of changes in the isotopic compositions/ratios following the hydraulic fracturing of a shale gas reservoir by obtaining high-quality flowback water, determining all potential endmembers (concentration and isotopic composition/ratio for water-soluble fraction, exchangeable fraction, and carbonate minerals), and conducting laboratory water–rock interactions to confirm the trends observed in the field. The results indicated that desorption-adsorption or/and ion change was a critical factor controlling the concentration and isotopic compositions (δ7Li and δ11B) and radiogenic 87Sr/86Sr ratios in this complex water–rock system. The contribution from the dissolution of carbonates to Li, B, and Sr in the flowback water beyond the mixing between fracturing water and formation water was estimated to be less than 16%. Desorption of B from shale would result in higher B/Cl ratios and changes in δ11B values of fluids rely on the exchangeable B in shale (δ11B values decreased in this study). Another new finding demonstrates that the increased 87Sr/86Sr values of flowback water beyond mixing was inferred from the re-equilibrium between the dissolved Sr and exchangeable Sr, rather than dissolution of silicates with a high 87Sr/86Sr value, which suggests a more complex process than previously expected. While clay minerals or/and organic matter are commonly distributed in groundwater systems, the findings are also beneficial for tracing water–rock interactions and the sources/sinks of a specific solute in a groundwater system that may be impacted by adsorption-desorption or/and ion exchange.

The Sr/Ba ratio response to salinity in clastic sediments of the Yangtze River Delta

The Sr/Ba ratio is a geochemical indicator that distinguishes between terrestrial and marine sedimentary environments in terrigenous clastic sediments. However, conclusions drawn from the Sr/Ba ratio is often inconsistent with those drawn from other sedimentary environment indicators during practical application; therefore, the usability of this indicator is not widely acknowledged. First, to verify this theory and accurately determine the differences between the sedimentary geochemical behaviours of Sr and Ba under different salinity conditions, 14 artificial deltaic sediments were prepared by mixing clean seawater and muddy river water in varying proportions to produce different constant salinities, ranging from 0.0 to 31.2 psu. The Sr and Ba concentrations in the different fractions were then examined via sequential extraction. The experimental results show that, on average, 87% of the total Ba and 65% of the total Sr in the 14 artificial deltaic sediments existed in the residual fraction that is independent of the geochemical conditions of the depositional environment. The experiment also revealed that the decrease in Ba in the sediments was caused by the desorption of adsorbed Ba, owing to the addition of seawater with increasing salinity. Furthermore, the Sr/Ba ratio in the exchangeable fraction yielded a strong linear correlation with salinity, and the Sr/Ba ratio in the carbonate-bound fraction tended to increase slightly with increasing salinity, the Sr/Ba ratios in the FeMn oxide-bound, organic and reductive, and residual fractions, and the total Sr/Ba ratio were not significantly correlated with salinity. Therefore, the Sr/Ba ratio (Sr/Ba-HAc) of exchangeable and carbonate-bound Sr and Ba extracted using dilute acetic acid (HAc) and the Sr/Ba ratio (Sr/Ba-NH4Ac) of exchangeable Sr and Ba extracted using ammonium acetate (NH4Ac) can be used to distinguish between marine and terrestrial sedimentary environments. If the Sr/Ba-HAc or Sr/Ba-NH4Ac ratio in the sediment is >1.0, it can be considered to have been deposited in a sedimentary environment affected by seawater with a salinity >5.0 psu. Second, to verify the aforementioned conclusions, the Sr and Ba in 24 field samples from the Yangtze River Delta were extracted using 10% HAc or 1.0 M NH4Ac. The results indicate that the increase in the amount of Sr in the estuarine terrigenous clastic sediments from the river to the marine environments is caused by the addition of Sr-rich, Ba-poor, marine carbonate shells. The extracted Sr/Ba-HAc ratios were < 1.0 in freshwater (fluvial), 1.0–3.0 in brackish water (delta front), 3.0–8.0 in saltwater (prodelta), and > 8.0 in normal seawater (neritic environments and relict sand); however, in the traditional bulk analysis, total Sr/Ba ratio did not change significantly with increasing salinity. Therefore, it is suggested that the selective extraction of sedimentogenic Sr and Ba is the only effective means of using the Sr/Ba ratio to discriminate between marine and terrestrial sedimentary environments in terrigenous clastic sediments, while the traditional method of using the total Sr/Ba ratio of whole samples is unsuitable.

87Sr/86Sr compositional linkage between geological and biological materials: A case study from the Toyota granite region of Japan

The strontium isotope value (87Sr/86Sr) is often used to identify the origins of agricultural products and the movements of ancient people. Maps delineating the distribution of 87Sr/86Sr values can therefore be of great use. Previous 87Sr/86Sr distribution maps of stream sediments covering small areas of Japan largely reflect the 87Sr/86Sr values of underlying bedrock. It remains unclear, however, whether the 87Sr/86Sr values of stream sediments may also reflect the 87Sr/86Sr values of the nearby vegetation and fauna. We sought to resolve this question by investigating the relationship between 87Sr/86Sr values in biological samples and in geological samples from the sites from which the biological samples grew. The geological samples consisted of soil samples from 2 paddy fields and 13 bed sediment samples (grain size <180 μm) from streams surrounding the paddy fields. All were collected from regions underlain by Inagawa granite bedrock, i.e., granite geology, around Toyota, Aichi Prefecture, Japan. The biological samples consisted of 4 rice plants and 1 wild boar bone, all collected in or near the same paddy fields.In the geological samples, the 87Sr/86Sr values of water-extracted Sr (water-soluble Sr) and ammonium-acetate-extracted Sr (exchangeable Sr) fractions in paddy soils were almost the same as each other (0.7098–0.7100) and slightly lower than the 87Sr/86Sr values of bulk Sr fractions (0.7099–0.7106). The 87Sr/86Sr values of biological samples (0.7098–0.7100) were also similar to those of the water-soluble Sr and exchangeable Sr fractions in paddy soils, though they varied among the parts of plants analyzed. This indicates that exchangeable Sr is absorbed by plants from soils through nutrients and water, and ingested by herbivorous animals from soils through plants and water. The 87Sr/86Sr values of bulk Sr in paddy soils ranged somewhat widely, while those from stream sediments collected around paddy fields were less variable (0.7099–0.7100). The difference in 87Sr/86Sr values between the paddy soils and stream sediments, samples similarly derived from Inagawa granite bedrock, may be related to the variable degree of weathering, and ultimately to the mineral composition, of the samples. This difference, however, was smaller than the range of 87Sr/86Sr values observed in the watershed geology. We therefore propose that in a silicate-dominated geology of the type typical of Japan, an 87Sr/86Sr spatial distribution map (using both exchangeable Sr and bulk Sr fractions in stream sediments) can provide useful guidance for identifying the geographic origins of food or for serving other related purposes.

Distribution coefficients (Kd) of strontium and significance of oxides and organic matter in controlling its partitioning in coastal regions of Japan

The Fukushima Daiichi Nuclear Power Plant accident in March 2011 resulted in the release of large quantities of a long-lived radioactive strontium (i.e. 90Sr; half-life: 28.8y) into the coastal areas of Japan. 90Sr release was dispersed and mixed into the water column, and will eventually be deposited into sediment. Because factors controlling seawater–sediment partitioning in the coastal marine environments are not fully understood, we developed seawater–sediment distribution coefficients, Kd (L/kg), for Sr in coastal regions of Japan by means of sediment–water partitioning experiments. 85Sr was used as a radiotracer and conditions were designed to mimic the environmental systems of the sampling sites as closely as possible. Experimentally determined Kd values (Kd-ex) varied between 0.3 and 3.3L/kg (mean, 1.4L/kg), and the variation in Kd-ex was attributed to the percentage of Sr in the exchangeable fraction in the sediment. Kd-ex values were used, along with the measured concentrations of 88Sr, a stable naturally occurring Sr isotope in seawater and sediment, to estimate the concentrations of exchangeable Sr in the sediment. Estimates ranged from 2.1 to 24.3μg/kg, or 1.3–15.7% of the total 88Sr concentration in the sediment. Significant correlations existed between the estimated concentrations of exchangeable Sr, and the organic matter and the oxide/hydrous oxide contents. When organic contents were greater than 0.38%, Sr binds to organic surface sites more strongly than to the other sites. Results indicate that binding of Sr to the surface of sedimentary particles was influenced by grain size, iron and manganese oxides, and organic matter. Furthermore, the information presented here could be useful to estimate Kd values for anthropogenic 90Sr in sediment in the coastal marine environment.

Behaviour of boron and strontium isotopes in groundwater–aquifer interactions in the Cornia Plain (Tuscany, Italy)

The Cornia Plain alluvial aquifer, in Tuscany, is exploited intensely to meet the demand for domestic, irrigation and industrial water supplies. The B concentration of groundwater, however, is often above the European limit of 1 mg L −1, with the result that exploitation of these water resources requires careful management. Boron and Sr isotopes have been used as part of a study on the origin and distribution of B dissolved in groundwater, and indirectly as a contribution to the development of appropriate water management strategies. The geochemistry of the Cornia Plain groundwater changes from a HCO 3 facies in the inland areas to a Cl facies along the coastal belt, where seawater intrusion takes place. The B concentration of groundwater increases towards the coastal areas, while the 11B/ 10B ratio decreases. This indicates that there is an increasing interaction between dissolved B and the sediments forming the aquifer matrix, whose B content is in the order of 100 mg kg −1. Adsorption–desorption exchanges take place between water and the sediment fine fraction rich in clay minerals, with a net release of B from the matrix into the groundwater, and a consequent δ 11B shift from positive to negative values. The aquifer matrix sediments therefore seem to be the major source of B dissolved in the groundwater. The groundwater–matrix interactions triggered by the ionic strength increase caused by seawater intrusion can also be detected in the Ca–Na ion exchanges. Dissolved Sr follows a trend similar to that of Ca, while the 87Sr/ 86Sr ratio is equal to that of the exchangeable Sr of the aquifer matrix and therefore does not change significantly. These results have helped to define a new strategy for groundwater exploitation, with the final objective of reducing B concentration in the water extracted from the aquifer.

Controls on the spatial and temporal variability of vadose dripwater geochemistry: Edwards aquifer, central Texas

A 4-yr study of spatial and temporal variability in the geochemistry of vadose groundwaters from caves within the Edwards aquifer region of central Texas offers new insights into controls on vadose groundwater evolution, the relationship between vadose and phreatic groundwaters, and the fundamental influence of soil composition on groundwater geochemistry. Variations in Sr isotopes and trace elements (Mg/Ca and Sr/Ca ratios) of dripwaters and soils from different caves, as well as phreatic groundwaters, provide the potential to distinguish between local variability and regional processes controlling fluid geochemistry, and a framework for understanding the links between climatic and hydrologic processes. The Sr isotope compositions of vadose cave dripwaters (mean 87Sr/ 86Sr = 0.7087) and phreatic groundwaters (mean 87Sr/ 86Sr = 0.7079) generally fall between values for host carbonates (mean 87Sr/ 86Sr = 0.7076) and exchangeable Sr in overlying soils (mean 87Sr/ 86Sr = 0.7088). Dripwaters have lower Mg/Ca and Sr/Ca ratios, and higher 87Sr/ 86Sr values than phreatic groundwaters. Dripwater 87Sr/ 86Sr values also inversely correlate with both Mg/Ca and Sr/Ca ratios. Mass-balance modeling combined with these geochemical relationships suggest that variations in fluid compositions are predominantly controlled by groundwater residence times, and water-rock interaction with overlying soils and host aquifer carbonate rocks. Consistent differences in dripwater geochemistry (i.e., 87Sr/ 86Sr, Mg/Ca, and Sr/Ca) between individual caves are similar to compositional differences in soils above the caves. While these differences appear to exert significant control on local fluid evolution, geochemical and isotopic variations suggest that the controlling processes are regionally extensive. Temporal variations in 87Sr/ 86Sr values and Mg/Ca ratios of dripwaters from some sites over the 4-yr interval correspond with changes in both aquifer and climatic parameters. These results have important implications for the interpretation of trace element and isotopic variations in speleothems as paleoclimate records, as well as the understanding of controls on water chemistry for both present-day and ancient carbonate aquifers.

Strontium isotope composition of runoff from a glaciated carbonate terrain

The relationship between subglacial chemical weathering processes and the Sr isotope composition of runoff from Robertson Glacier, Alberta, Canada, is investigated. This glacier rests on predominantly carbonate bedrock of Upper Devonian age, but silicate minerals are also present. The provenance of solute in meltwaters is found to vary systematically with solute concentration and, by inference, subglacial water residence time. In dilute waters, the principal process of solute acquisition is calcite dissolution fueled by protons derived from the dissolution of CO2 and subsequent dissociation of carbonic acid. At higher solute concentrations, dolomite dissolution coupled to sulfide oxidation is more important. Sr concentration is found to increase with total solute concentration in two separate meltwater streams draining from the glacier, but 87Sr/86Sr only increases in the eastern melt stream. Carbonate and K-feldspar sources are shown to dominate the Sr content of the western stream, irrespective of concentration. They also dominate the Sr content of the eastern stream at low and intermediate concentrations, but at higher concentrations, muscovite (with high 87Sr/86Sr) is also an important Sr source. This reflects the outcrop of muscovite-bearing lithologies in the catchment of the eastern stream and an increase in the rate of weathering of K-silicates relative to that of carbonates as more concentrated solutions approach saturation with respect to carbonates. Nonstoichiometric release of 87Sr/86Sr and preferential release of Sr over K from freshly ground K-silicate surfaces may also occur. This may help to explain the radiogenic nature of runoff from distributed subglacial drainage systems, which are characterized by long water:rock contact times and water flow through environments in which crushing and grinding of bedrock are active processes.Although the exchangeable Sr in tills has higher 87Sr/86Sr than local carbonate bedrock, only the more concentrated meltwaters from the eastern stream display similarly high values. The most dilute waters, which probably transport the bulk of the dissolved Sr flux from the glacier, have 87Sr/86Sr characteristic of local carbonate bedrock. Thus, the results suggest that although enhanced weathering of silicate minerals containing radiogenic Sr (such as muscovite) does occur in glaciated carbonate terrains, it is unlikely to contribute to any enhanced flux of radiogenic Sr from glaciated continental surfaces to the oceans.

Strontium as a tracer of weathering processes in a silicate catchment polluted by acid atmospheric inputs, Strengbach, France

This paper determines the weathering and atmospheric contributions of Ca in surface water from a small spruce forested silicate catchment (N–E France) receiving acid atmospheric inputs. The bedrock is a granite with K-feldspar and albite as dominant phases. The calcium content in plagioclase is low and the Ca/Na ratio in surface water is high, reflecting other sources of calcium from those expected from the weathering of major mineral phases. The biotite content is low. Only traces of apatite were detected while no calcite was found in spite of a major hydrothermal event having affected the granite. The strontium isotopic ratio 87Sr/ 86Sr and Sr content was used as a tracer of weathering and was determined in minerals and bulk bedrock, open field precipitation, throughfall, soil solution, spring and stream water. The Sr isotopic ratio of the reacting weathering end-member was predicted by simulating the alteration of the granite minerals by incorporating strontium into the water–rock interaction kinetic code KINDIS. In the early stages of water–rock interaction, K-feldspar and biotite strongly influence the isotopic composition of the weathering solution whereas, the Na-rich plagioclase appears to be the main long-term reactive weathering end-member. Approximately 50% of dissolved Sr in streamwater are atmospherically derived. The 87Sr/ 86Sr ratios of exchangeable Sr in the fine fraction at 1-m depth from a soil profile indicate that the amount of exchangeable Sr seems essentially controlled by atmospheric inputs. The exception is the deep saprolite where weathering processes could supply the Sr (and Ca). Na-Plagioclase weathering obviously control the chemistry and the isotopic composition of surface waters. The weathering of trace mineral plays a secondary role, the exception is for apatite when plagioclase is absent. Our hydrochemical, mineralogical and isotopic investigations show that a major part of the strong Ca losses detected in catchment hydrochemical budgets that result from the neutralization of acid precipitation has an atmospheric origin. Consequently, in the long term, in such areas, the availability of such an exchangeable base cation might be strongly limited and surface waters consequently acidified.

Strontium isotopic evidence on the chemical evolution of pore waters in the Milk River Aquifer, Alberta, Canada

Strontium isotope ratios were measured on 13 rock, 18 leachate and 28 pore-water samples from the Milk River aquifer, the confining argillaceous formations, and the glacial till mantling the recharge area. Strontium isotope ratios ( 87Sr/ 86Sr) of pore waters from the aquifer, confining units, and the glacial till ranged from 0.7069 to 0.7082. The 87Sr/ 86Sr ratios in aquifer pore waters decrease with increasing distance from the aquifer recharge area, and this is interpreted to be the result of mixing and water–rock interaction within the aquifer. The solute composition of the recharging groundwater is modified by the local lithology, causing distinct geochemical patterns along different flow paths within the aquifer. Whole-rock 87Sr/ 86Sr ratios indicate that the shales and till are generally more radiogenic than the aquifer sandstone. The authigenic carbonate cements and rock-forming minerals comprising the major lithologic units had little apparent influence on the pore-water Sr chemistry. Carbonate cement leachates from the till and the aquifer sandstone are more radiogenic than those from the confining shale formations. Feldspar separates from the aquifer sandstone have relatively radiogenic Sr isotope ratios, whereas bentonites from the Milk River and Colorado Shale Formations have whole-rock and leachate Sr isotope ratios that are relatively unradiogenic. Ratios of most Milk River aquifer pore waters are lower than those of any leachates or whole rocks analyzed, except the bentonites. The 87Sr/ 86Sr ratios of exchangeable Sr in the bentonites are similar to ratios found in the more evolved pore waters. Simple rock–water interaction models calculated for the whole-rock, leachate, and exchangeable-ion/pore-water pairs indicate that ion exchange with bentonite clays within the Milk River and Colorado Shale Formations appears to influence the isotopic evolution of the pore-water Sr in each of these units.

Chemical weathering of a soil chronosequence on granitoid alluvium: II. Mineralogic and isotopic constraints on the behavior of strontium

The use of strontium isotopes to evaluate mineral weathering and identify sources of base cations in catchment waters requires an understanding of the behavior of Sr in the soil environment as a function of time. Our approach is to model the temporal evolution of 87Sr/ 86Sr of the cation exchange pool in a soil chronosequence developed on alluvium derived from central Sierra Nevada granitoids during the past 3 Ma. With increasing soil age, 87Sr/ 86Sr of ammonium-acetate extractable Sr initially decreases from values typical of K-feldspar to those of plagioclase and hornblende and then remains constant, even though plagioclase and hornblende are absent from the soils after approximately 1 Ma of weathering. The temporal variation of 87Sr/ 86Sr of exchangeable Sr is modeled by progressively equilibrating Sr derived from mineral weathering and atmospheric deposition with Sr on exchange sites as waters infiltrate a soil column. Observed decreases in quartz-normalized modal abundances of plagioclase, hornblende, and K-feldspar with time, and the distinct 87Sr/ 86Sr values of these minerals can be used to calculate Sr flux from weathering reactions. Hydrobiotites in the soils have nearly constant modal abundances, chemistry, and 87Sr/ 86Sr over the chronosequence and provide negligible Sr input to weathering solutions. The model requires time and soil horizon-dependent changes in the amount of exchangeable Sr and the efficiency of Sr exchange, as well as a biologic cycling term. The model predicts that exchangeable Sr initially has 87Sr/ 86Sr identical to that of K-feldspar, and thus could be dominated by Sr leached from K-feldspar following deposition of the alluvium. The maximum value of 87Sr/ 86Sr observed in dilute stream waters associated with granitoids of the Yosemite region is likewise similar to that of the K-feldspars, suggesting that K-feldspar and not biotite may be the dominant source of radiogenic Sr in the streams. This study reveals that, when attempting to use Strontium isotopes to identify sources of base cations in catchment waters and biomass, both preferential leaching of Sr from minerals during incipient soil development and changing Sr exchange efficiency must be considered along with chemical contributions due to mineral dissolution.

Rapid exchange effects on isotope ratios in groundwater systems: 2. Flow investigation using Sr isotope ratios

Sr isotope ratios were measured in groundwater, whole rock digestions, and cation exchange extracts from a clay‐rich groundwater system at Ernest O. Lawrence Berkeley National Laboratory and were used to constrain flow velocities and search for preferential flow paths. In the Orinda formation siltstone, 87Sr/86Sr increases strongly over tens of meters along presumed flow paths, indicating slow groundwater flow. Dissolved Sr is close to isotopic equilibrium with the exchangeable Sr in the clays, and the observed 87Sr/86Sr increase is interpreted as a cation exchange front moving slowly through the unit combined with dissolution of minerals with relatively high 87Sr/86Sr ratios. The data are inverted using a one‐dimensional transport‐dissolution‐exchange model; the results indicate long‐term average flow velocities of less than 0.2 m/yr which are consistent with 14C measurements. The data suggest a lack of strong preferential flow paths through this unit.

A silicate weathering mechanism linking increases in marine 87Sr/ 86Sr with global glaciation

THE 87Sr/ 86Sr ratio of sea water is generally regarded as a proxy for the rate of chemical weathering of the continents. Interpretation of the 87Sr/86Sr record in marine limestones requires an understanding of how changes in this quantity are linked to climate and tectonic events. A connection between the strontium isotope record and the extent of global glaciation has been proposed1 –8, but without a well established mechanism. Glaciation may influence marine 87Sr/ 86Sr by changing global riverine Sr fluxes (that is, rock weathering rates) and/or 87Sr/86Sr ratios (that is, relative mineral weathering rates). Here we investigate how the 87Sr/86Sr release from silicate weathering is affected by glaciation, using the Sr isotope systematics of granitoid soils on alpine glacial moraines in the Wind River Range, Wyoming. We observe a negative correlation between the 87Sr/86Sr ratio of exchangeable Sr in soils and the soil age, indicating that the 87Sr/86Sr ratio of Sr released in the early stages of weathering is significantly higher than in later stages. We estimate that this mechanism can increase global riverine 87Sr/86Sr by an average of 0.0002 during periods of glacial–interglacial cycling. When superimposed on long-term trends controlled by tectonic processes2–7,9,10, this shift can account for the correlation between the rate of change of marine 87Sr/86Sr and the intensity of glaciation over at least the past 10 Myr.

Sr and Nd isotopic evidence for punctuated clay diagenesis, Texas Gulf Coast

We report Rb Sr, Sm Nd and mineralogical data for leached whole rock, < 2 μm and < 0.1 μm clay size fractions of lower- to mid-Tertiary shales spanning the smectite/illite transition, from a single deep well in the Texas Gulf Coast. The abrupt transition at 2400 m, from smectite- to illite-dominated assemblages is accompanied by a marked increase in 87Sr/ 86Sr from 0.708 to 0.711 in exchangeable sites. Leached authigenic illite from the < 0.1 μm fraction, sampled from below the smectite/illite transition, defines a Rb Sr isochron age of 34.8 ± 2.0 Ma. These data are consistent with a single episode of authigenic illite precipitation over a depth range of > 1200 m, independent of the duration of burial. The exchangeable Sr in the clay has remained an open system and has undergone isotopic exchange since that time. Minimum fluid/rock ratios of 0.2 to 2 are modeled from Sr isotopic data, consistent with rock-dominated, open system diagenesis. Leachates and residues of the < 0.1 μm fraction, below the smectite/illite transition appear to have achieved Nd isotopic equilibrium during clay diagenesis, and show no signs of subsequent exchange. The clay residues have low 147Sm/ 144Nd ( < 0.085), indicating that authigenic illite has a lower Sm/Nd ratio than its bulk rock protolith. Conversely the coexisting exchangeable REE inventory has elevated Sm/Nd, as reflected by leachate 147Sm/ 144Nd as high as 0.188. This Sm/Nd fractionation, in conjunction with the preserved isotopic equilibrium may be useful for dating diagenesis and associated fluid-rock interaction in older rocks.

Effects of energy straggling on surface analysis with fast ion beams: Akira Kawano and Yoshiaki Kido, J appl Phys, 63, 1988, 75–79

A 4-yr study of spatial and temporal variability in the geochemistry of vadose groundwaters from caves within the Edwards aquifer region of central Texas offers new insights into controls on vadose groundwater evolution, the relationship between vadose and phreatic groundwaters, and the fundamental influence of soil composition on groundwater geochemistry. Variations in Sr isotopes and trace elements (Mg/Ca and Sr/Ca ratios) of dripwaters and soils from different caves, as well as phreatic groundwaters, provide the potential to distinguish between local variability and regional processes controlling fluid geochemistry, and a framework for understanding the links between climatic and hydrologic processes.The Sr isotope compositions of vadose cave dripwaters (mean 87Sr/86Sr = 0.7087) and phreatic groundwaters (mean 87Sr/86Sr = 0.7079) generally fall between values for host carbonates (mean 87Sr/86Sr = 0.7076) and exchangeable Sr in overlying soils (mean 87Sr/86Sr = 0.7088). Dripwaters have lower Mg/Ca and Sr/Ca ratios, and higher 87Sr/86Sr values than phreatic groundwaters. Dripwater 87Sr/86Sr values also inversely correlate with both Mg/Ca and Sr/Ca ratios. Mass-balance modeling combined with these geochemical relationships suggest that variations in fluid compositions are predominantly controlled by groundwater residence times, and water-rock interaction with overlying soils and host aquifer carbonate rocks. Consistent differences in dripwater geochemistry (i.e., 87Sr/86Sr, Mg/Ca, and Sr/Ca) between individual caves are similar to compositional differences in soils above the caves. While these differences appear to exert significant control on local fluid evolution, geochemical and isotopic variations suggest that the controlling processes are regionally extensive. Temporal variations in 87Sr/86Sr values and Mg/Ca ratios of dripwaters from some sites over the 4-yr interval correspond with changes in both aquifer and climatic parameters. These results have important implications for the interpretation of trace element and isotopic variations in speleothems as paleoclimate records, as well as the understanding of controls on water chemistry for both present-day and ancient carbonate aquifers.

RbSr and oxygen isotopic study of alkalic rocks from the Trans-Pecos magmatic province, Texas: Implications for the petrogenesis and hydrothermal alteration of continental alkalic rocks

Rb-Sr and O isotopic data for mid-Tertiary alkalic rocks from the Trans-Pecos magmatic province of west Texas demonstrate that hydrothermal alteration and fluid/rock (cation exchange) interactions have affected the isotope geochemistry of these rocks. Strontium and O isotopic data for late-stage minerals in an alkali basalt (hawaiite) sill record two episodes of fluid/rock interactions. Firstly, Sr isotopic data for calcite precipitated in open spaces suggest that most of the Sr incorporated in this mineral was derived from the surrounding silicate minerals; however, O was derived from heated formation fluids ( δ 18 O = +13; t< 100° C) in isotopic equilibrium with surrounding Cretaceous marine limestones. Secondly, Sr isotopic data for analcime and natrolite lie along mixing lines between a magmatic 87Sr/ 86Sr and a Cretaceous marine 87Sr/ 86Sr. Oxygen isotopic data for natrolite suggest fluids in equilibrium with this mineral were isotopically light ( δ 18 O = −8), very similar to present-day geothermal waters of the Trans-Pecos region ( δ 18 O = −6 to −9). These data suggest that later meteoric fluids introduced Sr with a Cretaceous marine 87Sr/ 86Sr ratio into minerals with significant cation exchange capacity. Dilute HCl leaching experiments demonstrate the removal of this labile or exchangeable Sr from the alkali basalt. Rb-Sr isotopic data for the leached alkali basalt and handpicked calcite record a crystallization age of 42 Ma, consistent with K-Ar data for an unaltered alkali basalt (hawaiite) dike from the same area (42.6 ± 1.3 Ma). Leaching experiments on one phonolite suggest the Sr isotopic variability in unleached phonolite and nepheline trachyte samples may be attributed to Sr in secondary calcite and zeolites, which have an upper Cretaceous marine 87Sr/ 86Sr ratio. RbSr isotopic data for leached phonolite and sanidine separate yield an age of 36.5 ± 0.8 Ma, within analytical uncertainty of a K-Ar biotite age (36.0 ± 1.1 Ma) of another phonolite. These leaching experiments demonstrate that the Rb-Sr isotopic systematics of hydrothermally-altered continental alkalic rocks may be significantly improved, providing more reliable geochronologic and isotopic tracer information necessary in constructing precise models of mantle sources.

ストロンチウムの成層通気層土壌への吸着

The study is focused on the sorption characteristics of strontium on soils for predicting the migration into deeper layers. Soils were collected from the surface to 8m depth of layers by boring at three districts in Japan. All soil samples exhibited the sorption characteristics of Sr in accordance with the Freundlich type isotherm equilibrium within the range of the Sr concentration of 10-5 to 10-3μeq/ml. The 1/n values shown as the exponent for C in the Freundlich equation were close to 1.0. This finding led to the assumption that the sorption isotherm should be expressed by Henry type isotherm equilibrium for the Sr. The Kd values were positively correlated to CEC, the exchangeable Sr and Ca in soils excluding Konno soil. This result suggests that the Sr sorption reaction takes place on the surface of soil particles. The Kd values were also correlated to organic matter contents.

Rb-Sr Ages of Glauconite Recrystallization: Dating Times of Regional Emergence Above Sea Level

ABSTRACT Rb-Sr ages of glauconite pellets commonly fall short of the presumed age of deposition by 10-20%, according to current estimates of the geologic time scale. In part, the young ages are the result of preferential loss of exchangeable radiogenic Sr. Consequently, in pellets where tightly bound, nonexchangeable Rb and Sr have remained a closed chemical system since deposition, removal of the exchangeable cations with ammonium acetate (NH4OAc) can raise the apparent age to coincide with the expected depositional age. In other cases the age of pelletal glauconite remains younger than the depositional age after chemical treatment. Analyses of NH4OAc-treated, disaggregated clay-size fractions invariably yield young but rather consistent apparent ages. These are interpre ed as recording the time of diagenetic recrystallization of the glauconite crystallites which compose the pellets. Glauconite recrystallization appears to have taken place in the Llano Uplift of central Texas at 428, 350, 328, and 265-280 my ago. These ages do not coincide with times of elevated temperatures, deep burial, or tectonism. Instead, they correlate with times of regional emergence above sea level. Apparently a change in pore-water chemistry due to an influx of meteoric water promoted recrystallization, and this inference is supported by the oxygen isotope composition of the glauconite. Diagenesis of glauconite by this mechanism offers an opportunity to date sea-level changes by isotopic methods.