An assessment of solubility profiling as a decontamination procedure for the 87Sr/86Sr analysis of archaeological human skeletal tissue

⁸⁷Sr/⁸⁶Sr of archaeological skeletal tissues are increasingly used to reconstruct residential mobility and migration, but the post mortem preservation of biogenic Sr is often uncertain. Sample pre-treatment regimes, notably ‘solubility profiling’, have been used to remove diagenetic Sr prior to analysis, but doubts remain over their effectiveness. The investigation examines the effectiveness of solubility profiling by comparing the Sr content and ⁸⁷Sr/⁸⁶Sr composition of bone, dentine and enamel from two archaeological juveniles (Blackfriars, UK) before and after attempted decontamination. For both individuals leached samples of cortical bone and dentine had similar ⁸⁷Sr/⁸⁶Sr to those of soil leachates from the burial site, and are therefore thought to represent diagenetic ⁸⁷Sr/⁸⁶Sr. For both individuals samples of treated dental enamel have ⁸⁷Sr/⁸⁶Sr considerably more or less radiogenic than the soil leachates and other tissues. These are considered representative of biogenic Sr, i.e. Sr acquired in vivo. In effect, solubility profiling should have resulted in ⁸⁷Sr/⁸⁶Sr that were similar for all 3 tissues types and close to those of the untreated enamel. Experimental results show that tooth enamel ⁸⁷Sr/⁸⁶Sr remained largely unaffected by solubility profiling, and the process did not significantly alter the final ⁸⁷Sr/⁸⁶Sr of either dentine or cortical bone. It is concluded that the technique was ineffective in facilitating the recovery of biogenic Sr from these tissues.     

Patterns of Ca/Sr and Sr/Sr variation before and after a whole watershed CaSiO3 addition at the Hubbard Brook Experimental Forest, USA

Forty-one metric tons of the mineral wollastonite (CaSiO₃) was applied to an 11.8 hectare watershed at the Hubbard Brook Experimental Forest (HBEF; White Mountains, New Hampshire, USA) with the goal of restoring the Ca estimated to have been depleted from the soil exchange complex by acid deposition. This experiment provided an opportunity to gain qualitative information on whole watershed hydrologic flow paths by studying the response of stream water chemistry to the addition of Ca. Because the Ca/Sr and ⁸⁷Sr/⁸⁶Sr ratios of wollastonite strongly contrast that of other Ca sources in the watershed, the wollastonite-derived Ca can be identified and its amount estimated in various ecosystem components. Stream water chemistry at the HBEF varies seasonally due to shifts in the proportion of base flow and interflow. Prior to the wollastonite application, seasonal variations in ⁸⁷Sr/⁸⁶Sr ratios indicated that ⁸⁷Sr/⁸⁶Sr was higher during base flow than interflow, due largely to greater amounts of biotite weathering along deeper flow paths. After the application, Ca/Sr and ⁸⁷Sr/⁸⁶Sr changed markedly as the high Ca/Sr and low ⁸⁷Sr/⁸⁶Sr wollastonite dissolved and mixed with stream water. The Ca addition provided information on the response times of various flow paths and ion exchange processes to Ca addition in this small upland watershed. During the first year after the addition, wollastonite applied to the near stream zone dissolved and was partially immobilized by cation exchange sites in the hyporheic zone. In the second and third years after the addition we infer that much of this Ca and Sr was subsequently desorbed from the hyporheic zone and was exported from the watershed in stream flow. In the fourth through ninth years after the addition, Ca and Sr from wollastonite that had dissolved in upland soils was transported to the stream by interflow during wet periods when the ground water table was elevated. Between years three and nine the minimum annual Ca/Sr ratio (in late summer base flow) increased, providing evidence that Ca and Sr had increasingly infiltrated to the deepest flow paths. Strong seasonal variations in Ca/Sr and ⁸⁷Sr/⁸⁶Sr ratios of stream water resulted from the wollastonite addition to upland forest soils, and these ratios have become sensitive to changing flow paths during the annual cycle. Most notably, high flow events now produce large excursions in stream geochemistry toward the high Ca/Sr and low ⁸⁷Sr/⁸⁶Sr ratios of wollastonite. Nine years after the application we estimate that ∼360 kg of Ca from wollastonite has been exported from the watershed in stream flow. The rate of export of Ca from wollastonite dissolution has stabilized at about 11 kg of Ca per year, which accounts for ∼30% of the dissolved Ca in the stream water. Given that 19 metric tons of Ca were applied to the watershed, and assuming this current rate of loss, it should take over 1000 years for this added Ca to be transported from the watershed.     

Ostracod Mg/Sr/Ca and 87Sr/86Sr geochemistry from Tibetan lake sediments: Implications for early to mid‐Pleistocene Indian monsoon and catchment weathering

Jin, Z. D., Bickle, M. J., Chapman, H. J., Yu, J., An, Z., Wang, S. & Greaves, M. J. 2010: Ostracod Mg/Sr/Ca and ⁸⁷Sr/⁸⁶Sr geochemistry from Tibetan lake sediments: Implications for early to mid‐Pleistocene Indian monsoon and catchment weathering. Boreas, 10.1111/j.1502‐3885.2010.00184.x. ISSN 0300‐9483 Lacustrine sediment serves as a valuable archive for tracing catchment weathering processes associated with past climatic and/or tectonic changes. High‐resolution records of fossil ostracod Mg/Ca, Sr/Ca and ⁸⁷Sr/⁸⁶Sr ratios from a lake sediment core from the central Tibetan Plateau reveal a temporal link between lake‐water chemistry and catchment weathering and distinct monsoonal oscillations over the early to mid‐Pleistocene. Between 2.01 and 0.95 Ma, lake‐water chemistry was dominated by a high proportion of carbonate weathering related to variations in the Indian monsoon, resulting in relatively low and constant ostracod ⁸⁷Sr/⁸⁶Sr but obvious fluctuations in Mg/Ca, Sr/Ca and δ¹⁸O. Across the mid‐Pleistocene transition (MPT), a significant increase in ⁸⁷Sr/⁸⁶Sr and frequently fluctuating ratios of ostracod Mg/Ca, Sr/Ca and δ¹⁸O are coincident with increases in both Chinese loess grain size and Arabian Sea lithogenic flux. This correlation indicates an increased glaciation and a strong monsoon seasonal contrast over the plateau. The increase in lake‐water ⁸⁷Sr/⁸⁶Sr across the MPT highlights a change in catchment weathering patterns, rather than one in climate‐enhanced weathering intensity, with an increased weathering of ⁸⁷Sr‐rich minerals potentially induced by marked extensive glaciation and strong seasonality in the central plateau.     

Sr/Sr and Sr/Ca in speleothems for paleoclimate reconstruction in Central China between 70 and 280 kyr ago

Limestone cave deposits (speleothems) provide archives for past changes in regional climates over a range of timescales. While δ¹⁸O and δ¹³C in speleothem calcite have been commonly used for reconstruction of paleoclimates, we report here further efforts in the use of ⁸⁷Sr/⁸⁶Sr and Sr/Ca signals in speleothem calcite to deduce paleomonsoon variability near the Loess Plateau of central China. A two end-member mass-balance model of concentration and isotopic composition of strontium in a cave system is used to estimate variation of the ⁸⁷Sr/⁸⁶Sr ratio in sediments overlying a limestone cave. We show that this ratio reflects climate-driven variations in the provenance and the extent of chemical weathering of the epikarstic sediments. The measurements of ⁸⁷Sr/⁸⁶Sr made on a well-dated stalagmite, SFL, from Buddha Cave (33^o40N′ 109^o05′E) show ratios of 0.71092 to 0.71133 (±0.00001 as 2σ) during relatively cold periods (e.g., Marine Isotope Stage (MIS) 5b, 5d, and 8), lower than ratios of 0.71133 to 0.71194 during relatively warm periods (e.g., MIS 5a, 5c, 5e, and 7). As changes in the Sr/Ca ratio may affect speleothem ⁸⁷Sr/⁸⁶Sr, we show that the direct use of speleothem ⁸⁷Sr/⁸⁶Sr is less ideal than our modeled ⁸⁷Sr/⁸⁶Sr for the exogenic Sr source above the cave as a paleomonsoon proxy. Using the δ¹⁸O, δ¹³C, Sr/Ca, and ⁸⁷Sr/⁸⁶Sr records of the stalagmite, we reconstruct the variability of the East Asian monsoon for the time period between 70 and 280 kyr ago. The results show that summer monsoons were more intense during interglacial periods than during glacial periods.     

Ca/Sr and Sr isotope systematics of a Himalayan glacial chronosequence: carbonate versus silicate weathering rates as a function of landscape surface age

We explored changes in the relative importance of carbonate vs. silicate weathering as a function of landscape surface age by examining the Ca/Sr and Sr isotope systematics of a glacial soil chronosequence located in the Raikhot watershed within the Himalaya of northern Pakistan. Bedrock in the Raikhot watershed primarily consists of silicate rock (Ca/Sr ≈ 0.20 μmol/nmol, ⁸⁷Sr/⁸⁶Sr ≈ 0.77 to 1.2) with minor amounts of disseminated calcite (Ca/Sr ≈ 0.98 to 5.3 μmol/nmol, ⁸⁷Sr/⁸⁶Sr ≈ 0.79 to 0.93) and metasedimentary carbonate (Ca/Sr ≈ 1.0 to 2.8 μmol/nmol, ⁸⁷Sr/⁸⁶Sr ≈ 0.72 to 0.82). Analysis of the exchangeable, carbonate, and silicate fractions of seven soil profiles ranging in age from ∼0.5 to ∼55 kyr revealed that carbonate dissolution provides more than ∼90% of the weathering-derived Ca and Sr for at least 55 kyr after the exposure of rock surfaces, even though carbonate represents only ∼1.0 wt% of fresh glacial till. The accumulation of carbonate-bearing dust deposited on the surfaces of older landforms partly sustains the longevity of the carbonate weathering flux. As the average landscape surface age in the Raikhot watershed increases, the Ca/Sr and ⁸⁷Sr/⁸⁶Sr ratios released by carbonate weathering decrease from ∼3.6 to ∼0.20 μmol/nmol and ∼0.84 to ∼0.72, respectively. The transition from high to low Ca/Sr ratios during weathering appears to reflect the greater solubility of high Ca/Sr ratio carbonate relative to low Ca/Sr ratio carbonate. These findings suggest that carbonate weathering controls the dissolved flux of Sr emanating from stable Himalayan landforms comprising mixed silicate and carbonate rock for tens of thousands of years after the mechanical exposure of rock surfaces to the weathering environment.     

Determination of Sr/Sr mass-dependent isotopic fractionation and radiogenic isotope variation of Sr/Sr in the Neoproterozoic Doushantuo Formation

We measured both mass-dependent isotope fractionation of δ⁸⁸Sr (⁸⁸Sr/⁸⁶Sr) and radiogenic isotopic variation of Sr (⁸⁷Sr/⁸⁶Sr) for the Neoproterozoic Doushantuo Formation that deposited as a cap carbonate immediately above the Marinoan-related Nantuo Tillite. The δ⁸⁸Sr and ⁸⁷Sr/⁸⁶Sr compositions showed three remarkable characteristics: (1) high radiogenic ⁸⁷Sr/⁸⁶Sr values and gradual decrease in the ⁸⁷Sr/⁸⁶Sr ratios, (2) anomalously low δ⁸⁸Sr values at the lower part cap carbonate, and (3) a clear correlation between ⁸⁷Sr/⁸⁶Sr and δ⁸⁸Sr values. These isotopic signatures can be explained by assuming an extreme greenhouse condition after the Marinoan glaciation. Surface seawater, mixed with a large amount of freshwater from continental crusts with high ⁸⁷Sr/⁸⁶Sr and lighter δ⁸⁸Sr ratios, was formed during the extreme global warming after the glacial event. High atmospheric CO₂ content caused sudden precipitation of cap carbonate from the surface seawater with high ⁸⁷Sr/⁸⁶Sr and lighter δ⁸⁸Sr ratios. Subsequently, the mixing of the underlying seawater, with unradiogenic Sr isotope compositions and normal δ⁸⁸Sr ratios, probably caused gradual decrease of the ⁸⁷Sr/⁸⁶Sr ratios of the seawater and deposition of carbonate with normal δ⁸⁸Sr ratios. The combination of ⁸⁷Sr/⁸⁶Sr and δ⁸⁸Sr isotope systematics gives us new insights on the surface evolution after the Snowball Earth.     

Silicate and carbonate weathering in the drainage basins of the Ganga-Ghaghara-Indus head waters: Contributions to major ion and Sr isotope geochemistry

The role of silicate and carbonate weathering in contributing to the major cation and Sr isotope geochemistry of the headwaters of the Ganga-Ghaghara-Indus system is investigated from the available data. The contributions from silicate weathering are determined from the composition of granites/ gneisses, soil profiles developed from them and from the chemistry of rivers flowing predominantly through silicate terrains. The chemistry of Precambrian carbonate outcrops of the Lesser Himalaya provided the data base to assess the supply from carbonate weathering. Mass balance calculations indicate that on an average ∼ 77% (Na + K) and ∼ 17% (Ca + Mg) in these rivers is of silicate origin. The silicate Sr component in these waters average ∼40% and in most cases it exceeds the carbonate Sr. The observations that (i) the⁸⁷Sr/⁸⁶Sr and Sr/Ca in the granites/gneisses bracket the values measured in the head waters; (ii) there is a strong positive correlation between⁸⁷Sr/⁸⁶Sr of the rivers and the silicate derived cations in them, suggest that silicate weathering is a major source for the highly radiogenic Sr isotope composition of these source waters. The generally low⁸⁷Sr/⁸⁶Sr (< 0.720) and Sr/Ca (∼ 0.2 nM/ μM) in the Precambrian carbonate outcrops rules them out as a major source of Sr and⁸⁷Sr/⁸⁶Sr in the headwaters on a basin-wide scale, however, the high⁸⁷Sr/⁸⁶Sr (∼ 0.85) in a few of these carbonates suggests that they can be important for particular streams. The analysis of⁸⁷Sr/⁸⁶Sr and Ca/Sr data of the source waters show that they diverge from a low⁸⁷Sr/⁸⁶Sr and low Ca/Sr end member. The high Ca/Sr of the Precambrian carbonates precludes them from being this end member, other possible candidates being Tethyan carbonates and Sr rich evaporite phases such as gypsum and celestite. The results of this study should find application in estimating the present-day silicate and carbonate weathering rates in the Himalaya and associated CO₂ consumption rates and their global significance.     

Sr and Sr/Sr in the Yamuna River System in the Himalaya: sources, fluxes, and controls on sr isotope composition

Sr and ⁸⁷Sr/⁸⁶Sr have been measured in the Yamuna river headwaters and many of its tributaries (YRS) in the Himalaya. These results, with those available for major ions in YRS rivers and in various lithologies of their basin, have been used to determine their contributions to riverine Sr and its isotopic budget. Sr in the YRS ranges from 120 to 13,400 nM, and ⁸⁷Sr/⁸⁶Sr from 0.7142 to 0.7932. Streams in the upper reaches, draining predominantly silicates, have low Sr and high ⁸⁷Sr/⁸⁶Sr whereas those draining the lower reaches exhibit the opposite resulting from differences in drainage lithology. ⁸⁷Sr/⁸⁶Sr shows significant co-variation with SiO₂/TDS and (Na^* + K)/TZ⁺ (indices of silicate weathering) in YRS waters, suggesting the dominant role of silicate weathering in contributing to high radiogenic Sr. This is also consistent with the observation that streams draining largely silicate terrains have the highest ⁸⁷Sr/⁸⁶Sr, analogous to that reported for the Ganga headwaters. Evaluation of the significance of other sources such as calc-silicates and trace calcites in regulating Sr budget of these rivers and their high ⁸⁷Sr/⁸⁶Sr needs detailed work on their Sr and ⁸⁷Sr/⁸⁶Sr. Preliminary calculations, however, indicate that they can be a significant source to some of the rivers.It is estimated that on an average, ∼25% of Sr in the YRS is derived from silicate weathering. In the lower reaches, the streams receive ∼15% of their Sr from carbonate weathering whereas in the upper reaches, calc-silicates can contribute significantly (∼50%) to the Sr budget of rivers. These calculations reveal the need for additional sources for rivers in the lower reaches to balance their Sr budget. Evaporites and phosphorites are potential candidates as judged from their occurrence in the drainage basin. In general, Precambrian carbonates, evaporites, and phosphorites “dilute” the high ⁸⁷Sr/⁸⁶Sr supplied by silicates, thus making Sr isotope distribution in YRS an overall two end member mixing. Major constraints in quantifying contributions of Sr and ⁸⁷Sr/⁸⁶Sr from different sources to YRS rivers are the wide range in Sr and ⁸⁷Sr/⁸⁶Sr of major lithologies, limited data on Sr and ⁸⁷Sr/⁸⁶Sr in minor phases and on the behavior of Sr, Na, and Ca during weathering and transport.The Ganga and the Yamuna together transport ∼0.1% of the global Sr flux at the foothills of the Himalaya which is in the same proportion as their contribution to global water discharge. Dissolved Sr flux from the Yamuna and its mobilization rate in the YRS basin is higher than those in the Ganga basin in the Himalaya, a result consistent with higher physical and chemical erosion rates in the YRS.     

Initial 87Sr/86Sr ratios of plutonic rocks from Japan

Initial ⁸⁷Sr/⁸⁶Sr rations were determined for more than 80 plutonic rocks in Japan. The ⁸⁷Sr/⁸⁶Sr ratios of gabbroic and granitic rocks show no significant difference in plutonic terranes where both rocks occur closely associated, implying a genetic relationship between them (e.g., Green Tuff belt) or reequilibration at deep level (e.g., Ryoke belt). Wherever granitic rocks occur independently from gabbroic rocks, the granites have higher ratios than the gabbros.Initial ⁸⁷Sr/⁸⁶Sr ratios of the granitic rocks are low (<0.706) in Northeast Japan but high (<0.706) in Southwest Japan, the boundary being the Tanakura Tectonic Line. Within Southwest Japan, the ratios are low along the Japan Sea side of the southernmost area. This regional variation is generally correlated with thickness of the continental crust as deduced from the Bouguer anomaly.Initial ⁸⁷Sr/⁸⁶Sr ratios of the granitic rocks vary from 0.7037 to 0.7124. The low group (<0.706) is considered to consist of essentially mantle-derived magmas contaminated by crustal material in lesser but varying degree, because of its geological setting and initial ⁸⁷Sr/⁸⁶Sr values. The high group may have been formed by contamination of a deep-seated magmas by crustal material or by generation of the main part of the magmas within the continental crust. The ratios of individual belts reflect their own history depending upon age and Rb/Sr ratio of the crustal material.Initial ⁸⁷Sr/⁸⁶Sr ratios of granitic rocks are generally low for the magnetite-series but high for the ilmenite-series. Thus, a negative correlation is observed between initial ratios and ³⁴S for most Cretaceous-Paleogene granites. However, Neogene ilmenite-series granites are low in both initial ⁸⁷Sr/⁸⁶Sr and ³⁴S indicating interaction of the granitic magma with young sedimentary rocks enriched in ³²S.     

Determination of Sr isotopes in calcium phosphates using laser ablation inductively coupled plasma mass spectrometry and their application to archaeological tooth enamel

The determination of accurate Sr isotope ratios in calcium phosphate matrices by laser ablation multi-collector ICP-MS is demonstrated as possible even with low Sr concentration archaeological material. Multiple on-line interference correction routines for doubly-charged REE, Ca dimers and Rb with additional calibration against TIMS-characterised materials are required to achieve this. The calibration strategy proposed uses both inorganic and biogenic apatite matrices to monitor and correct for a ⁴⁰Ca-³¹P-¹⁶O polyatomic present at levels of 0.3-1% of the non-oxide peak, which interferes on ⁸⁷Sr causing inaccuracies of 0.03-0.4% in the ⁸⁷Sr/⁸⁶Sr isotope ratio. The possibility also exists for synthetic materials to be used in this calibration. After correction for interferences total combined uncertainties of 0.04-0.15% (2SD) are achieved for analyses of 13-24 μg of archaeological tooth enamel with Sr concentrations of ca. 100-500 ppm using MC-ICP-MS. In particular, for samples containing >300 ppm Sr, total uncertainties of ∼0.05% are possible utilising 7-12 ng Sr. Data quality is monitored by determination of ⁸⁴Sr/⁸⁶Sr ratios.When applied to an archaeological cattle tooth this approach shows Sr-isotope variations along the length of the tooth in agreement with independent TIMS data. The ⁴⁰Ca-³¹P-¹⁶O polyatomic interference is the root cause of the bias at mass 87 during laser ablation ICP-MS analysis of inorganic and biogenic calcium phosphate (apatite) matrices. This results in inaccurate ⁸⁷Sr/⁸⁶Sr ratios even after correction of Ca dimers and doubly charged rare earth elements. This interference is essentially constant at specific ablation conditions and therefore the effect on ⁸⁷Sr/⁸⁶Sr data varies in proportion to changes in the Sr concentration of the ablated material. Complete elimination of this interference is unlikely through normal analytical mechanisms and therefore represents a limitation on the achievable accuracy of LA-(MC-)ICP-MS ⁸⁷Sr/⁸⁶Sr data without rigorous calibration to known reference materials.     

Non-uniqueness and interpretation of the seawater Sr/Sr curve

Variations in the seawater ⁸⁷Sr/⁸⁶Sr curve through time can be caused by fluctuations in the strontium flux or variations in the isotopic ratio from at least six different sources and sinks. Thus, 12 or more parameters control each single measurement although widely accepted assumptions allow this to be reduced to typically six unknowns. Interpreting the causes of time-variation in the seawater ⁸⁷Sr/⁸⁶Sr curve is therefore hampered by inherent non-uniqueness. However, this problem is under-constrained rather than unconstrained. As a result, whilst there are an infinite number of possible interpretations, these all come from a few families of very similar solutions. Using this insight, it is possible to find solutions having the smallest possible variations in source flux or source ⁸⁷Sr/⁸⁶Sr ratio. Thus, lower-bounds can be placed upon the source variations responsible for the observed fluctuations in the seawater ⁸⁷Sr/⁸⁶Sr curve. When applied to the evolution of the Early Jurassic ⁸⁷Sr/⁸⁶Sr seawater curve, this approach demonstrates that a short-lived Toarcian event is genuine since it is present in all models, regardless of the values chosen for the unknown source fluxes and unknown source isotope ratios. However, the variations in strontium flux or isotopic ratio necessary to explain the Toarcian event may be significantly smaller than would be predicted assuming modern values for the unknown parameters.     

In situ Sr/Sr investigation of igneous apatites and carbonates using laser-ablation MC-ICP-MS

In situ Sr isotopic compositions of coexisting apatite and carbonate for carbonatites from the Sarfartoq alkaline complex, Greenland, have been determined by laser-ablation multicollector inductively coupled plasma mass spectrometry. This study is the first to examine the extent of Sr isotopic homogeneity among coexisting igneous minerals containing high Sr (>3000 ppm) and low Rb (?1 ppm) contents within a single ∼50-μm-thick thin-section mount. This technique is capable of producing measured ⁸⁷Sr/⁸⁶Sr values with analytical precision (∼0.005%, 2σ) approaching those obtained by thermal ionization mass spectrometry but in a much shorter interval of time (100 s vs. >1 h, respectively). The combined total analyses (n = 107) of apatite and carbonate yield ⁸⁷Sr/⁸⁶Sr compositions ranging from ∼0.7025 to ∼0.7031. This relatively large variation in Sr isotopic compositions (∼0.0006) is ∼1 order of magnitude larger than the estimated external reproducibility (∼0.00005, 2σ) of the method. The large range in ⁸⁷Sr/⁸⁶Sr values suggests that apatite and carbonate precipitated predominantly under nonequilibrium conditions. The isotopic variations observed within individual hand specimens may therefore reflect larger (regional) scale open-system processes, possibly involving mixing of carbonatitic melts derived from distinct mantle sources or from a common isotopically heterogeneous mantle.     

Ba/Sr,Ca/Sr and 87Sr/86Sr ratios in soil water and groundwater: implications for relative contributions to stream water discharge

Barium/Sr and Ca/Sr ratios have been used to model the relative importance of different sources of stream water. Major and trace element concentrations together with ⁸⁷Sr/⁸⁶Sr ratios were measured in precipitation, soil water, groundwater and stream water in a small (9.4 km²) catchment in northern Sweden. The study catchment is drained by a first order stream and mainly covered with podzolized Quaternary till of granitic composition. It is underlain by a 1.8 Ga granite. A model with mixing equations used in an iterative mode was developed in order to separate the stream water into 3 subsurface components: soil water, shallow groundwater, and deep groundwater. Contributions from precipitation are thus not included in the model. This source may be significant for the stream water generation, but it does not interfere with the calculations of the relative contributions from the subsurface components. The results show that the deep groundwater constitutes between 5 and 20% of the subsurface water discharge into the stream water. The highest values of the deep groundwater fraction occur during base flow. Soil water dominates during snowmelt seasons, whereas during base flow it is the least important fraction. Soil water accounts for 10–100% of the subsurface water discharge into the stream water. Shallow groundwater accounts for up to 80% of the subsurface water discharge with the lowest values at peak discharge during snowmelt seasons and the highest values during base flow. The validity of the model was tested by comparing the measured ⁸⁷Sr/⁸⁶Sr ratios in the stream water with the ⁸⁷Sr/⁸⁶Sr ratios predicted by the model. There was a systematic difference between the measured and modelled ⁸⁷Sr/⁸⁶Sr ratios which suggests that the fraction of soil water is overestimated by the model, especially during spring flood. As a consequence of this overestimation of soil water the amount of shallow groundwater is probably underestimated during this period. However, it is concluded that the differences between measured and predicted values are relatively small, and that element ratios are potentially effective tracers for different subsurface water flowpaths in catchments.     

柴达木察尔汗贝壳堤剖面Sr同位素及其环境意义

根据对柴达木盆地察尔汗古湖贝壳堤剖面酸不溶组分和酸溶组分的提取及其87Sr/86Sr的测定,结合沉积物中Rb/Sr的变化,指出Sr同位素组成特征可有效指示源区化学风化和沉积区古环境变化;依据酸溶组分中盐度指标Sr/Ca, Sr/Ba值与Sr同位素组成曲线的对比分析,揭示酸溶组分Sr同位素可以较好的指示水体形成时的盐度。通过分析贝壳堤剖面化学风化和部分盐度变化过程,探讨了研究区晚更新世39.6~ 17.1 ka BP(未经校正的AMS 14C测年结果,全文同)湖泊高湖面的演化历史。     

Magmatic 87Sr/86Sr relicts in hydrothermally altered quartz diorites (Brabant Massif,Belgium) and the role of epidote as a Sr filter

The porphyritic quartz diorites of the Caledonian Brabant Massif have been totally altered. Ca, Rb, Sr, Zr, Ce, Y measurements and Sr-Nd isotopic analyses were performed on the Quenast plug and the Lessines sill, in an attempt to study the relative mobility of Sr and evaluate the extent, direction and magnitude of the ⁸⁷Sr/⁸⁶Sr alterations. Sr electron microprobe analyses of epidote were also carried out to assess its role in the Sr distribution.The initial ⁸⁷Sr/⁸⁶Sr ratio is shown to have had an unsteady behaviour during the studied water/rock interactions since it has been sometimes enhanced, sometimes depressed and occasionally not modified. The possibility and magnitude of the ⁸⁷Sr contamination turn out to be strictly related to the degree of Sr accommodation in the secondary minerals. Epidote in particular has proved to be the main trap for the hydrothermal Sr and this mineral is thus regarded as the major controlling factor of ⁸⁷Sr hydrothermal contamination. The epidote-poor rocks (albite+chlorite-rich rocks) seem to have been unaffected by any Sr interchange with the aqueous solutions. Therefore, as alteration quickly follows the crystallization of the magma, their initial ⁸⁷Sr/ ⁸⁶Sr ratio, which is deduced from an isochron, might be a primary petrogenetic feature enabling interpretation of the genesis of their parental magmas. On the other hand, in the epidote-rich rocks, this ratio has been readily altered; it could thus generally be used only to trace the origin of the hydrothermal solutions. As a consequence, these rocks should not be selected for dating an alteration event by the Rb-Sr method.     

87Sr/86Sr isotope composition of bottled British mineral waters for environmental and forensic purposes

Mineral waters in Britain show a wide range of ⁸⁷Sr/⁸⁶Sr isotope compositions ranging between ⁸⁷Sr/⁸⁶Sr = 0.7059 from Carboniferous volcanic rock sources in Dunbartonshire, Scotland to ⁸⁷Sr/⁸⁶Sr = 0.7207 in the Dalradian aquifer of Aberdeenshire, Scotland. The ⁸⁷Sr/⁸⁶Sr composition of the waters shows a general correlation with the aquifer rocks, resulting in the waters from older rocks having a more radiogenic signature than those from younger rocks. This wide range of values means that the Sr isotope composition of mineral water has applications in a number of types of studies. In the modern commercial context, it provides a way of fingerprinting the various mineral waters and hence provides a method for recognising and reducing fraud. From an environmental perspective, it provides the first spatial distribution of bio-available ⁸⁷Sr/⁸⁶Sr in Britain that can be used in modern, historical and archaeological studies.     

Ge/Si and Sr/Sr tracers of weathering reactions and hydrologic pathways in a tropical granitoid system

Ge/Si and ⁸⁷Sr/⁸⁶Sr data from primary and secondary minerals, soil waters, and stream waters in a tropical granitoid catchment quantitatively reflect mineral alteration reactions that occur at different levels within the bedrock–saprolite–soil zone. Near the bedrock–saprolite interface, plagioclase to kaolinite reaction yields low Ge/Si and 87Sr/86Sr. Higher in the regolith column, biotite weathering and kaolinite dissolution drive Ge/Si and 87Sr/86Sr to high values. Data from streams at base flow sample the bedrock–saprolite interface zone, while at high discharge solutes are derived from upper saprolite–soil zone. Coupled Ge/Si and 87Sr/86Sr can be effective tools for quantifying the importance of specific weathering reactions, and for geochemical hydrograph separation.     

87Sr/86Sr in kimberlitic carbonates by ion microprobe: Hydrothermal alteration,crustal contamination and relation to carbonatite

Carbonates in a 30 cm wide zoned kimberlite dyke from the De Beers Mine, Kimberley, S. Africa were studied by cathodoluminescence and electron microprobe techniques and their ⁸⁷Sr/⁸⁶Sr ratios were measured using an AEI-IM20 ion microprobe. Primary carbonates (including calcite dendrites, rhombohedral calcites in segregation vesicles and mosaic dolomite) have high Sr (0.69–1.35 wt.% SrO) and Ba (0.24–0.44% BaO) and ⁸⁷Sr/⁸⁶Sr ratios in the range 0.7046 to 0.7056. Secondary sparry calcite in amygdales and veins is characterised by low Ba (<0.05% BaO) and ⁸⁷Sr/⁸⁶Sr near 0.72. Rhombohedral calcite 0.5 cm from a contact with 2,900 my. old biotite-gneiss has minor element chemistry like that of primary carbonate, but an elevated ⁸⁷Sr/⁸⁶Sr ratio of 0.7103, possibly indicating crustal contamination in a boundary layer of the kimberlite magma. Amygdale-like segregations of carbonate and/or serpentine originated as gas-cavities and were not formed by liquid immiscibility. They are now filled either by secondary calcite or by minerals precipitated from residual kimberlite liquid. However, dendritic calcite and primary dolomite and calcite with high Sr, Ba and low ⁸⁷Sr/⁸⁶Sr demonstrate shared chemical characteristics between these carbonates and carbonatite. The primary kimberlite magma had initial ⁸⁷Sr/⁸⁶Sr close to 0.7046.     

Input of Sr/Sr ratios and Sr geochemical signatures to update knowledge on thermal and mineral waters flow paths in fractured rocks (N-Portugal)

Strontium isotopes and other geochemical signatures are used to determine the relationships between CO₂-rich thermal (Chaves: 76 °C) and mineral (Vilarelho da Raia, Vidago and Pedras Salgadas: 17 °C) waters discharging along one of the major NNE–SSW trending faults in the northern part of mainland Portugal. The regional geology consists of Hercynian granites (syn-tectonic-310 Ma and post-tectonic-290 Ma) intruding Silurian metasediments (quartzites, phyllites and carbonaceous slates). Thermal and mineral waters have ⁸⁷Sr/⁸⁶Sr isotopic ratios between 0.716713 and 0.728035. ⁸⁷Sr/⁸⁶Sr vs. 1/Sr define three end-members (Vilarelho da Raia/Chaves, Vidago and Pedras Salgadas thermal and mineral waters) trending from rainfall composition towards that of the CO₂-rich thermal and mineral waters, indicating different underground flow paths. Local granitic rocks have ⁸⁷Sr/⁸⁶Sr ratios of 0.735697–0.789683. There is no indication that equilibrium was reached between the CO₂-rich thermal and mineral waters and the granitic rocks. The mean ⁸⁷Sr/⁸⁶Sr ratio of the thermal and mineral waters (0.722419) is similar to the Sr isotopic ratios of the plagioclases of the granitic rocks (0.71261–0.72087). The spatial distribution of Sr isotope and geochemical signatures of waters and the host rocks suggests that the thermal and mineral waters circulate in similar but not the same hydrogeological system. Results from this study could be used to evaluate the applicability of this isotope approach in other hydrogeologic investigations.     

Hydrologic controls on radiogenic Sr in meltwater from an alpine glacier system: Athabasca Glacier,Canada

Filtered subglacial meltwater samples were collected daily during the onset of melt (May) and peak melt (July) over the 2011 melt season at the Athabasca Glacier (Alberta, Canada) and analyzed for strontium-87/strontium-86 (⁸⁷Sr/⁸⁶Sr) isotopic composition to infer the evolution of subglacial weathering processes. Both the underlying bedrock composition and subglacial water–rock interaction time are the primary influences on meltwater ⁸⁷Sr/⁸⁶Sr. The Athabasca Glacier is situated atop Middle Cambrian carbonate bedrock that also contains silicate minerals. The length of time that subglacial meltwater interacts with the underlying bedrock and substrate is a predominant determining factor in solute concentration. Over the course of the melt season, increasing trends in Ca/K and Ca/Mg correspond to overall decreasing trends in ⁸⁷Sr/⁸⁶Sr, which indicate a shift in weathering processes from the presence of silicate weathering to primarily carbonate weathering.Early in the melt season, rates of carbonate dissolution slow as meltwater approaches saturation with respect to calcite and dolomite, corresponding to an increase in silicate weathering that includes Sr-rich silicate minerals, and an increase in meltwater ⁸⁷Sr/⁸⁶Sr. However, carbonate minerals are preferentially weathered in unsaturated waters. During the warmest part of a melt season the discharged meltwater is under saturated, causing an increase in carbonate weathering and a decrease in the radiogenic Sr signal. Likewise, larger fraction contributions of meltwater from glacial ice corresponds to lower ⁸⁷Sr/⁸⁶Sr values, as the meltwater has lower water–rock interaction times in the subglacial system. These results indicate that although weathering of Sr-containing silicate minerals occurs in carbonate dominated glaciated terrains, the continual contribution of new meltwater permits the carbonate weathering signal to dominate.