Strontium Isotope Systematics Research Articles

Uranium-series and strontium isotope systematics in soil carbonates from dryland Critical Zones: Implications for soil inorganic carbon storage and transformation

Soil carbonates are dominantly present in dryland Critical Zone (CZ) and their formation could lead to important long-term carbon sequestration in arid to semiarid soils if the Ca ions were derived from silicate weathering or other non-carbonate sources. In managed CZ systems such as agricultural areas converted from natural drylands, irrigation has profound effects on the dryland CZ inorganic carbon storage, especially by modifying dissolution and precipitation dynamics of soil carbonates via controls of irrigation intensity and water chemistry, soil properties, and hydrological flow paths. These processes could lead to transformation of inorganic carbon in soils and groundwater aquifers underneath drylands. One key knowledge gap in studying soil carbonates from managed dryland CZ systems is to detect the formation of soil carbonates under irrigated conditions and distinguish them effectively from soil carbonates formed under natural conditions.Here, we explore the potential of using U-series and strontium isotopes to investigate the formation timescales and conditions of soil carbonates in both natural and managed dryland CZ in Chihuahua Desert of American Southwest. We obtained new U-series and Sr isotope ratios from mature stage V natural soil carbonates in the Jornada Basin of southern New Mexico and compared to previously published U-series and Sr isotope results for younger Jornada soil carbonates as well as irrigation impacted soil carbonates from the Rio Grande floodplains in west Texas. Specifically, we applied the U-series dating method (238U-234U-230Th) to estimate the timescales of soil carbonate formation under both natural climatic conditions and impacts of modern agriculture irrigation. In addition, we showed that the initial (234U/238U) activity ratios recorded in these soil carbonates reflect the systematic changes of soil infiltration rates in both natural and managed dryland CZ. We also utilized Sr isotope ratios (87Sr/86Sr) in soil carbonates, dust, and irrigation water to trace the Ca sources in soil carbonates from natural and managed dryland CZ. Soil carbonates from both settings were characterized by slightly radiogenic 87Sr/86Sr ratios, consistent with their potential of long-term carbon storage in drylands.U-series and Sr isotope systematics characterize important differences in soil carbonates from natural and managed dryland CZ with respect to their Ca sources, timescales of carbonate formation and transformation, and availability of soil moisture. Soil carbonates formed, accumulated, and transformed in natural drylands with long time scales but under irrigated arid agricultural areas its formation is significantly modified by irrigation and other agriculture practices, with implications for long- and short-term inorganic carbon sequestration in both systems of drylands.

Origin of lithium in oilfield brines in continental petroliferous basin: Insights from Li and Sr isotopes in the Jianghan Basin, central China

Oilfield brines are a significant alternative lithium resource to meet the global demand in the next decades. However, their metallogenic characteristics and mechanism remain unclear. To constrain the source and enrichment process, this study presents chemical and multi-isotope data for oilfield brines from the Middle Eocene to Lower Oligocene Qianjiang Formation of the Jianghan Basin, central China. These formation water samples collected from 159 oil wells have salinities of 47–355 g/L and lithium contents of 5.36–150.00 mg/L. The good linear relationship between δD and δ18O values indicates that the salinities are achieved by dilution of the primary brine by meteoric water. Waters from the lower of the Qianjiang Formation have higher δD and δ18O values and 87Sr/86Sr ratios than those from the upper, in conjunction with the low correlation between the δD and δ18O value and the salinity and between the Sr content and the 87Sr/86Sr ratio, indicating the impact of associated water-rock interactions. Moreover, contributions from diagenetic processes of dolomitization, dissolution of evaporites and albitization are also indicated by the high Cl/Br mass and Na/Cl molar ratios and the Ca excess and Na deficit. Chemical geothermometers give reservoir temperature values of 118 °C and 138 °C on average for the oilfield brines from the upper and lower formation, respectively. The binary linear relationships between Li contents and TDS values and Br contents imply specific enrichment processes in addition to evapo-concentration. Variations in δ7Li values (+10.6‰–+21.3%) and 87Sr/86Sr ratios (0.709479–0.711516) were used to trace the process of Li enrichment in oilfield brines. The lower δ7Li values with more enriched Li in comparison to Na indicate a significant lithium input from clastic host rocks, which is also confirmed by the fact that more radiogenic waters have relatively lower δ7Li values. Overall, these results suggest that Li enrichment in the Qianjiang oilfield brines was generated by the interaction of Li-bearing minerals in the clastic host rocks with water formed by original evaporitic brines diluted by meteoric water. This work represents a significant role of lithium and strontium isotopic systematics in elucidating the Li origins of deep-seated oilfield brines in continental petroliferous basins.

Effective use of limestones to reconstruct seawater Li isotope compositions - A community standard proposal

Past variations in lithium concentration and isotope ratio of seawater recorded in marine carbonates have shown great potential in tracing carbon cycles during critical climate transitions, especially coupled with the radiogenic strontium isotope system. However, acquiring reliable seawater Li isotope compositions from marine carbonates can be technically challenging because non‑carbonate phases co-existed in ancient marine carbonates can easily contaminate carbonate Li signal during inappropriate sample dissolution. Each contaminant phase, including Mn oxides, clays, and silicate minerals, can have different Li isotope compositions than that preserved in the carbonate lattice. More importantly, a significant portion of Li is adsorbed on the Mn oxides and clay minerals. This fraction is considered exchangeable and can be liberated using weak acid without destruction of mineral structure. Cleaning procedures and partial digestion have been applied to remove the exchangeable Li fraction in contaminative phases and selectively dissolve the carbonates. However, previous methods cannot be directly applied to ancient marine carbonates of various carbonate purity, which lack systematic testing on the threshold of sample purity to maintain the efficiency of the precleaning and partial digestion. Here we report an efficient sequential leaching protocol to extract primary seawater Li and Sr isotope compositions from bulk marine carbonates. The procedure consists of pre-cleaning steps using 1 M ammonium acetate and 1 M ammonium carbonate to completely remove the exchangeable fraction, followed by partial digestion using 0.3 M acetic acid to selectively target carbonate-bounded Li. We validate this method by mixing carbonate and contaminant endmembers in different weight percentages. We found this protocol works effectively for carbonates of >80% purity. Additionally, we demonstrate filtration is necessary when separating leachates from the solids to quantitatively avoid contaminations from non‑carbonate phases. We also suggest NIST-SRM-1d to be a cross-lab carbonate standard for Li isotope analyses and report its long-term external precision (two years). To sum, we propose an effective community standard procedure to reconstruct δ7Lisw records using ancient bulk carbonate samples.

δ26Mg, δ44Ca and 87Sr/86Sr isotope differences among bedrock minerals constrain runoff generation in headwater catchments: An acidified granitic site in Central Europe as an example

Magnesium, calcium and strontium isotope systematics were studied in a Central European headwater catchment underlain by granite. The Uhlirska catchment (Czech Republic) is recovering from acidification following 40 years of acid rain. A combination of isotope and non-isotope data was used to constrain the origin of base cations in runoff. Whole-rock δ26Mg, δ44Ca values and 87Sr/86Sr ratios were complemented by isotope analysis of apatite, biotite, plagioclase, orthoclase and titanite. Isotope composition of Mg, Ca and Sr in precipitation, throughfall and runoff was monitored for 12 months.Soil, soil solutions and Norway spruce tissues were also analyzed. Non-isotope data included a 24-year time-series of input/output Mg and Ca fluxes. Biotite was the likely main source of geogenic Mg. Apatite and plagioclase were the likely main sources of geogenic Ca, and plagioclase was the likely main source of geogenic Sr. Magnesium in biotite was isotopically too heavy to dominate runoff. Calcium in apatite and plagioclase was isotopically indistinguishable from whole-rock Ca and could play a major role in runoff generation. Plagioclase had a significantly lower 87Sr/86Sr ratio than bulk bedrock, close to the low 87Sr/86Sr ratio of runoff. Plagioclase weathering was consistent with a sizeable geogenic Sr contribution to runoff but if only bulk-rock 87Sr/86Sr was considered predominance of geogenic Sr in runoff would be unlikely. Higher Mg, Ca and Sr runoff fluxes, compared to deposition, suggested geogenic control of runoff. A decrease in runoff fluxes of Mg and Ca coincided with a decrease in deposition fluxes but there may not be a causal relationship pointing to a large atmospheric contribution of base cations to runoff. Decreasing fluxes of base cations via runoff were mostly related to decreasing sulfate export accompanying retreat of acidification. Mg/Ca/Sr isotope systematics at Uhlirska are discussed in light of analogous data from four other headwater catchments.

Tourmaline chemistry, boron, and strontium isotope systematics trace multiple melt–fluid–rock interaction stages in deeply subducted continental crust

The generation, transport, and recrystallization of slab-derived melts/fluids play a critical role in the deep recycling of elements in subduction zones. While boron (B) isotope systematics have been invoked as an important tracer of these processes, its behavior during metamorphic dehydration and partial melting of deeply subducted continental slabs, and the partitioning of B isotopes between minerals and melts/fluids is not fully understood. Here, we investigate these processes through an in situ study of the major, trace-element, and B-Sr isotope variations in different occurrences of tourmaline in migmatite from the Yuka terrane, North Qaidam orogen (China), which resulted from partial melting of a continental slab at different stages during subduction and exhumation. Based on textural and detailed high-resolution X-ray mapping studies, tourmaline was classified into four paragenetic generations (Tur-I, Tur-II, Tur-III, and Tur-IV). Dravitic Tur-I occurs in melanosomes and shows increasing Fe, Ca, and Ti contents from the core to the outer rim. In addition, it has relatively homogeneous Sr isotope values (0.7407–0.7416) and decreasing δ11B values (-3.8 to −8.6 ‰) and XMg (Mg/(Mg + Fe)) ratios, indicating formation in a rock buffered by an aqueous fluid during the prograde to peak metamorphism. Schorlitic Tur-II occurs within selvage zones between melanosomes and leucosomes, and yields high-Fe values and low δ11B (-13.5 to −10.9 ‰) and more variable 87Sr/86Sr (0.7343–0.7418) values, indicating crystallization in the presence of a hydrous melt external derived from breakdown of Fe-rich mineral(s) during partial melting of the subducted slab. Dravitic Tur-III formed in the matrix and also enveloped Tur-II. It shows homogeneous 87Sr/86Sr values (0.7411–0.7420) and decreasing δ11B values (-6.8 to −9.9 ‰) and XMg ratios as well as increasing Fe and Ti contents from core to outer rim. Formation of Tur-III reflects a transitional stage from hydrous melt to aqueous fluid during exhumation. Tur-IV in the leucosomes is essentially a schorl-dravite solid solution with small amounts of Ca. Its 87Sr/86Sr values (0.7402–0.7416) and δ11B values (-11.4 to −8.5 ‰) are intermediate between the respective values of Tur-II and Tur-III. The formation of Tur-IV likely results from interaction between melt and fluid and, based on its chronological sequence, and is interpreted to have formed during the exhumation stage of the Yuka terrane.Overall, the variable XMg ratios and δ11B values in the different generations of tourmalines are a consequence of the evolution of the melt/fluid at different depths within the deeply subducted slab. Decreasing δ11B values from Tur-I to Tur-II and Tur-III are controlled by the breakdown of different minerals during partial melting or metamorphic dehydration of the subducted slab, while the co-variations of the elemental geochemistry and B isotopic compositions of tourmaline reflect different depths of formation during subduction and exhumation of the lithosphere. These observations suggest tourmaline may serve as a useful tracer of multiple melt/fluid–rock interactions and of boron cycling in continental subduction zones. The heterogeneity of δ11B in melts/fluids at different depth levels of the continental subducted slab may also result in locally variable B isotope values in syn- and post-collisional magmas.

Lithium and strontium isotopic systematics in the Nalenggele River catchment of Qaidam basin, China: Quantifying contributions to lithium brines and deciphering lithium behavior in hydrological processes

The salt lakes fed by the Nalenggele River in the Qaidam Basin, located on the northeastern Qinghai Tibetan Plateau (QTP), are major brine lithium reservoirs in China. The quantitative identifications of the Li sources and their dynamic behavior shed light on the formation mechanism of the brine Li deposits in closed-basin of arid regions. This study presents Li and Sr isotope ratios (δ7Li, 87Sr/86Sr) and other hydrogeochemical parameters for river waters, springs, spring river waters, shallow groundwaters, surface brines, and intercrystalline brines around the Nalenggele River Basin (NRB) and its adjacent small watersheds. The results indicate that the Nalenggele River water is a mixing product of hot springs and regional surface meltwater, and the source of dissolved Li in the NRB is mainly supplied by the hot springs. With multi-tracer models, it is confirmed that around 84.9 %−93.4 % of dissolved Li is provided by the Li-rich hot spring waters, which account for only 3–5 % of the total water input. Furthermore, the dissolved Li in the streams of different hydrological regions shows various isotopic fractionation behavior. In general, the lower average δ7Li values (average of +5.6 ‰) with more enriched Li with respect to Na in the waters of the piedmont Gobi region indicates a significant additional input of dissolved Li from bedrock and sediments. The second-stage alluvial fan of the NRB produces 7Li enrichments and Li-depleted waters (compared with Na), which can be explained by the adsorption of clay or Fe (Mn) oxide phases that fractionate Li isotopes. The highest average δ7Li values (+12.76 ‰) are observed when Li is not adsorbed by the scarce fine clastic sediments in the hypersaline environment of the salt lake region. It can be concluded that the continuous halite precipitation during the brine evolution not only causes the abnormal enrichment of Li in the brine (compared with Na) but also continuously sequesters the 6Li from the brines, resulting in a significant 7Li enrichment. Overall, these findings advance the understanding of the lithium sources of Li-rich brines and its enrichment and metallogenic process during water migration in hyper-arid regions and have crucial indicative significance for lithium cycling under a strong evaporation environment.

Tracing wedge-internal deformation by means of strontium isotope systematics of vein carbonates

AbstractRadiogenic strontium isotopes (87Sr/86Sr) of vein carbonates play a central role in the tectonometamorphic study of fold-and-thrust belts and accretionary wedges and have been used to document fluid sources and fluxes, for example, along major fault zones. In addition, the 87Sr/86Sr ratios of vein carbonates can trace the diagenetic to metamorphic evolution of pore fluids in accreted sediments. Here we present 87Sr/86Sr ratios of vein carbonates from the Infrahelvetic flysch units of the central European Alps (Glarus Alps, Switzerland), which were accreted to the North Alpine fold-and-thrust belt during the early stages of continental collision. We show that the vein carbonates trace the Sr isotopic evolution of pore fluids from an initial seawater-like signature towards the Sr isotopic composition of the host rock with increasing metamorphic grade. This relationship reflects the progressive equilibration of the pore fluid with the host rock and allows us to constrain the diagenetic to low-grade metamorphic conditions of deformation events, including bedding-parallel shearing, imbricate thrusting, folding, cleavage development, tectonic mélange formation and extension. The strontium isotope systematics of vein carbonates provides new insights into the prograde to early retrograde tectonic evolution of the Alpine fold-and-thrust belt and helps to understand the relative timing of deformation events.

Strontium Isotope Systematics of Tenerife Wines (Canary Islands): Tracing Provenance in Ocean Island Terroir

The production of fraudulent goods remains widespread and economically damaging. The high value of the wine industry makes it particularly vulnerable, and a number of geochemical methods have been developed to ensure traceability and identification of origin. Here, strontium (Sr) isotope data on wines from five defined regions in Tenerife (Canary Islands, Spain) show that the young volcanic geology imparts a clearly identifiable low 87Sr/86Sr signature (<0.7072). These values discriminate Tenerife wines from mainland Spanish and continental European produce, as these are much more radiogenic in general. However, unlike continental wine regions, wines from Tenerife show small but ubiquitous enrichments in 87Sr/86Sr above what is expected in the soils. Bentonite addition has not affected the 87Sr/86Sr signatures, with white wines at lower Sr concentrations than red wines in all regions. A number of natural contributions to the terroir are evaluated in relation to Tenerife’s unique combination of geology and geography. Atmospheric precipitation (rainfall) is likely a dominant influence on Sr isotope systematics in northern Denominación de Origen regions, and evaporation may play a role in buffering signatures in southern regions. Other natural additions of 87Sr are not precluded at a local scale, given the large range in climatic conditions of island terroir and known input of mineral dust from Africa. Despite natural explanations affecting the overall small shift observed, there are clear outliers with considerably higher 87Sr/86Sr and Sr concentration. This confirms the utility of Sr isotope systematics for oceanic-island viticulture and demonstrates the use of young volcanic soils for tracing natural inputs that may be masked in other regions.

Basalts From the Chukchi Borderland: 40Ar/39Ar Ages and Geochemistry of Submarine Intraplate Lavas Dredged From the Western Arctic Ocean

AbstractSubmarine volcanism in the western Arctic Ocean, known as Amerasia Basin, is attributed to a mantle plume based on geophysics and meager geochemical evidence. Basaltic samples dredged from Chukchi Borderland within the basin have produced minimum 40Ar/39Ar ages for eruption at circa 118–112, circa 105–100, and circa 90–70 Ma, which we use to constrain tectonic models for basin opening. Major oxide and trace element concentrations and Sr, Nd, and Hf isotopic ratios of the lavas show that the circa 118–112 Ma samples from Northwind Ridge are tholeiites (low‐Ti tholeiite I) with low degrees of rare‐earth element (REE) fractionation, high overall heavy rare‐earth element (HREE), and Mg# (Mg‐number), which suggests magma derivation from a garnet‐free source followed by minor crystal fractionation. Strontium, Nd, and Hf isotope systematics for these lavas and ratios of highly incompatible trace elements point toward a lithospheric source. Eruptions at circa 105–100 and circa 90–70 Ma, both at Healy Spur, produced two types of lavas: low‐Ti tholeiite II—which are generally older than high‐Ti tholeiite—both common in continental flood basalt (CFB) provinces and both with trace element abundance patterns typifying a garnet‐free source and significant crystal fractionation for the high‐Ti tholeiite. The isotope characteristics for both groups are common features of asthenospheric sources. Composition‐time relationships for the lavas suggest inception of melting in the subcontinental lithospheric mantle (SCLM)—probably due to introduction of a heat source by a plume—followed later (at ca. 105–100 and ca. 90–70 Ma) by asthenospheric melting possibly triggered by plume rise.

Tourmaline boron and strontium isotope systematics reveal magmatic fluid pulses and external fluid influx in a giant iron oxide-apatite (IOA) deposit

Tourmaline is a common boron-bearing mineral in hydrothermal system and has been widely used as a mineral probe to reconstruct geological processes because of its broad range in composition and resistance to metasomatic alteration. The origin of Kiruna-type iron oxide-apatite (IOA) deposits, commonly linked to andesitic subvolcanic or volcanic rocks, is highly controversial. Constraints on the evolution of these mineralizing systems are needed to advance understanding of the ore-forming process. In this study, we apply in situ elemental and combined B-Sr isotopic analyses of tourmaline to elucidate the nature and evolution of the subsurface hydrothermal system associated with IOA mineralization in the giant Taocun deposit, eastern China. Taocun is hosted at the top of a diorite intrusion and exhibits three stages of hydrothermal alteration that contain tourmaline: pre-ore Na alteration (Tur I), syn-ore magnetite formation and associated Ca-Fe alteration (Tur II), and post-ore Ca-Mg alteration with sulfide veins (Tur III). Compositional data for each stage of tourmaline plot along the “oxy-dravite”–povondraite join, which is indicative of precipitation from relatively oxidizing fluids. Ranges of Sr-isotopic compositions in Tur I (0.7065–0.7078) and Tur II (0.7068–0.7076) are identical to those of the igneous host rocks, indicating precipitation from magmatic-hydrothermal fluids. The range of B-isotopic compositions in Tur I (δ 11 B values of −6.3‰ to −1.2‰) is also consistent with a magmatic source. Higher δ 11 B values (−2.4‰ to 5.4‰) obtained from Tur II are mainly ascribed to Rayleigh fractionation in the magmatic-hydrothermal system as tourmaline precipitated. Post-ore Tur III has a wide range of mostly lower B-isotopic compositions (−8.5‰ to 0.8‰) that record another pulse of magmatic fluid input. This interpretation is supported by the enrichment of Na, Li, Be, W, Sn, V, and Ti in Tur III, relative to Tur I and II. However, the higher Sr-isotope composition (0.7076–0.7086) of Tur III and available O-isotope composition (−7‰ to 3.5‰) of fluids of this stage record the infiltration of meteoric ground water from adjacent sedimentary country rocks. The results suggest that the Taocun IOA deposit formed in a magmatic-hydrothermal system characterized by two (or more) pulses of magmatic fluid discharge from subvolcanic diorite intrusions, followed by the influx of external ground water as the system waned. This study highlights the utility of tourmaline as a robust geochemical and isotopic monitor of ore-forming processes in such systems.

Strontium isotope systematics for plagioclase of the Skaergaard intrusion (East Greenland): A window to crustal assimilation, differentiation, and magma dynamics

Author(s): Hagen-Peter, G; Tegner, C; Lesher, CE | Abstract: We present in situ laser ablation-multicollector-inductively coupled plasma-mass spectrometry Sr isotope data for plagioclase from a reference stratigraphic profile of the entire Layered Series and Upper Border Series of the Skaergaard intrusion (East Greenland). Plagioclase Sr isotope compositions and anorthite contents vary systematically from the margins of the intrusion inwards. The lowest 87Sr/86Sri (calculated at 56 Ma) values (~0.7041) occur near the base and top of the intrusion and systematically increase over several lithostratigraphic zones to a value of ~0.7044, which is uniform throughout the middle ~2000 m of intrusion. Across this same profile, anorthite content of plagioclase varies smoothly from An65-70 at the base and top to ~An25 approaching the purported Sandwich Horizon. Plagioclase near the roof and proximal to rafts of partially assimilated basement gneiss are markedly more radiogenic (87Sr/86Sri up to ~0.7046). We explain the stratigraphic relationships by progressive contamination of the magma during early stages of differentiation by basement gneiss rafts entrained during emplacement and accumulated near the top of the chamber. Contamination was transient, ceasing once the entrained gneiss was consumed or isolated from the main magma reservoir as the solidification front advanced. Modeling of fractionation-assimilation processes accounts for the observed isotopic trends with only a few percent assimilation (relative to the original magma mass). The record of contamination revealed by Sr in plagioclase supports the view that the bulk of the Skaergaard intrusion formed by closed-system differentiation with only minor in situ contamination and no magma recharge. Comparing plagioclase and bulk-rock Sr suggests that the latter may have witnessed late-stage metasomatic overprinting of phases other than plagioclase.

Calcium and strontium isotope systematics in the lagoon-estuarine environments of South Australia: Implications for water source mixing, carbonate fluxes and fish migration

This study uses Ca and Sr isotopes (δ44/40Ca and 87Sr/86Sr), coupled with elemental ratios, to better understand the water source apportionment and carbonate output in the Coorong, Lower Lakes and Murray Mouth Estuary, which represents the terminus of Australia’s longest river system. The geochemistry of waters in the Coorong (i.e., North and South Lagoon) can be explained by mixing of three major components, including: (i) the Southern Ocean seawater, (ii) local freshwaters, and (iii) hypersaline lagoon waters, the latter significantly modified by ongoing evaporation and carbonate formation. The Sr and Ca isotope composition of the North Lagoon is indistinguishable from that of the Southern Ocean (i.e., normal salinity of ∼35 PSU), with the exception of transient freshwater input events that can temporarily lower the salinity to brackish levels. Interestingly, our results from the hypersaline South Lagoon (salinity up to ∼120 PSU) confirmed that the latter is highly evaporated brackish water (with ≥40% contribution from continent-derived waters), which has been additionally modified by in-situ carbonate precipitation. Importantly, our Ca isotope and elemental constraints showed that about 15–17% of the dissolved Ca2+ in the South Lagoon has been removed as CaCO3 (primarily as aragonite). This in turn has implications for the local carbonate cycle and blue carbon studies, suggesting that the South Lagoon acts as a net sink for the dissolved inorganic carbon (DIC).Ca isotope data from the otoliths of smallmouth hardyhead fish species (Atherinosoma microstoma) collected in the Coorong indicate that δ44/40Ca is primarily controlled by biological processes (i.e., kinetic isotope fractionation effects related to growth rate), rather than by the Ca isotope composition of local lagoon waters. As to 87Sr/86Sr in otoliths, the latter confirmed the importance of continent-derived water sources in the Coorong, recorded over the life span of the fish.Overall, with suitable fossil carbonate archives (e.g., bivalve shells, foraminifera), our calibration of 87Sr/86Sr and δ44/40Ca in the modern hydrological system, with respect to a large salinity gradient (ranging from fresh to hypersaline, i.e., 0 to ∼120 PSU), implies potential future applications of these isotope tracers in carbonate-producing coastal systems, which include (i) tracing and apportioning different water sources, (ii) quantifying local carbonate outputs, and (iii) reconstructing paleo-salinity changes.

Strontium concentration, radiogenic (87Sr/86Sr) and stable (δ88Sr) strontium isotope systematics in a controlled feeding study

ABSTRACTTranshumance and palaeodiet are two central themes in archaeology and using chemical analysis of bones and teeth to reconstruct trends and patterns in diet and mobility has become a cornerstone of bioarchaeology. This study has investigated strontium concentration ([Sr]), radiogenic (87Sr/86Sr) and stable strontium (δ88Sr) isotope systematics in a controlled feeding experiment on domestic pigs designed to simulate terrestrial versus marine protein consumption. The results of the radiogenic (87Sr/86Sr) analysis offer a validation of the strontium isotope methodology. The study confirms that the radiogenic strontium isotope composition of dental enamel does represent the radiogenic strontium isotope composition of the diet. The results of the δ88Sr analysis have revealed a distinct shift of 0.322 ± 0.060 ‰ towards isotopically light Sr with trophic level. The magnitude of this shift is consistent with the predictions from the analogous shift observed in calcium isotopes. This is the first time that trophic level fractionation in δ88Sr has been identified in a controlled setting. Although still in its infancy, δ88Sr analysis has great potential to inform on trophic level systematics, to investigate dietary trends in early life and is potentially useful in examining diagenetic alteration.

An isotopic generation: four decades of stable isotope analysis in African archaeology

ABSTRACTThe application of stable isotope analysis in African archaeology has a long and distinguished pedigree. Many new developments were inspired by questions in the archaeology of the continent, helped considerably by the availability of African landscapes as natural laboratories for providing the essential underpinning of isotope ecology systematics. Stable isotopes are now regularly built into multi-disciplinary archaeological projects spanning the very long temporal and geographical span of human history on the African continent. On the fiftieth anniversary of Azania: Archaeological Research in Africa we evaluate these endeavours — the achievements, problems, constraints and potential of stable isotopic research in African archaeology. We begin with a discussion of the continent's role in developing understandings of stable carbon, nitrogen, oxygen and strontium isotope systematics, as used to interpret change in the archaeological record. We discuss the application of these principles within several areas of archaeological research, drawing on case studies from among late Pleistocene/Holocene hunter-gatherers, early food producers, complex societies of the African Iron Age and the individual life histories of the African slave trade. There is clearly much potential for further application in African archaeology.

Characterising the stable (δ88/86Sr) and radiogenic (87Sr/86Sr) isotopic composition of strontium in rainwater

Rainwater plays a major role in determining the transfer of elements and pollutants to soils and hydrological catchments, with compositional variations reflecting the extent of mixing between marine (sea-salt), terrestrial (dust and vegetation) and anthropogenic aerosols. While the radiogenic strontium isotope system (87Sr/86Sr) is often used to help constrain the different sources of atmospheric Sr, the degree of variability in the stable strontium isotope system (δ88/86Sr) remains unresolved. In order to determine how atmospheric Sr affects the δ88/86Sr composition of water entering the hydrological cycle this study analysed a suite of rainwater samples collected over the course of a year in central Paris. Rainwater δ88/86Sr compositions were found to range from 0.13‰ to 0.32‰, with 87Sr/86Sr ratios varying between 0.70796 and 0.71093. No seasonality was observed in δ88/86Sr, whereas 87Sr/86Sr ratios were observed to be generally more radiogenic in the summer and less radiogenic in the winter. Strontium enrichment factors significantly above seawater implied a predominantly terrestrial (non-sea-salt) origin of Sr in rainwater, with the δ88/86Sr and 87Sr/86Sr composition of this terrestrial fraction consistent with a mixture of Sr derived from carbonate dust and agricultural fertiliser. Evidence for a ~0.1‰ decrease in the δ88/86Sr composition of the River Seine as it passes through Paris suggests that anthropogenic pollutants may contribute to significant regional variations in the composition of Sr in the hydrological cycle. Together these results demonstrate that there is significant potential for both temporal and spatial variations in the δ88/86Sr and 87Sr/86Sr composition of rainwater that need to be taken into consideration when studying δ88/86Sr variations in localised catchments.

Reassessing the stable (δ88/86Sr) and radiogenic (87Sr/86Sr) strontium isotopic composition of marine inputs

The stable strontium isotope system (δ88/86Sr) has recently been suggested to be a suitable proxy for determining variations in the strength of the marine carbonate system, the principal output flux of oceanic Sr. However, in order to be able to interpret carbonate-driven variations in δ88/86Srseawater a robust understanding of δ88/86Srinput is required. Surprisingly only a limited amount of δ88/86Sr data currently exists for rivers and hydrothermal fluids, thus this study assesses the variability of δ88/86Sr and 87Sr/86Sr in global rivers, hydrothermal fluids and porewaters, as well as minor marine Sr sources such as continental dust, rainwater and glacial ice. Our analyses broadly confirm the findings of Krabbenhöft et al. (2010), and reveal flux-weighted δ88/86Srriverine and 87Sr/86Srriverine compositions of 0.32‰ and 0.71299 respectively. The hydrothermal fluids analysed in this study are consistent with an end-member δ88/86Srhydrothermal composition that is the same as the oceanic crust at ∼0.24‰, although three samples that display δ88/86Sr compositions offset from the seawater-hydrothermal mixing trend suggest that the precipitation of alteration phases such as anhydrite may drive δ88/86Srhydrothermal to higher values. Porewater fluids obtained from sediment cores in the Atlantic and Pacific Oceans have δ88/86Sr compositions within error of seawater (0.39‰), implying that the diagenetic flux of Sr may not significantly affect the δ88/86Sr composition of seawater. Continental loess samples have δ88/86Sr compositions that are consistently lighter than, or equal to, terrestrial silicates, with their tendency to lower values thought to reflect the preferential removal of heavier Sr isotopes into solution during weathering. Finally, rainwater and glacial ice samples have δ88/86Sr compositions that are also isotopically lighter than their associated water sources, a factor that may be attributed to interaction with isotopically light loess and additional Sr contributions from the bedrock. Together, the principal marine inputs define flux-weighted oceanic δ88/86Srinput and 87Sr/86Srinput compositions of 0.32‰ and 0.71161. These values are consistent with an elevated supply of riverine Sr to the oceans due to increased post-glacial weathering, but require the enhanced weathering of exposed carbonate shelves during glacial periods or significant changes in the rate of carbonate burial to match observed changes in the 87Sr/86Sr ratio of seawater. Our results confirm that, providing a diagenetically robust proxy can be found, the δ88/86Sr and 87Sr/86Sr isotope systems should provide a useful proxy for investigating changes in the marine carbonate system through time.

High-K Mafic Plinian Eruptions of Volcán de Colima, Mexico

The last Plinian-type eruption of Volcan de Colima, Mexico, occurred in 1913; this resulted in the removal of the top 100 m of the edifice and the deposition of a tephra layer that blanketed the slopes of the Colima Volcanic Complex (CVC). Road-cuts on the flanks of the nearby Nevado de Colima edifice expose pre-1913 air-fall tephra, pyroclastic flow and ash-rich surge deposits resulting from numerous highly explosive events throughout the Holocene. The majority of the pumice and scoria fallout deposits are medium-K subalkaline basaltic andesite and andesite in composition, defining a clear major element differentiation trend. In contrast, three newly discovered scoria fallout deposits are high-K subalkaline, transitional to alkaline, basaltic andesite in composition and are characterized by the presence of phlogopite; these deposits have high MgO (up to 7·9 wt %), K2O (up to 2·6 wt %) and P2O5 (up to 0·67 wt %) contents. They are also strongly enriched in fluid- and melt-mobile large ion lithophile elements (LILE; Rb, Ba, K, Sr and Th) and light REE (LREE; La, Ce, Pr and Nd) relative to the majority of the Colima tephra fallout deposits. Strontium and Nd isotope systematics reveal that the high-K mafic scoria have more radiogenic Sr (87Sr/86Sr = 0·70365–0·70408) and less radiogenic Nd (143Nd/144Nd = 0·51279–0·51294) compared with the majority of the subalkaline tephras (87Sr/86Sr = 0·70338–0·70371 and 143Nd/144Nd = 0·51290–0·51295). Two-component mixing models, using whole-rock geochemical data, indicate the importance of magma mixing in the petrogenesis of the Colima magmas, with addition of up to 50% by volume of an alkaline mafic magma component in the most potassic magmas. This is supported by mineral chemistry and textural data, which reveal multiple episodes of decompression and magma mingling within a shallow crustal magma storage region. The presence of these potassic tephra fall deposits among the otherwise prevailing medium-K subalkaline stratigraphy indicates that pulses of K-rich alkaline mafic magmas periodically enter the CVC plumbing system on timescales of a few thousand years and may trigger Plinian explosive eruptions.

The 87Sr/86Sr strontium isotopic systematics applied to Glera vineyards: A tracer for the geographical origin of the Prosecco

Glera vineyards from the Prosecco wine district in northern Italy have been characterised in terms of the 87Sr/86Sr isotope-ratio of musts from the 2010, 2011 and 2012 vintages, coupled with the isotopic analysis of Sr in the labile fraction of the soils of provenance. For a single vineyard, detailed Sr isotopic analyses were carried out in sequentially extracted soil fractions at three different depths, and in the grape components (skin, seeds, must and stem), in order to verify the lack of Sr isotopic fractionation within the plant. The 87Sr/86Sr in must, seeds and stem overlaps within experimental uncertainties; skins are shifted towards a lower Sr isotopic composition. A large range of Sr isotopic compositions (87Sr/86Sr between 0.70706 and 0.71215) characterizes musts from the different vineyards, notwithstanding the relatively limited extension of the investigated geographic area. A statistically significant correspondence between the soil labile fraction and must is observed.

The nature of the Palaeozoic oceanic basin at the southwestern margin of Gondwana and implications for the origin of the Chilenia terrane (Pichilemu region, central Chile)

The Coastal Accretionary Complex of central Chile constitutes the product of early Carboniferous to Late Triassic subduction at the rear of Chilenia, a continental terrane likely derived from Laurentia and accreted to southwestern margin of Gondwana during the Mid to Late Devonian. The complex contains basaltic metavolcanic sequences of the subducted oceanic lithosphere accreted to the active margin. In this paper, we address the tectonic setting of these rocks by means of a geochemical study in the coastal area of Pichilemu region, central Chile. The accreted fragments of oceanic crust occupy different structural levels, exhibit variable metamorphic grade, and have geochemical fingerprints that reveal a compositional heterogeneity of the subducted oceanic crust. The amphibolites have N to E-MORB compositions. Greenschist units include N-MORB and E-MORB transitional to OIB, and blueschists and greenschists interleaved within a single metavolcanosedimentary sequence have OIB signatures. Neodymium isotopic systematics indicate depleted and enriched mantle sources, whereas strontium isotopic systematics indicate seawater/rock interaction. The variety of rocks suggests formation in an oceanic setting characterized by shallow and deep mantle sources, such as plume-influenced ridge. Based on the geological, petrological, geochemical, and isotopic characteristics, we propose that the metavolcanic protoliths of the Pichilemu region formed relatively close to the western margin of the Chilenia terrane during the initial stage (late Cambrian–Early Devonian) of seafloor development and drifting of this continental block. Geochemical similarities with oceanic units accreted to the active margin south of the Pichilemu region indicate a regional pattern of the oceanic crust subducted under the Palaeozoic Chilean margin between, at least, 34°S and 39°S latitude, strongly supporting the activity of a mantle plume. This, in turn, can be correlated with the location of the Pacific plume generation zone in early Palaeozoic era, corroborating a Laurentian origin for the Chilenia terrane.

Compilation and Review of 87Sr/86Sr and Stable Isotopes from Groundwater, Calcite Fracture Fillings, Mineral, and Whole‐Rock Sampling at Äspö, Sweden

Integrated isotopic and hydrochemical studies of groundwater at Äspö, Sweden, support mixing models involving deep saline water, low-solute infiltration, and Baltic Sea water. Carbon, oxygen, and strontium isotope analyses of calcite fracture fillings indicate that paleohydrologic conditions were different than those of today in terms of the isotopic composition of water flowing through fractures. Sr isotopes of whole-rock and mineral (plagioclase, microcline, biotite, and epidote) samples are used to assess the effects of water-rock interaction. Biotite is a major reactant in the early stages of water-rock interaction. Strontium isotope systematics of groundwater from deep in the Hard Rock Laboratory, and underground research facility, and from low conductivity zones revealed a first order mixing line defined by a high-chloride saline component with 66 mg/L Sr and an ancient sea water component with approximately 4.5 mg/L. Dilution with low salinity recharge has produced groundwater with variable Sr contents and 87Sr/86Sr values between 0.7186 and 0.7160. Differences between Äspö and Laxemar groundwater are shown by trends in Sr concentrations plotted against 87Sr/86Sr. The Äspö trend shows increasing 87Sr/86Sr values with increasing concentrations of Sr, whereas the Laxemar groundwater trend shows little variability in 87Sr/86Sr with increasing Sr concentrations. These trends are controlled by the differences in 87Sr/86Sr composition of the saline end members in the two areas.