Strontium Isotopic Composition Research Articles

Oxygen and strontium isotopes from fossil shark teeth: Environmental and ecological implications for Late Palaeozoic European basins

Fossil shark remains occur in both marine and nonmarine Late Palaeozoic deposits, therefore their palaeoecology is controversial. The oxygen and strontium isotopic composition of biogenic fluorapatite in 179 teeth, scales and spines predominantly of hybodontid (Lissodus) and xenacanthiform (Orthacanthus, Xenacanthus, Bohemiacanthus, Triodus) sharks from various Late Carboniferous (Moscovian) to Early Permian (Artinskian) basins of Europe are used as ecological tracers to decipher diadromous or obligate freshwater lifestyle of the investigated taxa. The δ18OP values of the different shark teeth range from 11.7 to 20.2‰ within the different basins with mean values of 16.9±0.5‰ for the Bohemian Massif, 16.2±0.8‰ for eastern Germany, 18.2±1.0‰ for southwestern Germany, 18.5±0.7‰ for southern-central Spain, 17.6±0.4‰ for Sardinia, and 16.6±0.5‰ VSMOW for the French Massif Central. The tooth δ18OP values from the basins are mostly depleted by 1–5‰ relative to those of shark teeth from contemporaneous marine settings. Oxygen isotope signatures of co-occurring taxa do not show systematic differences excluding habitat effects for different shark groups. However, distinctly higher δ18OP values from Puertollano and Saar–Nahe can be attributed to significant evaporative enrichment in 18O of the ambient water in the ancient lacustrine environments due to a warm and dry climate and sufficient residence time in the basins. The strontium isotopic composition of the teeth varies between 0.70824 and 0.71216 with a mean value of 0.71031. These 87Sr/86Sr ratios are always more radiogenic in comparison to the 87Sr/86Sr record of seawater of their stratigraphic age. Overall, the investigated tooth samples yield low δ18OP and high 87Sr/86Sr values deviating from bioapatite values expected for contemporaneous marine vertebrates and typical for freshwater settings. This indicates a fully freshwater adapted lifestyle for a variety of fossil shark taxa in Late Palaeozoic European basins.

Geochemical and Visual Indicators of Hydrothermal Fluid Flow through a Sediment-Hosted Volcanic Ridge in the Central Bransfield Basin (Antarctica)

In the austral summer of 2011 we undertook an investigation of three volcanic highs in the Central Bransfield Basin, Antarctica, in search of hydrothermal activity and associated fauna to assess changes since previous surveys and to evaluate the extent of hydrothermalism in this basin. At Hook Ridge, a submarine volcanic edifice at the eastern end of the basin, anomalies in water column redox potential (Eh) were detected close to the seafloor, unaccompanied by temperature or turbidity anomalies, indicating low-temperature hydrothermal discharge. Seepage was manifested as shimmering water emanating from the sediment and from mineralised structures on the seafloor; recognisable vent endemic fauna were not observed. Pore fluids extracted from Hook Ridge sediment were depleted in chloride, sulfate and magnesium by up to 8% relative to seawater, enriched in lithium, boron and calcium, and had a distinct strontium isotope composition (87Sr/86Sr = 0.708776 at core base) compared with modern seawater (87Sr/86Sr ≈0.70918), indicating advection of hydrothermal fluid through sediment at this site. Biogeochemical zonation of redox active species implies significant moderation of the hydrothermal fluid with in situ diagenetic processes. At Middle Sister, the central ridge of the Three Sisters complex located about 100 km southwest of Hook Ridge, small water column Eh anomalies were detected but visual observations of the seafloor and pore fluid profiles provided no evidence of active hydrothermal circulation. At The Axe, located about 50 km southwest of Three Sisters, no water column anomalies in Eh, temperature or turbidity were detected. These observations demonstrate that the temperature anomalies observed in previous surveys are episodic features, and suggest that hydrothermal circulation in the Bransfield Strait is ephemeral in nature and therefore may not support vent biota.

Strontium Isotope Fractionation in Soils and Pedogenic Processes

The stable isotope composition of strontium (the ratio 88Sr/86Sr expressed as δ88/86Sr) showed significant fractionation in mountain soils of the Judea Highland. In order to understand this phenomenon, we studied the elemental composition and the stable and radiogenic Sr isotopic composition in soil transects conducted from semi-arid (desert fringe) to wetter (Mediterranean) climate zones. These transects were selected because the degree of soil leaching depends on the amount of precipitation and the permeability of the underlying bedrock. These soils are the pedogenic products of leaching of the accumulated desert dust and the underlying carbonate bedrocks resulting in, among others, enrichment of the residual soils in Al-clays. A clear negative correlation was found between the δ88/86Sr and Al2O3 (Al-clay content) values of the soils, the high δ88/86Sr-low Al2O3 being the dust end-member. This preliminary study demonstrates the feasibility of using stable 88Sr-86Sr isotopes as tracers of terrestrial weathering processes.

Carbon, oxygen and strontium isotopic composition of low-latitude Permian carbonates (Venezuelan Andes): climate proxies of tropical Pangea

Abstract The Late Palaeozoic period was an important time in Earth history, primarily as a result of the formation of Pangea and the Gondwana glaciations that had direct influences in high latitudes and indirect, but no less important, effects in low latitudes. This study documents and interprets the isotopic record for the Early–Middle Permian in northern Gondwana from detailed fieldwork in the Venezuelan Andes. Sr-isotope dating of well-preserved brachiopod shells suggests a Kungurian age from samples located within the middle part of the Palmarito Formation. δ 13 C data show a long-term trend through the section towards more positive values, but with some very negative δ 13 C values suggesting carbonates affected by pedogenic processes. In the middle part of the succession, major excursions in δ 13 C and δ 18 O show a strong connection with glacial–deglacial events during the Early–Middle Permian. Overall results presented here fill an important gap in the regional palaeogeography and, therefore, have significant implications for the palaeoclimate and palaeoceanography of this Late Palaeozoic time.

Strontium isotope composition of sedimentary rocks and its application to chemostratigraphy and palaeoenvironmental reconstructions

Strontium isotope composition of seawater is homogenous in all oceans and marginal seas but has varied through time with changing strontium fluxes. Strontium isotope ratio of ancient seawater may be reconstructed from well-preserved authigenic minerals and used for dating marine sediments. Because of a change in the strontium isotope ratio between sea- and freshwaters it can also be employed for the determination of palaeosalinities or the reconstruction of diagenetic processes of marine limestones. A review of methodology and practical hints concerning the utilization of strontium isotopes in geological studies of both marine and brackish sediments are presented in the current article.

The Kipushi Cu–Zn deposit (DR Congo) and its host rocks: A petrographical, stable isotope (O, C) and radiogenic isotope (Sr, Nd) study

Near the city of Kipushi, located in the southern part of the Central African Copperbelt, a major vein-type Cu–Zn ore deposit occurs. A combination of petrographic techniques and both stable (O, C) and radiogenic (Sr, Nd) isotope analysis is used to investigate the influence of the mineralisation on the Neoproterozoic dolomite host rocks. A quantification of the abundance and size of the different host rock constituents (dolomite types, quartz, phyllosilicates) revealed a lithostratigraphical controlled variation, without trends towards the ore body. The bulk oxygen isotopic composition of the host rock varies between −2.54‰ and −9.64‰ V-PDB, with most values within the range of Neoproterozoic marine dolomite. Samples with more positive δ18O all originate from the same stratigraphic interval and are interpreted as the result of reflux dolomitisation by an evaporated brine. Few samples with depleted δ18O signatures could indicate the influence of a depleted or high temperature fluid, but are not related to the ore deposit. Moreover, the presence of the ore body cannot be traced through the host rock oxygen isotopic composition. δ18O of gangue dolomite is significantly depleted in comparison with the host rocks and ranges between −7.67‰ and −12.46‰ V-PDB. For an estimated mineralisation temperature of 310°C, this implies a δ18Ofluid between 10.7‰ and 15.6‰ V-SMOW. This is a significant enrichment compared to Neoproterozoic seawater, indicating that the mineralising fluid underwent significant fluid–rock interactions. δ13C of both host rock and gangue dolomite are in range of Neoproterozoic marine dolomites. However, a limited stratigraphic interval has clearly more negative δ13C signatures, due to in situ maturation of carbonaceous material. At the time of mineralisation (450Ma), the host rock dolomite has a strontium isotopic composition partly more radiogenic than Neoproterozoic marine carbonates (0.70793<87Sr/86Sr<0.71167). Nevertheless, the signatures show no relation to the ore body. The gangue dolomite is significantly more radiogenic (0.71061<87Sr/86Sr<0.71332) than the host rock. The radiogenic signature may be due to the interaction of formational and mineralising fluids with Neoproterozoic siliciclastic-rich dolomites, e.g. the top the Kakontwe Supérieur which has 87Sr/86Sr values up to 0.72575 at 450Ma. Alternatively, the mineralising fluid could have interacted with basement rocks. According to εNd450 values between −5.4 and −0.9 for gangue dolomites, this basement was mafic in nature. However, mafic rocks also occur in the Roan Group near the Kipushi deposit. Taken into account previous fluid inclusion data, the mineralising fluid most likely derived metals both from mafic and felsic basement units and possibly interacted with Roan rocks.

Delineation of groundwater flow paths using hydrochemical and strontium isotope composition: A case study in high arsenic aquifer systems of the Datong basin, northern China

► We delineate the flow paths in high arsenic groundwater aquifer system. ► We model the evolution of chemical and Sr isotopic composition along the flow paths. ► Groundwater flow regime has effect on arsenic enrichment in groundwater. Strontium isotopic compositions and major ion chemistry were used to delineate flow paths in an arsenic affected groundwater flow system in the Datong basin, China. Total of 28 samples including one spring and one river water were collected for major ions and strontium isotopic compositions and concentration analysis. The ion ratios indicate that dissolution of silicates and carbonates is the dominant geochemical process controlling the hydrochemistry of groundwater from two recharge areas at the basin margins, while the water chemistry in the center of the basin is affected by the dissolution of evaporites (including gypsum and halite). Groundwater from eastern and western margin areas have relatively high 87 Sr/ 86 Sr values ranging from 0.72114 to 0.72604 and from 0.71119 to 0.71151, which are consistent with expected values for groundwater dominantly affected by dissolution of silicates. However, in the discharge area, the groundwater samples had lower 87 Sr/ 86 Sr values and varied between 0.71016 and 0.71753. The contour map of δ 87 Sr in groundwater shows the general tendency of decrease from the western and eastern margin areas to the discharge area. The plot of 87 Sr/ 86 Sr vs. Sr/Na indicates that interactions between Quaternary aquifer sediment and groundwater in the recharge areas along the flow paths control the hydrochemistry and strontium isotopic compositions of groundwater. By contrast, groundwater samples from the discharge area are plotted on the mixing line, indicating that mixing of groundwater from recharge area with low 87 Sr/ 86 Sr values groundwater could be the controlling factor on their hydrochemistry and strontium isotopic compositions. Four main flow paths of groundwater were inferred from hydrochemical and isotopic data. The results of PHREEQC inverse modeling matched quite well with the results of strontium isotopic and ion compositions along the flow paths. The distribution of high arsenic groundwater in this area could be attributed to the combined effect of the vertical flow and local groundwater flow on the redox conditions of the groundwater system.

SPATIALLY RESOLVED LA-MC-ICPMS STRONTIUM ISOTOPE MICROANALYSIS OF ARCHAEOLOGICAL FAUNA

The analysis of the strontium isotope composition of archaeological materials can provide important information about the mobility of a range of mammals, including humans. The basis of this method is that, prior to any postburial diagenesis, the Sr87/Sr86 ratio of bone and teeth reflects the geological environment from which food and water were sourced while these biominerals were forming. Teeth are particularly amenable to tracing the geographic origins of humans as they mineralize during the first 12–13 years of life (White and Folkens, 2005) and do not subsequently change strontium composition after this time (Schweissing and Grupe, 2003). Strontium isotope analysis can be used to determine if individuals are local or nonlocal by comparison to the isotopic composition in and around their burial location (i.e., Schweissing and Grupe, 2000; Bentley et al., 2007; Conlee et al., 2009). In order to quantify the extent of faunal mobility, the strontium isotope composition of biominerals from fossil samples needs to be compared with a regional map of values obtained either from local faunal material (Price et al., 2002) or from analysis of the bioavailable component of strontium from plants, regolith, or bedrock (Capo et al., 1998). Strontium isotope analysis has been extensively applied to the determination of archaeological mobility, as reviewed by Price et al. (2002), Budd et al. (2004), Bentley (2006), and Montgomery (2010). Although studies of Paleolithic or older archeological material are rare, some have been undertaken on hominins (Sillen et al., 1995, 1998; Richards et al., 2008; Copeland et al., 2011) and faunal material (Horn et al., 1994; Copeland et al., 2010; Britton et al., 2011). Enamel has been shown to be the most favorable material for strontium isotope analysis based on its ability to …

Using the 87Sr/86Sr of modern and paleoseep carbonates from northern Cascadia to link modern fluid flow to the past

Most authigenic carbonates previously recovered from the Cascadia slope have 87Sr/86Sr signatures that reflect shallow precipitation in equilibrium with coeval seawater. There is also evidence for carbonate formation supported by fluids that have been modified by reactions with the incoming Juan de Fuca plate (87Sr/86Sr=0.7071; Teichert et al., 2005) or with terrigenous turbidites (87Sr/86Sr=0.70975 to 0.71279; Sample et al., 1993). We report on the strontium isotopic composition of carbonates and fluids from IODP Site U1329 and nearby Barkley Canyon (offshore Vancouver Island), which have strontium isotope ratios as low as 0.70539. Whereas the strontium and oxygen isotopic compositions of carbonates from paleoseeps in the uplifted Coast Range forearc indicate formation in ambient bottom seawater, several samples from the Pysht/Sooke Fm. show a 87Sr-depleted signal (87Sr/86Sr=0.70494 and 0.70511) similar to that of the anomalous Site U1329 and Barkley Canyon carbonates.Our data, when analyzed in the context of published elemental and isotopic composition of these carbonates (Joseph et al., 2012), point to two formation mechanisms: 1) shallow precipitation driven by the anaerobic oxidation of methane (AOM) with δ13C values as low as −50‰ and contemporaneous 87Sr/86Sr seawater ratios, and 2) carbonate precipitation driven by fluids that have circulated through the oceanic crust, which are depleted in 87Sr. Carbonates formed from the second mechanism precipitate both at depth and at sites of deep-sourced fluid seepage on the seafloor. The 87Sr-depleted carbonates and pore fluids found at Barkley Canyon represent migration of a deep, exotic fluid similar to that found in high permeability conglomerate layers at 188mbsf of Site U1329, and which may have fed paleoseeps in the Pysht/Sooke Fm. These exotic fluids likely reflect interaction with the 52–57Ma igneous Crescent Terrane, which supplies fluids with high calcium, manganese and strontium enriched in the non-radiogenic nucleide. Tectonic compression and dehydration reactions then force these fluids updip, where they pick up the thermogenic hydrocarbons and 13C-enriched dissolved inorganic carbon that are manifested in fluids and carbonates sampled at Barkley Canyon and at Site U1329. The Crescent Terrane may have sourced cold seeps in this margin since at least the late Oligocene.

Strontium isotope variations in Middle Jurassic (Late Bajocian–Callovian) seawater: Implications for Earth's tectonic activity and marine environments

Strontium isotope ratios of well-preserved belemnite rostra and ammonite shells are presented from stratigraphically well-dated Upper Bajocian–Callovian sediments of the Polish Jura Chain in central Poland. Nekto-benthic belemnites show a narrow trend of changes of 87Sr/86Sr ratio that are interpreted to record secular variations in the strontium isotope composition of seawater. A new fit of the seawater strontium isotope curve for the Late Bajocian–Callovian is presented based on current and literature data. The data show a gradual decrease of seawater 87Sr/86Sr ratio throughout the Late Bajocian–earliest Middle Callovian, lowest 87Sr/86Sr ratios (0.706850) and the flat minimum of the curve were reached in the latest Middle and the Late Callovian. The Middle Jurassic decrease in seawater 87Sr/86Sr ratio is interpreted to have been a result of the increased volcanic activity of the seafloor. Studied nektonic ammonites are considered to have lived in surface or shallow near-coastal waters of the Polish Jura Chain basin and been affected by the freshwater influx. A slight enrichment of ammonite shells in 87Sr isotope is linked to a change in strontium isotope composition of the surface or near-coastal waters due to the riverine strontium input.

Conditions for veining and origin of mineralizing fluids in the Alpi Apuane (NW Tuscany, Italy): Evidence from structural and geochemical analyses on calcite veins hosted in Carrara marbles

This work deals with structural and geochemical (chemical and isotopic) analyses of calcite veins hosted in Carrara marbles in the Alpi Apuane, NW Tuscany, Italy. Geometric features and spatial distribution of veins provided estimations of stress ratio (Φ = (σ2 − σ3)/(σ1 − σ3)), driving stress ratio (R′ = (Pf − σ3)/(σ1 − σ3)) and fluid overpressure (Δsi = Pf − σ3) at the time of vein formation. The obtained values of Φ = 32 and R′ = 0.43 reveal that fluid pressure was higher than the intermediate principal stress at the time of veins formation, whereas the estimated Δsi ranging from 129 to 207 MPa indicates that veins formed under supra-hydrostatic to lithostatic pressure conditions. Carbon (δ13CV-PDB = 1.81–2.10‰ for veins and 1.95–2.51‰ for host marbles), oxygen (δ18OV-SMOW = 28.71–29.57‰ for veins and 28.90–29.36‰ for host marbles) and strontium (87Sr/86Sr = 0.707716–0.707985 for veins and 0.0707708–0.707900 for host marbles) isotope compositions in vein/host marble pairs were internally quite consistent. Combining our structural and geochemical data, a modeling approach was performed to investigate the compositional features and temperatures of calcite depositing fluids. The results of our studies give evidence that (1) pore-fluids in Carrara marble, consisting of metamorphic formation waters, were re-mobilized during veining event and migrated within the veins in closed system conditions, (2) veins formed after ductile folding phases and before high-angle brittle faulting events, at temperature and pressure around 250 °C and 210 MPa, and finally (3) about 12 g H2O/m3 marble are calculated to have been available as vein parental fluid at the time of vein formation.

Palaeoenvironments of the late Triassic Rhaetian Sea: Implications from oxygen and strontium isotopes of hybodont shark teeth

The oxygen and strontium isotopic composition of 125 teeth of the hybodont sharks Lissodus and Hybodus from various late Triassic (Rhaetian) bone bed localities in the Central European Basin are used as environmental and ecological tracers for the shallow epicontinental Rhaetian Sea. The preservation of tooth enameloid was ascertained by cathodoluminescence microscopy. Mean δ18OP values differ regionally across the Rhaetian Sea with values of 18.6±0.6‰ near the western gate, 16.1±0.8‰ in the east, 15.4±0.6‰ in the southeast, and 17.7±0.9‰ in the south. The δ18OP values in the eastern Rhaetian Sea are 4–5‰ lower relative to Tethyan seawater, while teeth from the southern and western parts close to the marine gates are less fractionated. The strontium isotopic composition of teeth from most localities is more radiogenic in comparison to late Triassic seawater. The observed spatial δ18OP trend in the Rhaetian Sea indicates a shift from marine to brackish conditions in the south and west towards extensively brackish conditions with salinities of less than 16‰ in the east, thus confirming the existence of two marine gates. Brackish conditions persisted throughout early Rhaetian times, and were maintained by increased fluvial discharge from the Vindelician–Bohemian and Fennoscandian Highs as a result of enhanced atmospheric moisture availability in the course of the Rhaetian transgression. The δ18OP values of Lissodus and Hybodus indicate euryhaline behaviour of Rhaetian hybodonts by exceeding modern shark intraspecific variability. The lack of accordance in δ18OP values between different isotopically conspicuous localities in the Rhaetian Sea contradicts a basin-wide migration pattern. Differences between co-site taxa might indicate some degree of niche partitioning.

Sources and physicochemical characteristics of fluids along a subduction‐zone megathrust: A geochemical approach using syn‐tectonic mineral veins in the Mugi mélange, Shimanto accretionary complex

The Mugi mélange in the Shimanto accretionary complex, southwest Japan, records faulting and fluid flow patterns at the updip limit of the seismogenic region of the Nankai subduction zone. To characterize the origin and behavior of syn‐tectonic fluids, we investigated the carbon, oxygen, and strontium isotopic compositions, and rare earth element (REE) patterns of syn‐tectonic calcite within veins along fault zones in the mélange, as well as the Sr isotopic compositions and REE patterns of surrounding host rocks. With the exception of intra‐basalt veins formed prior to subduction, the δ13C values of veins range from −10‰ to −19‰, suggesting a mixed carbon source (i.e., marine carbonate and organic matter). The vein‐forming fluids have positive oxygen isotopic compositions (+2‰ to +9‰ (SMOW)) and high 87Sr/86Sr values (0.70794–0.70850), suggesting that the source was rock‐buffered fluids affected both by terrigenous sediments and altered oceanic crust. The veins found in filling the fault zone associated with tectonic underplating have different REE patterns to those of the other veins, implying a difference in physicochemical processes affecting the fault zone near the subduction megathrust.

Were European steppe bison migratory? 18O, 13C and Sr intra-tooth isotopic variations applied to a palaeoethological reconstruction

This paper focuses on the palaeoethological study of steppe bison (Bison priscus), a Eurasiatic species that disappeared at the end of the Pleistocene. In the Southern steppe of Eastern Europe, this large bovid seems to have been a keystone species during the late Pleistocene, where it is omnipresent in archaeozoological assemblages and can constitute spectacular bone accumulations. Based on the principles of actualism, which assume that modern animal biology and their behavioural adaptations can be applied to the past, different models of bison palaeoethological reconstructions have been proposed to explain this important occurrence of bison in East European archaeological records. Considering the variability of extant bison behaviour, it appeared necessary to undertake a direct reconstruction of the behaviour of fossil bison, to assess how they were targeted.In order to reconstruct steppe bison ecology and habits stable isotope analyses were conducted of tooth enamel carbonate of 25 individuals from Amvrosievka, a Late Pleistocene (∼18,500 BP) archaeological site complex from Eastern Ukraine. Intra- and inter-individual variations in the stable oxygen, carbon and strontium isotope composition of tooth enamel were analysed to reconstruct the feeding behaviour and seasonal movements of steppe bison. This is the first large scale multi-proxy study of its kind for a Pleistocene cohort.The results show that bison δ13C values are wholly consistent with an exclusively C3 plant diet, typical of steppe/grassland environments, with a likely consumption of lichen during the cold season. δ18O, δ13C and 87Sr/86Sr values are typical of sedentary bison herds, with limited inter-seasonal movement. These results challenge the previous assumption that Pleistocene bison of the East European steppe were migratory, demonstrating instead that they occupied limited ranges, with no long distance seasonal movement, and that they occasionally incorporated low nutrient foods in their diet.These new palaeobiological results confirm the ecological and behavioural plasticity of bison and have important implications for archaeologists studying hunter-gatherers since human acquisition strategies are directly related to the social, seasonal and spatial behaviour of prey species. This research demonstrates that direct reconstructions of the eco-ethology of fossil prey species are essential, therefore, before attempting to make inferences about the hunting tactics and subsistence strategies developed by Prehistoric hunters.

Erratum: Estimating westslope cutthroat trout (Oncorhynchus clarkii lewisi) movements in a river network using strontium isoscapes

We used natural variation in the strontium concentration (Sr:Ca) and isotope composition (87Sr:86Sr) of stream waters and corresponding values recorded in otoliths of westslope cutthroat trout (Oncorhynchus clarkii lewisi) to examine movements during their life history in a large river network. We found significant spatial differences in Sr:Ca and 87Sr:86Sr values (strontium isoscapes) within and among numerous spawning and rearing streams that remained relatively constant seasonally. Both Sr:Ca and 87Sr:86Sr values in the otoliths of juveniles collected from nine natal streams were highly correlated with those values in the ambient water. Strontium isoscapes measured along the axis of otolith growth revealed that almost half of the juveniles had moved at least some distance from their natal streams. Finally, otolith Sr profiles from three spawning adults confirmed homing to natal streams and use of nonoverlapping habitats over their migratory lifetimes. Our study demonstrates that otolith geochemistry re...

Isotope geochemistry of Paleoproterozoic metacarbonates from Itatuba, Borborema Province, Northeastern Brazil: Evidence of marble melting within a collisional suture

Strongly-deformed marbles may be easily confused with linear and elongated carbonatite intrusions. Both rocks may present similar texture and foliation to the host rock, or even cross cutting field relationships, which could be interpreted either as igneous or high-grade metamorphosed marble. Diagnostic criteria are even more complex when there is evidence of melting of the metasedimentary carbonate rock, such as has been described in the Himalayas and in the Eastern Ghats, India.In the Alto Moxotó Terrane, a high-grade gneissic domain of the Borborema Province, Northeastern Brazil, there are metacarbonates associated with banded gneisses and different metaplutonic rocks. Field evidence indicates the absence of other metasedimentary rocks associated with these marbles, thus suggesting that these carbonates were separated from other siliciclastic metasedimentary rocks. The presence of marble also suggests that it may represent the initial stage of a crustal carbon recycling into the mantle. These marbles present many field similarities to carbonatites (e.g., fluid-flow structure) and, together with metagranites and metamafic intrusions, may represent a major collisional tectonic suture.A detailed study of the carbon, oxygen and strontium isotopic composition of these marbles is presented. This study aims to identify the origin of the different isotopic components. It is argued that these rocks were subjected to temperature and pressure conditions that were sufficiently high to have melted them. The isotopic data presented here support this interpretation and indicate the mixing of two components: (i) one characterized by radiogenic Sr isotopes and mantle-like carbon isotopes, which is associated with the gneissic and mafic rocks, and (ii) another characterized by low 87Sr/86Sr ratios and highly positive δ13C values. Available geochemical data for the upper Paleoproterozoic indicate that the 87Sr/86Sr ratio of ocean water, varying between 0.7050 (2.25±0.25Ga) and 0.7047 (1.91Ga), falls within the lower range of the samples from Itatuba and thus reinforces the interpretation that these marbles are sedimentary-derived and were partially contaminated by interaction with the host gneissic and mafic rocks.

Geochemistry, and carbon, oxygen and strontium isotope composition of brachiopods from the Khuff Formation of Oman and Saudi Arabia

ABSTRACT Brachiopods are abundant in the Oman Khuff Formation and similar brachiopod faunas are present at a few horizons in the same formation in Central Saudi Arabia. Following extensive systematic and biostratigraphic studies of these faunas, specimens from the base of the Midhnab Member of the Khuff Formation of Saudi Arabia (Buraydah Quadrangle), and from Member 3 of the Khuff Formation of the Huqf outcrop of Oman were assessed for isotope geochemistry (Sr, O and C). Dating using 87Sr/86Sr alone is not conclusive. Five pristine Oman brachiopods from biostratigraphically well-constrained lower Wordian horizons record a range of 87Sr/86Sr values that form a separate cluster offset from the current Sr isotope seawater curve, which defines the Early Permian and earliest Mid-Permian. The 87Sr/86Sr of the pristine Saudi Arabian brachiopod sits in an area which corresponds to a wide scatter of 87Sr/86Sr in the seawater curve data. However, the Saudi Arabian data does indicate that the Midhnab Member is likely younger than Member 3 of the Khuff Formation of the Huqf outcrop. The well-preserved brachiopod carbonate allows deductions to be made about the palaeotemperature of the Oman Khuff Formation Member 3 seawater using its oxygen isotope composition (δ18O). Assuming δ18O of seawater &amp;lt; –0.5‰, then palaeotemperature derived from brachiopods in the Oman horizons would be +25°C, +22°C and +17°C respectively. This is consistent with the trend of shallowing within Member 3, suggested by facies.

The influence of near-surface winds on the Sr isotope composition of aeolian sediments: a wind tunnel experiment

We used wind tunnel experiments to simulate the effect of near-surface wind velocity on the silicate strontium (Sr) isotopic compositions of the fractions <10μm in windblown dust using intact surface samples obtained from a potential dust source area in arid northern China. Our results showed that the 87Sr/86Sr ratios of the silicate fractions in the windblown dust varied with near-surface wind speed and with the total amount of the fraction <10μm. For the five samples that we analyzed, the variances of the 87Sr/86Sr ratio under a range of wind velocities were 0.00014, 0.00004, 0.00052, 0.00151, and 0.00012. In addition, although most samples were collected at the same site and were treated in the same way, there were still differences in the 87Sr/86Sr ratios, with an intersample variance of up to 0.00409. These observations suggest that although using the 87Sr/86Sr ratios of aeolian sediments to reconstruct climate change or as fingerprints for aeolian materials transported from dust source areas remains a valid technique, the effects of the near-surface wind on the 87Sr/86Sr ratios in the dust source regions must be carefully considered.

Iodine-129, 87Sr/86Sr, and trace elemental geochemistry of northern Appalachian Basin brines: Evidence for basinal-scale fluid migration and clay mineral diagenesis

Evidence for basin scale brine migration and clay mineral diagenesis in the northern Appalachian Basin was investigated using elemental and isotope (129I/I, 87Sr/86Sr) geochemistry of formation waters collected from the Middle to Upper Devonian section of the northern basin margin in western New York, northwest Pennsylvania, and eastern Kentucky. One sample from each of the Mississippian Berea sandstone and the Silurian Medina sandstone were analyzed for comparison. Measured iodine ratios range between 28 to 1,890 × 10−15 and are anomalously high compared to cosmogenic iodine sourced from Devonian age organic matter. Iodine-129 in the waters was largely derived from fissiogenic sources, the spontaneous fission of 238U to produce 129I, with estimated 129I/I values up to 270 × 10−15, which occur locally in the organic-rich shales. There are three water samples that have values of 490 × 10−15, 860 × 10−15, and 1,890 × 10−15, which are above the range for local fissiogenic 129I and may be accounted for by topographically driven, basin scale fluid flow through a regionally high fissiogenic source. Relatively large uranium occurrences lie along the structural front of the Appalachian Basin in the Blue Ridge Province and are situated within hypothesized flow paths parallel to the main compressional direction of the Alleghanian orogeny. Estimated 129I/I values for these uranium occurrences are in excess of 55,000 × 10−15. The strontium isotope composition and Sr concentration of brines display a mixing trend between a highly radiogenic end-member (0.7210) with low Sr (51 mg/L) and a non-radiogenic (0.7100), high Sr (4789 mg/L) end-member. Potassium and boron concentrations are notably depleted relative to evaporated Paleozoic seawater, the hypothesized source of Appalachian Basin brines. The K/Rb values of formation waters are depleted relative to seawater values, but in some cases are well above values indicative of water-rock reactions. The Sr isotopic composition, K and B depletion, and intermediate K/Rb ratios are consistent with smectite diagenesis and paleo-temperatures that are likely greater than approximately 150 °C. These temperatures may be high given the burial history of the study area and support the flow of formation waters from deeper within the basin. The combined isotopic and elemental results of formation waters provide compelling evidence for basin scale fluid migration in the northern Appalachian Basin and are consistent with previously published evidence documented from the rock record, including clay mineral diagenesis and ore deposition.

An emplacement mechanism for the mega‐block zone within the Chicxulub crater, (Yucatán, Mexico) based on chemostratigraphy

Abstract– To better constrain the emplacement mechanism of the so‐called “mega‐block zone,” a structurally complex unit of target rocks within the Chicxulub impact structure, the stratigraphic coherence of this zone is tested using its strontium isotopic composition. Forty‐eight samples across the 616 m sequence of deformed Cretaceous rocks in the lower part of the Yaxcopoil‐1 core, drilled by ICDP in 2002, were analyzed for their 87Sr/86Sr isotope ratio. The oceanic anoxic event 2 (OAE2 event), located near the base of the core forms the only stratigraphic anchor point. From this point upward to approximately 1050 m depth, the 87Sr/86Sr trend shows small oscillations, between approximately 0.7074 and 0.7073, characteristic of Cenomanian to Santonian values. This is followed by an increase to approximately 0.7075, similar to the one reported in the seawater strontium curve during the Campanian. Scattered Sr isotope ratios are attributed to local diagenetic effects, such as those expected from the possible presence of hot, impact‐induced dikes and hydrothermal fluid flow, originating from the thick central melt sheet. The absence of Upper Maastrichtian Sr isotope values may result from the removal of upper target lithologies during the impact cratering process. Based on these results, the displaced Cretaceous sequence in Yax‐1 appears to have preserved its stratigraphic coherence. During the modification stage, it probably moved as a whole into the annular basin during collapse of the crater wall, thereby breaking up into discrete units along previously weakened detachment zones. This model is consistent with the emplacement mechanism postulated by Kenkmann et al. (2004).