Isotopic Variations Research Articles

Petrology and source characteristics of the Arbarastakh alkaline ultramafic carbonatite-phoscorite complex, the Aldan-Stanovoy Shield

Late Neoproterozoic Arbarastakh alkaline ultramafic carbonatite-phoscorite complex in the southern margin of the Siberian craton (Aldan-Stanovoy shield), includes carbonatites and phoscorites closely associated with pyroxenites-ijolites and ultramafic lamprophyres. Major and trace elements data, Sr, Nd and Pb isotope compositions for the Arbarastakh rocks have been obtained to characterize the sources involved in their formation, primary melt composition and to build the petrogenetic model.All rock varieties, excluding nepheline syenites, are characterized by incompatible elements enrichments, including light rare earth elements, and strong fractionation of REEs. The initial isotope ratios of the analyzed samples, calculated at 645 Ma, display limited variations: εNd from +5.9 to +6.9 and 87Sr/86Sr from 0.70225 to 0.70272, excluding nepheline syenite with εNd +5.4. The initial Pb isotope ratios for the most studied samples overlap with each other within uncertainties. They yield an age of 642 ± 5 Ma. The Nd and Sr isotope data of the Arbarastakh rocks generally fit the patterns of the other Neoproterozoic alkaline ultramafic carbonatite complexes of the southwestern and southern margins of the Siberian craton. Observed Sr, Nd and Pb isotope variations indicate mixing of the asthenosphere and the depleted mantle components.It is supposed that the primary melts for the Arbarastakh rocks were generated directly by low-degree melting of metasomatic phlogopite‑carbonate veins with apatite and Ti-oxides in garnet peridotite, which formed shortly before the onset of melting. Ultramafic lamprophyre (aillikite) is closest to the primary melt composition in terms of high #MgO, Cr and Ni. The aillikite and latter pyroxenite crystallized from primitive melt by fractionating the olivine, phlogopite and clinopyroxene dominated mineral assemblage that was free of feldspar. This fractionation forced Na-enrichment in the magmas resulted in liquid silicate‑carbonate immiscibility. Major and trace element and isotope data indicate that the nepheline syenites are unlikely to be related with other alkaline silicate rocks through fractional crystallization and could have been formed through pre-emplacement interaction with the ambient crustal materials. Emplacement of calcite carbonatites resulted in metasomatism of surrounding pyroxenites with formation of clinopyroxene-phlogopite-calcite carbonatites. Subsequent portions of Fe-P-enriched carbonatite magmas crystallized phoscorites and apatite-dolomitic carbonatites.

The centennial-resolution loess δDwax record indicates summer precipitation variations in the marginal region of the East Asian monsoon during the last deglaciation

Precipitation isotopic composition variations that are directly recorded in geological samples play an important role in understanding past hydroclimatic changes. In arid regions, the hydrogen isotopes of leaf wax (δDwax) in loess sections can provide records of summer precipitation isotopes. However, previous studies have primarily focused on the orbital scale, and there was still a lack of clear understanding of variations in precipitation isotopes on shorter timescales and their implications for climate change. In this study, we report centennial-resolution loess δDwax records from the central Chinese Loess Plateau (CLP) from the last deglaciation to the early Holocene. We compared the loess optically stimulated luminescence chronology and U-Th ages conveyed by linking stalagmite δ18O and loess δDwax to validate the δDwax method as a potential tool for assessing loess chronology on the suborbital scale. Our findings also indicated that regional precipitation isotopes experienced a depletion of approximately 24‰ for δD and around 3.2‰ for δ18O during the last deglaciation. The overall precipitation isotope amplitude in the central CLP was similar to that of typical East Asian monsoon regions and lower than that of the Indian monsoon and westerly regions. However, the precipitation isotope oscillations on the millennial scale in the central CLP were significantly weaker than those in eastern to central China. The finding affirmed that although the summer precipitation in the CLP was primarily influenced by the East Asian monsoon system at the orbital scale, the northern boundary of the summer rain belt remained a distance away from the interior of the CLP during the last deglaciation. As a result, the short-term oscillations of atmospheric circulation were not strongly reflected in the precipitation isotopes of the central CLP. This result highlighted spatial and temporal differences in precipitation isotope variability between the dominant and marginal regions of the East Asian monsoon system.

The role of high oxygen fugacity on genesis of the late Cenozoic Abaga basalts in Xilin Gol League, Inner Mongolia, China

ABSTRACT The Abaga basalts, which erupted within an intraplate background in the late Cenozoic, are situated in eastern central Inner Mongolia. Previous studies have explored the genesis of these basalts, which were influenced by the subduction of slab. Although trace element ratios demonstrate that altered oceanic crust (AOC) and sediments have contributed to the genesis of the basalts, limited information about the detailed percentages of mixing among sediments, AOC, and depleted mantle (DM) based on Sr‒Nd‒Pb isotopic values has been reported. In this study, we modelled the possible mixing percentages among AOC, DM, and sediments. The calculated results illustrate that a proportion of 80% DM, 16% AOC, and 4% sediments is appropriate for interpreting the Sr‒Nd‒Pb isotopic variation of the basalts. In addition, the associated major and trace elements of olivine and the partition coefficient of V between olivine and melt are used to estimate the crystallization temperatures of the olivine phenocrysts and oxygen fugacity of the basalts. The calculated results display that the olivine mainly crystallized between 1248 and 1293°C (standard error estimate: 51°C), and the fO2 values of the Abaga basalts range from FMQ + 0.14 to FMQ + 1.46 (FMQ, fayalite–magnetite–quartz buffer). The estimated oxygen fugacity values plot in the range of island arc basalts (from FMQ to FMQ + 2) but are higher than those of both mid-ocean ridge basalt (MORB) (from FMQ-0.4 to FMQ + 0.5) and Abaga xenoliths (from FMQ-1.55 to FMQ + 0.53), which indicates that the basaltic melts were more oxidized than those of MORB and the Abaga lithospheric mantle. Furthermore, the primary magma and the corresponding mantle source were more oxidized than MORB. The Ba/Th and Ce/Pb ratios of the basalt present positive correlations with fO2 values, demonstrating that the addition of sediments and oceanic crust into the mantle source may have contributed to increasing the fO2 values of the basalts.

Interspecific, ontogenetic and temporal variations in stable isotopes of small tuna species in the northeast Atlantic Ocean

ABSTRACT In order to study the trophic level of small tuna species and their contribution to the carbon flow in pelagic food webs, an analysis of carbon and nitrogen stable isotopes was carried out. The investigation was focused on four small tuna species (Auxis rochei, Auxis thazard, Euthynnus alletteratus and Sarda sarda) commonly harvested in the northeast Atlantic Ocean. The isotope analysis showed how the results for S. sarda are different from the rest of the species analysed, with a higher trophic level, similar to other major tuna species. The greatest niche overlap in δ13C and δ15N occurs among A. rochei, A. thazard and E. alletteratus. Auxis rochei and E. alletteratus showed a size-dependent variability in δ15N, and in δ13C for S. sarda. The small tuna S. sarda exhibits the highest migration rates among various geographical areas in comparison to other small pelagic tunas, and the seasonal variability of isotope values in the area studied can be attributed to the incorporation of larger individuals with a higher lipid content. The results of this work provide new information on the ecological role played by small tuna in food webs, which is more complex and varied than currently thought. This knowledge is essential for a more effective management of fisheries.

Exploring uranium isotopes in shark teeth as a paleo-redox proxy

The uranium isotope composition (δ238U) of seawater is a powerful proxy for the extent of marine anoxia. For paleoredox reconstructions, carbonates are the most popular U isotope archive, but they have recently come under increased scrutiny as their δ238U values are subject to diagenetic alteration after deposition. Therefore, there is a need to investigate other archives that may record and preserve the original seawater δ238U signal. In this study, we explore whether shark teeth provide such an archive. Shark teeth enameloid consisting of crystalline fluorapatite is more resistant to post-depositional alteration and less sensitive to isotopic exchange than marine carbonates due to the lower solubility of the crystalline fluorapatite. Since U is readily incorporated into phosphate, shark teeth could incorporate and preserve the original δ238U signature of seawater.To assess whether U isotopes in shark teeth can record seawater signatures, we measured the U isotopes (both δ238U and δ234Usec) in 39 fossil shark teeth from various locations, including Banks Island (Arctic), the Gulf of Mexico (GOM), and Pisco Basin (Peru), and ranging in age from modern to Cretaceous. Our results show that U concentrations are negligible in modern shark teeth (<1 ppb) but elevated in fossil samples (up to several hundred ppm), indicating that U is incorporated into shark teeth postmortem during burial. The δ238U values range from −0.72 to +0.57 ‰, and the δ234U values from −162.1 to +969.7 ‰. The data indicate that (i) diagenetic overprinting of seawater U isotope ratios is common among shark teeth, and (ii) δ238U data are influenced by local depositional environments. Nonetheless, the U isotope variations observed in shark teeth are comparable to those seen in marine carbonates, indicating that the samples with less diagenetic alterations might offer useful insight into the past extent of ocean anoxia.

Linking Water Age, Nitrate Export Regime, and Nitrate Isotope Biogeochemistry in a Tile‐Drained Agricultural Field

AbstractAccurately quantifying and predicting the reactive transport of nitrate () in hydrologic systems continues to be a challenge, due to the complex hydrological and biogeochemical interactions that underlie this transport. Recent advances related to time‐variant water age have led to a new method that probes water mixing and selection behaviors using StorAge Selection (SAS) functions. In this study, SAS functions were applied to investigate storage, water selection behaviors, and export regimes in a tile‐drained corn‐soybean field. The natural abundance stable nitrogen and oxygen isotopes of tile drainage were also measured to provide constraints on biogeochemical transformations. The SAS functions, calibrated using chloride measurements at tile drain outlets, revealed a strong young water preference during tile discharge generation. The use of a time‐variant SAS function for tile discharge generated unique water age dynamics that reveal an inverse storage effect driven by the activation of preferential flow paths and mechanically explain the observed variations in isotopes. Combining the water age estimates with isotope fingerprinting shed new light on export dynamics at the tile‐drain scale, where a large mixing volume and the lack of a strong vertical contrast in concentration resulted in chemostatic export regimes. For the first time, isotopes were embedded into a water age‐based transport model to model reactive transport under transient conditions. The results of this modeling study provided a proof‐of‐concept for the potential of coupling water age modeling with isotope analysis to elucidate the mechanisms driving reactive transport.

Mobilization of Tungsten in Greenstone Belts of the Archean Kaapvaal and Singhbhum Cratons

AbstractDue to the inherently fluid‐mobile nature of W, the 182W record of the early Earth may have been obscured by fluid‐induced mobilization of W. To investigate W mobilization in Archean greenstone sequences, we analyzed 182W isotope systematics and major and trace element concentrations in samples from the 3.53 Ga old Onverwacht Group of the Kaapvaal Craton (South Africa) and the &gt;3.51 Ga old Badampahar Group of the Singhbhum Craton (India). Our results for mafic and ultramafic metavolcanic rocks show W/Th ratios significantly higher than primary magmatic values, which suggests fluid‐induced W enrichment. Samples least affected by secondary W enrichment (W/Th &lt; 0.26) show no resolvable W isotope anomalies from modern mantle values in both cratons. Samples from the Kaapvaal Craton with elevated W/Th exhibit deficits in 182W as low as −8.1 ± 4.3 ppm compared to the modern mantle. Covariations of μ182W with W/Th, and Ce/Pb suggest that negative isotope signatures were introduced during secondary fluid‐mediated processes. The enrichment of W is most evident in altered ultramafic rocks comprising serpentine, resulting in additional covariations between MgO, LOI, and W/Th. The W isotope composition of serpentinized komatiites reflects the composition of younger intruding granitoids. We therefore infer the latter as a possible source of W‐rich fluids. The Badampahar Group samples exhibit little W isotope variability. A well‐resolved 182W deficit of −6.2 ± 2.9 ppm was determined in a single komatiite sample, which indicates an unknown fluid source, currently not represented in any other unit of the Singhbhum Craton.

The covariation between Mg[sbnd]C isotopes and trace elements in dolostones: Implications for reconstructing the seawater chemistry

Magnesium isotopes of marine dolomites have the potential to reconstruct the Mg isotopic signals of coeval seawater. However, a massive dolostone sequence can form via the coupling effects of multiple dolomitization events, which can potentially reset the Mg isotopic compositions of dolomites during each migration of the dolomitization fluid. How to distinguish changes of seawater chemistry and early diagenetic effects which have been recorded in dolomites is crucial for the reconstruction of seawater Mg isotopic signals. Here we studied the Mg-Sr-C-O isotope systematics and trace elements of a dolostone sequence from the Qiangtang Basin in the hinterland of Tibet. Multiple lines of evidence consistently indicate the lack of deep-burial diagenesis and high-temperature alteration in the carbonates. The stratigraphic variations of isotopic and elemental compositions of the dolostone profile are generally characterized by the co-variation between MgC isotopes and trace element contents. Our modeling results indicated that a single downward dolomitization can cause the large Mg isotopic variability of dolomites, ca. 1 ‰, within a dolomitization front. However, when the limestones in the profile were completely dolomitized, the coupling effects of multiple downward dolomitizations will homogenize the δ26Mg values of dolostones. Therefore, we attribute the observed co-variation in isotopic and elemental signals in the dolostone sequence to the changes in the chemistry of evolved seawater within the platform. The reconstructed seawater δ26Mg values from the Carnian to Early Jurassic are lower than previous model-based estimations. This result implies an overestimation of the production of shallow-water carbonate in literature during Late Triassic to Early Jurassic. Our study highlights the importance of utilizing multiple geochemical proxies to identify the controlling factors driving Mg isotope variability in dolomites.

Zirconium stable isotope fractionation during intra-crustal magmatic differentiation in an active continental arc

Zirconium (Zr) stable isotope variations occur among co-existing Zr-rich accessory phases as well as at the bulk-rock scale, but the petrologic mechanism(s) responsible for Zr isotope fractionation during magmatic differentiation remain unclear. Juvenile magma generation and intra-crustal differentiation in convergent continental margins may play a crucial role in developing Zr isotope variations, and the Northern Volcanic Zone of the Andes is an ideal setting to test this hypothesis. To investigate the influence of these processes on Zr stable isotope compositions, we report δ94/90ZrNIST of whole rock samples from: 1) juvenile arc basalts from the Quaternary Granatifera Tuff, Colombia; 2) lower crust-derived garnet pyroxenites (i.e., arclogites), hornblendites, and gabbroic cumulates found in the same unit; and 3) felsic volcanic products from the Doña Juana Volcanic Complex, a dacitic composite volcano in close proximity to and partially covering the Granatifera Tuff. The basalts have δ94/90ZrNIST values ranging from −0.025 ± 0.018 ‰ to +0.003 ± 0.015 ‰ (n = 8), within the range of mid-ocean ridge basalts. The dacites have δ94/90ZrNIST values ranging from +0.008 ± 0.013 ‰ to +0.043 ± 0.015 ‰ (n = 14), slightly positive relative to the Granatifera and mid-ocean ridge basalts. In contrast, the (ultra)mafic cumulates have highly variable, predominantly positive δ94/90ZrNIST values, ranging from −0.134 ± 0.012 ‰ to +0.428 ± 0.012 ‰ (n = 15). Individual grains and mineral fractions of major rock-forming phases, including garnet (n = 21), amphibole (n = 9), and clinopyroxene (n = 18), were analyzed from 8 (ultra)mafic cumulates. The mineral fractions record highly variable Zr isotopic compositions, with inter-mineral fractionation (Δ94/90Zrgarnet-amphibole) up to 2.067 ‰. Recent ab initio calculations of Zr–O bond force constants in rock-forming phases predict limited inter-mineral Zr isotope fractionation in high-temperature environments, suggesting that the large fractionations we observe are not the product of vibrational equilibrium processes. Instead, we propose a scenario in which large Zr isotopic fractionations develop kinetically, induced by sub-solidus Zr diffusion between coexisting phases via changes in Zr distribution coefficients that arise from changes in temperature. Altogether, Zr isotope variability in this calc-alkaline continental arc setting exhibits no correlation with indices of magmatic differentiation (e.g., Mg#, SiO2), and is not a simple function of fractional crystallization. Furthermore, the garnet clinopyroxenite cumulates studied here represent density-unstable lower arc crust material; consequently, material with isotopically variable δ94/90Zr can be recycled into the mantle as a consequence of lower crustal foundering.

Water isotopic characterisation of the cloud–circulation coupling in the North Atlantic trades – Part 1: A process-oriented evaluation of COSMOiso simulations with EUREC4A observations

Abstract. Naturally available, stable, and heavy water molecules such as HDO and H218O have a lower saturation vapour pressure than the most abundant light water molecule H216O; therefore, these heavy water molecules preferentially condense and rain out during cloud formation. Stable water isotope observations thus have the potential to provide information on cloud processes in the trade-wind region, in particular when combined with high-resolution model simulations. In order to evaluate this potential, nested COSMOiso (isotope-enabled Consortium for Small Scale Modelling; Steppeler et al., 2003; Pfahl et al., 2012) simulations with explicit convection and horizontal grid spacings of 10, 5, and 1 km were carried out in this study over the tropical Atlantic for the time period of the EUREC4A (Elucidating the role of clouds-circulation coupling in climate; Stevens et al., 2021) field experiment. The comparison to airborne in situ and remote sensing observations shows that the three simulations are able to distinguish between different mesoscale cloud organisation patterns as well as between periods with comparatively high and low rain rates. Cloud fraction and liquid water content show a better agreement with aircraft observations with higher spatial resolution, because they show strong spatial variations on the scale of a few kilometres. A low-level cold-dry bias, including too depleted vapour in the subcloud and cloud layer and too enriched vapour in the free troposphere, is found in all three simulations. Furthermore, the simulated secondary isotope variable d-excess in vapour is overestimated compared to observations. Special attention is given to the cloud base level, which is the formation altitude of shallow cumulus clouds. The temporal variability of the simulated isotope variables at cloud base agrees reasonably well with observations, with correlations of the flight-to-flight data as high as 0.7 for δ2H and d-excess. A close examination of isotopic characteristics under precipitating clouds, non-precipitating clouds, clear-sky and dry-warm patches at the altitude of cloud base shows that these different environments are represented faithfully in the model with similar frequencies of occurrence, isotope signals, and specific-humidity anomalies as found in the observations. Furthermore, it is shown that the δ2H of cloud base vapour at the hourly timescale is mainly controlled by mesoscale transport and not by local microphysical processes, while the d-excess is mainly controlled by large-scale drivers. Overall, this evaluation of COSMOiso, including the isotopic characterisation of different cloud base environments, suggests that the simulations can be used for investigating the role of atmospheric circulations on different scales for controlling the formation of shallow cumulus clouds in the trade-wind region, as will be done in part 2 of this study.

The bimodal Fii-A2-type and calc-alkaline volcanic sequence of the Aljustrel brownfield region, Iberian Pyrite Belt, SW Iberian Massif

The Iberian Pyrite Belt (IPB) is a late Devonian – Early Carboniferous world-class polymetallic VMS province that includes significant Cu-(Sn)-Pb-Zn-(Ag) deposits of massive sulphides and feeder zones. The Aljustrel brownfield region contains one of the highest concentrations of ore in the IPB in 6 known deposits (Gavião, São João, Moinho, Algares, Estação and Feitais). To delve into the petrogenesis of the Aljustrel early Carboniferous (∼355 Ma) felsic-dominated bimodal volcanism, new whole-rock trace elements and SmNd isotopes, and UPb in zircon were obtained.Based on Ga/Al and Y/Nb ratios, it is shown that Aljustrel felsic magmatism has the geochemical features of A2-type melts, typical of post-collisional and back-arc settings. UPb in zircon for a juvenile felsic volcanic rock point to antecrysts ages spanning from 387.9 to 366.6 Ma and a maximum emplacement age of 354.3 ± 2.6 Ma. These long-lasting melting events, present in both juvenile (ƐNdi = +1.79) and evolved felsic rocks (ƐNdi = −5.07), imply heterogeneous sources dominated by zircon-bearing igneous rocks. The SmNd model ages are in accordance with previous LuHf model ages in zircon, reinforcing that the isotopic variability is related to the same petrogenetic process.Subordinated Aljustrel mafic rocks, coeval with the abundant felsic volcanism, show orogenic signatures, namely Nb-Ta-Ti negative anomalies and calc-alkaline affinities, whereas SmNd isotopic data (ƐNdi = +1.54 to +5.48) points to variable to no contamination with crustal material. These geochemical results suggest derivation from an enriched mantle source modified by subduction metasomatism. In addition, the mafic rocks did not provide zircons for geochronological analysis, with the exception of one sample, in which a Concordia age of 402.1 ± 15.5 Ma was obtained from a single grain.The combined geochemical signatures of mafic and felsic volcanic rocks suggest asthenospheric rise, but this solely does not explain the abundance of zircon antecrysts in the felsic rocks. Therefore, a geodynamic model that includes a continuous evolution from Devonian to Carboniferous times is inferred. This more complex and broader geodynamic model for the Iberian Pyrite Belt in which successive metal remobilization occurred after successive melting events, fits the present geochemical data and is more likely to explain why the Iberian Pyrite Belt is a unique metallogenetic province.

Power-law patterns in the Phanerozoic sedimentary records of carbon, oxygen, sulfur, and strontium isotopes

Power-law patterns appear in a variety of natural systems on the modern Earth; nevertheless, whether such behaviors appeared in the deep-time environment has rarely been studied. Isotopic records in sedimentary rocks, which are widely used to reconstruct the geological/geochemical conditions in paleoenvironments and the evolutionary trajectories of biogeochemical cycles, offer an opportunity to investigate power laws in ancient geological systems. In this study, I focus on the Phanerozoic sedimentary records of carbon, oxygen, sulfur, and strontium isotopes, which have well documented and extraordinarily comprehensive datasets. I perform statistical analyses on these datasets and show that the variations in the sedimentary records of the four isotopes exhibit power-law behaviors. The exponents of these power laws range between 2.2 and 2.9; this narrow interval indicates that the variations in carbon, oxygen, sulfur, and strontium isotopes likely belong to the same universality class, suggesting that these systematic power-law patterns are governed by universal, scale-free mechanisms. I then derive a general form for these power laws from a minimalistic model based on basic physical principles and geosystem-specific assumptions, which provides an interpretation for the power-law patterns from the perspective of thermodynamics. The fundamental mechanisms regulating such patterns might have been ubiquitous in paleoenvironments, implying that similar power-law behaviors may exist in the sedimentary records of other isotopes.

Mammal communities of primeval forests as sentinels of global change.

Understanding the drivers and consequences of global environmental change is crucial to inform predictions of effects on ecosystems. We used the mammal community of Białowieża Forest, the last lowland near-primeval forest in temperate Europe, as a sentinel of global change. We analyzed changes in stable carbon (δ13 C) and nitrogen (δ15 N) isotope values of hair in 687 specimens from 50 mammal species across seven decades (1946-2011). We classified mammals into four taxonomic-dietary groups (herbivores, carnivores, insectivores, and bats). We found a significant negative trend in hair δ15 N for the mammal community, particularly strong for herbivores. This trend is consistent with temporal patterns in nitrogen deposition from (15 N depleted) industrial fertilizers and fossil fuel emissions. It is also in line with global-scale declines in δ15 N reported in forests and other unfertilized, non-urban terrestrial ecosystems and with local decreases in N foliar concentrations. The global depletion of 13 C content in atmospheric CO2 due to fossil fuel burning (Suess effect) was detected in all groups. After correcting for this effect, the hair δ13 C trend became non-significant for both community and groups, except for bats, which showed a strong decline in δ13 C. This could be related to an increase in the relative abundance of freshwater insects taken by bats or increased use of methane-derived carbon in food webs used by bats. This work is the first broad-scale and long-term mammal isotope ecology study in a near-primeval forest in temperate Europe. Mammal communities from natural forests represent a unique benchmark in global change research; investigating their isotopic temporal variation can help identify patterns and early detections of ecosystem changes and provide more comprehensive and integrative assessments than single species approaches.

The influence of environmental parameters on spatial variation in zoobenthic density and stable isotopes (δ13C, δ15N, and δ34S) within a large lake

AbstractThe use of baselines in stable isotope studies to interpret food web structure is essential, but baseline isotope values are often assumed to be spatially homogeneous, even in large aquatic ecosystems. To test this assumption in large lakes, we quantified spatial variation in δ13C, δ15N (deposit‐feeding Oligochaeta and filter‐feeding Dreissena spp.), and δ34S (Dreissena spp. only) and density in Lake Erie between 2014 and 2016. Lake Erie's three distinct basins differ in size, bathymetry, and nutrient loading, making it an excellent system for exploring spatial variation in stable isotopes of baseline organisms. Dreissena spp. densities were highest in the western and lowest in the seasonally hypoxic central basin, while Oligochaeta densities were relatively consistent throughout Lake Erie. Values of δ13C, δ15N, and δ34S exhibited distinct spatial trends that were not related to population densities but followed the west to east direction of water flow within the lake. For both taxa, δ13C was lower in the deeper, oligotrophic east basin than the shallow, mesotrophic west basin, and δ15N and δ34S increased from west to east. Spatial patterns of low δ34S in Dreissena spp. in the western and central basins were likely related to hypoxia, whereas patterns of δ15N in both taxa were probably related to the greater influence of agricultural land uses in the western basin. Spatial trends of stable isotopes in large lake zoobenthos are driven by complex interactions of environmental gradients, which could introduce bias in evaluations of trophic structures within aquatic ecosystems that use stable isotopes.

Calcium isotope compositions of subduction-related leucite-bearing rocks: Implications for the calcium isotope heterogeneity of the mantle and carbonate recycling in convergent margins

Large Ca isotope variations recorded in mantle xenoliths and mantle-derived igneous rocks have been widely used to decode the origin of mantle heterogeneity, especially to trace recycled sedimentary carbonates. However, it is still unclear whether and how the subduction of sedimentary carbonate causes significant Ca isotope fractionation in the mantle in correspondence of convergent margins. In this study, we present Ca isotope data for sixteen silica-undersaturated ultrapotassic leucite-bearing rocks and their clinopyroxene (Cpx) phenocrysts from the “Colli Albani” volcano (Italy), and for six carbonate-bearing sediments from nearby Apennine Chain. These subduction-related volcanic rocks have been widely accepted as deriving from a mantle source that involves recycled carbonated sediments indicated by their enriched incompatible trace elements and radiogenic isotopes, and olivine geochemistry. The δ44/40Ca values (normalized to NIST SRM 915a) of leucite-bearing rocks are uniform (from 0.64 to 0.79 ‰, with one exception of 0.86 ‰) with an average of 0.72 ± 0.03 ‰, 2SE, N = 16, which is 0.12 ± 0.03 ‰ (2SE) and 0.22 ± 0.03 ‰ (2SE) lower than MORBs (0.84 ± 0.02 ‰, 2SE, N = 25) and BSE (0.94 ± 0.01 ‰, 2SE, N = 14) respectively. In contrast, the carbonate-bearing sediments show variable and notably low δ44/40Ca values, ranging from 0.44 to 0.77 ‰. The δ44/40Ca of Cpx (0.70 to 0.76 ‰) is indistinguishable from their host rocks (0.73 to 0.76 ‰). In addition, the lack of correlations between δ44/40Ca and SiO2, CaO, Ni and P2O5, and the Ca isotopic simulation result indicate that Ca isotope fractionation caused by fractional crystallization is insignificant. Given that the Ca isotope fractionation is limited during the melting of spinel wehrlite-like mantle source (within ∼0.04 to 0.11 ‰), the slightly low δ44/40Ca values of the Italian leucite-bearing rocks require 10 % to 20 % addition of carbonated sediments in their mantle source. This indicates that recycled carbonated sediments can cause slight δ44/40Ca heterogeneity (∼0.1 to 0.2) of the mantle wedge in local/regional scales.

The impact history and prolonged magmatism of the angrite parent body

AbstractAs some of the oldest differentiated materials in our solar system, angrite meteorites can provide unique insights into the earliest stages of planetary evolution. However, the timing of planetary mixing, as evidenced by oxygen isotope variations in the quenched angrites, and the extent of magmatism on the angrite parent body (APB) remain poorly understood. Here, we report on microstructurally guided in situ geochemical and Pb–Pb isotopic measurements on angrites aimed at better understanding of the timing and nature of magmatic processes, as well as impact events, on the APB. The quenched angrite Northwest Africa (NWA) 12320 yielded a Pb–Pb date of 4571.2 ± 9.4 Ma, which we interpret as corresponding to the timing of planetary mixing. The only known shocked quenched angrite, NWA 7203, also yielded an ancient Pb–Pb date of 4562.9 ± 9.3 Ma, which is identical to the Pb–Pb date of 4563.6 ± 7.9 Ma obtained for the texturally intermediate angrite NWA 10463. Pb–Pb analyses in phosphates in the dunitic angrite NWA 8535 yielded a much younger date of 4514 ± 30 Ma, representing the youngest Pb–Pb date ever recorded for an angrite. Based on the evidence from the lack of shock deformation, olivine major and trace element compositions, and no apparent contamination in the oxygen isotope composition of NWA 8535, our findings are consistent with prolonged magmatism on the APB. This finding is consistent with a large size for the APB.

Single-species dinoflagellate cyst carbon isotope fractionation in core-top sediments: environmental controls, CO2 dependency and proxy potential

Abstract. Sedimentary bulk organic matter and various molecular organic components exhibit strong CO2-dependent carbon isotope fractionation relative to dissolved inorganic carbon sources. This fractionation (εp) has been employed as a proxy for paleo-pCO2. Yet, culture experiments indicate that CO2-dependent εp is highly specific at genus and even species level, potentially hampering the use of bulk organic matter and non-species-specific organic compounds. In recent years, significant progress has been made towards a CO2 proxy using controlled growth experiments with dinoflagellate species, also showing highly species-specific εp values. These values were, however, based on motile specimens, and it remains unknown whether these relations also hold for the organic-walled resting cysts (dinocysts) produced by these dinoflagellate species in their natural environment. We here analyze dinocysts isolated from core tops from the Atlantic Ocean and Mediterranean Sea, representing several species (Spiniferites elongatus, S. (cf.) ramosus, S. mirabilis, Operculodinium centrocarpum sensu Wall and Dale (1966) (hereafter referred to as O. centrocarpum) and Impagidinium aculeatum) using laser ablation–nano-combustion–gas-chromatography–isotope ratio mass spectrometry (LA/nC/GC-IRMS). We find that the dinocysts produced in the natural environment are all appreciably more 13C-depleted compared to the cultured motile dinoflagellate cells, implying higher overall εp values, and, moreover, exhibit large isotope variability. Where several species could be analyzed from a single location, we often record significant differences in isotopic variance and offsets in mean δ13C values between species, highlighting the importance of single-species carbon isotope analyses. The most geographically expanded dataset, based on O. centrocarpum, shows that εp correlates significantly with various environmental parameters. Importantly, O. centrocarpum shows a CO2-dependent εp above ∼ 240 µatm pCO2. Similar to other marine autotrophs, relative insensitivity at low pCO2 is in line with active carbon-concentrating mechanisms at low pCO2, although we here cannot fully exclude that we partly underestimated εp sensitivity at low pCO2 values due to the relatively sparse sampling in that range. Finally, we use the relation between εp and pCO2 in O. centrocarpum to propose a first pCO2 proxy based on a single dinocyst species.

Influence of atmospheric circulation on the stable isotope of precipitation in the monsoon margin region

Precipitation stable isotope is a good medium for studying the water cycle and atmospheric water vapor transport process. To better understand the influence and control mechanism of atmospheric circulation on the stable isotope of precipitation in the East Asian monsoon margin region. We set up observation points in the Shiyang River basin in the East Asian monsoon marginal region to observe precipitation isotopes and analyze the mechanism and process of the control of atmospheric circulation on precipitation isotopes in the monsoon marginal region. The results show that: The results show that the stable isotope of precipitation in the monsoon margin region is greatly affected by the local meteorological conditions, and there are obvious temperature effects. The main water vapor affecting the monsoon margin region throughout the year are west wind water vapor, southeast monsoon water vapor, plateau wind water vapor and local circulation water vapor. In July and August, 25.33% and 65% of the southeast monsoon water vapor reached the monsoon margin region, respectively. The variation of stable isotope of precipitation in the monsoon margin region is mainly controlled by the East Asian Monsoon and the Northwest Pacific Monsoon Trough, but is less dependent on the Westerly Wind and Plateau Wind.

Calcium isotopic variability in hotspot lavas controlled by partial melting and source lithological heterogeneity

Crustal recycling can induce significant chemical and lithological heterogeneity of the mantle. Ocean island basalts (OIBs) are the surface expressions of rising mantle plumes from the Earth's deep interior and thus can be a probe of the compositional heterogeneity in the deep mantle. Endmember OIBs sampled the most isotopically extreme mantle components including EM1 (Enriched mantle 1), EM2 (Enriched mantle 2), HIMU (high μ; μ =238U/204Pb), and FOZO (Focus zone), but the lithological properties of their source regions are still not well understood. Here we explore the potential of stable calcium (Ca) isotopes in identifying mantle source lithology by investigating OIBs from the type localities for the several mantle components. We find that the δ44/40Ca values of classic HIMU OIBs (0.77 ± 0.09‰; 2SD, N = 15) and Louisville FOZO OIBs (0.78 ± 0.06‰; 2SD, N = 4) are indistinguishable from the average MORB (mid-ocean ridge basalt) value (0.84 ± 0.09‰; 2SD, N = 31), while the δ44/40Ca values of the Pitcairn EM1 (0.65–0.72‰) and the Samoan (Tutuila) EM2 OIBs (0.61–0.71‰) are distinctly lower than those of MORBs. Such Ca isotopic distinction cannot be attributed to magmatic differentiation because the EM1 and EM2 OIBs have obviously lower δ44/40Ca values than the HIMU and FOZO OIBs even at a same MgO content. Equilibrium fractionation during partial melting of garnet peridotite with bulk silicate Earth (BSE) δ44/40Ca (0.94 ± 0.05‰) can explain the Ca isotopic compositions of the HIMU and FOZO OIBs but is unable to reproduce the Ca isotopic variation of all OIBs in this study. Quantitative modeling suggests that the coupling of low δ44/40Ca (< 0.7‰) and high Gd/Yb (> 3.5) ratios in the EM1 and EM2 OIBs most likely points to an eclogitic source component with MORB-like or even lower δ44/40Ca (e.g., < 0.8‰), possibly reflecting contributions from recycled oceanic crust and sediments. A negative correlation between average δ44/40Ca and δ57Fe values (a useful proxy for mantle source lithology) of all endmember OIBs further confirms this proposal and highlights the significant contribution of distinct source lithologies to the Ca isotopic variations in these basalts. Therefore, our study indicates that Ca isotopes of basalts can be another promising tracer of lithological heterogeneity in the mantle.

Development of a coffee bean certified reference material (KRISS CRM 108-10-023) for elemental analysis.

The development of a novel coffee bean matrix certified reference material (CRM) for elemental analysis is described. The CRM was prepared by processing green coffee beans into a dry homogeneous powder. Mass fractions of elements in the CRM were measured using double isotope dilution inductively coupled plasma mass spectrometry (double ID-ICP-MS), and measurement results for eight elements (Mg, Ca, Fe, Cu, Zn, Cd, Hg, and Pb) of sufficient quality were certified. The mass fraction range was from 0.09476mg/kg (Cd) to 1908mg/kg (Mg), with relative expanded uncertainty range of 0.66% (Cd) to 12% (Pb). Measurement results of two elements (Cr and Ni) with insufficient quality were provided for information only. During characterization, an effective approach for the measurement of isotopic abundances and molar masses of elements with high natural isotopic variations for double ID-ICP-MS was developed and applied. The CRM developed in the present study is expected to be a useful measurement standard for assuring the quality of measurement procedures for coffee beans or related materials.