Variations In Isotopic Composition Research Articles

Neoproterozoic sedimentary rocks in the Liantuo Formation in South China and their implication based on geochronological and Sr–Nd–Hf–Pb isotope insights

Deposition of the Liantuo Formation is closely associated with the evolutionary history of the Rodinia supercontinent, as indicated by the breakup of the Yangtze Craton in South China. In this paper, we also carried out a detailed UPb zircon examination of the Liantuo Formation. Radiometric dating of sandstone within the Liantuo Formation suggested that it was deposited ca. 790 Ma, which coordinated with the age-paleopole contradictions in the Liantuo Formation for exploring the evolutionary history of the Yangtze Block during the breakup of the Rodinia supercontinent. All the Liantuo sandstones and shales have variable Sr isotopic compositions, with initial Sr isotope ratios ranging from 0.6920 to 0.7217, which support significant fluvial contributions during the deposition of the Liantuo Formation. Samples of the Liantuo Formation show distinct variations in NdHf isotopic compositions, suggesting that the Nantuo Formation received relatively juvenile materials (e.g., the newly formed Neoproterozoic rocks from the northern margin of the Yangtze Block), whereas the Liantuo Formation received relatively mature inputs (e.g., the Huangling granitoids and/or Kongling complex). The samples from the Liantuo Formation also contain various Pb isotopes (206Pb/204Pb = 17.56–17.76, 207Pb/204Pb = 15.05–15.43 and 208Pb/204Pb = 35.51–36.79). These values are also close to the Pb isotopic compositions of the Kongling complex, which suggest that the sedimentary rocks were enriched (the Kongling complex) and are consistent with the NdHf isotope data. That is, the major provenance of the Liantuo Formation is from the South China Block basement, which is the local Kongling complex. In contrast, the Luoquan diamictite can represent the well-mixed composition of the upper continental crust (UCC) in the North China Block during the Neoproterozoic.

Temporal variations of stable isotopic compositions in atmospheric water vapor on the Southeastern Tibetan Plateau and their controlling factors

Stable isotopic compositions in atmospheric water vapor (δ18OV, δDV and d-excessV) are effective tracers that provide new insights into the local and regional hydrological cycle on the Tibetan Plateau. One-year continuous measurements of the isotopic compositions in near-surface atmospheric water vapor on the Southeastern Tibetan Plateau are reported. Our results demonstrated the relative impacts of moisture source shifts, large scale moisture transport, and local processes on the variability of the isotopic compositions in atmospheric water vapor. The observed seasonal variations in δ18OV and d-excessV exhibit low values in the monsoon period and high values in the non-monsoon period due to the predominance of large-scale atmospheric circulations. Local processes significantly modulate diurnal cycles of δ18OV and d-excessV. The sharp decrease in δ18OV and increase in d-excessV is related to the synoptic process. Moreover, the seasonality of moisture sources and transport, including moisture uptake and upstream rainout strongly affects the atmospheric moisture and the isotopes. These findings will improve our understanding of the regional hydrological cycle on the Southeastern Tibetan Plateau and provide reference to validate the simulated stable water isotopes.

Diagenetic evolution of secondary evaporites and associated host-rock dolostones in the Huron Domain (Michigan Basin): insights from petrography, geochemistry, and C-O-S-Sr isotopes

ABSTRACT In this study, we present a detailed investigation of C-O-S-Sr isotope systematics and elemental analysis of secondary sulfates and associated host-rock carbonates of Cambrian to Devonian sedimentary successions along the eastern flank of the Michigan Basin, Ontario. This study evaluates the diagenetic evolution of pore fluids and their sources in fracture-fill and replacement sulfate minerals in low-permeability carbonate units in the Michigan Basin. Secondary sulfates, represented by gypsum and anhydrite, contain various petrographic types, represented by vug- and fracture-filling fibrous anhydrite in the Cambrian (δ18O vary between 16.8 to 17.6‰ VPDB and δ34S 28.3 to 29.0‰ CDT, 87Sr/86Sr ratios vary from 0.70834 to 0.70991, respectively) and Ordovician fibrous anhydrite (δ18O 16.8‰, δ34S 28.2‰, 87Sr/86Sr ratios 0.70829). These phases display mainly uniform REE patterns with Y/Ho and Zr/Hf ratios, flat La*, Gd*, and Y*, and enriched LREEs. In the Silurian strata, idiotopic satin-spar δ18O values vary between 4.0 and 8.3‰, δ34S 23.4 and 31.4‰ with 87Sr/86Sr ratios 0.70816–0.70866, xenotopic porphyroblast gypsum δ18O values vary between 4.5 and 13.3‰, δ34S 22.6 to 33.1‰, with 87Sr/86Sr ratios of 0.70850–0.70880, alabastrine gypsum δ18O values vary between 4.2 and 11.7‰, δ34S 23.1 to 26.9‰, with 87Sr/86Sr ratios of 0.70816–0.70876 and felted anhydrite δ18O values of 11.1‰, δ34S 27.4‰, with a 87Sr/86Sr ratio of 0.70849. These geochemical proxies suggest a comparable sulfate-rich source for both age groups under similar geochemical conditions. Gypsum in the Silurian Salina Group displays a wide range of REE values with Y/Ho, and Zr/Hf ratios, Nd/YbN and Pr/YbN, flat to strong Ce/Ce*, and a flat to strong Eu/Eu* anomaly, Gd/LaN, Tb/LaN, and Sm/LaN. These sulfates formed at burial from fluids of variable isotope compositions, chemical compositions, and temperatures. The sources of these fluids range from brines migrating from a deeper part of the basin forming anhydrite in the Cambrian and Ordovician sequences which were also affected by dolomitization and later hydrothermal fluid influx, to rehydration of gypsum at shallower burial depths and affected by salt dissolution and incursion of meteoric water during and following the Alleghenian orogeny.

Comparative Analysis of Water Isotopic Compositions: Evaluating Isotope Analyzer for Soil and Extraction Method for Stem Water

Stable isotopes of water (δ2H and δ18O) are reliable tracers for the investigation of plant–soil–water interactions in forest ecosystems. However, variations in isotopic compositions may arise due to differences in analytical instruments and water extraction methods. In this study, we conducted three different experiments to identify isotopic differences caused by analytical and methodological variations. First, we analyzed soil water by using the two most commonly applied methods: isotope ratio mass spectrometry (IRMS) and cavity ring-down spectroscopy (CRDS). Second, we compared the isotopes in xylem water extracted from the stems of nine tree species using cryogenic vacuum distillation (CVD) with different heating times. Third, we compared the compositions in xylem water extracted with three different methods: mechanical squeezing using a pressure chamber (PC), an induction module (IM), and CVD. The differences in isotopic composition between IRMS and CRDS were significant but minimal. Soil properties were not significant factors contributing to differences between the two instruments. For the xylem water extraction with CVD, each of the nine tree species required heating for more than three hours. Significant differences were observed in δ2H among the three extraction methods for xylem water. Xylem water extracted by CVD showed more depleted values compared to those obtained by PC and IM. Our results highlight the importance of considering analytical and methodological variations in stable isotope analysis.

Isotopic signatures of snow, sea ice, and surface seawater in the central Arctic Ocean during the MOSAiC expedition

The Arctic Ocean is an exceptional environment where hydrosphere, cryosphere, and atmosphere are closely interconnected. Changes in sea-ice extent and thickness affect ocean currents, as well as moisture and heat exchange with the atmosphere. Energy and water fluxes impact the formation and melting of sea ice and snow cover. Here, we present a comprehensive statistical analysis of the stable water isotopes of various hydrological components in the central Arctic obtained during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in 2019–2020, including the understudied Arctic winter. Our dataset comprises >2200 water, snow, and ice samples. Snow had the most depleted and variable isotopic composition, with δ18O (–16.3‰) increasing consistently from surface (–22.5‰) to bottom (–9.7‰) of the snowpack, suggesting that snow metamorphism and wind-induced transport may overprint the original precipitation isotope values. In the Arctic Ocean, isotopes also help to distinguish between different sea-ice types, and whether there is a meteoric contribution. The isotopic composition and salinity of surface seawater indicated relative contributions from different freshwater sources: lower δ18O (approximately –3.0‰) and salinities were observed near the eastern Siberian shelves and towards the center of the Transpolar Drift due to river discharge. Higher δ18O (approximately –1.5‰) and salinities were associated with an Atlantic source when the RV Polarstern crossed the Gakkel Ridge into the Nansen Basin. These changes were driven mainly by the shifts within the Transpolar Drift that carried the Polarstern across the Arctic Ocean. Our isotopic analysis highlights the importance of investigating isotope fractionation effects, for example, during sea-ice formation and melting. A systematic full-year sampling for water isotopes from different components strengthens our understanding of the Arctic water cycle and provides crucial insights into the interaction between atmosphere, sea ice, and ocean and their spatio-temporal variations during MOSAiC.

Barbary macaques show sex-related differences in body weight based on anthropogenic food exposure despite comparable female–male stable isotope ratios

As the human–primate interface expands, many nonhuman primate (NHP) populations exploit anthropogenic foods to survive, while some populations opportunistically target them. Though anthropogenic food consumption is sometimes associated with greater reproductive output and survival in these populations, there is a dearth of research on possible health effects. We explore how differential exposure to anthropogenic foods is linked to variation in isotopic compositions (δ13C and δ15N) and body weights in Barbary macaques (Macaca sylvanus) in the Upper Rock Nature Reserve, Gibraltar. We placed monkeys into three categories based on anthropogenic food exposure. We then analyzed individuals for isotopic signatures (N = 147) and body weight measurements (N = 80). Using the lowest exposure category as the comparison, we found body weights and δ15N values, but not δ13C values, significantly differed across key categories. Within categories, we found no significant associations between sex and δ13C or δ15N values, suggesting that individuals within categories consumed similar foods regardless of sex. We found a significant interaction effect between category and sex for predicting body weights. These results suggest that sex plays a role in how anthropogenic foods are accessed and consumed regardless of exposure, which may result in differential health profiles for female and male macaques.

Characteristics of water isotopes and their indication for water budget in a closed reservoir, Southeast China

Abstract Xili reservoir is one of the most important drinking water reservoirs in southeast China, ensuring the safety of water supply in Shenzhen city. Understanding the water balance in the reservoir system under climate change and human activities is essential for making optimal water use planning. To assess the water balance of the Xili reservoir system, the water isotope characteristics in rainfall, reservoir water, groundwater, soil, and plant were investigated. The variation of isotope composition in rainfall, reservoir water, and groundwater indicates the different recharging and mixing water sources. A mathematical mass-balance model was established to evaluate the water budget in the Xili reservoir. Roughly 35% of total incoming water was consumed by evaporation, which reminds managers to pay more attention to making scientific planning.

Evolution of the composition of Н, О and C stable isotopes in the groundwaters of oil and gas bearing sediments in the northern regions of West Siberia

The results of a study of the isotopic composition of oxygen and hydrogen of groundwater and dissolved inorganic carbon in oil and gas deposits of the northern regions of Western Siberia, covering a wide stratigraphic range – Mesozoic and Paleozoic are presented. The obtained values are very diverse, covering the intervals from –120 to –50‰ (δD) and from –17 to –2‰ (δ18O). Such a variable isotopic composition of waters indicates the absence of a unified mechanism for the accumulation of waters and their transformation during the geological evolution of the West Siberian sedimentary basin. The main feature of most of the studied waters is the pronounced values of oxygen isotope shifts relative to GMWL, which generally increase with the age of water-bearing deposits and reach 9‰, which indicates their ancient sedimentary origin. Variations in the isotopic composition of DIC (from –51.8 to +21.8‰) and its content in waters (from 0.2 to 38.6 mmol/I) indicate a wide range of water burial conditions and processes involved in the conversion of water-soluble carbon dioxide. A general pattern of changes in the isotopic composition of carbon in CO2 with time, expressed in general form by damped oscillations of the “isotope oscillator”, is revealed. A model of transformation of the carbon isotope composition in carbon dioxide in the closed cycle “atmosphere-soils-deep aquifers” for oil and gas deposits of the northern regions of Western Siberia based on the obtained isotope data is proposed.

Изотопный состав и условия образования фаменских карбонатолитов Центрально-Хорейверского вала (Хорейверская впадина, Печорская плита)

The article presents results of the distribution of the isotopic composition of 13C and 18O (119 samples) across four facies zones to substantiate the conditions of Famennian carbonation accumulation. The Lower Famennian carbonates of the shoal zone showed values of 13C (1.5 ± 0.15 %), 18O (25.01 ± 0.29 %) reflecting evaporative processes in a shallow basin in a warm and dry climate. For the rocks of the microbial mounds zone, the values of 13C (2.47 ± 1.12 %) and 18O (23.51 ± 1.12 %) reflect an increase in bioproductivity and some fluctuation in water salinity. In the limestones of the zone of transition to depression, the variations of isotopic composition are more distinct in the section. Thus, the Early Famennian is characterized by the average values of 13C (0.94 ± 0.59 %) and 18O (23.73 ± 2.18 %) for marine carbonates. The transition to the Middle Famennian is accompanied by isotopic weighting of 13C (1.30 ± 0.47%) and 18O (24.52 ± 1.45%), which shows an increase in evaporation processes under aridization conditions. In the Late Famennian, the isotopic composition abruptly lightens by 13C (–1.05 ± 0.66 %), which is associated with desalination under conditions of climate humidification. At the same time, the value of 18O (25.75 ± 0.31 %) corresponds to the average values for carbonates of the Devonian. The values of 13C (1.04 ± 0.89 %) and 18O (26.01 ± 0.99 %) in the carbonates of the zone slope of the carbonate bank towards the shallow shelf reflect the conditions of the normal sea basin with a slight increase in the evaporation regime during climate aridization. The obtained results indicate that the Famennian sedimentation basin characterized by fluctuations in sea level, facies, water hydrochemistry and climate.

Ore genesis of the Tieshajie Cu deposit: Implications for Cu mineralization in the Qin-Hang Metallogenic Belt, South China

The Tieshajie Cu deposit, located in the northeastern part of the Qin-Hang Metallogenic Belt (QHMB), South China, has long been regarded as a representative Meso-Neoproterozoic volcanogenic massive sulfide (VMS) deposit. Here we present a hydrothermal titanite U-Pb age, Re-Os and in-situ S-Cu isotope data for chalcopyrite to constrain the timing and ore genesis of the Tieshajie deposit. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating of titanite from the disseminated Cu ore yielded a weighted mean 206Pb/238U age of 160.1 ± 4.4 Ma. Chalcopyrite from different ore types has low 187Os/188Os (0.85–3.60) and 187Re/188Os (46.1–614.0) ratios, combined with initial 187Os/188Os (0.74–2.00), excluding a mantle source. A Re-Os isochron age (188 ± 30 Ma) for five chalcopyrite samples is consistent with the titanite U-Pb age within errors. Moreover, the variations in Cu isotope compositions (δ65Cu: −1.13 to +0.12 ‰) and δ34S values (+3.8 to +7.7 ‰) of chalcopyrite are inconsistent with those reported from the ancient VMS deposits in previous studies. Therefore, our results are indicative of a Late Jurassic magmatic-hydrothermal origin instead of a VMS origin for the Tieshajie deposit. In combination with previous studies, we propose that the Tieshajie Cu deposit belongs to the distal part of the Mid-Late Jurassic (170–150 Ma) porphyry-skarn Cu mineralization event in the QHMB, likely triggered by the subduction of the Paleo-Pacific plate during the Late Mesozoic. This study also has new insights into the genesis of Cu mineralization in the QHMB and further provides implications for future exploration.

Long-lived shallow crustal magma reservoirs beneath Ulleung Island volcano (South Korea) and their rejuvenation recorded in zircon

The microanalytical data obtained from zircons collected on Ulleung Island, an emergent peak of a Quaternary alkaline magma system in the southwestern East Sea (Sea of Japan) back-arc basin, provide new insights into the origin of magma, magmatic processes, and the trigger of explosive eruptions. The U-Th ages of zircons extracted from a pumice layer formed during the intense Plinian eruption at ∼11 ka, known as the Ulleung-Oki tephra, show a wide range spanning the pre-eruption equilibrium state (>350 ka) up to the eruption event itself. Zircon grains derived from syenite and gabbro fragments entrained within the tephra yield weighted mean U-Th and U-Pb ages of 34.7 ± 2.3 ka and 1378 ± 22 ka, respectively. Under cathodoluminescence, zircons from the syenite reveal dark, featureless, or oscillatory-zoned cores, containing numerous inclusions of britholite-like Y-rich phases and pyrochlore. These cores are partially or entirely replaced by inclusion-free, brighter domains, suggesting that in situ dissolution and precipitation processes significantly influenced the transformation of zircon. The consistently low δ18O values (5.4–2.6‰) of zircons from Middle Pleistocene trachyte lavas, the Ulleung-Oki tephra, and lithic fragments indicate the persistent presence of shallow crustal magma reservoirs beneath Ulleung Island. These reservoirs have undergone substantial assimilation of hydrothermally altered rocks, with assimilation rates typically around 10–30% and reaching a maximum of approximately 50%. Zircon core-to-rim variations in oxygen isotopic composition, along with textural observations, suggest the rejuvenation of magma reservoirs through the injection of new melt, which likely served as a trigger for the explosive eruptions at Ulleung Island. The similarity in the range of εHf values between the core and rim (+7.4 to −4.3) implies the Hf isotopic consistency of the recharge melt.

New Insights on Micro‐Scale Variations of Geochemical and Oxygen Isotope Compositions in Conodont and Shark Tooth Bioapatite

To help understand bioapatite microstructures and related chemical variations, their impact on O‐isotope compositions measured and give insights on sample preparation, this study analysed conodonts and shark teeth prepared in different orientations through microanalytical and bulk sampling techniques: scanning electron microscopy (SEM); electron probe microanalysis (EPMA); continuous‐flow and high‐temperature reduction – isotope ratio mass spectrometry; and secondary ion mass spectrometry (SIMS). The SEM and EPMA measurements in conodonts allowed to distinguish the tissues commonly analysed by SIMS, which included albid and hyaline crowns but given their often small‐scale intergrowth, mixtures of these are difficult to avoid. In situ SIMS O‐isotope analyses provided different δ18O values: lower values with higher variance (16 ± 1‰ n = 13, 15.7 ± 1.9‰ n = 11) for mixed albid‐hyaline tissues, and higher, homogeneous values (17.1 ± 0.2‰, n = 13) for mainly hyaline tissues. Recent shark teeth δ18OSIMS value for dentine of the same tooth was 10‰ lower than the mean δ18OSIMS value for enameloid whereas the δ18OPO4 values measured for enameloid and dentine using the HTR method were identical. The variation of δ18O seems sensitive to analytical artefacts related to sample textures, caused during the sample preparation over more porous biomineral surfaces.

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.

Strontium and Neodymium Isotopic Signatures in Basalt Glasses of the Mid-Atlantic Ridge, 12°–31° N

This paper presents the new geochemical isotope (Sr–Nd system, H2O, Cl) data obtained for basalt glasses of the Mid-Atlantic Ridge (MAR) sampled from six areas of the MAR axial zone between 31° and 12° N. The data are consistent with the existing ideas about large-scale geochemical segmentation of the MAR. It is shown that samples from predominantly serpentinite segments have a narrower range of variations of strontium isotopic composition (87Sr/86Sr = 0.7027–0.7032) in comparison with samples collected from the areas where the crustal section is dominated by basalts (87Sr/86Sr = 0.7024–0.7041). The variation ranges of the neodymium isotopic composition in these two groups of samples are almost identical (εNd = +4.9 to +10.9 and +5.9 to +11.6 in serpentinite and basalt segments, respectively), although, in general, serpentinite segments have a slightly more enriched composition. The wide variations of the neodymium isotopic composition and increased contents of Cl, H2O, and U, as well as increased K2O/TiO2 and La/Sm ratios, in samples from serpentinites can most probably be related to the participation of different geochemically heterogeneous sources in the magmatism of the MAR axial zone. The influence of enriched plume-type matter cannot be excluded in some segments. The isotopic composition of noble gases may shed light on the subject.

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.

Variations in the Stable Carbon Isotopic Composition of Soil Organic Matter in Mountain Depressions of the Cis-Baikal Region

Variations in the Stable Carbon Isotopic Composition of Soil Organic Matter in Mountain Depressions of the Cis-Baikal Region

Petrogenesis of early Paleozoic granitoids in the North Qinling Orogen, Central China: Implications for crustal evolution in an accretionary orogen

Accretionary orogens are sites of extensive continental crustal growth and modification. The mechanism by which mafic crust is transformed into silicic melts (i.e., maturation of continental crust) is important for understanding the formation of the continental crust. The North Qinling Orogen (NQO) is a composite orogenic belt and contains an early Paleozoic accretion-dominated orogenic system, which is ideal for investigating continental crustal maturation. We obtained zircon and monazite U–Pb age and O isotope data, zircon Lu–Hf isotope data, and whole-rock major- and trace-element and Sr–Nd isotope data for early Paleozoic granitoids of the NQO. The granitoids are divided into three groups. Group 1 includes the Taiping tonalite (445 ± 3 Ma), the Manziying syenogranite (445 ± 2 Ma), and the Huoshenmiao granodiorite (436 ± 2 Ma). The Taiping and Huoshenmiao plutons have relatively high SiO2 contents (68.64–71.67 wt.%) and Na2O/K2O ratios (1.15–4.19), with enrichments in Rb, Ba, Th, and U and depletions in Nb, Ta, P, and Ti, and they are geochemically similar to sodic arc magmas. The Manziying syenogranite is a peraluminous potassic granite with high K2O contents (4.59–5.27 wt.%). Grantioids from Group 1 have similarly depleted Sr–Nd–Hf–O isotopic features (εHf[t] = +5.9 to +8.8; δ18O = 3.98‰–5.41‰), indicative of derivation via partial melting of oceanic arc crust, which suggests that partial melting of oceanic arc crust in a subduction system contributes to the generation of continental crust and causes its maturation. Group 2 consists of the Wuduoshan monzogranite (418 ± 2 Ma) and the Sikeshu granodiorite (423 ± 3 Ma). These plutons have relatively high SiO2 (65.59–72.06 wt.%), K2O (3.26–4.79 wt.%), and Al2O3 (14.65–16.12 wt.%) contents and Sr/Y (33–87) and (La/Yb)N (23–48) ratios. The Wuduoshan monzogranite has positive zircon εHf(t) (+0.4 to +3.1) and uniform δ18O (6.38‰–8.07‰) values, but the Sikeshu granodiorite has more variable isotopic compositions (εHf[t] = −1.9 to +5.0; δ18O = 6.37‰–10.60‰). The Wuduoshan monzogranite and Sikeshu granodiorite have similar whole-rock Sr–Nd isotopic compositions to basement rocks of the NQO. These features indicate that the two plutons formed by partial melting of basement rocks (i.e., subducted into the lower crust) of the North Qinling unit, along with juvenile crustal material. Group 3 is represented by the Xiaguan monzogranite, which formed at 434–430 Ma, and can be subdivided into heavy rare earth element (REE)-depleted and -enriched units. The former has high Sr/Y (56–98) and (La/Yb)N (34–73) ratios and low MgO (0.13–0.24 wt.%), Cr (0.37–1.69 ppm), and Ni (0.32–1.09 ppm) contents, similar to adakites derived from metabasaltic sources. The heavy REE-enriched nature of the Xiaguan monzogranite may reflect modification of its source by melt or fluid. Our results show that partial melting of enriched oceanic arc crust contributed to crustal maturation in an accretionary orogen. The addition of evolved crustal material also facilitated this process; therefore, the basement rocks and crustal thickness should be considered when assessing crustal dynamics in an accretionary orogen.

Spatial variation of water stable isotopes of multiple rivers in southeastern Qaidam Basin, northeast Qinghai-Tibetan Plateau: Insights into hydrologic cycle

River contains plenty of information on hydrological cycle. The water stable isotope approach is a powerful tool to unravel the hydrological processes such as river water sources, evaporation and river water-groundwater interaction. However, the spatial variation and influencing factors of stable isotope compositions in multiple rivers within a large basin and their indication to hydrological cycle have been less documented. This work reports on the spatial variation of stable water isotopic compositions of river water of four rivers and the underlying hydrological processes in the southeastern Qaidam Basin, northeast Qinghai-Tibetan Plateau, China. The results show that the δD and δ18O values of individual river vary along the river flow directions (generally from south to north), and that of multiple rivers also exhibit an obvious spatial variation, which is mainly influenced by evaporation, groundwater discharge, moisture source and transport pattern and local precipitation. As for an individual river, the δD and δ18O of river water are gradually enriched by evaporation along the river flow direction, which are rapidly depleted due to groundwater discharge into river in the groundwater overflow zone. For multiple rivers, the stable isotopic compositions of river water are gradually depleted from the west to the east of the study area (i.e. approximately perpendicular to rivers’ flow directions), which are mainly controlled by moisture source and transport pattern. This study helps to enhance our understanding of the hydrological cycle in a large basin through combined analysis of water stable isotopic data from individual river and multiple rivers. The data evaluation methods and results of this study could be useful in other areas to understand hydrological processes and moisture transport pattern.

Mixing plant residues of different quality reduces priming effect and contributes to soil carbon retention

Cellulose-rich rice straw (RS) decomposes readily in soils, with much of C lost as CO2 and only a small fraction stabilized as soil organic carbon (SOC). Additionally, the addition of high-energy containing residues may stimulate native SOC decomposition (priming effect, PE) and the high C/N ratio of RS may lead to soil N-mining. We hypothesized that mixing high-C/N ratio RS with low-C/N ratio groundnut stover (GN) would improve the chemical composition of the input, thereby stimulating microbial growth and hence increasing the retention of residue-derived C in the soil as well as decreasing PE. To prove this, we designed a 112-day incubation experiment with unamended 13C-enriched C4 soil, soil amended with single 13C-depleted residues of C3 plants, RS and GN, or their mixture. The partitioning of applied C and native soil C between CO2 and microbial biomass was performed using their differences in 13C abundance, while the uncertainties in our estimates of soil C fluxes were quantified by a sensitivity analysis of the two-end member isotopic model. From 42 days after incorporation, the mixture reduced CO2–C loss by 9–18% compared to either residue applied alone. Compared to RS, the mixture respired less CO2, inducing a 29% reduction in PE, and increased C retention as microbial biomass by 22% during the first 56 days due to the preferential utilization of residue C. This resulted in reduced native soil C and N mining during the later decomposition stages. According to the isotopic sensitivity analysis, our conclusions were valid despite variations in isotopic composition of the chemical components of the residues. However, this might not be the case if the δ13C values of preferentially used readily available components (e.g., cellulose) and/or of whole residues are very different.

Sr-ISOTOPIC COMPOSITION OF PALEOOCEAN WATER ON THE BORDER OF 960 Ma (DATA FOR NIZHNY TUNGUSKA FORMATION OF TURUKHAN UPLIFT OF SIBERIAN PLATFORM)

At the boundary of the Meso- and Neoproterozoic, Grenville collision events were widely manifested on the margins of ancient continental blocks. Most of the sedimentary sequences that had accumulated by that time had undergone significant thermal–metamorphic changes. In many ways, this is the main reason for the lack of isotopic data for carbonate deposits in the interval of 1200–900 Ma in world practice. Sr–isotopic composition in carbonate rocks with an age of 980–920 Ma was determined in the only section of the World – the Huainan Formation of the North China Platform with accumulation time determined by U–Pb dating of detrital zircon grains. In the upper part of the Nizhny Tunguska suite of the Turukhansk uplift, among carbonate deposits, we established the presence of altered volcanic rocks, as well as weathering crust products along them – poor bauxite and chamosite ores. Based on U–Pb zircon isotope dating, the age of the volcanic rocks is 964 Ma. For the least altered limestones of the upper part of the Nizhny Tungusska Formation, received 87Sr/86Sr values are 0.70532–0.70578, which are close enough to those found in the rocks of the Huainan Formation. Geochronological age of the studied limestones gives us more correct data. These data can be used to refine the previously proposed configuration of the Sr–isotopic composition variation curve in the Early Neoproterozoic. The Nizhny Tunguska Formation is the only carbonate section of the Early Neoproterozoic in the world with correctly geochronologically substantiated dating (based on zircon from subsynchronous volcanic rocks). This compares favorably with the Huainan formation.