Carbon Isotopic Composition Research Articles

Resetting of Shallow-Water Carbonate Boron Isotope Values During Marine Burial Diagenesis

The boron isotopic composition (δ11B) of bulk carbonates may provide an archive to reconstruct changes in ocean pH. Reconstructions from ancient carbonates typically assume that no significant resetting of δ11B occurred during marine burial diagenesis. However, our understanding of B isotopic behavior associated with this process remains limited. Here we provide measurements of B/Ca and B isotopic composition (δ11B) from a modern peri-platform carbonate sequence near the Great Bahama Bank that has undergone marine burial diagenesis. Our results reveal significant decreases in both δ11B (~13 ‰) and B/Ca (~80 %) of bulk carbonates with depth. We attribute this pattern to the release of isotopically light B (δ11B ~20 ‰) to porewater during aragonite dissolution, with uptake of substantially isotopically lighter borate ions (δ11B ~-1 ‰) from porewater by newly forming low-Mg calcite. A quantitative model adds further support for this interpretation and provides an estimate of average neomorphism rate ( k0) in the range of 1×10-6 to 5×10-6 yr-1, which is comparable to previous rate estimates for neomorphism and/or recrystallization during meteoric diagenesis. Our results demonstrate the strong potential for resetting δ11B signatures in bulk carbonates during recrystallization, which must be considered in future attempts to reconstruct pH and pCO2 from these records and may require reinterpretation of existing records. Our results also suggest the potential of B isotopes as a proxy for carbonate recrystallization/neomorphism and original carbonate mineralogy.

Establishing the global isoscape of leaf carbon in C3 plants through the integrations of remote sensing, carbon, geographic, and physiological information

The carbon isotope composition (δ13CLeaf) of C3 plant leaves provides valuable information on the carbon-water cycle of vegetation and their responses to climate change within terrestrial ecosystems. However, global applications of δ13CLeaf are hindered by a lack of global long-term spatial maps (isoscapes) that capture vegetation δ13CLeaf variations. The ways in which δ13CLeaf varies over time and across regions are still unknown. In this study, we collected leaf carbon isotope samples across the globe and selected the optimal predictive model from three machine learning algorithms to construct long-term annual global δ13CLeaf isoscapes at a spatial resolution of 0.05° for natural C3 plants between 2001 and 2020. We also assessed the potential of remotely sensed spectral bands, atmospheric CO2 characteristics, geographic, and physiological information to estimate the δ13CLeaf of the global C3 plants. Our results show that the random forest (RF) algorithm can more accurately construct the δ13CLeaf isoscape (R2 = 0.61, Nash Sutcliffe = 0.61, RMSE = 1.21‰, MAE = 0.91‰) than the multilayer perceptron (MLP) and support vector machine (SVM). The inclusion of atmospheric CO2 characteristics, geographical, and physiological information greatly improves prediction compared to relying only on spectral bands. Among the variables, elevation, band 3 spectral reflectance, and solar-induced chlorophyll fluorescence were the three most important variables for constructing the isoscape model, and their relative importance all exceeded 85%. The predicted isoscape revealed strong spatial heterogeneity of δ13CLeaf for C3 plants at a global scale between different continental regions, with enriched values occurring in high-altitude cold and arid regions, and depleted values occurring in warm, humid, or tropical regions. For the first time, we estimated the global depleted rate of δ13CLeaf in C3 plant (−0.0491 ± 0.07 ‰ year−1 from 2001 to 2020). Over the past two decades, 86.49% and 1.00% of global grid cells showed more depleted and more enriched trends in δ13CLeaf of C3 plants, respectively. Our results demonstrated the potential for establishing isoscapes by combining remote sensing, atmospheric CO2 characteristics, physiological, and geographic variables using the RF machine learning algorithm. The isoscape products generated in this study are valuable for assessing carbon-water coupling of terrestrial ecosystems and for improving land surface models.

Systematic characterization of the influence of petroleum spill on terpene metabolism in Suaeda salsa (L.) Pall from coastal wetland: Implication by compound-specific stable isotope

With the increasing industrialization and urbanization, the ecological environment is suffering from severe deterioration in Liaohe coastal wetland, and petroleum spill is one of the pollution sources. Suaeda salsa (L.) Pall (S. salsa), one of the predominant plants in Liaohe coastal wetland, is facing the increasing degradation. Terpenes are a class of inherent compounds in plants, and play key role in maintain the growth of plants. However, the environmental stress on the terpene metabolism remained unclear in the plants. In the present study, the influence of petroleum spill on terpene metabolism in S. salsa was systematically investigated by analysis of concentrations, compositions and stable carbon isotope. Under the stress of petroleum spill, terpene concentrations showed the decreasing trend, indicating the inhibition effect of petroleum spill on terpene synthesis in S. salsa. The proportions of Sabinene and A-humulene showed the obviously increased with the influence of petroleum spill, implying that these congeners were more sensitive to petroleum spills. The significant changes in stable carbon isotope compositions were observed for Borneol, Dl-menthol, A-humulene and (-) -@-bisabolol, with the enrichment in heavier isotopes in residual fractions. This result indicated that the heavier 13C was preferentially fixed on terpene by S. salsa under the petroleum stress. The similar change trends along the incubation time was observed for A-humulene and (-) - trans caryophyllene, which might imply that A-humulene was one of the products of (-) - trans caryophyllene in S. salsa. Overall, the findings of present study verified the influence of petroleum spill on terpene metabolism in S. salsa, and were meaningful for protecting the plants in the petroleum-pollution wetlands.

Indian summer monsoon variability during the past ~75 ka based on stable isotope records of sediments from NW India

Here we report continuous and chronologically well-constrained records of stable oxygen (δ18O) and carbon (δ13C) isotope compositions of carbonate nodules (nodular calcretes), and δ13C of sediment organic matters (δ13CSOM) in samples from two (~48 m deep) drill-sediment cores, collected from a paleo-Yamuna channel in the NW Indo-Gangetic Plain in north Haryana. The results were used to reconstruct Indian summer monsoon (ISM) precipitation variability and paleovegetation pattern during the past ~75 ka and present a longer record of ISM variability in the Indo-Gangetic Plain. The δ18O of carbonate nodules (−7.80‰ to −4.04‰, average −6.01‰) shows a good negative covariation with variability in the model-derived intensity of ISM precipitation, indicating it to be a good proxy for ISM variability. We infer intense ISM precipitation during late marine isotope stage (MIS) 5, early and late MIS 3, and early MIS 1 periods, marked by relatively lower δ18O of carbonates. These changes in the ISM intensity are in-phase with the precession-induced changes in the Northern Hemisphere summer insolation. The δ13CSOM (−27.4‰ to −22.3‰, average −25.3‰) indicates C3-dominated vegetation in the Himalayan catchment with a noticeable increase in δ13CSOM during late MIS 4 to early MIS 3, late MIS 3, and early MIS 1 periods, which are characterized by the intense ISM (warm and moist condition). This suggests an increased abundance of C4 plants during the periods of intensified ISM precipitation. The δ13C of carbonate nodules varies from −3.63‰ to 1.56‰ (average −1.55‰). The enrichment in 13C of the carbonates is most likely due to the influence of dissolved inorganic carbon derived from the Himalayan source and/or atmospheric CO2 on the carbonate nodules. Therefore, we suggest that the δ13C of carbonate nodules from the Indo-Gangetic Plain requires a careful evaluation.

Degradation of peat-forming plants at the first stages of decomposition in natural and post-pyrogenic peatlands of West Siberia

The aim of the study. Assessment of the phytomass stocks and decomposition rate of peat-forming plants’ residues in peat deposits of natural and post-pyrogenic peatlands at the initial stages of decomposition. Location and time of the study. The study was carried out in 2022 (May-September) at two oligotrophic bog sites: “Bakcharskoye” (field station “Vasyuganye”, IMCES SB RAS) and “Iksinskoye”, which are located in the north eastern spurs of the Great Vasyugan mire in the Bakcharsky and Shegarsky districts of the Tomsk region, Russia. Methods. The phytomass stocks were determined by the cutting method. The following fractions were separated: living phytomass (annual and perennial photosynthetic phytomass, i.e. green parts of grasses, shrubs leaves, mosses); perennial non-photosynthetic phytomass, i.e. shrubs stems, roots of grasses and shrubs; and dead phytomass (mortmass), i.e. litter, moss litter, standing dead phytomass. The decomposition rate of plant residues was determined by the method of litter bags placement in peat (Chamaedaphne calyculata, Sphagnum fuscum, Mixed sample consisting of 60% S. fuscum and 40% Ch. calyculata). In the initial and decomposed samples, the ash content was determined by the dry combustion method. The total carbon and nitrogen content, as well as the carbon and nitrogen isotopic composition, were determined using a DELTA V Advantage isotope ratio mass spectrometer. Results. This short-term study conducted during the growing season in conditions of undisturbed (Natural Ryam) and post-pyrogenic (Gar) phytocenoses showed that due to a significant amount of mortmass (2402 g/m2) Natural Ryam had organic matter stock exceeding that in the Gar by 1.7 times. On average, 59% of organic matter loss from the total losses during the growing season occurred in the first month. In both Gar and Natural Ryam conditions, minimal losses of organic matter were characteristic for Sphagnum fuscum (3.1 and 3.5% respectively). For the remaining samples the Gar conditions were more favorable for the degradation at the initial stages of decomposition. The Mixed sample occupied an intermediate position between its individual components in terms of mass losses. Carbon losses correlated well with mass losses both over 1 month of degradation (r=0.87) and over 4 months (r=0.96). In contrast to carbon, nitrogen accumulated in the Mixed sample at the initial stages of degradation, both after 1 month and 4 months. Conclusions. With smaller phytomass stocks in areas of post-pyrogenic succession, more active decay at the initial stages of organic matter decomposition occurs exactly under these conditions. The impact of post-pyrogenic succession on the rate of organic matter decomposition of Chamaedaphne calyculata plant residues and the Mixed sample was demonstrated. The most intensive decomposition of litter occured during the first month of degradation. Additionally, in the conditions of a burnt bog, a more intense release of nitrogen from all plant residues and accumulation of ash elements in Sphagnum fuscum samples were observed. The mixing of litter components influenced both the rate and dynamics of decomposition. The carbon and nitrogen isotopic composition began to change during the initial stages of decomposition, leading to enrichment with heavier isotopes 15N and 13C. This research emphasizes the importance of studying the processes of plant litter decomposition at the initial stages and considering the plant residue composition when studying the processes of organic matter transformation.

Spectroscopic techniques to analyze stable carbon isotopic compositions of carbon dioxide, methane and volatile organic compounds

Spectroscopic techniques have been increasingly applied to determine stable carbon isotopic compositions of different compounds.

Geochemistry of Crude Oils from the Bazhenov Horizon (West Siberia)

Abstract —The Bazhenov horizon is the main source rock stratum of the West Siberian petroleum province, which possesses significant resources of hard-to-recover hydrocarbons, making it a unique exploration target. The study summarizes geochemical data on crude oils (39 samples) collected from pools within the Bazhenov and Tutleima formations. A comparative analysis of the carbon isotope composition and geochemical parameters calculated from the composition of the identified saturated and aromatic compounds (pristane/phytane, C29/C27 steranes, C35/C34 homohopanes, tricyclane index, relative concentrations of dibenzothiophenes) confirm a common aquatic genotype of the studied oils and their genetic affinity to the organic matter of the same age. At the same time, significant variations in the main physical and chemical characteristics of the studied Bazhenov oils may be caused their fractionation during migration processes.

РАСПРЕДЕЛЕНИЕ И ГЕНЕЗИС УГЛЕВОДОРОДНЫХ ГАЗОВ ДОННЫХ ОТЛОЖЕНИЙ ЛАПТЕВО-СИБИРОМОРСКОЙ ЗОНЫ ВОСТОЧНО-АРКТИЧЕСКОГО ШЕЛЬФА

Gases from bottom sediments of the Laptev–Siberian Sea zone contain methane (up to 8.3047 cm3/kg) and its homologues (down to and including pentane) totaling 0.0259 cm3/kg. The established values of the carbon isotope composition of methane (δ13С), the molecular weight of the hydrocarbon fraction, the weight concentrations of С1–С5 hydrocarbons and their ratios, the «wetness» and «transformation» coefficients of the hydrocarbons fraction, indicate the presence of various syngenetic and epigenetic gases in bottom sediments. Potential gas sources are modern sediments, peat, coal, gas and gas hydrate? deposits, solid bitumen, igneous rocks, and inferred oil and gas accumulations. It has been established that the distribution of hydrocarbon gases in sediments vertically and horizontally depends on the influence of a complex of geological factors – fold and fault tectonics, magmatism and seismic activity in the region, coal, oil and gas potential and bituminosity, organic saturation and lithological composition, hydrogeological, geocryological, and other conditions for accumulation of gases or their degassing.

Modeling position specific carbon isotopologue fractionation of thermogenic propane and precursors

<p>Position specific isotope analysis (PSIA) of thermogenic propane allows to track carbon isotopic compositions at different molecular positions, and thus providing new evidence to investigate propane��s origin, fate, and mechanisms of formation. However, the link between observed ��<sup>13</sup>C PSIA signals of propane and carbon isotopologue signatures of precursors in source organics still remains unclear, and understanding the underlying mechanisms requires a more sophisticated model. Here we developed a mathematical framework to simulate position specific carbon isotopologues of propane and its precursors based on mechanistic understanding of thermogenic propane��s bond-cleavage pathways. Besides, our model also allows integrating multiple signals including temperature, and isotopic characteristics of source compounds. Our model is validated by precisely reproducing propane��s experimental PSIA data obtained during cracking of different kerogens, and also correctly quantified the initial carbon isotopic signatures and the initial fraction of the different precursors in the source materials. Our model allows to include more complex reaction mechanisms to elucidate unknown reaction pathways, and could also guide and optimize future experimental studies to test different hypothesis.</p>

New radiocarbon age constraints on the eruption history of the Quill volcano, St. Eustatius, Dutch Caribbean

Abstract The late Pleistocene to Holocene subaerial pyroclastic deposits of the Quill stratovolcano on the Caribbean island of St Eustatius form seven stratigraphic divisions. New radiocarbon ages of charcoal are presented for the second, third and seventh divisions in order to better constrain the Quill’s eruption history. Three samples from the same layer of Division 2 at two localities on the northeast coast yield ages of 18,020 ± 40 (1σ), 18,310 ± 45 and 18,490 ± 45 14C yr BP (∼19,800–20,600 yr cal BC). These are considerably younger (∼4400 yr) than a previously published result for this division. A single sample of Division 3 gave an age of 8090 14C yr BP (∼7100 yr cal BC) and overlaps with previously published 14C ages for this division. A charred root in the pyroclastic unit deposited by the last eruption of the Quill (Division 7) gave an age of 919 14C yr BP (∼1100–1200 yr cal AD). This result is ∼600 years younger than a previously published age, and its origin is attributed to human activity. The timing of the last eruption of the Quill therefore remains poorly constrained but is older than 600 AD. Terrestrial gastropods found in paleosols and organic material found in small streams that developed in Division 3 indicate that Division 4 must be younger than 6100 ± 500 yr cal BC. The oxygen and carbon isotope composition of the terrestrial gastropods derived from Division 3 paleosols indicates that the C4 and CAM-type vegetation was dominant and that the climate subsequently changed to wetter conditions. The minimum eruption frequency for the Quill volcano is one eruption every ∼1400 years during the past 22,000 years. This eruption frequency of the Quill volcano is of the same order of magnitude as other recent northern Lesser Antilles volcanoes, Soufrière Hills (Montserrat, ∼5000 years) and Mt Liamuiga (St. Kitts, ∼2500 years).

Modeling position specific carbon isotopologue fractionation of thermogenic propane and precursors

<p>Position specific isotope analysis (PSIA) of thermogenic propane allows to track carbon isotopic compositions at different molecular positions, and thus providing new evidence to investigate propane��s origin, fate, and mechanisms of formation. However, the link between observed ��<sup>13</sup>C PSIA signals of propane and carbon isotopologue signatures of precursors in source organics still remains unclear, and understanding the underlying mechanisms requires a more sophisticated model. Here we developed a mathematical framework to simulate position specific carbon isotopologues of propane and its precursors based on mechanistic understanding of thermogenic propane��s bond-cleavage pathways. Besides, our model also allows integrates multiple signals including temperature, and isotopic characteristics of source compounds. Our model is validated by precisely reproducing propane��s PSIA experimental data obtained during cracking of different kerogens, and also correctly quantified the initial carbon isotopic signatures and the initial fraction of the different precursors in the source materials. Our model allows to include more complex reaction mechanisms to elucidate and unknown reaction pathways, and could also guide and optimize future experimental studies to test different hypothesis.</p>

The Possible Use of Stable Carbon and Nitrogen Isotope Signal and Spectral Analysis to Identify Habitat Condition of Aquatic Plants

Many macrophyte species exhibit a high degree of plasticity, enabling them to thrive in various aquatic ecosystems. Identifying the growth conditions of individual aquatic plant species during research or specimen collection is not always possible. In many cases, the nature of the planned research does not necessitate recognizing environmental conditions. However, the scope of identifying the habitat parameters of the collections of submerged aquatic plant herbariums provides an opportunity for further research. This paper explores the possibilities of using isotopic signals of plants, supported by spectral analyses of powdered plant materials, to ascertain the environmental conditions from which the samples were collected. The results obtained from the stable carbon and nitrogen isotope compositions (δ13CORG and δ15NORG) and the analysis of spectral spectra via FTIR-ART (Fourier Transform Infrared Spectroscopy with Attenuated Total Reflectance) of plant material (Elodea canadensis Michx. species) collected from various habitat ecosystems, including rivers and both hard- and softwater lakes, exhibited significant distinctions between these habitats. Particularly high values of δ15NORG were recorded in the material from rivers. The stable carbon and nitrogen isotope compositions did not differentiate between the material collected from softwater and hardwater lakes. Nevertheless, when comparing the isotopic findings with the FTIR-ATR spectral analysis focused on identifying characteristic peaks associated with the presence of calcium carbonate, noticeable differences were observed in the presence and intensity of calcium carbonate peaks in the material. These differences were only evident when nondecarbonated plant material from hardwater lakes was used for the FTIR-ATR analysis. To the best of the author’s knowledge, the combination of methods applied in this study to identify the origin of E. canadensis from various freshwater environments is the first application of its kind that could enable the rapid identification of plant material origin. Such identification could prove useful in environmental, ecological, and paleoenvironmental research. The increased knowledge of macrophytes’ δ13CORG and δ15NORG values might also be essential in further tracking accelerated eutrophication based on aquatic vegetation’s isotopic signals. This might be important due to the assumption that the increased rate of eutrophication influences organic matter sedimentation in aquatic ecosystems, especially lakes.

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.

Genesis of the Large-Scale Kamado Magnesite Deposit on the Tibetan Plateau

Lacustrine strata-bound magnesite deposits associated with Alpine-type ultramafic rocks are hydrothermal in origin. The magnesite ores of the Kamado deposit are unconformably underlain by mid-Jurassic marine carbonate and ultramafic rocks of the Bangong-Nujiang ophiolite suite and are in fault contact with hanging wall rocks composed of siliceous sinter. Three types of cryptocrystalline magnesite ores can be identified in Kamado: (1) strata-bound massive magnesites, representing the main ore type in the upper part; (2) banded ores in the lower part; and (3) some vein and stockwork ore in the ultramafic wall rocks. Integrated scanning electron microscopy, C–O isotope analysis, and geochemical analyses were carried out on the Kamado deposit. The results indicate that: (1) the orebody is composed of magnesite, with accessory minerals of aragonite, opal, and chromite; (2) the siliceous sinter and relatively high B (32.0–68.1 ppm) and Li (14.7–23.4 ppm) contents of the magnesite ores reflect long-term spring activity in Kamado; (3) the light carbon (δ13CV-PDB: −4.7 ± 0.3‰ to −4.1 ± 0.6‰) and oxygen isotopic compositions (δ18OV-SMOW: +12.3 ± 0.3 to +16.3 ± 0.1‰) of the stockwork ores in the foot wall rocks indicated that the carbon in fractures in the ultramafic rocks is from a mixture of marine carbonate and oxidized organic-rich sedimentary rocks, reflecting a typical “Kraubath-type” magnesite deposit; and (4) the relatively heavy carbon isotopic (δ13CV-PDB: +8.7 ± 0.4‰ to +8.8 ± 0.3‰) composition of the banded magnesite ores in the lower segment may have formed from heavy CO2 generated by anaerobic fermentation in the lakebed. Additionally, the carbon isotopic (δ13CV-PDB: +7.3 ± 0.3‰ to +7.7 ± 0.7‰) composition of the massive magnesite ores in the upper segment indicates a decline in the participation of anaerobic fermentation. As this economically valuable deposit is of the strata-bound massive ore type, Kamado can be classified as a lacustrine hydrothermal-sedimentary magnesite deposit, formed by continuous spring activities under salt lakes on the Tibetan Plateau, with the Mg mainly being contributed by nearby ultramafic rocks and the carbon mainly being sourced from atmosphere-lake water exchange, with minor amounts from marine carbonate strata.

Carbon isotopic composition of solid carbon and C–H–O fluid produced through the decomposition of stearic acid under HP-HT condition

ABSTRACT We estimated δ13C of the C–H–O fluid produced by the decomposition of stearic acid at high-pressure and high-temperature conditions (10 and 17 GPa, 800–1600°C) from δ13C of the solid carbon products and the initial stearic acid. The δ13C of the stearic acid and the recovered graphite/diamond were −28.0‰ and −27.08 to −29.35‰, respectively. Then, the δ13C of the coexisting C–H–O fluid was estimated to be −26.3 to −29.2‰ based on the stoichiometry of the decomposition reaction C18H36O2 → 10C + 8CH4 + 2H2O. In the studied P-T-t conditions, carbon isotope fractionation through the stearic acid decomposition is as small as < 3‰. This means that the δ13C of the C–H–O fluid is almost identical to that of the initial stearic acid and can be a useful tracer of carbon source for diamond forming reaction such as in fluid/carbonate systems.

The world-class carbonate-hosted Fankou Zn-Pb deposit in China. Part II. Alterations: C-O isotopic halos footprint the nature and transfer pathway of ore-fluids, and quantitative water-rock interaction process

The giant Fankou Zn-Pb deposit (more than 10 Mt Zn + Pb metals, grading at 15 %), situated in northern margin of the Quren Basin, is one of the largest Zn-Pb deposits in China. The stratiform orebodies are jointly bounded by both the strata (in the specific layers of Middle to Upper Devonian Donggangling and Tianziling carbonate sequences) and the fault system (a three-order thrusting fault system). Ore textures of carbonate dissolution, replacement-infilling and cementation are widespread in Fankou, analogous to a MVT (Mississippi Valley-type) deposit. Owing to the poorly developed fluid inclusions, the nature of the Fankou ore fluids remains indeterminate. To address this problem, we herein utilize micro-petrography, carbon-oxygen (C-O) isotopes and quantitative isotopic exchange modelling to trace the ore-fluids and water-rock interaction process. A total of ninety-three (93) carbonate samples, covering the majorities of stratigraphic horizons, planar locations, and various alteration intensities, were systematically collected. Micro-petrography reveals the primitively sedimentary bioclastic limestone suffer from heterogeneous dolomitization and carbonatization followed by massive Zn-Pb precipitation. These carbonate samples display a widely-ranged, δ18O- and δ13C-depleted isotope signature (δ18O = 12.57 to 23.82 ‰, δ13C = −7.79 to −0.46 ‰), relative to normal marine carbonates (δ18O = 25 ‰, δ13C = 0 ‰). Further modelling of isotopic exchange indicates a low-temperature (100 to 250 °C), δ18O-depleted (−3 to 7 ‰) and δ13C-depleted (−10 to −7‰) ore fluid equilibrated with the host rock by a relatively low water-rock ratio (W/R = 1 to 10). In terms of space, we identified a relatively decreasing trend of δ18O and δ13C isotopic compositions of carbonates, from southeast to northwest, and from bottom to up. Based upon the previous structural analysis, we interpret that the δ18O and δ13C isotopic distribution patterns are caused by a northwestwards transfer pathway of ore-fluids along the thrusting fault system, and the metalliferous, δ18O- and δ13C-depleted fluids are sourced from the interior of the Quren Basin. Collectively, we clarify the deep southeastern part of the Fankou Zn-Pb deposit and the interiors of the Quren Basin is the favorable exploration target in future.

Trophic characteristics of pelagic and bottom nekton in certain water masses in the western Chukchi Sea, by δ15N and δ13C data

To compare the trophic characteristics of pelagic- and bottom-dwelling fish in the western Chukchi Sea, isotopic composition of carbon and nitrogen (δ13С and δ15N) is considered separately for the Subarctic water mass (invaded from the Bering Sea) and local Arctic water mass. In summer of 2018 and 2019, the proper water mass occupied mostly the area southward from 71оN and the latter one – northward from this latitude. The isotopic composition of both pelagic and bottom nekton in the southern area was characterized by higher values of δ13C (up to 2 % higher) and low values of δ15N (up to 1 % lower) in comparison with those in the northern area. That was interpreted that in the southern area, within the water from the Bering Sea, more than 90 % of the sampled bottom fish species were linked with prey from the bottom community. In the northern area, within the Arctic water, all samples of bottom fish were characterized by minimal content of heavy carbon δ13C isotope, comparable with the values in samples of pelagic fish that indicated a significant role of pelagic prey in nutrition of the bottom nekton.

Source identification of sedimentary organic carbon in coastal wetlands of the western Bohai Sea

Coastal wetlands play a vital role in mitigating climate change, yet the characteristics of buried organic carbon (OC) and carbon cycling are limited due to difficulties in assessing the composition of OC from different sources (allochthonous vs. autochthonous). In this study, we analyzed the total organic carbon (TOC) to total nitrogen (TN) ratio (C/N), stable carbon isotope (δ13C) composition, and n-alkane content to distinguish different sources of OC in the surface sediments of the coastal wetlands on the western coast of the Bohai Sea. The coupling of the C/N ratio with δ13C and n-alkane biomarkers has been proved to be an effective tool for revealing OC sources. The three end-member Bayesian mixing model based on coupling C/N ratios with δ13C showed that the sedimentary OC was dominated by the contribution of terrestrial particulate organic matter (POM), followed by freshwater algae and marine phytoplankton, with relative contributions of 47 ± 21 %, 41 ± 18 % and 12 ± 17 %, respectively. The relative contributions of terrestrial plants, aquatic macrophytes and marine phytoplankton assessed by n-alkanes were 56 ± 8 %, 35 ± 9 % and 9 ± 5 % in the study area, respectively. The relatively high salinity levels and strong hydrodynamic conditions of the Beidagang Reservoir led to higher terrestrial plants source and lower aquatic macrophytes source than these of Qilihai Reservoir based on the assessment of n-alkanes. Both methods showed that sedimentary OC was mainly derived from terrestrial sources (plant-dominated), suggesting that vegetation plays a crucial role in storing carbon in coastal wetlands, thus, the coastal vegetation management needs to be strengthened in the future. Our findings provide insights into the origins and dynamics of OC in coastal wetlands on the western coast of the Bohai Sea and a significant scientific basis for future monitoring of the blue carbon budget balance in coastal wetlands.

Land use determines the composition and stability of organic carbon in earthworm casts under tropical conditions

Environmental conditions play an important role in controlling the fate of organic carbon (OC) in earthworm casts. However, the mechanisms that lead to the destabilization of OC in casts deposited in different land use systems are poorly understood. In this study, we investigated the impact of land use on the fate and composition of particulate organic matter (POM) and mineral-associated organic matter (MAOM) in earthworm casts under tropical conditions. We conducted a 400-day field exposure experiment in a woodland and a meadow in northern Vietnam with earthworm casts and soil aggregates without trace of earthworm activity (reference soil) under natural rainfall conditions. We analyzed the element content, stable carbon isotope composition and mid-infrared signatures of the two types of materials after 9 and 400 days of field exposure. The results showed that the casts were initially enriched in OC as compared to the reference soils, with the MAOM fraction accounting for over 90% of the earthworm-induced OC accumulation. The POM fraction occluded in casts consisted of fresh plant material and disappeared during the 400 days of field exposure. Enrichment and composition of OC differed between woodland and meadow casts. POM and MAOM showed contrasting persistence in the two land use systems. While woodland casts showed the highest potential to stabilize OC, the actual amount of cast OC stabilized for more than 400 days was more important under meadow. The processes affecting cast OC dynamics were contrasting in both systems. Under woodland, cast OC destabilization was most probably related to microbial degradation, while under meadow, OC accumulation was most probably related to root activity. Our study highlights that the impact of earthworms on the origin, composition, and fate of cast OC in tropical environments is strongly influenced by land use.

Mechanism of carbonate assimilation by intraplate basaltic magma and liquid immiscibility: example of Wangtian’e volcano (Changbaishan volcanic area, NE China)

The balance of CO2 during abundant basaltic magma production is an important factor of volcanic hazards and climate. In particular, this can be explored based on CO2-rich mantle-derived magmas or carbonate assimilation by basaltic melts. To reconstruct the origin of Fe-rich carbonates hosted by Cenozoic basalts from Wangtian’e volcano (northeast China), we studied elemental compositions of melt, crystalline and fluid inclusions in magmatic minerals as well as the oxygen and carbon isotope compositions of the plagioclase and carbonates from basalts. The crystallization of basaltic magmas occurred in shallow chamber (∼4 km) at temperatures of 1,180°C–1,200°C and a pressure of 0.1 ± 0.01 GPa. Stable Fe-rich carbonates occur in the Wangtian’e tholeiite basalts as groundmass minerals, crystalline inclusions in plagioclase and globules in melt inclusions, which suggests that they crystallized from a ferrocarbonate melt. The values of δ18О and δ13С in the minerals analyzed by laser fluorination method are in line with the sedimentary source of Fe-rich carbonates, indicating assimilation and partial decomposition of carbonate phases. The parent ferrocarbonate melt could be produced during interactions between the basaltic magma and the crustal marbles. The phase diagram and thermodynamic calculations show that the ferrocarbonate melt is stable at a temperature of 1,200°C and a pressure of 0.1 GPa. Our thermodynamic calculations show that carbonate melt containing 73 wt% FeCO3, 24 wt% MgCO3 and 3 wt% CaCO3 is in thermodynamic equilibrium with silicate melt in agreement with our natural observations. The proposed mechanism is crustal carbonate sediment assimilation by the intraplate basaltic magma resulting in the melt immiscibility, production of the ferrocarbonate melt and the following Fe-rich carbonate mineral crystallization during magma residence and cooling.