Isotopic Equilibrium Research Articles

Decoupled whole-rock and zircon Hf isotopes in young evolved post-collisional lavas from Dayingshan (SE Tibet): Evidence for open-system magmatic processes

Young zircons from volcanic rocks provide ideal samples for evaluating whole-rock and zircon Hf isotope equilibrium. We report zircon and whole-rock Hf isotopic compositions for Holocene post-collisional lavas (trachyandesites and trachytes) from Dayingshan volcano, SE Tibetan Plateau. Individual zircon εHf values differ from the host whole-rock value by up to 7.4ε units, and average zircon εHf values in individual samples are about 3ε units lower than the corresponding whole-rock εHf values, indicating discrepancy of zircon and whole-rock Hf isotopes. δ18O values for Dayingshan zircons vary from 6.1 to 7.8‰ and are negatively correlated with zircon εHf values. The decoupling of zircon/whole-rock Hf isotopic compositions is attributed to open-system magmatic processes rather than disequilibrium partial melting, as indicated by the negative correlation of zircon Hf-O isotopic compositions, as well as the correlations between whole-rock SiO2 content and Nd-Sr-Hf isotope ratios. The decoupling of zircon/whole-rock Hf isotopic compositions in Dayingshan lavas suggest that care must be taken when using individual zircon εHf values to study crust-mantle evolution. This Hf isotope decoupling might be more common than previously thought in continental post-collisional volcanic rocks where open-system magmatic processes are common.

Calibration of the dual clumped isotope thermometer for carbonates

The Δ47 (paleo)thermometer has opened a new avenue to determine carbonate formation temperatures independent of the oxygen isotopic composition of the fluid from which the carbonate crystallized. A major limitation of this thermometer is related to kinetic effects if homogeneous isotopic equilibrium is not attained during carbonate precipitation. Dual clumped isotope thermometry – the high-precision analysis of Δ48 along with Δ47 in CO2 evolved from phosphoric acid digestion of carbonates – makes it possible to resolve temperature from the kinetic information recorded in an individual carbonate phase. Therefore, it provides a new opportunity to identify (bio)mineralization pathways and to determine carbonate formation temperatures devoid of a kinetic bias, based solely on isotopic analysis of a single carbonate phase.Identification of the nature and extent of kinetic effects as well as the reconstruction of accurate formation temperatures requires knowledge of the position of equilibrium in Δ47 vs Δ48 space. Here, we present Δ47 and Δ48 data of carbonates that were previously considered as having crystallized closest to equilibrium in a temperature range of 8 to 1100 °C. Across this range, the temperature dependences of Δ47 and Δ48 are best expressed by the following fourth order polynomials of 1/T:Δ47 (CDES 90) (‰) = 1.038 (−5.897 1/T − 3.521 103 1/T2 + 2.391 107 1/T3 − 3.541 109 1/T4) + 0.1856Δ48 (CDES 90) (‰) = 1.028 (6.002 1/T − 1.299 104 1/T2 + 8.996 106 1/T3 − 7.423 108 1/T4) + 0.1245with CDES 90 representing the Carbon Dioxide Equilibrium Scale at a reaction temperature of 90 °C. In its entire temperature range, our Δ47 (CDES 90) - T - relationship agrees within 2 ppm with two previous Δ47 (I-CDES) - T - relationships reported by Jautzy et al. (2020) and Anderson et al. (2021). Accuracy of our proposed Δ47 (CDES 90) − Δ48 (CDES 90) equilibrium relationship is independently confirmed by additional dual clumped isotope data of experimental and geothermal carbonates which precipitated from potentially equilibrated dissolved inorganic carbon pools at a temperature range of 25–100 °C. Furthermore, we reprocessed original dual clumped isotope data of natural carbonates (Bajnai et al., 2020) and compared their composition to the position of equilibrium in Δ47 vs Δ48 space. These results corroborate preliminary evidence that the hydration/hydroxylation reactions became rate-limiting during the calcification of a speleothem-like sample, a warm water coral, a cold water coral and a brachiopod, finally evoking significant departures of carbonate-Δ47 and -Δ48 from dual clumped isotope equilibrium.An anti-clumped Δ48 value of −419 (±16) ppm (95% confidence interval level) is obtained for a technical calcite that was precipitated by the injection of CO2 into a Ca(OH)2-saturated solution. Its negative Δ48 value largely arises from a combinatorial effect, i.e. the carbonate oxygen derives from two sources with different bulk isotopic compositions. Besides the identification of the nature and the extent of (bio)mineralization kinetics and the reconstruction of carbonate formation temperatures unbiased by kinetics, dual clumped isotope analysis, therefore, allows tracing the isotopic heterogeneity of oxygen pools contributing to carbonate formation.

Effects of Ozone Isotopologue Formation on the Clumped‐Isotope Composition of Atmospheric O2

AbstractTropospheric 18O18O is an emerging proxy for past tropospheric ozone and free‐tropospheric temperatures. The basis of these applications is the idea that isotope‐exchange reactions in the atmosphere drive 18O18O abundances toward isotopic equilibrium. However, previous work used an offline box‐model framework to explain the 18O18O budget, approximating the interplay of atmospheric chemistry and transport. This approach, while convenient, has poorly characterized uncertainties. To investigate these uncertainties, and to broaden the applicability of the 18O18O proxy, we developed a scheme to simulate atmospheric 18O18O abundances (quantified as ∆36 values) online within the GEOS‐Chem chemical transport model. These results are compared to both new and previously published atmospheric observations from the surface to 33 km. Simulations using a simplified O2 isotopic equilibration scheme within GEOS‐Chem show quantitative agreement with measurements only in the middle stratosphere; modeled ∆36 values are too high elsewhere. Investigations using a comprehensive model of the O‐O2‐O3 isotopic photochemical system and proof‐of‐principle experiments suggest that the simple equilibration scheme omits an important pressure dependence to ∆36 values: the anomalously efficient titration of 18O18O to form ozone. Incorporating these effects into the online ∆36 calculation scheme in GEOS‐Chem yields quantitative agreement for all available observations. While this previously unidentified bias affects the atmospheric budget of 18O18O in O2, the modeled change in the mean tropospheric ∆36 value since 1850 CE is only slightly altered; it is still quantitatively consistent with the ice‐core ∆36 record, implying that the tropospheric ozone burden increased less than 40% over the twentieth century.

Ore Genesis of the Takatori Tungsten–Quartz Vein Deposit, Japan: Chemical and Isotopic Evidence

The Takatori hypothermal tin–tungsten vein deposit is composed of wolframite-bearing quartz veins with minor cassiterite, chalcopyrite, pyrite, and lithium-bearing muscovite and sericite. Several wolframite rims show replacement textures, which are assumed to form by iron replacement with manganese postdating the wolframite precipitation. Lithium isotope ratios (δ7Li) of Li-bearing muscovite from the Takatori veins range from −3.1‰ to −2.1‰, and such Li-bearing muscovites are proven to occur at the early stage of mineralization. Fine-grained sericite with lower Li content shows relatively higher δ7Li values, and might have precipitated after the main ore forming event. The maximum oxygen isotope equilibrium temperature of quartz–muscovite pairs is 460 °C, and it is inferred that the fluids might be in equilibrium with ilmenite series granitic rocks. Oxygen isotope ratios (δ18O) of the Takatori ore-forming fluid range from +10‰ to +8‰. The δ18O values of the fluid decreased with decreasing temperature probably because the fluid was mixed with surrounding pore water and meteoric water. The formation pressure for the Takatori deposit is calculated to be 160 MPa on the basis of the difference between the pressure-independent oxygen isotope equilibrium temperature and pressure-dependent homogenization fluid inclusions temperature. The ore-formation depth is calculated to be around 6 km. These lines of evidence suggest that a granitic magma beneath the deposit played a crucial role in the Takatori deposit formation.

High temperature generation and equilibration of methane in terrestrial geothermal systems: Evidence from clumped isotopologues

Fluids emanating from geothermal areas contain trace quantities of methane and other simple hydrocarbons. These hydrocarbons are thought to derive from thermal cracking of organic matter dissolved in circulating meteoric or seawater or found in pre-existing organic-rich sedimentary rocks, but an abiotic origin has also been proposed. We measured the relative abundances of four CH4 isotopologues (12CH4, 13CH4, 12CH3D, and 13CH3D) in hydrothermal gases discharged by steam vents and geothermal wells from Iceland and Nisyros island (Greece) in order to investigate the origin of methane. Measured methane samples yielded consistently low Δ13CH3D values (13CH3D abundance relative to stochastic) of 0.82–1.77‰, which correspond to high apparent temperatures of isotopologue equilibrium (TΔ13CH3D = 278–490 °C). Hydrothermal well fluids from the Krafla and Námafjall geothermal fields in Iceland yielded the lowest Δ13CH3D values, and thus the highest Δ13CH3D-based temperatures averaging 438-45+55 °C. Those samples also show the most pronounced departures in δDCH4 and δ13CCH4 values expected for isotopic equilibrium with respect to δDH2O and δ13CCO2. In contrast, CH4 samples from natural steam vents in other Iceland locations and in Nisyros have slightly higher Δ13CH3D values (with TΔ13CH3D=351-35+42 °C) and have δDCH4 and δ13CCH4 values that are consistent with those expected for isotopic equilibrium with both H2O and CO2. The short fluid residence times (1–50 years) in systems that are exploited for geothermal energy, such as Krafla and Námafjall, combined with the proximity of a hot magma chamber, favor the preservation of kinetic signals. The initial disequilibrium δDCH4 and δ13CCH4 values are consistent with a thermogenic origin from immature organics dissolved in hydrothermally heated groundwater, but an abiotic origin cannot be excluded. The high apparent Δ13CH3D-based temperatures at Krafla and Námafjall could therefore represent nonequilibrium signals associated with either pyrolysis or abiotic generation of CH4 in a superheated vapor or supercritical water phase (>374 °C), considered to exist in the roots of the system above the magmatic heat source. Isotopologue equilibration calculations demonstrate that under such conditions (e.g. > 400 °C) kinetic signals would be erased in days to months, implying rapid migration and quenching of CH4 into the overlying subcritical (<300 °C) hydrothermal reservoir fluids. In systems with longer fluid residence times such as Nisyros, equilibrium isotopologue distributions at temperatures of ~ 350 °C are consistent with long fluid residence times on the order of > 100 years. Our calculations further reveal that CO2–CH4 isotopic equilibration requires unreasonably long fluid residence times, suggesting that any apparent 13C equilibrium may be coincidental.

Can oxygen and carbon isotope ratios of Jurassic foraminifera be used in palaeoenvironmental reconstructions?

Oxygen and carbon isotope ratios of calcareous tests of foraminifera can serve as a tool for reconstructing seawater temperatures and bioproductivity. The foraminiferal isotope studies are, however, largely limited to Neogene, Paleogene and Cretaceous strata. In the current study are presented δ18O and δ13C values of well-preserved Jurassic representatives of different groups of benthic foraminifera: low-Mg calcite Lenticulina (Lagenata), high-Mg calcite Paleomiliolina (Miliolida, Tubothalamea) and aragonite Epistomina (Robertinida, Globothalamea) derived from clay deposits of Poland. Moreover, it provides the detailed comparison of isotopic data of Jurassic foraminifera with those of various co-occurring macrofossils (belemnite rostra, ammonite and bivalve shells). Our study and comparative data substantiate the existence of the offset of Lenticulina's δ18O values from isotope equilibrium of ca. 1‰, which seems to be relatively constant over geological time. This value may be used for calculation of ancient water temperatures from oxygen isotope ratios of well-preserved specimens of this very common and widespread Jurassic foraminiferal taxon. δ18O values of Epistomina and δ13C values of all studied foraminifera are significantly biased from isotope equilibrium.

Devils Hole Calcite Was Precipitated at ±1°C Stable Aquifer Temperatures During the Last Half Million Years

AbstractSubaqueous carbonates from the Devils Hole caves (southwestern USA) provide a continuous Holocene to Pleistocene North American paleoclimate record. The accuracy of this record relies on two assumptions: That carbonates precipitated close to isotope equilibrium and that groundwater temperature did not change significantly in the last 570 thousand years. Here, we investigate these assumptions using dual clumped isotope thermometry. This method relies on simultaneous analyses of carbonate ∆47 and ∆48 values and provides information on the existence and extent of kinetic isotope fractionation. Our results confirm the hypothesis that calcite precipitation occurred close to oxygen and clumped isotope equilibrium during the last half million years in Devils Hole. In addition, we provide evidence that aquifer temperatures varied by less than ±1°C during this interval. Thus, the Devils Hole calcite δ18O time series exclusively represents changes in groundwater δ18O values.

Are chlorine isotopologues of polychlorinated organic pollutants binomially distributed? Theoretical evaluation, numerical simulation, experimental evidences and implications for chlorine isotope analysis and source identification

Relative abundances of chlorine isotopologues of polychlorinated organic compounds (POCs) are commonly recognized to comply with binomial distribution. This study investigated whether chlorine isotopologue distributions of polychlorinated organic pollutants are binomial and evaluated implications of the distributions to relevant analytical and environmental research by theoretical derivation, numerical simulation and experiment. Chlorine kinetic isotope effects and equilibrium isotope effects vary in stepwise chlorination reactions, leading to inconsistent chlorine isotope ratios on different reaction positions of products, which results in non-binomial chlorine isotopologue distributions of the products. After physical changes and dechlorination, chlorine isotopologues of POCs are unlikely binomially distributed. The experimental results demonstrated that the chlorine isotopologue distributions of perchloroethylene, trichloroethylene, methyl-triclosan, and 2,3,7,8-tetrachlorodibenzofuran in standards and four polychlorinated biphenyls in both standard solutions and sediments were non-binomial. The patterns of chlorine isotope ratios derived from pairs of neighboring chlorine isotopologues of POCs from different sources were different, implying different isotopologue distributions, which might cause biases in compound-specific isotope analysis of chlorine (CSIA-Cl) and source identification. A complete-isotopologue scheme for isotope ratio calculation is recommended to CSIA-Cl for obtaining accurate data. Gas chromatography-double focusing magnetic-sector high resolution mass spectrometry is a promising instrument for CSIA-Cl that uses the complete-isotopologue scheme due to its excellent sensitivity, selectivity and ruggedness. This study yields new insights into chlorine isotopologue distributions of polychlorinated organic pollutants and proposes practicable solutions to improve CSIA-Cl that uses gas chromatography-mass spectrometry and facilitate source identification of polychlorinated organic pollutants.

Rapid Attainment of Isotopic Equilibrium after Mercury Reduction by Ferrous Iron Minerals and Isotopic Exchange between Hg(II) and Hg(0)

Rapid Attainment of Isotopic Equilibrium after Mercury Reduction by Ferrous Iron Minerals and Isotopic Exchange between Hg(II) and Hg(0)

Competition of equilibrium and kinetic silicon isotope fractionation during silica precipitation from acidic to alkaline pH solutions in geothermal systems

As an important volcanic geothermal region in China, Tengchong belongs to the Mediterranean-Himalayan geothermal belt and is characterized by a wide distribution of geothermal systems. The study area- Rehai- is the sole geothermal area in Tengchong in mainland China that discharges both acidic and alkaline hot springs with pH values ranging from 1.6 to 9.7 at temperatures between 50 and 97 °C. Natural geothermal environments with large variations in pH and temperature are ideal study sites to investigate silicon (Si) isotope fractionation mechanisms which are still insufficiently understood. In this study, we investigated 3 different geothermal systems, which have distinct fluid characteristics: (1) type I fluid from Diretiyan Spring (pH: 1.6–2.5, HSO4 water, saturated with respect to quartz), (2) type II fluid from Zhenzhuquan Spring (pH: 6.4, NaClSO4 water, saturated with respect to quartz), and (3) type III fluid from Dagunguo Spring (pH: 7.8–9.7, NaCl water, oversaturated with respect to amorphous silica: SiO2am). The mechanisms controlling Si isotope fractionation during Si precipitation differ significantly between the three fluid types. In type I fluid, equilibrium fractionation controls Si isotope variability during silica precipitation, causing the highest natural Si isotope equilibrium fractionation (Δ30Si) detected to date, ranging in Δ30Siquartz-fluid from +1.09‰ to +2.78‰. The equilibrium isotope fractionation is essentially caused by balanced precipitation and dissolution rates and high ionic strength of the geothermal fluids. In type II fluid, amorphous silica formed during evaporation and/or cooling of the spring water during capillary rise and a kinetically controlled, uni-directional Si isotope fractionation is observed. In type III fluid, the Δ30SiSiO2am-fluid ranges from −1.57‰ to +0.55‰ at the temperature range of 80–100 °C. The Δ30SiSiO2am-fluid show a shift from kinetic to equilibrium controlled fractionation and demonstrate that isotopic re-equilibration can reset the original kinetic Si isotope values in dependence of the hydrochemical conditions like pH, temperature and ionic strength as well as reaction time. Here we show that isotopic re-equilibration after solid formation can shift the isotopic signatures originally inherited during precipitation, challenging the interpretation of Si isotope values in the ancient rock record.

Isotopic equilibrium between raindrops and water vapor during the onset and the termination of the 2005–2006 wet season in the Bolivian Andes

The isotopic equilibrium state between precipitation and low-level water vapor is a common assumption in numerous paleoclimate and atmospheric studies based on water stable isotopes. However, the paucity of field observations limits the validation of this assumption. This study examines the isotopic equilibrium state from event-based precipitation and daily near-surface water vapor samples collected during the onset and the termination of the 2005–2006 wet season in the Bolivian Andes (Zongo valley, 16°09′S, 68°07′W). Our observations show that the observed isotopic composition of precipitation (δDp) deviates from the theoretical isotopic composition of precipitation at equilibrium with water vapor (δDp_eq). Disequilibriums (ΔDp_eq = δDp - δDp_eq) are mostly negative (73%), indicating that precipitation is more depleted than a condensate that would have been formed from surface water vapor, and half of them are between −10 and + 10‰. They are significantly correlated to δDp (r2 = 0.30, n = 70, p < 0.001) suggesting that controls on δDp also impact ΔDp_eq. Although equilibrium state does not prevail at the individual rain event scale, a strong relationship is observed between δDp and δDp_eq over the whole period of field samplings (r2 = 0.86, n = 70, p < 0.001). The review of possible causes to explain the disequilibriums shows that below-cloud rain evaporation and diffusive exchanges are little involved. Other local processes such as rain type, condensation conditions and surface water recycling appear as better candidates to explain ΔDp_eq. Lastly, we explore how local processes affect δDp. We show that large-scale dynamic along air masses history is dominant (nearly 80%) to explain δDp whereas local effects are dominant to explain deuterium excess in precipitation. In consequence, we conclude that δDp is a correct candidate to examine and to reconstruct large-scale atmospheric processes from past to present time scales.

In situ carbon and oxygen isotopes measurements in carbonates by fiber coupled laser diode-induced calcination: A step towards field isotopic characterization

Natural stable isotopes ratios (δ13Ccarb and δ18Ocarb) of carbonates archived in the geological record are routinely used to reconstruct local and global paleo temperatures and the secular evolution of the biogeochemical carbon cycle. The state-of-the-art technique, employed since the mid 20th century, to measure these isotopic ratios starts with field sampling followed by several steps of physical and chemical laboratory preparation including: (i) microdrilling and/or sawing and crushing, (ii) CO2 release by wet acid digestion, (iii) gas equilibration, purification and transfer, before (iv) gas phase IRMS measurements. While these steps are time and resource consuming, they provide accurate measurements of δ13Ccarb, δ18Ocarb and carbonate contents. This study presents a new protocol involving a compact and modernized laser calcination system that decreases drastically the analyses time by reducing the number of preparations steps together with offering the possibility of performing spatially resolved analysis at the mm scale. This new method is based on the use of a fiber coupled laser diode device emitting 30 W in the near infrared at 880 nm. The energy provided by the laser source induces the decomposition of calcium carbonate into lime and carbon dioxide. In this work, the CO2 was collected in sample tubes under a controlled atmosphere for offline analysis, however additional developments should permit online analysis in the near future.We analyzed 9 different types of carbonate minerals encompassing a range of isotopic compositions VPDB between +3.3 and − 18.2‰ and between −1.7 and − 14.6‰ for δ13Ccarb and δ18Ocarb, respectively. A comparison of isotopic results was performed for carbonate zones analyzed both by classic methods (micro-drilling followed by acid digestion) and laser calcination. This isotopic cross-calibration exercise shows a direct positive co-variation between both methods with a correlation coefficient of 0.99 and a regression slope of 1 within uncertainties for the δ13Ccarb values. The δ18Ocarb values also compared well with a correlation coefficient of 0.96, suggesting a constant gas-solid phase isotopic equilibrium between carbon dioxide and lime. The reproducibility of our laser calcination method performed on replicate analyses of dolomite, siderite and malachite shows a 1σ standard deviation of 0.31 and 0.77 for δ13Ccarb and δ18Ocarb, respectively. These reproducibilities are within the observed isotopic natural inhomogeneity of samples (up to 1.3 and 0.57‰ for the δ13Ccarb and δ18Ocarb, respectively) as assessed by microdrilling and acid digestion.Based on the suit of samples analyzed in this study, we demonstrate that (i) fiber coupled laser diode calcination enables accurate and reproducible C and O isotopic characterization of natural carbonates, (ii) physical effects during calcination do not introduce any isotopic fractionation for C and is accompanied by a constant isotopic offset for O over a range of isotopic compositions and mineral matrices. These findings pave the way for a new range of possibilities for carbonate δ13C and δ18O measurements directly in the field using rapid, portable, and easy to manipulate laser preparation devices paired with CRDS/IRIS optical-mass spectrometers.

Experimental and theoretical determinations of hydrogen isotopic equilibrium in the system CH4[sbnd]H2[sbnd]H2O from 3 to 200 °C

The stable isotopic composition of methane (CH4) is commonly used to fingerprint natural gas origins. Over the past 50 years, there have been numerous proposals that both microbial and thermogenic CH4 can form in or later attain hydrogen isotopic equilibrium with water (H2O) and carbon isotopic equilibrium with carbon dioxide (CO2). Evaluation of such proposals requires knowledge of the equilibrium fractionation factors between CH4 and H2O or CO2 at the temperatures where microbial and thermogenic CH4 form in or are found in the environment, which is generally less than 200 °C. Experimental determinations of these fractionation factors are only available above 200 °C, requiring extrapolation of these results beyond the calibrated range or the use of theoretical calculations at lower temperatures. Here, we provide a calibration of the equilibrium hydrogen isotopic fractionation factor for CH4 and hydrogen gas (H2) (DαCH4(g)–H2(g)) based on experiments using γ-Al2O3 and Ni catalysts from 3 to 200 °C. Results were regressed as a 2nd order polynomial of 1000 × lnDαCH4(g)–H2(g) vs. 1/T (K−1) yielding:1000×lnDαCH4(g)-H2(g)=3.5317×107T2+2.7749×105T-179.48We combine this calibration with previous experimental determinations of hydrogen isotope equilibrium between H2, H2O(g), and H2O(l) and we provide an interpolatable experimental calibration of 1000 × lnDαCH4(g)–H2O(l) from 3 to 200 °C. Our resulting 4th order polynomial is the following equation:1000×lnDαCH4(g)-H2Ol=-7.9443×1012T4+8.7772×1010T3-3.4973×108T2+5.4398×105T-382.05At 3 °C, the value from our calibration differs by 93‰ relative to what would be calculated based on the extrapolation of the only experimental calibration currently available to temperatures below its calibrated range (lowest temperature of 200 °C; Horibe and Craig, 1995). We additionally provide new theoretical estimates of hydrogen isotopic equilibrium between CH4(g), H2(g), and H2O(g) and carbon isotopic equilibrium between CH4(g) and CO2(g) using Path Integral Monte Carlo (PIMC) calculations. Our PIMC calculations for hydrogen isotopic equilibrium between CH4 and H2 agree 1:1 with our experiments. Finally, we compile carbon and hydrogen isotopic measurements of CH4, CO2, and H2O from various environmental systems and compare observed differences between carbon and hydrogen isotopes to those expected based on isotopic equilibrium. We find that isotopic compositions of some microbial gases from marine sedimentary, coalbed, and shale environments are consistent with those expected for CH4H2O(l) hydrogen and CH4CO2 carbon isotopic equilibrium. In contrast, microbial terrestrial and pure culture gases are not consistent with both CH4H2O(l) hydrogen and CH4CO2 carbon isotopic equilibrium. These results are explained qualitatively using previously developed conceptual models that link free energy gradients available to microorganisms to the degree that their enzymes can promote isotope-exchange reactions between CH4, CO2, and H2O.

Potassium isotopic fractionation during clay adsorption

Clay adsorption is a critical process responsible for the mobilization and cycling of potassium (K) on Earth’s surface. Recent studies emphasized the potential of using stable K isotopes (δ41K) to understand chemical weathering. However, the direction, degree, and mechanism of K isotopic fractionation linked to clay K uptake during chemical weathering remain poorly constrained.This work investigated the mechanism of K adsorption on clays (kaolinite and smectite) and the isotopic fractionation in three experimental sets with K-containing solutions. The time-series experiments revealed that the adsorption and isotope equilibria were attainedafter less than 12-hour reaction. Potassium adsorption rate slowed down and its isotopic fractionation approached the steady-state during 15-day reaction. The pH-dependent experiments demonstrated that the percentage of clay K adsorption and the isotopic composition of adsorbed K (and aqueous K) display negative linear correlations. Net isotopic fractionation between adsorbed and aqueous phases (Δ41Kad-aq) remained near-constant (0.6–0.8‰), regardless of variations in pH ranging from 4 to 10. The concentration-control experiments demonstrated that the percentage of K adsorption decreased with increasing KCl concentrations from 0.005 to 20 mM. The δ41K values of aqueous K reached the minimum of −0.53‰ after 92.7% K adsorbed (initial KCl of 0.005 mM). Potassium adsorption was substantially suppressed as ionic strength (fixed by Na+) increased from 0.001 to 0.5 M without apparent Δ41Kad-aq variations. The K K-edge XANES demonstrated that primary K incorporated in clay lattice and surface KCl derived from sorbed K+ and Cl− synchro-dehydration can be identified after drying of clays. These features indicate that adsorbed K+ was bounded onto clays as outer-sphere complexes, which can be replaced with excess Na+ at high ionic strength. Based on experimental results, we cannot distinguish specific mineralogy regulation on K isotopic fractionation. In sum, isotopically heavy K is preferentially sorbed on clay minerals. The results confirm an equilibrium fractionation path independent of reaction time, pH, ionic strength, and initial KCl concentration. Observed K isotopic fractionation is best fitted by an equilibrium isotopic fractionation law with a fractionation factor αad-aq of 1.00075. We highlight the opposite direction of K isotopic fractionation in clay adsorption and structural incorporation during chemical weathering, and their comparative contributions should be considered for future field investigations.

Nutritional contribution of seaweed Ulva lactuca single-cell detritus and microalgae Chaetoceros calcitrans to the growth of the Pacific oyster Crassostrea gigas

The Pacific oyster Crassostrea gigas (Thunberg, 1793) is the most cultivated bivalve in the world. Nonetheless, the massive production of microalgae as feed represents a substantial cost during laboratory production stages. The use of single cell detritus (SCD) from seaweed Ulva lactuca, cultivated in fish farm effluents, has been proposed as an alternative to microalgae Chaetoceros calcitrans in oyster culture, with the aim of reducing microalgae production costs. Seaweed meal was subjected to an acid and enzymatic digestion process to obtain SCD particles smaller than 20 μm to feed the oysters. Five levels of SCD (w:w) replacing microalgae were evaluated: 0, 25, 50, 75, and 100% in a feeding assay of 5 weeks. At the end of the experiment, growth parameters, condition index, enzymatic activity in the digestive gland (amylase, protease, lipase, and aminopeptidase) were analysed. In addition, during the course of the experiment, the stable isotope ratio of nitrogen (δ15N) was analysed at natural abundance levels in both feed sources and in the mantle tissue of oysters reared on different feeding regimes. Contribution to growth was estimated using an isotope mixing model. Better growth (74 to 94% dry weight gain) and condition indexes (81–75) were observed in oysters fed on experimental regimes having from 0 to 50% substitution of microalgae with SCD, showing no significant differences among them. Oysters under the latter treatments also showed similar enzymatic activity for amylase, alkaline-protease, and lipase. At higher substitution levels of microalgae (75–100%), oysters presented lower growth (13 to 34% dry weight gain) and poor condition indexes (<60); 100% of SCD also elicited higher amylase, alkaline-protease, and lipase activities, whereas (Leu) aminopeptidase N activity was lower. The digestive capacity of lipase was improved in oysters fed with 50 and 75% levels of SCD. Isotopic equilibrium for nitrogen (δ15N) was reached by day 14 in the 50% substitution treatment. Metabolic turnover rates of nitrogen decreased (0.15 to 0.08 day−1) whereas elemental half times in tissue increased (4.1 to 8.2 days) with higher microalgae substitution. Oysters under treatment with 50% microalgae substituted by SCD incorporated similar amounts of dietary nitrogen and dry matter from the microalgal biomass as from SCD to meet the nitrogen requirement for oyster growth. In conclusion, results suggest that SCD from U. lactuca can substitute up to 50% of microalgae C. calcitrans without modifying C. gigas productivity.

Isotopic carbon turnover in pig hoof and rib

The objective of this study was to evaluate the behavior of carbon incorporation and turnover in hoof and ribs of pigs at different periods of development in the search for tissues that reflect longer the former diet. We used 132 commercial hybrids (barrows and females), weaned at an average age of 21 days, distributed in a completely randomized design with four treatments on different days of substitution of corn (C4 cycle plant grain) diets with broken rice (C3 cycle plant grain) at 21, 42, 63 and 110 days of age to change the carbon-13 isotope signal. By means of isotopic dilution curves, we observed that animals whose C4 diet was replaced with C3 diet at 21, 42, 63 and 110 days of age, for hoof and rib, reached a new level of isotope equilibrium. Bone samples are better choices to reflect the former diet, due to conservation of the isotopic signal for longer.

Protogenetic sulfide inclusions in diamonds date the diamond formation event using Re-Os isotopes

Sulfides are the most abundant inclusions in diamonds and a key tool for dating diamond formation via Re-Os isotopic analyses. The manner in which fluids invade the continental lithospheric mantle and the time scale at which they equilibrate with preexisting (protogenetic) sulfides are poorly understood yet essential factors to understanding diamond formation and the validity of isotopic ages. We investigated a suite of sulfide-bearing diamonds from two Canadian cratons to test the robustness of Re-Os in sulfide for dating diamond formation. Single-crystal X-ray diffraction (XRD) allowed determination of the original monosulfide solid-solution (Mss) composition stable in the mantle, indicating subsolidus conditions of encapsulation, and providing crystallographic evidence supporting a protogenetic origin of the inclusions. The results, coupled with a diffusion model, indicate Re-Os isotope equilibration is sufficiently fast in sulfide inclusions with typical grain size, at mantle temperatures, for the system to be reset by the diamond-forming event. This confirms that even if protogenetic, the Re-Os isochrons defined by these minerals likely reflect the ages of diamond formation, and this result highlights the power of this system to date the timing of fluid migration in mantle lithosphere.

Are oxygen isotope fractionation factors between calcite and water derived from speleothems systematically biased due to prior calcite precipitation (PCP)?

The equilibrium oxygen isotope fractionation factor between calcite and water (18αcalcite/H2O) is an important quantity in stable isotope geochemistry and allows in principle to infer temperature variations from carbonate δ18O if carbonate formation occurred in thermodynamic equilibrium. For this reason, many studies intended to determine the value of the oxygen isotope fractionation factor between calcite and water (18αcalcite/H2O) for a wide range of temperatures using modern cave calcite and the corresponding cave drip water or ancient speleothem carbonate and fluid inclusion samples. However, the picture that emerges from all of these studies indicates that speleothem calcite is not formed in thermodynamic equilibrium but under kinetic conditions, provoking a large variability of determined 18αcalcite/H2O values. Here we present a conceptual framework that can explain the variability of 18αcalcite/H2O values obtained by cave studies. Prior calcite precipitation (PCP) is calcite precipitation before cave drip water is dripping from the cave ceiling and impinges on the surface of a stalagmite or watch glass. Prior to the karst water dripping from the cave ceiling, PCP can occur in the karst above the cave as well as on the cave ceiling, the cave walls and on the surface of stalactites. We argue that PCP leads to increasing the δ18O value of the dissolved HCO3− (δ18OHCO3-), resulting in an oxygen isotope disequilibrium of the δ18OHCO3- values with respect to the δ18O value of water (δ18OH2O). The oxygen isotope disequilibrium between HCO3− and H2O is re-equilibrated by oxygen isotope exchange between H2O and HCO3. Depending on the temperature, the re-equilibration time varies from hours to days and is usually much longer than the residence time of the drip water on stalactites, but much shorter than the time required to percolate through the karst. Therefore, while the oxygen isotope equilibrium between HCO3− and H2O is very likely re-established when PCP occurred in the karst, oxygen isotope disequilibrium conditions between HCO3− and H2O still prevail when PCP occurred inside a cave, e.g., on stalactites. If the oxygen isotope disequilibrium conditions between HCO3− and H2O is not re-established, the precipitated calcite will inherit the elevated δ18O value of the HCO3− and not be in oxygen isotope equilibrium with the corresponding drip water. Consequently, if the 18αcalcite/H2O value is calculated from cave calcite samples affected by PCP, the derived value will be systematically biased.

Continuous and simultaneous measurement of triple-oxygen and hydrogen isotopes of liquid and vapor during evaporation experiments.

Oxygen and hydrogen isotopes are important tools for studying the modern and past hydrological cycle. Previous evaporation experiments used episodic measurement of liquid and/or vapor or did not measure all isotopologues of water. Here, we describe an evaporation experimental system that allows all isotopologues of liquid and water vapor to be measured simultaneously and near-continuously at high precision using cavity ring-down laser spectroscopy (CRDS). Evaporating liquid is periodically sampled from a closed recirculating loop by a syringe pump that delivers a constant supply of water to the vaporizer, achieving a water vapor concentration of 20,000 ppmV H2 O (±132, 1σ). Vapor is sampled directly from the evaporation chamber. Isotope ratios are measured simultaneously with a Picarro L2140-i CRDS instrument. For liquid measurements, Allan variance analysis indicates an optimum data collection window of 34 min for oxygen isotopes and 27 min for hydrogen isotopes. During these periods, the mean standard error is ±0.0081‰ for δ17 O values, ±0.0081‰ for δ18 O values, and ±0.019‰ for δ2 H values. For the derived parameters 17 O-excess and d-excess, the standard error of the mean is 5.8 per meg and 0.07‰, respectively. For the vapor phase a 12.5 min data window for all isotopologues results in a mean standard error of ±0.012‰ for δ17 O values, ±0.011‰ for δ18 O values, and ±0.023‰ for δ2 H values. For the derived parameters, the standard error of the mean is 9.2 per meg for 17 O-excess and 0.099‰ for d-excess. These measurements result in consistently narrow 95% confidence limits for the slopes of ln(δ17 O + 1) vs ln(δ18 O + 1) and ln(δ2 H + 1) vs ln(δ18 O + 1). The experimental method permits measurement of fractionation of triple-oxygen and hydrogen isotopes of evaporating water under varying controlled conditions at high precision. Application of this method will be useful for testing theoretical models of evaporation and conducting experiments to simulate evaporation and isotopic equilibration in natural systems.

Seasonal deposition of authigenic calcite out of isotopic equilibrium with DIC and water, and implications for paleolimnological studies

We conducted year-round, monthly monitoring of the stable isotope composition of DIC and water in hypereutrophic Lake Kierskie, western Poland, along with isotope measures of calcite collected in sediment traps installed at 16 and 30 m water depth in the lake. Isotope data were supplemented by previously published data on physico-chemical variables in the lake water column. We sought to determine how carbon and oxygen isotopic disequilibria in calcite deposited in the lake’s laminated sediments vary seasonally, and what factors drive this variability. Deposition of calcite out of equilibrium with DIC and water was documented over the entire study period. For δ18O, the disequilibrium difference between successive months far exceeded the amplitude of the seasonal variability in the isotope composition of water. The biggest difference between the measured and calculated δ13Ccalcite and δ18Ocalcite values was observed during late autumn and winter sediment resuspension and redeposition (2.4‰ and 5.4‰, respectively). In the spring, δ13Ccalcite and δ18Ocalcite offsets from equilibria, 0.5‰ and 1.3‰, respectively, resulted from rapid precipitation of large calcite crystals. During summer, intense productivity and processes related to calcifying algae (“vital effects”) caused lower δ13C (0.5–1.8‰) and δ18O (2.8–2.9‰) in calcite. Differences between isotope values of calcite collected from the two water depths were small, and might have resulted from different settling velocities of small and large crystals, and/or preferential dissolution of smaller grains. We suggest that winter laminae should be excluded from isotope studies of varved sediments whenever possible, as they likely contain redeposited carbonate in which the isotope value is not indicative of conditions in the lake at the time of laminae formation. We also recommend supplementing isotope analysis of calcite in varved lake sediments with seasonally resolved analysis of carbonate content. It appears that major shifts in the proportion of carbonate deposited across seasons can cause notable changes in mean annual values of δ18Ocalcite and δ13Ccalcite, even if DIC and water isotopic compositions remain stable.