Displays

High-precision measurements of the triple oxygen isotope ratios (δ¹⁸O and Δ‘¹⁷O) in terrestrial waters, rocks and minerals opened new and exciting applications in the field of stable oxygen isotope geochemistry. After giving a short overview over measurement techniques and various applications, it will be emphasized on tracing the atmospheric composition in rocks and minerals.

Atmospheric samples from ice cores only date back ~1 Myrs. To obtain information about the atmosphere for the 99.98% of Earth history that is not covered by ice cores, we need to look for rocks. The oxygen isotope composition of the atmosphere younger than 2.4 Gyrs is dominated by molecular oxygen (O₂). Molecular O₂ is one of few components on Earth that has a mass-independent oxygen isotope signature. The anomaly in ¹⁷O provides information about the presence of an ozone layer, the global biosphere primary production, or the atmospheric CO₂ mixing ratio. A few rocks and fossils provide information about the ¹⁷O anomaly of air O₂. Sedimentary sulfates may form by precipitation from SO₄^2- that formed by subaerial oxidation of pyrite. In that process, a part of the oxygen in the sulfate originates from air O₂. Mobilizing of the sulfate oxygen can carry this anomaly over to other minerals like Fe oxides. The isotope signature of fossil tooth enamel also provides information about the atmospheric composition. Air O₂ is inhaled and used to oxidize carbohydrates and fat to (mainly) CO₂ and H₂O, which equilibrate with body water. Tooth apatite then precipitates from body water and inherits an anomaly in ¹⁷O from the inhaled air O₂. Manganese oxides are known to form by oxidation of Mn under participation of O₂. If the isotope composition of dissolved O₂ in the aqueous environment, in which the manganese oxides form is controlled by air, manganese oxides can be used to trace the composition of air O₂. It has been shown that some meteorite impact melts (tektites) have exchanged with ambient air O₂. As result of that exchange, they carry a ¹⁷O anomaly that may be used to trace the composition of air O₂. Also, I-type cosmic spherules have been shown to be indicators for the isotope anomaly of air O₂. These spherules form by aerial oxidation of asteroidal metallic Fe,Ni particles and thus can carry the anomaly of air O₂. Such recent discoveries open insights into the composition of the Earth atmosphere beyond the 1 Myrs limit from the ice core record.

How to cite: Pack, A., Fischer, M., Stübler, C., Peters, S., and Feng, D.: Tracing the triple isotope composition of air by high-precision analyses of meteorites, rocks and fossils, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18899, https://doi.org/10.5194/egusphere-egu2020-18899, 2020.

The Δʹ¹⁷O value of O₂ gas (the analyte for nearly all oxygen triple isotope measurements) can currently be measured to a precision of about 6 ppm; significantly better than the precision of the corresponding δ¹⁷O and δ¹⁸O data. However, reporting Δʹ¹⁷O measurements of silicate rocks to this degree of accuracy, relative to (for example) the VSMOW-SLAP line on the ln(1 + δ¹⁷O) versus ln(1 + δ¹⁸O) plot, poses practical challenges. Regardless of the reference line assigned, Δʹ¹⁷O values are still inextricably linked to the δ¹⁷O and δ¹⁸O calibration of the ‘working standard’ O₂ on the VSMOW–SLAP scale. Yet few laboratories have the capability to make such measurements on waters and on silicates. Even when direct calibration to VSMOW and SLAP is possible, there is not yet consensus on the Δʹ¹⁷O values of widely used silicate standards such as UWG-2 garnet or San Carlos olivine, when reported to a common reference line.

Fluorination of silicate rocks and minerals, to produce O₂ for isotope ratio measurements, requires a different procedure from that used for the fluorination of waters. Thus, there is the possibility of systematic errors being introduced by using a water reference material for reporting δ¹⁷O and δ¹⁸O data of silicates. Furthermore, fractionation arrays of natural silicates on the three-isotope plot are generally offset from VSMOW, which introduces an additional complication. To eliminate such potential sources of error, some authors have chosen to report δ¹⁷O and δ¹⁸O data relative to San Carlos olivine (as representative of Earth’s mantle) rather than to VSMOW, in conjunction with a reference line of assigned slope, for characterizing Δʹ¹⁷O values. However, a two-point scale, such as VSMOW–SLAP for waters, is preferable to a single point calibration

We have therefore characterized Δʹ¹⁷O values (and with inter-laboratory comparison) of two silicates spanning a greater δ¹⁸O range than VSMOW–SLAP and suggest that these materials may be used for accurate determinations of silicate Δʹ¹⁷O values. Our high-δ¹⁸O standard is a flint, designated SKFS, with δ¹⁸O = 33.93 ± 0.08 ‰ (standard error) and Δʹ¹⁷O = –69 ± 3 ppm relative to the VSMOW-SLAP reference line. This material can therefore be used to calibrate the position of an assigned reference line such that it passes through VSMOW. Alternatively, in combination with our low-δ¹⁸O silicate standard, designated as KRS (δ¹⁸O = –25.20 ± 0.03, Δʹ¹⁷O = –114 ± 2 ppm relative to the VSMOW-SLAP reference line), an empirical two-point silicate reference line may be defined from high precision δ¹⁷O and δ¹⁸O measurements of these proposed standards. Δʹ¹⁷O data of silicate rock and mineral samples reported relative to this reference line are independent of whether the δ¹⁷O and δ¹⁸O measurements are reported relative to VSMOW or to the ‘working standard’ O₂, of any isotopic composition. This confers significant advantages for inter-laboratory comparisons.

How to cite: Miller, M., Pack, A., Bindeman, I., and Greenwood, R.: Standardizing high precision Δʹ17O data from silicate rocks and minerals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19830, https://doi.org/10.5194/egusphere-egu2020-19830, 2020.

Oxygen isotopes are a widely used tracer in the field of paleoceanography and provide unique information on mineral formation and environmental conditions. Carbonate sediments record a shift in δ¹⁸O of 10 to 15‰ from the Archean towards higher values in the Phanerozoic. Three different scenarios are suggested to explain this observation: (I) hot Archean oceans, (II) depletion of ¹⁸O in Archean oceans compared to present day and (III) diagenetic alteration of the primary isotopic signature [1]. Recent advances in high-resolution gas source isotope ratio mass spectrometry provide a new tool that may allow to decipher the origin of this isotopic shift observed in the early rock record. We performed high-precision ¹⁸O/¹⁶O and ¹⁷O/¹⁶O measurements on oxygen ion fragments (¹⁶O⁺, ¹⁷O⁺, ¹⁸O⁺) generated in the ion source from CO₂ gas [2]. Isobaric interferences on m/z=17 (¹⁶OH⁺) and m/z=18 (H₂¹⁶O⁺) are separated by means of high mass resolution. The CO₂ gas is first liberated from carbonate samples by orthophosphoric acid digestion and then analyzed on a Thermo Scientific Ultra dual-inlet gas source isotope ratio mass spectrometer [3]. By adding the dimension of ¹⁷O/¹⁶O to the classical ¹⁸O/¹⁶O system, equilibrium trajectories of carbonates that are defined by the equilibrium fractionation factor (¹⁸a_eq) and the triple isotope fractionation exponent (θ) can be predicted as a function of temperature. Minerals that were altered by or formed in meteoric water can be distinguished from those that precipitated in equilibrium with ambient sea water. Therefore, triple oxygen isotope analysis of carbonates does not only hold the potential for a new single-phase paleothermometer, but may also be used to trace the origin of carbonates. Here, we present high-precision triple oxygen isotope data for carbonates from the Pilbara and the Kaapvaal cratons that cover nearly one billion years from the Paleoarchean to the Paleoproterozoic. Marine carbonates from the Phanerozoic complement the dataset. The carbonates were formed in different marine settings, from shallow marine stromatolites to carbonates grown in the interstitial space of basaltic pillows. Phanerozoic carbonates record equilibrium conditions with modern sea water at moderate temperatures. The majority of Precambrian carbonates plot below the predicted equilibrium curve in the δ’¹⁸O-Δ‘¹⁷O space and do not reflect equilibrium conditions with modern sea water at elevated temperatures that were proposed for the Archean oceans. Modeling the triple oxygen isotope composition of carbonates in equilibrium with sea water, that is depleted in ¹⁸O also cannot explain the observed isotopic shift. Further modeling of post-depositional alteration suggests that most carbonates interacted and re-equilibrated with meteoric waters at variable water-rock ratios and temperatures.

[1] Shields and Veizer, 2002, Geochem., Geophy., Geosyst., 10.1029/2001GC000266
[2] Getachew et al., 2019, Rapid Commun. Mass. Spectrom., 10.1002/rcm.847
[3] Eiler et al., 2013, Int. J. Mass. Spectrom., 335, 45-56.

How to cite: Jäger, O., Surma, J., Albrecht, N., Marien, C. S., Xiang, W., Schier, K., Bau, M., Reitner, J., and Pack, A.: High-precision triple oxygen isotope analysis of Archean and Proterozoic carbonates , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17881, https://doi.org/10.5194/egusphere-egu2020-17881, 2020.

Hydrothermal circulation of seawater at mid-ocean ridges cools the oceanic crust and modulates the oceanic chemistry over multimillion-year time scales. Recent research on mass-dependent fractionation of triple oxygen isotopes allows us to gain a new insight into the seawater-basalt exchange reactions that occur within the oceanic crust. To understand the systematics of triple oxygen isotope exchange, we present a novel combined dataset for Δ¹⁷O and ⁸⁷Sr/⁸⁶Sr isotope values measured in modern seawater-derived vent fluids at the Axial Seamount volcano located on the Juan de Fuca Ridge and oceanic epidotes extracted from altered mid-ocean ridge basalts. Upon reaction with fresh oceanic crust, seawater evolves towards the low Mg compositions characteristic of fluids in equilibrium with basalt. In concert with decreasing Mg content and with decreasing ⁸⁷Sr/⁸⁶Sr, the vent fluids at Axial Seamount shift towards values that are 0.04 ‰ lower in Δ¹⁷O and 2 ‰ higher in δ¹⁸O compared to initial seawater. The ⁸⁷Sr/⁸⁶Sr and Δ¹⁷O values of epidotes extracted from modern hydrothermally altered basalts reveal a trend of isotope exchange similar to the one defined by the fluids. We suggest that epidotes record isotope shifts that were experienced by fluids in the areas of focused flow within the oceanic crust. Both fluids and epidotes display similar trajectories of Δ¹⁷O and ⁸⁷Sr/⁸⁶Sr shifts which are modeled using a Monte-Carlo simulation of reactive transport in dual-porosity medium. These trajectories provide important constraints on the physical complexity of reactive circulation of seawater within the oceanic crust. We show how the contribution of hydrothermal circulation to the isotope budget of seawater can be changed during geologic history and evaluated based on the studies of fragments of ancient oceanic crust.

How to cite: Zakharov, D., Tanaka, R., Lundstrom, C., Butterfield, D., Reed, M., and Bindeman, I.: Hydrothermal seawater-basalt exchange reactions traced by triple oxygen and strontium isotope values of fluids and epidotes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21469, https://doi.org/10.5194/egusphere-egu2020-21469, 2020.

Water isotopic composition is a key proxy for past climate reconstructions using deep ice cores from Antarctica. As precipitation forms, the local temperature is imprinted in the snowfalls δ¹⁸O. However, this climatic signal can be erased after snow deposition when snow is exposed to the atmosphere for a long time in regions with extremely low accumulation. Understanding this effect is crucial for the interpretation of ice core records from the extremely dry East Antarctic Plateau, where post-deposition processes such as blowing snow or metamorphism affect the physical and chemical properties of snow during the long periods of snow exposure to the atmosphere. Despite the importance of these processes for the reliable reconstruction of temperature from water isotopic composition in ice cores, the tools required to quantify their impacts are still missing. Here, we present a first year-long comparison between (a) time series of surface snow isotopic composition including d-excess and ¹⁷O-excess at Dome C and (b) satellite observations providing information on snow grain size, a marker of surface metamorphism. Long summer periods without precipitation tend to produce a surface snow metamorphism signature erasing the climatic signal in the surface snow δ¹⁸O. Using a simple model, we demonstrate that d-excess and ¹⁷O-excess allow the identification of the latent fluxes induced by metamorphism, and their impact on surface snow isotopic composition. In turn, their measurements can help improve climate reconstructions based on δ¹⁸O records ice by removing the influence of snow metamorphism.

How to cite: Casado, M., Landais, A., Picard, G., Arnaud, L., Dreossi, G., Stenni, B., and Prie, F.: Using triple water isotopes signatures of surface snow to gauge metamorphism in Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-44, https://doi.org/10.5194/egusphere-egu2020-44, 2020.

The oxygen isotope signature of leaf water is used to trace several processes at the soil-plant-atmosphere interface. During photosynthesis, it is transferred to the oxygen isotope signature of atmospheric CO₂ and O₂, which can be used for reconstructing past changes in gross primary production. The oxygen isotope signature of leaf water additionally imprints leaf organic and mineral compounds, such as phytoliths, used as paleoclimate and paleovegetation proxies when extracted from sedimentary materials.

Numerous experimental and modelling studies were dedicated to constrain the main parameters responsible for changes in the δ¹⁸O of leaf water. Although these models usually correctly depict the main trends of ¹⁸O-enrichment of the leaf water when relative humidity decreases, the calculated absolute values often depart from the observed ones by several ‰. Moreover, the δ¹⁸O of leaf water absorbed by plants is dependent on the δ¹⁸O value of meteoric and soil waters that can vary by several ‰ at different space and time scales. These added uncertainties make our knowledge of the parameters responsible for changes in the δ¹⁸O of leaf water and phytoliths flawed.

Changes in the triple oxygen isotope composition of leaf water, expressed by the ¹⁷O-excess, are controlled by fewer variables than changes in δ¹⁸O. In meteoric water the ¹⁷O-excess varies slightly as it is weakly affected by temperature or phase changes during air mass transport. This makes the soil water fed by meteoric water and the atmospheric vapour in equilibrium with meteoric water changing little from a place to another. Hence the ¹⁷O-excess of leaf water is essentially controlled by the evaporative fractionation. The latest depends on the ratio of vapor pressure in the air to vapor pressure in the stomata intercellular space, close to relative humidity. Leaf water evaporative fractionation can lead to ¹⁷O-excess negative values that can exceed most of surficial water ones.

Here we present the outcomes of several recent growth chamber and field studies, for the purpose of i) refining the grass leaf water and phytoliths δ¹⁸O and ¹⁷O-excess modelling, ii) assessing whether the δ¹⁸O and ¹⁷O-excess of grass leaf water can be reconstructed from phytoliths, and iii) examining the precision of the ¹⁷O-excess of phytoliths as a new proxy for past changes in continental atmospheric relative humidity. Atmospheric continental relative humidity is an important climate parameter poorly constrained in global climate models. A model-data comparison approach, applicable beyond the instrumental period, is essential to progress on this issue. However, there is currently a lack of proxies allowing quantitative reconstruction of past continental relative humidity. The ¹⁷O-excess signature of phytoliths could fill this gap.

How to cite: Alexandre, A., Outrequin, C., Vallet-Coulomb, C., Landais, A., Piel, C., Devidal, S., Peugeot, C., Ouani, T., Afouda, S., Couapel, M., Sonzogni, C., Mazur, J.-C., Prié, F., and Webb, E.: Factors controlling the triple oxygen isotope composition of grass leaf water and phytoliths: insights for paleo-environmental reconstructions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6641, https://doi.org/10.5194/egusphere-egu2020-6641, 2020.

The carbonate ¹⁷O anomaly (Δ¹⁷O) has recently been developed as a geochemical proxy for estimating the relative humidity of moisture at the source point of evaporation, which can be a vital tool in paleoclimate research. Speleothem Δ¹⁷O variability in particular may provide a quantitative constraint on moisture regimes at millennial and orbital timescales—far longer than can be addressed by analyzing ¹⁷O in other materials, such as tree rings. Modern observations and calibration studies have established a robust negative correlation between the Δ¹⁷O (¹⁷O excess) of rainfall and relative humidity, so that Δ¹⁷O is enhanced during arid conditions at the moisture source. Herein, we report novel triple oxygen isotope data across the Last Glacial in speleothems collected from Cherrapunji Cave, northeastern India. Triple oxygen isotope measurements were obtained by an O₂-CO₂ Pt-catalyzed oxygen-isotope equilibration method. Preliminary results suggest lower Δ¹⁷O in speleothems during cold periods (e.g. Heinrich Events), which would imply higher relative humidity over the oceanic moisture source. Importantly, higher source relative humidity does not necessarily imply changes in precipitation amount at the cave site. While it is possible that a substantial geographic shift in the moisture source region (or additional contributions from secondary sources) could obfuscate the Δ¹⁷O signal, we argue that this explanation is unlikely for our study site. Alternatively, we cannot exclude the effects of excessive moisture recycling in the tropical ocean, which can enhance the ¹⁷O anomaly in cloud vapor (particularly if combined with large temperature swings) and thereby alter speleothem Δ¹⁷O. To refine our interpretation of the Δ¹⁷O signal in Cerrapunji Cave samples, we investigate multiple cold periods, as well as coeval samples from the Asian Summer Monsoon region. Finally, we compare our results with novel data from westernmost Asia, where temperature variations during cold events are more likely to be accompanied by large shifts in predominant moisture source, due to the migration of wintertime westerlies.

How to cite: Mahata, S., Duan, P., Sha, L., Baker, J., Kathayat, G., Dong, X., Zong, B., Ning, Y., Zhang, H., and Cheng, H.: Control on millennial scale events (H-events) inferred from triple oxygen isotope ratios of speleothems from a Northeast Indian cave, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7701, https://doi.org/10.5194/egusphere-egu2020-7701, 2020.

Recent studies showed that the ¹⁷O-excess of plant leaf biogenic silicates (phytoliths) can be used to quantify the atmospheric relative humidity occurring during leaf water transpiration. The ¹⁷O-excess vs ∂¹⁸O signature of phytoliths can also be used to trace back to the signature of leaf water. In a similar way, the signature of lacustrine diatoms is expected to record the signature of the lake water in which they formed. Therefore, the triple oxygen isotope composition of biogenic silicates extracted from well-dated sedimentary cores may bring new insights for past climate and hydrological reconstructions. However, for high time-resolution reconstructions, we need to be able to measure microsamples (300 to 800 µg) of biogenic silica. In another context, the triple oxygen isotope composition of micro-meteorites constitutes an efficient tool to determine their parent-body. In this case too, micro-samples need to be handled.

Here we report the results of new ∂¹⁸O and ∂¹⁷O measurements of macro- and micro-samples of international and laboratory silicate standards (e.g. NBS28 quartz, San Carlos Olivine, Boulangé quartz, MSG phytoliths and PS diatoms). Molecular O₂ is extracted from silica and purified in a laser-fluorination line, passed through a 114°C slush to condense potential interfering gasses and sent to the dual-inlet Isotope Ratio Mass Spectrometer (IRMS) Thermo-Scientific Delta V. In order to get sufficient 34/32 and 33/32 signals for microsamples the O₂ gas is concentrated within the IRMS in an additional auto-cooled 800 ml microvolume tube filled with silica gel. Accuracy and reproducibility of the ∂¹⁸O, ∂¹⁷O and ¹⁷O excess measurements are assessed. Attention is payed to determine the concentration from which O₂ gas yields offsets in ∂¹⁸O, ∂¹⁷O and ¹⁷O-excess are measured and whether these offsets are reproducible and can be corrected for.

How to cite: Couapel, M., Sonzogni, C., Alexandre, A., and Sylvestre, F.: Accuracy and reproducibility of the triple oxygen isotope measurement of silicate micro-samples by laser-fluorination-IRMS, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14088, https://doi.org/10.5194/egusphere-egu2020-14088, 2020.

Oxygen isotopes are powerful proxies that are commonly used to decipher the formation of terrestrial and extraterrestrial rocks. Most of modern scientific approaches imply the determination of the oxygen isotopic composition at the mineral scale, thus requiring instruments enable to perform in situ, multi-collection, isotopic analyses in complex mineralogical assemblages and zoned minerals. Among them, large-geometry secondary ion mass spectrometer (LG-SIMS) is the most versatile with unique advantages such as (i) high spatial resolution (10–20 μm beam diameter and 1–2 μm depth); (ii) high sensitivity (detection limits below the ppm level for most elements) and (iii) high mass-resolution analysis allowing to remove most isobaric interferences (Villeneuve et al., 2019). Thanks to these capabilities, analytical uncertainties were significantly reduced for oxygen isotopes and reproducibilities much better that 1 ‰ on d¹⁷O and d¹⁸O are commonly obtained (e.g., Vacher et al. 2016; Marrocchi et al., 2018). Reaching such precisions is, however, linked to the use of 10¹¹ Ω Faraday Cups (FCs) that require minimum count rates of > 10⁶ cp/s for reaching permil precisions. This implies performing measurements with relatively large primary beam (i.e., 15-20 μm) that limits the minerals that can be targeted, especially in extraterrestrial samples (e.g., chondrule olivine crystals, Marrocchi et al., 2019).

Latest generation LG-SIMS instruments have been recently equipped with 10¹² Ω FCs that enable isotopic measurements to be performed at count rates significantly lower (i.e., 3 × 10⁵ cp/s) while maintaining good precision. This implies that high-precision oxygen isotopic measurements can be now performed with a less intense and smaller primary beam (~1 nA; 5 μm), In this contribution, we will report the specific characteristics of measurements using 10¹² Ω FCs and the reproducibilities obtained for oxygen isotope measurements. Few scientific examples where the use of 10¹² Ω FCs can represent a significant beakthrough will also be presented.

Marrocchi Y., Bekaert D.V. & Piani L. (2018). Origin and abundance of water in carbonaceous asteroids. Earth and Planetary Science Letters 482, 23-32.

Marrocchi Y., Euverte R., Villeneuve J., Batanova V., Welsch B., Ferrière L. & Jacquet E. (2019) Formation of CV chondrules by recycling of amoeboid olivine aggregate-like precursors. Geochimica et Cosmochimica Acta 247C, 121-141.

Villeneuve J., Chaussidon M., Marrocchi Y., Deng Z. & Watson B.E. (2019). High-precision silicon isotopic analyses by MC-SIMS in olivine and low-Ca pyroxene. Rapid Communication in Mass Spectrometry 33, 1589-1597.

Vacher L.G., Marrocchi Y., Verdier-Paoletti M., Villeneuve J. & Gounelle M. (2016) Inward radial mixing of interstellar water ices in the solar protoplanetary disk. The Astrophysical Journal Letters, 826, 1-6.

How to cite: Marrocchi, Y., Villeneuve, J., Peres, P., and Fernandes, F.: Recent developments and applications of triple oxygen isotope measurements by secondary ion mass spectrometry , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5798, https://doi.org/10.5194/egusphere-egu2020-5798, 2020.

Triple oxygen isotope data (denoted as ¹⁷O-excess) have been used to constrain meteorological processes, plant fractionation processes, animal metabolism, and a variety of other physical and chemical processes. Measurement precision is key in order to successfully apply this promising new tracer to a range of scientific questions. Up to date, the highest measurement precision for ¹⁷O-excess on water was achieved by converting water to O₂ and subsequent mass spectrometric analysis of O₂ (Barkan and Luz, 2005). This approach allows to reach a measurement precision of about 5-6permeg. However, it is very difficut to setup and only a few laboratories worldwide succesfully use this methodology. A far simpler approach is to use Cavity Ring-Down Spectroscopy (CRDS), i.e. the Picarro L2140-i analyzer that measures δ¹⁸O, δ¹⁷O, δD and determines ¹⁷O-excess. To date, the ¹⁷O-excess measurement precision of CRDS was limited to 10-15permeg. Here, we will present a new metholodology that allows to reach a similar or even better precision compared to the mass spectrometric approach. The improved methodology does not require any hardware changes but is solely based on modifications of the injection procedure. 

How to cite: Hofmann, M. E. G., Lin, Z., Doherty, T., Bent, J. D., and Lucic, G.: Improved precision and throughput for 17O-excess measurements on water with Cavity Ring-Down Spectroscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20088, https://doi.org/10.5194/egusphere-egu2020-20088, 2020.

Can we not use the Δ value to measure a triple isotope system?

Huiming Bao^{1 ,2, 3} and Xiaobin Cao^{1 ,2}

¹ International Center for Isotope Effects Research, Nanjing University, Nanjing 210023, P. R. China

² School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, PR China

³ Department of Geology and Geophysics, Louisiana State University, E235 Howe Russell Kniffen, Baton Rouge, LA 70803

In a triple isotope system, taking oxygen for example, the deviation of the δ¹⁷O (or δ’) from a defined δ¹⁷O-δ¹⁸O relationship is measured by the term Δ, defined as the value of δ¹⁷O - C×δ¹⁸O, in which “C” is a reference slope number. The use of Δ has generated two problems. First, there is a spectrum of C values currently being adopted in the community, for reasons of end-member cases (e.g. 0.5305 at high-temperature limit), legacy (0.52), or compound-specificity (e.g. 0.528 for water cycle or 0.524 for silicates). These practices have brought confusions especially when we deal with small Δ values and when we must compare Δ values among different compounds. A second, more serious problem is the lack of appreciation that a Δ value scales with its corresponding δ¹⁸O value. That means even for the same process we may get different Δ values depending on the magnitude of fractionation and/or laboratory references used.

A pair of radial-angular parameters in a polar coordinate system or a pair of δ¹⁸O and δ¹⁷O in Cartesian space uniquely describe a triple isotope data point in 2D space. Either of the two ways would thaw any debates on the choice of reference slope value C necessary for calculating the Δ. In addition, a polar coordinate system is usually preferred when studying behaviors centering around an origin, in this case, isotope composition deviating from a reference point (0, 0). The angular coordinate φ of a triple isotope composition stays the same for the same fractionation process regardless of its radial coordinate r which is determined by δ¹⁸O and δ¹⁷O values. Thus, the use of a polar coordinate (r, φ) to describe a triple isotope composition in 2D space would avoid the δ¹⁸O scaling issue for Δ values of the same process. Unfortunately, polar coordinate does not offer straightforward representation of process-specific δs or fractionation factors. Using just a pair of δ¹⁸O and δ¹⁷O values to describe a triple isotope system also eliminates additional symbols. Unfortunately, the direct use of the δ¹⁸O and δ¹⁷O presents an apparently larger uncertainty for a data point than the other approaches. And it could not take advantage of the use of an accurate Δ value in case when the analytical yield is not 100%.

The limitations of a polar coordinate system or a pair of δ¹⁸O and δ¹⁷O in Cartesian space outweigh their advantages and we are left with no better alternative than the use of Δ. Therefore, when reporting small Δ values, we must report their corresponding δ¹⁸O values as well to avoid scaling bias when dealing with small Δ values.

How to cite: Bao, H. and Cao, X.: Can we not use the Δ value to measure a triple isotope system?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6191, https://doi.org/10.5194/egusphere-egu2020-6191, 2020.

The  Δ¹⁷O value of sedimentary sulfate can provide a direct, stable, geological archive of atmospheric-biospheric evolution. Negative Δ¹⁷O values in gypsum/anhydrite are inherited from the negative Δ¹⁷O value of atmospheric O₂ which is transferred to sulfate during sulfide weathering. The magnitude of the O₂ Δ¹⁷O value reflects pCO₂, pO₂ and gross primary productivity, hence modelling of the geological Δ¹⁷O record has led to estimates of changing atmospheric composition and primary productivity over Earth history. However, sulfate Δ¹⁷O values represent a conservative estimate of atmospheric Δ¹⁷O values as the magnitude of negative Δ¹⁷O in sulfate can be diluted (or erased) through sulfur cycling. As sulfate is transported away from the site of sulfide oxidation the likelihood of this happening increases.

Although this effect is acknowledged, the extent to which Δ¹⁷O values may vary within and between palaeoenvironments, and how evaporite sedimentology may affect stratigraphic interpretations of Δ¹⁷O values, remains unclear. We present the preliminary results of two case-studies probing the spatiotemporal variability of Δ¹⁷O values.

Case-study 1: temporally correlative Tournaisian (Lower Mississippian) evaporites within Carboniferous rift basins of Britain and Ireland were deposited in a range of settings: coastal wetland (Ballagan Fm.); supratidal sabkha on margin of a restricted basin (Ballycultra Fm.); and coastal sabkha on open ocean margin (Middleton Dale Anhydrite Fm.) All three settings plot on a positive slope in d³⁴S vs Δ¹⁷O space with values ranging between δ³⁴S ≈ +15 ‰, Δ¹⁷O ≈ -0.08 ‰ and δ³⁴S ≈ +24 ‰, Δ¹⁷O ≈ -0.2 ‰. We discuss whether this trend (and intraformational trends) represents a spatial variability in sulfate Δ¹⁷O as controlled by fluctuating fluvial and marine dominance in evaporite depositional environments, or whether this might represent a temporal change in δ³⁴S and Δ¹⁷O.   

Case study 2: non-marine evaporites of the early Permian Cedar Mesa Sandstone (CMS) Formation in Utah were deposited in continental saline pans in an erg-margin setting that fluctuated through arid and humid cycles. These evaporites record negative Δ¹⁷O values as low as -270 per meg, however δ³⁴S values lie along the marine curve. We interpret the signal preserved in the CMS as recycling of the underlying marine evaporites of the late Carboniferous Paradox Formation which have been uplifted on the basin margin. Hence, we discuss how in non-marine settings the recycling of evaporites can decouple the age of the succession from the age of the atmospheric Δ¹⁷O signal.

How to cite: Warke, M., Pettigrew, R., Millward, D., Raine, R., Clarke, S., Peng, Y., Bao, H., and Claire, M.: Spatiotemporal Δ17O variability in the rock record, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5752, https://doi.org/10.5194/egusphere-egu2020-5752, 2020.

Atmospheric oxygen and ozone over geological time have been constrained using various geochemical proxies and modelling studies, but ambiguity remains. Triple oxygen isotope measurements from Phanerozoic and Proterozoic rocks (e.g. Crockford et al., 2019) provide a direct record of ancient atmospheric composition, and as such are an exciting novel proxy. The only known source of mass-independent fractionation of oxygen isotopes (O-MIF) on Earth is in the formation of stratospheric ozone. A large positive O-MIF signal is imparted to ozone, while the larger reservoir of oxygen gains a much smaller negative O-MIF signal. These species interact with other gases in the atmosphere, and oxidised end products including nitrate, sulphate and perchlorate can persist in various geological archives such as ice, arid desert soil, and marine evaporites. As a result, the magnitude of the O-MIF signature detected in the geological record could be used to quantify levels of atmospheric ozone (and closely-related molecular oxygen) over certain time intervals. Here we develop a one-dimensional photochemical model to incorporate the three isotopes of oxygen, in order to trace oxygen isotope anomalies from stratospheric ozone through other atmospheric species, and into the geological record. This model, ‘Atmos,’ has been calibrated over 40 years to provide credible estimates of atmospheric composition deviating from the modern. We use the model to show the lowest oxygen levels at which the anomaly can be produced and transferred, putting a potential lower limit on oxygen levels for parts of the Phanerozoic and mid-Proterozoic.

Reference:

Crockford, P.W., Kunzmann, M., Bekker, A., Hayles, J., Bao, H., Halverson, G.P., Peng, Y., Bui, T.H., Cox, G.M., Gibson, T.M. and Wörndle, S., 2019. Claypool continued: Extending the isotopic record of sedimentary sulfate. Chemical Geology.

How to cite: Gregory, B., Claire, M., and Rugheimer, S.: 1-D photochemical model predicts oxygen isotope anomalies in early Earth atmospheres, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22196, https://doi.org/10.5194/egusphere-egu2020-22196, 2020.

Understanding the origin of ocean island basalts (OIB) has important bearings on Earth’s deep mantle. Although it is widely accepted that subducted oceanic crust, as a consequence of plate tectonics, contributes material to OIB’s formation, its exact fraction in OIB’s mantle source remains ambiguous largely due to uncertainties associated with existing geochemical proxies. We have shown, through theoretical calculation and examining published data, that unlike many known proxies, triple oxygen isotope compositions (i.e. Δ¹⁷O) in olivine samples are not affected by crystallization and partial melting. This unique feature allows olivine Δ¹⁷O values to identify and quantify the fractions of subducted ocean sediments and hydrothermally altered oceanic crusts in OIB’s mantle source. In this work, new Δ¹⁷O measurements for OIB will be presented, and the implications will be discussed. 

How to cite: Cao, X., Bao, H., and Peng, Y.: Triple oxygen isotope constraints on the origin of ocean island basalts , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17738, https://doi.org/10.5194/egusphere-egu2020-17738, 2020.

Tracing the sources of pollutants in Ganga river water using conventional and non-conventional isotope analysis in nitrates

Abhayanand S. Maurya¹, Amzad H. Laskar², Nityanand S. Maurya³, Mao-Chang Liang⁴,

¹Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee-247667, Uttarakhand, India

²Geosciences Division, Physical Research Laboratory, Ahmedabad 380009, Gujarat, India

³Department of Civil Engineering, National Institute of Technology Patna, India

⁴Institute of Earth Sciences, Academia Sinica, Taiwan

Ganga is the largest river in India providing fresh water to ~40 % of India’s population which is more than any other river in the world. It is also one of the most polluted rivers in the world. Pollution, mainly from human and industrial wastes in the Ganga poses significant threats to human health and environment. This is an attempt to identify and quantify the contribution of different sources in the river water pollution using stable isotopes in nitrate (NO₃^-). We measured non-conventional triple oxygen isotopes (∆¹⁷O_NO3=δ¹⁷O_NO3-λδ¹⁸O_NO3) along with the conventional isotopes (δ¹⁵N and δ¹⁸O) in NO₃^- and concentrations of major ions and metals (both heavy and trace ones) in Ganga river water to understand the sources and contribution from different pollution sectors. We also measured stable water isotopes (δD and δ¹⁸O) to understand the secondary processes such as in stream evaporation and inflow over the course of the river.  Water samples were collected from multiple locations starting from the clean water in the upstream region to all the way to the estuaries before the onset of monsoon, to best capture anthropogenic signals. ∆¹⁷O in NO₃^- is used to partition the atmospheric depositions from other sources such as human and industrial wastes and δ¹⁵N and δ¹⁸O values are used to partition the contribution of pollutants from different land sources such as municipal wastes and agricultural fertilizers. ∆¹⁷O in NO₃^- is also used to understand reaction processes which affect the isotopic composition such as nitrification, denitrification, volatilization, assimilation and mineralization as those processes mostly follow mass dependent fractionation without affecting ∆¹⁷O but influence the conventional isotopic compositions. We will present the results along with some recommendations for reducing the pollution level of the Ganga water.

How to cite: Maurya, A. S.: Tracing the sources of pollutants in Ganga river water using conventional and non-conventional isotope analysis in nitrates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2854, https://doi.org/10.5194/egusphere-egu2020-2854, 2020.

    High precision triple oxygen isotope measurement of meteoric water is a newly added tracer in hydrological and paleoclimate research. However, it is prerequisite to study the controls on precipitation ¹⁷O-excess for proper application of it. Here we report two years highly precise precipitation data from Nanjing, a southeast China station dominated by Asian monsoon. All the water isotopes (δ¹⁷O, δ¹⁸O and δD) reported here are based on mass spectrometer measurements and optical measurements (cavity ring-down spectroscopy). Nanjing receives moisture from different vapor sources and experiences different rainout mechanisms during various monsoonal sessions. Combined use of above parameters can help us to delineate processes occurring during evaporation, transport, condensation and re-evaporation. Year to year ¹⁷O-excess variability is observed in the obtained dataset and no notable seasonal variation is observed. However, the ¹⁷O-excess seasonal amplitude is little larger in the first year than the subsequent year. So far, it is known that the precipitation ¹⁷O-excess depends on three values: ¹⁷O-excess of the source water bodies, amount of ¹⁷O-excess gain during evaporation and ¹⁷O-excess loss during raindrops evaporation. During dry months ¹⁷O-excess gain is balanced by ¹⁷O-excess loss, which might lead to the near absence of seasonal cycle at Nanjing. From the comparison of observed data and model simulation, the amount of re-evaporation on falling raindrop is estimated to be about 10% at Nanjing. In addition, correlation with available meteorological parameters has been discussed. Except temperature no significant correlation has been found with other metrological variables (relative humidity and rainfall amount). This study will serve as a baseline to understand some of issues in paleoclimate that have puzzled the scientific community for years.

How to cite: Duan, P., Mahata, S., Sha, L., and Cheng, H.: Triple oxygen isotope variations in precipitation from southeast China and its hydrological significance, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6734, https://doi.org/10.5194/egusphere-egu2020-6734, 2020.

This study aims at evaluating the information carried by the ¹⁷O-excess composition of precipitation in the sub-humid part of West Africa. Located at the southern border of the Sahelian band, the so-called “Sudanian Climatic Zone”, characterized by annual precipitation of 1200-1400mm, plays a crucial role in providing water to large African watersheds such as the Niger river’s one, and the Lake Chad catchment. Surface-atmosphere interactions were shown to influence convective processes in the semi-arid Sahelian band, with positive feedbacks between vegetation land cover and rainfall. Less focus has been put on the more humid Sudanian Zone, although surface-atmosphere interactions may have an important influence on the control of rainfall variations, and therefore on water resource availability in these watersheds.

The stable isotope composition of precipitation reflects the combination of different processes associated with phase changes over the atmospheric water cycle, from the initial water vapor formation above the ocean to the raindrop on the ground surface. Classical tracers (δ¹⁸O, δ²H, and d-excess) are affected by multiple factors (i.e. Rayleigh process, temperature, humidity) changing during these successive steps. In contrast, ¹⁷O-excess variations mainly records evaporation processes controlled by the humidity conditions that prevail during phase change. There are few available ¹⁷O-excess studies focusing on precipitation in tropical and sub-tropical areas. They show that the ¹⁷O-excess in precipitation provides information on 1) relative humidity at oceanic moisture sources, and 2) secondary processes, such as raindrop re-evaporation. The contribution of vapor of continental origin, produced either by plant transpiration or soil water evaporation, should additionally affect the ¹⁷O-excess signature of precipitation, although no data are available so far to evaluate the magnitude of this process.

For the study presented here, we collected precipitation from two sampling stations, both located in Benin and affected by a similar oceanic moisture source in the Gulf of Guinea. The first station (lat. 6°26’ N; long. 2°21’ E) is located along the coast and is essentially subject to oceanic influence. The second station (lat. 9°44’ N; long. 1°34’ E) is located 400 km inland and may be additionally affected by continental vapor recycling. The stable isotope composition of rainfall samples (δ²H, δ¹⁸O and δ¹⁷O) are measured on a WS-CRDS Picarro L2140-i, using three replicates per sample. Comparison between those two records allow to investigate how humidity at the oceanic source, raindrop re-evaporation and continental vapor­ contribute to the ¹⁷O-excess signature of precipitation.

How to cite: Vallet-Coulomb, C., Alexandre, A., Peugeot, C., Alassane, A., Gbewezoun, V., Couapel, M., Outrequin, C., Ouani, T., and Afouda, S.: Contribution of the triple oxygen isotope composition of precipitation to the identification of surface-atmosphere interactions in the sub-humid part of West Africa, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13543, https://doi.org/10.5194/egusphere-egu2020-13543, 2020.

Few studies of rainfall isotopic composition are available in the northern Iberian Peninsula, and up to now none of them has provided detailed analyses of the triple oxygen isotopes (d¹⁷O,  d¹⁸O and derived parameter ¹⁷O_excess), preventing from the complete understanding of some atmospheric processes and their relationship with the current climate in this region. This information, together with the characterization of dripwater isotopic composition once transferred throughout the epikarst, is essential to the correct interpretation of paleoclimate records based on speleothem isotopic data.

We provide the first database of triple oxygen and hydrogen stable isotopes of rainwater in Central-South Pyrenees. We characterize local rainfall isotopic variability in a high altitude site and identify the main factors controlling the isotopic composition of rainwater. The samples were collected on a rainfall-event basis from July 2017 to June 2019 (n=216) at the interpretation center of “Las Güixas” touristic cave in Villanúa (Huesca, Spain), where other monitoring surveys are in progress. This site (42º40’59’’N; 0º31’55’’W; 957 m a.s.l.) is characterized by a transitional Mediterranean – Oceanic climate with a highly contrasted seasonality, mean annual temperature of 10ºC and mean annual precipitation of 1100 mm. We analyzed d¹⁷O, d¹⁸O and dD, and derived parameters ¹⁷O_excess and d-excess in rainwaters using a Picarro L2140-i analyzer at the University of Almería (Spain), with mean precisions (1-standard error) of 5 per meg for ¹⁷O_excess and 0.1‰ for d-excess. Meteorological variables (temperature, RH and rainfall amount) were monitored (every 10 min) at the sampling site during the length of this study.

During the two-years monitoring period, d¹⁸O ranged from -21.7 to 8.7‰ and dD did from -170.8 to 34.1‰, with average values of -7.4‰ and -52.3‰, respectively. The ¹⁷O_excess averaged 21±24 per meg and the mean d-excess was 7.1±7.7‰. The local meteoric water line is defined by dD= 7.3·d¹⁸O+1.9 (R²=0.96) and d´¹⁷O= 0.524·d´¹⁸O+0.0088 (R²=1). The d¹⁷O, d¹⁸O and dD values were higher during summer (June to September; -2.1, -3.9 and -26.6‰, respectively; n=68) and were lower during the rest of the year (-4.7, -9.0 and -63.8‰, respectively; n=164). In contrast, the ¹⁷O_excess and d-excess were lower during summer (3 per meg and 4.6‰, respectively) and higher (29 per meg and 8.2‰, respectively) during the remaining months. We found that the isotopic parameters are weakly correlated with rainfall amount during each event, but they strongly depend on seasonal changes in air temperature and relative humidity. The extremely low ¹⁷O_excess and d-excess values observed in summer (down to -75 per meg and -35.6‰, respectively), cannot be explained by particular conditions at the source of moisture during water vapor formation, but by local meteorological parameters and rain drops re-evaporation during rainfall events.

Further processing of this database will consider other influencing factors in the isotopic composition of rainfall events, such as changes in the source moisture, synoptic pattern and type of rainfall, to further understand the complexity of atmospheric processes through the information stored in the triple oxygen isotopes of rainfall, with application to future ¹⁷O_excess studies in speleothems.

How to cite: Giménez, R., Gázquez, F., Bartolomé, M., and Moreno, A.: Triple oxygen (16O, 17O, 18O) and hydrogen (1H, 2H) isotope analyses of rainfall events in Central-South Pyrenees., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7795, https://doi.org/10.5194/egusphere-egu2020-7795, 2020.

Gypsum (CaSO₄∙2H₂O) speleothems (i.e. stalactites, stalagmites, etc.) in caves form frequently through dissolution of the gypsum host-rock by seepage water and subsequent secondary mineral re-precipitation from gypsum-saturated solutions [1]. Gypsum takes its structurally-bound hydration water (GHW) from the liquid; the isotopic composition (δ¹⁷O, δ¹⁸O and δ²H) of GHW reflects that of cave dripwater at the time of mineral crystallization, with insignificant effect of temperature on the liquid-GHW isotope fractionation factors [2]; therefore, GHW may be used to reconstruct the isotopic composition of paleo-dripwater in caves. Here we investigate the triple oxygen and hydrogen isotopic composition of GHW in speleothems from circum-Mediterranean gypsum caves, including the gypsum karsts of Emilia Romagna (NE Italy), Sorbas (SE Spain), Sicily and Mesaoria (Cyprus), all of them hosted in gypsum of Messinian age (ca. 5.5 Ma). The climatic settings of the studied caves range from semiarid (i.e. Sorbas and Mesaoria, <300 mm·yr^-1) to relatively wet (i.e. Emilia Romagna and Sicily >600 mm·yr^-1).

Our results reveal that most gypsum speleothems in these caves precipitated from unevaporated solutions (e.g. d-excess >8‰ and ¹⁷O_excess >10 per meg), with isotopic compositions similar to those of local meteoric/seepage waters and close to the local meteoric water lines (LMWL) of each region. Gypsum crystallization in absence of evaporation can be explained by the mechanism known as Ostwald ripening [3], a solution-mediated recrystallization under constant temperature by which older crystals (i.e. Messinian gypsum) dissolve to feed new crystals (i.e. gypsum speleothems). Only GHW in speleothems from the Sorbas caves show evidence for solution evaporation prior mineral precipitation. Gypsum speleothems in several caves of Emilia Romagna crystallized from unevaporated waters with significantly different triple oxygen and hydrogen isotopic compositions (e.g. Ca´ Castellina cave: δ¹⁸O=-8.3±0.3‰, δ²H=-55.2±1.7‰, ¹⁷O_excess=33±9 per meg; Abisso Bentini cave: δ¹⁸O=-10.6±0.3‰, δ²H=-73.4±1.8‰, ¹⁷O_excess=47±13 per meg). In absence of chronological data, this can be interpreted as (1) gypsum speleothems formed in different climatic periods or (2) do at present from waters that seepage into the epikarst during different times of the year. Either way, gypsum records the mean isotopic composition of seepage water under distinct environmental conditions in this region.

The δ¹⁸O and ¹⁷O_excess values across the entire dataset are negatively correlated, unlike δ¹⁸O and d-excess values that are, positively correlated for δ¹⁸O<-6‰ and negatively correlated for δ¹⁸O>-6‰. We suggest that the different behaviors of ¹⁷O_excess and d-excess derive from their distinct sensitivities to environmental parameters (i.e. RH and temperature) during formation of water vapor at the moisture source of rain and local effects during rainfall events in each area. We conclude that gypsum speleothems of known ages may be useful as archives for triple oxygen and hydrogen isotope reconstructions of paleo-rainfall.

[1] Gázquez et al. 2017. Chemical Geology, v. 452, p. 34–46; [2] Gázquez et al. 2017. Geochimica et Cosmochimica Acta, v. 198, p. 259–270; [3] Kahlweit, 1975. Advances in Colloid and Interface Science, v. 5, p. 1–35.

How to cite: Gazquez, F., Chiarini, V., Columbu, A., De Waele, J., Audra, P., Cailhol, D., Vattano, M., Madonia, G., Giesche, A., Calaforra, J.-M., and Hodell, D. A.: Gypsum speleothems record the triple oxygen (δ17O and δ18O) and hydrogen (δ2H) isotopic composition of cave dripwater: potential paleoenvironmental implications, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6938, https://doi.org/10.5194/egusphere-egu2020-6938, 2020.

Triple oxygen isotope compositions have become one of critical proxies in characterizing a wide range of geochemical and hydroclimate processes. However, Δ¹⁷O (carbonate ¹⁷O anomaly) has only been barely used in the last decade because it is difficult to measure δ¹⁷O of natural samples to a sufficient precision in order to resolve small natural variability. In this study, we present triple oxygen isotope data from speleothems obtained by an O₂-CO₂ Pt-catalyzed oxygen-isotope equilibration method. The high precision (9 per meg or better, 1σ SD) of our new speleothem Δ¹⁷O data is sufficient to resolve subtle hydroclimatic signals. Based on this method, we established triple-oxygen-isotope records of TON cave in westerly region since the last 135ka, providing the evolution history of water vapor source and water vapor cycle in the orbit-millennium scale atmospheric precipitation in the Central Asia. In addition, the triple-oxygen-isotope records of speleothem from Asian and South American monsoonal regions were established in the key periods, such as glacial and interglacial periods. Our speleothem Δ¹⁷O data indicate a 20 per meg difference between Marine Isotope Stage 5d and 5e in samples from Central Asia, suggesting a shift in moisture source and/or fractionation history. Unexpectedly, there were no measurable Δ¹⁷O differences between glacial and interglacial samples from both the South American (western Amazon) and Asian (southern China) monsoon domains, implying consistent moisture-source conditions across glacial and interglacial cycles, at least in terms of relative humidity. Speleothem Δ¹⁷O data may thus provide new and important constraints for understanding regional and global hydroclimate dynamics.

How to cite: sha, L., Mahata, S., Duan, P., Luz, B., Zhang, P., Baker, J., Zong, B., Ning, Y., Ait Brahim, Y., Zhang, H., Edwards, R. L., and Cheng, H.: A novel application of triple oxygen isotope ratios of speleothems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6791, https://doi.org/10.5194/egusphere-egu2020-6791, 2020.

Here, we describe a system for measuring triple oxygen and hydrogen isotopic ratios of both the liquid and vapour during evaporation of water in a dry gas stream (N2 or dry air) at constant temperature and relative humidity (RH).  The hardware consists of a polymer glove box (COY), peristaltic pump (Ismatec), and Picarro L2140-i cavity ring-down laser spectrometer (CRDS) with Standard Delivery Module (SDM). Liquid water from the evaporation pan is sampled via a closed recirculating loop and syringe pump that delivers a constant rate of water to the vaporizer, maintaining a constant concentration of water vapour in the cell (20,000 ±103, 1 s.d.) over an injection cycle. Liquid measurements alternate with vapour from the glove box which is introduced to the CRDS using a diaphragm gas pump. Important for high-precision measurements, both cavity pressure and outlet valve stability are maintained throughout the liquid injection and subsequent vapour phase. Experiments are bookended by two in-house standards which are calibrated to the SMOW-SLAP scales. An additional drift corrector is introduced periodically.

To test the precision and stability of the liquid injections, we sampled from an isotopically homogeneous volume of water and introduced it to the cavity over a period of ~48h. To minimise the standard deviation derived from noise, we chose an optimum integration time of ~2000s (~33 minutes) based on σ_Allan minimisation. Therefore, for combined liquid-vapour experiments we use an injection/vapour sampling window of 40-minutes (140ug water is consumed per injection), which provides a data collection period of 33-minutes after a 7-min waiting time for equilibration.

Across a single liquid injection, the mean standard error for d¹⁷O, d¹⁸O, and dD is 0.008‰, 0.007‰, and 0.02‰, respectively. For the vapour phase equivalent, the mean standard error for d¹⁷O, d¹⁸O, and dD is 0.01‰, 0.009‰, 0.03‰ respectively. For the d-excess in the liquid and the vapour across one 33-minute cycle, the standard error is 0.07‰ and 0.08‰, respectively. For the O17-excess in the liquid and the vapour across one 33-minute cycle, the standard error is 6 per meg and 8 per meg, respectively.

A single evaporation experiment produces in excess of 100,000 measurements each of d¹⁷O, d¹⁸O, and dD for both the evaporating liquid and resulting vapour. These measurements result in 95% confidence limits for the slope of ln(d¹⁷O+1) vs ln(d¹⁸O+1) of ±0.0002 and ±0.0003 for the liquid and vapour, respectively.  For the slope of ln(dD+1) vs ln(d¹⁸O+1) we obtain a 95% confidence interval of ±0.001 and ±0.002 for the liquid and vapour, respectively. The experimental method permits measurement of fractionation of triple oxygen and hydrogen isotopes of water under varying experimental conditions (e.g., RH, temperature, turbulence) at very high precision. It will be useful for testing numerical models of evaporation and conducting experiments to simulate evaporation and isotopic equilibration in natural systems. An application to closed-basin lakes will be presented.

How to cite: Brady, M. and Hodell, D.: Continuous and simultaneous measurement of triple oxygen and hydrogen isotopes of liquid and vapour during evaporation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16872, https://doi.org/10.5194/egusphere-egu2020-16872, 2020.

The Thar Desert (NW India) has numerous evaporative saline playa lakes. Some are still active and others are dry and preserve up to several meters of sedimentary deposits. These deposits feature a variety of evaporite minerals, including the hydrated mineral gypsum (CaSO₄ 2H₂O). Assuming no secondary exchange, the isotopic composition of the gypsum hydration water preserves the δ¹⁸O, δ¹⁷O and δ D of palaeolake water at the time of gypsum formation. This method provides a way to understand the hydrologic balance in a part of the world where it is typically very difficult to obtain palaeoclimate records. Our 36-hour pan evaporation experiment on site shows that triple oxygen isotopes track changes in evaporative conditions, which vary diurnally due to fluctuating temperature and relative humidity, and appear to reflect night-time condensation. We present new palaeohydrological records from two dry playas (Karsandi, Khajuwala) and one active playa (Lunkaransar) in the Thar Desert using the triple oxygen and hydrogen isotopic composition of gypsum hydration water. Results show that a source of water maintained active playa lake basins in the central Thar Desert for much of the Holocene, either by enhanced direct precipitation and/or fluvial sources. The derived ¹⁷O-excess and d-excess data potentially enable modelling of past changes in relative humidity, once other parameters (windiness, evaporation/inflow, etc.) are set.

How to cite: Giesche, A., Dixit, Y., Gázquez, F., Bauska, T., Brady, M., Singh, V. K., Singh, R. N., Petrie, C. A., and Hodell, D. A.: A paleoclimatic perspective of triple oxygen isotopes from gypsum in Holocene Thar Desert playa lakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21681, https://doi.org/10.5194/egusphere-egu2020-21681, 2020.

Gypsum crystals capture the isotopic composition (δ²H, δ¹⁷O, and δ¹⁸O) of ambient water in their structurally bonded water and may serve as a useful tool to reconstruct paleoclimate. Relative humidity, water temperature, wind speed, along with the isotopic composition of atmospheric vapor and inflowing water control, to a variable degree, the relative proportion of equilibrium and kinetic isotope fractionation during evaporation, and, thus, ultimately determine the d-excess and ¹⁷O-excess of gypsum-bonded water. Here, we demonstrate that the respective best fit of these variables through measured gypsum-bonded water isotope data using the classic Craig-Gordon evaporation model provides apparent absolute values for the fundamental climate mean state variables humidity and temperature and an empirical wind speed parameter of the geologic past.

In this proof-of-concept study, we sampled gypsum crystals within individual stratigraphic units of Pliocene lacustrine deposits from the Atacama Desert, extracted their structurally bonded water, and analyzed the hydrogen and triple oxygen isotope composition. The spread of measured isotope data within each sampled stratigraphic unit suggests variable degrees of evaporation between individual gypsum samples along a common evaporation trajectory. We used the Craig-Gordon evaporation model together with a Monte Carlo simulation to determine the limits of climate mean state variables that fit the measured isotopic data.

Our results demonstrate that primary isotope signatures of marine and continental source waters are preserved in structurally bonded gypsum waters. The data coherently suggest a slightly warmer (18-35°C), less windy and much more humid (50-75%) climate for the Pliocene Atacama, which is consistent with marine records and global circulation climate models that agree on “permanent El Niño” conditions for the Pliocene in the equatorial East Pacific.

Under the assumption that mixing of different brines or multiple sources is insignificant - as would be evident from scattering of isotopic data below the evaporation trajectory in ¹⁷O-excess over δ¹⁸O – the combined hydrogen and triple oxygen isotope analyses of gypsum-bonded water provides a powerful tool to quantify past mean states of humidity and temperature, and to estimate paleo-wind conditions.

How to cite: Voigt, C., Herwartz, D., and Staubwasser, M.: Quantitative reconstruction of past climate mean states in the Atacama Desert using hydrogen and triple oxygen isotopes of gypsum hydration water, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18469, https://doi.org/10.5194/egusphere-egu2020-18469, 2020.

Ca-Sulfates (Gypsum and Anhydrite) are the most common salts accumulating in the soil of the Chilean Atacama Desert. Sulfate sources include sea spray, redeposition of playa sediments, terrestrial weathering, and deposition of sulfate formed in the atmosphere (secondary atmospheric sulfate = SAS). Sulfate from sea spray, playa lakes, and terrestrial weathering have a triple oxygen isotope composition (Δ¹⁷O_SO4) at or slightly below zero reflecting reaction with water and oxygen. Positive Δ¹⁷O_SO4 are generally the result of atmospheric SO₂ oxidation by ozone or hydrogen peroxide. Sulfate oxygen is only altered with ambient water by cycling through biological activity resulting in Δ¹⁷O_SO4 ≈ 0‰. Therefore, Δ¹⁷O_SO4 aids in quantifying the relative contribution of SAS to the desert soil and in identifying bioactivity and water availability in the hyperarid Atacama Desert. The spatial quantification of different sulfate sources may serve to improve the understanding of sulfate deposition in this region.

Samples were analysed by continuous flow IRMS using the pyrolysis of Ag₂SO₄ to determine Δ¹⁷O_SO4 from O₂. An optimized sample preparation to form clean silver sulfate and intra-day calibration against two in-house standards resulted in an external reproducibility of 0.12‰. An inter laboratory comparison including data derived from the laser-fluorination method confirmed the accuracy of our analyses.

We analyzed desert soil surface samples from four E-W transects in the Atacama Desert reaching from the Pacific coast across the Coastal Cordillera, the Central Depression, and up the alluvial fans protruding from the Pre-Andean Cordillera. Transects begin at Pisagua (19.5°S), Salar Grande (21.0°S), Antofagasta (24.0°S), and Paposo (25.0°S). Values of Δ¹⁷O_SO4 vary between 0.3 and 1.1‰. The lowest Δ¹⁷O_SO4 values were measured near Salar Grande and on the Pre-Andean alluvial fans. The highest Δ¹⁷O_SO4 values are observed at the highest altitudes in the Coastal Cordillera - east of Paposo - well above the coastal fog zone (> 1200 m). At similar or higher altitudes on the Pre-Andean fans, Δ¹⁷O_SO4 converge towards zero.

The spatial distribution is the result of source contributions and subsequent biological reset. Positive Δ¹⁷O_SO4 values throughout suggest a significant contribution from SAS. We quantified sea spray contributions using Cl- concentration, which drop dramatically above the fog-zone [1]. Furthermore, salt distribution suggests minimal weathering and redistribution in recent times. The lowest contribution from such near zero Δ¹⁷O_SO4 sulfate sources are expected in the Coastal Cordillera, which is consistent with our data. Within the Coastal Cordillera there is a north to south Δ¹⁷O_SO4 trend, which is also an elevation trend. Increased water availability from fog at lower elevations facilitates more efficient resetting of Δ¹⁷O_SO4 via microbial activity. These observations suggest that the driest place in the Atacama Desert is situated within the Coastal Cordillera above the fog zone.

[1] Voigt et al. (2020) Global and Planetary Change 184

How to cite: Klipsch, S., Herwartz, D., and Staubwasser, M.: Identifying water availability in the Atacama Desert (Chile) by triple oxygen isotope analyses of sulfates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18698, https://doi.org/10.5194/egusphere-egu2020-18698, 2020.

This study reports on measurements of Δ¹⁷O (derived from the triple oxygen isotopes) in sulphate from black crust sampled in Sicily. Atmospheric oxidants, such as O₃, H₂O₂, OH and O₂ carry specific ¹⁷O-anomalies, which are partly transferred to the sulphate during sulphur gas (e.g. SO₂) oxidation. Hence, the Δ¹⁷O in sulphate can be used as a tracer of sulphur oxidation pathways. So far, this method has been mostly applied on sulphate from aerosols, rainwaters, volcanic deposits and ice cores. Here we propose a new approach, that aims to investigate the dominant oxidants of gaseous sulphur precursors into sulphate extracted from black crust material. Black crusts are mostly found on building/monument/sculpture and are the result of the reaction between sulphur compounds (SO₂, H₂SO₄) and carbonate (CaCO₃) from the substrate, which leads to the formation of gypsum (CaSO₄, 2H₂O). Sicilian black crust from sites under different emission influences (anthropogenic, marine and volcanic) were collected. Multi oxygen and sulphur isotope analyses were performed to better assess the origins of black crust sulphate in these different environments. This is crucial for both a better understanding of the sulphur cycle and the preservation of historical monument.

Multi sulphur isotopes show mostly negative values ranging from -0.4 ‰ to 0.02 ‰ ± 0.01 and from -0.59 ‰ to 0.41‰ ± 0.3 for Δ³³S and Δ³⁶S respectively. This is unique for natural samples and different from sulphate aerosols measured around the world (Δ³³S > 0‰). This tends to indicate that sulphate from black crust is not generated by the same processes as sulphate aerosols in the atmosphere. Instead of SO₂ oxidation in the atmosphere, dry deposition of SO₂ and its oxidation on the substratum is preferred. The multi oxygen isotopes show a clear dependence with the geographical repartition of the samples. Indeed, black crusts from Palermo (the biggest Sicilian city) show small ¹⁷O-anomalies ranging between -0.16 ‰ to 1.02 ‰ with an average value of 0.45 ‰ ± 0.26 (n=12; 2σ). This is consistent with Δ¹⁷O values measured in black crust from the Parisian Basin (Genot et al., 2020), which are also formed in an environment influenced by anthropogenic and marine emissions. On the other hand, samples from the eastern part of the Mount Etna region, which are downwind of the volcanic emissions, show the highest ¹⁷O-anomalies ranging from 0.48 ‰ to 3.87 ‰ with an average value of 2.7 ‰ ± 0.6 (n=11; 2σ).

These results indicate that volcanic emissions influence the oxygen isotopic signature of black crust sulphate. In standard urban areas, SO₂ deposited on the substratum is mostly oxidised by O₂-TMI and H₂O₂ to generate the black crust. Yet, under the influence of volcanic emissions, O₃ may play the main role in the SO₂ oxidation.

How to cite: Aroskay, A., Martin, E., Bekki, S., Montana, G., Randazzo, L., and Cartigny, P.: The multi oxygen isotope analyses on black crust from Sicily highlight the volcanic emission influence from Mount Etna on urban areas , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20136, https://doi.org/10.5194/egusphere-egu2020-20136, 2020.