Displays

High precision measurements of triple isotopic composition of oxygen in water is a useful tool to infer the dynamic of past hydrological cycle when measured in ice core together with δ¹⁸O and δD. In particular, the triple isotopic composition of oxygen in water provides information on the climatic conditions of the evaporative sources. In parallel, it has been shown that the triple isotopic composition of oxygen in the atmospheric dioxygen can be a useful tracer of the global biosphere productivity and hence reconstruct the dynamic of the global biosphere productivity in the past from measurements performed in the air bubbles. Measuring triple isotopic composition of oxygen both in the water and in the atmospheric dioxygen trapped in bubbles in ice cores is thus a strong added value to study the past variability of water cycle and biosphere productivity in parallel to climate change.

Here, we first present new laboratory experiments performed in closed biological chambers to show how the triple isotopic composition of oxygen in atmospheric dioxygen can be used for quantification of the biosphere productivity with determination of fractionation coefficients. Then, we present new records of triple isotopic composition of oxygen in water and O₂ trapped in bubbles from the EPICA Dome C ice core over the deglaciations of the last 800 ka.

How to cite: Landais, A., Yang, J.-W., Pasquier, N., Grisart, A., Brandon, M., Extier, T., Prié, F., Minster, B., Piel, C., Sauze, J., Milcu, A., Stenni, B., and Blunier, T.: Triple isotopic composition of oxygen in water and dioxygen during deglaciations recorded in the EPICA Dome C ice core to link climate, biosphere productivity and water cycle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5961, https://doi.org/10.5194/egusphere-egu2020-5961, 2020.

Understanding the processes affecting the triple oxygen isotope composition of atmospheric CO₂ during photosynthesis can help to constrain the interaction and fluxes between the atmosphere and the biosphere. We conducted leaf cuvette experiments under controlled conditions, using sunflower (Helianthus annuus), an annual C₃ species with high photosynthetic capacity and stomatal conductance for CO₂, an evergreen C₃ species, ivy (Hedera hybernica) with lower values for these traits, and a C₄ species maize (Zea mays) that has a high photosynthetic capacity and low stomatal conductance. The experiments were conducted at different light intensities and using CO₂ with different ¹⁷O- excess. Our results demonstrate that two key factors determine the effect of photosynthetic gas exchange on Δ¹⁷O of atmospheric CO₂: The relative difference in Δ¹⁷O of the CO₂ entering the leaf and Δ¹⁷O of leaf water, and the back-diffusion flux from the leaf to the atmosphere, which can be quantified by the c_m/c_a ratio.  At low c_m/c_a the discrimination is governed by diffusion into the leaf, and at high c_m/c_a by back-diffusion of CO₂ that has equilibrated with the leaf water. Plants with a higher c_m/c_a ratio modify the Δ¹⁷O of atmospheric CO₂ more strongly than plants with lower c_m/c_a. 

Based on the leaf cuvette experiments using both C₄ and C₃ plants, the global discrimination in ¹⁷O-excess of atmospheric CO₂ due to assimilation is estimated to be -0.6±0.2‰. The main uncertainty in the global estimation is due to the uncertainty in the c_m/c_a ratio.

How to cite: Adnew, G., Pons, T., Koren, G., Peters, W., and Röckmann, T.: Leaf-scale quantification of the effect of photosynthetic gas exchange on Δ17O of atmospheric CO2 , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3064, https://doi.org/10.5194/egusphere-egu2020-3064, 2020.

Evaluating atmospheric transport simulations against observations helps refining bottom-up estimates of greenhouse gas fluxes and identifying gaps in our understanding of regional and category-specific contributions to atmospheric mole fractions. This insight is critical in the efforts to mitigate anthropogenic environmental impact. Beside total mole fractions, stable isotope ratios provide further constraints on source-sink processes [1-3].

Here, we present two receptor-oriented model simulations for carbon dioxide (CO₂) mole fraction and δ¹³C-CO₂ stable isotope ratios for a nine year period (2009-2017) at the High Altitude Research Station Jungfraujoch (Switzerland, 3580 m asl). The model simulations of CO₂ were performed on a 3-hourly time-resolution with two backward Lagrangian particle dispersion models driven by two different numerical weather forecast fields: FLEXPART-COSMO and STILT-ECMWF. Anthropogenic CO₂ fluxes were based on the EDGAR v4.3 emissions inventory aggregated into 14 source categories representing fossil and biogenic fuel uses as well as emissions from cement production. Biospheric CO₂ fluxes representing the photosynthetic uptake and respiration of 8 plant functional types were based on the Vegetation Photosynthesis and Respiration Model (VPRM). The simulated CO₂ emissions per source and sink category were weighted with category-specific δ¹³C-CO₂ signatures from published experimental studies. Background CO₂ values at the boundaries of both model domains were taken from global model simulations and the corresponding δ¹³C-CO₂ values were constructed as suggested in Ref. [3]. We compare the simulations to a unique data set of continuous in-situ observations of CO₂ mole fractions and δ¹³C-CO₂ stable isotope ratios by quantum cascade laser absorption spectroscopy as described in previous work [1, 4-5], available for the whole nine year period at the site.

The simulated atmospheric CO₂ and δ¹³C-CO₂ time-series are in good agreement with the observations and capture the observed variability at the models' 3-hourly time-resolution. This allows for an in-depth evaluation of the contribution of different CO₂ emission sources and for an allocation of source regions when Jungfraujoch is influenced by air masses from the planetary boundary layer. In brief, the receptor-oriented model simulations suggest that anthropogenic CO₂ contributions are primarily of fossil origin (90%). Anthropogenic emissions contribute between 60% in February, and 20% in July/August, to the CO₂ enhancements observed at Jungfraujoch. The remaining fraction is due to biosphere respiration, which thus largely dominates emissions during the summer season. However, intense photosynthetic CO₂ uptake during June, July and August roughly outweighs CO₂ contributions from anthropogenic activities and biosphere respiration at JFJ.

REFERENCES

[1] Tuzson et al., 2011. ACP, 11, 1685

[2] Röckmann et al., 2016. ACP, 16, 10469

[3] Vardag et al., 2016. Biogeosciences, 13, 4237

[4] Tuzson et al., 2008. Appl. Phys. B, 92, 451

[5] Sturm et al., 2013. AMT 6, 1659

How to cite: Pieber, S. M., Tuzson, B., Henne, S., Karstens, U., Brunner, D., Steinbacher, M., and Emmenegger, L.: Simulations of atmospheric CO2 and δ13C-CO2 compared to real-time observations at the high altitude station Jungfraujoch , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10588, https://doi.org/10.5194/egusphere-egu2020-10588, 2020.

Carbon dioxide clumped isotope thermometry is one of the most developed applications of the geochemistry of multiply substituted isotopologues. The degree of heavy isotope clumping (e.g., ¹⁶O¹³C¹⁸O) beyond an expected random distribution can be related to the temperature of calcite precipitation. This provides an independent temperature estimate that, when combined with carbonate δ¹⁸O values, can constrain paleowater δ¹⁸O values. However, the use of isotope ratio mass spectrometry (IRMS) to do these measurements remains relatively rare because it is time-consuming and costly. We have developed an isotope ratio laser spectrometry method using tunable infrared laser differential absorption spectroscopy (TILDAS) and describe our latest results using both gaseous carbon dioxide samples and CO₂ derived from carbonate minerals. The TILDAS instrument has two continuous wave lasers to directly and simultaneously measure four isotopologues involved in the ¹⁶O¹³C¹⁸O equilibrium calculation. Because each isotopologue is independently resolved, this approach does not have to correct for isobaric peaks. The gas samples are trapped in a low volume (~250 ml) optical multi-pass cell with a path length of 36 meters. Raw data are collected at 1.6 kHz, providing 96,000 peak-area measurements of each CO₂ isotopologue per minute. With a specially designed sampling system, each sample measurement is bracketed with measurements of a working reference gas, and a precision of 0.01‰ is achieved within 20 minutes, based on four repeated measurements. The total sample size needed for a complete measurement is approximately 15 μmol of CO₂, or 1.5 mg of calcite equivalent. TILDAS reported ∆_16O13C18O values show a linear relationship with theoretical calculations, with a very weak dependence on bulk isotope composition. The performance of the TILDAS system demonstrated in this study is competitive with the best IRMS systems and surpasses typical IRMS measurements in several key respects, such as measurement duration and isobaric interference problems. This method can easily be applied more widely in stable isotope geochemistry by changing spectral regions and laser configurations, leading to rapid and high precision (0.01‰) measurement of conventional stable isotope ratios and δ¹⁷O in CO₂ gas samples.

How to cite: Nelson, D., Wang, Z., Dettman, D., McManus, B., Quade, J., Yanay, N., Huntington, K., and Schauer, A.: Rapid and Precise Carbon Dioxide Clumped Isotope Composition Analysis by Tunable Infrared Laser Differential Absorption Spectroscopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11501, https://doi.org/10.5194/egusphere-egu2020-11501, 2020.

Nitrous oxide (N₂O) has been for long a major focus of all greenhouse gas accounting agreements. Understanding the mechanisms of its formation and clarifying its sources and sinks are highly important for mitigating N₂O emissions. In this context, measuring the doubly substituted isotopocules of N₂O can add new and unique opportunities to fingerprint and constrain the biogeochemical N₂O cycle, similar to other atmospheric species such as CO₂, CH₄, and O₂.

We address this challenging research field by developing and validating a laser spectroscopic technique for selective analysis of the eight most abundant N₂O isotopic species including the doubly substituted isotopocules ¹⁴N¹⁵N¹⁸O, ¹⁵N¹⁴N¹⁸O, and ¹⁵N¹⁵N¹⁶O. This method is based on quantum cascade laser absorption spectroscopy (QCLAS) and reaches a precision of 0.01 – 0.20 ‰ with 1 – 2 min spectral averaging on samples of 4 μmol of N₂O in N₂ at 4 hPa.

Furthermore, we have established a new reference frame combining two independent approaches: (1) clumped N₂O isotopocule abundances were linked to stochastic distribution by equilibrating the N–O bond in the N₂O molecule over activated Al₂O₃ at 100 and 200 °C, and (2) individual isotopocule concentrations were calibrated using a set of high-accuracy gravimetric N₂O-in-N₂ gas mixtures. The latter approach, applied for the first time to clumped isotope measurements, has a particular potential in realizing regular multi-point calibration for species like ¹⁵N¹⁵N¹⁶O, because no procedure for equilibration of the N–N bond has been successful yet.

Results of the validation measurements, using the QCLAS method and calibration approach, are presented for a large range of δ values (approx. 100 ‰ for d¹⁵N and d¹⁸O). Inter-comparison measurements with high-resolution mass spectrometry show compatible results for bulk isotopic composition (d¹⁵N, d(458+548)), but superior performance of QCLAS for determining site-selectivity (SP, SP¹⁸). In summary, this work provides new methodological basis for the measurements of clumped N₂O isotopes and has a high potential to stimulate future research in the N₂O community by establishing a new class of reservoir-insensitive tracers and molecular-scale insights.

How to cite: Kantnerová, K., Yu, L., Zindel, D., Zahniser, M. S., Nelson, D. D., Tuzson, B., Emmenegger, L., Nakagawa, M., Toyoda, S., Yoshida, N., Bernasconi, S. M., and Mohn, J.: Clumped isotope analysis in nitrous oxide by mid-IR laser spectroscopy: analytical developments and validation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6726, https://doi.org/10.5194/egusphere-egu2020-6726, 2020.

Instrumental development in measurement technique now allows continuous in-situ isotope analysis of ¹³CH₄ by Cavity Ring-Down Spectroscopy (CRDS). Analyses of the isotopic composition of methane in ambient air can potentially be used to partition between different CH₄ source categories.

Since 2014 a CRDS G2201-i analyser has been used to continuously measure CH₄ and its ¹³C/¹²C ratio in ambient air at the Institute of Environmental Physics (IUP) in Heidelberg (116m a.s.l.), South-West Germany. Furthermore, the CRDS G2201-i analyser was installed twice for a month at the measurement station of the German Environment Agency at Schauinsland (1205m a.s.l.). In September 2018 and in February 2019 the analyser was moved to Schauinsland to examine the validity of evaluations of continuous δ¹³CH₄ measurements at a semi-rural station.

As an urban station, the seasonal and daily variations of the measured CH₄ mole fraction and isotopic composition in Heidelberg vary much stronger than at the mountain station Schauinsland. The precision of the isotopic source signature calculation using a Keeling plot strongly depends on the CH₄ peak height and instrumental precision. Therefore, at Schauinsland station the lower variability in the CH₄ mole fraction makes the evaluation challenging. Different methods such as monthly/weekly interval evaluations and moving Keeling/Miller Tans methods has been used to calculate the isotopic source signature in ambient air.

The isotopic methane source signatures of the air in Heidelberg was found to be between -75 ‰ and -35 ‰, with an average of (-54 ± 2) ‰. An annual cycle can be noticed with more depleted values (-56 ‰) in summer and more enriched values (-51 ‰) in winter, due to larger biogenic emissions in summer and more thermogenic (e.g. natural gas) emissions in winter. The mean isotopic source signature calculated at Schauinsland shows variations, too, with more enriched values (−56 ‰) in winter and more depleted (−60 ‰) ones in autumn. The more depleted values in summer/autumn at Schauinsland corresponds to more biogenic methane and can be explained by dairy cows grazing near the station especially during this time.

The generally more enriched values at Schauinsland are caused by the more rural surrounding. Emission estimates of county provided by the LUBW Landesanstalt für Umwelt Baden-Württemberg shows that around Schauinsland 60 % of the CH₄ emissions are emitted by livestock farming and around Heidelberg only 28 %. The mean isotopic source signature calculated using these emissions is (-58 ± 2) ‰ for Schauinsland and (-53 ± 2) ‰ for Heidelberg. These results agreed well with the mean source signatures determined out of continuous isotopic measurements.

How to cite: Hoheisel, A., Meinhardt, F., and Schmidt, M.: Evaluation of continuous δ13CH4 measurements in Heidelberg and at Schauinsland, Germany, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5761, https://doi.org/10.5194/egusphere-egu2020-5761, 2020.

Hydrocarbons are the main constituents of natural gas. Their chemical and isotope abundance is a window to biogeochemical processes occurring in the subsurface. Stable isotopes of natural gas hydrocarbons are traditionally measured through compound-specific isotope analysis (CSIA) where each hydrocarbon is separated before its isotope ratio is determined.

Recently a variety of methods have been developed to determine position-specific isotope composition of propane, the first hydrocarbon with two distinct isotopomers: central and terminal [1][2][3][4]. The relative abundance of propane isotopomers (e.g. Δ¹³C_central = δ¹³C_central - δ¹³C_terminal) is a promising tool for tracing sources and sinks of hydrocarbons in natural gas reservoirs. In particular, anaerobic oxidation of propane starts with a fumarate addition at the central position, which is expected to lead to a specific enrichment of the central ¹³C-isotopomer of the remaining propane.

We measured Δ¹³C_central values of propane throughout the course of its oxidation by bacteria BuS5 [5] and showed that the isotope fractionation is located mainly on the central position, which differs from the signature expected for thermogenic evolution [6]. The approach has been used to detect anaerobic oxidation of propane in several natural gas reservoirs: Southwest Ontario (Canada), Carnarvon Basin (Australia), Michigan (USA) [6], and more recently Tokamachi mud volcano in Japan [7]. In addition, isotopomers of n-butane and i-butane analysed using the same technique allows gaining insights into the mechanism of their microbial oxidation.

The isotopomer approach presented here can thus shed light on the fate of natural gas hydrocarbons. In combination with clumped isotope measurements of methane and ethane, the approach can provide unprecedented information regarding carbon cycling in the subsurface.

[1] Gilbert et al., 2016 GCA v177, p205

[2] Piasecki et al., 2016 GCA v188 p58

[3] Gao et al., 2016 Chem Geol. v435, p1

[4] Liu et al., 2018 Chem Geol. v491, p14

[5] Kniemeyer et al., 2007 Nature v449, p898

[6] Gilbert et al., 2019 PNAS v116, p6653

[7] Etiope et al., 2011 Appl. Geochem. v26, p348

How to cite: Gilbert, A., Julien, M., Yoshida, N., and Ueno, Y.: Isotopomer approaches to the detection of anaerobic oxidation of natural gas hydrocarbons, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13618, https://doi.org/10.5194/egusphere-egu2020-13618, 2020.

Oxygen isotope analysis of plant material, such as sugars in different tissues, provides an important tool to understand how plants function, interact with their environment and also cope with climate change. Knowing how to extract and purify carbohydrates without artificially altering their oxygen isotope ratio (δ¹⁸O) is therefore essential.

We aimed to resolve the impact of different steps on sugars' δ¹⁸O values during their extraction and purification from leaf and phloem tissue. More precisely, we investigated (1) different drying processes (oven- vs freeze-drying), and (2) how extraction and purification affect leaf sugars. To clearly see fractionation and exchange processes, these experiments were performed using ¹⁸O-labelled water. We further examined (3) the influence of different EDTA media and immersion times to facilitate sugar exudation and subsequent yield from twig phloem tissue. Finally, we analysed (4) the sugar phloem composition, as well as the individual compounds’ carbon isotopic signatures (δ¹³C).

Comparison of freeze- and oven-dried sugars showed lower δ¹⁸O memory effects and more consistent oxygen isotopic signatures across different sugars, indicating lyophilisation as the more reliable method. The extraction and purification can be conducted without significant oxygen isotope fractionation. However, ¹⁸O-depletion was observed when sugars were dissolved and dried multiple times. This suggests that additional dissolution and drying steps should best be avoided whenever possible. Different immersion times and exudation media during twig phloem extraction revealed to have a substantial influence on the phloem sugars' overall oxygen isotopic signature, their composition, and the individual compounds' δ¹³C values.

Our research illustrates which precautions during sample preparation – from drying to extracting and purifying – need to be taken when plant sugars and their oxygen isotopic signature are of interest. Regarding the preservation of the phloem sugars' original δ¹⁸O values and stabilising their composition (prevention of sucrose degradation) as much as possible, we recommend a short immersion time of approx. 1 hour. After a thorough initial rinse of the tissue, the sap should be eluted in pure water without any additives (no EDTA). This further reduces the possibility of hexoses to exchange oxygen with that of the surrounding water.

How to cite: Egli, M., Lehmann, M. M., Brinkmann, N., Werner, R. A., Saurer, M., and Kahmen, A.: Testing different methods for the extraction and purification of leaf and phloem sugars for oxygen isotope analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22570, https://doi.org/10.5194/egusphere-egu2020-22570, 2020.

Biomass burning on the African continent emits large amounts of CO₂, CO, and aerosols. Our aim is to use measurements of the stable carbon isotope ¹³C in organic carbon, CO and CO₂ in biomass burning smoke to estimate the contribution of C3 plants (trees and bushes) and C4 plants (mainly Savannah grass), which have very distinct ¹³C/¹²C ratios. This is possible, if ¹³C/¹²C ratios are not significantly altered by the combustion process. This assumption is investigated in a series of laboratory experiments, where C3 and C4 plants (corn and willow wood), or C3-C4 plant mixtures are burned. The laboratory results are used to interpret the results of pilot studies of smoke sampled in African savannah fires.

First results from the laboratory studies indicate that organic carbon (OC) from combustion of willow or corn shows ¹³C/¹²C ratios comparable to the burned plant material. For combustion of willow (C3), the ¹³C/¹²C ratios in OC tend to be slightly higher than in the wood fuel, depending on combustion conditions. For combustion of corn ¹³C/¹²C ratios of OC tend to be slightly lower than in the fuel. For mixtures of willow and corn the relationship between ¹³C/¹²C ratios in the emitted organic carbon and the fuel mixture is slightly non-linear: For a 50-50% oak wood and corn mixture the ¹³C/¹²C ratio in OC is closer to that of corn than that of willow. First results from pilot field studies indicate that a larger fraction of OC comes from trees and bushes, although mainly Savannah grass is burned in the investigated fires.

How to cite: Dusek, U., Vernooij, R., Shaikat, A., Qiu, C., Popa, E., Winiger, P., Schutgens, N. A. J., Yao, P., and van der Werf, G. R.: Stable carbon isotopic composition of biomass burning emissions – implications for estimating the contribution of C3 and C4 plants , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10484, https://doi.org/10.5194/egusphere-egu2020-10484, 2020.

Excessive automotive engine exhaust emissions of gases and particulate matter (PM) pose a threat to public health and urban air quality. In an effort to reduce automotive emissions modern cars use a variety of engine modifications, catalytic systems and filters which in turn alter the isotope ratio of carbonaceous particles (isotope fractionation effect). Diesel engines are of particular interest due to higher production of particulates (soot) in comparison to gasoline engines [1].

The aim of this work was to examine particulate matter δ¹³C variation in automotive emissions using stable carbon isotope ratio measurements. Emission experiments were performed in dynamometer laboratory using four light passenger vehicles with differing liquid fuels - diesel, diesel with additives, 92 RON and 95 RON. Vehicles were tested with varying engine power and using simulated transient cycles in urban and rural areas. Engine exhaust particulate matter was collected on quartz filters. Later, isotope ratio δ¹³C values of fuel and exhaust carbonaceous particulates were measured using IRMS. δ¹³C values were then compared and level of isotope fractionation determined.

The obtained results show particulate matter δ¹³C values ranging from -28.8 ‰ to -27.2 ‰ during separate driving modes. Isotope fractionation Δ (particulates-fuel) values varied between 1.8 ‰ and 3.5 ‰. It was determined that δ¹³C values of automotive emissions depend on the type of fuel used, applied engine power, driving modes (urban, rural) and can be used to characterize automotive carbonaceous particle emissions.

[1]             M. V. Twigg, “Progress and future challenges in controlling automotive exhaust gas emissions,” Appl. Catal. B Environ., 2007.

How to cite: Bučinskas, L., Matijošius, J., and Garbaras, A.: Stable carbon δ13C analysis of automotive particulate matter emissions under controlled conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21320, https://doi.org/10.5194/egusphere-egu2020-21320, 2020.

A three-year-long methane mole fraction and d¹³C_CH4 measurement campaign was performed at the Hungarian tall tower station, Hegyhátsál, between 2013-2016. The results were compared with that of two NOAA atmospheric monitoring sites Mace Head and Zeppelin to determine the continental methane excess and the relative isotopic shift. The data then were used for bac trajectory analyses to identify potential methane source regions in Europe coupled with d¹³C_CH4 results. The Hungarian station can be separated from the coastal and polar areas based on the mole fraction results having higher maxima and seasonal amplitude, but the d¹³C_CH4 results match well with the NOAA stations’ results. Our study shows that although the local, regional anthropogenic and natural sources are major influences, more distant regions can also influence the measured CH₄ level and d¹³C_CH4 signal in the Pannonian Basin.

How to cite: Varga, T., Haszpra, L., Major, I., Nisbet, E. G., Lowry, D., Fisher, R. E., Jull, T. A. J., Molnár, M., and László, E.: Identification of potential methane source regions in Europe using d13C-CH4 measurements and back trajectory modeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4529, https://doi.org/10.5194/egusphere-egu2020-4529, 2020.

Due to concerns regarding potential impacts of the development of natural gas from unconventional hydrocarbon resources on groundwater systems in North America and elsewhere, it has been crucial to improve methods of Environmental Baseline Assessment (EBA). Any subsequent deviations from the EBA could indicate migration of natural gas into the monitored groundwater systems. In collaboration with Alberta Environment and Parks, over 800 groundwater samples have been collected from dedicated monitoring wells since 2006 resulting in an extensive high-quality database of aqueous and gaseous geochemical and isotopic compositions. Because methane is the main component of natural gas, it had been the principal target of our groundwater studies. Our objectives were a) to assess the occurrence of methane in groundwater throughout the province of Alberta (Canada), b) to use isotope techniques to track the predominant sources of methane, c) to use a combination of chemical and multi-isotopic techniques and models to assess the fate of methane in groundwater, and d) to use probability for predicting the presence of methane in groundwater based on hydrogeochemical parameters in regions where no gas data exist.

Methane was found to be ubiquitous in groundwater samples throughout the province of Alberta with concentrations varying from 2.9 10^-4 to >2.4 mmol/l. The highest methane concentrations were found in Na-HCO₃ and Na-Cl water-types where the sulfate concentrations were <1 mmol/l. Analyses of the isotopic compositions of sulfate, dissolved inorganic carbon (DIC) and methane revealed that in some groundwater systems bacterial sulfate reduction occurred (δ³⁴S_SO4 >+10‰ associated with lowest sulfate concentrations) and evidence for methane oxidation was also detected (highest δ¹³C_CH4 values > ‑55‰ associated with lowest methane concentrations). Moreover, some δ¹³C_DIC values were as high as +13.8‰ associated with the highest methane concentrations. A geochemical and multi-isotope model using long-term monitoring data was developed and revealed two different sources of methane: 1) microbial methane resulting from in-situ methanogenesis within the aquifer for a subset of the samples; 2) migration of microbial methane into aquifers characterized by various redox conditions, followed by methane oxidation potentially coupled with bacterial sulfate reduction within sulfate-rich zones causing a pseudo-thermogenic carbon isotopic fingerprint for the remaining methane. So far, no evidence of unambiguously thermogenic methane in the groundwater samples collected from dedicated monitoring wells has been found. Efforts to assess the probability of regional occurrence of methane in groundwater systems in Alberta have then focused on a model for methane prediction model based on logistic regression (LR) for regions of Alberta where no gas data exist. Using basic hydrogeochemical parameters such as occurrence of electron donors, well depth and total dissolved solids of groundwater, the LR approach shows excellent performance metrics e.g. model sensitivity, specificity >80% regarding the prediction of methane occurrence in groundwater of Alberta.

How to cite: Humez, P., Osselin, F., Kloppmann, W., McClain, C., Nightingale, M., and Mayer, B.: Sources, trends, and fate of methane in shallow aquifers of Alberta, Canada, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10777, https://doi.org/10.5194/egusphere-egu2020-10777, 2020.

The atmospheric burden of methane (CH₄) has more than doubled since the 18^th Century. Currently the abundance of CH₄ in the atmosphere is well known, but emission rates from different source sectors are uncertain. CH₄ is emitted to the atmosphere from various sources. To better understand the changes in atmospheric CH₄ abundance before and after 2006, it is important to study the contribution from these different sources separately. Most CH₄ source have process specific δ¹³C-CH₄ values, which can be used to broadly identify source sectors.

This study examines the seasonal cycle of atmospheric δ¹³C-CH₄ in recent decades using the TM5 atmospheric transport. TM5 is driven by ECMWF ERA-Interim meteorological fields, and uses pre-calculated OH-fields and reaction rates with Cl and O(¹D) to account for the CH₄ sink processes in the atmosphere. TM5 is run at a 1^ox1^o resolution over Europe and globally at 6^ox4^o. Emissions for enteric fermentation and manure management, landfills and waste water treatment, rice cultivation, coal industry, oil and gas industry, and residential are taken from the EDGAR inventory. Natural emission for wetlands, peatlands and mineral soils, and soil sinks are taken from the LPX-Bern DYPTOP ecosystem model. Emissions for geological seeps including onshore hydrocarbon macro-seeps (including mud volcanoes), submarine (offshore) seeps, diffuse microseepage and geothermal manifestations are included. Emissions for fires (GFED v4), termites, wild animals and from the ocean are also included. Several sensitivity analyses are carried out. The sensitivity analyses include simulations with and without seasonal cycles in the anthropogenic emission fields (EDGAR v4.2 FT2010 vs EDGAR v4.3.2), and with and without spatial variations in source specific  δ¹³C-CH₄ values, which are used to calculate ¹³CH₄/CH₄ emission ratios. The effect of including the seasonal cycle in the anthropogenic emissions were not significant, which means natural sources probably play more important role in determining the seasonal cycle of δ¹³C-CH₄. The global observations of atmospheric CH₄ and δ¹³C-CH₄, provided by NOAA’s GMD, the INSTAAR and Royal Holloway, the University of London, are used for evaluation. We present the importance of having reasonable initial fields of atmospheric 13CH₄, which will be later used as inputs for CarbonTracker Europe-δ¹³CH₄ (CTE-δ¹³CH₄) data assimilation system to optimise CH₄ emissions by source category.

How to cite: Kangasaho, V., Tsuruta, A., Aalto, T., Backman, L., Houweling, S., Krol, M., Peters, W., Luijkx, I., Lienert, S., Joos, F., Dlugokencky, E., Michael, S., White, J., and Fisher, R.: Modelling the seasonal cycle of atmospheric δ13C-CH4 using source specific δ13C-CH4 values, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13274, https://doi.org/10.5194/egusphere-egu2020-13274, 2020.

We investigate the seasonal cycle of δ¹³CO₂ using the Earth system model of intermediate complexity Bern3D-LPX. Using a model of atmospheric transport (TM3), the spatial fields of simulated ¹³CO₂ and CO₂ exchange are translated to local δ¹³CO₂ anomalies, which are then compared to atmospheric measurements. We discuss the ability of the model to accurately simulate the atmospheric seasonal δ¹³CO₂ cycle_, which could prove to be a valuable novel observational constraint. The coupled simulation allows us to distinguish the relative importance of the biosphere and ocean in determining the seasonal cycle of δ¹³CO₂ at different measurement sites across the world.

The amplitude of the seasonal cycle of δ¹³CO₂ is of particular importance to quantify land biosphere processes. The decreasing δ¹³CO₂ of the atmosphere during the last decades (Suess effect) leads to a divergence of the δ¹³C signature in assimilation and heterotrophic respiration, because of the long lifetime of soil pools. This is expected to lead to a high sensitivity of the seasonal amplitude to the amount of soil respiration. The effect of changes in soil turnover times on the simulated seasonal cycle is explored with factorial simulations of the Dynamic Global Vegetation Model LPX-Bern.

How to cite: Lienert, S., Köstler, C., Zaehle, S., and Joos, F.: Simulating the seasonal cycle of 13C, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7841, https://doi.org/10.5194/egusphere-egu2020-7841, 2020.

The stable oxygen isotope composition of atmospheric CO₂ and the mixing ratio of carbonyl sulphide (COS) are potential tracers of biospheric CO₂ fluxes at large scales. However, the use of these tracers hinges on our ability to understand and better predict the activity of the enzyme carbonic anhydrase (CA) in different soil microbial groups, including phototrophs. Because different classes of the CA family (α, β and γ) may have different affinities to CO₂ and COS and their expression should also vary between different microbial groups, differences in the community structure could impact the ‘community-integrated’ CA activity differently for CO₂ and COS. Four soils of different pH were incubated in the dark or with a diurnal cycle for forty days to vary the abundance of native phototrophs. Fluxes of CO₂, CO¹⁸O and COS were measured to estimate CA activity alongside the abundance of bacteria, fungi and phototroph genes. The abundance of soil phototrophs increased most at higher soil pH. In the light, the strength of the soil CO₂ sink and the CA-driven CO₂-H₂O isotopic exchange rates correlated with phototroph abundance. COS uptake rates were attributed to fungi whose abundance was positively enhanced in alkaline soils but only in the presence of increased phototrophs. In addition we developed a metabarcoding approach to reveal the interactions of specific taxonomic groups incuding photosynthetic eukaryotic algae and cyanobacteria when exposed to light and their impact on flux rates. Our findings demonstrate that soil-atmosphere CO₂, COS and CO¹⁸O fluxes are strongly regulated by the microbial community structure in response to changes in soil pH and light availability and support the idea that different members of the microbial community express different classes of CA, with different affinities to CO₂ and COS.

How to cite: Wingate, L., Foucault, C., Fanin, N., Sauze, J., Maron, P.-A., Nowak, V., Terrat, S., Mondy, S., van Schaik, E., Crouzet, O., Ogée, J., and Wohl, S.: The influence of light on soil community structure and consequences for soil CO2, CO18O and COS exchange , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20422, https://doi.org/10.5194/egusphere-egu2020-20422, 2020.

Carbonyl sulfide (COS) is the most abundant sulfur-containing trace gas in the atmosphere, with an average mixing ratio of 500 parts per trillion (ppt). It has a relatively long lifetime of about 2 years, which permits it to travel into the stratosphere. There, it likely plays an important role in the formation of stratospheric sulfur aerosols (SSA), which have a cooling effect on the Earth’s climate. Furthermore, during photosynthetic uptake by plants, COS follows essentially the same pathway as CO₂, and therefore COS could be used to estimate gross primary production (GPP). Unfortunately, significant uncertainties still exist in the sources, sinks and global cycling of COS, which need to be overcome. Isotopic measurements of COS could be a promising tool for constraining the COS budget, as well as for investigating its role in the formation of stratospheric sulfur aerosols.

Within the framework of the COS-OCS project, we developed a new pre-concentration and measurement system at Utrecht University, that can measure d³³S and d³⁴S from COS from 2 to 5 L air samples, with a current precision of about 5‰ and 2‰ for d³³S and d³⁴S, respectively. The aim of the project is to perform a global-scale characterization of COS isotopes by measuring seasonal, latitudinal and altitudinal variations in the troposphere and stratosphere. Here, I will present the details of the new measurement system and results from various atmospheric samples.

How to cite: Baartman, S., Popa, E., Krol, M., and Röckmann, T.: Isotopic Measurements: A New Tool for Studying Global Carbonyl Sulfide, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3528, https://doi.org/10.5194/egusphere-egu2020-3528, 2020.

Ecosystem assimilation and respiration result in anti-correlated fluxes of oxygen (O₂) and carbon dioxide (CO₂). While the ecosystem O₂:CO₂ molar exchange ratio is usually assumed constant at ≈1.1 on longer timescales, variations for individual ecosystem compartments or shorter timescales have been reported in the past. We hypothesize that these exchange ratio variations can reveal information about underlying biotic and abiotic processes in plants or soil that cannot be inferred from traditional net ecosystem exchange measurements. To date, oxygen measurements have not been widely implemented in ecosystem research due to the technical challenge of detecting very small variations (ppm-level) against an atmospheric background of ≈21% (≈210,000 ppm).

We evaluate the performance and applicability of two commercial oxygen analyzers Integrated into custom-built gas handling and calibration systems, and report first results from measurements of O₂:CO₂ exchange ratios in a managed European beech forest in central Germany.

System 1, consisting of a relatively slow response differential fuel cell O₂ analyzer (Oxzilla FC-2, Sable Systems Inc., USA) together with a non-dispersive infrared CO₂ analyzer (LI-840, LI-COR Biosciences, USA), was used to simultaneously measure O₂ and CO₂ mole fractions in air sampled from soil, stem, and branch chambers. Chambers were operated in an open flow-through steady-state design aimed at equilibrium mole fractions within a few hundred ppm of atmospheric background. Using a multiplexer valve design, we measured chambers sequentially by directing chamber air at a controlled flow rate to the gas analyzing system.

Preliminary analysis of August to December 2018 data show that chamber-based flux estimates for O₂ and CO₂ were anti-correlated at all times, and that the O₂:CO₂ molar exchange ratios (defined as ‑Δ[O₂]/Δ[CO₂]) varied considerably over time and between the different ecosystem compartments (soil, stems, and branches) with a median (interquartile range) of 0.94 (0.75 to 1.09).

In system 2, CO₂, O₂ and water vapor (H₂O) measurements were performed with a fast response (5 Hz) gas analyzer using tunable infrared laser direct absorption spectroscopy (TILDAS, Aerodyne Research Inc., USA). We measured fluctuations in O₂:CO₂ exchange ratios in air sampled at 1.5 times the canopy height, i.e. a typical eddy covariance set-up.

Analysis of the high-frequency data revealed instrumental noise levels of ≈±12 ppm O₂. Fourier transformation of high-frequency data obtained during well-mixed boundary layer conditions indicate that turbulent fluctuations of the O₂ signal were insufficiently resolved when compared to the CO₂ power spectra. When averaging high-frequency data to 2-min aggregates, instrumental noise was reduced to ≈±1 ppm, similar to the precision of system 1. At this timescale, contemporaneous measurements of above-canopy air revealed agreement between the fuel cell and the laser systems, both in O₂ mole fraction (R² = 0.6 slope = 0.7, MAE = 1.6 ppm) and in estimated O₂:CO₂ exchange ratios of 1.01 and 0.97 for system 1 and 2, respectively.

Our presentation will expand on the applicability of both O₂ and CO₂ measurement systems with regard to micrometeorological flux techniques. Specifically, we elucidate on the potential of using O₂ flux measurements as a constraint for estimating ecosystem-scale gross primary production.

How to cite: Knohl, A., Muhr, J., Deventer, M. J., Blei, E., Braden-Behrens, J., Tunsch, E., Bonazza, M., Pickers, P. A., Nelson, D., Zahniser, M., and Manning, A. C.: Measuring oxygen fluxes in a European beech forest - results from the OXYFLUX project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18822, https://doi.org/10.5194/egusphere-egu2020-18822, 2020.

Abundances of ¹⁷O¹⁸O and ¹⁸O¹⁸O (also called clumped isotopes and denoted by Δ₃₅ and Δ₃₆) of O₂  in firn and ice core air are novel tracers that can be useful to study past changes in atmospheric photochemistry and temperature. We present Δ₃₅ and Δ₃₆ values measured in firn and ice core air O₂ from North Greenland (NEEM; 77.45°N 51.06°W). The aim is to reconstruct the preindustrial-industrial, Holocene and glacial-interglacial variation in the tropospheric ozone photochemistry and temperature. Measurements of Δ₃₅ and Δ₃₆ are carried out using a high-resolution stable isotope ratio mass spectrometer Thermo Fisher 253 ULTRA[1]. Our measurements of Δ₃₅ and Δ₃₆  across past air, from archive samples, to the modern-day show significant changes in the atmospheric photochemistry via ozone burdening and stratospheric- tropospheric transport processes. We will present the measurement results along with a detailed discussion on the dominant process using explicit dynamic simulations of ∆₃₆ in the AC-GCM EMAC model [2,3,4].

How to cite: Laskar, A., Peethambaran, R., Gromov, S., Blunier, T., and Roeckmann, T.: 17O18O and 18O18O in ice core O2 from Greenland: implications to reconstruct past atmospheric photochemistry , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20741, https://doi.org/10.5194/egusphere-egu2020-20741, 2020.

Stable isotopes of hydrogen and oxygen (²H and ¹⁸O) are often used for quantification of water budgets of lakes and other surface water bodies, in particular for the assessment of underground components of those budgets [1]. Recent advances in laser spectroscopy enabled simultaneous analyses of ²H, ¹⁸O and ¹⁷O content in water, with measurement uncertainties comparable (δ¹⁸O) or surpassing (δ²H) those routinely achieved by off-line sample preparation methods combined with conventional IRMS technique [2]. This open up the doors for improving reliability of isotope-aided budgets of surface water bodies by adding third isotope tracer (¹⁷O). This, however, requires adequate information on triple isotope effects accompanying evaporation of water, in particular the kinetic isotope effect related to evaporation of ¹H₂¹⁷O isotopologue.

Here we present the results of dedicated laboratory experiments aimed at quantification of triple isotope effects accompanying evaporation of water under fully developed diffusive sublayer [3]. Identical containers with predefined mass of water of known isotopic composition were placed in an isolated chamber with controlled atmosphere during the experiment (temperature, relative humidity). The chamber was flushed with synthetic air. At regular time intervals (approximately one week) containers were removed one by one from the chamber, the remaining mass of water in the removed container was determined gravimetrically, and stored for subsequent isotope analyses. The flow rate was adjusted at each step of the process to keep humidity inside the chamber constant. Evaporation continued until approximately half of the initial mass of water was removed from the containers. The experiment was repeated under diiferent conditions inside the chamber (two different temperatures and three different values of relative humidty).

The results of the experiments were interpreted in the framework of Craig-Gordon model of evaporation [3]. It turned out that the assumption often used in the description of isotopic effects accompanying evaporation that liquid phase is isotopically homogeneous during the process, leads to conflicting results for three isotope systems in use. However, if surface enrichment of the liquid phase, different for each heavy isotopologue (¹H²H¹⁶O, ¹H₂¹⁸O, ¹H₂¹⁷O) is included in the model, consistent results for all three isotopes can be achieved, with calculated kinetic fractionation factor for ¹H₂¹⁷O isotopologue equal 14.76 ± 0.48 ‰,. This value agrees, within the quoted uncertainty, with the value of 14.60 ± 0.30 ‰ obtained by Barkan and Luz [4].  

Acknowledgements: The presented work was supported by National Science Centre (research grant No. 2016/23/B/ST10/00909) and by the Ministry of Science and Higher Education (project no. 16.16.220.842 B02)

References:
[1]   Rozanski K. Froehlich K. Mook WG. Technical Documents in Hydrology, No. 39, Vol. III, UNESCO, Paris, 2001 117 pp.
[2]   Pierchala A, Rozanski K, Dulinski M, Gorczyca Z, Marzec M, Czub R, Isotopes in Environmental and Health Studies, 2019 (55) 290-307.
[3]   Horita, J. Rozanski K. Cohen S. 2007. Isotopes in Environmental and Health Studies, 2007 (44) 23-49.
[4]   Barkan E. Luz B. Rapid Commun. Mass Spectrom., 2007(21) 2999-3005.

How to cite: Pierchala, A., Rozanski, K., Dulinski, M., Gorczyca, Z., and Czub, R.: Triple isotope effects accompanying evaporation of water: new insights from laboratory experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6708, https://doi.org/10.5194/egusphere-egu2020-6708, 2020.

Stable water isotopes are widely-used tracers to investigate hydrological processes occurring in the atmosphere and to determine the geospatial origin of water, i.e. to acquire useful information about the hydrological cycles over catchment basins and to find the origin of water recharging rivers, aquifers, and springs. Mapping the isotopic composition of precipitation provides hydrological and climate information at regional and global scales. However, the isotopic composition of precipitation is usually analyzed at large scales with a limited spatial resolution. In Italy, a few studies mapped the oxygen stable isotopes using annually-averaged data, not accounting for the strong seasonality of the isotopic composition linked to climatic and weather factors. To partially fill this gap, the present study proposes a detailed analysis of more than 2250 isotope data (δ¹⁸O, δ²H, and deuterium excess) related to precipitations collected in the Friuli Venezia Giulia (FVG) region (Italy) with monthly or seasonal frequency in 36 sites between 1984 and 2015.

The FVG region lies at the north-eastern end of Italy, bordering Austria in the North and Slovenia in the East, and extends over ~7.9·10³ km². From a hydrogeological point of view, FVG is an interesting case study. Large highly-permeable carbonate aquifers are present in the Alps and Prealps, while the southern part of the region is characterized by an alluvial plain, split by the spring belt into two sectors: the High Plain in the North, characterized by an highly-permeable unconfined aquifer, and the Low Plain in the South, characterized by a system of confined and artesian aquifers. All the aquifers are recharged by the effective precipitations which in the FVG exhibits among the highest annual precipitation rates in Italy (with peaks >3000 mm/year).

For the present research, the isotopic data were used: (i) to analyze the spatial and seasonal variability of isotopic composition; (ii) to relate water isotopes with orography and weather parameters collected from meteorological stations as well as using ECMWF ERA5 reanalysis; (iii) to reconstruct the local meteoric water lines across the FVG at annual and seasonal bases; (iv) to quantify interannual trends and analyze their spatial distribution; and (iv) to model the spatial distribution of isotope content in precipitation and create annual and seasonal maps.

How to cite: Masiol, M., Zannoni, D., Stenni, B., Dreossi, G., Zini, L., Calligaris, C., Karlicek, D., Michelini, M., Flora, O., Cucchi, F., and Treu, F.: Interannual analysis of high spatially-resolved δ18O and δ2H data in precipitation across North-East Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8881, https://doi.org/10.5194/egusphere-egu2020-8881, 2020.