Water Isotopologues Research Articles

Differential hydro-climatic evolution of East Javanese ecosystems over the past 22,000 years

The Maritime Continent, home to widespread tropical rainforest and millions of people, is the primary region of deep atmospheric convection on the Earth. However, debate exists whether the isotopologues of water reflect rainfall amount during the Last Glacial Maximum (LGM), resulting in different interpretations of the LGM climate of the Maritime Continent. Here we present paired leaf wax δ13C and δD records together with pollen data from a sediment core retrieved off East Java dating back to 22,000 years before present. We use three n-alkane homologues (n-C29, n-C31 and n-C33) in order to reconstruct past changes in vegetation types and seasonal rainfall. Our results suggest that in East Java, evergreen rainforest remained the dominant vegetation type in montane regions since the seasonality there remained relatively unaltered over the entire period. In contrast, the East Javanese lowlands were characterised by C4 grass expansion and an extended dry season but a wetter rainy season, thus stronger seasonality, during the LGM.

Custom-built light-pipe cell for high-resolution infrared absorption spectroscopy of tritiated water vapor and other hazardous gases.

We present a new custom-built cell for high-resolution absorption spectroscopy of hazardous gases. The use of an aluminum light-pipe enables sensitive detection due to the small tube diameter and an increased particle density in the interaction volume for a limited analyte amount in the cell, while avoiding additional surfaces such as mirrors. To demonstrate this, we have used the cell to measure tritiated water isotopologues (HTO and traces of T2O) for which spectroscopic data is scarce, due to the challenge of performing spectroscopy of these highly radio-chemical aggressive substances. For this purpose, the new cell also features the efficient inline-production of tritiated water. In this paper we present the concept of the light-pipe cell and demonstrate its performance with a high-resolution absorption spectrum of gaseous HTO generated inside of this cell.

Terrestrial deuterium-to-hydrogen ratio in water in hyperactive comets

The D/H ratio in cometary water has been shown to vary between 1 and 3 times the Earth’s oceans value, in both Oort cloud comets and Jupiter-family comets originating from the Kuiper belt. This has been taken as evidence that comets contributed a relatively small fraction of the terrestrial water. We present new sensitive spectroscopic observations of water isotopologues in the Jupiter-family comet 46P/Wirtanen carried out using the GREAT spectrometer aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). The derived D/H ratio of (1.61 ± 0.65)×10−4 is the same as in the Earth’s oceans. Although the statistics are limited, we show that interesting trends are already becoming apparent in the existing data. A clear anti-correlation is seen between the D/H ratio and the active fraction, defined as the ratio of the active surface area to the total nucleus surface. Comets with an active fraction above 0.5 typically have D/H ratios in water consistent with the terrestrial value. These hyperactive comets, such as 46P/Wirtanen, require an additional source of water vapor in their coma, explained by the presence of subliming icy grains expelled from the nucleus. The observed correlation may suggest that hyperactive comets belong to a population of ice-rich objects that formed just outside the snow line, or in the outermost regions of the solar nebula, from water thermally reprocessed in the inner disk that was transported outward during the early disk evolution. The observed anti-correlation between the active fraction and the nucleus size seems to argue against the first interpretation, as planetesimals near the snow line are expected to undergo rapid growth. Alternatively, isotopic properties of water outgassed from the nucleus and icy grains may be different due to fractionation effects at sublimation. In this case, all comets may share the same Earth-like D/H ratio in water, with profound implications for the early solar system and the origin of Earth’s oceans.

Azeotropic isotopologues

Isotopologues are molecular species differing only in their stable isotope composition. For example, the isotopologues of water include HOH, DOD, and HOD. The possibility of reactive azeotropy among isotopologues of the same molecule is explored within the ideal gas/ideal solution assumption. The chemical equilibrium enabling azeotropy may involve isotope exchange reactions or may involve more complicated dissociation/solvation chemistry. Azeotropy among isotopologues appears possible in the presence of isotope exchange reactions and with simple dissociation or combination reactions. However, for such chemical reactions, the isotope fractionation factors must be unity, that is, isotope ratios are identical in the vapor and liquid phases. This is not consistent with experimental observations that isotope fractionation factors can differ from unity at an azeotrope. More complex solvation chemistry, wherein the isotopic composition in the solvation shell differs slightly from that of the bulk solvent, can lead to an azeotrope for which the isotope fractionation factor is not unity. Possible azeotropy among the isotopologues of water is then considered. Experiments to directly quantify water isotopologue compositions at the elevated temperatures and pressures anticipated for azeotropy are described. The paper concludes with a discussion of the potential impact of azotropic isotopologues on the azeotropic extraction and isotopic analysis of soil water and on the interpretation of isotopic signatures in geological and cosmological samples.

Evidence of Isotopic Fractionation During Vapor Exchange Between the Atmosphere and the Snow Surface in Greenland.

Several recent studies from both Greenland and Antarctica have reported significant changes in the water isotopic composition of near‐surface snow between precipitation events. These changes have been linked to isotopic exchange with atmospheric water vapor and sublimation‐induced fractionation, but the processes are poorly constrained by observations. Understanding and quantifying these processes are crucial to both the interpretation of ice core climate proxies and the formulation of isotope‐enabled general circulation models. Here, we present continuous measurements of the water isotopic composition in surface snow and atmospheric vapor together with near‐surface atmospheric turbulence and snow‐air latent and sensible heat fluxes, obtained at the East Greenland Ice‐Core Project drilling site in summer 2016. For two 4‐day‐long time periods, significant diurnal variations in atmospheric water isotopologues are observed. A model is developed to explore the impact of this variability on the surface snow isotopic composition. Our model suggests that the snow isotopic composition in the upper subcentimeter of the snow exhibits a diurnal variation with amplitudes in δ18O and δD of ~2.5‰ and ~13‰, respectively. As comparison, such changes correspond to 10–20% of the magnitude of seasonal changes in interior Greenland snow pack isotopes and of the change across a glacial‐interglacial transition. Importantly, our observation and model results suggest, that sublimation‐induced fractionation needs to be included in simulations of exchanges between the vapor and the snow surface on diurnal timescales during summer cloud‐free conditions in northeast Greenland.

Far-infrared self-continuum absorption of H216O and H218O (15–500 cm−1)

The water vapor continuum absorption is studied in a spectral range covering most of the pure rotational spectrum of water molecule up to 500 cm−1. The continuum absorption was derived from the broadband water vapor spectra recorded by Fourier transform spectrometer equipped with the 151-m multipass gas cell at the AILES beam line of the SOLEIL synchrotron. The coherent (10–35 cm−1) and standard (40–500 cm−1) radiation modes of the synchrotron were used. In order to refine the magnitude and clarify the physical origin of the continuum, spectra of the two major water isotopologues, H216O and H218O, were considered. Recordings at several water vapor pressures were used to check the expected quadratic pressure dependence of the continuum.The new data extend and supplement previous measurements filling, in particular, the gap between 200 and 350 cm−1, which was never studied before. The H216O and H218O absorption continua in the range of 50–650 cm−1 show similar frequency dependence and magnitude. In particular, both continua exhibit a clear water dimer spectral signature near 15 cm−1, in good agreement with previous ab initio calculations. The present data confirm that the MT-CKD empirical continuum model widely used in atmospheric applications, overestimates importantly the continuum magnitude in the whole range of the rotational band. The observed irregular frequency dependence of the retrieved self-continuum cross-section values is tentatively interpreted as due to uncertainties on the resonance lines of the water monomer spectrum which is subtracted from the recorded spectra. On the basis of spectra simulations, the inadequate description of the line shapes in the range of the intermediate wings (detuning of 5–10 cm−1 from line center) and the uncertainties on the self-broadening coefficients of water monomer lines are identified as possible mechanisms responsible of the observed irregular fluctuations.

Moist Entropy and Water Isotopologues in a Zonal Overturning Circulation Framework of the Madden‐Julian Oscillation

AbstractThe Madden‐Julian oscillation (MJO) is the principal source of tropical intraseasonal variability, yet challenges remain to accurately simulate its observed convective behavior and eastward propagation. There is specific need for evaluating the role of water within the MJO, including evaporation, vertical transport, precipitation, and latent heating of the coupled atmosphere‐ocean system. Hydrogen isotope ratios are particularly useful for investigating these aspects of the water cycle. This study complements previous characterization of MJO joint distributions for water vapor and isotopologue concentrations (δD) with consideration of moist entropy in the tropical zonal overturning circulation framework of the MJO. The goals are to distinguish the roles of convective and large‐scale dynamic processes through the life cycle of the MJO and identify shortcomings for modeling MJO humidity, clouds, and convection. From MJO composite analyses, wet equivalent potential temperature (θq) anomalies are largest at 500 hPa and tilt westward with altitude. A positive θq anomaly co‐occurs with the precipitation maxima, and negative θq anomalies co‐occur with subsidence both trailing and leading the convective center. Out of phase with θq, δD anomalies are positive east and negative west of the convective center, coherent with the regional zonal overturning stream function. These results point to a decoupling in the MJO between midtroposphere water vapor, which is tied to convective processes, and the isotopologue ratios, which are tied to the large‐scale circulation. A conceptual model is presented to describe the physical processes that explain the MJO life‐cycle for joint distributions of humidity (q) and water vapor δD.

Bond rearrangement during Coulomb explosion of water molecules

Bond rearrangement, namely the dissociation of water ions into ${{\mathrm{H}}_{2}}^{+}+{\mathrm{O}}^{(q\ensuremath{-}1)+}$ ($q=1$--4) following fast ion-impact ionization, unexpectedly occurs following multiple ionization of water in spite of the presumably fast ``Coulomb explosion'' of the transient molecular ion. Furthermore, the branching ratio of bond rearrangement is found to be nearly equal for each level of ionization, $q$. In addition, formation of ${{\mathrm{H}}_{2}}^{+}$ is more than twice as likely to occur from the lighter water isotopologue ${\mathrm{H}}_{2}{\mathrm{O}}^{+}$ than ${{\mathrm{D}}_{2}}^{+}$ from ${\mathrm{D}}_{2}{\mathrm{O}}^{+}$. These findings are consistent with the ground state dissociation mechanism in which a fast projection of the ground state nuclear wave function onto the vibrational continuum of the cation potential energy surface is sometimes followed by ${{\mathrm{H}}_{2}}^{+}$ formation.

Water fluxes mediated by vegetation: emerging isotopic insights at the soil and atmosphere interfaces.

Plants mediate water fluxes within the soil-vegetation-atmosphere continuum. This water transfer in soils, through plants, into the atmosphere can be effectively traced by stable isotopologues of water. However, rapid dynamic processes have only recently gained attention, such as adaptations in root water uptake depths (within hours to days) or the imprint of transpirational fluxes on atmospheric moisture, particularly promoted by the development of real-time in-situ water vapour stable isotope observation techniques. We focus on open questions and emerging insights at the soil-plant and plant-atmosphere interfaces, as we believe that these are the controlling factors for ecosystem water cycling. At both interfaces, complex pictures of interacting ecophysiological and hydrological processes emerge: root water uptake dynamics depend on both spatiotemporal variations in water availability and species-specific regulation of adaptive root conductivity within the rooting system by, for example, modulating soil-root conductivity in response to water and nutrient demands. Similarly, plant water transport and losses are a fine-tuned interplay between species-specific structural and functional strategies of water use and atmospheric processes. We propose that only by explicitly merging insights from distinct disciplines - for example, hydrology, plant physiology and atmospheric sciences - will we gain a holistic picture of the impact of vegetation on processes governing the soil-plant-atmosphere continuum.

Cavity ring down spectroscopy of 17O enriched water vapor near 1.73 µm

The room temperature absorption spectrum of water vapour highly enriched in 17O is recorded by cavity ring down spectroscopy (CRDS) near 1.73 µm. Two series of recordings were performed with pressure values of about 0.12 and 8.5 Torr from 5693 to 5850 cm−1. The noise equivalent absorption (αmin) of the spectra is better than 10−10 cm−1 allowing for the measurements of water lines with intensity down to 10−29 cm/molecule. A total of 1132 lines were assigned to 1220 transitions of five water isotopologues (H216O, H217O, H218O, HD16O, and HD17O). Their intensities span near five orders of magnitude from 10−29 to 10−24 cm/molecule at 296 K. The assignments were performed using known experimental energy levels as well as calculated line lists based on the results of Schwenke and Partridge.Most of the new results concern the H217O isotopologue: 483 transitions were measured while only the 27 strongest were previously known. Overall 104 levels were newly determined for H217O (64 levels), H218O (16), HD17O (23) and HD16O (1) and a few additional levels were corrected compared to the literature. The obtained results are compared to the water line list in the HITRAN2016 database, to the ExoMol energy levels and to the empirical energy levels recommended by an IUPAC task group.

Low–temperature exchange of hydrogen and deuterium between molecular ethanol and water on Au(111)

Proton transfer and hydrogen-bonding networks are critical in biochemical processes including protein folding and protein–protein interactions. Surface science can simplify these complex systems by studying the exchange of hydrogen and deuterium atoms between molecules supported on surfaces. Temperature programmed desorption (TPD) was used to show that the exchange of hydrogen and deuterium between co–adsorbed isotopologues of ethanol and water occurs readily at temperatures below 200 K on Au(111). Since Au(111) is an inert substrate that does not cause dissociation of water or ethanol at any temperature, the results herein suggest that Au(111) acts as a support to facilitate H/D exchange via hydrogen-bonded networks of molecular ethanol and water at low temperatures.

Structure and Spectroscopy of Furan:H2O Complexes.

An analysis of the 1:1 complex of furan and water is presented. In this study, computation and matrix isolation FTIR were used to determine stable complexes of furan:water. Density functional theory and Møller-Plesset second-order, perturbation theory calculations found four, unique geometries for the complex. Two complexes were characterized by C-H···O interactions, one complex was characterized by O-H···O, and the fourth complex was characterized by O-H···π. Optimizations completed using B3LYP, B3LYP-GD3BJ, M05-2X, and MP2 showed the most stable species to be bound by O-H···O interactions. Matrix isolation experiments of mixtures of furan and water held in nitrogen at 15 K showed evidence of stable complexes when probed by FTIR. These signatures grew in intensity when matrices were annealed at 30 K. These vibrational features were predominately associated with perturbation of the water monomer. Additionally, the spectra of complexes containing water isotopologues were recorded. Analysis of spectral features pointed to the presence of a single geometry formed in the matrix, which is best described as a 1:1 complex stabilized by a O-H···O interaction.

Below‐ground resource partitioning alone cannot explain the biodiversity–ecosystem function relationship: A field test using multiple tracers

Abstract Below‐ground resource partitioning is among the most prominent hypotheses for driving the positive biodiversity–ecosystem function relationship. However, experimental tests of this hypothesis in biodiversity experiments are scarce, and the available evidence is not consistent. We tested the hypothesis that resource partitioning in space, in time or in both space and time combined drives the positive effect of diversity on both plant productivity and total community resource uptake. At the community level, we predicted that total community resource uptake and biomass production above‐ and below‐ground will increase with increased species richness or functional group richness. We predicted that, at the species level, resource partition breadth will become narrower, and that overlap between the resource partitions of different species will become smaller with increasing species richness or functional group richness. We applied multiple resource tracers (Li and Rb as potassium analogues, the water isotopologues—H218O and 2H2O, and 15N) in three seasons at two depths across a species and functional group richness gradient at a grassland biodiversity experiment. We used this multidimensional resource tracer study to test if plant species partition resources with increasing plant diversity across space, time or both simultaneously. At the community level, total community resource uptake of nitrogen and potassium and above‐ and below‐ground biomass increased significantly with increasing species richness but not with increasing functional group richness. However, we found no evidence that resource partition breadth or resource partition overlap decreased with increasing species richness for any resource in space, time or both space and time combined. Synthesis. These findings indicate that below‐ground resource partitioning may not drive the enhanced resource uptake or biomass production found here. Instead, other mechanisms such as facilitation, species‐specific biotic feedback or above‐ground resource partitioning are likely necessary for enhanced overall ecosystem function.

Stable Water Isotopologues in the Stratosphere Retrieved from Odin/SMR Measurements

Stable Water Isotopologues (SWIs) are important diagnostic tracers for understanding processes in the atmosphere and the global hydrological cycle. Using eight years (2002–2009) of retrievals from Odin/SMR (Sub-Millimetre Radiometer), the global climatological features of three SWIs, H216O, HDO and H218O, the isotopic composition δD and δ18O in the stratosphere are analysed for the first time. Spatially, SWIs are found to increase with altitude due to stratospheric methane oxidation. In the tropics, highly depleted SWIs in the lower stratosphere indicate the effect of dehydration when the air comes through the cold tropopause, while, at higher latitudes, more enriched SWIs in the upper stratosphere during summer are produced and transported to the other hemisphere via the Brewer–Dobson circulation. Furthermore, we found that more H216O is produced over summer Northern Hemisphere and more HDO is produced over summer Southern Hemisphere. Temporally, a tape recorder in H216O is observed in the lower tropical stratosphere, in addition to a pronounced downward propagating seasonal signal in SWIs from the upper to the lower stratosphere over the polar regions. These observed features in SWIs are further compared to SWI-enabled model outputs. This helped to identify possible causes of model deficiencies in reproducing main stratospheric features. For instance, choosing a better advection scheme and including methane oxidation process in a specific model immediately capture the main features of stratospheric water vapor. The representation of other features, such as the observed inter-hemispheric difference of isotopic component, is also discussed.

Anharmonic vibrational spectroscopy calculations using the ab initio CSP method: Applications to H2CO3, (H2CO3)2, H2CO3-H2O and isotopologues

The anharmonic frequencies of the O–H and the C–O stretching modes of carbonic acid are calculated using the ab initio classical separable potentials approximation. In this approach, ab initio molecular dynamics simulations are used to determine an effective classical potential for each of the normal modes of the system. The frequencies are calculated by solving the time-independent Schrödinger equation for each mode using time-averaged potentials. Hydrogen-bonded carbonic acid dimer, carbonic acid–water complex and isotopologues of these systems were also studied, showing good agreement with experiments. Useful insights are obtained by Fourier transforming the effective potentials, which relates the computed quantum frequencies to the classical dynamics of the system. This approach is illustrated for the systems studied, including for the challenging anharmonic isotope effect. In conclusion, the ab initio CSP approximation yields results in good accord with experiments, and the method provides interpretations in terms of the dynamics of vibrational motions.

Effects of variation in background mixing ratios of N<sub>2</sub>, O<sub>2</sub>, and Ar on the measurement of <i>δ</i><sup>18</sup>O–H<sub>2</sub>O and <i>δ</i><sup>2</sup>H–H<sub>2</sub>O values by cavity ring-down spectroscopy

Abstract. Cavity ring-down spectrometers have generally been designed to operate under conditions in which the background gas has a constant composition. However, there are a number of observational and experimental situations of interest in which the background gas has a variable composition. In this study, we examine the effect of background gas composition on a cavity ring-down spectrometer that measures δ18O–H2O and δ2H–H2O values based on the amplitude of water isotopologue absorption features around 7184 cm−1 (L2120-i, Picarro, Inc.). For background mixtures balanced with N2, the apparent δ18O values deviate from true values by −0.50 ± 0.001 ‰ O2 %−1 and −0.57 ± 0.001 ‰ Ar %−1, and apparent δ2H values deviate from true values by 0.26 ± 0.004 ‰ O2 %−1 and 0.42 ± 0.004 ‰ Ar %−1. The artifacts are the result of broadening, narrowing, and shifting of both the target absorption lines and strong neighboring lines. While the background-induced isotopic artifacts can largely be corrected with simple empirical or semi-mechanistic models, neither type of model is capable of completely correcting the isotopic artifacts to within the inherent instrument precision. The development of strategies for dynamically detecting and accommodating background variation in N2, O2, and/or Ar would facilitate the application of cavity ring-down spectrometers to a new class of observations and experiments.

Water isotopologues in the circumstellar envelopes of M-type AGB stars

AIM: In this study we examine rotational emission lines of two isotopologues of water: H$_2$$^{17}$O and H$_2$$^{18}$O. By determining the abundances of these molecules, we aim to use the derived isotopologue --- and hence oxygen isotope --- ratios to put constraints on the masses of a sample of M-type AGB stars that have not been classified as OH/IR stars. METHODS: We use detailed radiative transfer analysis based on the accelerated lambda iteration method to model the circumstellar molecular line emission of H$_2$$^{17}$O and H$_2$$^{18}$O for IK Tau, R Dor, W Hya, and R Cas. The emission lines used to constrain our models come from Herschel/HIFI and Herschel/PACS observations and are all optically thick, meaning that full radiative transfer analysis is the only viable method of estimating molecular abundance ratios. RESULTS: We find generally low values of the $^{17}$O/$^{18}$O ratio for our sample, ranging from 0.15 to 0.69. This correlates with relatively low initial masses, in the range $\sim1.0$ to 1.5 M$_\odot$ for each source, based on stellar evolutionary models. We also find ortho-to-para ratios close to 3, which are expected from warm formation predictions. CONCLUSIONS: The $^{17}$O/$^{18}$O ratios found for this sample are at the lower end of the range predicted by stellar evolutionary models, indicating that the sample chosen had relatively low initial masses.

Laboratory measurements of HDO/H2O isotopic fractionation during ice deposition in simulated cirrus clouds

The stable isotopologues of water have been used in atmospheric and climate studies for over 50 years, because their strong temperature-dependent preferential condensation makes them useful diagnostics of the hydrological cycle. However, the degree of preferential condensation between vapor and ice has never been directly measured at temperatures below 233 K (-40 °C), conditions necessary to form cirrus clouds in the Earth's atmosphere, routinely observed in polar regions, and typical for the near-surface atmospheric layers of Mars. Models generally assume an extrapolation from the warmer experiments of Merlivat and Nief [Merlivat L, Nief G (1967) Tellus 19:122-127]. Nonequilibrium kinetic effects that should alter preferential partitioning have also not been well characterized experimentally. We present here direct measurements of HDO/H2O equilibrium fractionation between vapor and ice ([Formula: see text]) at cirrus-relevant temperatures, using in situ spectroscopic measurements of the evolving isotopic composition of water vapor during cirrus formation experiments in a cloud chamber. We rule out the recent proposed upward modification of [Formula: see text], and find values slightly lower than Merlivat and Nief. These experiments also allow us to make a quantitative validation of the kinetic modification expected to occur in supersaturated conditions in the ice-vapor system. In a subset of diffusion-limited experiments, we show that kinetic isotope effects are indeed consistent with published models, including allowing for small surface effects. These results are fundamental for inferring processes on Earth and other planets from water isotopic measurements. They also demonstrate the utility of dynamic in situ experiments for studying fractionation in geochemical systems.

Evaluating hydrological processes in the Community Atmosphere Model Version 5 (CAM5) using stable isotope ratios of water

AbstractWater isotope‐enabled climate and earth system models are able to directly simulate paleoclimate proxy records to aid in climate reconstruction. A less used major advantage is that water isotopologues provide an independent constraint on many atmospheric and hydrologic processes, allowing the model to be developed and tuned in a more physically accurate way. This paper describes the new isotope‐enabled CAM5 model, including its isotopic physics routines, and its ability to simulate the modern distribution of water isotopologues in vapor and precipitation. It is found that the model has a negative isotopic bias in precipitation. This bias is partially attributed to model overestimates of deep convection, particularly over the midlatitude oceans during winter. This was determined by examining isotope ratios both in precipitation and vapor, instead of precipitation alone. This enhanced convective activity depletes the isotopic water vapor in the lower troposphere, where the majority of poleward moisture transport occurs, resulting in the insufficient transport of water isotopologue mass poleward and landward. This analysis also demonstrates that large‐scale dynamical or moisture source changes can impact isotopologue values as much as local shifts in temperature or precipitation amount. The diagnosis of limitations in the large‐scale transport characteristics has major implications if one is using isotope‐enabled climate models to examine paleoclimate proxy records, as well as the modern global hydroclimate.

Cycle bases to the rescue

Cycle bases of graph theory are introduced for the analysis of transition data deposited in line-by-line rovibronic spectroscopic databases. The principal advantage of using cycle bases is that outlier transitions –almost always present in spectroscopic databases built from experimental data originating from many different sources– can be detected and identified straightforwardly and automatically. The data available for six water isotopologues, H216O, H217O, H218O, HD16O, HD17O, and HD18O, in the HITRAN2012 and GEISA2015 databases are used to demonstrate the utility of cycle-basis-based outlier-detection approaches. The spectroscopic databases appear to be sufficiently complete so that the great majority of the entries of the minimum cycle basis have the minimum possible length of four. More than 2000 transition conflicts have been identified for the isotopologue H216Oin the HITRAN2012 database, the seven common conflict types are discussed. It is recommended to employ cycle bases, and especially a minimum cycle basis, for the analysis of transitions deposited in high-resolution spectroscopic databases.