Hydrogen Isotope Composition Of Water Research Articles

Hydrogen isotopic evidence for nebular pre-hydration and the limited role of parent-body processes in CM chondrites

The conditions and environments in which hydrated phases in unequilibrated meteorites formed remain debated. Among carbonaceous chondrites, Mighei-type chondrites (CMs) display a large range in the degree of aqueous alteration, and thus record different stages of hydration and alteration. Here, we report the bulk H, C, and N contents, H and C isotopic compositions, and thermogravimetric signatures of the most- and least-altered CMs known so far, Kolang and Asuka 12236, respectively. We also report in-situ SIMS measurements of the hydrogen isotopic compositions of water in both chondrites. Compared to other CMs, Asuka 12236 has the lowest bulk water content (3.3 wt.% H2O) and the most D-rich water and bulk isotopic compositions (δD = 180‰ and 280‰, respectively). Combined with literature data, our results show that phyllosilicate-bearing CMs altered to varying degrees accreted water-ice grains with similar isotopic compositions. These results demonstrate that the hydrogen isotopic variations in CM chondrites (i) are not controlled by secondary alteration processes and (ii) were mostly shaped by interactions between the protoplanetary disk and the molecular cloud that episodically fed the disk over several million years. The minimally altered CM chondrites Paris and Asuka 12236 display peculiar, D-rich, hydrogen isotopic compositions that imply the presence of another H-bearing component in addition to insoluble organic matter and phyllosilicates. This component is most likely the hydrated amorphous silicates that are ubiquitous in these chondrites. CM bulk H and O isotopic compositions are linearly correlated, implying that (i) amorphous silicates in CM matrices were already hydrated by disk processes before the onset of CM parent-body alteration, and (ii) the quest for a hypothetically water-free CM3 is illusory.

Determination of the initial hydrogen isotopic composition of the solar system

The initial isotopic composition of water in the Solar System is of paramount importance to understanding the origin of water on planetary bodies but remains unknown, despite numerous studies1–5. Here we use the isotopic composition of hydrogen in calcium–aluminium-rich inclusions (CAIs) from primitive meteorites, the oldest Solar System rocks, to establish the hydrogen isotopic composition of water at the onset of Solar System formation. We report the hydrogen isotopic composition of nominally anhydrous minerals from CAI fragments trapped in a once-melted host CAI. Primary minerals have extremely low D/H ratios, with δD values down to −850‰, recording the trapping of nebular hydrogen. Minerals rich in oxidised iron formed before the capture of the fragments record the existence of a nebular gas reservoir with an oxygen fugacity substantially above the solar value and a D/H ratio within 20% of that of the Earth’s oceans. Hydrogen isotopes also correlate with oxygen and nitrogen isotopes, indicating that planetary reservoirs of volatile elements formed within the first 2 × 105 years of the Solar System, during the main CAI formation epoch. We propose that the isotopic composition of inner Solar System water was established during the collapse of the protosolar cloud core owing to a massive admixture of interstellar water. The hydrogen isotopic composition of the oldest Solar System rocks demonstrates that a gaseous reservoir of terrestrial isotopic composition existed as early as the onset of Solar System formation and coexisted with the solar gas.

Mixing beech with fir or pubescent oak does not help mitigate drought exposure at the limit of its climatic range

In the context of climate change, it remains unclear whether mixed-species forests will help mitigate the impacts of future droughts and, if so, through which processes. As European beech (Fagus sylvatica L.) is one of the major European species, it is crucial to evaluate its response to drought when mixed with species with contrasted functional traits and in contrasted climatic conditions, particularly at the limit of its climatic range. This study aimed to (i) characterize the effects of tree species interactions on the drought exposure of beech in south-eastern France, and (ii) determine whether belowground water uptake complementarity underlies these effects. We focused on beech-silver fir (Abies alba Mill.) and beech-pubescent oak (Quercus pubescens Willd.) forests across six sites in the French pre-Alps, a region at the limit of the climatic range for beech. We used a triplet approach to compare the tree-ring carbon isotope composition (δ13C) of these species in pure and two-species mixed stands during a period of dry years, and used water hydrogen isotope composition (δ2H) in the xylem to identify water uptake sources. Overall, we found no clear mixture effect pattern on beech physiological functioning among sites and triplets. In beech-fir sites, mixing beech with fir had no effect on beech δ13C values during dry years. In beech-oak sites, mixture effects on beech were mostly neutral, although sometimes beech suffered from a stronger exposure to drought in mixed stands. Our study emphasizes the impact of the tree sampling design on the outcome of studies on forest biodiversity-ecosystem functioning relationships. Limiting tree sampling to dominant trees when analyzing stand-level relationships may bias these outcomes. We evidenced differences in water uptake sources between beech and fir, but not between beech and oak during a dry summer. However, these patterns did not help explain the lack of species mixture effects, or existence thereof, at the triplet scale. Our study demonstrates that managing beech in mixed stands with silver fir or pubescent oak at the limit of beech climatic range does not buffer drought impacts on beech during dry years. In the long term, with more frequent extreme droughts, promoting beech-fir mixtures will not be detrimental to beech drought response, while beech may suffer in mixtures with pubescent oak.

Preservation of primordial signatures of water in highly-shocked ancient lunar rocks

Spurred by the discovery of water in lunar volcanic glasses about a decade ago, the accessory mineral apatite became the primary target to investigate the abundance and source of lunar water. This is due to its ability to contain significant amounts of OH in its structure, along with the widespread presence of apatite in lunar rocks. There is a general understanding that crustal cumulate rocks of the lunar magnesian (Mg) suite are better candidates for recording the original isotopic compositions of volatile elements in their parental melts compared to eruptive rocks, such as mare basalts. Consequently, water-bearing minerals in Mg-suite rocks are thought to be ideal candidates for discerning the primary hydrogen isotopic composition of water in the lunar interior. Mg-suite rocks and most other Apollo samples that were collected at the lunar surface display variable degrees of shock-deformation. In this study, we have investigated seven Apollo 17 Mg-suite samples that include troctolite, gabbro and norite lithologies, in order to understand if shock processes affected the water abundances and/or H isotopic composition of apatite. The measured water contents in apatite grains range from 31 to 964 ppm, with associated δD values varying between −535 ± 134‰ and +147 ± 194‰ (2σ). Considering the full dataset, there appears to be no correlation between H2O and δD of apatite and the level of shock each apatite grain has experienced. However, the lowest δD was recorded by individual, water-poor (<∼100 ppm H2O) apatite grains that are either directly in contact with an impact melt or in its proximity. Therefore, the low-δD signature of apatite could be a result of interactions with D-poor regolith (solar wind derived H), facilitated by shock-induced nanostructures that could have provided pathways for migration of volatiles. In contrast, in relatively water-rich apatites (>∼100 ppm H2O), regardless of the complexity of the shock-induced nanostructures, there appears to be no evidence of water-loss or alteration in their δD. The weighted average δD value of 24 such water-rich apatites is −192 ± 71‰, and, of all 36 analyzed spots is −209 ± 47‰, indistinguishable from that of other KREEPy lunar lithologies or the Earth's deep mantle. Despite experiencing variable degrees of shock-deformation at a later stage in lunar history, water-rich apatite in some of the earliest-formed lunar crustal material appears to retain the original isotopic signature of H in the Moon.

Experimental evaluation of oxygen isotopic exchange between inclusion water and host calcite in speleothems

Abstract. The oxygen and hydrogen isotopic compositions of water in fluid inclusions in speleothems are important hydroclimate proxies because they provide information on the isotopic compositions of rainwater in the past. Moreover, because isotopic differences between fluid inclusion water and the host calcite provide information on the past isotopic fractionation factor, they are also useful for quantitative estimation of past temperature changes. The oxygen isotope ratio of inclusion water (δ18Ofi), however, may be affected by isotopic exchange between the water and the host carbonate. Thus, it is necessary to estimate the bias caused by this postdepositional effect for precise reconstruction of paleotemperatures. Here, we evaluate the isotopic exchange reaction between inclusion water and host calcite based on a laboratory experiment involving a natural stalagmite. Multiple stalagmite samples cut from the same depth interval were heated at 105 ∘C in the laboratory from 0 to 80 h. Then, the isotopic compositions of the inclusion water were measured. In the 105 ∘C heating experiments, the δ18Ofi values increased from the initial value by 0.7 ‰ and then remained stable after ca. 20 h. The hydrogen isotope ratio of water showed no trend in response to the heating experiments, suggesting that the hydrogen isotopic composition of fluid inclusion water effectively reflects the composition of past drip water. We then evaluated the process behind the observed isotopic variations using a partial equilibration model. The experimental results are best explained by the assumption that a thin CaCO3 layer surrounding the inclusion reacted with the water. The amount of CaCO3 that reacted with the water is equivalent to 2 % of the water inclusions in molar terms. These results suggest that the magnitude of the isotopic exchange effect has a minor influence on paleotemperature estimates for Quaternary climate reconstructions.

Hydrogen isotopic ratios of long-chain diols reflect salinity

Long-chain diols (LCDs) are ubiquitous lipids produced by freshwater and marine algae. A combination of semi-preparative high performance liquid chromatography and gas chromatography isotope ratio monitoring mass spectrometry, allowed the measurement of δ2H of individual LCDs from cultures, which indicated a correlation with the hydrogen isotope composition of the growth water and a species-specific effect. Results from environmental samples along a salinity gradient indicated the potential of δ2H ratios of LCDs to trace the hydrogen isotopic composition of water and sea surface salinity.

Oxygen and hydrogen isotopic composition of waters in a past-mining area of southern Apuan Alps (Italy): Hydrogeological characterization and implications on the fate of potentially toxic elements

The oxygen and hydrogen isotopic composition of stream water, springs, groundwater tapped from irrigation wells and acid mine drainages were determined during two different surveys in 2015 in an area highly impacted by past-mining activity of Apuan Alps (Italy), as a guidance on good practices for water management. The isotopic local meteoric water line (LMWL) was built by monthly collecting rainwater between 2014 and 2018, given by δD = 7.02 ± 0.35 × δ18O + 8.54 ± 2.89. The obtained results indicate that acid mine drainages (AMD) are supplied by freshwater from karst systems which flow throughout the post-mining workings. Such waters contaminate by interactions with sulfides (pyrite) that remained unmined in the ore-bodies. During rainstorms, infiltration rainwater displaces water ponding within mine, sharply increasing the outflow rate of highly-contaminated AMD. Acidic drippings in tunnels show an isotopic shift in both δ18O and δD values attributable to pyrite oxidation and Fe hydrolysis. The data reveal that karst-springs represents the primary supply for the stream. The isotopic data also reveal that waters flowing in the bedrock carbonate aquifer represent the main feeding component for the overlying alluvial aquifer tapped by wells. The prevailing transfer of clean freshwaters from the carbonate aquifer towards the alluvial aquifer system mitigates the possible influence of contaminated water from stream seepage. However, these observations require a monitoring program on water quality to be established.

Hydrogen isotopic composition of water in CV-type carbonaceous chondrites

Among the different groups of carbonaceous chondrites, variable concentrations of hydrous minerals and organic matter are observed that might be related to the time and/or place of formation of their asteroidal parent bodies. However, the precise distribution of these volatile-bearing components between chondrite groups and their chemical and isotopic compositions remain fairly unknown. In this study, we used a novel secondary ion mass spectrometry analytical protocol to determine the hydrogen isotopic composition of water-bearing minerals in CV-type carbonaceous chondrites. This protocol allows for the first time the D/H ratio of CV chondrite hydrous minerals to be determined without hindrance by hydrogen contributions from adjacent organic material. We found that water in the altered CV chondrites Kaba, Bali, and Grosnaja has an average D/H ratio of D/HCV-water = [144−21+8] × 10−6 (or δDCV-water = −77−131+54‰, 2σ), significantly higher than water in most CM-type carbonaceous chondrites (D/HCM-water = [101 ± 6] × 10−6 or δDCM-water = −350 ± 40‰, 2σ). We show that because organic matter in CV chondrites is depleted in deuterium compared to that in CM chondrites, such differences could result from isotopic exchange between water and organics. Another possibility is that the CM and CV parent bodies sampled different reservoirs of water ice and organics characterized by variable isotopic compositions due to their different time and/or place of accretion.

The δ18 O and δ2 H of water in the leaf growth-and-differentiation zone of grasses is close to source water in both humid and dry atmospheres.

The oxygen and hydrogen isotope composition of water in the leaf growth-and-differentiation zone, LGDZ, (δ18 OLGDZ , δ2 HLGDZ ) of grasses influences the isotopic composition of leaf cellulose (oxygen) and wax (hydrogen) - important for understanding (paleo)environmental and physiological information contained in these biological archives - but is presently unknown. This work determined δ18 OLGDZ and δ2 HLGDZ , 18 O- and 2 H-enrichment of LGDZ (∆18 OLGDZ and ∆2 HLGDZ ), and the 18 O- and 2 H-enrichment of leaf blade water (∆18 OLW, ∆2 HLW ) in two C3 and three C4 grasses grown at high and low vapor pressure deficit (VPD). The proportion of unenriched water (px ) in the LGDZ ranged from 0.9 to 1.0 for 18 O and 1.0 to 1.2 for 2 H. VPD had no effect on the proportion of 18 O- and 2 H-enriched water in the LGDZ, and species effects were small or nonsignificant. Deuterium discrimination caused depletion of 2 H in LGDZ water, increasing (apparent) px -values >1.0 in some cases. The isotopic composition of water in the LGDZ was close to that of source water, independent of VPD and much less enriched than previously supposed, but similar to reported xylem water in trees. The well-constrained px will be useful in future investigations of oxygen and hydrogen isotopic fractionation during cellulose and wax synthesis, respectively.

Volcano–hydrothermal system of Ebeko volcano, Paramushir, Kuril Islands: Geochemistry and solute fluxes of magmatic chlorine and sulfur

Ebeko volcano at the northern part of Paramushir Island in the Kuril island arc produces frequent phreatic eruptions and relatively strong fumarolic activity at the summit area ~1000m above sea level (asl). The fumaroles are characterized by low-temperature, HCl- and S-rich gas and numerous hyper-acid pools (pH<1) without drains. At ~550m asl, in the Yurieva stream canyon, many hot (up to 87°C) springs discharge ultra-acidic (pH1–2) SO4–Cl water into the stream and finally into the Sea of Okhotsk. During quiescent stages of degassing, these fumaroles emit 1000–2000t/d of water vapor, <20t/d of SO2 and <5t/d of HCl. The measurement of acidic hot Yurieva springs shows that the flux of Cl and S, 60–80t/d each, is independent on the volcanic activity in the last two decades. Such high flux of Cl is among the highest ever measured in a volcano–hydrothermal system. Oxygen and hydrogen isotopic composition of water and Cl concentration for Yurieva springs show an excellent positive correlation, indicating a mixing between meteoric water and magmatic vapor. In contrast, volcanic gas condensates of Ebeko fumaroles do not show a simple mixing trend but rather a complicated data suggesting evaporation of the acidic brine. Temperatures calculated from gas compositions and isotope data are similar, ranging from 150 to 250°C, which is consistent with the presence of a liquid aquifer below the Ebeko fumarolic fields. Saturation indices of non-silicate minerals suggest temperatures ranging from 150 to 200°C for Yurieva springs. Trace elements (including REE) and Sr isotope composition suggest congruent dissolution of the Ebeko volcanic rocks by acidic waters. Waters of Yurieva springs and waters of the summit thermal fields (including volcanic gas condensates) are different in Cl/SO4 ratios and isotopic compositions, suggesting complicated boiling–condensation–mixing processes.

New evidence for chondritic lunar water from combined D/H and noble gas analyses of single Apollo 17 volcanic glasses

In order to assess the proportion of solar, cosmogenic, and indigenous water (hydrogen) trapped in individual Ti-rich lunar volcanic glasses (LVGs) from the 74002 core obtained during the Apollo 17 mission, we coupled ion microprobe measurements of water abundances and D/H ratios with CO2 laser extraction-static mass spectrometry analyses of noble gases (He, Ne, Ar). The large (∼300–400μm in diameter) LVGs studied here contain a small amount of solar wind (SW) volatiles implanted at the grain surfaces, as indicated by the small concentrations of solar helium and neon that represent ⩽5% of the respective total noble gas abundances. The large proportion of volume-correlated cosmogenic gases reflects an exposure duration of ∼28Ma, on average, of the glasses at the lunar surface. Hydrogen abundances determined in the grain interiors of glassy and partially-crystalline LVGs are equivalent to between 6.5 and 54.3ppm H2O. Based on the noble gas exposure ages, the correction of the measured hydrogen isotope composition for in situ production of cosmogenic deuterium by spallation reactions varies between −5‰ to −254‰ for the different grains. Corrected δD values range from +38‰ to +809‰ in the LVGs and are anti-correlated with the water content, consistent with extensive hydrogen isotope fractionation during kinetic H2 loss from a lunar melt with an inferred initial isotope signature of the order of −100‰ and a water content of 100–300ppm. The detection of water in these primitive lunar melts confirms the presence of a non-anhydrous mantle reservoir within the Moon. Furthermore, our results reveal that the hydrogen isotope composition of water in the melt source of the 74002 LVGs is similar to that of carbonaceous chondrites. These observations indicate that the contribution of deuterium-enriched cometary water to the Earth–Moon system is negligible.

Hydrogen isotope fractionation during lipid biosynthesis by Tetrahymena thermophila

Hydrogen isotope ratio values of lipids are increasingly used to reconstruct past variation in hydrological conditions. However, apart from recording the hydrogen isotope composition of ambient water, δD values of lipids also depend on specific biosynthetic pathways and growth conditions. We have evaluated the hydrogen isotope fractionation by the ciliated protozoan, Tetrahymena thermophila, grown in pure culture at three temperatures (24°C, 30°C and 36°C) and in water with a range of hydrogen isotope composition. T. thermophila synthesizes tetrahymanol, a pentacyclic triterpenoid alcohol and the diagenetic precursor of the biomarker gammacerane. We focused our attention on the isotopic controls on tetrahymanol and various fatty acids (FAs).The δD values of FAs and tetrahymanol correlated linearly with the hydrogen isotope composition of water, but growth temperature was also clearly an important factor controlling lipid D/H compositon. Hydrogen isotope fractionation during tetrahymanol biosynthesis changed with higher growth temperatures, resulting in D-depleted signatures relative to water at 24°C and 30°C and D-enriched composition at 36°C. T. thermophila grown at 36°C – a temperature above opitmum growth conditions – showed a significant change in lipid composition, with the abundance of tetrahymanol increasing relative to total FAs. We suggest the change is a response to temperature stress and a decrease in the stability of the cell membrane. The temperature effect is also presumed to impact δDlipid by altering the hydrogen isotopic composition of NADPH and potentially intracellular water.

The influence of metabolic effects on stable hydrogen isotopes in tissues of aquatic organisms

A steady-state mass-balance model describing controls on the stable hydrogen isotopic ratios (δ2H) of tissues in fish was previously developed but physiological effects related to fish size and growth had not been tested. Here, we assessed the influence of size (or growth rate) on tissue δ2H composition of a fish species (Poecilia reticulata) and the incorporation of metabolic products derived from dietary lipids (water, NADH). Sampled tissues were obtained from individuals that grew at different rates while raised on an isotopically homogeneous commercial diet (lipid-free fraction, δ2H=−95±2 ‰; and dietary lipids,−198±11 ‰) under different controlled water hydrogen isotopic composition (δ2H=−128±3 ‰;+17±5 ‰; and+202±5 ‰). Our findings suggested that fish growth rate was correlated positively with the degree of incorporation of metabolic products from dietary lipids that, in turn, influenced both fish tissue protein and lipid δ2H values. We conclude that δ2H measurements of lipids (and, subsequently, of body water) in fish could become a physiological tool that provides insights into fish growth rates.

Isotope Approach to Assess Hydrologic Connections During Marcellus Shale Drilling

Water and gas samples were collected from (1) nine shallow groundwater aquifers overlying Marcellus Shale in north-central West Virginia before active shale gas drilling, (2) wells producing gas from Upper Devonian sands and Middle Devonian Marcellus Shale in southwestern Pennsylvania, (3) coal-mine water discharges in southwestern Pennsylvania, and (4) streams in southwestern Pennsylvania and north-central West Virginia. Our preliminary results demonstrate that the oxygen and hydrogen isotope composition of water, carbon isotope composition of dissolved inorganic carbon, and carbon and hydrogen isotope compositions of methane in Upper Devonian sands and Marcellus Shale are very different compared with shallow groundwater aquifers, coal-mine waters, and stream waters of the region. Therefore, spatiotemporal stable isotope monitoring of the different sources of water before, during, and after hydraulic fracturing can be used to identify migrations of fluids and gas from deep formations that are coincident with shale gas drilling.

Water transport in protoplanetary disks and the hydrogen isotopic composition of chondrites

The D/H ratios of carbonaceous chondrites, believed to reflect the hydrogen isotopic composition of water in the inner early Solar System, are intermediate between the protosolar value and that of most comets. The isotopic composition of cometary water has been accounted for by several models where the isotopic composition of water vapor evolved by isotopic exchange with hydrogen gas in the protoplanetary disk. However, the position and the large range of variation of the distribution of D/H ratios in carbonaceous chondrites have yet to be explained. In this paper, we assume that the D/H composition of cometary ice was achieved in the disk building phase and model the further isotopic evolution of water in the inner disk in the classical T Tauri stage. Reaction kinetics compel isotopic exchange between water and hydrogen gas to stop at ∼500K, well inside the snow line. However, the equilibrated water can be transported to the snow line (and beyond) via turbulent diffusion and consequently mix with isotopically comet-like water. Thus the competition between outward diffusion and net inward advection established an isotopic gradient, which is at the origin of the large isotopic variations in the carbonaceous chondrites and other water-bearing objects accreted in the protoplanetary disk.Under certain simplifying assumptions, we calculate analytically the probability distribution function of the D/H ratio of ice accreted in planetesimals and compare it with observational data. The distribution is found to essentially depend on two parameters: the radial Schmidt number ScR, which ratios the efficiencies of angular momentum transport and turbulent diffusion, and the range of heliocentric distances over which currently sampled chondrite parent bodies were accreted. The minimum D/H ratio of the distribution corresponds to the composition of water condensed at the snow line, which is a function of both the composition of equilibrated water having diffused from hotter disk regions and the efficiency of this outward transport as measured by ScR. Observations constrain the latter to low values (0.1–0.3), which suggests that turbulence in the planet-forming region was hydrodynamical in nature, as would be expected in a dead zone. Such efficient outward diffusion would also account for the presence of high-temperature minerals in comets.

Hydrogen isotopic composition of water from fossil micrometeorites in howardites

We have measured the hydrogen isotopic composition (D/H ratios) of the water from 13 carbonaceous chondritic microclasts (CCMs, size <1 mm) trapped in two howardites (Kapoeta and Yamato-793497) early in the evolution of Solar System. The division into tochilinite-rich; magnetite-rich, olivine-poor; magnetite-rich, olivine-rich CCM types is corroborated by the hydrogen isotopic compositions. Both mineralogy and hydrogen isotopic compositions demonstrate that tochilinite-rich CCMs represent CM2 chondritic matter. In contrast, there is no good match between the isotopic and mineralogical properties of the magnetite-rich CCMs and the known groups of carbonaceous chondrites, suggesting that magnetite-rich CCMs represent a new kind of chondritic matter, not yet sampled in meteorite collections. This demonstrates that the view of the asteroid belt revealed by the collection of meteorites is incomplete. The study of (micro)clasts offers a unique opportunity to better decipher the nature and relative abundance of asteroids. The average hydrogen isotopic composition of water belonging to CCMs, D/H = (152.0 ± 4.8) × 10 −6 (1σ m), is similar to that of Antarctic micrometeorites (AMMs), D/H = (161.2 ± 3.8) × 10 −6 (1σ m). The similarity, in terms of mineralogy and hydrogen isotopic composition, between CCMs and AMMs demonstrates that the composition of the micrometeorites has not been modified over the whole history of the Solar System. It indicates that the composition of the micrometeorite flux onto Earth has been, and is, dominated by a mixture of CM2-like; magnetite-rich, olivine-poor; magnetite-rich, olivine-rich carbonaceous chondritic matter exemplified by CCMs found in howardites. Because CCMs have not suffered atmospheric entry, they provide an abundant source of pristine micrometeorites. The average D/H ratio of the whole population of CCMs is identical within errors to that of the Earth (149 ± 3 × 10 −6). The match between the CCMs D/H ratio and that of the Earth is especially remarkable because 1) three different populations of CCMs are needed to make the D/H ratio of the Earth; 2) there is no single carbonaceous chondrite group for which a similar match exists. This observation suggests that CCMs population might be representative of the late veneer agent(s) that delivered water to the Earth.

The global influence of the hydrogen isotope composition of water on that of bacteriogenic methane from shallow freshwater environments

We propose that the hydrogen isotope composition of recently produced microbial methane, δD(CH4), in sulfate-poor, shallow freshwater environments, is directly related to the hydrogen isotopic composition of the system water δD(H2O). As δD(H2O) varies globally, systematic differences in δD(CH4) as a function of δD(H2O) should be observed.From available mean paired measurements from 46 sites, the relationship for δD(CH4) and δD(H2O) in the natural environment can be defined as δD(CH4) = 0.675δD(H2O) −284‰ (p < 0.0001). This relationship is statistically distinct from that generated by considering three separate laboratory-based anaerobic inoculations that contain similar methanogenic communities to the natural freshwater samples and therefore, are likely to produce methane by similar metabolic pathways: δD(CH4) = 0.444δD(H2O) −321‰ (p < 0.0001). We suggest that the relationship arising from the laboratory incubations defines the δD(CH4) of methane produced at source in shallow freshwater environments. We can approximate that 50% of the variation in natural δD(CH4) samples can be explained by δD(H2O), with isotopic fractionation postproduction, or mixing with gas already fractionated likely responsible for most of the noise in the natural system and difference of the natural sample relationship to the laboratory relationship. Methanogenic pathway may also influence δD(CH4), but the foundations for this hypothesis need to be reconsidered, and field and laboratory data exist that do not support it. The relationships presented here describe δD of methane from only shallow (subsurface) freshwater environments; paired δD(CH4)–δD(H2O) values from other environments (e.g., marine, glacial drift) suggest that a different relationship is needed to describe the influence of δD(H2O) on δD(CH4).

Hydrogen isotope geochemistry of SNC meteorites

We report the yields and hydrogen isotopic compositions of water extracted by stepwise heating of eight whole-rock SNC meteorites. D/H values are consistent with terrestrial water at low temperatures and increase with temperature to values that far exceed terrestrial. The data are consistent with the water in the samples originating from two sources: a terrestrial component/contaminant, released largely at low temperature, and an extraterrestrial component, released at high temperature. The variation in δD values of the high-temperature hydrogen (∼+250 to +900 for the nakhlites, ∼+1200 to +2100 for the shergottites, and ∼+800 for ALH84001) could represent true variation of the δD of the extra-terrestrial water in the samples, or may reflect varying contributions of the terrestrial endmember, even at high temperature. The high δD values are consistent with a martian origin for the meteorites since the current martian atmosphere contains water with a δD of ∼+4000. The presence of alteration products in at least some of the samples suggests the D-enriched water was probably incorporated into the rocks through interaction at low temperature with aqueous crustal fluids that had exchanged with the martian atmosphere. Chassigny contains water that has terrestrial δD values at all temperatures (with the possible exception of the highest temperature step, with δD up to ∼+50), suggesting contamination of this sample by terrestrial water. Carbon and oxygen isotopic results for CO 2 extracted by stepwise heating are also reported. These data are consistent with formation of carbonates in the SNC meteorites by secondary processes on Mars (from fluids that had exchanged C and O with the atmosphere), perhaps in the same alteration events that formed the D-enriched minerals.

Hydrogen isotope exchange reaction rates: Origin of water in the inner solar system

Rate constants for hydrogen isotope exchange between molecular hydrogen and water ( k( T) H 2- HDO ) and hydrogen and methane ( k( T) H 2- CH 3 D ) have been determined experimentally. The temperature dependence of the constants can be expressed by: with k( T) in cm 3/sec. Catalytic effects on charcoal, silica, phyllosilicates, and iron were found to be negligible. A model describing the evolution (with time) of the hydrogen isotopic composition of water in the solar nebula is proposed: while the temperature is decreasing in the gas, isotope exchange between H 2 and H 2O yields a deuterium enrichment in water. Using Cameron's numerical models (1972, 1978, 1985) of the solar nebula, we show that the observed difference between the protosolar and the carbonaceous chondrite D/H ratios (i.e., between 30 and 150 10 −6, respectively) is well accounted for by our model, provided that (1) water was accreted as ice at a temperature of ≤ 160 K. and (2) the hydrogen pressure was higher than 10 −5 atm somewhere between 1 and 3 AU. In such conditions, the calculations show that meteoritic phyllosilicates did not form in the gas phase, by reactions between water vapour and silicate grains, but by the subsequent circulation of liquid water, originally condensed as ice. Water on Earth ( D/ H = 155 · 10 −6) also results from such a cold nebula chemistry.

Fractionnement isotopique lors des changements d‘état solide-vapeur et liquide-vapeur de l'eau à des températures inférieures à 0°C

Measurements of the hydrogen isotopic composition of water in atmospheric precipitations make possible the study of their formation and evolution, since deuterium is a natural tracer for the water molecule. In a hydrologic cycle, each change of state is accompanied by an isotopic fractionation expressed in terms of a factor ?, which is a function of temperature. This fractionation is due to differences in the physical properties of the molecules H 2 O and HDO. In the work described in this paper we have measured ? between ?40°C and 0°C for the phase equilibrium ice-water vapor and between ?15°C and 0°C for the equilibrium liquid-water vapor. The values obtained may be expressed in the relations log ?solide-vapeur= ?4.10 · 10 ?2 + 7.074/ T 2 log ?liquide-vapeur= ? 4.34 · 10 ?2 + 6.520/ T 2 . DOI: 10.1111/j.2153-3490.1967.tb01465.x