Stable Noble Gas Research Articles

Noble gas geochemistry and chronology of groundwater in an active rift basin in central China

Stable noble gas isotopes are excellent groundwater tracers. Radioactive noble gases are emerging new tools in the study of groundwater circulation dynamics. Among these, the 85Kr and 81Kr, and 39Ar have advanced very fast in recent years and exhibit strong potential in the reconstruction of the history of groundwater recharge and evolution in sedimentary basins at different scales. Here, we report the findings in groundwater circulation dynamics as relative to intensive water-rock interactions, heat transfer and He gas flux in Guanzhong Basin located in Xi’an, the geographical centre of China, which is a rift basin created by collision between the Eurasia and Indian plates, with active neotectonic activities. The recent technological breakthrough in noble gas isotope measurements, i.e. the atomic trap trace analysis (ATTA) techniques on Kr and Ar gas radionuclei, has revolutionized groundwater dating. Noble gas samples from shallow and deep wells to 3000 m depth have been collected to study isotope variations to reconstruct the history of groundwater recharge and understand the water-rock interaction processes. Stable isotopes of water show strong water-rock interaction in the formation, creating a strong positive O-isotope shift up to 10 ‰, a phenomenon that is rarely seen in a fairly low temperature environment. Analysis of 85Kr and 81Kr show groundwater ages up to 1.3 million years old along both North-South and a West-East cross sections, which offers strong evidence about the slow moving flow, strong water-rock interaction, rich geothermal resources as well as He gas resources.

South African crustal fracture fluids preserve paleometeoric water signatures for up to tens of millions of years

Fracture fluids in Earth's crust may remain isolated for millions to billions of years, and contain information on paleohydrogeology, subsurface microbial life, and conservative components that help elucidate the atmospheric evolution of the early Earth. Examples include fluids in the South African Kaapvaal craton which host chemolithoautotrophic microbial communities that survive independent of the photosphere, and billion-year-old fluids in the Canadian Shield, which preserve the Xe isotopic signature of an evolving early atmosphere. Stable isotope analyses of the aqueous phase combined with isotopic analyses of the dissolved noble gases provide unrivalled insight into the time-alteration history of aqueous fracture fluids. Here we report stable isotope and noble gas data for fracture fluids in the Witwatersrand Basin and Bushveld Igneous Province systems, South Africa. We determine closed-system radiogenic noble gas residence times of 0.77–97 million years (Myr). Open-system residence times range between 6.0 kyr and 10.8 Myr. One sample from Masimong Mine has a mean closed-system residence time of 85 Myr, making it one of oldest paleometeoric waters ever recorded. The δ2H and δ18O of water in this sample, and in previously reported samples from the same mining district that are shown to have similar ages, require an isotopically depleted source of groundwater recharge. This could reflect a recharge regime at a higher paleolatitude, elevation, or with higher rainfall, established up to tens of Myr ago, and perhaps similar to the recharge regime in the modern Lesotho Highlands. These data suggest that groundwater isotopes can provide useful paleoclimatic information for many Myr.

Tracers Reveal Recharge Elevations, Groundwater Flow Paths and Travel Times on Mount Shasta, California

Mount Shasta (4322 m) is famous for its spring water. Water for municipal, domestic and industrial use is obtained from local springs and wells, fed by annual snow melt and sustained perennially by the groundwater flow system. We examined geochemical and isotopic tracers in samples from wells and springs on Mount Shasta, at the headwaters of the Sacramento River, in order to better understand the hydrologic system. The topographic relief in the study area imparts robust signatures of recharge elevation to both stable isotopes of the water molecule (δ18O and δD) and to dissolved noble gases, offering tools to identify recharge areas and delineate groundwater flow paths. Recharge elevations determined using stable isotopes and noble gas recharge temperatures are in close agreement and indicate that most snowmelt infiltrates at elevations between 2000 m and 2900 m, which coincides with areas of thin soils and barren land cover. Large springs in Mt Shasta City discharge at an elevation more than 1600 m lower. High elevation springs (>2000 m) yield very young water (<2 years) while lower elevation wells (1000–1500 m) produce water with a residence time ranging from 6 years to over 60 years, based on observed tritium activities. Upslope movement of the tree line in the identified recharge elevation range due to a warming climate is likely to decrease infiltration and recharge, which will decrease spring discharge and production at wells, albeit with a time lag dependent upon the length of groundwater flow paths.

Differentiating between biogenic and thermogenic sources of natural gas in coalbed methane reservoirs from the Illinois Basin using noble gas and hydrocarbon geochemistry

Abstract While coalbed methane (CBM) is a significant source of natural gas production globally, uncertainties regarding the proportions of biogenic and thermogenic natural gas in CBM reservoirs still remain. We integrate major gases, hydrocarbon composition, hydrocarbon stable isotopes and noble gases in fluids from 20 producing CBM wells to more accurately constrain the genetic source of natural gases in the eastern Illinois Basin, USA. Previous studies have indicated primarily biogenic production of methane (>99.6%) with negligible contributions from thermogenic natural gases. However, by integrating noble gases, we identify quantifiable (up to 19.2%) contributions of exogenous thermogenic gas in produced gases from the Seelyville and Springfield coal seams. Thermogenic gases are distinguished by a positive relationship between methane, ethane and helium-4, lower C 1 /C 2 +, heavier δ 13 C-CH 4 , more radiogenic noble gases ( 4 He, 21 Ne*, 40 Ar*), and lower abundances of atmospherically derived gases ( 20 Ne, 36 Ar). Biogenic gases displayed lighter δ 13 C-CH 4 , higher C 1 /C 2 +, higher levels of atmospheric gases and lower abundances of radiogenic noble gases. Our data suggest that natural gases from a deeper, exogenous thermogenic source likely migrated to the Pennsylvanian-aged coals at an unknown time and later mixed with biogenic methane diluting the geochemical signature of the thermogenic methane within the Springfield and Seelyville coal seams.

Using noble gas fingerprints at the Kerr Farm to assess CO2 leakage allegations linked to the Weyburn-Midale CO2 monitoring and storage project

For carbon capture and storage technology to successfully contribute to climate mitigation efforts, the stored CO2 must be securely isolated from the atmosphere and oceans. Hence, there is a need to establish and verify monitoring techniques that can detect unplanned migration of injected CO2 from a storage site to the near surface. Noble gases are sensitive tracers of crustal fluid input in the subsurface due to their low concentrations and unreactive nature. Several studies have identified their potential to act as tracers of deep fluid migration to the shallow subsurface, but they have yet to be used in a contested situation. In January 2011 it was reported extensively in global media that high CO2 concentrations in soils and related groundwater pollution had been identified on a farm property belonging to the Kerr family, located near to the town of Weyburn in Saskatchewan, Canada. The origin of this CO2 pollution was cited to be the nearby Weyburn-Midale CO2 Monitoring and Storage Project. Here, as part of an investigation funded independently of the Weyburn-Midale field operators, we present δ13CDIC, 3He/4He, 4He/20Ne, 20Ne, 36Ar, 40Ar and Kr measured in waters obtained from four groundwater wells located on and surrounding the Kerr property. We aim to establish if stable carbon and noble gas natural tracers are effective at determining if migration of CO2 from the storage project was responsible for the alleged high CO2 concentrations and water pollution measured on the Kerr farm. We compare the stable carbon isotope and noble gas ‘fingerprints’ of the Kerr groundwaters to those expected in a water equilibrated with the atmosphere under local recharge conditions, the produced CO2 obtained from production wells, and the CO2 injected into the Weyburn and Midale oil fields. We find that the stable carbon isotope data do not constrain the origin of the dissolved CO2 in the Kerr groundwaters. Due to low noble gas concentrations in the captured CO2 we are unable to completely rule out the presence of 20–34% contribution from injected CO2 to the groundwaters surrounding the Kerr property. However, we find that all of the Kerr groundwater samples exhibit noble gas fingerprints that would be expected in a shallow groundwater in contact with the atmosphere and hence there is no evidence for the addition of a deep radiogenic component or dilution from the addition of a gas phase low in atmospheric derived noble gases. Our findings corroborate previous studies that indicate that elevated CO2 concentrations found on the Kerr property are almost certainly of biological origin, and not migrated from the deep subsurface. The comprehensive follow up to these CO2 leakage allegations outlined in this study provides a robust framework for responses to any future leakage allegations at CO2 storage sites and further highlights that no single technique can categorically identify the origin of CO2 in the shallow subsurface. Hence, it is essential that the full range of geochemical tracers (stable carbon and 14C isotopes, noble gases, water chemistry, process based gas ratios) are integrated with a good understanding of geological and engineering data in response to CO2 leakage allegations in the future.

Genesis of Paleogene gas in the Dongpu Depression, Bohai Bay Basin, East China

The Dongpu Depression, located in the southwest of the Bohai Bay Basin, east China, is enriched in natural gas with proven reserves of about 480 billion m3. However, their genesis and distribution are not completely clear. On the basis of molecular, stable carbon isotope and noble gas isotope (Ar) composition of a large amount of gas samples and combined with the geological background, this paper analyzed the genesis and migration features of Paleogene gas from major gas fields in the Dongpu depression. The results show that Paleogene gases in the Dongpu Depression are of thermogenic origin, and can be divided into coal-derived gas, oil-associated gas and their mixture. Furthermore, Natural gases from the E2s3L and E2s3M are predominantly oil-associated gas generated from E2s3 source rocks, while those from E2s4, and E2s3U and E2s2 in the Baimiao area are derived from the Carboniferous-Permian coaly source rocks. In addition, the coal-derived gas can also mix with the oil-associated gas, which can be called mixed coal-derived gas and oil-associated gas. Carbon isotope data suggest that gases of E2s3 and most E2s2 are predominately unaltered, while those of E2s4 and several E2s2 mostly from Baimiao area are altered and undergoing strong dissipation. The geographical variation in the gas composition can be attributed to the geochemical fraction caused by vertical migration through the basement faults for the coal-derived gas in E2s4 and several E2s3U and E2s2 in Baimiao area. While the oil-associated gas in E2s3L and E2s3M are resulted from the near-source or even in situ accumulation.

Stable isotope and noble gas constraints on the source and residence time of spring water from the Table Mountain Group Aquifer, Paarl, South Africa and implications for large scale abstraction

Large scale groundwater abstraction is increasingly being used to support large urban centres especially in areas of low rainfall but presents particular challenges in the management and sustainability of the groundwater system. The Table Mountain Group (TMG) Aquifer is one of the largest and most important aquifer systems in South Africa and is currently being considered as an alternative source of potable water for the City of Cape Town, a metropolis of over four million people. The TMG aquifer is a fractured rock aquifer hosted primarily in super mature sandstones, quartzites and quartz arenites. The groundwater naturally emanates from numerous springs throughout the cape region. One set of springs were examined to assess the source and residence time of the spring water. Oxygen and hydrogen isotopes indicate that the spring water has not been subject to evaporation and in combination with Na/Cl ratios implies that recharge to the spring systems is via coastal precipitation. Although rainfall in the Cape is usually modelled on orographic rainfall, δ18O and δ2H values of some rainfall samples are strongly positive indicating a stratiform component as well. Comparing the spring water δ18O and δ2H values with that of local rainfall, indicates that the springs are likely derived from continuous bulk recharge over the immediate hinterland to the springs and not through large and/or heavy downpours. Noble gas concentrations, combined with tritium and radiocarbon activities indicate that the residence time of the TMG groundwater in this area is decadal in age with a probable maximum upper limit of ∼40years. This residence time is probably a reflection of the slow flow rate through the fractured rock aquifer and hence indicates that the interconnectedness of the fractures is the most important factor controlling groundwater flow. The short residence time of the groundwater suggest that recharge to the springs and the Table Mountain Group Aquifer as a whole is vulnerable to climate change and reductions in regional precipitation. Any plans for large scale abstraction to supplement the City of Cape Town water supply would need to factor this into models of maximum sustainable yield.

Characterizing the Noble Gas Isotopic Composition of the Barnett Shale and Strawn Group and Constraining the Source of Stray Gas in the Trinity Aquifer, North-Central Texas.

This study presents the complete set of stable noble gases for Barnett Shale and Strawn Group production gas together with stray flowing gas in the Trinity Aquifer, Texas. It places new constraints on the source of this stray gas and further shows that Barnett and Strawn gas have distinct crustal and atmospheric noble gas signatures, allowing clear identification of these two sources. Like stray gas, Strawn gas is significantly more enriched in crustal 4He*, 21Ne*, and 40Ar* than Barnett gas. The similarity of Strawn and stray gas crustal noble gas signatures suggests that the Strawn is the source of stray gas in the Trinity Aquifer. Atmospheric 22Ne/36Ar ratios of stray gas mimic also that of Strawn, further reinforcing the notion that the source of stray gas in this aquifer is the Strawn. While noble gas signatures of Strawn and stray gas are consistent with a single-stage water degassing model, a two-stage oil modified groundwater exsolution fractionation model is required to explain the light atmospheric noble gas signature of Barnett Shale production gas. These distinct Strawn and Barnett noble gas signatures are likely the reflection of distinct evolution histories with Strawn gas being possibly older than that of Barnett Shale.

Constraining the recent history of the perennially ice-covered Lake Bonney, East Antarctica using He, Kr and Xe concentrations

Lake Bonney is a perennially ice-covered lake in the McMurdo Dry Valleys (MDVs) that has long been studied in order to provide constraints on the paleoclimate of West Antarctica. The lake is divided into two lobes, West Lake Bonney (WLB) and East Lake Bonney (ELB) that are separated by a narrow ridge. The two lobes currently receive surface melt water during austral summers from glacier-fed ephemeral streams and this meltwater enters the lake via a narrow ring, or moat, of liquid water that forms around the lake during summer. The West Lobe also receives water from direct input of melt water from Taylor glacier and saline water from irregular subglacial discharge.Here, we combine previously published He data from Lake Bonney with new Kr and Xe concentration data to examine the signatures of water recharge via the seasonal moat and these data are used to constrain a model for He, Kr and Xe transport within both WLB and ELB over about the last 5000–6000yrs. A detailed numerical simulation is presented that combines diffusive transport of noble gases within the stratified water column of Lake Bonney, along with ice ablation at the top of the ice cover, partitioning of noble gases between water and ice, plus exchange of noble gases between WLB and ELB. Results strongly suggest that open moats have only operated for about 2–3 centuries within the last millennium. These results are corroborated by the high concentration of He, especially within WLB, which points to a history of ice cover with no open moats operating for both lobes for at least about 5 millennia. In addition, the distribution of He, Kr and Xe suggest that a significant rise of the water level of Lake Bonney associated with a warmer period may have been interrupted by a roughly 4–5 century long cold period during which the moats were not large enough to allow air saturated water into the lake, with this cold period ending about one century ago. In addition, during this cold period, there is evidence for a thickening of the ice cover. In the last century Lake Bonney levels have been rising, pointing either to more extended austral summers or elevated summer temperatures, a rise that is compatible with the heavy noble gas record in the upper portion of the lake.A preliminary analysis that includes all stable noble gases heavier than He, suggests that glacial melt water (GMW) directly from Taylor Glacier below the ice cover may have been a significant input into the deepest parts of WLB, but not ELB.

Groundwater responses to recharge in the Gacka Area, Croatia, as revealed by stable isotopes, tritium, CFCs and noble gases

Due to their extreme anisotropy and heterogeneity, the hydrogeological characterization of karst terrains is notoriously difficult. Approximately 50% of Croatia consist of karst aquifers, in particular the touristically important region along the Adriatic coast. Being an important water reservoir for this area, we have chosen the Gacka area of Croatia as a typical example. We studied the mixing of groundwater and the responses to recharge in several karst springs by using environmental tracers, including stable isotopes in water (O-18, H-2), tritium (H-3), chlorofluorocarbons (CFCs) and noble gases (He-3, He-4, Ne, Ar, Kr, Xe). The variation of the stable isotopic signal (delta O-18 and delta H-2) in the spring discharges provides qualitative information on the mixing of waters with different transit time. Assuming piston flow in the system, the CFC and H-3 data indicate that the waters contain mainly post-1980 recharge. The CFC concentrations in the spring waters were found to be significantly different between wet and dry weather periods. The higher CFC values during the wet weather period indicate that the waters contained some water components that have relatively short mean residence times. An important finding of the study was that the sole application of tritium-helium methods does not permit an accurate age determination. This is due to mass exchange between liquid and gaseous phases occurring in karst aquifer systems particularly in dry weather periods. During such a dry weather period, more space in the karst groundwater system is not filled with water compared with a wet weather period. It is shown in this study, however, that the combined use of stable isotopes (O-18, H-2), tritium, CFCs and noble gases is able to account for groundwater mixing and responses to recharge in this specific scenario. Compared to single-tracer studies, multi-tracer techniques will allow a much better understanding and characterization of these systems. This information is vital for the assessment of resources and potential contamination of the groundwater in this special environment.

Characterizing glacial meltwater sources in the Athabasca Glacier, Canada, using noble gases as tracers

This study is the first comprehensive noble gas study in meltwater of an alpine glacier. It uses stable noble gases' (He, Ne, Ar, Kr, and Xe) concentrations and isotopic ratios from the Athabasca Glacier meltwater (AGMW), Canada, in an attempt to identify the original source location of ice melt and the relative contributions of modern surface melt versus basal melt and/or groundwater. It also estimates first order water residence times of the glacial meltwater (GMW) resulting from a mixture of modern surface and basal melt and/or groundwater. Two patterns are apparent with respect to noble gas concentrations: 1) a mass-dependent depletion pattern with stronger depletion of the heavier noble gases compared to the lighter ones, and 2) a pattern displaying a relative Ne depletion with respect to Ar. Ratios of noble gas concentrations suggest that different gases have different degrees of equilibration and samples are far from equilibration with the atmosphere at any temperature compatible with the glacial environment. Xe concentrations alone suggest that all AGMW samples equilibrated with the atmosphere at altitudes between 2500 m and 3400 m, altitudes that lie within the altitude range (1900 m–3500 m) of the Columbia Icefield. Most samples display Xe equilibration altitudes above the maximum altitude of the AG (∼2700 m), suggesting that a significant portion of the current AGMW originates in the Columbia Icefield which contributes to both the AG per se and current subglacial meltwater discharge. All AGMW samples are largely dominated by surface melt as opposed to basal melt with surface melt representing at least 71%–96% of the total GMW. Basal melt and/or groundwater represent at most 4%–29% of the total GMW. All AGMW samples exhibit tritiogenic 3He (3Hetrit) levels varying between 0 and 12 TU. Based on estimated 3Hetrit levels, 4He concentrations, and average U and Th concentrations in carbonates, we conclude that the bulk of our AGMW is likely a mixture between pre-bomb and present time GMW with a most likely average residence time of 160 ± 5 years with the exclusion of one present day sample.

Post-irradiation analysis of an ISOLDE lead-bismuth target: Stable and long-lived noble gas nuclides

We measured the isotopic concentrations of long-lived and stable He, Ne, Ar, Kr, and Xe isotopes in a sample from a lead-bismuth eutectic target irradiated with 1.0 and 1.4 GeV protons. Our data indicate for most noble gases nearly complete release with retention fractions in the range of percent or less. Higher retention fractions result from the decay of long-lived radioactive progenitors from groups 1, 2, or 7 of the periodic table. From the data we can calculate a retention fraction for 3H of 2–3%. For alkaline metals we find retention fractions of about 10%, 30%, and 50% for Na, Rb, and Cs, respectively. For the alkaline earth metal Ba we found complete retention. Finally, the measured Kr and Xe concentrations indicate that there was some release of the halogens Br and I during and/or after the irradiation.

Potential interstellar noble gas molecules: ArOH[formula omitted] and NeOH[formula omitted] rovibrational analysis from quantum chemical quartic force fields

The discovery of ArH+ in the interstellar medium has shown that noble gas chemistry may be of more chemical significance than previously believed. The present work extends the known chemistry of small noble gas molecules to NeOH+ and ArOH+. Besides their respective neonium and argonium diatomic cation cousins, these hydroxyl cation molecules are the most stable small noble gas molecules analyzed of late. ArOH+ is once again more stable than the neon cation, but both are well-behaved enough for a complete quartic force field analysis of their rovibrational properties. The Ar–O bond in ArOH+, for instance, is roughly three-quarters of the strength of the Ar–H bond in ArH+ highlighting the rigidity of this system. The rotational constants, geometries, and vibrational frequencies for both molecules and their various isotopologues are computed from ab initio quantum chemical theory at high-level, and it is shown that these cations may form in regions where peroxy or weakly-bound alcohols may be present. The resulting data should be of significant assistance for the laboratory or observational analysis of these potential interstellar molecules.

Preface: The Mars Atmosphere and Volatile Evolution (MAVEN) Mission

The solar system began in the early solar nebula when the gas and dust accreted to form planetesimals which accumulated into the larger bodies we now know as the planets. Mars, the fourth planet from the sun was less massive than its two neighbors, Jupiter and Earth. But it had grown large enough to differentiate, forming a core and mantle and topped by a crust. It also was surrounded by an atmosphere. This early atmosphere appears to have provided a relatively benign environment, with surficial water that was much more abundant and geologically active than is possible on Mars today. Now, the surface of Mars is cold and dry, and any liquid water will very quickly either freeze or evaporate/sublimate into the atmosphere. We want to understand how this transition in climate occurred, and what role loss of gas from the atmosphere to space played in this climate change. The records of how the atmosphere evolved are difficult to decipher. We can determine how loss processes operate today, and try to extrapolate backward in time. It is not sufficient to simply use the present day loss rate because the conditions that led to that atmospheric evolution were not the same throughout Mars’ history but changed as the Sun evolved. So we must study the loss of atmosphere as a function of solar conditions and apply what we know about the evolution of the Sun to account for these changing conditions and make an accurate estimate of the losses with time. We also can examine characteristics such as the ratios of isotopes of light stable gases and noble gases for clues to the integrated loss to space over time. A major step in being able to do this was the development of the Mars Atmosphere and Volatile Evolution (MAVEN) mission as part of NASA’s Scout program. MAVEN is led by a Principal Investigator, and involves collaboration between the University of Colorado’s Laboratory for Atmospheric and Space Physics, the University of California at Berkeley’s Space Sciences Laboratory, NASA’s Goddard Space Flight Center and Jet Propulsion Laboratory, and Lockheed Martin. While many missions have flown to Mars, none has had such

Assessing compositional variability and migration of natural gas in the Antrim Shale in the Michigan Basin using noble gas geochemistry

This study uses stable noble gases' (He, Ne, Ar, Kr, Xe) volume fractions and isotopic ratios from Antrim Shale natural gas to assess compositional variability and vertical fluid migration within this reservoir, in addition to distinguishing between the presence of thermogenic versus biogenic methane. R/Ra values, where R is the measured 3He/4He ratio and Ra is the atmospheric value of 1.384±0.013×10−6, vary from 0.01 to 0.34 suggesting a largely dominant crustal 4He component with minor atmospheric and mantle contributions. Crustal 21Ne, 40Ar and 136Xe contributions are also present but the atmospheric component is largely dominant for these gases. Crustal contributions for 21Ne, 40Ar and 136Xe vary between 1.1% and 12.5%, between 0.7% and 19% and between 0.1% and 2.7%, respectively. A few samples present higher than atmospheric 20Ne/22Ne ratios pointing to the presence of a small mantle Ne component. High horizontal and vertical variability of noble gas signatures in the Antrim Shale are observed. These are mainly due to variable noble gas input from deep brines and, to a smaller extent, variable in-situ production within different layers of the Antrim Shale, in particular, the Lachine and Norwood members. Estimated 4He ages, considering external 4He input for Antrim Shale water, vary between 0.9ka and 238.2ka and match well for most samples with the timing of the major Wisconsin glaciation, suggesting that Antrim Shale water was influenced by glaciation-induced recharge. Consistency between measured and predicted 40Ar/36Ar ratios assuming Ar release temperatures ≥250°C supports a thermogenic origin for most of the methane in these samples. This thermogenic methane is likely to originate at greater depths, either from the deeper portion of the Antrim Shale in the central portion of the Michigan Basin or from deeper formations given that the thermal maturity of the Antrim Shale in the study area is rather low.

Groundwater recharge and age‐depth profiles of intensively exploited groundwater resources in northwest India

AbstractIntensive irrigation in northwest India has led to growing concerns over the sustainability of current and future groundwater abstraction. Environmental tracers and measurements of groundwater residence times can help quantify the renewal processes. Results from 16 paired locations show the interquartile ranges for residence times in shallow alluvial groundwater (8–50 m deep) to be 1–50 years and significantly less than those from deeper groundwater (76–160 m deep) at 40–170 years. The widespread occurrence of modern tracers in deep groundwater (>60% of sites had >10% modern recharge) suggests that there is low regional aquifer anisotropy and that deep aquifers are recharged by a significant component of recent recharge via vertical leakage. Stable isotope and noble gas results at all depths conform to modern meteoric sources and annual average temperatures, with no evidence of significant regional recharge from canal leakage in this study area close to the Himalayas.

The 18-electron rule and electron counting in transition metal compounds: theory and application

The 18-electron rule and the corresponding methods for counting the total valence electrons of transition metal complexes are among the most useful basic tools in modern inorganic chemistry, particularly in its application to organometallic species. While in its simplest representation, the 18-electron rule is explained in that a closed, stable noble gas configuration of ns 2(n-1)d 10 np 6 is achieved with 18 valence electrons, this does not adequately explain the trends and exceptions seen in practice. As such, this report presents a deeper discussion of the 18-electron rule via molecular orbital models, stressing the roles of both σ- and π-bonding effects. This discussion thus aims to provide a better understanding of the relationship between electron count and stability, while also illustrating which factors can determine adherence (or not) to this commonly utilized rule. Lastly, the two common methods for electron counting (ionic and covalent models) are also presented with practical examples to provide the complete ability to apply the 18-electron rule.

Determination of natural in vivo noble-gas concentrations in human blood.

Although the naturally occurring atmospheric noble gases He, Ne, Ar, Kr, and Xe possess great potential as tracers for studying gas exchange in living beings, no direct analytical technique exists for simultaneously determining the absolute concentrations of these noble gases in body fluids in vivo. In this study, using human blood as an example, the absolute concentrations of all stable atmospheric noble gases were measured simultaneously by combining and adapting two analytical methods recently developed for geochemical research purposes. The partition coefficients determined between blood and air, and between blood plasma and red blood cells, agree with values from the literature. While the noble-gas concentrations in the plasma agree rather well with the expected solubility equilibrium concentrations for air-saturated water, the red blood cells are characterized by a distinct supersaturation pattern, in which the gas excess increases in proportion to the atomic mass of the noble-gas species, indicating adsorption on to the red blood cells. This study shows that the absolute concentrations of noble gases in body fluids can be easily measured using geochemical techniques that rely only on standard materials and equipment, and for which the underlying concepts are already well established in the field of noble-gas geochemistry.

The five stable noble gases are sensitive unambiguous tracers of glacial meltwater

AbstractThe five inert noble gases—He, Ne, Ar, Kr, and Xe—exhibit a unique dissolved gas saturation pattern resulting from the formation and addition of glacial meltwater to seawater. He and Ne become oversaturated, and Ar, Kr, and Xe become undersaturated to varying percentages. For example, addition of 10‰ glacial meltwater to seawater results in a saturation anomaly of ΔHe = 12.8%, ΔNe = 8.9%, ΔAr = −0.5%, ΔKr = −2.2%, and ΔXe = −3.3%. This pattern in noble gas saturation reflects a unique meltwater signature that is distinct from the other major physical processes that modify the gas concentration and saturation, namely, seasonal changes in temperature at the ocean surface and bubble mediated gas exchange. We use Optimum Multiparameter analysis to illustrate how all five noble gases can help distinguish glacial meltwater from wind‐driven bubble injection, making them a potentially valuable suite of tracers for glacial melt and its concentration in the deep waters of the world ocean.

Noble gas, CFC and other geochemical evidence for the age and origin of the Bath thermal waters, UK

The city of Bath is a World Heritage site and its thermal waters, the Roman Baths and new spa development rely on undisturbed flow of the springs (45°C). The current investigations provide an improved understanding of the residence times and flow regime as basis for the source protection. Trace gas indicators including the noble gases (helium, neon, argon, krypton and xenon) and chlorofluorocarbons (CFCs), together with a more comprehensive examination of chemical and stable isotope tracers are used to characterise the sources of the thermal water and any modern components. It is shown conclusively by the use of 39Ar that the bulk of the thermal water has been in circulation within the Carboniferous Limestone for at least 1000years. Other stable isotope and noble gas measurements confirm previous findings and strongly suggest recharge within the Holocene time period (i.e. the last 12 kyr). Measurements of dissolved 85Kr and chlorofluorocarbons constrain previous indications from tritium that a small proportion (<5%) of the thermal water originates from modern leakage into the spring pipe passing through Mesozoic valley fill underlying Bath. This introduces small amounts of O2 into the system, resulting in the Fe precipitation seen in the King’s Spring. Silica geothermometry indicates that the water is likely to have reached a maximum temperature of between 69–99°C, indicating a most probable maximum circulation depth of ∼3km, which is in line with recent geological models. The rise to the surface of the water is sufficiently indirect that a temperature loss of >20°C is incurred. There is overwhelming evidence that the water has evolved within the Carboniferous Limestone formation, although the chemistry alone cannot pinpoint the geometry of the recharge area or circulation route. For a likely residence time of 1–12kyr, volumetric calculations imply a large storage volume and circulation pathway if typical porosities of the limestone at depth are used, indicating that much of the Bath-Bristol basin must be involved in the water storage.