Noble Gas Tracers Research Articles

Helium isotope evidence for cryptic magmatism along Central France

Determining the source(s) of helium in regions of the crust where 3He anomalies occur can be challenging when no surface manifestations of magmatic activity or a clear active extension regime are present. This is the case of the Paris basin (France), where geothermal fluids and oils 3He/4He have been previously shown to range from 0.02 Ra up to 0.14 Ra (Marty et al., 1993; Pinti and Marty, 1995, 1998), whereas natural gases to the south of the Paris Basin (near the Massif central) show unambiguous evidence for mantle helium contribution, with 3He/4He up to 6.4 Ra (Bräuer et al., 2017). However, higher 3He/4He than the crustal endmember (∼ 0.01 Ra), as observed in the Paris Basin, could be a sign of either (i) a weak contribution of mantle He (enriched in primordial 3He) or (ii) the contribution of 3He naturally produced by nuclear reactions of lithium, for which commercially exploitable concentrations have been reported in rare-metal-rich granites. One way to distinguish between these two possibilities is to combine He isotope systematics with other noble gas tracers of mantle inputs, such as xenon isotopes. Here, we report the isotope compositions of helium and xenon, measured by static and dynamic mass spectrometry, respectively, in gas samples collected in the Nièvre County, between the Massif Central and the Paris Basin. 3He/4He ratios range between 0.16 and 0.22 Ra. Combined with previous studies, our data are consistent with a southward increase of He isotope ratios along major fault systems from the Paris Basin (∼ 0.02 Ra) towards the Massif Central (6.4 Ra). This geographical trend, paired with high precision Xe isotopes exhibiting an excess of mantle 129Xe, shows a clear mantle input that is most likely restricted to the major fault systems in Central France. The occurrence of mantle-derived helium and xenon in Central France highlights the presence of cryptic magmatism in areas with no apparent volcanic manifestations.

New Experimental Tools to Use Noble Gases as Artificial Tracers for Groundwater Flow

Labeling groundwater by injecting an artificial tracer is a standard and widely used method to study groundwater flow systems. Noble gases dissolved in groundwater are potentially ideal artificial tracers, as they are not subject to biogeochemical transformations, do not adsorb onto the aquifer matrix, are colorless, and have no negative impact on the quality of groundwater resources. In addition, combining different noble-gas species in multi-tracer tests would allow direct analysis of the spatio-temporal heterogeneity of groundwater flow systems. However, while the handling of noble gases is safe and straightforward for injection into groundwater, conventional methods to analyse dissolved noble gases tend to be impractical for groundwater tracer tests. The sampling and subsequent lab-based analysis of dissolved noble gases are laborious, expensive and time intensive. Therefore, only researchers with access to specialized noble-gas labs have attempted such tracer tests. The recently developed gas-equilibrium membrane-inlet mass spectrometers (GE-MIMS) allow efficient on-site analysis of dissolved gases at high temporal resolution. The GE-MIMS instruments thereby eliminate most of the analytical and logistical constraints of conventional lab-based techniques and therefore provide new opportunities for groundwater tests using artificially injected gases. We used a GE-MIMS to systematically test the applicability of He, Kr, and Xe as artificial groundwater tracers. These gas species were injected into groundwater as Dirac-like pulses at three piezometers located at various locations upstream of a pumping well, where dissolved gas concentrations were continuously monitored with the GE-MIMS instrument. The groundwater travel times observed in these tracer tests ranged from a few hours to several weeks, and were consistent with the groundwater flow field at the experimental test site. Travel times determined from the noble gas tracer tests were also consistent with those obtained traditional dye tracers.

Transfer of dissolved gases through a thick karstic vadose zone – Implications for recharge characterisation and groundwater age dating in karstic aquifers

This study analyses the impact of a thick vadose zone on gaseous transfers and Excess Air (EA) formation using dissolved noble and anthropogenic gas tracers (Ne, Ar, N2, CFC, SF6, O2 and CO2). Understanding gas transfers integrates the issues of equilibrium and water/air exchanges along the vadose zone. In this study, the noble and anthropogenic gases were analysed in air and water over vadose zone depths of 120 m and over two contrasted hydrodynamic periods therefore enabling a rare insight into the spatial and seasonal variations of equilibrium and water/air exchanges in a heterogeneous karst environment. The results highlight that both noble and anthropogenic gases of the caves are in equilibrium with the atmosphere up to at least depths of 120 m. In water, the Noble Gas Temperature (NGT) remains stable at 8 °C whilst some EA is formed (up to 1.3 cm3 STP/kg at 120 m) during transfers in the vadose zone. The high EA concentrations measured at the spring outlet, until 10.7 cm3 STP/kg, cannot be explained by an increase during transfers via the vadose zone. This high EA concentration is due to rapid and high water level variations during recharge. The implications of these results are important to advance our understanding in the use of noble and anthropogenic gases in characterising recharge and quantifying groundwater residence times. In particular, these results help to identify the parts of the aquifer actively involved in dissolved gas transfers, highlight the futility of using a time-lag factor for groundwater residence times in karstic systems, and improve our understanding of the EA formation processes.

Tracing Water from River to Aquifer

A new technique using dissolved noble gas tracers sheds light on how water moves through an aquifer, with implications for water resources and their vulnerability to climate change.

Noble gas tracers in gas streams at Norwegian CO2 capture plants

Carbon capture and storage (CCS) may play a significant role in reducing greenhouse gas emissions. Noble gases are potential tracers to monitor subsurface CO2 storage sites and verify their containment. Naturally occurring noble gases have been used successfully to refute alleged CO2 leakage in the past.We present results from several sampling campaigns at two Norwegian CO2 capture facilities, the demonstration plant Technology Centre Mongstad (TCM) and the natural gas processing plant with CO2 capture and storage on Melkøya. The gas streams in the capture plants were monitored with a combination of on-site mass spectrometry and subsequently analysed discrete samples. This allows us to define the factors controlling noble gas concentrations in captured CO2, to monitor temporal variation of noble gas concentrations and finally evaluate the potential to use noble gases as inherent environmental tracers for labelling CO2 in storage reservoirs.At both sites, CO2 is captured using amine gas treatment. Noble gas concentrations in the gas streams were observed to decrease by several orders of magnitude during the processing. Isotopic ratios are air-like for CO2 captured after natural gas combustion at TCM and natural gas-like for CO2 captured from natural gas processing on Melkøya. Further, we detected a solubility trend caused by the amine solvent at TCM with higher solubility for heavier noble gases.We find that the relative concentrations of noble gases in the captured CO2 are defined by the gas from which the CO2 is captured and the design of the amine gas treatment process. Both factors were observed to cause temporal variation in the captured CO2.Using mixing and noble gas partitioning calculations we show that the significant depletion in noble gas concentrations, together with degassing of noble gas enriched formation water, means that the injected CO2 will inherit the noble gas signature of the storage formation, even following the injection of significant CO2 volumes. Any CO2 leaked from the storage formation is thus likely to have a crustal noble gas signature, characteristic of the storage site, which can be targeted for monitoring.

Application of noble gas tracers to identify the retention mechanisms of CO2 migrated from a deep reservoir into shallow groundwater

Carbon Capture and Storage (CCS) is a valuable climate-mitigation technology, which offers the potential to cost-effectively reduce the emissions associated with the burning of fossil fuels. However, there is a potential risk of a small portion of the stored CO2 unintentionally migrating from a storage site to a shallow groundwater aquifer which is the final retaining zone for any migrated CO2 before it escapes to the atmosphere. Hence, it is imperative to identify the physical retention mechanisms of CO2 within a shallow aquifer. In this study 1.70 × 102 kg of CO2 and noble gas tracers (He, Ar and Kr) were continuously injected into a groundwater aquifer over 28 days with the aim of identifying the mechanisms and amount of CO2 retention. Among the tracers, Kr was found to be the earliest indicator of CO2 migration. The other tracers – He and Ar – arrived later and exhibited diluted signals. The diluted signals were attributed to degassing of the plume mass (1.6 % of CO2) during the early stages of CO2 migration. Diffusion accelerated the dilution of the lighter elements at the plume boundaries. Consequently, the clear relation of the noble gases with the CO2 proved that degassing and mixing primarily control the mass retention of CO2 in shallow groundwater, and the relative importance of these processes varies along the evolving path of migrating CO2.

Migration of noble gas tracers at the site of an underground nuclear explosion at the Nevada National Security Site

As part of an underground gas migration study, two radioactive noble gases (37Ar and 127Xe) and two stable tracer gases (SF6 and PFDMCH) were injected into a historic nuclear explosion test chimney and allowed to migrate naturally. The purpose of this experiment was to provide a bounding case (natural transport) for the flow of radioactive noble gases following an underground nuclear explosion. To accomplish this, soil gas samples were collected from a series of boreholes and a range of depths from the shallow subsurface (3 m) to deeper levels (~160 m) over a period of eleven months. These samples have provided insights into the development and evolution of the subsurface plume and constrained the relative migration rates of the radioactive and stable gas species in the case when the driving pressure from the cavity is low. Analysis of the samples concluded that the stable tracer SF6 was consistently enriched in the subsurface samples relative to the radiotracer 127Xe, but the ratios of SF6 and 37Ar remained similar throughout the samples.

Application of natural and artificial tracers to constrain CO2 leakage and degassing in the K-COSEM site, South Korea

Although Carbon Capture and Storage (CCS) has been demonstrated successfully on many occasions, the potential leakage of deep sequestrated CO2 into shallow groundwater remains a concern. To address this, an artificial injection experiment was performed at the K-COSEM test site in Eumseong, South Korea, that involved the release of CO2–infused water (16.9 kg of CO2 in 5 m3) containing He and Kr tracers into a shallow, heterogeneous, weathered-granite aquifer. The initial CO2–fluid was slightly oversaturated at the subsurface injection point, and thus the plume was expected to initially degas CO2 before equilibrating at in-situ conditions. Monitoring of carbonate system parameters in nearby observation wells helped define the evolution of the injected fluids, while the noble gas tracers were used to clearly define the physical behavior of the CO2 plume (including an estimate of degassed CO2 equal to 0.9–3.1%). This study demonstrates the potential use of noble gases for monitoring CO2 leakage in shallow aquifers, constraining mass balance and phase changes of leaking fluids, and better understanding local flow pathways. Furthermore, breakthrough of noble gases in this study was different from some previous experiments, suggesting that monitoring efficiency of these tracers may depend on leakage and site conditions.

Adsorptive transport of noble gas tracers in porous media

The transport of noble gas radionuclides in porous media is relevant to the detection of underground nuclear detonations as well as the sequestration of reprocessing off-gases. However, in field tests releasing radioxenon underground, the quantity of radioxenon observed at the surface has fallen well below expectations.[Formula: see text] This study examined the diffusivity of noble gases (Kr and Xe) and the inert molecular gas sulfur hexafluoride (SF[Formula: see text] in porous media to observe any unexpected behavior. To replicate the transport of radiogenic noble gases in underground media, a two-bulb gaseous diffusion apparatus was constructed. The two bulbs were connected with a column of 10–30 Ottawa sand and ordinary atmosphere filled both the bulbs and pore spaces. The tracer gases were diluted in an isolated bulb to approximately 1000 ppm. Once released, the gases were allowed to diffuse through the column. Aliquots were withdrawn at regular time intervals from both bulbs and concentrations were quantified using a Shimadzu QP2010 SE gas chromatograph-mass spectrometer. The effective diffusivity was then calculated using a maximum likelihood estimate on the quasi-steady state model. The effective diffusivity of Xe in the silica sand was observed to be 135.2% that of SF[Formula: see text] whereas the effective diffusivity of Kr was observed to be 161.4% that of sulfur hexafluoride. These findings are consistent with the binary diffusivities in N[Formula: see text]: 132.6% and 161.7%, respectively. However, the apparent volume of the system was inconsistent amongst the species, with Xe converging at slightly lower gas-phase concentrations than Kr or SF[Formula: see text]. This apparent reduction in gas-phase concentration occurred within the first few measurements and is consistent with transient accumulation of an adsorbed phase. As the effective diffusivities in the silica sand were shown to be consistent with the binary diffusivities in N[Formula: see text], a porosity-tortuosity model appears to be sufficient when considering similar geological materials. However, with the observation of significant gas adsorption, consideration of adsorbed-phase accumulation is necessary when scaling to larger geological systems.

Determining CO2 storage potential during miscible CO2 enhanced oil recovery: Noble gas and stable isotope tracers

Rising atmospheric carbon dioxide (CO2) concentrations are fueling anthropogenic climate change. Geologic sequestration of anthropogenic CO2 in depleted oil reservoirs is one option for reducing CO2 emissions to the atmosphere while enhancing oil recovery. In order to evaluate the feasibility of using enhanced oil recovery (EOR) sites in the United States for permanent CO2 storage, an active multi-stage miscible CO2 flooding project in the Permian Basin (North Ward Estes Field, near Wickett, Texas) was investigated. In addition, two major natural CO2 reservoirs in the southeastern Paradox Basin (McElmo Dome and Doe Canyon) were also investigated as they provide CO2 for EOR operations in the Permian Basin. Produced gas and water were collected from three different CO2 flooding phases (with different start dates) within the North Ward Estes Field to evaluate possible CO2 storage mechanisms and amounts of total CO2 retention. McElmo Dome and Doe Canyon were sampled for produced gas to determine the noble gas and stable isotope signature of the original injected EOR gas and to confirm the source of this naturally-occurring CO2. As expected, the natural CO2 produced from McElmo Dome and Doe Canyon is a mix of mantle and crustal sources. When comparing CO2 injection and production rates for the CO2 floods in the North Ward Estes Field, it appears that CO2 retention in the reservoir decreased over the course of the three injections, retaining 39%, 49% and 61% of the injected CO2 for the 2008, 2010, and 2013 projects, respectively, characteristic of maturing CO2 miscible flood projects. Noble gas isotopic composition of the injected and produced gas for the flood projects suggest no active fractionation, while δ13CCO2 values suggest no active CO2 dissolution into formation water, or mineralization. CO2 volumes capable of dissolving in residual formation fluids were also estimated along with the potential to store pure-phase supercritical CO2. Using a combination of dissolution trapping and residual trapping, both volumes of CO2 currently retained in the 2008 and 2013 projects could be justified, suggesting no major leakage is occurring. These subsurface reservoirs, jointly considered, have the capacity to store up to 9 years of CO2 emissions from an average US powerplant.

Noble gas tracers of ventilation during deep-water formation in the Weddell Sea

To explore the dynamics and implications of incomplete air-sea equilibration during the formation of abyssal water masses, we simulated noble gases in the Estimating the Circulation & Climate of the Ocean (ECCO) global ocean state estimate. A novel computation approach utilizing a matrix-free Newton-Krylov (MFNK) scheme was applied to quickly compute the periodic seasonal solutions for noble gas tracers. MFNK allows for quick computation of a cyclo-stationary solution for tracers (i.e., a spun-up, repeating seasonal cycle), which would otherwise be computationally infeasible due to the long time scale of dynamic adjustment of the abyssal ocean (1000’s of years). A suite of experiments isolates individual processes, including atmospheric pressure effects, the solubility pump and air-sea bubble fluxes. In addition to these modeled processes, a volumetric contribution of 0.28 ± 0.07% of glacial melt water is required to reconcile deep-water observations in the Weddell Sea. Another primary finding of our work is that the saturation anomaly of heavy noble gases in model simulations is in excess of two-fold more negative than is suggested from Weddell Sea observations. This result suggests that model water masses are insufficiently ventilated prior to subduction and thus there is insufficient communication between atmosphere and ocean at high latitudes. The discrepancy between noble gas observations and ECCO simulations highlights that important inadequacies remain in how we model high-latitude ventilation with large implications for the oceanic uptake and storage of carbon.

Using noble gas tracers to constrain a groundwater flow model with recharge elevations: A novel approach for mountainous terrain

AbstractEnvironmental tracers provide information on groundwater age, recharge conditions, and flow processes which can be helpful for evaluating groundwater sustainability and vulnerability. Dissolved noble gas data have proven particularly useful in mountainous terrain because they can be used to determine recharge elevation. However, tracer‐derived recharge elevations have not been utilized as calibration targets for numerical groundwater flow models. Herein, we constrain and calibrate a regional groundwater flow model with noble‐gas‐derived recharge elevations for the first time. Tritium and noble gas tracer results improved the site conceptual model by identifying a previously uncertain contribution of mountain block recharge from the Coast Mountains to an alluvial coastal aquifer in humid southwestern British Columbia. The revised conceptual model was integrated into a three‐dimensional numerical groundwater flow model and calibrated to hydraulic head data in addition to recharge elevations estimated from noble gas recharge temperatures. Recharge elevations proved to be imperative for constraining hydraulic conductivity, recharge location, and bedrock geometry, and thus minimizing model nonuniqueness. Results indicate that 45% of recharge to the aquifer is mountain block recharge. A similar match between measured and modeled heads was achieved in a second numerical model that excludes the mountain block (no mountain block recharge), demonstrating that hydraulic head data alone are incapable of quantifying mountain block recharge. This result has significant implications for understanding and managing source water protection in recharge areas, potential effects of climate change, the overall water budget, and ultimately ensuring groundwater sustainability.

Residual CO2 saturation estimate using noble gas tracers in a single-well field test: The CO2CRC Otway project

This paper presents the results of a partitioning tracer test using noble gases to measure residual CO2 saturation in the field as one part of the CO2CRC residual saturation and dissolution test sequence. Noble gas tracers were used in a sequential partitioning tracer test. The first tracer test was performed prior to CO2 injection to characterise the aquifer under single-phase conditions. The second tracer test was performed after CO2 had been driven to residual saturation. The noble gas tracers can be regarded as non-partitioning aqueous-phase tracers in the undisturbed aquifer and subsequently as water/CO2 partitioning tracers in the presence of the residual CO2 saturation.One-dimensional radial simulations are used to estimate residual saturation from the tracer tests by fitting only two independent parameters: the apparent dispersivity of each tracer in the aqueous phase and the residual CO2 saturation. This analysis method is validated by successful application to krypton (Kr) and xenon (Xe) production curves from the field data.Both tracers give the same estimate of residual saturation to within the accuracy of the method. Furthermore the estimated residual saturation from the noble gas tracer tests is consistent with independent measurements. However the analysis of the two tracer production curves indicates that Xe has higher dispersivity than Kr, despite having been injected and produced simultaneously and measured from the same samples. Finally the application of this sequential tracer test method to other field settings is discussed.

An assessment of near surface CO2 leakage detection techniques under Australian conditions

Abstract Geoscience Australia and the CO 2 CRC operate a greenhouse gas controlled release facility at an experimental agricultural station maintained by CSIRO Plant Industry in Canberra, Australia. The facility is designed to simulate surface emissions of CO 2 and other greenhouse gases from the soil into the atmosphere. Over 10 different near surface monitoring techniques were trialled at the Ginninderra controlled release site during 2012-2013. These included soil gas, soil CO 2 flux, soil analysis, eddy covariance, CO 2 laser, noble gas tracers, airborne hyperspectral, in-field phenotyping (thermal, hyperspectral and 3D imaging), and microbial soil genomics. Result highlights are presented. Different climatic conditions for the early 2012 release experiment (wet) and late 2013 release experiment (dry) resulted in markedly different sub-surface plume behaviour and surface expression of CO 2 . The differences between the years are attributed to changes in groundwater levels and drier conditions leading to a larger vadose zone during the 2013 experiment.

A membrane inlet mass spectrometry system for noble gases at natural abundances in gas and water samples

Noble gases dissolved in groundwater can reveal paleotemperatures, recharge conditions, and precise travel times. The collection and analysis of noble gas samples are cumbersome, involving noble gas purification, cryogenic separation and static mass spectrometry. A quicker and more efficient sample analysis method is required for introduced tracer studies and laboratory experiments. A Noble Gas Membrane Inlet Mass Spectrometry (NG-MIMS) system was developed to measure noble gases at natural abundances in gas and water samples. The NG-MIMS system consists of a membrane inlet, a dry-ice water trap, a carbon-dioxide trap, two getters, a gate valve, a turbomolecular pump and a quadrupole mass spectrometer equipped with an electron multiplier. Noble gases isotopes (4)He, (22)Ne, (38)Ar, (84)Kr and (132)Xe are measured every 10 s. The NG-MIMS system can reproduce measurements made on a traditional noble gas mass spectrometer system with precisions of 2%, 8%, 1%, 1% and 3% for He, Ne, Ar, Kr and Xe, respectively. Noble gas concentrations measured in an artificial recharge pond were used to monitor an introduced xenon tracer and to reconstruct temperature variations to within 2 °C. Additional experiments demonstrated the capability to measure noble gases in gas and in water samples, in real time. The NG-MIMS system is capable of providing analyses sufficiently accurate and precise for introduced noble gas tracers at managed aquifer recharge facilities, groundwater fingerprinting based on excess air and noble gas recharge temperature, and field and laboratory studies investigating ebullition and diffusive exchange.

Mechanisms of subglacial groundwater recharge as derived from noble gas, 14C, and stable isotopic data

Noble gas, stable isotope and 14C data from samples collected along groundwater flow path within a confined Paleozoic aquifer in northeastern Wisconsin, USA are used to deduce the effect of the Laurentide Ice Sheet (LIS) on the underlying groundwater and its recharge dynamics. During the last glacial maximum the investigated area was near the center of the Green Bay Lobe of the LIS. 14C ages that extend to 26k.a. and low δ18O derived temperatures during the time that the LIS was present indicate that aquifer recharge continued when ice covered the area. δ18O values as low as −17.5‰ and δ2H values as low as −127.7‰ indicate that a significant portion of aquifer recharge was derived from glacial meltwater that maintained its glacial isotopic signature during melting and subsequent recharge. Noble gas temperatures that remain above freezing at a constant ~3°C, unusually high excess air (ΔNe) values and noble gas fractionation patterns indicate that recharge occurred across a very dynamic water table located within the ice sheet. This englacial hydrologic system experienced recharge heads of as much as 7.8m. Evidence for direct recharge of basal meltwater into the aquifer is not seen. To the authors' knowledge this is the first time that noble gas and isotope tracers have been used to deduce the provenance of aquifer water beneath continental ice sheets.

The potential of noble gas tracers for CO2 monitoring and early warning tracers for leakage

Greenhouse Gases: Science and TechnologyVolume 3, Issue 2 p. 99-102 Interview The potential of noble gas tracers for CO2 monitoring and early warning tracers for leakage Dr Stuart Gilfillan, Dr Stuart Gilfillan [email protected] Scottish Carbon Capture and Storage (SCCS), School of GeoSciences, University of EdinburghSearch for more papers by this author Dr Stuart Gilfillan, Dr Stuart Gilfillan [email protected] Scottish Carbon Capture and Storage (SCCS), School of GeoSciences, University of EdinburghSearch for more papers by this author First published: 09 April 2013 https://doi.org/10.1002/ghg.1342Citations: 1Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Citing Literature Volume3, Issue2April 2013Pages 99-102 RelatedInformation

An atom counter for measuring 81Kr and 85Kr in environmental samples

Due to its simple production and transport processes in the terrestrial environment, the long-lived noble-gas isotope 81Kr is the ideal tracer for old water and ice in the age range of 105–106 years, a range beyond the reach of 14C. 81Kr-dating, a concept pursued in the past four decades by numerous laboratories employing a variety of techniques, is now available for the first time to the earth science community at large. This is made possible by the development of ATTA-3, an efficient and selective atom counter based on the Atom Trap Trace Analysis method and capable of measuring both 81Kr/Kr and 85Kr/Kr ratios of environmental samples in the range of 10−14–10−10. The instrument was calibrated with 12 samples whose 85Kr/Kr ratios were independently measured using Low Level Decay Counting, including six samples that were measured in a blind arrangement. Compared to the previously reported ATTA-2 instrument, the counting rates of ATTA-3 are higher by two orders of magnitude and the required sample size lower by one order of magnitude. For 81Kr-dating in the age range of 150–1500kyr, the required sample size is 5–10μL STP of krypton gas, which can be extracted from approximately 100–200kg of water or 40–80kg of ice. Moreover, a laser-induced quenching scheme was developed to enable measurements of both the rare 81,85Kr and the abundant 83Kr, whose isotopic abundances differ by 11 orders of magnitude. This scheme allows ATTA-3 to directly determine 81Kr/Kr and 85Kr/Kr ratios without other supplemental measurements. Combining the significant reduction in sample size with numerous advances in the measurement procedure, ATTA-3 represents the state-of-the-art instrument for routine analysis of these rare noble gas tracers in a wide range of earth science applications.

Denitrification and inference of nitrogen sources in the karstic Floridan Aquifer

Abstract. Aquifer denitrification is among the most poorly constrained fluxes in global and regional nitrogen budgets. The few direct measurements of denitrification in groundwaters provide limited information about its spatial and temporal variability, particularly at the scale of whole aquifers. Uncertainty in estimates of denitrification may also lead to underestimates of its effect on isotopic signatures of inorganic N, and thereby confound the inference of N source from these data. In this study, our objectives are to quantify the magnitude and variability of denitrification in the Upper Floridan Aquifer (UFA) and evaluate its effect on N isotopic signatures at the regional scale. Using dual noble gas tracers (Ne, Ar) to generate physical predictions of N2 gas concentrations for 112 observations from 61 UFA springs, we show that excess (i.e. denitrification-derived) N2 is highly variable in space and inversely correlated with dissolved oxygen (O2). Negative relationships between O2 and δ15NNO3 across a larger dataset of 113 springs, well-constrained isotopic fractionation coefficients, and strong 15N:18O covariation further support inferences of denitrification in this uniquely organic-matter-poor system. Despite relatively low average rates, denitrification accounted for 32 % of estimated aquifer N inputs across all sampled UFA springs. Back-calculations of source δ15NNO3 based on denitrification progression suggest that isotopically-enriched nitrate (NO3–) in many springs of the UFA reflects groundwater denitrification rather than urban- or animal-derived inputs.

Evaluating the accretion of meteoritic debris and interplanetary dust particles in the GPC-3 sediment core using noble gas and mineralogical tracers

Extraterrestrial (ET) noble gases (helium and neon) in 35 sediment samples from Central Pacific core LL-44 GPC-3 demonstrate the variable flux of interplanetary dust particles (IDPs) and major meteorite impacts over the past 70Ma (21–72Ma). Spinel mineralogical and chemical compositions clearly distinguish major impact events from the continuous flux of IDPs, including the well-established Cretaceous/Tertiary (K/T) and late Eocene (E/O) impact boundaries. No spinel grains with chemical or mineralogical evidence of a distinctly ET origin were found in an extensive survey of ‘background’ samples (i.e. non E/O or K/T boundary) suggesting that either the carrier grains for ET noble gas occur within the Fe–Ti oxide mineral fraction observed in this study (found to include ilmenite and ulvospinel) or are too small for identification by SEM. The presence of ilmenite and ulvospinel suggest lunar regolith is a potential source for ET noble gas-rich particles. Noble gas analysis on both the EMF (extractable magnetic fraction) and the Bulk minus EMF (Bulk−EMF) show that the He and Ne compositions are consistent with partially degassed noble gas signatures of zero-age magnetic grains (Z-MAG) and stratospheric interplanetary dust particles (IDPs). Conclusive evidence for a ‘planetary’ (Ne-A) noble gas signature is found only in the bulk sediments at the K/T boundary, although all GPC-3K/T fractions (Bulk, EMF, and HF Digestion) plot along a mixing line between planetary (Ne-A) and solar wind (SW). Spinels from major impact boundaries (K/T; E/O) exhibit dendritic texture and elevated [Ni], consistent with previous reports. In contrast to the otherwise consistent [3He] signal from IDPs, the [3He] at the known impact boundaries (K/T and E/O) actually decreases. These anomalously low [3He] are accompanied by significantly elevated [Ne] and significantly lower (3He/20Ne)solar ratios (∼10× lower) produced by both preferentially degassing of He relative to Ne at times of increased flux of larger ET material. Degassed (“degassed-He/enriched-Ne profile”) noble gas characteristics occur in two sample intervals that do not correspond to any known impact events (47 and 71Ma), explained by an influx of larger particles. SEM analysis of the 47Ma sample shows spinels with dendritic textures, but without distinctive markers of large meteorite impacts (e.g. elevated Ni). Particle size increases and degassed signatures may be caused by major bolides, micrometeorites, comet showers; or simply a flux of larger IDPs, potentially with a different source.