Dual Isotope Measurements Research Articles

Isotopic characterisation and mobile detection of methane emissions in a heterogeneous UK landscape

Characterising methane (CH4) sources and their stable isotope values at the regional level is important for taking effective mitigation actions as well as more accurately constraining global atmospheric CH4 budgets. We performed dual stable isotope (13C, 2H) analysis of CH4 emission sources as well as mobile 13C measurements in North-West England, in a region with a mix of natural and anthropogenic emission sources as well as potentially exploitable shale gas deposits. Dual isotope analysis was performed for enteric fermentation, animal waste, landfill gas, wetlands, and natural gas from the regional distribution network. Microbial emission sources’ δ13C values ranged from −72.1 ± 0.31‰ to −53.1 ± 1.17‰ with agricultural sources and landfills showing partially overlapping values (−65.3 ± 0.41‰ to −72.1 ± 0.31‰ and −59.2 ± 0.26‰ to −70.4‰, respectively). However, the use of a dual isotope approach with δ2H provided additional separation between agricultural (−340 ± 0.8‰ to −322 ± 19.5‰) and landfill (−312 ± 0.3‰ to −282‰) CH4. All microbial sources were clearly distinct from natural gas with mean values of −39.5 ± 1.38‰ and −184 ± 4.9‰ for δ13C and δ2H, respectively. Mobile measurements conducted over a distance of 557 km detected emissions from two out of four surveyed managed landfills in the region. Multiple gas leaks were detected, which may confound emissions from other thermogenic sources. When separating the surveyed area by land-use into agricultural and urban, we found that background levels of CH4 were more depleted by around 1‰ in areas with agricultural land use compared to urban areas, but emissions from gas leaks and landfills are present in both categories. Our findings highlight the complexity of isoscapes in regions with multiple types of emission sources and the value of dual-isotope measurements in source attribution.

Application of concentration and 2-dimensional stable isotope measurements of methane to constrain sources and sinks in a seasonally stratified freshwater lake

Methane (CH4) emissions from aquatic systems have recently been comprised to account for up to 50% of global CH4 emissions, with lakes representing one of the largest CH4 sources within this pool. However, there is large uncertainty associated with CH4 emissions from freshwater environments to the atmosphere, because of a lack of understanding in the spatial and temporal dynamics of CH4 sources and sinks, as well as underlying mechanisms and processes. In this study, we investigated the concentrations and stable carbon (δ13C-CH4) and hydrogen (δ2H-CH4) isotope composition of CH4 in a small eutrophic lake (Lake Willersinnweiher) with seasonal stratification and its spatial and temporal variation. We found that while supersaturation of CH4 in the entire water column was present throughout the whole year, the isotopic composition of CH4 in sediment and water column varied depending on lake stratification, physiochemical conditions, and lake depth. During the stratification period, isotopic characteristics of pelagic surface water CH4 differed from littoral and sedimentary CH4, suggesting likely mixing of CH4 from different sources including vertical and lateral input as well as groundwater input and potentially oxic methane production in the mixed surface water layer. Aerobic CH4 oxidation indicated by a strong increase in both δ13C-CH4 and δ2H-CH4 values at the bottom of the oxycline was found to significantly reduce upward migrating CH4 released at the sediment-water interface. In the sediment, stable isotope characteristics of CH4 showed an increasing dominance of the acetoclastic CH4 formation pathway from the pelagic towards the littoral area. Furthermore, the occurrence of sulfate-dependent anaerobic methane oxidation in the sediment was suggested by an increase in δ13C-CH4 and δ2H-CH4 values. During the mixing period, the isotopic CH4 composition of the water column was distinctively less negative than during the stratification period potentially resulting from a greater impact of groundwater CH4 input compared to the stratification period. Our findings implicate that the application of concentrations and dual isotope measurements of CH4 is a promising approach for constraining CH4 sinks and sources in Lake Willersinnweiher and potentially other small lakes to clearly disentangle the complex CH4 dynamics in lakes both spatially and seasonally.

Molecular diffusion of stable water isotopes in polar firn as a proxy for past temperatures

Polar precipitation archived in ice caps contains information on past temperature conditions. Such information can be retrieved by measuring the water isotopic signals of δ18O and δD in ice cores. These signals have been attenuated during densification due to molecular diffusion in the firn column, where the magnitude of the diffusion is isotopologue specific and temperature dependent. By utilizing the differential diffusion signal, dual isotope measurements of δ18O and δD enable multiple temperature reconstruction techniques. This study assesses how well six different methods can be used to reconstruct past surface temperatures from the diffusion-based temperature proxies. Two of the methods are based on the single diffusion lengths of δ18O and δD, three of the methods employ the differential diffusion signal, while the last uses the ratio between the single diffusion lengths. All techniques are tested on synthetic data in order to evaluate their accuracy and precision. We perform a benchmark test to thirteen high resolution Holocene data sets from Greenland and Antarctica, which represent a broad range of mean annual surface temperatures and accumulation rates. Based on the benchmark test, we comment on the accuracy and precision of the methods. Both the benchmark test and the synthetic data test demonstrate that the most precise reconstructions are obtained when using the single isotope diffusion lengths, with precisions of approximately 1.0°C. In the benchmark test, the single isotope diffusion lengths are also found to reconstruct consistent temperatures with a root-mean-square-deviation of 0.7°C. The techniques employing the differential diffusion signals are more uncertain, where the most precise method has a precision of 1.9°C. The diffusion length ratio method is the least precise with a precision of 13.7°C. The absolute temperature estimates from this method are also shown to be highly sensitive to the choice of fractionation factor parameterization.

Dual Isotope Measurements Reveal Zoning of Nitrate Processing in the Summer Changjiang (Yangtze) River Plume

AbstractRiverine nitrogen input into the coastal zone has increased remarkably in recent decades. Yet its transformation and recycling within hydrodynamically active regions remains unclear. Using observations of nitrate concentration and dual isotopic composition across the Changjiang River plume within a three end‐member mixing model, we found deviations between the observed and expected values for mixing alone, revealing the nonconservative behavior of nitrate. Using cross correlations between concentrations and dual isotope deviations, we identified three nitrogen transformation zones, which correspond to separate portions of the plume. Nitrification and sedimentary denitrification occurred near the river mouth, nitrification prevailed further offshore under the plume, and finally, phytoplankton assimilation occurred in the outer surface plume (>100 km offshore), where it was fueled by nitrate that had already been strongly modified by microbial processes. Information was assembled into a conceptual model offering an overview of nitrogen transformations in a large river plume.

Nitrate sources and nitrogen biogeochemical processes in the Feng River in West China inferred from the nitrogen and oxygen dual isotope measurements of nitrate

A portion of the nitrogen from watersheds entering river networks, which degrades river water quality and causes subsequent eutrophication of the downstream river. Identifying the source of nitrate in rivers is the primary method of controlling excessive nitrogen input into rivers. In this paper, nitrate sources in the Feng River were inferred from nitrate dual isotopes (δ15Nnitrate and δ18Onitrate) combined with the water quality data, and nitrogen biogeochemical processes were further explored. The results indicated that NO3--N is the dominant form of nitrogen in the Feng River system. The annual average content of nitrate was 3.21 mg/L, representing approximately 83% of the total nitrogen content. In September of 2012, the δ15Nnitrate and δ18Onitrate values suggested that the atmospheric deposition and soil organic nitrogen were the main sources of nitrate in the upstream region, and that the atmospheric deposition, chemical fertilizers, sewage, and manure inputs were the sources in the downstream region, with average contribution ratios of 68.4, 19, and 12.6%, respectively. In December of 2011, the δ15Nnitrate and δ18Onitrate values suggested that the nitrate upstream was mainly derived from soil organic nitrogen and that the nitrate downstream was derived mainly from sewage and manure inputs, with a contribution ratio of 68%. The δ18Onitrate data indicated that most of the nitrate from microbial nitrification could make a remarkable contribution to the Feng River system. The variations in the isotopic nitrate values suggested that denitrification enriched the heavier isotopes of nitrate in the upstream and midstream Feng River in the summer. The isotopic characteristics of the phytoplankton uptake of nitrogen were obvious in the downstream Feng River in April 2012. Nitrogen biogeochemical processes of the Feng River water system are the results of a mixture of denitrification and phytoplankton uptake and other processes.

Insights on the marine microbial nitrogen cycle from isotopic approaches to nitrification

The microbial nitrogen (N) cycle involves a variety of redox processes that control the availability and speciation of N in the environment and that are involved with the production of nitrous oxide (N2O), a climatically important greenhouse gas. Isotopic measurements of ammonium (NH+4), nitrite (NO−2), nitrate (NO−3), and N2O can now be used to track the cycling of these compounds and to infer their sources and sinks, which has lead to new and exciting discoveries. For example, dual isotope measurements of NO−3 and NO−2 have shown that there is NO−3 regeneration in the ocean's euphotic zone, as well as in and around oxygen deficient zones (ODZs), indicating that nitrification may play more roles in the ocean's N cycle than generally thought. Likewise, the inverse isotope effect associated with NO−2 oxidation yields unique information about the role of this process in NO−2 cycling in the primary and secondary NO−2 maxima. Finally, isotopic measurements of N2O in the ocean are indicative of an important role for nitrification in its production. These interpretations rely on knowledge of the isotope effects for the underlying microbial processes, in particular ammonia oxidation and nitrite oxidation. Here we review the isotope effects involved with the nitrification process and the insights provided by this information, then provide a prospectus for future work in this area.

Isotopic composition of carbon dioxide from a boreal forest fire: Inferring carbon loss from measurements and modeling

Fire is an important pathway for carbon (C) loss from boreal forest ecosystems and has a strong effect on ecosystem C balance. Fires can range widely in severity, defined as the amount of vegetation and forest floor consumed by fire, depending on local fuel and climatic conditions. Here we explore a novel method for estimating fire severity and loss of C from fire using the atmosphere to integrate ecosystem heterogeneity at the watershed scale. We measured the δ13C and Δ14C isotopic values of CO2 emitted from an experimental forest fire at the Caribou‐Poker Creek Research Watershed (CPCRW), near Fairbanks, Alaska. We used inverse modeling combined with dual isotope measurements of C contained in aboveground black spruce biomass and soil organic horizons to estimate the amount of C released by this fire. The experimental burn was a medium to severe intensity fire that released, on average, about 2.5 kg Cm−2, more than half of the C contained in vegetation and soil organic horizon pools. For vegetation, the model predicted that approximately 70–75% of pools such as needles, fine branches, and bark were consumed by fire, whereas only 20–30% of pools such as coarse branches and cones were consumed. The fire was predicted to have almost completely consumed surface soil organic horizons and burned about half of the deepest humic horizon. The ability to estimate the amount of biomass combusted and C emission from fires at the watershed scale provides an extensive approach that can complement more limited intensive ground‐based measurements.

A large source of atmospheric nitrous oxide from subtropical North Pacific surface waters

Nitrous oxide (N2O), a trace gas whose concentration is increasing in the atmosphere, plays an important role in both radiative forcing and stratospheric ozone depletion1,2. Its biogeochemical cycle has thus come under intense scrutiny in recent years. Despite these efforts, the global budget of N2O remains unresolved, and the nature and magnitude of the sources and sinks continue to be debated3,4,5 despite the constraints that can be provided by characterizations of the gas6,7. We report here the results of dual-isotope measurements of N2O from the water column of the subtropical North Pacific Ocean. Nitrous oxide within the lower-euphotic and upper-aphotic zones is depleted in both 15N and 18O relative to its tropospheric and deep-ocean composition. These findings are consistent with a prediction, based on global mass-balance considerations, of a near-surface isotopically depleted oceanic N2O source4. Our results indicate that this source, probably produced by bacterial nitrification, contributes significantly to the ocean–atmosphere flux of N2O in the oligotrophic subtropical North Pacific Ocean. This source may act to buffer the isotopic composition of tropospheric N2O, and is quantitatively significant in the global tropospheric N2O budget. Because dissolved gases in near-surface waters are more readily exchanged with the atmospheric reservoir than those in deep waters, the existence of a quantitatively significant N2O source at a relatively shallow depth has potentially important implications for the susceptibility of the source, and the ocean–atmosphere flux, to climatic influences.

Relative response ratios for dual-isotope measurements via coelution and GC/MS

Dual-isotope internal standard measurements by GC/MS which mimic isotope dilution may suffer from non-linear response relations, irreproducibilities or unduly large uncertainties because of variations in ionization efficiences for the respective isotopic forms in the MS source. Such variations may sometimes be avoided via extensive pretreatments, high resolution GC separations and careful control of instrumental parameters. However, an alternative approach is feasible which instead exploits advantages of decreasing GC resolution. By forcing both forms of each analyte to coelute, their relative ionization efficiencies in the MS source should be nearly constant, thereby effectively allowing for constant relative sensitivities over several orders of magnitude in concentration. Thus, constant relative response ratios, required for internal standard calculations, may be attained as a consequence of dramatically lowered GC resolution. Coelution results described herein show linear relative sensitivity relations over much broader ranges than observed for corresponding conventional calibrations with separated components. Coelution methods for dual-isotope GC/MS determinations are compatible with internal standard calculations and thereby offer a powerful alternative to the conventional approach of requiring expensive and labor-intensive additional pretreatments and separations to assure resolution of measured eluates.

Quantitative measurements via co-elution and dual-isotope detection by gas chromatography-mass spectrometry

Dual-isotope measurements by gas chromatography-mass spectrometry (GC-MS) which mimic isotope dilution may suffer from irreproducibilities or unduly large uncertainties because of variations in ionization efficacies for the respective forms in the MS source. Such variations are sometimes avoided via extensive pretreatments and high-resolution GC separations. However, in some circumstances, an alternative approach is feasible which instead exploits the advantages of decreasing GC resolution. By forcing both forms of each analyte to co-elute, their ionization efficacies in the MS source will be virtually identical, thereby allowing for highly reproducible relative response ratios to be attained despite dramatically lowered GC resolution. The co-elution results described here are nearly as precise as results from moderate-resolution separations in the absence of interferents. Thus, dual-isotope GC-MS measurements with co-elution of the target analytes and their respective isotopically labeled internal standards offer a powerful alternative to the conventional approach of requiring expensive and labor-intensive additional pretreatments and separations; however, the effects of interferences may be exacerbated by the forced co-elution and must also be considered.

Measurements using gas chromatography with coelution and dual-isotope atomic emission detection

Dual-isotope measurements by gas chromatography (GC)—atomic emission detection (AED) may enhance results for quantitative analyse. Adding a known amount of an isotopically labelled form of target analytes in each sample can compensate for irreproducibilities or uncertainties associated with sample pretreatments and sample loading. Similarly, fluctuations in AED temperatures, flows and interferants can be compensated via the added labelled forms if each target analyte and its isotopically labelled for coelute. Under these conditions they are subject to identical excitation environments and are measured from the same viewed volumes. Consequently, improved quantitative results may be attained by coellution in GC—AED methods which mimic isotope dilution.

Sodium ion diffusion through liposome membranes containing cerebroside

Na + efflux from liposomes (small unilamellar vesicles, SUV) of various compositions was studied, using 22Na + and 3H-labelled stachyose in simultaneous dual isotope measurements, stachyose being used as a measure of liposome disintegration. Dialysis was utilised to separate liposomes from extra-liposomal activity. Liposomes were made from egg lecithin and sphingomyelin and from mixtures of egg lecithin, sphingomyelin, cerebroside, sulphatide and cholesterol. All mixtures produced more leaky and less stable SUVs than pure lecithin and pure sphingomyelin. The incorporation of cerebroside is significantly smaller than that of the phospholipids including sphingomyelin. It was found that membranes containing cerebroside had a significantly higher Na + permeability than membranes without cerebroside.