Isotope Fractionation Effects Research Articles

Diurnal variation of oxygen isotopic enrichment in asymmetric‐18 ozone observed by the SMILES from space

AbstractAtomic oxygen is a key oxidant for greenhouse gases such as H2O, CH4, and N2O whose amounts are currently increasing in the stratosphere. Ozone photolysis is a major source of atomic oxygen generation. We found, for the first time, that the ozone photolysis of 48O3 and 18O16O16O generated a sharp diurnal variation of δ18OOO in the middle stratosphere (37 km), in contrast to the temporal variation of 48O3. The average isotopic enrichment δ18OOO derived from the Superconducting Submillimeter‐Wave Limb‐Emission Sounder (SMILES) observation was higher during daytime (16.2 ± 3.3%) than at night (13.7 ± 3.6%). Isotopic fractionation effects were simulated for both ozone formation reaction and ozone photolysis with the ozone photolysis contribution estimated to be 80% of the daytime δ18OOO enhancement. Significant solar radiation dependence was observed in the isotopic fractionation of ozone photolysis derived from the SMILES observation and theoretical calculations. These results show that δ18O can be a powerful tool to investigate oxidant chemistry.

Carbon and hydrogen isotope fractionation during aerobic biodegradation of quinoline and 3-methylquinoline.

Compound-specific isotope analysis has been used extensively to investigate the biodegradation of various organic pollutants. To date, little isotope fractionation information is available for the biodegradation of quinolinic compounds. In this study, we report on the carbon and hydrogen isotope fractionation during quinoline and 3-methylquinoline aerobic microbial degradation by a Comamonas sp. strain Q10. Degradation of quinoline and 3-methylquinoline was accompanied by isotope fractionation. Large hydrogen and small carbon isotope fractionation was observed for quinoline while minor carbon and hydrogen isotope fractionation effects occurred for 3-methylquinoline. Bulk carbon and hydrogen enrichment factors (ε bulk) for quinoline biodegradation were -1.2±0.1 and -38±1‰, respectively, while -0.7±0.1 and -5±1‰ for 3-methylquinoline, respectively. This reveals a potential advantage for employing quinoline as the model compound and hydrogen isotope analysis for assessing aerobic biodegradation of quinolinic compounds. The apparent kinetic isotope effects (AKIEC) values of carbon were 1.008±0.0005 for quinoline and 1.0048±0.0005 for 3-methylquinoline while AKIEH values of hydrogen of 1.264±0.011 for quinoline and 1.0356±0.0103 for 3-methylquinoline were obtained. The combined evaluation of carbon and hydrogen isotope fractionation yields Λ values (Λ=Δδ2H/Δδ13C≈εHbulk/εCbulk) of 29±2 for quinoline and 8±2 for 3-methylquinoline. The results indicate that the substrate specificity may have a significant influence on the isotope fractionation for the biodegradation of quinolinic compounds. The substrate-specific isotope enrichment factors would be important for assessing the behavior and fate of quinolinic compounds in the environment.

Sulfur isotopic fractionation and its implication: Sulfate formation in PM2.5 and coal combustion under different conditions

In order to exactly explore sulfur source and sulfate formation under highly polluted atmosphere, we determined δ34S values of sulfate in PM2.5 and atmospheric SO2 in Nanjing region from 1 to 23 Jan. 2014. The secondary sulfate formation mechanism was discussed based on sulfur isotopic fractionation in the process of SO2 oxidation. Meanwhile, we synchronously studied δ34S values of raw coals used locally as well as sulfur isotopic fractionation during the combustion under coal burning and smoldering. The results show that δ34S average values of SO2 and sulfate in PM2.5 were 1.5‰ and 5.1‰, respectively. δ34S values of sulfate in PM2.5 were consistent with those of coals widely used in Nanjing region and Northern China, indicating coal combustion was an important sulfur source for PM2.5. Sulfur isotopic fractionation factors ranged from 1.0014 to 1.0075, implying that SO2 heterogeneous and homogeneous oxidation were coexisting during the formation of the secondary sulfate. The contribution of SO2 heterogeneous oxidation to sulfate varied from 40.7% to 64.8% during the observation period. δ34S values of coals presented moderately positive sulfur isotopic signatures due to organic sulfur in low sulfur coals were mainly formed by plant assimilation. Besides, the negative relationship between δ34S values of coals and total sulfur contents was also found. In addition, there existed a significant sulfur isotopic fractionation effect during coal combustion. Sulfate in PM2.5 in flue gas enriched 34S, while SO2 in flue gas enriched 32S. There was presence of the difference of δ34S values in PM2.5 and SO2 in flue gas between coal burning and smoldering, which was related to coal property and combustion temperature.

Stable hydrogen isotope composition of n-alkanes in urban atmospheric aerosols in Taiyuan, China

The hydrogen isotope compositions (δD) of n-alkanes associated with particulate matter with a diameter of ≤10 μm from Taiyuan, China, during heating and non-heating periods were measured via gas chromatography–isotope ratio mass spectrometry to reveal the spatial and temporal characteristics of five functional zones and to provide another constraint on atmospheric pollutants. The δD values of n-C16 to n-C31 during the heating and non-heating periods ranged from −235.9‰ to −119.8‰ and from −231.3‰ to −129.2‰, respectively, but these similar spans had different distribution features. During the heating period, the δD distributions between non–central heating and commercial districts were consistent, as were those between residential and industrial districts; the n-alkanes came from two or more types of emission sources. Coal soot might be the primary local emission source, but not the only source. During the non-heating period, the n-alkanes of n-C16 to n-C20 were more depleted in D with the increasing carbon number in all functional zones, but there was no rule for n-C21 to n-C31. Specifically, coal soot and vehicle exhaust might be the primary sources of n-alkanes for non–central heating districts in the heating and non-heating periods, respectively, according to the δD distribution of n-C18 to n-C22; gasoline vehicle exhaust might be an n-alkane source, and the hydrogen isotope fractionation effect during the condensation process should be a pollution mechanism for the commercial district during the heating period; the δD distribution difference of n-C16 to n-C18 between the two periods in the residential and industrial districts was consistent, which indicates a similar source of fossil fuel combustion and a similar isotope fractionation effect during the non-heating period.

Diverging effects of isotopic fractionation upon molecular diffusion of noble gases in water: mechanistic insights through ab initio molecular dynamics simulations.

Atmospheric noble gases are routinely used as natural tracers to analyze gas transfer processes in aquatic systems. Their isotopic ratios can be employed to discriminate between different physical transport mechanisms by comparison to the unfractionated atmospheric isotope composition. In many applications of aquatic systems molecular diffusion was thought to cause a mass dependent fractionation of noble gases and their isotopes according to the square root ratio of their masses. However, recent experiments focusing on isotopic fractionation within a single element challenged this broadly accepted assumption. The determined fractionation factors of Ne, Ar, Kr and Xe isotopes revealed that only Ar follows the prediction of the so-called square root relation, whereas within the Ne, Kr and Xe elements no mass-dependence was found. The reason for this unexpected divergence of Ar is not yet understood. The aim of our computational exercise is to establish the molecular-resolved mechanisms behind molecular diffusion of noble gases in water. We make the hypothesis that weak intermolecular interactions are relevant for the dynamical properties of noble gases dissolved in water. Therefore, we used ab initio molecular dynamics to explicitly account for the electronic degrees of freedom. Depending on the size and polarizability of the hydrophobic particles such as noble gases, their motion in dense and polar liquids like water is subject to different diffusive regimes: the inter-cavity hopping mechanism of small particles (He, Ne) breaks down if a critical particle size achieved. For the case of large particles (Kr, Xe), the motion through the water solvent is governed by mass-independent viscous friction leading to hydrodynamical diffusion. Finally, Ar falls in between the two diffusive regimes, where particle dispersion is propagated at the molecular collision time scale of the surrounding water molecules.

Variations of Stable Carbon Isotopes of CH4 Emission from Three Typical Rice Fields in China

Little is known about the stable carbon isotopes of methane (CH4) emitted (δ13CH4emitte4) from permanently flooded rice fields and double rice-cropping fields. The CH4 emission and corresponding δ13CH4emitte4 under various field managements (mulching, water regime, tillage, and nitrogen (N) fertilization) were simultaneously measured in three typical Chinese rice fields, a permanently flooded rice field in Ziyang City, Sichuan Province, Southwest China, a double-rice cropping field in Yingtan City, Jiangxi Province, Southeast China, and a rice-wheat rotation field in Jurong City, Jiangsu Province, East China, from 2010 to 2012. Results showed different seasonal variations of δ13CH4emitte4 among the three fields during the rice-growing season. The values of δ13CH4emitte4 were negatively correlated with corresponding CH4 emissions in seasonal variation and mean, indicating the importance of CH4 production, oxidation, and transport associated with isotopic fractionation effects to the δ13CH4emitte4. Seasonal variations of δ13CH4emitte4 were slightly impacted by mulching cultivation, tillage, and N application, but highly controlled by drainage. Meanwhile, tillage, N application, and especially mulching cultivation had important effects on seasonal mean CH4 emissions and corresponding δ13CH4emitte4, with low emissions accompanied by high values of δ13CH4emitte4. Seasonal mean values of δ13CH4emitte4 from the three fields were similar, mostly ranging from −60‰ to −50‰, which are well in agreement with previously published data. These demonstrated that seasonal variations of δ13CH4emitte4 mainly depended on the changes in CH4 emission from rice fields and further indicated the important effects of methanogenic pathways, CH4 oxidation, and CH4 transport associated with isotope fractionation effects influenced by field managements on δ13CH4emitte4.

Stable isotope analyses of oxygen (18 O:17 O:16 O) and chlorine (37 Cl:35 Cl) in perchlorate: reference materials, calibrations, methods, and interferences.

Perchlorate (ClO4- ) is a common trace constituent of water, soils, and plants; it has both natural and synthetic sources and is subject to biodegradation. The stable isotope ratios of Cl and O provide three independent quantities for ClO4- source attribution and natural attenuation studies: δ37 Cl, δ18 O, and δ17 O (or Δ17 O or 17 Δ) values. Documented reference materials, calibration schemes, methods, and interferences will improve the reliability of such studies. Three large batches of KClO4 with contrasting isotopic compositions were synthesized and analyzed against VSMOW-SLAP, atmospheric O2 , and international nitrate and chloride reference materials. Three analytical methods were tested for O isotopes: conversion of ClO4- to CO for continuous-flow IRMS (CO-CFIRMS), decomposition to O2 for dual-inlet IRMS (O2-DIIRMS), and decomposition to O2 with molecular-sieve trap (O2-DIIRMS+T). For Cl isotopes, KCl produced by thermal decomposition of KClO4 was reprecipitated as AgCl and converted into CH3 Cl for DIIRMS. KClO4 isotopic reference materials (USGS37, USGS38, USGS39) represent a wide range of Cl and O isotopic compositions, including non-mass-dependent O isotopic variation. Isotopic fractionation and exchange can affect O isotope analyses of ClO4- depending on the decomposition method. Routine analyses can be adjusted for such effects by normalization, using reference materials prepared and analyzed as samples. Analytical errors caused by SO42- , NO3- , ReO42- , and C-bearing contaminants include isotope mixing and fractionation effects on CO and O2 , plus direct interference from CO2 in the mass spectrometer. The results highlight the importance of effective purification of ClO4- from environmental samples. KClO4 reference materials are available for testing methods and calibrating isotopic data for ClO4- and other substances with widely varying Cl or O isotopic compositions. Current ClO4- extraction, purification, and analysis techniques provide relative isotope-ratio measurements with uncertainties much smaller than the range of values in environmental ClO4- , permitting isotopic evaluation of environmental ClO4- sources and natural attenuation. Copyright © 2016 John Wiley & Sons, Ltd.

Spatial distribution of organic carbon fractions and δ13C in urban soils, Shanghai, China

Urban areas are characterized by diverse land-use patterns and are strongly influenced by anthropogenic activities. However, few studies have examined the effects of urbanization on concentrations of soil organic carbon (SOC) and its various components or δ 13C in urban soils. The aim of this study was to assess the spatial distribution of SOC fractions and δ 13C signatures of urban soils in Shanghai, China. The results showed that SOC fractions and δ 13C compositions differ over a range of spatial scales. The concentrations of SOC, readily oxidizable organic carbon (C), black carbon (BC) and δ 13C in surface soils (0–20 cm) were 10.5, 3.5, 6.9 g kg–1 and –24.9%, respectively, and the corresponding concentrations in deep soils (20–100 cm) were 8.4, 2.8, 6.0 g kg–1 and –23.9%. In urban soils, BC accounted for a higher proportion of the SOC pools. Concentrations of SOC, readily oxidizable organic C and BC were higher, whereas the 13C ratio was much lower in the city centre. The effects of carbon isotope fractionation were also more evident in the central urban area. The results also indicated that not only the concentrations of readily oxidizable organic C and BC but also the δ 13C values were related to the time since the soils were converted to urban use. Differences in the time since urbanization and the severity of the associated environmental impacts can be assessed using SOC fractions and δ 13C isotopic compositions because observed changes in these quantities can be attributed to the strong influence of anthropogenic activities.

Calcium-aluminum-rich inclusions with fractionation and unidentified nuclear effects (FUN CAIs): II. Heterogeneities of magnesium isotopes and 26Al in the early Solar System inferred from in situ high-precision magnesium-isotope measurements

Calcium-aluminum-rich inclusions with isotopic mass fractionation effects and unidentified nuclear isotopic anomalies (FUN CAIs) have been studied for more than 40years, but their origins remain enigmatic. Here we report in situ high precision measurements of aluminum-magnesium isotope systematics of FUN CAIs by secondary ion mass spectrometry (SIMS). Individual minerals were analyzed in six FUN CAIs from the oxidized CV3 carbonaceous chondrites Axtell (compact Type A CAI Axtell 2271) and Allende (Type B CAIs C1 and EK1-4-1, and forsterite-bearing Type B CAIs BG82DH8, CG-14, and TE). Most of these CAIs show evidence for excess 26Mg due to the decay of 26Al. The inferred initial 26Al/27Al ratios [(26Al/27Al)0] and the initial magnesium isotopic compositions (δ26Mg0) calculated using an exponential law with an exponent β of 0.5128 are (3.1±1.6)×10−6 and 0.60±0.10‰ (Axtell 2271), (3.7±1.5)×10−6 and −0.20±0.05‰ (BG82DH8), (2.2±1.1)×10−6 and −0.18±0.05‰ (C1), (2.3±2.4)×10−5 and −2.23±0.37‰ (EK1-4-1), (1.5±1.1)×10−5 and −0.42±0.08‰ (CG-14), and (5.3±0.9)×10−5 and −0.05±0.08‰ (TE) with 2σ uncertainties. We infer that FUN CAIs recorded heterogeneities of magnesium isotopes and 26Al in the CAI-forming region(s). Comparison of 26Al-26Mg systematics, stable isotope (oxygen, magnesium, calcium, and titanium) and trace element studies of FUN and non-FUN igneous CAIs indicates that there is a continuum among these CAI types. Based on these observations and evaporation experiments on CAI-like melts, we propose a generic scenario for the origin of igneous (FUN and non-FUN) CAIs: (i) condensation of isotopically normal solids in an 16O-rich gas of approximately solar composition; (ii) formation of CAI precursors by aggregation of these solids together with variable abundances of isotopically anomalous grains—possible carriers of unidentified nuclear (UN) effects; and (iii) melt evaporation of these precursors accompanied by crystallization under different temperatures and gas pressures, leading to the observed variations in mass-dependent isotopic fractionation (F) effects.

Episodic speleothem deposition tracks the terrestrial impact of millennial-scale last glacial climate variability in SW Ireland

Abstract Eighty four new U-Th ages are presented for twenty randomly selected broken, displaced and reworked calcite speleothems retrieved from clastic sedimentary fill and from isolated bedding-plane shelves in Crag cave (SW Ireland). The dated pre-Holocene samples span much of the last glacial, ranging in age from 85.15 ± 0.60 to 23.45 ± 0.17 ka. Speleothem deposition requires the presence of liquid water, and because Crag cave is a shallow system, deposition is considered likely only when mean annual air temperatures (MAAT) exceed the freezing point of water. Deposition at this mid-latitude ocean-marginal site occurred episodically during MIS5a through to MIS2, synchronously within dating uncertainties, with the timing of Greenland Interstadials (GI). In the latter part of Marine Isotope Stage 3 (MIS3), deposition was particularly intense, consistent with regional scale climate amelioration inferred previously from radiocarbon ages for sparse MIS3 organic and freshwater surficial deposits in N. Ireland. A brief episode of speleothem deposition at c.23.40 ± 0.22 ka coincides with GI-2, demonstrating the sensitivity of the site to brief climate amelioration episodes in Greenland during MIS2. Conditions favourable for speleothem deposition occurred periodically during the last glacial, indicating temperature changes of at least 10 °C between stadials and interstadials at this mid-latitude site. Deposition ceased during Greenland Stadials (GS), including during periods of ice-rafting in the adjacent N. Atlantic Ocean (Heinrich events). Oxygen and carbon isotope ratios of the last glacial speleothems are generally elevated, reflecting non-equilibrium isotope fractionation effects. However, establishment of low δ13C values often occurred within a few decades of climate amelioration, indicating that biogenic CO2 production resumed rapidly at this site, particularly during MIS3. Speleothem δ18O variability was driven largely by long-term changes in the δ18O value of the adjacent North Atlantic surface water, in turn largely reflecting changes in global ice volume. In common with published speleothem datasets, warming episodes in Ireland associated with GI events typically pre-date their timing in the GICC05 and GICC05modelext time scales, but lie comfortably within the maximum counting uncertainties of these ice core age models.

Anoxic nitrate reduction coupled with iron oxidation and attenuation of dissolved arsenic and phosphate in a sand and gravel aquifer

Nitrate has become an increasingly abundant potential electron acceptor for Fe(II) oxidation in groundwater, but this redox couple has not been well characterized within aquifer settings. To investigate this reaction and some of its implications for redox-sensitive groundwater contaminants, we conducted an in situ field study in a wastewater-contaminated aquifer on Cape Cod. Long-term (15year) geochemical monitoring within the contaminant plume indicated interacting zones with variable nitrate-, Fe(II)-, phosphate-, As(V)-, and As(III)-containing groundwater. Nitrate and phosphate were derived predominantly from wastewater disposal, whereas Fe(II), As(III), and As(V) were mobilized from the aquifer sediments. Multiple natural gradient, anoxic tracer tests were conducted in which nitrate and bromide were injected into nitrate-free, Fe(II)-containing groundwater. Prior to injection, aqueous Fe(II) concentrations were approximately 175μM, but sorbed Fe(II) accounted for greater than 90% of the total reactive Fe(II) in the aquifer. Nitrate reduction was stimulated within 1m of transport for 100μM and 1000μM nitrate additions, initially producing stoichiometric quantities of nitrous oxide (>300μMN). In subsequent injections at the same site, nitrate was reduced even more rapidly and produced less nitrous oxide, especially over longer transport distances. Fe(II) and nitrate concentrations decreased together and were accompanied by Fe(III) oxyhydroxide precipitation and decreases in dissolved phosphate, As(III), and As(V) concentrations. Nitrate N and O isotope fractionation effects during nitrate reduction were approximately equal (ε15N/ε18O=1.11) and were similar to those reported for laboratory studies of biological nitrate reduction, including denitrification, but unlike some reported effects on nitrate by denitrification in aquifers. All constituents affected by the in situ tracer experiments returned to pre-injection levels after several weeks. Additionally, Fe(II)-oxidizing, nitrate-reducing microbial enrichment cultures were obtained from aquifer sediments. Growth experiments with the cultures sequentially produced nitrite and nitrous oxide from nitrate while simultaneously oxidizing Fe(II). Field and culture results suggest that nitrogen oxide reduction and Fe(II) oxidation in the aquifer are a complex interaction of coupled biotic and abiotic reactions. Overall, the results of this study demonstrate that anoxic nitrate-dependent iron oxidation can occur in groundwater; that it could control iron speciation; and that the process can impact the mobility of other chemical species (e.g., phosphate and arsenic) not directly involved in the oxidation–reduction reaction.

The forensic analysis of office paper using oxygen Isotope Ratio Mass Spectrometry, part 2: Characterising the source materials and the effect of production and usage on the δ18O values of cellulose and paper

For casework applications, understanding the source processes used to create a material and the effects of those sources on the results obtained by Isotope Ratio Mass Spectrometry (IRMS) of a bulk material is important. Likewise, understanding the effect of environment, home/office printing processes and some forensic testing in at least a basic context, ensures that in casework, enough information on the effects of these variables is available during comparison and interpretation.In this study, which focuses on oxygen isotopic abundance measurements, both fractionation and mixing effects were observed within the pulping and production process. Also observed in the carbon isotopic experiments, sampling that included toner changed the measured isotopic abundance values of the paper and should be avoided in casework. Inkjet printing processes were not shown to have an effect on the paper oxygen abundance values. Samples that were treated for fingerprints using 1,2-Indandione-Zn prior to sampling showed the greatest risk for misinterpretation of whether two samples had originated from the same source. While this study provides a good basis and understanding of the effects of a range of factors on document paper oxygen isotope values, further testing for a range of specific casework scenarios is required and should be undertaken on a case by case basis as the need arises.

Active layer hydrology in an arctic tundra ecosystem: quantifying water sources and cycling using water stable isotopes

AbstractClimate change and thawing permafrost in the Arctic will significantly alter landscape hydro‐geomorphology and the distribution of soil moisture, which will have cascading effects on climate feedbacks (CO2 and CH4) and plant and microbial communities. Fundamental processes critical to predicting active layer hydrology are not well understood. This study applied water stable isotope techniques (δ2H and δ18O) to infer sources and mixing of active layer waters in a polygonal tundra landscape in Barrow, Alaska (USA), in August and September of 2012. Results suggested that winter precipitation did not contribute substantially to surface waters or subsurface active layer pore waters measured in August and September. Summer rain was the main source of water to the active layer, with seasonal ice melt contributing to deeper pore waters later in the season. Surface water evaporation was evident in August from a characteristic isotopic fractionation slope (δ2H vs δ18O). Freeze‐out isotopic fractionation effects in frozen active layer samples and textural permafrost were indistinguishable from evaporation fractionation, emphasizing the importance of considering the most likely processes in water isotope studies, in systems where both evaporation and freeze‐out occur in close proximity. The fractionation observed in frozen active layer ice was not observed in liquid active layer pore waters. Such a discrepancy between frozen and liquid active layer samples suggests mixing of meltwater, likely due to slow melting of seasonal ice. This research provides insight into fundamental processes relating to sources and mixing of active layer waters, which should be considered in process‐based fine‐scale and intermediate‐scale hydrologic models. Copyright © 2016 John Wiley & Sons, Ltd.

Stable silver isotope fractionation in the natural transformation process of silver nanoparticles.

Nanoparticles in the environment can form by natural processes or be released due to human activities. Owing to limited analytical methods, the behaviour of nanoparticles in the natural environment is poorly understood and until now they have only been described by the variations in the nanoparticle size or the concentration of the element of interest. Here we show that by using inductively coupled plasma mass spectrometry to measure silver (Ag) isotope ratios it is possible to understand the transformation processes of silver nanoparticles (AgNPs) in the environment. We found that the formation and dissolution of AgNPs under natural conditions caused significant variations in the ratio of natural Ag isotopes ((107)Ag and (109)Ag) with an isotopic enrichment factor (ε) up to 0.86‰. Furthermore, we show that engineered AgNPs have distinctly different isotope fractionation effects to their naturally formed counterparts. Further studies will be needed to understand whether isotope analysis can be used to reveal the sources of AgNPs in the environment.

Nuclear field shift effects on stable isotope fractionation: a review

An anomalous isotope effect exists in many heavy element isotope systems (e.g., Sr, Gd, Zn, U). This effect used to be called the “odd–even isotope effect” because the odd mass number isotopes behave differently from the even mass number isotopes. This mass-independent isotope fractionation driving force, which originates from the difference in the ground-state electronic energies caused by differences in nuclear size and shape, is currently denoted as the nuclear field shift effect (NFSE). It is found that the NFSE can drive isotope fractionation of some heavy elements (e.g., Hg, Tl, U) to an astonishing degree, far more than the magnitude caused by the conventional mass-dependent effect (MDE). For light elements, the MDE is the dominant factor in isotope fractionation, while the NFSE is neglectable. Furthermore, the MDE and the NFSE both decrease as temperatures increase, though at different rates. The MDE decreases rapidly with a factor of 1/T2, while the NFSE decreases slowly with a factor of 1/T. As a result, even at high temperatures, the NFSE is still significant for many heavy element isotope systems. In this review paper, we begin with an introduction of the basic concept of the NSFE, including its history and recent progress, and follow with the potential implications of the inclusion of the NFSE into the kinetic isotope fractionation effect (KIE) and heavy isotope geochronology.

Physical inversion of the full IASI spectra: Assessment of atmospheric parameters retrievals, consistency of spectroscopy and forward modelling

Spectra observed by the Infrared Atmospheric Sounder Interferometer (IASI) have been used to assess both retrievals and the spectral quality and consistency of current forward models and spectroscopic databases for atmospheric gas line and continuum absorption. The analysis has been performed with thousands of observed spectra over sea surface in the Pacific Ocean close to the Mauna Loa (Hawaii) validation station. A simultaneous retrieval for surface temperature, atmospheric temperature, H2O, HDO, O3 profiles and gas average column abundance of CO2, CO, CH4, SO2, N2O, HNO3, NH3, OCS and CF4 has been performed and compared to in situ observations. The retrieval system considers the full IASI spectrum (all 8461 spectral channels on the range 645–2760cm−1). We have found that the average column amount of atmospheric greenhouse gases can be retrieved with a precision better than 1% in most cases. The analysis of spectral residuals shows that, after inversion, they are generally reduced to within the IASI radiometric noise. However, larger residuals still appear for many of the most abundant gases, namely H2O, CH4 and CO2. The H2O ν2 spectral region is in general warmer (higher radiance) than observations. The CO2ν2 and N2O/CO2ν3 spectral regions now show a consistent behavior for channels, which are probing the troposphere. Updates in CH4 spectroscopy do not seem to improve the residuals. The effect of isotopic fractionation of HDO is evident in the 2500–2760cm−1 region and in the atmospheric window around 1200cm−1.

Marine Mo biogeochemistry in the context of dynamically euxinic mid-depth waters: A case study of the lower Cambrian Niutitang shales, South China

Molybdenum (Mo) concentrations and Mo isotopes have been widely used as proxies for local and global redox conditions in early oceans (>520Ma) that were stratified and characterized by dynamically euxinic mid-depth waters. However, the nature of the Mo cycle and accompanying isotopic fractionations in such oceans remain poorly known. To fill this gap, we conducted an integrated study of Mo isotopes and redox-sensitive trace element (RSTE) abundances in the Lower Cambrian Niutitang Formation at Yangjiaping, South China. This section accumulated on the northern shelf margin of the Nanhua Basin, a failed intracontinental rift basin with a good connection to the open ocean during the early Cambrian. The Niutitang Formation contains a ∼18-m-thick lower black shale member, and a ∼56-m-thick upper gray silty shale member. The lower member (LM) is moderately to strongly enriched in Mo, U and V, and heterogeneous in Mo isotopic composition (δ98Mo=−0.65‰ to +2.14‰), indicative of dominantly euxinic depositional conditions punctuated by ferruginous episodes (as shown by previously reported Fe speciation data). The upper member (UM) shows lesser enrichment of Mo, U and V, higher Mo/U ratios, and intermediate and more uniform Mo-isotopic compositions (δ98Mo=+1.16‰ to +1.71‰), indicative of weakly oxic to anoxic-euxinic depositional conditions. Geochemical profiles suggest that the LM-to-UM transition reflects a shift of the O2/H2S chemocline from the water column to the sediment. Large δ98Mo fluctuations in the LM may record variations of H2S concentrations in the mid-depth euxinic waters. The intermediate and relatively uniform δ98Mo values of the UM are attributed to the effects of a local Fe–Mn particulate shuttle.In light of the observations at Yangjiaping and other contemporaneous sections in the Nanhua Basin, we propose a new marine Mo biogeochemical model for the early Cambrian ocean. In this model, an Fe–Mn reduction zone may have developed above mid-depth euxinic waters in nearshore areas of the Nanhua Basin due to relatively high availability of fluvial Fe–Mn oxides and their subsequent reduction below the chemocline. Within this redox framework, large sedimentary δ98Mo variations observed for the Niutitang shales can be explained through the combined effects of Mo isotopic fractionation associated with adsorption onto Fe–Mn-oxides and spatially variable H2S concentrations in the euxinic watermass. By compiling published data, we found that the large sedimentary δ98Mo variations previously observed in the early Cambrian Nanhua Basin and in other Neoproterozoic-Cambrian marine basins can be similarly explained. Our model provides a novel interpretation of the Mo biogeochemistry of early Earth oceans, within the framework of which Mo isotopes may serve to further our understanding of the oxygenation history of the Earth-surface system.

Nitrogen cycling in shallow low-oxygen coastal waters off Peru from nitrite and nitrate nitrogen and oxygen isotopes

Abstract. O2 deficient zones (ODZs) of the world's oceans are important locations for microbial dissimilatory nitrate (NO3−) reduction and subsequent loss of combined nitrogen (N) to biogenic N2 gas. ODZs are generally coupled to regions of high productivity leading to high rates of N-loss as found in the coastal upwelling region off Peru. Stable N and O isotope ratios can be used as natural tracers of ODZ N-cycling because of distinct kinetic isotope effects associated with microbially mediated N-cycle transformations. Here we present NO3− and nitrite (NO2−) stable isotope data from the nearshore upwelling region off Callao, Peru. Subsurface oxygen was generally depleted below about 30 m depth with concentrations less than 10 µM, while NO2− concentrations were high, ranging from 6 to 10 µM, and NO3− was in places strongly depleted to near 0 µM. We observed for the first time a positive linear relationship between NO2−δ15N and δ18O at our coastal stations, analogous to that of NO3− N and O isotopes during NO3− uptake and dissimilatory reduction. This relationship is likely the result of rapid NO2− turnover due to higher organic matter flux in these coastal upwelling waters. No such relationship was observed at offshore stations where slower turnover of NO2− facilitates dominance of isotope exchange with water. We also evaluate the overall isotope fractionation effect for N-loss in this system using several approaches that vary in their underlying assumptions. While there are differences in apparent fractionation factor (ε) for N-loss as calculated from the δ15N of NO3−, dissolved inorganic N, or biogenic N2, values for ε are generally much lower than previously reported, reaching as low as 6.5 ‰. A possible explanation is the influence of sedimentary N-loss at our inshore stations which incurs highly suppressed isotope fractionation.

Identifying organic matter sources using isotopic ratios in a watershed impacted by intensive agricultural activities in Northeast China

Knowledge about the origin and variation of OM (organic matter) pools in a receiving river might provide a basis for addressing non-point sources of pollution resulting from intensive agricultural activity. In this study, an isotopic approach (δ13C, δ15N and C/N ratios) was used to identify potential OM sources and C and N cycle processes occurring in the Abujiao River watershed located in the Sanjiang Plain. A Bayesian model (stable isotope analysis in R, SIAR) was utilized to apportion the contributions of potential sources. In this watershed, isotopic ratios of δ13C ranged from −30.62‰ to −23.71‰ and from −28.22‰ to −23.98‰ in particulate organic matter (POM) and sedimentary organic matter (SOM), respectively. Slightly lower δ15N values (3.12±4.12‰ in POM; 3.7±2.42% in SOM) and C/N ratios (6.62±5.28 in POM; 10.73±6.58 in SOM) were found in SOM pools. Considerable spatio-temporal variability of the organic carbon and nitrogen isotopic ratios was present in the POM pool, which was controlled by storms and land use. The unusually low δ15N-POM values accompanied by high chlorophyll-a concentrations suggested that an isotopic fractionation effect caused by phytoplankton existed in the POM pool. However, isotopic characteristics indicated that the isotopic fractionation was absent in the SOM pool. SIAR modeling result showed that source contributions differed significantly among different sampling periods. During storms, ”soil OM” was the major source (accounting for 45.50%) in the POM pools. During other periods, effluent detritus and phytoplankton sources were predominant in the downstream region. Compared with POM, almost half of SOM came from terrestrial sources, and the effluent detritus and “soil OM” sources predominated in the SOM pool. This study suggested that assessment of behaviors and sources of OM could lead to the implementation of effective non-point source pollution reduction strategies and management practices for protecting water quality.

Diffusion related isotopic fractionation effects with one-dimensional advective–dispersive transport

Aqueous phase diffusion-related isotope fractionation (DRIF) for carbon isotopes was investigated for common groundwater contaminants in systems in which transport could be considered to be one-dimensional. This paper focuses not only on theoretically observable DRIF effects in these systems but introduces the important concept of constraining “observable” DRIF based on constraints imposed by the scale of measurements in the field, and on standard limits of detection and analytical uncertainty. Specifically, constraints for the detection of DRIF were determined in terms of the diffusive fractionation factor, the initial concentration of contaminants (C0), the method detection limit (MDL) for isotopic analysis, the transport time, and the ratio of the longitudinal mechanical dispersion coefficient to effective molecular diffusion coefficient (Dmech/Deff). The results allow a determination of field conditions under which DRIF may be an important factor in the use of stable carbon isotope measurements for evaluation of contaminant transport and transformation for one-dimensional advective–dispersive transport. This study demonstrates that for diffusion-dominated transport of BTEX, MTBE, and chlorinated ethenes, DRIF effects are only detectable for the smaller molar mass compounds such as vinyl chloride for C0/MDL ratios of 50 or higher. Much larger C0/MDL ratios, corresponding to higher source concentrations or lower detection limits, are necessary for DRIF to be detectable for the higher molar mass compounds. The distance over which DRIF is observable for VC is small (less than 1m) for a relatively young diffusive plume (<100years), and DRIF will not easily be detected by using the conventional sampling approach with “typical” well spacing (at least several meters). With contaminant transport by advection, mechanical dispersion, and molecular diffusion this study suggests that in field sites where Dmech/Deff is larger than 10, DRIF effects will likely not be observable for common groundwater contaminants. Importantly, under most field conditions, Dmech/Deff≥10 is usually satisfied in the longitudinal direction, suggesting that DRIF is not likely to be observable in most groundwater systems in which contaminant transport is predominantly one-dimensional. Given the importance in the MDL it is recommended that MDL should always be explicitly reported in both modeling and field studies.