Barium isotope fractionation during witherite (BaCO3) dissolution, precipitation and at equilibrium

This study examines the behavior of Ba isotope fractionation between witherite and fluid during mineral dissolution, precipitation and at chemical equilibrium. Experiments were performed in batch reactors at 25 oC in 10-2 M NaCl solution where the pH was adjusted by continuous bubbling of a water saturated gas phase of CO2 or atmospheric air. During witherite dissolution no Ba isotope fractionation was observed between solid and fluid. In contrast, during witherite precipitation, caused by a pH increase, a preferential uptake of the lighter 134Ba isotopomer in the solid phase was observed. In this case, the isotope fractionation factor αwitherite-fluid is calculated to be 0.99993 ± 0.00004 (or Δ137/134Bawitherite-fluid ≈ -0.07 ± 0.04 ‰, 2sd). The most interesting feature of this study, however, is that after the attainment of chemical equilibrium, the Ba isotope composition of the aqueous phase is progressively becoming lighter, indicating a continuous exchange of Ba2+ ions between witherite and fluid. Mass balance calculations indicate that the detachment of Ba from the solid is not only restricted to the outer surface layer of the solid, but affects several (~7 unit cells) subsurface layers of the crystal. This observation comes in excellent agreement with the concept of a dynamic system at chemical equilibrium in a mineral-fluid system, denoting that the time required for the achievement of isotopic equilibrium in the witherite-fluid system is longer compared to that observed for chemical equilibrium. Overall, these results indicate that the isotopic composition of Ba bearing carbonates in natural environments may be altered due to changes in fluid composition without a net dissolution/precipitation to be observed.

Optimized Demineralization Technique for the Measurement of Stable Isotope Ratios of Nonexchangeable H in Soil Organic Matter

To make use of the isotope ratio of nonexchangeable hydrogen (δ2Hn (nonexchangeable)) of bulk soil organic matter (SOM), the mineral matrix (containing structural water of clay minerals) must be separated from SOM and samples need to be analyzed after H isotope equilibration. We present a novel technique for demineralization of soil samples with HF and dilute HCl and recovery of the SOM fraction solubilized in the HF demineralization solution via solid-phase extraction. Compared with existing techniques, organic C (Corg) and organic N (Norg) recovery of demineralized SOM concentrates was significantly increased (Corg recovery using existing techniques vs new demineralization method: 58% vs 78%; Norg recovery: 60% vs 78%). Chemicals used for the demineralization treatment did not affect δ2Hn values as revealed by spiking with deuterated water. The new demineralization method minimized organic matter losses and thus artificial H isotope fractionation, opening up the opportunity to use δ2Hn analyses of SOM as a new tool in paleoclimatology or geospatial forensics.

Chlorine isotope composition in chlorofluorocarbons CFC-11, CFC-12 and CFC-113 in firn, stratospheric and tropospheric air

The stratospheric degradation of chlorofluorocarbons (CFCs) releases chlorine, which is a major contributor to the destruction of stratospheric ozone (O3). A recent study reported strong chlorine isotope fractionation during the breakdown of the most abundant CFC (CFC-12, CCl2F2, Laube et al., 2010a), similar to effects seen in nitrous oxide (N2O). Using air archives to obtain a long-term record of chlorine isotope ratios in CFCs could help to identify and quantify their sources and sinks. We analyse the three most abundant CFCs and show that CFC-11 (CCl3F) and CFC-113 (CClF2CCl2F) exhibit significant stratospheric chlorine isotope fractionation, in common with CFC-12. The apparent isotope fractionation (ϵapp) for mid- and high-latitude stratospheric samples are respectively −2.4 (0.5) and −2.3 (0.4) ‰ for CFC-11, −12.2 (1.6) and −6.8 (0.8) ‰ for CFC-12 and −3.5 (1.5) and −3.3 (1.2) ‰ for CFC-113, where the number in parentheses is the numerical value of the standard uncertainty expressed in per mil. Assuming a constant isotope composition of emissions, we calculate the expected trends in the tropospheric isotope signature of these gases based on their stratospheric 37Cl enrichment and stratosphere–troposphere exchange. We compare these projections to the long-term δ (37Cl) trends of all three CFCs, measured on background tropospheric samples from the Cape Grim air archive (Tasmania, 1978–2010) and tropospheric firn air samples from Greenland (North Greenland Eemian Ice Drilling (NEEM) site) and Antarctica (Fletcher Promontory site). From 1970 to the present day, projected trends agree with tropospheric measurements, suggesting that within analytical uncertainties, a constant average emission isotope delta (δ) is a compatible scenario. The measurement uncertainty is too high to determine whether the average emission isotope δ has been affected by changes in CFC manufacturing processes or not. Our study increases the suite of trace gases amenable to direct isotope ratio measurements in small air volumes (approximately 200 mL), using a single-detector gas chromatography–mass spectrometry (GC–MS) system.

Stable N isotope composition of nitrate reflects N transformations during the passage of water through a montane rain forest in Ecuador

Knowledge of the fate of deposited N in the possibly N-limited, highly biodiverse north Andean forests is important because of the possible effects of N inputs on plant performance and species composition. We analyzed concentrations and fluxes of NO3 −–N, NH4 +–N and dissolved organic N (DON) in rainfall, throughfall, litter leachate, mineral soil solutions (0.15–0.30 m depths) and stream water in a montane forest in Ecuador during four consecutive quarters and used the natural 15N abundance in NO3 − during the passage of rain water through the ecosystem and bulk δ15N values in soil to detect N transformations. Depletion of 15N in NO3 − and increased NO3 −–N fluxes during the passage through the canopy and the organic layer indicated nitrification in these compartments. During leaching from the organic layer to mineral soil and stream, NO3 − concentrations progressively decreased and were enriched in 15N but did not reach the δ15N values of solid phase organic matter (δ15N = 5.6–6.7‰). This suggested a combination of nitrification and denitrification in mineral soil. In the wettest quarter, the δ15N value of NO3 − in litter leachate was smaller (δ15N = −1.58‰) than in the other quarters (δ15N = −9.38 ± SE 0.46‰) probably because of reduced mineralization and associated fractionation against 15N. Nitrogen isotope fractionation of NO3 − between litter leachate and stream water was smaller in the wettest period than in the other periods probably because of a higher rate of denitrification and continuous dilution by isotopically lighter NO3 −–N from throughfall and nitrification in the organic layer during the wettest period. The stable N isotope composition of NO3 − gave valuable indications of N transformations during the passage of water through the forest ecosystem from rainfall to the stream.

Magma differentiation fractionates Mo isotope ratios: Evidence from the Kos Plateau Tuff (Aegean Arc)

We investigated high temperature Mo isotope fractionation in a hydrous supra-subduction volcano-plutonic system (Kos, Aegean Arc, Greece) in order to address the debate on the δ98/95Mo variability of the continental crust. In this igneous system, where differentiation is interpreted to be dominated by fractional crystallization, bulk rock data from olivine basalt to dacite show δ98/95Mo ratios increasing from +0.3 to +0.6‰ along with Mo concentrations increasing from 0.8 to 4.1 μg g−1. Data for hornblende and biotite mineral separates reveal the extraction of light Mo into crystallizing silicates, with minimum partition coefficients between hornblende- silicate melt and biotite-silicate melt of 0.6 and 0.4 δ98/95Mo, respectively. Our data document significant Mo isotope fractionation at magmatic temperatures, hence, the igneous contribution to continental runoff is variable, besides probable source-related variability. Based on these results and published data an average continental δ98/95Mo of +0.3 to +0.4‰ can be derived. This signature corresponds more closely to the average of published data of dissolved Mo loads of large rivers than previous estimates and is consistent with an upper limit of δ98/95Mo = 0.4‰ of the Earth's upper crust as derived from the analysis of molybdenites.

Proposal for an International Molybdenum Isotope Measurement Standard and Data Representation

Molybdenum isotopes are increasingly widely applied in Earth Sciences. They are primarily used to investigate the oxygenation of Earth's ocean and atmosphere. However, more and more fields of application are being developed, such as magmatic and hydrothermal processes, planetary sciences or the tracking of environmental pollution. Here, we present a proposal for a unifying presentation of Mo isotope ratios in the studies of mass-dependent isotope fractionation. We suggest that the δ98/95Mo of the NIST SRM 3134 be defined as +0.25‰. The rationale is that the vast majority of published data are presented relative to reference materials that are similar, but not identical, and that are all slightly lighter than NIST SRM 3134. Our proposed data presentation allows a direct first-order comparison of almost all old data with future work while referring to an international measurement standard. In particular, canonical δ98/95Mo values such as +2.3‰ for seawater and −0.7‰ for marine Fe–Mn precipitates can be kept for discussion. As recent publications show that the ocean molybdenum isotope signature is homogeneous, the IAPSO ocean water standard or any other open ocean water sample is suggested as a secondary measurement standard, with a defined δ98/95Mo value of +2.34 ± 0.10‰ (2s). Les isotopes du molybdène (Mo) sont de plus en plus largement utilisés dans les sciences de la Terre. Ils sont principalement utilisés pour étudier l'oxygénation de l'océan et de l'atmosphère de la Terre. Cependant, de plus en plus de domaines d'application sont en cours de développement, tels que ceux concernant les processus magmatiques et hydrothermaux, les sciences planétaires ou encore le suivi de la pollution environnementale. Ici, nous présentons une proposition de présentation unifiée des rapports isotopiques du Mo dans les études du fractionnement isotopique dépendant de la masse. Nous suggérons que le δ98/95Mo du NIST SRM 3134 soit définit comme étant égal à +0.25 ‰. La raison est que la grande majorité des données publiées sont présentés par rapport à des matériaux de référence qui sont similaires, mais pas identiques, et qui sont tous légèrement plus léger que le NIST SRM 3134. Notre proposition de présentation des données permet une comparaison directe au premier ordre de presque toutes les anciennes données avec les travaux futurs en se référant à un standard international. En particulier, les valeurs canoniques du δ98/95Mo comme celle de +2,3 ‰ pour l'eau de mer et de -0,7 ‰ pour les précipités de Fe-Mn marins peuvent être conservés pour la discussion. Comme les publications récentes montrent que la signature isotopique moyenne du molybdène de l'océan est homogène, le standard de l'eau océanique IAPSO ou tout autre échantillon d'eau provenant de l'océan ouvert sont proposé comme standards secondaires, avec une valeur définie du δ98/95 Mo de 2.34 ± 0.10 ‰ (2s).

Atypical Mo isotope signatures in eastern Mediterranean sediments

Suboxic sapropel S1 sediments of the eastern Mediterranean carry an untypically light δ98/95Mo signal, being even lighter than oxic Mn-enriched sediments and Mn-crusts/nodules, which were previously thought to carry the lightest Mo isotope values. The evaluation and comparison of oxic S1, diagenetically overprinted oxic S1, and suboxic S1 sediments indicates that this light isotope signal is primarily an oxic signal that has been overprinted by secondary diagenetic processes occurring after sediment burial. Such secondary processes bear the potential of additional Mo isotope fractionation in particular in non-steady state diagenetic environments that involve the discontinuous re-location of the redox boundary.

Isotopic effects of nitrate photochemistry in snow: a field study at Dome C, Antarctica

Stable isotope ratios of nitrate preserved in deep ice cores are expected to provide unique and valuable information regarding paleoatmospheric processes. However, due to the post-depositional loss of nitrate in snow, this information may be erased or significantly modified by physical or photochemical processes before preservation in ice. We investigated the role of solar UV photolysis in the post-depositional modification of nitrate mass and stable isotoperatios at Dome C, Antarctica, during the austral summer of 2011/2012. Two 30 cm snow pits were filled with homogenized drifted snow from the vicinity of the base. One of these pits was covered with a plexiglass plate that transmits solar UV radiation, while the other was covered with a different plexiglass plate having a low UV transmittance. Samples were then collected from each pit at a 2–5 cm depth resolution and a 10-day frequency. At the end of the season, acomparable nitrate mass loss was observed in both pits for the top-level samples (0–7 cm) attributed to mixing with the surrounding snow. After excluding samples impacted by the mixing process, we derived an average apparent nitrogen isotopic fractionation (15" app/of role in driving the isotopic fractionation of nitrate in snow.We have estimated a purely photolytic nitrogen isotopic fractionation (15"photo) of -55.8 12.0 ‰ from the difference in the derived apparent isotopic ractionations of the two experimental fields, as both pits were exposed to similar physical processes except exposure to solar UV. This value is in close agreement with the 15" photo value of -47.9 6.8 ‰ derived in a laboratory experiment simulated for Dome C conditions (Berhanu et al., 2014). We have also observed an insensitivity of 15" with depth in the snowpack under the given experimental setup. This is due to the uniform attenuation of incoming solar UV by snow, as 15" is strongly dependent on the spectral distribution of the incoming light flux. Together with earlier work, the results presented here represent a strong body of evidence that solar UV photolysis is the most relevant post-depositional process modifying the stable isotope ratios of snow nitrate at low-accumulation sites, where many deep ice cores are drilled. Nevertheless, modeling the loss of nitrate in snow is still required before a robust interpretation of ice core records can be provided.

Selenium isotope analysis by N-TIMS: Potential and challenges

The isotope composition of selenium (Se) can provide important constraints on biological, geochemical, and cosmochemical processes taking place in different reservoirs on Earth and during planet formation. To provide precise qualitative and quantitative information on these processes, accurate and highly precise isotope data need to be obtained. The currently applied ICP-MS methods for Se isotope measurements are compromised by the necessity to perform a large number of interference corrections. Differences in these correction methods can lead to discrepancies in published Se isotope values of rock standards which are significantly higher than the acclaimed precision. An independent analytical approach applying a double spike (DS) and state-of-the-art TIMS may yield better precision due to its smaller number of interferences and could test the accuracy of data obtained by ICP-MS approaches. This study shows that the precision of Se isotope measurements performed with two different Thermo Scientific™ Triton™ Plus TIMS is distinctly deteriorated by about ±1‰ (2 s.d.) due to δ80/78Se by a memory Se signal of up to several millivolts and additional minor residual mass bias which could not be corrected for with the common isotope fractionation laws. This memory Se has a variable isotope composition with a DS fraction of up to 20% and accumulates with increasing number of measurements. Thus it represents an accumulation of Se from previous Se measurements with a potential addition from a sample or machine blank. Several cleaning techniques of the MS parts were tried to decrease the memory signal, but were not sufficient to perform precise Se isotope analysis. If these serious memory problems can be overcome in the future, the precision and accuracy of Se isotope analysis with TIMS should be significantly better than those of the current ICP-MS approaches.

Isotope Study of Moisture Sources, Recharge Areas and Groundwater Flowpaths within the Eastern Batinah Coastal Plain, Sultanate of Oman

Oxygen and hydrogen isotope analyses of rainfall samples collected on the eastern Batinah coastal plain of northern Oman between 1995 and 1998 indicate two different principal water vapor sources for precipitation in the area: a northern, Mediterranean source and a southern, Indian Ocean source. As a result, two new local meteoric water lines were defined for the study area. Isotopic analyses of groundwater samples from over 200 springs and wells indicate that the main source of water to the Batinah coastal alluvial aquifer is high-altitude rainfall from the adjacent Jabal Akhdar Mountains, originating from a combination of northern and southern moisture sources. The groundwater recharged at high-altitude forms two plumes of water which is depleted in the heavy isotopes 18O and 2H and stretches from the mountains across the coastal plain to the sea, thereby retaining a chemical homogeneity horizontally and vertically down to a depth exceeding 300 m. In contrast, in areas adjacent to these two plumes the alluvial aquifer is geochemically stratified. Near the coast, saline intrusion results in abrupt changes in chloride concentrations and isotope values.

The isotope composition of selenium in chondrites constrains the depletion mechanism of volatile elements in solar system materials

Solar nebula processes led to a depletion of volatile elements in different chondrite groups when compared to the bulk chemical composition of the solar system deduced from the Sun's photosphere. For moderately-volatile elements, this depletion primarily correlates with the element condensation temperature and is possibly caused by incomplete condensation from a hot solar nebula, evaporative loss from the precursor dust, and/or inherited from the interstellar medium. Element concentrations and interelement ratios of volatile elements do not provide a clear picture about responsible mechanisms. Here, the abundance and stable isotope composition of the moderately- to highly-volatile element Se are investigated in carbonaceous, ordinary, and enstatite chondrites to constrain the mechanism responsible for the depletion of volatile elements in planetary bodies of the inner solar system and to define a δ(82/78)Se value for the bulk solar system. The δ(82/78)Se of the studied chondrite falls are identical within their measurement uncertainties with a mean of −0.20±0.26‰ (2 s.d., n=14n=14, relative to NIST SRM 3149) despite Se abundance depletions of up to a factor of 2.5 with respect to the CI group. The absence of resolvable Se isotope fractionation rules out a kinetic Rayleigh-type incomplete condensation of Se from the hot solar nebula or partial kinetic evaporative loss on the precursor material and/or the parent bodies. The Se depletion, if acquired during partial condensation or evaporative loss, therefore must have occurred under near equilibrium conditions to prevent measurable isotope fractionation. Alternatively, the depletion and cooling of the nebula could have occurred simultaneously due to the continuous removal of gas and fine particles by the solar wind accompanied by the quantitative condensation of elements from the pre-depleted gas. In this scenario the condensation of elements does not require equilibrium conditions to avoid isotope fractionation. The results further suggest that the processes causing the high variability of Se concentrations and depletions in ordinary and enstatite chondrites did not involve any measurable isotope fractionation. Different degrees of element depletions and isotope fractionations of the moderately-volatile elements Zn, S, and Se in ordinary and enstatite chondrites indicate that their volatility is controlled by the thermal stabilities of their host phases and not by the condensation temperature under canonical nebular conditions.

Calcium isotopic composition of high-latitude proxy carrier Neogloboquadrina pachyderma (sin.)

The accurate reconstruction of sea surface temperature (SST) history in climate-sensitive regions (e.g. tropical and polar oceans) became a challenging task in palaeoceanographic research. Biogenic shell carbonate SST proxies successfully developed for tropical regions often fail in cool water environments. Their major regional shortcomings and the cryptic diversity now found within the major high latitude proxy carrier Neogloboquadrina pachyderma (sin.) highlight an urgent need to explore complementary SST proxies for these cool-water regions. Here we incorporate the genetic component into a calibration study of a new SST proxy for the high latitudes. We found that the calcium isotopic composition (δ44/40Ca) of calcite from genotyped net catches and core-top samples of the planktonic foraminifera Neogloboquadrina pachyderma (sin.) is related to temperature and unaffected by genetic variations. The temperature sensitivity has been found to be 0.17 (±0.02)‰ per 1°C, highlighting its potential for downcore applications in open marine cool-water environments. Our results further indicate that in extreme polar environments, below a critical threshold temperature of 2.0 (±0.5)°C associated with salinities below 33.0 (±0.5)‰, a prominent shift in biomineralization affects the δ44/40Ca of genotyped and core-top N. pachyderma (sin.), becoming insensitive to temperature. These findings highlight the need of more systematic calibration studies on single planktonic foraminiferal species in order to unravel species-specific factors influencing the temperature sensitivity of Ca isotope fractionation and to validate the proxies' applicability.

Pelagic molybdenum concentration anomalies and the impact of sediment resuspension on the molybdenum budget in two tidal systems of the North Sea

The seasonal dynamics of molybdenum (Mo) were studied in the water column of two tidal basins of the German Wadden Sea (Sylt-Rømø and Spiekeroog) between 2007 and 2011. In contrast to its conservative behaviour in the open ocean, both, losses of more than 50% of the usual concentration level of Mo in seawater and enrichments up to 20% were observed repeatedly in the water column of the study areas. During early summer, Mo removal by adsorption on algae-derived organic matter (e.g. after Phaeocystis blooms) is postulated to be a possible mechanism. Mo bound to organic aggregates is likely transferred to the surface sediment where microbial decomposition enriches Mo in the pore water. First δ98/95Mo data of the study area disclose residual Mo in the open water column being isotopically heavier than MOMo (Mean Ocean Molybdenum) during a negative Mo concentration anomaly, whereas suspended particulate matter shows distinctly lighter values. Based on field observations a Mo isotope enrichment factor of ε = −0.3‰ has been determined which was used to argue against sorption on metal oxide surfaces. It is suggested here that isotope fractionation is caused by biological activity and association to organic matter. Pelagic Mo concentration anomalies exceeding the theoretical salinity-based concentration level, on the other hand, cannot be explained by replenishment via North Sea waters alone and require a supply of excess Mo. Laboratory experiments with natural anoxic tidal flat sediments and modelled sediment displacement during storm events suggest fast and effective Mo release during the resuspension of anoxic sediments in oxic seawater as an important process for a recycling of sedimentary sulphide bound Mo into the water column.