Silicon isotopes in meteorites and planetary core formation

The silicon (Si) isotope compositions of 42 meteorite and terrestrial samples have been determined using MC-ICPMS with the aim of resolving the current debate over their compositions and the implications for core formation. No systematic δ³⁰Si differences are resolved between chondrites (δ³⁰Si = −0.49 ± 0.15‰, 2σ_SD) and achondrites (δ³⁰Si = −0.47 ± 0.11‰, 2σ_SD), although enstatite chondrites are consistently lighter (δ³⁰Si = −0.63 ± 0.07‰, 2σ_SD) in comparison to other meteorite groups. The data reported here for meteorites and terrestrial samples display an average difference Δ³⁰Si_{BSE−meteorite∗} = 0.15 ± 0.10‰, which is consistent within uncertainty with previous studies. No effect from sample heterogeneity, preparation, chemistry or mass spectrometry can be identified as responsible for the reported differences between current datasets. The heavier composition of the bulk silicate Earth is consistent with previous conclusions that Si partitioned into the metal phase during metal-silicate equilibration at the time of core formation. Fixing the temperature of core formation to the peridotite liquidus and using an appropriate metal silicate fractionation factor (ε ∼0.89), the Δ³⁰Si_{BSE−meteorite∗} value from this study indicates that the Earth core contains at least 2.5 and possibly up to 16.8 wt% Si.     

Trace element partitioning in the Fe-S-C system and its implications for planetary differentiation and the thermal history of ureilites

Element partitioning in metal-light element systems is important to our understanding of planetary differentiation processes. In this study, solid-metal/liquid-sulfide, liquid-metal/liquid-sulfide and solid-metal/troilite partition coefficients (D) were determined for 18 elements (Ag, As, Au, Co, Cr, Cu, Ge, Ir, Ni, Os, Pd, Pt, Mo, Mn, Re, Ru, Se and W) in the graphite-saturated Fe-S-C system at 1 atm. Compared at the same liquid S concentration, the solid/liquid partition coefficients are similar to those in the Fe-S system, but there are systematic differences that appear to be related to interactions with carbon dissolved in the solid metal. Elements previously shown to be “anthracophile” generally have larger solid/liquid partition coefficients in the Fe-S-C system, whereas those that are not have similar or smaller partition coefficients in the Fe-S-C system. The partitioning of trace elements between C-rich and S-rich liquids is, in most cases, broadly similar to the partitioning between solid metal and S-rich liquid. The highly siderophile elements Os, Re, Ir and W are partitioned strongly into the C-rich liquid, with D ? 100. The partition coefficients for Pt, Ge and W decrease significantly at the transition to liquid immiscibility, while the partition coefficient for Mo increases sharply. The bulk siderophile element patterns of ureilite meteorities appear to be better explained by separation of S-rich liquid from residual C-rich metallic liquid at temperatures above the silicate solidus, rather than by separation of S-rich liquid from residual solid metal at lower temperatures.     

Stable isotopes and archaeological geology: the Carrara marble,northern Italy

The principal marble quarries of Italy for the past 2000 years have been those of Carrara. In Roman times, the marble was exported all over the ancient world. Renaissance quarries were opened up in Seravezza; both the Seravezza and the Carrara quarries have been exploited until today. The correct identification of Carrara marble has always been a problem because it was traded so widely in Roman times and was later used to fabricate broken or lost pieces of classical statuary. Many types of geochemical analysis have been tried in order to find distinctive signatures for the recognition of classical marbles, including trace elements, ESR spectroscopy of Mn, thermoluminescence, and stable isotopic ratios. To date, the most promising has been isotopic ratio analysis of oxygen and carbon.In this study, stable isotopic signatures were investigated as a means for distinguishing: (1) the principal quarry areas of the Carrara Commune: Fossacava, Miseglia, and Torano, and the Seravezza Commune; (2) or merely the largely classical quarries of Carrara from the Renaissance quarries of Seravezza; (3) and, most important, the Carrara quarries from other quarries of Greece, Turkey, Italy, and Tunisia in the classical marble data base.Discriminant analysis (DA) of the isotopic analyses showed that the three quarry areas of a Carrara could not be told apart but that they could be told easily from Seravezza. In addition, a “mineralized” Carrara quarry at Mandria, with buff to yellow rather than white marble, was also distinctive. DA also suggests that Carrara can be told from those quarries with which it is presently confused: Paros, in the Aegean Sea; Marmara, in Turkey; and Pentelikon, near Athens.Sr isotopic analysis showed a range in the Carrara and other marbles of Hettangian (Lower Jurassic) age from 0.70778 to 0.70810 which is very different from values found in Paros and Pentelikon. This suggests that Sr might make a good third discriminator for classical marble.     

Stable carbon isotopes in selected granitic,mafic, and ultramafic igneous rocks

Noncarbonate (combustion) and carbonate (acid decomposition) carbon were separately analyzed in 18 granitic rocks from a group of related Tertiary intrusions near Crested Butte, Colorado, and 14 mafic and ultramafic rocks from various localities in the western United States. Among the granites, carbonate carbon ranges from nil to 0.76 per cent with δC¹³-values from ?5.6 to ? 9.0‰ (vs PDB); noncarbonate carbon varies from 32–360 ppm with δC¹³-values from ?19.7 to ?26.6‰, The mafic and ultramafic rocks have carbonate carbon contents ranging from 53 ppm to about 2 per cent with δC¹³-values from + 2.9 to ?10.3‰; noncarbonate carbon varies from 26 to 150 ppm with δC¹³-values of ?22.2 to ? 27.l‰ For these samples, carbonate carbon ranges from 12.0 to 29.4‰ heavier than coexisting noncarbonate carbon. This consistent difference between δC¹³ of carbonate and noncarbonate carbon may be an isotopic fractionation effect. Because the specific indigenous form of noncarbonate (combustion) carbon is in doubt, conclusive interpretations regarding isotopic equilibration and fractionation cannot be made.These results have bearing on the assessment of the isotopic composition of mantle carbon and consequently are germane to the question of the origin (source) and history of crustal carbon. If mantle carbon is isotopically similar to noncarbonate (combustion) carbon, i.e. δC¹³-values from ?19.7 to ? 27.1‰, then a simple mantle degassing source for crustal carbon is improbable. Such a result would indicate an additional source of crustal carbon such as from a primitive atmosphere or extra-terrestrial accretion.     

无定河流域碳氮稳定同位素研究

对黄河中游一条很有特点的高悬沙支流--无定河流域进行了碳氮稳定同位素调查,发现该流域植被的碳氮同位素组成分布范围很广,碳同位素组成呈现出明显的双峰特征,说明该流域生态系统属于C3和C4混合植被类型.植物的氮同位素组成与碳同位素组成之间并不存在相关关系.沿河沉积物中的碳氮同位素组成之间存在有弱负相关性.河水悬浮体中碳氮同位素组成显示出良好的负相关关系,反映出该河悬浮体中主要有2种物质来源以及具保守性的物理混合特征.根据无定河流域植被、土壤、沉积物的碳同位素调查结果,估算出该河流颗粒有机质中C3植物碳的贡献可占75%～80%.河水悬浮体中有机质的控制性来源是中下游陆地侵蚀物质,同样也反映了流域植被的同位素特征,C3植物碳的贡献占优势.初步的研究结果已经证明,在研究河流中生源要素的输送通量与降水及径流过程之间的关系和河流物质与陆地生态环境变化之间的关系时碳氮稳定同位素示踪应该是十分有效的手段.     

Stable sulfur isotopes in the water column of the Cariaco Basin

Previous geochemical and microbiological studies in the Cariaco Basin indicate intense elemental cycling and a dynamic microbial loop near the oxic-anoxic interface. We obtained detailed distributions of sulfur isotopes of total dissolved sulfide and sulfate as part of the on-going CARIACO time series project to explore the critical pathways at the level of individual sulfur species. Isotopic patterns of sulfate (δ³⁴S_SO4) and sulfide (δ³⁴S_H2S) were similar to trends observed in the Black Sea water column: δ³⁴S_H2S and δ³⁴S_SO4 were constant in the deep anoxic water (varying within 0.6‰ for sulfide and 0.3‰ for sulfate), with sulfide roughly 54‰ depleted in ³⁴S relative to sulfate. Near the oxic-anoxic interface, however, the δ³⁴S_H2S value was ∼3‰ heavier than that in the deep water, which may reflect sulfide oxidation and/or a change in fractionation during in situ sulfide production through sulfate reduction (SR). δ³⁴S_H2S and Δ³³S_H2S data near the oxic-anoxic interface did not provide unequivocal evidence to support the important role of sulfur-intermediate disproportionation suggested by previous studies. Repeated observation of minimum δ³⁴S_SO4 values near the interface suggests ‘readdition’ of ³⁴S-depleted sulfate during sulfide oxidation. A slight increase in δ³⁴S_SO4 values with depth extended over the water column may indicate a reservoir effect associated with removal of ³⁴S-depleted sulfur during sulfide production through SR. Our δ³⁴S_H2S and Δ³³S_H2S data also do not show a clear role for sulfur-intermediate disproportionation in the deep anoxic water column. We interpret the large difference in δ³⁴S between sulfate and sulfide as reflecting fractionations during SR in the Cariaco deep waters that are larger than those generally observed in culturing studies.     

Stable isotopes in the ostracod shell: a preliminary study

Oxygen and carbon isotope ratios of Recent ostracods from six localities are presented. The δO¹⁸ data are consistent with precipitation of the shells in isotopic equilibrium with seawater. although additional data are necessary to confirm equilibrium precipitation. No strong correlation between carbon isotope ratios and temperature or salinity were observed for ostracods.     

Stable isotopes, chemistry and petrology of carbonate concretions (Mishash formation, Israel)

Carbonate concretions of variable sizes occur in the upper member of the Senonian (Upper Cretaceous) Mishash formation in Israel. Eight concretions and their surrounding country rocks were examined in the field, in thin sections and by X-rays. The isotopic composition of carbon and oxygen in the carbonates, the amount of acid insoluble residue, as well as the concentrations of P₂O₅, Ca, Mg, Sr and Fe were determined. Other concretions and country rocks were analyzed for oxygen and carbon isotopes only.

The concretions are composed of almost pure, microsparitic calcite, whereas the country rocks are porcellanitic-phosphoritic chalks. Compared with the surrounding rocks, the concretions are strongly enriched in ¹²C and are depeleted in insoluble residue, P₂O₅, SiO₂, Fe, Sr and Mg.

It is postulated that the concretions were formed by addition of CaCO₃ to sites of anaerobic decomposition of organic matter, while CaCO₃ was mobilized in the surrounding sediments, in which aerobic decomposition of organic matter prevailed.

Several consequences of this model are considered, concerning the quantitative volume changes, as well as the distribution of SiO₂ and Mg between concretions and country rocks.     

Noble gases in ureilites: cosmogenic, radiogenic, and trapped components

Abundances and isotopic compositions of Ne (in bulk samples only), Ar, Kr, and Xe have been investigated in 6 monomict, 3 polymict, and the diamond-free ureilite ALH78019 and their acid-resistant, C-rich residues. Isotopic ratios of Kr and Xe are very uniform and agree with data for ureilites from the literature. The measured ratio ³⁸Ar/³⁶Ar showed large variations due to an experimental artifact. This is shown to be connected to the pressure dependence of the instrumental mass discrimination, which for ureilites with their low abundance of ⁴⁰Ar is different from that of the usual air standard. This observation necessitates a reassessment for the recently reported ³⁶Ar excesses due to possible decay of extinct ³⁶Cl in the Efremovka meteorite.Trapped ²²Ne in the range of (1.4-2.5) × 10⁻⁸ cc STP/g is present in bulk ureilites. A Ne three-isotope plot for polymict ureilites indicates the presence of solar Ne. ²¹Ne-based cosmic ray exposure ages for the 10 ureilites studied range from 0.1 Ma (for ALH78019) to 46.8 Ma (for EET83309)All ureilites may have started with nearly the same initial elemental ratio (¹³²Xe/³⁶Ar)₀, established in the nebula during gas trapping into their carbon carrier phases (diamond, amorphous C) by ion implantation. Whereas diamonds are highly retentive, amorphous C has suffered gas loss due to parent body metamorphism. The correlation of the elemental ratios ¹³²Xe/³⁶Ar and ⁸⁴Kr/³⁶Ar along the mass fractionation line could be understood as a two-component mixture of the unaffected diamond gases and the fractionated (to varying degrees) gases from amorphous C. In this view, the initial ratio (¹³²Xe/³⁶Ar)₀ is a measure of the plasma temperature in the nebula at the formation location of the carbon phases. Its lack of correlation with Δ¹⁷O (a signature of the silicate formation location) indicates that carbon phases and silicates formed independently in the nebula, and not from a carbon-rich magmaThe elemental ratios ¹³²Xe/³⁶Ar and ⁸⁴Kr/³⁶Ar in carbon-rich acid residues show a decreasing trend with depth (inferred from carbon consumption during combustion), which can be interpreted as a consequence of the ion implantation mechanism of gas trapping that leads to greater depth of implantation for lighter mass ionThe similarity between trapped gases in phase Q in primitive chondrites and the C phases in ureilites—for both elemental and isotopic compositions—strongly suggests that phase Q might also have received its noble gases by ion implantation from the nebula. The slight differences in the elemental ratios can be explained by a plasma temperature at the location of phase Q gas loading that was about 2000 K lower than for ureilite C phases. This inference is also consistent with the finding that the trapped ratio ¹²⁹Xe/¹³²Xe (1.042 ± 0.002) in phase Q is slightly higher, compared to that of ureilite C phases (1.035 ± 0.002), as a consequence of in situ decay of ¹²⁹I, and becomes observable due to higher value of I/Xe in phase Q as a result of ion implantation at about 2000 K lower plasma temperature.     

Stable silicon isotopes of groundwater, feldspars, and clay coatings in the Navajo Sandstone aquifer, Black Mesa, Arizona, USA

We present some of the first analyses of the stable isotopic composition of dissolved silicon (Si) in groundwater. The groundwater samples were from the Navajo Sandstone aquifer at Black Mesa, Arizona, USA, and the Si isotope composition of detrital feldspars and secondary clay coatings in the aquifer were also analyzed. Silicon isotope compositions were measured using high-resolution multi-collector inductively coupled mass spectrometry (HR-MC-ICP-MS) (Nu1700 & NuPlasma HR). The quartz dominated bulk rock and feldspar separates have similar δ³⁰Si of −0.09 ± 0.04‰ and −0.15 ± 0.04‰ (±95% SEM), respectively, and clay separates are isotopically lighter by up to 0.4‰ compared to the feldspars. From isotopic mass-balance considerations, co-existing aqueous fluids should have δ³⁰Si values heavier than the primary silicates. Positive δ³⁰Si values were only found in the shallow aquifer, where Si isotopes are most likely fractionated during the dissolution of feldspars and subsequent formation of clay minerals. However, δ³⁰Si decreases along the flow path from 0.56‰ to −1.42‰, representing the most negative dissolved Si isotope composition so far found for natural waters. We speculate that the enrichment in ²⁸Si is due to dissolution of partly secondary clay minerals and low-temperature silcretes in the Navajo Sandstone. The discovery of the large range and systematic shifts of δ³⁰Si values along a groundwater flow path illustrates the potential utility of stable Si isotopes for deciphering the Si cycling in sedimentary basins, tracing fluid flow, and evaluating global Si cycle.     

Stable hydrogen isotopes in iron oxides—isotope effects associated with the dehydration of a natural goethite

The dehydration of a natural goethite to hematite is accompanied by a systematic hydrogen isotope fractionation. Closed system dehydration at, and below, 250°C results in a significantly greater degree of isotopic fractionation than does open system dehydration. This relationship is apparently reversed at 300°C. Both processes produce a progressive decrease in the $\text{D}\text{H}$ ratio of the mineral hydrogen with increasing degree of dehydration. At temperatures of 160°C to 250°C the closed system mineralvapor fractionation factor is independent of temperature, while above 250°C, it varies strongly with temperature. The mineral-vapor fractionation factor associated with open system dehydration appears to be independent of temperature over the interval 160°C to 300°C. The closed system $\text{D}\text{H}$ fractionation suggests that natural goethite undergoing dehydration in the presence of water can isotopically exchange with that water.CO₂ loss from goethite during dehydration is correlated with the loss of H₂O. The CO₃ is thought to be present in carbonates which exist as impurities in the goethite. Loss of both H₂O and CO₂ appears to be diffusion-controlled.     

Chemical evidence for the genesis of the ureilites, the achondrite Chassigny and the nakhlites

Major element and REE, Cr, Sc, V, Ni, Co, Ir, Au, Sr, Ba abundances were determined in three ureilites and the unique achondrite, Chassigny. Chondritic-normalized REE abundance patterns for the ureilites are v-shaped, similar to pallasites, indicating a possible deep-seated origin. The lithophile trace element abundances and v-shaped REE patterns of the ureilites are consistent with a two-stage formation process, the first of which is an extensive partial melting of chondrite-like matter to yield ureilite precursors in the residual solid, which is enriched in Lu relative to La. The second step consists of an admixture of small and variable amounts of material enriched in the light REE. Such contaminating material may be magmas derived from the first formed melt of partial melting of chondrite-like matter.

In contrast to the ureilites, Chassigny has a chondritic-normalized REE pattern which decreases smoothly from La(1.8 × ) to Lu(0.4 × ) and is parallel to and ˜0.25 × the REE pattern in the nakhlitic achondrites. The composition of the magma from which Chassigny crystallized was highly enriched in the light REE; e.g. chondritic normalized La/Lu ˜ 7. The similarity in the fractionated REE patterns (no Eu anomalies) for the olivine-pyroxene Chassigny and for the nakhlites suggests a genetic relationship.

Siderophile trace element relationships in ureilites can be interpreted by three components: (1) ultramafic silicates enriched in Co relative to Ni, (2) an indigenous metal phase remaining after the partial melting event, and (3) a component of the carbon-rich vein material added after the partial melting.     

Stable isotopes of carbon dioxide in soil gas over massive sulfide mineralization at Crandon, Wisconsin

Stable isotope ratios of oxygen and carbon were determined for CO₂ in soil gas in the vicinity of the massive sulfide deposit at Crandon, Wisconsin with the objective of determining the source of anomalously high CO₂ concentrations detected previously by McCarthy et al. (1986). Values of δ¹³C in soil gas CO₂ from depths between 0.5 and 1.0 m were found to range from −12.68‰ to −20.03‰ (PDB). Organic carbon from the uppermost meter of soil has δ¹³C between −24.1 and −25.8‰ (PDB), indicating derivation from plant species with the C₃ (Calvin) type of photosynthetic pathway. Microbial decomposition of the organic carbon and root respiration from C₃ and C₄ (Hatch-Slack) plants, together with atmospheric CO₂ are the likely sources of carbon in soil gas CO₂. Values of δ¹⁸O in soil-gas CO₂ range from 32 to 38‰ (SMOW). These δ¹⁸O values are intermediate between that calculated for CO₂ gas in isotopic equilibrium with local groundwaters and that for atmospheric CO₂. The δ¹⁸O data indicate that atmospheric CO₂ has been incorporated by mixing or diffusion. Any CO₂ generated by microbial oxidation of organic matter has equilibrated its oxygen isotopes with the local groundwaters.The isotopic composition of soil-gas CO₂ taken from directly above the massive sulfide deposit was not distinguishable from that of background samples taken 1 to 2 km away. No enrichment of the δ¹³C value of soil-gas CO₂ was observed, contrary to what would be expected if the anomalous CO₂ were derived from the dissolution of Proterozoic marine limestone country rock or of Paleozoic limestone clasts in glacial till. Therefore, it is inferred that root respiration and decay of C₃ plant material were responsible for most CO₂ generation both in the vicinity of the massive sulfide and in the “background” area, on the occasion of our sampling. Interpretation of our data is complicated by the effects of rainfall, which significantly reduced the magnitude of the CO₂ anomaly. Therefore, we cannot rule out the possible mechanism of carbonate dissolution driven by pyrite oxidation, as proposed by Lovell et al. (1983) and McCarthy et al. (1986). Further work is needed on seasonal and daily variations of CO₂ concentrations and stable isotope ratios in various hydrogeologic and ecologic settings so that more effective sampling strategies can be developed for mineral exploration using soil gases.     

High-precision EA-IRMS analysis of S and C isotopes in geological materials

The continuous flow elemental analyzer-isotopic ratio mass spectrometer (EA-IRMS) technique provides a significant improvement over conventional extraction methods for the determination of C- and S-isotope ratios in geological materials. It is faster and requires much smaller quantities of material for analysis. Sample preparation is simple, with little or no need for chemical or complete mechanical mineral phase separation, and EA-IRMS sample introduction is rapid. However, because of its simplicity and the fact that data quality depends on the control of a variety of factors, the technique requires rigid adherence to a careful sample analysis protocol.The matching of sample and reference peak heights by sample weight adjustment minimizes non-linearity effects. An accurate calibration should be determined by using standards having a wide range of isotopic compositions, ideally bracketing the samples analysed to both low and high isotopic ratios, and a regular analysis of standards undertaken to maintain the accuracy of the calibration. The calibration equation must be monitored throughout the run by regular analysis of standards, and performance of the Cu-reduction reactor regularly checked to avoid O₂ saturation. With this level of attention to analytical detail, measured precision on replicates of isotopic standards is in the order of ±0.1‰ for both C and S isotopic analyses. For S this is a significant improvement over conventional techniques, with 53% of natural samples analysed replicating to better than ±0.1‰.     

Stable isotopes of cherts and carbonate cements in the Lake Valley Formation (Mississippian), Sacramento Mts, New Mexico

Oxygen isotopic compositions of chert and calcite cements in the Lake Valley Formation indicate that these diagenetic features cannot be equilibrium co-precipitates in spite of their coexistence in the same interstices. Petrography of megaquartz and non-ferroan calcite cements indicates that both are original precipitates that formed during pre-Pennsylvanian time at shallow burial depths (< 215m) implying precipitation temperatures less than 30°C. Under these constraints the δ¹⁸Os of megaquartz (mean =+27.0^0/00 SMOW; range =+ 24.8 to + 28.9^0/00) and calcite (mean =+ 28.0^0/00 SMOW; range =+ 27.3 to + 28.4^0/00) are best interpreted as unaltered since precipitation; thus, they must reflect the oxygen isotopic composition of pre-Pennsylvanian pore waters. Microquartz and chalcedony are interpreted to have formed from recrystallization of pre-Pennsylvanian opal-CT precursors, and therefore probably re-equilibrated during recrystallization in late or post-Mississippian time. We propose a model integrating the isotopic data with regional petrographic and sedimentological data that explains the greater consistency and generally greater δ¹⁸Os values of the calcites compared to those of the cherts. This model is one of chertification and calcite cementation in a regional meteoric phreatic ground-water system, the seaward terminus of which moved southward during lowering of pre-Pennsylvanian sea level. The calcite cements and some of the opal-CT precursor to microquartz and chalcedony are interpreted to have formed in the more seaward portions of the groundwater system. The megaquartz precipitated in the more inland parts of the phreatic groundwater system where rainfall was isotopically lighter and more variable. As such, the δ¹⁸Os of the megaquartz reflect the isotopic composition of groundwaters in areas undersaturated with respect to calcite.     

Stable carbon and oxygen isotopes distribution in carbonates from the Central Barents Sea shelf sediments

Carbonate macrofaunal remains and diagenetic tubes collected from a number of structures in the Central Barents Sea area during the 18th TTR (Training Through Research) cruise were subdivided into three groups according to the results of stable carbon and oxygen isotopes analysis. The first group includes carbonates that were formed from bicarbonate only from surrounding sea waters. The carbonates of the second group were formed during diagenesis with use of mixed sources of bicarbonate. The fourth group includes methane-derived carbonates that were formed as a result of anaerobic oxidation of methane (AOM).     

Stable chlorine isotopes in arid non-marine basins: Instances and possible fractionation mechanisms

Stable chlorine isotopes are useful geochemical tracers in processes involving the formation and evolution of evaporitic halite. Halite and dissolved chloride in groundwater that has interacted with halite in arid non-marine basins has a δ³⁷Cl range of 0 ± 3‰, far greater than the range for marine evaporites. Basins characterized by high positive (+1 to +3‰), near-0‰, and negative (−0.3 to −2.6‰) are documented. Halite in weathered crusts of sedimentary rocks has δ³⁷Cl values as high as +5.6‰. Salt-excluding halophyte plants excrete salt with a δ³⁷Cl range of −2.1 to −0.8‰. Differentiated rock chloride sources exist, e.g. in granitoid micas, but cannot provide sufficient chloride to account for the observed data. Single-pass application of known fractionating mechanisms, equilibrium salt-crystal interaction and disequilibrium diffusive transport, cannot account for the large ranges of δ³⁷Cl. Cumulative fractionation as a result of multiple wetting-drying cycles in vadose playas that produce halite crusts can produce observed positive δ³⁷Cl values in hundreds to thousands of cycles. Diffusive isotope fractionation as a result of multiple wetting-drying cycles operating at a spatial scale of 1–10 cm can produce high δ³⁷Cl values in residual halite. Chloride in rainwater is subject to complex fractionation, but develops negative δ³⁷Cl values in certain situations; such may explain halite deposits with bulk negative δ³⁷Cl values. Future field studies will benefit from a better understanding of hydrology and rainwater chemistry, and systematic collection of data for both Cl and Br.     

Stable water isotopes of precipitation in China simulated by SWING2 models

The stable water isotope ratio in precipitation is a useful tracer of atmospheric circulation. Such observations, however, are very limited in space and time. To solve this problem, many isotope-enabled general circulation models (GCMs) are used to help the interpretation of isotope proxies. In this paper, several isotope-enabled GCMs released by the second Stable Water Isotope Intercomparison Group (SWING2) were selected to assess the spatial pattern of deuterium (δD) and the deuterium excess (d) of precipitation in China. The isotopic data of the Global Network of Isotopes in Precipitation (GNIP) and the Chinese Network of Isotopes in Precipitation (CHNIP) were also applied to verify the simulations. The results indicate that these models accurately simulate the spatial characteristics of δD and d of precipitation in China. The correlation between the observations and simulations for LMDZ is the highest among these models, while the root-mean-square (RMS) and standard deviation are not perfect. In addition, LMDZ is worse than other models in capturing the low signal in certain regions, such as CAM, GISS_E, and MIROC. For the monthly variation, most SWING2 models underestimate δD of the precipitation but overestimate the value of d, except for isoGSM. The simulated monthly variation of the water isotopes from SWING2 models is in general similar to the observations, and the trend corresponds to the monthly variation in the Northern Hemisphere. Moreover, all models are good at illustrating the temperature and precipitation amount effects, while they exhibit varying skills in interpreting the altitude and continental effects.     

Stable isotopes reveal ecological differences amongst now‐extinct proboscideans from the Cincinnati region,USA

The mobility and dietary preferences of now‐extinct proboscideans have not been comprehensively examined in the central USA. We used stable carbon (δ¹³C), oxygen (δ¹⁸O) and strontium (⁸⁷Sr/⁸⁶Sr) isotopic signatures in molar enamel to investigate the foraging ecology of four mastodons (Mammut americanum) and eight mammoths (Mammuthus spp.) from southwestern Ohio and northwestern Kentucky. We tested two hypotheses: (i) these individuals were nomadic migrants that were passing through the region when they died; and (ii) mammoths and mastodons foraged in different environments. Unexpectedly, our results suggest that 11 of the 12 sampled individuals were regional residents. With the exception of one mastodon, ⁸⁷Sr/⁸⁶Sr ratios for proboscideans and regional water samples were statistically indistinguishable; slightly lower ratios for waters suggest glacial loess has an impact on modern samples. Amongst the individuals identified as residents, ⁸⁷Sr/⁸⁶Sr ratios indicate that mammoths and mastodons foraged in discrete geographical areas, and δ¹³C values imply dietary differences between the genera, which is consistent with our expectations. Oxygen isotope values may be able to distinguish animals that lived during the Last Glacial Maximum (LGM) from those that lived more recently. Three mammoths and one mastodon yielded δ¹⁸O values that are similar to modern regional precipitation and surface water, but too high for estimated drinking water during the LGM. We propose that these individuals lived during a relatively warm period following the LGM. Compellingly, the mammoth with the highest δ¹⁸O value also has the lowest δ¹³C value, suggesting that this individual was alive after regional vegetation shifted from open parkland to deciduous forest dominated by C₃ species. Our results demonstrate that a wealth of information can be gleaned from fossil museum specimens and lay a foundation for future work on the foraging ecology of proboscideans and other extinct megafauna from the Midwest USA.