Shifts In Isotope Ratios Research Articles

Using geostatistical analysis for simultaneous estimation of isoscapes and ontogenetic shifts in isotope ratios of highly migratory marine fish

Tracking migration of highly migratory marine fish using isotope analysis (iso-logging) has become a promising tool in recent years. However, application of this method is often hampered by the lack of essential information such as spatial variations in isotope ratios across habitats (isoscapes) and ontogenetic shifts of isotope ratios of target animals. Here, we test the utility of geostatistical analysis to generate isoscapes of δ13C and δ15N in the western Pacific and estimate the ontogenetic shifts in δ13C and δ15N values of a target species. We first measured δ13C and δ15N in the white muscle of juvenile (n = 210) and adult (n = 884) skipjack tuna Katsuwonus pelamis sampled across the northwest Pacific. Next we fitted a geostatistical model to account for the observed spatial variations in δ13C and δ15N of skipjack by fork length and other environmental variables with spatial random effects. We then used the best-fit models to predict the isoscapes of δ13C and δ15N in 2021. Furthermore, we measured δ15N of amino acids (δ15NAAs) of skipjack (n = 5) to determine whether the observed spatial variation of isotope ratios resulted from baseline shifts or differences in trophic position. The geostatistical model reasonably estimated both isoscapes and ontogenetic shifts from isotope ratios of skipjack, and the isoscapes showed that δ13C and δ15N can clearly distinguish the latitudinal migration of skipjack in the western Pacific. The δ15NAAs supported the results of the geostatistical model, that is, observed variations in skipjack δ15N were largely derived from a baseline shift rather than regional differences in trophic position. Thus, we showed that geostatistical analysis can provide essential basic information required for iso-logging without compound-specific isotope analysis.

Plant litter composition and stable isotope signatures vary during decomposition in blue carbon ecosystems

The ratio of isotopes of carbon (13C:12C or δ13C) and nitrogen (15N:14N or δ15N) are common indicators of the flow and storage of organic matter in coastal wetland research. Effective use of these indicators requires quantification and understanding of: (1) the variability of isotope signatures of potential organic matter source materials; and (2) the influence of organic matter decomposition on isotopic signatures. While it is well-established that organic matter characteristics change during the decomposition process, there has been little direct quantification of any concurrent shifts in isotope signatures for coastal detritus. In this study, we addressed this by quantifying: (1) shifts in sample composition using solid-state 13C Nuclear Magnetic Resonance (NMR) spectroscopy; and (2) shifts in δ13C and δ15N signatures of coastal plant tissues from field litterbag experiments. We observed significant shifts in 13C NMR spectra across the course of deployment for all four plant tissues assessed (leaves of mangrove Avicennia marina; branchlets of supratidal tree Casuarina glauca; leaf wrack and roots/rhizomes of the seagrass Zostera muelleri), driven largely by the preferential loss of labile constituents and concentration of more resistant macromolecules, such as lignin and leaf waxes. While there were shifts in isotope ratios for all species, these varied in direction and magnitude among species, tissue type and isotopes. This included δ13C enrichments of up to 3.1‰ and 2.4‰ in leaves of A. marina, and branchlets of C. glauca, respectively, but δ13C depletions of up to 4.0‰ for Z. muelleri. Shifts in δ15N varied among species and tissue types, with few clear temporal patterns. Partial least squares regression analyses showed that some tissue isotope signatures can be reliably predicted on the basis of sample composition (13C NMR spectra), however, multiple inter- and intra-species variations preclude a simple explanation of isotopic signature shifts on the basis of plant-material molecular shifts alone. Further, we cannot preclude the potential influence of microbe-associated organic matter on sample composition or isotopic signatures. Our findings emphasise the importance of considering decomposition effects on stable isotope signatures in blue carbon ecosystems. Isotope approaches will remain a valuable tool in coastal ecosystem research, but require robust experimental approaches (including appropriate use of decomposed end-members or fractionation correction factors; quantification of microbial organic matter) and quantification of decomposition dynamics for specific plant tissues and environmental settings.

Does sorption influence isotope ratios of chlorinated hydrocarbons under field conditions?

This study aims to investigate the effect of sorption on isotope ratios of chlorinated hydrocarbons migrating through the subsurface. For this purpose concentration and isotope ratio profiles (carbon and chlorine) were determined in saturated low permeability sediments below two DNAPL sources (1,2-Dichloroethane (1,2-DCA) and Dichloromethane (DCM)). The sources had been emplaced artificially as part of a long-term, emplaced source field experiment 15.5 years (5673 days) ago. Low permeable sediments are well-suited for investigating sorption-induced isotope fractionation under field conditions. The advancing concentration front, where isotope fractionation due to sorption is expected, can be localized precisely and sampled at a high spatial resolution. Along a concentration profile below the 1,2-DCA and DCM DNAPL sources, opposite isotope trends were observed with an enrichment of heavy carbon isotopes (Δδ13C = 1.9‰ for 1,2-DCA and 2.4‰ for DCM) and a depletion of heavy chlorine isotopes (Δδ37Cl = 1.3‰ for 1,2-DCA). For field data interpretation laboratory experiments were conducted to determine sorption and diffusion-induced isotope fractionation factors for 1,2-DCA and DCM and included in a numerical model. When considering only diffusive isotope fractionation, numerical simulation failed to reproduce the opposite isotope trends. In contrast when sorption-induced isotope fractionation was also included, the model reproduced the data well. Hence, the observed isotope trends reflect a superposition between competing isotope effects due to sorption and diffusion. For chlorine the diffusive isotope effect is larger than for carbon due to the mass difference of two between the stable isotopes overruling the sorption effect, while for carbon the sorption effect dominates. The observed shifts of isotope ratios due to sorption are in the range of the 2‰ threshold value, which is often used for identifying reactive processes. Numerical modelling showed that under specific conditions (strong sorption behavior, early transient diffusion) even higher shifts of isotope ratios can occur. Hence, when shifts of isotope ratios in the range of 2‰ are observed under field conditions where sorption prevails, their attribution to reactive processes should be made with caution. This is especially crucial if a reactive process is slow and associated with a small isotope fractionation factor.

Evaluation of the long-term fluctuation in isotope ratios measured by TIMS with the static, dynamic, and multistatic methods: A case study for Nd isotope measurements

We investigated the long-term fluctuation in high-precision Nd isotope measurements by using a modern thermal ionization mass spectrometer (Triton plus) to evaluate the effect of Faraday cup deterioration caused by the accumulation of strong ion beams into the cups. A number of measurements (n=76) were conducted by applying three distinct methods (static, dynamic, and multistatic) during an eight-month analytical period, which was further divided into seven short analytical campaigns. The reproducibilities of 142Nd/144Nd ratios for campaigns 1–4 were 23, 6.4, and 17ppm (2 SD) for the static (Jump 1), dynamic, and multistatic methods, respectively. After the replacement of six out of nine Faraday cups (campaigns 5–7), the reproducibilities of 142Nd/144Nd ratios were improved to 18, 4.1, and 5.4ppm (2 SD) for the static, dynamic, and multistatic methods, respectively. This implies that high-precision Nd isotope analysis with not only the static but also the multistatic method is susceptible to the deterioration of Faraday cups. Consequently, the dynamic method is the most effective to minimize the influence of Faraday cup degradation, although it is recommended to ensure that Faraday cups are as fresh as possible to avoid small, but resolvable, shifts in isotope ratios obtained by this method. In addition to 142Nd/144Nd, we demonstrate that it is possible to determine 148Nd/144Nd and 150Nd/144Nd ratios with the dynamic method. The reproducibilities of 148Nd/144Nd and 150Nd/144Nd for campaigns 5–7 were 6.3 and 8.8ppm, respectively, which are 1.5–4.1 times better than those obtained in previous studies.

Compound-Specific Stable Isotope Analysis: Implications in Hexachlorocyclohexane in-vitro and Field Assessment.

Assessment of biotic and abiotic degradation reactions by studying the variation in stable isotopic compositions of organic contaminants in contaminated soil and aquifers is being increasingly considered during the last two decades with development of Compound specific stable isotope analysis (CSIA) technique. CSIA has been recognized as a potential tool for evaluating both qualitative and quantitative degradation with measurement of shifts in isotope ratios of contaminants and their degradation products as its basis. Amongst a wide variety of environmental pollutants including monoaromatics, chlorinated ethenes and benzenes etc., it is only recently that its efficacy is being tested for assessing biodegradation of a noxious pollutant namely hexachlorocyclohexane (HCH), by pure microbial cultures as well as directly at the field site. Anticipating the increase in demand of this technique for monitoring the microbial degradation along with natural attenuation, this review highlights the basic problems associated with HCH contamination emphasizing the applicability of emerging CSIA technique to absolve the major bottlenecks in assessment of HCH. To this end, the review also provides a brief overview of this technique with summarizing the recent revelations put forward by both in vitro and in situ studies by CSIA in monitoring HCH biodegradation.

Changes in the trophic status of fish feeding guilds in response to flow modification

AbstractRivers are among the most extensively modified ecosystems globally. Identifying linkages between critical drivers of change and ecological response is challenging because of the myriad of ways rivers are modified. This study examines longer‐term relationships (>70 years) between the trophic status of fish and historical flow changes in rivers of the Mississippi Basin. The flow regime of each river is regulated but differs in terms of character of hydrological modification. Tissue samples from specimens obtained from museum collections were used for determination of carbon and nitrogen stable isotope ratios. Questions asked were as follows: (1) Are there differences in stable isotope ratios of fish feeding guilds prior to and following hydrological modification? (2) What hydrological attributes are associated with isotopic changes? And (3) are corresponding changes in stable isotope ratios and hydrology identifiable across different spatial scales? Significant and sustained changes in stable isotope ratios of fish feeding guilds occurred immediately following flow regulation in all rivers. These changes were not associated with human‐induced biogeochemical alterations. The subsequent response was complex because (1) different guilds exhibited pronounced shifts in isotopic ratios, (2) hydrological modification differed between rivers, and (3) differences in stable isotopic ratios varied between spatial scales.

Spatial, seasonal and individual variation in the diet of White‐tailed Eagles Haliaeetus albicilla assessed using stable isotope ratios

Many raptor species are considered to be generalists, taking a range of prey species. However, longitudinal dietary records are often scarce, although necessary for characterizing niche width of species at population and individual levels. Quantifying raptor diets at large spatio‐temporal scales is often necessary for refining conservation efforts, although it can be particularly difficult and may involve a great effort by conventional means. Therefore, we adopted the analysis of stable isotopes in tissues of predators and their potential food sources as a complementary methodology for assessing animals' diet. We examined the isotopic composition (δ13C and δ15N) of White‐tailed Eagles Haliaeetus albicilla from Germany, Finland and Greenland to detect patterns of dietary variation and quantify diet composition. The isotopic analysis included liver and muscle samples from Eagles of the three populations together with 16 potential food sources in the German population. Our results suggested dietary differences between German and Greenlandic Eagles, in accordance with the availability of freshwater and marine habitats in each population. Within the German population, we found seasonal shifts in isotopic ratios, suggesting the birds responded to temporal changes in food availabilities, and age‐related isotopic differences, indicating different diets in adults and juveniles. Isotopic values of liver and muscle tissues collected from the same animal showed intra‐individual short‐term changes in the German and Finnish but not Greenlandic population. This suggests that local feeding niches of this generalist predator may vary with local food supplies, which determines the niche width (from generalist to specialist) at the individual level. Our results also revealed that game mammal carcasses constitute an important food source (29.5% of diet) for the German Eagle population during the winter half‐year corresponding to the hunting season. This result is of relevance to management and conservation because the White‐tailed Eagle and other raptor species are affected by the ingestion of lead ammunition from shot mammalian carcasses.

Human baby hair amino acid natural abundance 15N-isotope values are not related to the 15N-isotope values of amino acids in mother’s breast milk protein

Since exclusively breast-suckled infants obtain their nutrient only from their mother's milk, it might be anticipated that a correlation will exist between the (15)N/(14)N isotope ratios of amino acids of protein of young infants and those supplied by their mother. The work presented here aimed to determine whether amino nitrogen transfer from human milk to infant hair protein synthesized within the first month of life conserves the maternal isotopic signature or whether post-ingestion fractionation dominates the nitrogen isotope spectrum. The study was conducted at 1month post-birth on 100 mother-infant pairs. Isotope ratios (15)N/(14)N and (13)C/(12)C were measured using isotope ratio measurement by Mass Spectrometry (irm-MS) for whole maternal milk, and infant hair and (15)N/(14)N ratios were also measured by GC-irm-MS for the N-pivaloyl-O-isopropyl esters of amino acids obtained from the hydrolysis of milk and hair proteins. The δ(15)N and δ(13)C (‰) were found to be significantly higher in infant hair than in breast milk (δ(15)N, P<0.001; δ(13)C, P<0.001). Furthermore, the δ(15)N (‰) of individual amino acids in infant hair was also significantly higher than that in maternal milk (P<0.001). By calculation, the observed shift in isotope ratio was shown not to be accounted for by the amino acid composition of hair and milk proteins, indicating that it is not simply due to differences in the composition in the proteins present. Rather, it would appear that each pool-mother and infant-turns over independently, and that fractionation in infant N-metabolism even in the first month of life dominates over the nutrient N-content.

Role of water contamination within the GC column of a GasBench II peripheral on the reproducibility of 18O/16O ratios in water samples

The GasBench II peripheral along with MAT 253 combination provides a more sensitive platform for the determination of water isotope ratios. Here, we examined the role of adsorbed moisture within the gas chromatography (GC) column of the GasBench II on measurement uncertainties. The uncertainty in 18O/16O ratio measurements is determined by several factors, including the presence of water in the GC. The contamination of GC with water originating from samples as water vapour over a longer timeframe is a critical factor in determining the reproducibility of 18O/16O ratios in water samples. The shift in isotope ratios observed in the experiment under dry and wet conditions correlates strongly with the retention time of analyte CO2, indicating the effect of accumulated moisture. Two possible methods to circumvent or minimise the effect of adsorbed water on isotope ratios are presented here. The proposed methodology includes either the regular baking of the GC column at a higher temperature (120 °C) after analysis of a batch of 32 sample entries or conducting the experiment at a low GC column temperature (22.5 °C). The effects of water contamination on long-term reproducibility of reference water, with and without baking protocol, have been described.

Sensitivity of stable isotope mixing models to variation in isotopic ratios: evaluating consequences of lipid extraction

Summary 1. Stable isotopes of carbon and nitrogen are increasingly used in studies of animal diet reconstruction via mixing models. However, isotope ratios of both consumer and source tissues can be altered by various amounts of lipids, potentially leading to biased estimates of diet composition when they are not taken into account. 2. We investigated the consequences of lipid correction on the estimation of diet composition with mixing models. Using empirical data from three northern terrestrial trophic systems, we illustrated the direct effects of lipid extraction (LE) on the δ13C and δ15N of source and consumer tissues and its ultimate effects on the reconstruction of the consumer’s diet. 3. In parallel, we developed a simulation tool in R, called fatsim, to assess sensitivity of mixing models to variation in isotopic ratios of samples from source or consumer tissues. This tool can be used to assess the effect of shifts in isotopic ratios caused by LE, or other sources of variation, in any trophic system and thus aid in decision making regarding lipid removal. 4. Using fatsim, we showed that the potential effects of LE on estimates of diet composition cannot be predicted without simulations, even in relatively simple systems. The sensitivity of a mixing model isotopic shift depends on the complexity of the system (number of sources) and on the relative positions of sources and consumers within the isotopic mixing space. 5. Our study confirms that the presence of lipids in tissues can bias the interpretation of diet reconstruction results. In a given trophic system, testing the sensitivity of a mixing model to LE can help decide whether lipid removal is required in order to avoid this bias.

Online high‐precision δ2H and δ18O analysis in water by pyrolysis

AbstractA method for online simultaneous δ2H and δ18O analysis in water by high‐temperature conversion is presented. Water is injected by using a syringe into a high‐temperature carbon reactor and converted into H2 and CO, which are separated by gas chromatography (GC) and carried by helium to the isotope ratio mass spectrometer for hydrogen and oxygen isotope analysis. A series of experiments was conducted to evaluate several issues such as sample size, temperature and memory effects. The δ2H and δ18O values in multiple water standards changed consistently as the reactor temperature increased from 1150 to 1480°C. The δ18O in water can be measured at a lower temperature (e.g. 1150°C) although the precision was relatively poor at temperatures &lt;1300°C. Memory effects exist for δ2H and δ18O between two waters, and can be reduced (to &lt;1%) with proper measures. The injection of different amounts of water may affect the isotope ratio results. For example, in contrast to small injections (100 nL or less) from small syringes (e.g. 1.2 µL), large injections (1 µL or more) from larger syringes (e.g. 10 µL) with dilution produced asymmetric peaks and shifts of isotope ratios, e.g. 4‰ for δ2H and 0.4‰ for δ18O, probably resulting from isotope fractionation during dilution via the ConFlo interface. This method can be used to analyze nanoliter samples of water (e.g. 30 nL) with good precision of 0.5‰ for δ2H and 0.1‰ for δ18O. This is important for geosciences; for instance, fluid inclusions in ancient minerals may be analyzed for δ2H and δ18O to help understand the formation environments. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Variation in stable isotope ratios of carbon and nitrogen in Hokkaido sika deer Cervus nippon during 1990–2000: possible causes and implications for management

Sika deer Cervus nippon populations in eastern Hokkaido, Japan, increased rapidly during 1990–1998. This increase appeared to have halted in 1999–2000, probably due to increased hunting and nuisance control. The period of rapid increase was associated with a disproportionately rapid increase in compensation paid for deer damage to crops. We studied changes in diet during 1990–2000, as reflected by stable isotope ratios of C and N in tooth collagen. We hypothesised that isotope ratios would demonstrate dietary shifts related to population levels and/or time, and that shifts in isotope ratios would be consistent with increasing individual consumption of pasture grasses at higher population levels. δ13C isotope ratios of tooth collagen in 3-year-old sika indicated a diet dominated by C3 plants throughout the period, and that forage species other than pasture grasses and dwarf bamboo Sasa nipponica (the main crop and woodland understory plants, respectively) were important elements. There was a significant decline in the δ13C isotope ratio during 1990–2000 in both males and females. δ15N values showed no trend with time for males, but increased over time in females. Indices of population (Sightings per Unit Effort, SPUE) were negatively correlated with female δ13C, and positively correlated with female δ15N, values, indicating a shift in diet over the period, especially among females. This shift may be related to population and/or offtake levels, in particular the rapid increase in female offtake for nuisance control and hunting during the period. The data are consistent with a relative increase in pasture grass consumption per individual at higher population levels; however, other explanations of the data are equally plausible. Possible dietary changes, and other factors, influencing the observed shift in isotope ratios are discussed. Although statistically significant, the magnitude of dietary shifts nevertheless appeared small, and did not provide evidence which would justify modifying the current policy, of limiting crop damage through managed population reduction to about 25% of peak levels.

A stable isotopic investigation into the causes of decline in a sub‐Antarctic predator, the rockhopper penguin Eudyptes chrysocome

AbstractThe rockhopper penguin (Eudyptes chrysocome) is a conspicuous apex marine predator that has experienced marked population declines throughout most of its circumpolar breeding distribution. The cause(s) for the declines remain elusive, but the relatively large spatio‐temporal scale over which population decreases have occurred implies that ecosystem‐scale, at‐sea factors are likely to be involved. We employ stable isotope analyses of carbon (13C/12C, expressed as δ13C) and nitrogen (15N/14N, δ15N) in time‐series of rockhopper penguin feather samples, dating back to 1861, in order to reconstruct the species' ecological history. Specifically, we examine whether rockhopper penguin population decline has been associated with a shift towards lower primary productivity in the ecosystem in which they feed, or with a shift to a diet of lower trophic status and lower quality, and we use long‐term temperature records to evaluate whether shifts in isotope ratios are associated with annual variations in sea surface temperature. Having controlled temporally for the Suess Effect and for increases in CO2 concentrations in seawater, we found that overall, δ13C signatures decreased significantly over time in rockhopper penguins from seven breeding sites, supporting the hypothesis that decreases in primary productivity, and hence, carrying capacity, for which δ13C signature is a proxy, have been associated with the decline of penguin populations. There was some evidence of a long‐term decline in δ15N at some sites, and strong evidence that δ15N signatures were negatively related to sea surface temperatures across sites, indicative of a shift in diet to prey of lower trophic status over time and in warm years. However, a site‐by‐site analysis revealed divergent isotopic trends among sites: five of seven sites exhibited significant temporal or temperature‐related trends in isotope signatures. This study highlights the utility of stable isotope analyses when applied over relatively long timescales to apex predators.

Assessment of Degradation Pathways in an Aquifer with Mixed Chlorinated Hydrocarbon Contamination Using Stable Isotope Analysis

The demonstration of monitored natural attenuation (MNA) of chlorinated hydrocarbons in groundwater is typically conducted through the evaluation of concentration trends and parent-daughter product relationships along prevailing groundwater flow paths. Unfortunately, at sites contaminated by mixtures of chlorinated ethenes, ethanes, and methanes, the evaluation of MNA by using solely concentration data and parent-daughter relationships can result in erroneous conclusions regarding the degradation mechanisms that are truly active at the site, since many of the daughter products can be derived from multiple parent compounds. Stable carbon isotope analysis was used, in conjunction with concentration data, to clarify and confirm the active degradation pathways at a former waste solvent disposal site where at least 14 different chlorinated hydrocarbons have been detected in the groundwater. The isotope data indicate that TCE, initially believed to be present as a disposed product and/or a PCE dechlorination intermediate, is attributable to dehydrochlorination of 1,1,2,2-PCA. The isotope data further support that vinyl chloride and ethene in the site groundwater result from dichloroelimination of 1,1,2-trichlorethane and 1,2-dichloroethane, respectively, rather than from reductive dechlorination of the chlorinated ethenes PCE, TCE, or 1,2-DCE. The isotope data confirm that the chlorinated ethanes and chlorinated methanes are undergoing significant intrinsic degradation, whereas degradation of the chlorinated ethenes may be limited. In addition to the classical trend of enriched isotope values of the parent compounds with increasing distance associated to biodegradation, shifts of isotope ratios of degradation byproduct in the opposite direction due to mixing of isotopically light byproducts of biodegradation with compounds from the source are shown to be of high diagnostic value. These data underline the value of stable isotope analysis in confirming transformation processes at sites with complex mixtures of chlorinated compounds.

Geochemical characterization of selected Western Canada oils by C 5–C 8 Compound Specific Isotope Correlation (CSIC)

Forty-two oils/condensates from the Western Canada Sedimentary Basin (WCSB) ranging in reservoir age from Devonian to Cretaceous are compared using the stable carbon isotopic ratio of individual hydrocarbons in the gasoline-range fraction (C 5–C 8) of these oils. Continuous Flow-Isotope Ratio Mass Spectrometry (CF-IRMS), using a modified solid dilution, purge and trap inletting system can provide reproducible isotopic ratios for the ca. 26 dominant n-, iso-, and cyclo-alkanes and aromatics in the gasoline range. The resulting isotopic signatures, or gasoline “isotopograms”, for oils/condensates provide a diagnostic geochemical technique for oil–oil and oil–source correlations. In many instances, the non-straight-chained hydrocarbons are more diagnostic than n-alkanes. Hierarchical cluster analysis provides a non-subjective approach to the treatment of individual isotopic information. This study confirms that oils from similar or the same sources produce isotopic signatures that are highly correlated. Clear carbon isotopic relationships between individual hydrocarbon compounds in the oils suggest a steady-state kinetic relationship for the generation of oil and gas, as opposed to a thermodynamic equilibrium relationship. This mechanism explains the consistent shift in isotopic ratios between similar compounds, such as the methylalkanes and dimethylalkanes.

Temporal changes in Nd isotopic composition of sedimentary rocks in the Sevier and Taconic foreland basins: Increasing influence of juvenile sources

Initial 143 Nd/ 144 Nd ratios of clastic sedimentary rocks in the Ordovician Taconic and Sevier foreland basins increase with decreasing stratigraphic age. This shift in isotopic composition reflects an increased influx of juvenile sediment to both basins. For the Taconic basin, geologic and isotopic data are consistent with the juvenile sediment being derivedfromupliftedGrenvillecrust,upthrustaspartoftheTaconicallochthons,sediment derivedfromtheBronsonHillarc,oracombinationofboth.FortheSevierforelandbasin, there are few geologic constraints on source regions, but the similar shift in isotopic ratios with time suggests a similar increase in the influence of juvenile sources such as exotic terranes. Sevier basin sedimentary rocks have higher «Nd values than those within the Taconic basin, thus juvenile source areas were of greater importance to Sevier basin sediments.

The isotope and trace element geochemistry of basalts from the volcanic islands of the southern Red Sea

Abstract Alkali basalts from the Hanish-Zukur and Zubair island groups in the southern Red Sea have been analysed for major, trace elements and Sr, Nd and Pb isotopes. They have close to primary compositions with high Mg# (60–70) and Ni and Cr concentrations up to 298 ppm and 597 ppm respectively. Many have only experienced olivine fractionation and this can be readily corrected. Compositional variations in the corrected analyses are interpreted to result from variations in the degree of melting, and a simple inversion of the corrected incompatible element abundances reveals a source composition intermediate between that of N-MORB and primitive mantle. This is suggested to result from the interaction of depleted asthenospheric mantle with less depleted material derived from the nearby Afar plume. The overall geochemical characteristics of the basalts are comparable with ocean island basalts erupted through thin (young) lithosphere. SiO 2 varies from 44.0–47.9%, Nb/Y from 0.8–1.4 and Ce/Y averages 1.64, close to values from Ascension Island. 143 Nd/ 144 Nd ratios show minimal variation between 0.51301–0.51309 whereas 87 Sr/ 86 Sr ranges between 0.70323–0.70396, and the analyses define a horizontal trend extending to the right of the mantle array. Pb isotopes also show marked variation, 206 Pb/ 204 Pb values varying from 18.675–19.077, but the analyses remain within the ranges defined by MORB and are generally comparable with samples from the Red Sea axial trough and the Gulf of Aden. Comparison with basalts from Afar and the Main Ethiopian Rift (MER), together with those from other parts of the Red Sea and Gulf of Aden reveal systematic shifts in isotope ratios with degree of extension. Along the constructive plate margins and including the island basalts, isotope results are dominantly asthenospheric, showing variations from N-MORB towards a possible plume end-member. In Afar, where β factors are lower (&lt;2.5), isotope ratios show a greater deviation from oceanic basalts and this is most marked in the MER basalts where β factors are lowest (&lt; 1.2). These data are tentatively related to a model of passive extension over a mantle plume, driven by plate motions, with the possibility of active, plume-driven extension being restricted to the early stages of Ethiopian flood basalt volcanism and the subsequent development of the MER during the Miocene.

Strontium isotope diagenesis of biogenic aragonite and low-Mg calcite

Using petrographie and geochemical criteria, it is possible to identify marine Sr isotopic compositions and diagenetic overprints in carbonate samples. The 87Sr 86Sr ratio of originally low-Mg calcite brachiopods and aragonite molluscs from the Boggy, Magoffin, and Brush Creek Formation/Members increases with progressive post-depositional alteration in the presence of diagenetic meteoric waters. In contrast, brachiopods from the Codroy Formation exhibit an increase in non-radiogenic Sr with diagenetic alteration. Based on the seawater 87Sr 86Sr curve, this alteration probably occurred in the presence of marine water during the mid-Permian, Triassic, or late-Cretaceous-early-Tertiary. The degree of change in radiogenic Sr probably reflects the physicochemical nature of the diagenetic microsystem. Diffusion controlled altered material shows the smallest degree of change in Sr isotopic ratios as well as microstructural and other geochemical modifications. In contrast, fluid-controlled altered material shows the greatest shift in isotopic ratios with concomitant microstructural and geochemical changes. Data of diagenetically well-defined biogenic carbonates show that Mississippian seawater 87Sr 86Sr was constant with a mean of about 0.70773 and a range of 0.7076–0.7078. The revised high-resolution curve shows significantly higher variations for Pennsylvanian seawater 87Sr 86Sr (0.7076–0.7093). A low perturbation marks the midVirgilian (0.7076–0.7078), and high perturbations are observed for the early Missourian (0.70846) and latest Morrowan (0.70913). The mean value determined for the latest Morrowan is similar to 87Sr 86Sr ratios measured for modern carbonates and seawater. It is further suggested that seawater 87Sr 86Sr varies by as much as 0.000154 at any given time.

The rate of lake acidification in four lakes North of lake superior and its relationship to downcore sulphur isotope ratios

Surficial sediments (0 to 12 cm) were removed from three lakes located within 20 km of a large siderite sintering plant which annually releases 140 000 tonne S and sinter fly ash up its stacks. A fourth lake located approximately 100 km upwind of the sintering plant was chosen as a control. Lake sediments were analyzed for sulphate, pyrite, organic S, total S and subfossil diatoms. The rate of lake acidification inferred from stratigraphic changes in diatom species composition indicated that one of the lakes located in a granitic basin near the sintering plant was undergoing rapid acidification. The other two study lakes located near the sintering plant were situated on carbonate-rich greenstones. Their pH as inferred from diatom stratigraphy indicated that both lakes were actually increasing in pH over the last 30 yr. The diatom inferred pH of the control lake's deeper sediments (4.8 ± 0.2) did not differ significantly from the lakes' present day observed pH (4.9). Lakes closest to the sintering plant had the highest S concentrations in their sediments. In addition, their surface sediments (0 to 2 cm) had significantly higher pyrite concentrations than their deeper sediments. Organic S and pyrite S comprised the major fraction of S in the sediments of all four lakes. The S isotope ratios (δ34S) in the various forms of S in the lake sediments were also measured. The δ34S values in the deeper (10 to 12 cm) sediments were found to be fairly uniform in all four lakes at + 4.0%., almost identical to those of the lake sulphates upwind from the Wawa sintering plant. This suggests little or no S isotope fractionation during sulphate reduction and precipitation of sulphides at the time of sediment deposition roughly 100 yr age. However, δ34S values in the surface sediments (0 to 2 cm) were shifted toward negative values. In the lakes downwind from the Wawa smelter, the isotope shift ranged from 14 to 16%, whereas in the upwind lakes the shift was very much smaller ranging from 1.5 to 5.0%. This shift in δ34S in the recent sediments wass presumed to be largely a result of isotope fractionation in the dissimilatory reduction of lake sulphate by bacteria stimulated by anthropogenic inputs of S. The possibility of anthropogenic inputs of S with high negative δ34S values to explain the shifts in isotopic ratios seems unlikely in view of known δ34S values for the ores in the area. Additional δ34S measurements on lake sediments, lake sulphates and ore samples (smelter effluent), etc., have been undertaken. This should make it possible to interpret the 534 shift in the sediments in terms of environmental changes since the Ambrosia rise (circa 1890).

Kinetische Kohlenstoff‐Isotopenfraktionierung im CO2 – H2O‐System des Laacher Sees

AbstractUnexpected shifts of 13C concentrations observed with carbondioxide emerging from the water close to the eastern shore of Lake Laach could be explained by kinetic C isotopefractionation. This was caused by preferential 12C incorporation into HCO3‐ions as long as no saturation of CO2 was reached and time was not sufficient for establishing isotopic equilibrium. – Laboratory experiments with tank carbon‐dioxide bubbling through triple distilled water in columns up to 4 m high provided fractionations as large as 7.2‰. 13Carbon was found to be enriched in the passing carbondioxide. The effect decreased with time (i.e. with increasing saturation). – The fractionation becomes larger the higher the water column is. It depends also on the size of the carbondioxide bubbles and the water temperature. – These shifts of isotope ratios which occur frequently in nature invalidate fast conclusions regarding the origin of carbondioxide. Careful investigations of the degree of saturation and time available for equilibration are required before genetic conclusions from C isotope ratios are allowed.