Study of the trophic web of San Simón Bay (Ría de Vigo) by using stable isotopes

Based on the stable isotope composition in ¹⁵N and ¹³C of different potential sources of organic matter and consumers of an intertidal Zostera marina meadow located in San Simón Bay (Ría de Vigo, NW of Spain), a simplified food web of this community was reconstructed. For this purpose, some alternatives in different steps of the most used methodology of stable isotope dietary analysis were developed that cope with some of the limitations associated to the interpretation of isotopic signals for food web analysis, those of uncertainty on the fractionation value, mathematical model to use for the diet resolution and shortage of the isotope number for discriminating many food sources. The application of this protocol to the studied community reported similar results to those from other studies based on similar trophic webs, emphasizing the importance of local primary producers, especially microphytobenthos, which could be available for several primary consumers through resuspension forced by tidal hydrodynamic. The good agreement with previous results suggests that the proposed protocol is a feasible alternative to elucidate the most plausible trophic relationships in complex trophic webs using stable isotopes analysis.     

Multiple sulfur and oxygen isotope compositions in Beijing aerosol

Multiple sulfur isotopes(32S, 33 S, 34 S, 36S) and oxygen isotopes(16O, 18O) in Beijing aerosols were measured with MAT-253 isotope mass spectrometer. The δ34S values of Beijing aerosol samples range from 1.68‰ to 12.57‰ with an average value of 5.86‰, indicating that the major sulfur source is from direct emission during coal combustion. The δ18O values vary from 5.29‰ to 9.02‰ with an average value of 5.17‰, revealing that the sulfate in Beijing aerosols is mainly composed of the secondary sulfate. The main heterogeneous oxidation of SO2 in atmosphere is related to H2O2 in July and August, whereas H2O2 oxidation and Fe3+ catalytic oxidation with SO2 exist simultaneously in September and October. Remarkable sulfur isotope mass-independent fractionation effect is found in Beijing aerosols, which is commonly attributed to the photochemical oxidation of SO2 in the stratosphere. In addition, thermochemical reactions of sulfur-bearing compounds might be also a source of sulfur isotope anomalies based on the correlation between ?33S and CAPE.     

Minimal incorporation of Deepwater Horizon oil by estuarine filter feeders

Natural abundance carbon isotope analyses are sensitive tracers for fates and use of oil in aquatic environments. Use of oil carbon in estuarine food webs should lead to isotope values approaching those of oil itself, −27‰ for stable carbon isotopes reflecting oil origins and −1000‰ for carbon-14 reflecting oil age. To test for transfer of oil from the 2010 Deepwater Horizon spill into estuarine food webs, filter-feeding barnacles (Balanus sp.) and marsh mussels (Geukensia demissa) were collected from Louisiana estuaries near the site of the oil spill. Carbon-14 analyses of these animals from open waters and oiled marshes showed that oil use was <1% and near detection limits estimated at 0.3% oil incorporation. Respiration studies showed no evidence for enhanced microbial activity in bay waters. Results are consistent with low dietary impacts of oil for filter feeders and little overall impact on respiration in the productive Louisiana estuarine systems.     

Characteristics of Cd isotopic compositions and their genetic significance in the lead-zinc deposits of SW China

Up to now, the evaporation and condensation, as well as the biological absorption and inorganic absorptions, have been proved to be major factors in Cd isotope fractionation. And Cd isotopes have been widely applied in studies on the universal evolution and marine environment and so on. However, only a few researches have been conducted in applying Cd isotopes to trace the source of metallogenic material and the evolution of the ore-forming fluid in a complex mineralization environment, especially in a hydrothermal ore-formation system. We measured the Cd isotopic compositions of sphalerite, galena, and ores from five lead-zinc deposits in SW China, and found that the δ ^114/110Cd values varied from ?1.53‰ to 0.34‰, with a total range of 1.87‰, which is greater than most of measured geological samples. Meanwhile, through contrasting the Cd content with Cd isotopic compositions of different deposits, it may be concluded that different genetic lead-zinc deposits have different Cd content and isotopic compositions, which could be a tool for the studies on the origin of ore deposits. Also, the biomineralization and crystal fractionation may also result in Cd isotope fractionation. In a word, although the research of Cd isotopes is presently at the preliminary stage (especially in hydrothermal ore-formation system), this study demonstrated that Cd isotopes can give a clue in tracing the evolution of ore-forming fluid and metallogenic environment.     

Hydrogen isotopes in palmitic and stearic acids in suspended particles from the Changjiang River Estuary

Hydrogen isotopes in lipid biomarkers can trace past changes in the hydrologic cycle. Recent studies have revealed the potential of hydrogen isotopes in microalgal lipids for quantitatively reconstructing water δ~2H(δD) values and salinity. In this study we collected suspended particles along a salinity gradient from the Changjiang River Estuary(CRE), and measured δD values in fatty acids in these particles. The results indicated that δD values of water were correlated highly with salinity from the CRE, in agreement with the results from other estuaries. δD values in palmitic acid and stearic acid had a positive correlation with δD values of water from the CRE. Nevertheless, in the CRE, hydrogen isotope fractionation in fatty acids relative to water increased as salinity increased, opposite the trend in hydrogen isotope fractionation with salinity found in microalgal culture and field studies. We attribute the increase in hydrogen isotope fractionation as salinity increased to light availability, which was likely lower in the particle rich mixing zone at the end of the estuary, and potentially as well to multiple sources of fatty acids in the CRE.     

Carbon isotope fractionation during CH4 transport in paddy fields

In this study,to further promote the application of the stable carbon isotope natural abundance(SCINA)method to the study of CH4in paddy fields in China,field experiments were carried out to investigate carbon isotope fractionation during CH4transport in both rice-and non-rice-growing seasons.More importantly,two new methods for the measurement of the CH4transport fractionation factor(εtransport)in paddy fields were introduced.The results indicated that the closed chamber+syringe method was much better for the determination ofεtransport during the non-rice-growing season.Presently,εtransport was calculated using theδ13C value of the CH4emitted from a rice field minus that of the CH4in the floodwater(–6.7‰to–3.0‰).In addition,there were three methods available for estimatingεtransport during the rice-growing season:deduction of theδ13C value of the CH4in the floodwater from that of the CH4emitted from the field(–16.6‰to–15.2‰);deduction of theδ13C value of the CH4in the soil pore water from that of the CH4emitted from the field(–13.2‰to–1.1‰);and deduction of theδ13C value of the CH4in the aerenchyma of plants from that of the CH4emitted from plants(–16.3‰to–10.9‰).Unfortunately,the first two methods showed relatively large uncertainties.Only the last one,the dividing+cutting method,was not only scientific and reliable but also provided accurate measurements.     

Oxygen isotope fractionation factors between anhydrite and water from 100 to 550°C

Oxygen isotope exchange between anhydrite and water was studied from 100 to 550°C, using the partial equilibrium method. The exchange rate was extremely low in NaCl solution. In the lower-temperature range, acid solutions were used to produce sufficient reaction to determine the oxygen isotope fractionation factors. The fractionation factors obtained in the present study are definitely different from those given by Lloyd [8]. They are similar to those for the HSO₄^?-water system studied by Mizutani and Rafter [19], and are consistently 2‰ higher than those of the barite-water system by Kusakabe and Robinson [5]. The temperature dependence of the oxygen isotope fractionation factors was calculated by the least squares method in which the weight was taken to be inversely proportional to the experimental error. The fractionation is given by:10³lnαanhydrite-water=3.21×(10³/T)²?4.72Available δ¹⁸O values of natural anhydrite were used to test the validity of this expression. It is shown that this newly revised geothermometer can be successfully applied to natural hydrothermal anhydrite.     

Diagenetic fluid evolution and water-rock interaction model of carbonate cements in sandstone: An example from the reservoir sandstone of the Fourth Member of the Xujiahe Formation of the Xiaoquan-Fenggu area,Sichuan Province,China

Carbonate cement is the most abundant cement type in the Fourth Member of the Xujiahe Formation in the Xiaoquan-Fenggu area of the West Sichuan Depression. Here we use a systematic analysis of carbonate cement petrology, mineralogy, carbon and oxygen isotope ratios and enclosure homogenization temperatures to study the precipitation mechanism, pore fluid evolution, and distribution of different types of carbonate cement in reservoir sand in the study area. Crystalline calcite has relatively heavy carbon and oxygen isotope ratios(δ13C = 2.14‰, δ18O = -5.77‰), and was precipitated early. It was precipitated directly from supersaturated alkaline fluid under normal temperature and pressure conditions. At the time of precipitation, the fluid oxygen isotope ratio was very light, mainly showing the characteristics of a mixed meteoric water-seawater fluid(δ18O = -3‰), which shows that the fluid during precipitation was influenced by both meteoric water and seawater. The calcite cement that fills in the secondary pores has relatively lighter carbon and oxygen isotope ratios(δ13C = -2.36‰, δ18O = -15.68‰). This cement was precipitated late, mainly during the Middle and Late Jurassic. An important material source for this carbonate cement was the feldspar corrosion process that involved organic matter. The Ca2+, Fe3+ and Mg2+ ions released by the clay mineral transformation process were also important source materials. Because of water-rock interactions during the burial process, the oxygen isotope ratio of the fluid significantly increased during precipitation, by about 3‰. The dolomite cements in calcarenaceous sandstone that was precipitated during the Middle Jurassic have heavier carbon and oxygen isotope ratios, which are similar to those of carbonate debris in the sandstone(δ13C = 1.93‰, δ18O = -6.11‰), demonstrating that the two are from the same source that had a heavier oxygen isotope ratio(δ18O of about 2.2‰). The differences in fluid oxygen isotope ratios during cement precipitation reflect the influences of different water-rock interaction systems or different water-rock interaction strengths. This is the main reason why the sandstone containing many rigid particles(lithic quartz sandstone) has a relatively negative carbon isotope ratio and why the precipitation fluid in calcarenaceous sandstone has a relatively heavier oxygen isotope ratio.     

云南4个湖泊浮游生物碳、氮稳定同位素的季节变化及其影响因子

浮游生物是湖泊食物网的重要组成,其碳、氮稳定同位素能够反映元素地球化学循环和食物来源的波动,是了解水域生态系统结构变化的重要手段之一.本文选取云南4个不同类型湖泊,开展浮游生物碳、氮稳定同位素组成(δ¹³C、δ¹⁵N)的季节变化与湖泊对比研究.大型深水湖泊(抚仙湖和阳宗海)中,浮游植物δ¹³C值在夏、秋季(-20.34‰±1.98‰)显著高于冬、春季(-28.00‰±2.51‰),反映夏秋季藻类生长速率较高、HCO3-无机碳源利用增多等的影响.而小型浅水湖泊(长桥海和大屯海)中浮游植物δ¹³C值在夏季最高(-21.24‰±0.88‰),可能与雨季流域输入增强、陆源有机质占比增加有关.4个湖泊浮游生物δ¹⁵N值具有一致的变化特征,春季显著高于其他季节.分析表明,云南地区雨季以面源污染为主向旱季以点源污染为主的转变,导致氮素营养盐季节性来源差异,并通过生物吸收作用影响了浮游生物δ¹⁵N值的季节变化.在浮游动物与浮游植物的稳定同位素差值(即富集度)方面,营养水平高的小型浅水湖泊中δ¹³C富集度为1.61‰±0.90‰、δ¹⁵N富集度为2.71‰±1.22‰,显著小于营养水平低的大型深水湖泊(分别为2.60‰±0.98‰和4.19‰±1.25‰),表明随着湖泊营养水平的增加,浮游动物更多地以浮游植物为食,导致有机碳在不同营养级之间的传输过程中具有更强的耦合作用,且相邻营养级之间具有更低的δ¹⁵N富集度特征.     

The δ13C–δ18O variations in marble in the Hida Belt,Japan

Marble has a great potential to understand a history of various geological events occurring during tectonic processes. In order to decode metamorphic–metasomatic records on C–O isotope compositions of marble at mid-crustal conditions, we conducted a C–O–Sr isotope study on upper amphibolite-facies marbles and a carbonate–silicate rock from the Hida Belt, which was once a part of the crustal basement of the East Asian continental margin. Carbon and oxygen isotope analyses of calcite from marbles (Kamioka area) and a carbonate–silicate rock (Wadagawa area) show a large variation of δ¹³C [VPDB] and δ¹⁸O [VSMOW] values (from −4.4 to +4.2 ‰ and +1.6 to +20.8 ‰, respectively). The low δ¹³C values of calcites from the carbonate–silicate rock (from −4.4 to −2.9 ‰) can be explained by decarbonation (CO₂ releasing) reactions; carbon–oxygen isotope modeling suggests that a decrease of δ¹³C strongly depends on the amount of silicate reacting with carbonates. The occurrence of metamorphic clinopyroxene in marbles indicates that all samples have been affected by decarbonation reactions. All δ¹⁸O values of calcites are remarkably lower than the marine-carbonate values. The large δ¹⁸O variation can be explained by the isotope exchange via interactions between marble, external fluids, and/or silicates. Remarkably low δ¹⁸O values of marbles that are lower than mantle value (~+5 ‰) suggest the interaction with meteoric water at a later stage. Sr isotope ratios (⁸⁷Sr/⁸⁶Sr = 0.707255–0.708220) might be close to their protolith values. One zircon associated with wollastonite in a marble thin-section yields a U–Pb age of 222 ± 3 Ma, which represents the timing of the recrystallization of marble, triggered by H₂O-rich fluid infiltration at a relatively high-temperature condition. Our isotope study implies that the upper amphibolite-facies condition, like the Hida Belt, might be appropriate to cause decarbonation reactions which can modify original isotope compositions of marble if carbonates react with silicates.     

Differences in stable isotope compositions of freshwater snails from surface sediments of two Polish shallow lakes

The study reports and discusses the differences in δ¹³C and δ¹⁸O values of shells between several species of freshwater snails. Shells were derived from sediment samples collected from depths of 0.5, 1, 2 and 3 m along transects in two shallow eutrophic lakes located in mid-western Poland. Mean δ¹³C values of the shells ranged between −7.5 and −3.8‰ in Lake Jarosławieckie and between −8.1 and −5.2‰ in Lake Rosnowskie Duże, whereas mean δ¹⁸O values ranged between −2.2 and −0.2‰ and between −2.2 and 0.4‰ respectively in the studied lakes. A similar order of species in terms of shell isotope values, from least to most ¹³C and ¹⁸O-depleted was observed in both lakes and seems to indicate constancy of the factors controlling the stable isotope compositions of snail shells. We postulate that the nearly 4‰ difference in the mean carbon stable isotope values between the species was primarily controlled by the amount of metabolic carbon incorporated into the shells and the δ¹³C values of the snail food. Different growth cessation temperatures and microhabitats of the species studied result in temporally and spatially varied DIC δ¹³C values, water δ¹⁸O values and water temperature of shell precipitation, and may thus differentiate the δ¹³C and δ¹⁸O values of shells. The range of δ¹³C and δ¹⁸O values of individual shells from a sediment sample (mean 2.35 and 2.15‰, respectively) is interpreted as reflecting an intraspecific variability of isotope compositions in shells from a population and changes of the ambient conditions during the accumulation of the sediment layer. The species-specificity and intraspecific variability in C and O isotopic compositions of shells allow concluding that in palaeolimnological studies, stable isotope analyses should be performed on a set of mono-specific shells representing mean isotope compositions of the species for the interval studied rather than single shells or multispecific bulk shell material.     

δ53Cr values of catchment runoff exhibit seasonality regardless of bedrock Cr concentrations: Role of periodically changing chromium export fluxes

One pre-requisite for the construction of a global chromium isotope mass balance is detailed understanding of Cr isotope systematics in the critical zone where redox-processes can modify the isotope signature of geogenic Cr input into the hydrosphere. A Cr isotope inventory of bedrock, soil, and runoff was performed in a Central European headwater catchment underlain by amphibolite, situated in the vicinity of two previously studied catchments underlain by different bedrock types (serpentinite and leucogranite). Fresh bedrock in the amphibolite catchment NAZ contained ~300 mg/kg Cr, serpentinite at PLB contained ~800 mg/kg Cr, and leucogranite at LYS contained ~2 mg/kg Cr. Monthly hydrochemical monitoring at all three sites revealed higher Cr(VI) export fluxes in winter than in summer. NAZ was characterized by a distinct seasonality in the δ⁵³Cr values, with minima during winter/spring snowmelts (−0.35‰) and maxima during dry summers (0.40‰). Similar seasonality in δ⁵³Cr values had been reported from PLB and LYS. Bedrock at all three sites had similar Cr isotope composition close to −0.10‰, a value indistinguishable from the δ⁵³Cr value of bulk silicate Earth (BSE). Positive mean δ⁵³Cr value of NAZ runoff indicated Cr-isotope fractionations during weathering of geogenic Cr(III), combined with adsorption of the resulting Cr(VI) on soil particles during pedogenesis. However, the mass-weighted mean δ⁵³Cr of NAZ runoff was lower (−0.08‰), indistinguishable from the Cr isotope signature of bedrock. The same pattern of lower mass-weighted mean δ⁵³Cr values of runoff, compared to arithmetic mean δ⁵³Cr values of runoff, were observed also at PLB and LYS. We suggest that elevated Cr runoff fluxes in winter remove some of the residual isotopically light Cr that accumulated in the soil during summer. Seasonality in runoff δ⁵³Cr values appears to be a relatively widespread phenomenon, de-coupled from Cr availability for chemical weathering.     

Oxygen and hydrogen isotope ratios in tree rings: how well do models predict observed values?

We have measured annual oxygen and hydrogen isotope ratios in the α-cellulose of the latewood of oak (Quercus robur L.) growing on well-drained ground in Norfolk, UK. We compare the observed values of isotope ratios with those calculated using equations that allow for isotopic fractionation during the transfer of oxygen and hydrogen from source water taken by the tree to cellulose laid down in the cambium. The equations constitute a model in which isotopic fractionation occurs during evaporative enrichment within the leaf and during isotopic change between carbohydrates and water in the trunk during cellulose synthesis. From the relationship between isotope ratios in precipitation and α-cellulose, we deduce that the source water used by the tree comprises a constant mixture of groundwater and precipitation, chiefly from the months of May, June and July of the growth year. By selection of isotopic fractionation factors and the degree of isotope exchange within the trunk, we are able to model the observed annual values of oxygen isotope ratios of α-cellulose to a significant level (r=0.77, P<0.01). When we apply the same model to hydrogen isotope ratios, however, we find that, although we can predict the average value over the time series, we can no longer predict the year-to-year variation. We suggest that this loss of environmental signal in the hydrogen isotopes is caused by differences in the kinetic isotope effects of the biochemical reactions involved in the fixation of hydrogen in different positions of the glucose molecule. Owing to these effects, the hydrogen isotope ratios of cellulose can vary in a way not anticipated in current models and hence may induce non-climatic ‘noise’ in the hydrogen isotope time series.     

Investigating the distribution and sources of organic matter in surface sediment of Coombabah Lake (Australia) using elemental,isotopic and fatty acid biomarkers

Extensive physical and biological measurements were made of the surface sediments within the shallow, semi-urbanised Coombabah Lake in southern Moreton Bay, Australia. Sediment bulk parameters (C/N ratios, δ¹³C and δ¹⁵N) and fatty acid biomarkers were used to determine distributions and sources of organic matter in the intertidal sediments. The determination of organic matter sources within coastal and estuarine settings is important in understanding the roles of organic matter as energy and nutrient sources. Spatial variability of biomarker values within the sediments were interpreted by thematic maps employing the Krigging algorithm. Grain size analysis indicated the lake was dominated by mud (<63 μm) in the southern (landward) and sand (>63 μm) in the northern (seaward) lake regions, respectively. Surface sediment organic C and N values ranged from 0.12% to 1.76% and 0.01% to 0.12% dry weight, respectively, and C/N ratios averaged 16.3±3.19%. Sedimentary δ¹³C values ranged from −26.1‰ to −20.9‰, with an average value of −23.9±1.0‰. Sedimentary δ¹⁵N values ranged from +1.7‰ to +4.8‰, with an average value of +2.8±0.8‰. Bulk sediment parameters suggested that sedimentary organic matter is provided predominantly by allochthonous sources in the form of fringing mangroves. Thirty-nine individual fatty acids were identified using gas chromatography–mass spectrometry. The mean contributions of long chain fatty acids (LCFAs), polyunsaturated fatty acids (PUFAs), saturated fatty acids (SAFAs) and bacterial fatty acids (BAFAs) were, respectively, 13.9±11.4%, 7.6±4.1%, 53.6±8.6% and 18.2±4.6% of the identified fatty acid methyl esters (FAMEs), with BAFAs occurring in all sampled sediments. Fatty acid compositions varied throughout lake sediments, which indicated spatial differences in autochthonous and allochthonous organic matter sources, including terrestrial and planktonic (i.e. zooplankton, diatoms and other algal species) sources. The contribution of organic matter from shoreline mangroves was confirmed by the presence of LCFAs and 18:2ω6 and 18:3ω3, which are markers for mangroves in this ecosystem. BAFAs were identified in increased proportions in sediments adjacent to urban developments and dominated by mud. Grain size was identified as a dominant factor in the fatty acid compositions and contributing values to FAME pool. Spatial patterns of C/N ratios, δ¹³C and δ¹⁵N values, and fatty acid biomarker contributions illustrated that there is a greater contribution of autochthonous and labile organic matter to the sedimentary organic matter pool in the northern (marine entrance) sediments compared to the more allochthonous sourced organic matter of the southern region of the lake. This study details the distribution and sources of organic matter within Coombabah Lake and illustrates the usefulness of a multiple biomarker approach in discriminating organic matter sources within estuarine environments.     

Resource partitioning among cladocerans in a littoral macrophyte zone: implications for the transfer of essential compounds

The maintenance of species diversity in a given environment is strongly linked to resource partitioning. Littoral macrophyte zones are heterogeneous environments with high microcrustacean diversity, where zooplankton have dietary access to seston as well as organisms growing on macrophytic surfaces (epiphyton). We conducted a field study in a macrophyte-rich backwater of the river Allier to examine how seston and epiphyton were used as potential food sources by four dominant cladoceran species. Fatty acids were analyzed in these two food sources to assess how their differential uptake affects the trophic trajectory of essential compounds from these resources to cladocerans. Our results showed resource partitioning among the four cladocerans studied; while Eurycercus fed mostly on epiphyton, Daphnia mostly consumed phytoplankton, and Ceriodaphnia and Simocephalus were able to forage on sestonic and epiphytic resources. Based on their polyunsaturated fatty acid (PUFA) content, it was evident that epiphyton was of higher food quality than seston in this macrophyte-rich backwater system. Variability of PUFA compositions of seston and epiphyton, and diversity of foraging strategies of cladoceran species, which represent the major link between microorganisms and consumers at higher trophic levels, affect dietary energy pathways and point to a variable PUFA transfer efficiency in backwater food webs.     

D/H fractionation factors between serpentine and water at 100° to 500°C and 2000 bar water pressure,and the D/H ratios of natural serpentines

D/H fractionation factors between serpentine (clinochrysotile) and water were experimentally determined to be: 1000 In α_ser-w = 2.75 × 10 ⁷/T² ? 7.69 × 10⁴/T + 40.8 in the temperature range from 100 to 500°C. The present results do not support the semi-empirical fractionation factors employed by Wenner and Taylor [1] for the interpretation of δD values of natural serpentines. About 100 serpentines from the Japanese Islands have δD values from ?110 to ?40‰ SMOW, with antigorite being from ?40 to ?60‰. The results are in accord with the two conclusions by Wenner and Taylor [1,2], that is, the presence of a latitude ?δD correlation and the more uniform and higher δD values of antigorite than chrysotile and lizardite.According to the present fractionation factors, almost none of the continental lizardite-chrysotile serpentines could have formed at a temperature below 500°C under equilibrium with fluids of δD values similar to the present-day local meteoric waters. The fluid responsible for oceanic serpentinization could be either a mixture of oceanic and magmatic water or oceanic water alone. However, full interpretation of the δD values of natural serpentines should wait until kinetic behaviors of hydrogen isotopes in serpentinization are better understood.     

Modern stable isotopic (δ18O, δ2H, δ13C) variation in terrestrial,fluvial, estuarine and marine waters from north‐central Sarawak,Malaysian Borneo

Stable isotope data on humid tropical hydrology are scarce and, at present, no such data exist for Borneo. Delta¹⁸O, δ²H and δ¹³C were analysed on 22 water samples from different parts of the Sungai (river) Niah basin (rain, cave drip, rainforest pool, tributary stream, river, estuary, sea) in north‐central Sarawak, Malaysian Borneo. This was done to improve understanding of the modern stable isotope systematics of the Sungai Niah basin, essential for the palaeoenvironmental interpretation of the Late Quaternary stable isotope proxies preserved in the Great Cave of Niah. The Niah hydrology data are put into a regional context using the meteoric water line for Southeast Asia, as derived from International Atomic Energy Agency/World Meteorological Organization isotopes in precipitation network data. Although the Niah hydrological data‐set is relatively small, spatial isotopic variability was found for the different subenvironments of the Sungai Niah basin. A progressive enrichment occurs towards the South China Sea (δ¹⁸O ?4·6‰; δ²H ?29·3‰; δ¹³C ?4·8‰) from the tributary stream (δ¹⁸O ?8·4‰; δ²H ?54·7‰; δ¹³C ?14·5‰) to up‐river (δ¹⁸O c. ?8‰; δ²H c. ?51‰; δ¹³C c. ?12‰) and down‐river values (δ¹⁸O c. ?7·5‰; δ²H c. ?45‰; δ¹³C c. ?11‰). This is thought to reflect differential evaporation and mixing of different components of the water cycle and a combination of depleted biogenic δ¹³C (plant respiration and decay) with enriched δ¹³C values (due to photosynthesis, atmospheric exchange, mixing with limestone and marine waters) downstream. Cave drip waters are relatively enriched in δ¹³C as compared to the surface waters. This may indicate rapid degassing of the cave drips as they enter the cave atmosphere. Copyright © 2005 John Wiley & Sons, Ltd.     

From precipitation to runoff: stable isotopic fractionation effect of glacier melting on a catchment scale

Stable isotope variability and fractionation associated with transformation of precipitation/accumulation to firn to glacial river water is critical in a variety of climatic, hydrological and paleoenvironmental studies. This paper documents the modification of stable isotopes in water from precipitation to glacier runoff in an alpine catchment located in the central Tibetan Plateau. Isotopic changes are observed by sampling firnpack profiles, glacier surface snow/ice, meltwater on the glacier surface and catchment river water at different times during a melt season. Results show the isotopic fractionation effects associated with glacier melt processes. The slope of the δD‐δ¹⁸O regression line and the deuterium excess values decreased from the initial precipitation to the melt‐impacted firnpack (slope from 9.3 to 8.5 and average d‐excess from 13.4‰ to 7.4‰). The slope of the δD‐δ¹⁸O line further decreased to 7.6 for the glacier runoff water. The glacier surface snow/ice from different locations, which produces the main runoff, had the same δD‐δ¹⁸O line slope but lower deuterium excess (by 3.9‰) compared to values observed in the firnpack profile during the melt season. The δD‐δ¹⁸O regression line for the river water exhibited a lower slope compared to the surface snow/ice samples, although they were closely located on the δD‐δ¹⁸O plot. Isotope values for the river and glacier surface meltwater showed little scatter around the δD‐δ¹⁸O regression line, although the samples were from different glaciers and were collected on different days. Results indicate a high consistency of isotopic fractionation in the δD‐δ¹⁸O relationships, as well as a general consistency and temporal covariation of meltwater isotope values at the catchment scale. Copyright © 2013 John Wiley & Sons, Ltd.     

Sulfur isotope ratios of Icelandic lava incrustations and volcanic gases

Sulfur isotope ratios were measured in eight lava incrustations and three volcanic gas samples and their corresponding lava flows. The lava incrustations of sulfate composition are from five recent eruptions and occur as thenardite or as aphtitalite-thenardite mixtures, with abundant trace elements. The incrustations show small sulfur isotope fractionation of 1–2‰ compared with corresponding lavas and the volcanic gas samples. The sulfate incrustations are formed through oxidation of SO₂ from the emitted volcanic gas and subsequent reaction with metal halides. The volcanic gas samples show a distribution of decreasing δ³⁴S through time from +3.4 to −1.8‰; sulfate was preferentially degassed compared to sulfide. The data indicate that sulfate incrustations serve as a late-stage volcanic gas sample with respect to sulfur isotopes.     

Coupled silicon–oxygen isotope fractionation traces Archaean silicification

Silica alteration zones and cherts are a conspicuous feature of Archaean greenstone belts worldwide and provide evidence of extensive mobilisation of silica in the marine environment of the early Earth. In order to understand the process(es) of silicification we measured the silicon and oxygen isotope composition of sections of variably silicified basalts and overlying bedded cherts from the Theespruit, Hooggenoeg and Kromberg Formations of the Barberton Greenstone Belt, South Africa.The δ³⁰Si and δ¹⁸O values of bulk rock increase with increasing amount of silicification from unsilicified basalts (?0.64‰ < δ³⁰Si < ?0.01‰ and + 8.6‰ < δ¹⁸O < + 11.9‰) to silicified basalts (δ³⁰Si and δ¹⁸O values as high as + 0.81‰ and + 15.6‰, respectively). Cherts generally have positive isotope ratios (+ 0.21‰ < δ³⁰Si < + 1.05‰ and + 10.9 < δ¹⁸O < + 17.1), except two cherts, which have negative δ³⁰Si values, but high δ¹⁸O (up to + 19.5‰).The pronounced positive correlations between δ³⁰Si, δ¹⁸O and SiO₂ imply that the isotope variation is driven by the silicification process which coevally introduced both ¹⁸O and ³⁰Si into the basalts. The oxygen isotope variation in the basalts from about 8.6‰ to 15.6‰ is likely to represent temperature-dependent isotope fractionation during alteration. Our proposed model for the observed silicon isotope variation relies on a temperature-controlled basalt dissolution vs. silica deposition process.