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Isotopic Signatures (isotopic + signature)
Kinds of Isotopic Signatures Selected AbstractsBURIAL HISTORY RECONSTRUCTION USING LATE DIAGENETIC PRODUCTS IN THE EARLY PERMIAN SILICICLASTICS OF THE FARAGHAN FORMATION, SOUTHERN ZAGROS, IRANJOURNAL OF PETROLEUM GEOLOGY, Issue 2 2009S. M. Zamanzadeh In spite of the increasing importance to hydrocarbon exploration and production of the Palaeozoic succession in the Zagros area of SW Iran, few burial history and palaeothermal modelling studies of the interval have been carried out. This paper attempts to assess the burial and palaeotemperature history of the Lower Permian Faraghan Formation which is composed of stromatolitic dolomites overlain by mainly cross-bedded sandstones. The formation grades up into the thick bedded carbonates of the Upper Permian Dalan Formation. The Faraghan and Dalan Formations are major hydrocarbon reservoir units in SW Iran and are time-equivalents of the Unayzah and Khuff Formations in Saudi Arabia, respectively. The Faraghan Formation consists of shallow-marine siliciclastics and foreshore deposits, including tidal-flat and tidal-channel, estuarine, sabkha, shoreface and offshore facies. In this study, diagenetic constituents are used to evaluate the formation's burial history in the Southern Zagros, an area for which only limited subsurface data is available. A burial history diagram for the formation was constructed for well Finu # 1 using WinBuryTM software. The diagram shows that the formation underwent progressive burial at variable rates between its deposition and the mid-Tertiary, since when it has undergone rapid uplift. Burial diagenetic products in the Faraghan Formation comprise saddle, ferroan and zoned dolomites, together with dickite, illite/sericite and chlorite minerals. Additional burial-related features include stylolites and dissolution seams. Isotopic signatures (,18O versus ,13C) of the ferroan dolomites suggest a burial trend for the formation. Reconstruction of the paragenetic sequence together with the burial history diagram suggests a maximum burial depth of about 5000 m and a wide palaeotemperature range of 80-160°C. However considering the saddle dolomites as a palaeothermometer, the temperature range narrows to 78 to 138 °C. The burial depth and temperature ranges closely correlate with the main stage of oil generation to the dry gas zone. [source] Noble gas and boron isotopic signatures of the Bacon-Manito geothermal fluid, PhilippinesGEOFLUIDS (ELECTRONIC), Issue 4 2008F. E. B. BAYON Abstract Noble gas isotopic composition and abundances were determined on dry gas sampled in geothermal wells from the Bacon-Manito (BGPF) geothermal field in the Philippines. The most significant findings come from the 3He/4He ratio; a mantle-He source is evidenced by ratio close to 7 Ra. Peripheral fluid from the west and south of the geothermal system is relatively enriched in 4He (R/Ra slightly > 2), most probably sourced from U and Th decay in old igneous or crustal rocks. The two end-members mix, producing the range of R/Ra ratios observed in the other wells included in this study. Preliminary data on the ,11B signature of the Bacon Manito fluid separated from vapour range from 7, to 9,. These values suggest that the local magmatic rocks could represent the main boron source, in agreement with the boron isotopic signature of Pacific arc lavas. [source] Origin of deep saline groundwaters in the Vienne granitic rocks (France): constraints inferred from boron and strontium isotopesGEOFLUIDS (ELECTRONIC), Issue 2 2001J. Casanova Abstract As part of a preliminary geological characterization programme to assess the feasibility of an underground laboratory in granitic rock, a series of 17 deep boreholes (maximum depth, 900 m) was drilled by ANDRA in the Vienne district, France. A salinity gradient was demonstrated in the granitic waters with concentrations varying from approximately 1 g L,1 at 150 m depth at the top of the basement (beneath the sedimentary cover) to 10 g L,1 in the deeper part (from 400 to 600 m depth). Sr and B isotope ratios were measured in order to better understand the origin of the salinity and to evaluate the degree of water,rock interaction in the system. The results obtained were compared to those of mineral spring waters emerging from the granitic basement in the Massif Central. Evidence in support of a significant marine contribution include: (i) the Cl,Br investigations agree with a marine origin for the saline groundwaters without evolution from seawater; (ii) the 87Sr/86Sr ratio of the Vienne deep groundwaters (0.7078,0.7084) is in agreement with a palaeo-seawater isotopic signature; (iii) measured ,11B values for the deepest brine samples are enriched in 11B (up to 36.1,) relative to the granitic springs. The combined use of ,11B, Cl, B, Br, Sr contents and 87Sr/86Sr ratios makes it possible to define and quantify a mixing model between marine and crustal end-members in order to explain the origin of the deep saline groundwaters in the Vienne granitic rocks. [source] C4-derived soil organic carbon decomposes faster than its C3 counterpart in mixed C3/C4 soilsGLOBAL CHANGE BIOLOGY, Issue 10 2007JONATHAN G. WYNN Abstract The large difference in the degree of discrimination of stable carbon isotopes between C3 and C4 plants is widely exploited in global change and carbon cycle research, often with the assumption that carbon retains the carbon isotopic signature of its photosynthetic pathway during later stages of decomposition in soil and sediments. We applied long-term incubation experiments and natural 13C-labelling of C3 and C4-derived soil organic carbon (SOC) collected from across major environmental gradients in Australia to elucidate a significant difference in the rate of decomposition of C3- and C4-derived SOC. We find that the active pool of SOC (ASOC) derived from C4 plants decomposes at over twice the rate of the total pool of ASOC. As a result, the proportion of C4 photosynthesis represented in the heterotrophic CO2 flux from soil must be over twice the proportional representation of C4-derived biomass in SOC. This observation has significant implications for much carbon cycle research that exploits the carbon isotopic difference in these two photosynthetic pathways. [source] Linking microbial activity and soil organic matter transformations in forest soils under elevated CO2GLOBAL CHANGE BIOLOGY, Issue 2 2005S. A. Billings Abstract Soil organic matter (SOM) dynamics ultimately govern the ability of soil to provide long-term C sequestration and the nutrients required for ecosystem productivity. Predicting belowground responses to elevated CO2 requires an integrated understanding of SOM transformations and the microbial activity that governs them. It remains unclear how the microorganisms upon which these transformations depend will function in an elevated CO2 world. This study examines SOM transformations and microbial metabolism in soils from the Duke Free Air Carbon Enrichment site in North Carolina, USA. We assessed microbial respiration and net nitrogen (N) mineralization in soils with and without elevated CO2 exposure during a 100-day incubation. We also traced the depleted C isotopic signature of the supplemental CO2 into SOM and the soils' phospholipid fatty acids (PLFA), which serve as biomarkers for living cells. Cumulative net N mineralization in elevated CO2 soils was 50% that in control soils after a 100-day incubation. Respiration was not altered with elevated CO2. C : N ratios of bulk SOM did not change with elevated CO2, but incubation data suggest that the C : N ratios of mineralized organic matter increased with elevated CO2. Values of SOM ,13C were depleted with elevated CO2 (,26.7±0.2 vs. ,30.2±0.3,), reflecting the depleted signature of the supplemental CO2. We compared ,13C of individual PLFA with the ,13C of SOM to discern incorporation of the depleted C isotopic signature into soil microbial groups in elevated CO2 plots. PLFA i15:0, a15:0, and 10Met18:0 reflected significant incorporation of recently produced photosynthate, suggesting that the bacterial groups defined by these biomarkers are active metabolizers in elevated CO2 soils. At least one of these groups (actinomycetes, 10Met18:0) specializes in metabolizing less labile substrates. Because control plots did not receive an equivalent 13C tracer, we cannot determine from these data whether this group of organisms was stimulated by elevated CO2 compared with these organisms in control soils. Stimulation of this group, if it occurred in the elevated CO2 plot, would be consistent with a decline in the availability of mineralizable organic matter with elevated CO2, which incubation data suggest may be the case in these soils. [source] Effects of climate change on labile and structural carbon in Douglas-fir needles as estimated by ,13C and Carea measurementsGLOBAL CHANGE BIOLOGY, Issue 11 2002ERIC A. HOBBIE Abstract Models of photosynthesis, respiration, and export predict that foliar labile carbon (C) should increase with elevated CO2 but decrease with elevated temperature. Sugars, starch, and protein can be compared between treatments, but these compounds make up only a fraction of the total labile pool. Moreover, it is difficult to assess the turnover of labile carbon between years for evergreen foliage. Here, we combined changes in foliar Carea (C concentration on an areal basis) as needles aged with changes in foliar isotopic composition (,13C) caused by inputs of 13C-depleted CO2 to estimate labile and structural C in needles of different ages in a four-year, closed-chamber mesocosm experiment in which Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings were exposed to elevated temperature (ambient + 3.5 °C) and CO2 (ambient + 179 ppm). Declines in ,13C of needle cohorts as they aged indicated incorporation of newly fixed labile or structural carbon. The ,13C calculations showed that new C was 41 ± 2% and 28 ± 3% of total needle carbon in second- and third-year needles, respectively, with higher proportions of new C in elevated than ambient CO2 chambers (e.g. 42 ± 2% vs. 37 ± 6%, respectively, for second-year needles). Relative to ambient CO2, elevated CO2 increased labile C in both first- and second-year needles. Relative to ambient temperature, elevated temperature diminished labile C in second-year needles but not in first-year needles, perhaps because of differences in sink strength between the two needle age classes. We hypothesize that plant-soil feedbacks on nitrogen supply contributed to higher photosynthetic rates under elevated temperatures that partly compensated for higher turnover rates of labile C. Strong positive correlations between labile C and sugar concentrations suggested that labile C was primarily determined by carbohydrates. Labile C was negatively correlated with concentrations of cellulose and protein. Elevated temperature increased foliar %C, possibly due to a shift of labile constituents from low %C carbohydrates to relatively high %C protein. Decreased sugar concentrations and increased nitrogen concentrations with elevated temperature were consistent with this explanation. Because foliar constituents that vary in isotopic signature also vary in concentrations with leaf age or environmental conditions, inferences of ci/ca values from ,13C of bulk leaf tissue should be done cautiously. Tracing of 13C through foliar carbon pools may provide new insight into foliar C constituents and turnover. [source] ,Distribution of oxygen-18 and deuterium in river waters across the United StatesHYDROLOGICAL PROCESSES, Issue 7 2001Carol Kendall Abstract Reconstruction of continental palaeoclimate and palaeohydrology is currently hampered by limited information about isotopic patterns in the modern hydrologic cycle. To remedy this situation and to provide baseline data for other isotope hydrology studies, more than 4800, depth- and width-integrated, stream samples from 391 selected sites within the USGS National Stream Quality Accounting Network (NASQAN) and Hydrologic Benchmark Network (HBN) were analysed for ,18O and ,2H (http://water.usgs.gov/pubs/ofr/ofr00-160/pdf/ofr00-160.pdf). Each site was sampled bimonthly or quarterly for 2·5 to 3 years between 1984 and 1987. The ability of this dataset to serve as a proxy for the isotopic composition of modern precipitation in the USA is supported by the excellent agreement between the river dataset and the isotopic compositions of adjacent precipitation monitoring sites, the strong spatial coherence of the distributions of ,18O and ,2H, the good correlations of the isotopic compositions with climatic parameters, and the good agreement between the ,national' meteoric water line (MWL) generated from unweighted analyses of samples from the 48 contiguous states of ,2H=8·11,18O+8·99 (r2=0·98) and the unweighted global MWL of sites from the Global Network for Isotopes in Precipitation (GNIP) of the International Atomic Energy Agency and the World Meteorological Organization (WMO) of ,2H=8·17,18O+10·35. The national MWL is composed of water samples that arise in diverse local conditions where the local meteoric water lines (LMWLs) usually have much lower slopes. Adjacent sites often have similar LMWLs, allowing the datasets to be combined into regional MWLs. The slopes of regional MWLs probably reflect the humidity of the local air mass, which imparts a distinctive evaporative isotopic signature to rainfall and hence to stream samples. Deuterium excess values range from 6 to 15, in the eastern half of the USA, along the northwest coast and on the Colorado Plateau. In the rest of the USA, these values range from ,2 to 6,, with strong spatial correlations with regional aridity. The river samples have successfully integrated the spatial variability in the meteorological cycle and provide the best available dataset on the spatial distributions of ,18O and ,2H values of meteoric waters in the USA. Published in 2001 by John Wiley & Sons, Ltd. [source] Photosynthetic pathways, spatial distribution, isotopic ecology, and implications for pre-Hispanic human diets in central-western ArgentinaINTERNATIONAL JOURNAL OF OSTEOARCHAEOLOGY, Issue 2 2009C. Llano Abstract In a number of areas around the world researchers have begun to use the isotopic values of subsistence resources as a means of determining diets of human populations. The objective of the present study is to classify the plant species present at distinct altitudes in southern Mendoza Province, Argentina, considering photosynthetic pathways in order to determine their ,13C isotopic signature. This will help to understand the relationships between diets and the isotopic values observed in archaeological human remains. Data compiled from various sources are used to establish the photosynthetic pathways and mean ,13C values. The results indicate that C3 species are dominant at high-altitude settings, and that the few identified C4 species were found primarily at lower altitudes. These results are intended to serve as a foundation for future isotopic studies designed to address relationships among species at different trophic levels. Copyright © 2009 John Wiley & Sons, Ltd. [source] Carbon and nitrogen stable isotopes as tracers of change in diet breadth during Middle and Upper Palaeolithic in EuropeINTERNATIONAL JOURNAL OF OSTEOARCHAEOLOGY, Issue 3-4 2004D. Drucker Abstract Carbon and nitrogen stable isotope ratios in fossil bone collagen have been used as evidence for an increase of diet breadth between Middle Palaeolithic Neanderthals and Early Upper Palaeolithic anatomically modern humans. In this paper, we revisit the rules of palaeodietary reconstruction using collagen stable isotopes and reassess the possible isotopic signatures of potential protein resources available to prehistoric humans. It appears that the interpretation of the human's isotopic signature does not necessarily imply a significant proportion of aquatic-derived protein in the diet neither for Neandertal nor for first anatomically modern humans in Europe. Exploitation of aquatic ecosystems by humans needs to be supported by further zooarchaeological evidence. Nevertheless, isotopic biogeochemistry of fossil human collagen can be very useful in palaeodietary reconstructions provided that basic rules are followed while selecting samples of coeval fauna, in order to establish the end members of different food resources. Significant progress investigating the evolution of subsistence strategies in fossil hominids is expected from a combination of zooarchaeological and isotopic data. Copyright © 2004 John Wiley & Sons, Ltd. [source] Differences in stable isotope composition within and among zooplanktivorous Utaka cichlid populations from Lake MalawiAFRICAN JOURNAL OF ECOLOGY, Issue 2 2010Dieter Anseeuw Abstract Stable isotope analysis was used to determine whether five sympatric zooplanktivorous cichlids of the Utaka assemblage from Lake Malawi vary in isotopic signature as an indication of possible differences in food resource composition. The isotopic composition of the Utaka in combination with literature data about diet composition suggests that these species exploit a narrow range of zooplanktonic prey types. At five sampling locations, significant differences in ,13C and ,15N were detected among species but a consistent pattern across locations was absent. Significant intraspecific differences between locations were found. These differences were relatively low in view of the large geographic distances between the populations and there was no consistent spatial pattern among the species. The observed differences may be indicative of local variations in diet composition, which may help in reducing niche overlap among these zooplanktivores. Résumé Nous avons utilisé une analyse des isotopes stables pour déterminer si cinq cichlides sympatriques zooplanctivores appartenant à l'assemblage Utaka du lac Malawi ont une signature isotopique différente, ce qui serait une indication de différences possibles dans la composition de leurs ressources alimentaires. La composition isotopique d'Utaka, combinée aux données publiées au sujet de la composition du régime alimentaire, suggère que ces espèces exploitent une gamme réduite de types de proies zooplanctoniques. À cinq endroits échantillonnés, on a détecté des différences significatives de ,13C et ,15N parmi les espèces, mais il n'y avait pas de schéma cohérent parmi tous les sites. On a découvert des différences intraspécifiques significatives entre les sites. Ces différences étaient relativement faibles compte tenu des grandes distances entre les populations et il n'y avait pas de schéma spatial cohérent parmi les espèces. Les différences observées peuvent indiquer des variations locales de la composition du régime alimentaire, ce qui pourrait aider à réduire le recouvrement des niches chez ces zooplanctivores. [source] Dispersal abilities of adult click beetles in arable land revealed by analysis of carbon stable isotopesAGRICULTURAL AND FOREST ENTOMOLOGY, Issue 3 2009Nikolaus Schallhart Abstract 1,The dispersal abilities of agrioted beetles, serious pests on a variety of crops, are poorly known under natural conditions. This hampers their control. We used, for the first time, a stable isotope approach to assess dispersal of adult Agriotes obscurus in arable land. 2,After a diet switch from a C3- to a C4-plant, carbon isotope ratios of A. obscurus larvae significantly changed towards the isotopic signature of the new diet. Moreover, the larval ,13C signatures were transferred to the wing covers of the adult beetles with little distortion. 3,To assess the dispersal abilities under natural conditions, pheromone traps, lured for Agriotes sp., were installed at two study sites in Western Austria. Each site comprised a maize field (= C4-plant) and adjacent C3-grasslands with traps established along a transect of increasing distance to the maize. 4,,13C signatures of wing covers revealed that adult male A. obscurus were able to migrate at least 80 m, which was the maximum distance that dispersal could be traced in the present study. The dispersal behaviour might have been influenced by site-specific factors. 5,The results obtained demonstrate a higher potential of adult male Agriotes to disperse than previously assumed. Moreover, the combination of pheromone trapping and stable isotope analysis proved to be an effective approach to study insect movement and dispersal in arable systems harbouring C3- and C4-crops. [source] POTENTIAL TOOLS FOR TRACKING OCEAN CLIMATE: VARIABILITY IN STABLE ISOTOPES IN LIVING CORALLINE ALGAEJOURNAL OF PHYCOLOGY, Issue 2000R.A. Dunn Our ability to track long term climate change in coastal regions is limited in temperate and polar regions. Physical oceanographic dynamics in temperature and upwelling events can be recorded as carbon and oxygen stable isotope signals in carbonate producing organisms. Because coralline algae photosynthesize, produce calcium carbonate and are widely distributed, they may provide a new tool for detecting short-term change. However, little is known about how coralline algae incorporate stable isotopes into their calcite thallus structure. The objectives of this study were to determine if growth and isotopic signature differ in articulated coralline algae grown in different oceanographic regimes in Monterey Bay. The articulated alga Calliarthron cheiliospororioides was outplanted at three locations varying in seawater temperature and upwelling strength. New algal growth was measured by staining the algae with Alizarin Red and enumerating the amount of accumulated material at the branch tips. Growth rates varied seasonally and spatially. Low-upwelling daily growth rates averaged 0.044,0.056 mm day,1, while high-upwelling growth rates were 0.083 mm day,1. Isotope ratios were obtained by analyzing microsampled portions of the alga in a mass spectrometer. Changes in the 18O/16O and 13C/12C ratios appear to reflect change in seawater temperature and upwelling strength, respectively. [source] Prediction of model pools for a long-term experiment using near-infrared spectroscopyJOURNAL OF PLANT NUTRITION AND SOIL SCIENCE, Issue 1 2010Kerstin Michel Abstract Fourty-one soil samples from the "Eternal Rye" long-term experiment in Halle, Germany, were used to test the usefulness of near-infrared spectroscopy (NIRS) to differentiate between C derived from C3 and C4 plants by using the isotopic signature (,13C) and to predict the pools considered in the Rothamsted Carbon (RothC) model, i.e., decomposable plant material, resistant plant material, microbial biomass, humified organic matter, and inert organic matter. All samples were scanned in the visible-light and near-infrared region (400,2500 nm). Cross-validation equations were developed using the whole spectrum (first to third derivative) and a modified partial least-square regression method. ,13C values and all pools of the RothC model were successfully predicted by NIRS as reflected by RSC values (ratio between standard deviation of the laboratory results and standard error of cross-validation) ranging from 3.2 to 3.4. Correlations analysis indicated that organic C can be excluded as basis for the successful predictions by NIRS in most cases, i.e., 11 out of 16. [source] Stable carbon isotopes as an indicator for soil degradation in an alpine environment (Urseren Valley, Switzerland)RAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 10 2009Monika Schaub Analyses of soil organic carbon (SOC) content and stable carbon isotope signatures (,13C) of soils were assessed for their suitability to detect early stage soil erosion. We investigated the soils in the alpine Urseren Valley (southern central Switzerland) which are highly impacted by soil erosion. Hill slope transects from uplands (cambisols) to adjacent wetlands (histosols and histic to mollic gleysols) differing in their intensity of visible soil erosion, and reference wetlands without erosion influence were sampled. Carbon isotopic signature and SOC content of soil depth profiles were determined. A close correlation of ,13C and carbon content (r,>,0.80) is found for upland soils not affected by soil erosion, indicating that depth profiles of ,13C of these upland soils mainly reflect decomposition of SOC. Long-term disturbance of an upland soil is indicated by decreasing correlation of ,13C and SOC (r,,,0.80) which goes in parallel with increasing (visible) damage at the site. Early stage soil erosion in hill slope transects from uplands to adjacent wetlands is documented as an intermediate ,13C value (,27.5,) for affected wetland soil horizons (0,12,cm) between upland (aerobic metabolism, relatively heavier ,13C of ,26.6,) and wetland isotopic signatures (anaerobic metabolism, relatively lighter ,13C of ,28.6,). Carbon isotopic signature and SOC content are found to be sensitive indicators of short- and long-term soil erosion processes. Copyright © 2009 John Wiley & Sons, Ltd. [source] Basin geochemistry and isotopic ratios of fishes and basal production sources in four neotropical riversECOLOGY OF FRESHWATER FISH, Issue 3 2007David B. Jepsen Abstract,,, We analysed stable carbon and nitrogen isotopic ratios of dissolved inorganic carbon (DIC), plants, detritus and fishes to estimate the relative importance of dominant production sources supporting food webs of four Venezuelan rivers with divergent geochemical and watershed characteristics. Based on samples taken during the dry season at each site, fishes from two nutrient-poor, blackwater rivers had significantly lower ,13C values (mean = ,31.4, and ,32.9,) than fishes from more productive clearwater and whitewater rivers (mean = ,25.2, and ,25.6, respectively). Low carbon isotopic ratios of fishes from blackwaters were likely influenced by low ,13C of DIC assimilated by aquatic primary producers. Although floodplains of three savanna rivers supported high biomass of C4 grasses, relatively little carbon from this source appeared to be assimilated by fishes. Most fishes in each system assimilated carbon derived mostly from a combination of microalgae and C3 macrophytes, two sources with broadly overlapping carbon isotopic signatures. Even with this broad overlap, several benthivorous grazers from blackwater and whitewater rivers had isotopic values that aligned more closely with algae. We conclude that comparative stable isotopic studies of river biota need to account for watershed geochemistry that influences the isotopic composition of basal production sources. Moreover, isotopic differences between river basins can provide a basis for discriminating spatial and temporal variation in the trophic ecology of fishes that migrate between watersheds having distinct geochemical characteristics. [source] Methane-derived carbon flows through methane-oxidizing bacteria to higher trophic levels in aquatic systemsENVIRONMENTAL MICROBIOLOGY, Issue 5 2007Peter Deines Summary Recent investigations have shown that biogenic methane can be a carbon source for macro invertebrates in freshwater food webs. Stable carbon isotopic signatures, used to infer an organism's food source, indicated that methane can play a major role in the nutrition of chironomid larvae. However, the pathway of methane-derived carbon into invertebrate biomass is still not confirmed. It has been proposed that chironomid larvae ingest methane-oxidizing bacteria (MOB), but this has not been experimentally demonstrated to date. Using 13C-labelled methane we could show for the first time that chironomid larvae assimilate methane-derived carbon through MOB. Chironomid larval biomass was significantly 13C-enriched after dwelling for 10 days in lake sediment enriched with labelled methane. Moreover, phospholipid fatty acids diagnostic for MOB were detected in larval tissue and were significantly 13C-enriched, which encompasses the 13C-uptake predicted for a methane-based nutrition. Additionally, chironomid larvae fed on sediment and water-column derived MOB biomass. [source] Microbiological investigation of methane- and hydrocarbon-discharging mud volcanoes in the Carpathian Mountains, RomaniaENVIRONMENTAL MICROBIOLOGY, Issue 4 2006Karine Alain Summary Paclele Mici is a terrestrial mud volcano field located in the Carpathian Mountains (Romania), where thermal alteration of sedimentary organic compounds leads to methane, higher hydrocarbons and other petroleum compounds that are continuously released into the environment. The hydrocarbons represent potential substrates for microorganisms. We studied lipid biomarkers, stable isotope ratios, the effect of substrate (methane, other organic compounds) addition and 16S rRNA genes to gain insights into the hitherto unknown microbial community at this site. Quantitative real-time polymerase chain reaction analysis demonstrated that bacteria were much more abundant than archaea. Phylogenetic analyses of 16S rDNA clone sequences indicated the presence of bacterial and archaeal lineages generally associated with the methane cycle (methanogens, aerobic and anaerobic methanotrophs), the sulfur cycle (sulfate reducers), and groups linked to the anaerobic degradation of alkanes or aromatic hydrocarbons. The presence of sulfate reducers, methanogens and methanotrophs in this habitat was also confirmed by concurrent surveys of lipid biomarkers and their isotopic signatures. Incubation experiments with several common and complex substrates revealed the potential of the indigenous microbial community for sulfate reduction, methanogenesis and aerobic methanotrophy. Additionally, consistently to the detection of methane-oxidizing archaea (ANME) and 13C-depleted archaeal lipids, a weak but significant activity of anaerobic methane oxidation was measured by radiotracer techniques and in vitro. This survey is the first to report the presence and activity of ANME in a terrestrial environment. [source] The dynamics of organic matter in rock fragments in soil investigated by 14C dating and measurements of 13CEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 1 2002A. Agnelli Summary Rock fragments in soil can contain significant amounts of organic carbon. We investigated the nature and dynamics of organic matter in rock fragments in the upper horizons of a forest soil derived from sandstone and compared them with the fine earth fraction (<2 mm). The organic C content and its distribution among humic, humin and non-humic fractions, as well as the isotopic signatures (,14C and ,13C) of organic carbon and of CO2 produced during incubation of samples, all show that altered rock fragments contain a dynamic component of the carbon cycle. Rock fragments, especially the highly altered ones, contributed 4.5% to the total organic C content in the soil. The bulk organic matter in both fine earth and highly altered rock fragments in the A1 horizon contained significant amounts of recent C (bomb 14C), indicating that most of this C is cycled quickly in both fractions. In the A horizons, the mean residence times of humic substances from highly altered rock fragments were shorter than those of the humic substances isolated in the fine earth. Values of ,14C of the CO2 produced during basal respiration confirmed the heterogeneity, complexity and dynamic nature of the organic matter of these rock fragments. The weak 14C signatures of humic substances from the slightly altered rock fragments confirmed the importance of weathering in establishing and improving the interactions between rock fragments and surrounding soil. The progressive enrichment in 13C from components with high- 14C (more recent) to low- 14C (older) indicated that biological activity occurred in both the fine and the coarse fractions. Hence the microflora utilizes energy sources contained in all the soil compartments, and rock fragments are chemically and biologically active in soil, where they form a continuum with the fine earth. [source] Incorporating life histories and diet quality in stable isotope interpretations of crustacean zooplanktonFRESHWATER BIOLOGY, Issue 7 2008MARC VENTURA Summary 1. Stable isotope studies have been extremely useful for improving general food web descriptions due to their ability to simultaneously summarize complex trophic networks and track the energy flow through them. However, when considering trophic relationships involving only two or few species, application of general isotopic interpretations based on average fractionation values may easily lead to misleading conclusions. In these cases a more accurate consideration of the current processes involved in the isotopic fractionation should be considered. 2. We investigated the trophic relationships of the crustacean zooplankton assemblage in an alpine lake (Lake Redon, Pyrenees) by means of stable isotopes of carbon and nitrogen and applied information on their life history and biochemical composition in the interpretation. 3. The three species occurring in the lake had distinct isotopic signatures: the two copepod species (the cyclopoid Cyclops abyssorum and the calanoid Diaptomus cyaneus) had higher nitrogen isotopic composition than the cladoceran (Daphnia pulicaria), indicative of a higher trophic position of the two copepods. Most intra-specific isotopic variability was associated with growth, while the effect of metabolic turnover was negligible. The effects of changes in the proportion of lipids was restricted to the adults of the two copepods. 4. Daphnia Juveniles showed ontogenetic shifts in their carbon, and nitrogen isotopic composition. Cyclops copepodites only showed changes in carbon isotopic composition. These isotopic shifts with changes in size were the result of: (i) the prevalence of growth over metabolic turnover as the main factor for isotopic variability and (ii) feeding, during the growth period, on isotopically depleted food in the case of Daphnia, and on isotopically enriched food in the case of Cyclops. 5. The carbon isotopic variation in Cyclops juveniles could be explained by fitting an isotopic growth model that considered that they fed entirely on Daphnia. However this was not the case for nitrogen isotopic variability. Cyclops nitrogen isotopic composition variation and the Cyclops to Daphnia nitrogen isotopic enrichment were closely correlated to the quantity of Daphnia protein and to the dissimilarity in the essential amino acid composition between the two species, which can be interpreted as an indication of consumer nitrogen limitation. [source] Taxon-specific variation in the stable isotopic signatures (,13C and ,15N) of lake phytoplanktonFRESHWATER BIOLOGY, Issue 5 2006KRISTIINA VUORIO Summary 1. The variability in the stable isotope signatures of carbon and nitrogen (,13C and ,15N) in different phytoplankton taxa was studied in one mesotrophic and three eutrophic lakes in south-west Finland. The lakes were sampled on nine to 16 occasions over 2,4 years and most of the time were dominated by cyanobacteria and diatoms. A total of 151 taxon-specific subsamples covering 18 different phytoplankton taxa could be isolated by filtration through a series of sieves and by flotation/sedimentation, followed by microscopical identification and screening for purity. 2. Substantial and systematic differences between phytoplankton taxa, seasons and lakes were observed for both ,13C and ,15N. The values of ,13C ranged from ,34.4, to ,5.9, and were lowest in chrysophytes (,34.4, to ,31.3,) and diatoms (,30.6, to ,26.6,). Cyanobacteria were most variable (,32.4, to ,5.9,), including particularly high values in the nostocalean cyanobacterium Gloeotrichia echinulata (,14.4, to ,5.9,). For ,13C, the taxon-specific amplitude of temporal changes within a lake was usually <1,8, (<1,4, for microalgae alone and <1,8, for cyanobacteria alone), whereas the amplitude among taxa within a water sample was up to 31,. 3. The values of ,15N ranged from ,2.1, to 12.8, and were high in chrysophytes, dinophytes and diatoms, but low in the nitrogen-fixing cyanobacteria Anabaena spp., Aphanizomenon spp. and G. echinulata (,2.1, to 1.6,). Chroococcalean cyanobacteria ranged from ,1.4, to 8.9,. For ,15N, the taxon-specific amplitude of temporal changes within a lake was 2,6,, (2,6, for microalgae alone and 2,4, for cyanobacteria alone) and the amplitude among taxa within a water sample was up to 11,. 4. The isotopic signatures of phytoplankton changed systematically with their physical and chemical environment, most notably with the concentrations of nutrients, but correlations were non-systematic and site-specific. 5. The substantial variability in the isotopic signatures of phytoplankton among taxa, seasons and lakes complicates the interpretation of isotopic signatures in lacustrine food webs. However, taxon-specific values and seasonal patterns showed some consistency among years and may eventually be predictable. [source] Lipid biomarker and carbon isotopic signatures for stromatolite-forming, microbial mat communities and Phormidium cultures from Yellowstone National ParkGEOBIOLOGY, Issue 1 2004Linda L. Jahnke ABSTRACT The molecular and isotopic compositions of lipid biomarkers from cultured filamentous cyanobacteria (Phormidium, also known as Leptolyngbya) have been used to investigate the community and trophic structure of photosynthetic mats from alkaline hot springs of the Lower Geyser Basin at Yellowstone National Park. We studied a shallow-water coniform mat from Octopus Spring (OS) and a submerged, tufted mat from Fountain Paint Pots (FPP) and found that 2-methylhopanepolyols and mid-chain branched methylalkanes were diagnostic for cyanobacteria, whereas abundant wax esters were representative of the green non-sulphur bacterial population. The biomarker composition of cultured Phormidium -isolates varied, but was generally representative of the bulk mat composition. The carbon isotopic fractionation for biomass relative to dissolved inorganic carbon (DIC; ,CO2) for cultures grown with 1% CO2 ranged from 21.4 to 26.1 and was attenuated by diffusion limitation associated with filament aggregation (i.e. cell clumping). Isotopic differences between biomass and lipid biomarkers, and between lipid classes, depended on the cyanobacterial strain, but was positively correlated with overall fractionation. Acetogenic lipids (alkanes and fatty acids) were generally more depleted than isoprenoids (phytol and hopanoids). The ,13CTOC for OS and FPP mats were somewhat heavier than for cultures (,16.9 and ,23.6, respectively), which presumably reflects the lower availability of DIC in the natural environment. The isotopic dispersions among cyanobacterial biomarkers, biomass and DIC reflected those established for culture experiments. The 7-methyl- and 7,11-dimethylheptadecanes were from 9 to 11 depleted relative to the bulk organic carbon, whereas 2-methylhopanols derived from the oxidation-reduction of bacteriohopanepolyol were enriched relative to branched alkanes by approximately 5,7. These isotopic relationships survived with depth and indicated that the relatively heavy isotopic composition of the OS mat resulted from diffusion limitation. This study supports the suggestion that culture studies can establish valid isotopic relationships for interpretation of trophic structure in modern and ancient microbial ecosystems. [source] Noble gas and boron isotopic signatures of the Bacon-Manito geothermal fluid, PhilippinesGEOFLUIDS (ELECTRONIC), Issue 4 2008F. E. B. BAYON Abstract Noble gas isotopic composition and abundances were determined on dry gas sampled in geothermal wells from the Bacon-Manito (BGPF) geothermal field in the Philippines. The most significant findings come from the 3He/4He ratio; a mantle-He source is evidenced by ratio close to 7 Ra. Peripheral fluid from the west and south of the geothermal system is relatively enriched in 4He (R/Ra slightly > 2), most probably sourced from U and Th decay in old igneous or crustal rocks. The two end-members mix, producing the range of R/Ra ratios observed in the other wells included in this study. Preliminary data on the ,11B signature of the Bacon Manito fluid separated from vapour range from 7, to 9,. These values suggest that the local magmatic rocks could represent the main boron source, in agreement with the boron isotopic signature of Pacific arc lavas. [source] Chemical and isotopic signatures of Na/HCO3/CO2 -rich geofluids, North PortugalGEOFLUIDS (ELECTRONIC), Issue 4 2006J. M. MARQUES Abstract Geochemical and isotopic studies have been undertaken to assess the origin of CO2 -rich waters issuing in the northern part of Portugal. These solutions are hot (76°C) to cold (17°C) Na,HCO3 mineral waters. The ,2H and ,18O signatures of the mineral waters reflect the influence of altitude on meteoric recharge. The lack of an 18O-shift indicates there has been no high temperature water,rock interaction at depth, corroborating the results of several chemical geothermometers (reservoir temperature of about 120°C). The low 14C activity (up to 9.9 pmC) measured in some of the cold CO2 -rich mineral waters (total dissolved inorganic carbon) is incompatible with the presence of 3H (from 1.7 to 4.1 TU) in those waters, which indicates relatively short subsurface circulation times. The ,13C values of CO2 gas and dissolved inorganic carbon range between ,6, and ,1, versus Vienna-Peedee Belemnite, indicating that the total carbon in the recharge waters is being diluted by larger quantities of CO2 (14C-free) introduced from deep-seated (upper mantle) sources, masking the 14C-dating values. The differences in the 87Sr/86Sr ratios of the studied thermal and mineral waters seem to be caused by water,rock interaction with different granitic rocks. Chlorine isotope signatures (,0.4, < ,37Cl < +0.4, versus standard mean ocean chloride) indicate that Cl in these waters could be derived from mixing of a small amount of igneous Cl from leaching of granitic rocks. [source] Ancient hydrocarbon seeps from the Mesozoic convergent margin of California: carbonate geochemistry, fluids and palaeoenvironmentsGEOFLUIDS (ELECTRONIC), Issue 2 2002K. A. Campbell Abstract More than a dozen hydrocarbon seep-carbonate occurrences in late Jurassic to late Cretaceous forearc and accretionary prism strata, western California, accumulated in turbidite/fault-hosted or serpentine diapir-related settings. Three sites, Paskenta, Cold Fork of Cottonwood Creek and Wilbur Springs, were analyzed for their petrographic, geochemical and palaeoecological attributes, and each showed a three-stage development that recorded the evolution of fluids through reducing,oxidizing,reducing conditions. The first stage constituted diffusive, reduced fluid seepage (CH4, H2S) through seafloor sediments, as indicated by Fe-rich detrital micrite, corroded surfaces encrusted with framboidal pyrite, anhedral yellow calcite and negative cement stable isotopic signatures (,13C as low as ,35.5, PDB; ,18O as low as ,10.8, PDB). Mega-invertebrates, adapted to reduced conditions and/or bacterial chemosymbiosis, colonized the sites during this earliest period of fluid seepage. A second, early stage of centralized venting at the seafloor followed, which was coincident with hydrocarbon migration, as evidenced by nonluminescent fibrous cements with ,13C values as low as ,43.7, PDB, elevated ,18O (up to +2.3, PDB), petroleum inclusions, marine borings and lack of pyrite. Throughout these early phases of hydrocarbon seepage, microbial sediments were preserved as layered and clotted, nondetrital micrites. A final late-stage of development marked a return to reducing conditions during burial diagenesis, as implied by pore-associated Mn-rich cement phases with bright cathodoluminescent patterns, and negative ,18O signatures (as low as ,14, PDB). These recurring patterns among sites highlight similarities in the hydrogeological evolution of the Mesozoic convergent margin of California, which influenced local geochemical conditions and organism responses. A comparison of stable carbon and oxygen isotopic data for 33 globally distributed seep-carbonates, ranging in age from Devonian to Recent, delineated three groupings that reflect variable fluid input, different tectono-sedimentary regimes and time,temperature-dependent burial diagenesis. [source] Development of Cu and Zn Isotope MC-ICP-MS Measurements: Application to Suspended Particulate Matter and Sediments from the Scheldt EstuaryGEOSTANDARDS & GEOANALYTICAL RESEARCH, Issue 2 2008Jérôme C.J. Petit isotopes de Cu et Zn; interférences spectrales et non spectrales; fractionnement de masse instrumental; MC-ICP-MS; sédiments The present study evaluates several critical issues related to precision and accuracy of Cu and Zn isotopic measurements with application to estuarine particulate materials. Calibration of reference materials (such as the IRMM 3702 Zn) against the JMC Zn and NIST Cu reference materials were performed in wet and/or dry plasma modes (Aridus I and DSN-100) on a Nu Plasma MC-ICP-MS. Different mass bias correction methods were compared. More than 100 analyses of certified reference materials suggested that the sample-calibrator bracketing correction and the empirical external normalisation methods provide the most reliable corrections, with long term external precisions of 0.06 and 0.07, (2SD), respectively. Investigation of the effect of variable analyte to spike concentration ratios on Zn and Cu isotopic determinations indicated that the accuracy of Cu measurements in dry plasma is very sensitive to the relative Cu and Zn concentrations, with deviations of ,65Cu from ,0.4, (Cu/Zn = 4) to +0.4, (Cu/Zn = 0.2). A quantitative assessment (with instrumental mass bias corrections) of spectral and non-spectral interferences (Ti, Cr, Co, Fe, Ca, Mg, Na) was performed. Titanium and Cr were the most severe interfering constituents, contributing to inaccuracies of ,5.1, and +0.60, on ,68/64Zn, respectively (for 500 ,g l,1 Cu and Zn standard solutions spiked with 1000 ,g l,1 of Ti or Cr). Preliminary isotopic results were obtained on contrasting sediment matrices from the Scheldt estuary. Significant isotopic fractionation of zinc (from 0.21, to 1.13, for ,66Zn) and copper (from ,0.38, to 0.23, for ,65Cu), suggest a control by physical mixing of continental and marine water masses, characterized by distinct Cu and Zn isotopic signatures. These results provide a stepping-stone to further evaluate the use of Cu and Zn isotopes as biogeochemical tracers in estuarine environments. L'étude présentée ici porte sur l'évaluation critique d'un certain nombre de paramètres contrôlant la précision et la justesse des mesures des isotopes de Cu et Zn, dans le cadre d'une application à du matériel particulaire estuarien. Une calibration de matériaux de référence (tels que le Zn IRMM 3702) par rapport aux matériaux de référence JMC Zn et NIST Cu a été effectuée avec des plasmas humides et secs (avec Aridus I et DSN-100) sur un MC-ICP-MS Nu. Différentes méthodes de correction de biais de masse ont été comparées. Plus de 100 analyses de matériaux de référence certifiés ont montré que la correction par l'intercalation d'un calibrateur entre chaque échantillon et la calibration externe empirique fournissaient les corrections les plus fiables, avec des précisions externes sur le long terme de 0.06 et 0.07, (2SD) respectivement. Les effets de la variation des rapports de concentrations entre analyte et spike sur les mesures des rapports isotopiques de Cu et Zn ont montré que la justesse des mesures pour Cu en plasma sec est très tributaire des concentrations relatives de Cu et Zn, avec des déviations de ,65Cu allant de ,0.4, (Cu/Zn = 4) à+0.4, (Cu/Zn = 0.2). Une estimation quantitative des interférences spectrales et non spectrales (Ti, Cr, Co, Fe, Ca, Mg, Na) a été faite. Ti et Cr se sont révélés être les constituants interférents les plus importants pouvant entraîner des erreurs de ,5.1, et +0.60, sur ,68/64Zn respectivement (pour des solutions standards à 500 ,g l,1 de Cu et Zn dopées avec 1000 ,g l,1 de Ti ou Cr). Des données isotopiques préliminaires ont été obtenues sur des matrices sédimentaires très différentes provenant de l'estuaire de Scheldt. Les fractionnements significatifs du zinc (de 0.21,à 1.13, pour ,66Zn) et du cuivre (de ,0.38,à 0.23, pour ,65Cu) suggèrent un contrôle par un processus physique de mélange entre des masses d'eaux continentales et marines ayant des signatures isotopiques de Cu et Zn distinctes. Ces résultats constituent un tremplin vers une utilisation future des isotopes de Cu et Zn comme traceurs biogéochimiques des environnements estuariens. [source] Geochemical Tracers to Evaluate Hydrogeologic Controls on River SalinizationGROUND WATER, Issue 3 2008Stephanie J. Moore The salinization of rivers, as indicated by salinity increases in the downstream direction, is characteristic of arid and semiarid regions throughout the world. Historically, salinity increases have been attributed to various mechanisms, including (1) evaporation and concentration during reservoir storage, irrigation, and subsequent reuse; (2) displacement of shallow saline ground water during irrigation; (3) erosion and dissolution of natural deposits; and/or (4) inflow of deep saline and/or geothermal ground water (ground water with elevated water temperature). In this study, investigation of salinity issues focused on identification of relative salinity contributions from anthropogenic and natural sources in the Lower Rio Grande in the New Mexico-Texas border region. Based on the conceptual model of the system, the various sources of water and, therefore, salinity to the Lower Rio Grande were identified, and a sampling plan was designed to characterize these sources. Analysis results for boron (,11B), sulfur (,34S), oxygen (,18O), hydrogen (,2H), and strontium (87Sr/86Sr) isotopes, as well as basic chemical data, confirmed the hypothesis that the dominant salinity contributions are from deep ground water inflow to the Rio Grande. The stable isotopic ratios identified the deep ground water inflow as distinctive, with characteristic isotopic signatures. These analyses indicate that it is not possible to reproduce the observed salinization by evapotranspiration and agricultural processes alone. This investigation further confirms that proper application of multiple isotopic and geochemical tracers can be used to identify and constrain multiple sources of solutes in complex river systems. [source] Carbon and nitrogen stable isotopes as tracers of change in diet breadth during Middle and Upper Palaeolithic in EuropeINTERNATIONAL JOURNAL OF OSTEOARCHAEOLOGY, Issue 3-4 2004D. Drucker Abstract Carbon and nitrogen stable isotope ratios in fossil bone collagen have been used as evidence for an increase of diet breadth between Middle Palaeolithic Neanderthals and Early Upper Palaeolithic anatomically modern humans. In this paper, we revisit the rules of palaeodietary reconstruction using collagen stable isotopes and reassess the possible isotopic signatures of potential protein resources available to prehistoric humans. It appears that the interpretation of the human's isotopic signature does not necessarily imply a significant proportion of aquatic-derived protein in the diet neither for Neandertal nor for first anatomically modern humans in Europe. Exploitation of aquatic ecosystems by humans needs to be supported by further zooarchaeological evidence. Nevertheless, isotopic biogeochemistry of fossil human collagen can be very useful in palaeodietary reconstructions provided that basic rules are followed while selecting samples of coeval fauna, in order to establish the end members of different food resources. Significant progress investigating the evolution of subsistence strategies in fossil hominids is expected from a combination of zooarchaeological and isotopic data. Copyright © 2004 John Wiley & Sons, Ltd. [source] Diet reconstruction and historic population dynamics in a threatened seabirdJOURNAL OF APPLIED ECOLOGY, Issue 4 2007D. RYAN NORRIS Summary 1For the overwhelming majority of species, we lack long-term information on the dynamics of populations. As a consequence, we face considerable uncertainty about how to discriminate among competing hypotheses of population decline and design conservation plans. 2The marbled murrelet Brachyramphus marmoratus is a small seabird that nests in coastal old-growth forest but feeds year-round in near-shore waters of the north-eastern Pacific. Although a decline in nesting habitat is the primary reason why marbled murrelets are listed as threatened in Canada, nest predation and food availability may also influence population abundance. To examine the hypothesis that murrelet populations are influenced by variation in diet quality, we analysed stable-carbon and -nitrogen isotopes in feathers of museum specimens collected in the Georgia Basin, British Columbia. 3Between 1889 and 1996, we found a decline in stable isotopic signatures that was approximately equal to a 62% drop in trophic feeding level. We also found that the estimated proportion of fish in murrelet diet was related closely to murrelet abundance over the past 40 years, as estimated from volunteer surveys. Using these isotopic data, we modelled population size as a function of variation in reproductive rate due to changes in diet quality and found that our model matched closely the 40-year field estimates. We then applied our 107-year isotopic record to the model to back-cast estimates of population growth rate to 1889. 4Our results suggest that, up to the 1950s, murrelet populations in the Georgia Basin were capable of growing and were probably limited by factors other than diet quality. After this period, however, our results imply that murrelets were often, but not solely, limited by diet quality. 5Synthesis and applications. Protecting nesting habitat may not be sufficient to rebuild populations of this highly secretive and threatened seabird and recovery might also require the restoration of marine habitat quality, as well as a better understanding of how ocean climate affects prey abundance and reproductive rate. Combined with contemporary demographic data, stable isotope analysis of historic samples provides a unique opportunity to reconstruct population histories for species where we lack long-term information. [source] Intra-lake stable isotope ratio variation in selected fish species and their possible carbon sources in Lake Kyoga (Uganda): implications for aquatic food web studiesAFRICAN JOURNAL OF ECOLOGY, Issue 3 2010Dismas Mbabazi Abstract The stable isotopes of nitrogen (,15N) and carbon (,13C) provide powerful tools for quantifying trophic relationships and carbon flow to consumers in food webs; however, the isotopic signatures of organisms vary within a lake. Assessment of carbon and nitrogen isotopic signatures in a suite of plants, invertebrates, and fishes in Lake Kyoga, indicated significant variation between two sites for ,13C (paired t = 6.305; df = 14, P < 0.001 and ,15N paired t = 1.292; df = 14; P < 0.05). The fish fauna in Bukungu was generally more 13C enriched (mean ,13C = ,16.37 ± 1.64,) than in Iyingo (mean ,13C = ,20.80 ± 2.41,) but more ,15N depleted (mean ,15N = 5.57 ± 0.71,) than in Iyingo (mean ,15N = 6.92 ± 0.83,). The simultaneous shifts in phytoplankton and consumer signatures confirmed phytoplankton as the major source of carbon for the food chain leading to fish. Limited sampling coverage within lakes may affect lake wide stable isotope signatures, and the same error is transferred into trophic position estimation. Consideration of potential intra-lake spatial variability in isotope ratios and size is essential in evaluating the spatial and trophic structure of fish assemblages. Résumé Les isotopes stables d'azote (,15N) et de carbone (,13C) sont des outils intéressants pour quantifier les relations trophiques et le flux de carbone vers les consommateurs de chaînes alimentaires; cependant, la signature isotopique des organismes varie au sein d'un même lac. L'évaluation des signatures isotopiques du carbone et de l'azote dans une suite de plantes, d'invertébrés et de poissons du lac Kyoga indiquait une variation significative entre deux sites pour ,13C (test t apparié = 6.305; df = 14; P < 0.05). La faune piscicole de Bukungu était généralement plus enrichie en ,13C (moyenne de ,13C = ,16.37 ± 1.64,) qu'à Iyingo (moyenne de ,13C = ,20.80 ± 2.41,) mais plus dépourvue de ,15N (moyenne de ,15N = 5.57 ± 0.71,) qu'Inyingo (moyenne de ,15N = 6.92 ± 0.83,). Les glissements simultanés des signatures du phytoplancton et des consommateurs confirmaient que le phytoplancton est la source principale de carbone de la chaîne alimentaire qui aboutit aux poissons. Une couverture limitée de l'échantillonnage dans les lacs peut affecter la signature des isotopes stables de tout le lac, et cette même erreur est reportée dans l'estimation de la situation trophique. Il est essentiel de tenir compte de la variabilité spatiale possible des taux et de la taille des isotopes dans les lacs lorsque l'on évalue la structure spatiale et trophique des assemblages de poissons. [source] DOLOMITIZATION OF THE EARLY EOCENE JIRANI DOLOMITE FORMATION, GABES-TRIPOLI BASIN, WESTERN OFFSHORE, LIBYAJOURNAL OF PETROLEUM GEOLOGY, Issue 4 2000I. Y. Mriheel Dolomitization in the early Eocene Jirani Formation in the Gabes-Tripoli Basin (offshore western Libya) occurred in two stages. Stage I dolomites are composed of two types, one associated with anhydrite (Type I) the other anhydrite free (Type II,). The stratigraphic and sedimentological settings together with petrographic and geochemical criteria suggest that dolomitization was effected by refluxed evaporative seawater. Stable isotope and trace element analyses suggest dolomitization of both Types from a fluid of near-surface seawater composition under oxidising conditions modified by evaporation. Non-luminescence and lack ofzonation of all the dolomite indicate that the dolomitizing fluids maintained a relatively constant composition. The geologic setting during the early Eocene, interpreted as hypersaline lagoon, supports an evaporative reflux origin for the anhydritic dolomite Type I. Type II developed under less saline conditions in the transition zone between lagoon and open marine shelf. Stage II dolomitization is recorded by negative isotope values in both Types I and II indicating their dissolution and recrystallization (neomorphism) by dilute solutions. A period of exposure of the overlying Jdeir Formation following a relative sea-level fall allowed ingress of meteoric waters into both the Jdeir and the underlying Jirani Formations. Flushing by meteoric waters also resulted in development of excellent secondaly porosity and caused major dissolution of anhydrite to form the anhydritic-free dolomite facies typical of Type II. Following, and possibly during, both Stages I and II, low temperature dolomites (Type IIIa) precipitated in pore spaces from residual jluids at shallow burial depths, partially occluding porosity. In the late stage of basin evolution, medium clystalline, pore-filling saddle dolomite precipitated, causing some filling of mouldic and vuggy porosity (Type IIIb). Very light oxygen isotopic signatures confirm that it developed from high temperature fluids during deep burial diagenesis. Calculation of temperatures and timings of the dolomitization and cement phases show that the main dolomitization phases and Type IIIa cements occurred in the early Eocene, and that the saddle dolomite precipitated in the Miocene; these results are consistent with age relationships established from stratigraphic, petrographic and geochemical signatures. The most common porosity includes intercrystal, vuggy and mouldic types. Porosity is both pre-dolomitization and syn-dolomitization in origin, but the latter is the most dominant. Hence, reservoir quality is largely controlled by fluid dynamics. [source] | |||||||||||||||||||