Article

Hydrogen isotope assimilation and discrimination in green turtles

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  • NOAA - Southwest Fisheries Science Center
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Abstract

Although hydrogen isotopes (δ ² H) are commonly used as tracers of animal movement, minimal research has investigated the use of δ ² H as a proxy to quantify resource and habitat use. While carbon and nitrogen are ultimately derived from a single source (food), the proportion of hydrogen in consumer tissues originates from two distinct sources: body water and food. Before hydrogen isotopes can be effectively used as a resource and habitat tracer, we need estimates of (net) discrimination factors (Δ ² H Net ) that account for the physiologically mediated differences in the δ ² H values of animal tissues relative to that of the food and water sources they use to synthesize tissues. Here we estimated Δ ² H Net in captive green turtles ( Chelonia mydas ) by measuring δ ² H values of tissues (epidermis and blood components) and dietary macromolecules collected in two controlled feeding experiments. Tissue δ ² H and Δ ² H Net values varied systematically among tissues, with epidermis having higher δ ² H and Δ ² H Net values than blood components, which mirrors patterns between keratinaceous tissues (feathers, hair) and blood in birds and mammals. Serum/plasma of adult female green turtles had significantly lower δ ² H values compared to that of juveniles, likely due increased lipid mobilization associated with reproduction. This is the first study to quantify Δ ² H Net values in a marine ectotherm, and we anticipate our results will further refine the use of δ ² H analysis to better understand animal resource and habitat use in marine ecosystems, especially coastal areas fueled by a combination of marine (e.g., micro/macroalgae and seagrass) and terrestrial (e.g., mangroves) primary production.

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Stable isotope ratios in organisms can be used to estimate source contributions to the organism. However, during lipid synthesis light isotopes of carbon ( ¹² C) and hydrogen ( ¹ H) are preferentially incorporated into the lipids, potentially causing source contributions to be poorly estimated. Contrary to expectations and other published examples in animals, larval lampreys, which are basal vertebrates, have lipids which are enriched in heavy isotopes of carbon ( ¹³ C), but still depleted in heavy hydrogen (deuterium; ² H). Four lamprey species were collected and their isotopes ratios of δ ² H, δ ¹³ C, δ ¹⁵ N were measured in their muscle before and after lipid extraction. Larval lamprey of one species was collected every three months for a year from two streams in Maryland and the isotope ratios of muscle before and after lipid extraction, as well as the extracted lipid were measured. Muscle δ ¹³ C was positively related to C:N ratios in samples when lipids were not removed and δ ² H was negatively associated with the percent hydrogen in a sample. As expected, the measured difference between muscle and lipid δ ² H (Δ ML δ ² H) was the same for all months and was 111‰ (SE = ± 21, n = 35), but the Δ ML δ ¹³ C was different between months (ANOVA, F 3,53 = 5.05, p < 0.005) and was always negative. Our work suggests that while lipids are often enriched in ¹² C relative to muscle, this is not a universal rule. The physiological mechanism(s) for generating heavy carbon-backbones in lipids remains unknown and requires exploration.
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We investigated the relationships between the δdeuterium (δD) and the δ(18)oxygen (δ(18)O) of drinking water and the δD and δ(18)O of blood plasma, red blood cells and feathers in house sparrows (Passer domesticus) fed on diets with identical hydrogen and oxygen isotopic compositions and five isotopically distinct drinking water treatments. We expected and, with only one exception ((18)O in blood plasma), found linear relationships between the δD and δ(18)O values of drinking water and those of bird tissues. The slopes of these relationships, which estimate the percentage contributions of drinking water to the tissue isotopic signatures, were lower than those of previous studies. We found significant differences in the δD and δ(18)O values of feathers, red blood cells and plasma solids. In feathers and red blood cells, δD and δ(18)O values were linearly correlated. Our results have three implications for isotopic field studies: (1) if the isotopic composition of drinking water differs from that of food, its effect on tissue isotope values can confound the assignment of animals to a site of origin; (2) comparisons of the δD and δ(18)O values of different tissues must account for inter-tissue discrimination factors; and (3) δD/δ(18)O linear relationships are probably as prevalent in animal systems as they are in geohydrological systems. These relationships may prove to be useful tools in animal isotopic ecology.
Article
Hydrogen isotope (δ²H) analysis has become a valuable tool in the study of animal migration; however, the biochemical framework required for ecologists to confidently apply δ²H to quantify resource use has yet to be adequately resolved. In contrast to carbon (d13C) and nitrogen (d15N) isotopes where food is the only source of these elements, there are two distinct sources of hydrogen available to consumers for tissue synthesis: food and water. To effectively use tissue δ²H values as a tracer of food and water resources, two fundamental questions need to be examined in animals that inhabit different environments: (1) What proportion of hydrogen in tissues is derived from sources of food vs. water? and (2) What is the magnitude and degree of variation in total δ²H discrimination (Δ²Hnet) between consumer tissues and these sources? We completed a 3 9 3 controlled feeding experiment on Nile tilapia (Oreochromis niloticus) in which we varied the δ²H of tank water and dietary macromolecules to examine these questions in two tissues commonly analyzed by ecologists: muscle and liver. We found that the proportion of hydrogen in tilapia tissue derived from tank water was similar for muscle (∼23%) and liver (∼25%). We then used linear regression and an isotope mixing model based on accompanying d13C data to estimate the proportion of hydrogen in muscle and liver tissue derived from dietary protein (34-44%), cornmeal (21-27%), corn syrup (4-5%), and lipids (≤1%). With this information and the δ²H values of water, protein, carbohydrates, and lipids supplied to fish in each treatment, we calculated Δ²Hnet values of-47& + 5& for muscle and-41& + 5& for liver. Our experiment is the first to quantify the relative proportion of hydrogen from different dietary macromolecules used by an omnivore to synthesize its tissues. Such information is needed to further refine the use of δ²H analysis as a dietary tracer for aquatic animals.
Article
A common use of stable isotope analysis in mammalogy is to make inferences about diet from isotope values (typically 13C and 15N) measured in tissues and food sources of a consumer. Mathematical mixing models are used to estimate the proportional contributions of food sources to the isotopic composition of the tissues of a consumer, which reflect the assimilated diet. This paper reviews basic mixing models and how they work; additional refinements also are described that include addressing uncertainty, larger numbers of sources, combining sources, concentration effects, and Bayesian statistical frameworks. Information is provided on where to access software for the various models. Numerous examples are cited to show application of these models in the mammal research literature.
Chapter
The use of stable carbon isotopes for diet reconstruction is predicated on the assumption that you are what you eat. In other words, the carbon isotopic composition of animal tissues is assumed to be a direct and constant function of the diet. Is this assumption valid? Precise dietary reconstruction requires as accurate knowledge of the isotopic composition of locally available dietary resources, as well as an adequate understanding of the effects of nutrition, environment, and physiology on the diet-tissue function (van der Merwe 1982, 1989; Chisholm 1989; Norr 1990; Matson and Chisholm 1991; Tieszen 1991; Ambrose 1992). There is a systematic but poorly defined difference between the isotopic composition of the consumer tissues and that of the diet (an enrichment factor, expressed as Δ diet-tissue). Given the isotopic composition of a specific tissue, that of the diet or of other tissues may be calculated if the Δ diet-tissue difference factors are known. The dietary proportions of isotopically distinct food resources (e.g., C3 vs C4, or C3 vs marine) have thus been calculated from the δ 13C value of bone collagen (Δ13Cd-co) and bone apatite carbonate (Δ13Cd-ca). Deviations from actual or assumed average δ 13C values for dietary endmembers, and incorrect values for diet-to-tissue isotopic relationships, will lead to errors in the estimation of consumption of specific classes of resources. Experiments and observations designed to determine the diet-to-collagen stable isotope functions (Δ13Cd-co) however, have provided widely different values.
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Bivalves influence both the ecology and the economy of coastal regions. By filterfeeding on particles in the water column, these organisms reduce turbidity and link benthic and pelagic production. In addition, production and sales of harvested bivalves are a source of income in coastal areas like the Eastern Shore of Virginia (USA). Phytoplankton are known to be a main food source to many bivalves; however, ocean-side lagoons off the coast of Virginia support extensive aquaculture of Mercenaria mercenaria (hard clams) in waters with relatively low chlorophyll concentrations. The ultimate energy sources supporting these clams are uncertain but significant because seagrass restoration, sea level rise, and climate change will potentially change the quality and quantity of primary production available to these populations. We measured the C, N, and H isotopic ratios of aquaculture clams and a variety of primary producers in a Virginia coastal lagoon over an annual cycle and conducted a Bayesian mixing model analysis to identify current energy sources for clams. By adding a third isotopic ratio (hydrogen), we were able to improve precision over a 2-isotope model based on C and N isotopes. Our analysis reveals that field-cultured clams in Virginia coastal lagoons are significantly supported by microalgae (23 to 44%) but gain most of their energy from macroalgae (55 to 66%), and only a small fraction from macrophytes (0 to 14%). While macroalgae are often an indicator of coastal eutrophication, these algae can be an important food source to bivalves when abundant in low nitrogen, oligotrophic systems. Our results also indicate hydrogen stable isotopes are useful in concert with other isotopes for tracing sources in coastal food webs.
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The importance of terrestrial-derived organic matter for lake zooplankton communities remains debated, partly because little is known about the basic pathways by which allochthonous carbon is transferred to zooplankton, and whether these vary among the major taxonomic and functional groups. We quantified allochthony of three zooplankton groups (Cladocera, Calanoida, and Cyclopoida) across 18 lakes in Quebec, spanning broad gradients of dissolved organic matter (DOM) and lake trophy, using a multi-isotope (delta2H + delta13C), multi-source (terrestrial, phytoplanktonic, benthic) approach. All three zooplankton groups had significant levels of allochthony, but differed greatly in their respective patterns across lakes. Allochthony in Calanoida and Cyclopoida was linked to detrital food chains based on particulate organic matter (POM) and on DOM, respectively, whereas in Cladocera it appeared related to both pathways; not surprisingly this latter group had the highest mean allochthony (0.31; compared to 0.18 in Cyclopoida and 0.16 in Calanoida). This study highlights the complexity of the pathways of delivery and transfer of terrestrial organic matter in freshwaters, and underscores the role that microbial food webs play in this transfer.
Article
The relative global 2H-content of natural plant products is correlated to that of the primary hydrogen source, i.e. water, to the site of their biosynthesis (C3-, C4- and CAM-plants; chloroplasts, cytosol), and to their biosynthetic pathways. A relative global 2H-content sequence can be established in the order phenylpropanoids > carbohydrates > bulk material > hydrolysable lipids > steroids. A detailed analysis of the 2H-patterns of the main groups of secondary compounds reveals regularities, in that they are correlated to the primary precursors and to the origin of hydrogen from four main pools with the mean d2H-values [‰]V-SMOW: leaf H2O ~+30; carbohydrates ~-70; NADPH ~-250; flavoproteins ~-350. Aside from the 2H-discrimination between these pools, kinetic isotope effects on defined reactions only become effective in connection with metabolic branching events. So, the 2H-pattern of natural aromatic compounds can be correlated to the 2H-pattern of the precursor carbohydrates and a reduction step in the course of the shikimic acid pathway, furthermore to the implication of the NIH-shift. The pattern of aromatic compounds from the polyketide is different from that of the shikimate pathway. The alternating 2H-abundance of fatty acid chains is caused by the origin of their hydrogen atoms from carbohydrates and from NADPH, directly or via a flavoprotein, respectively. This is similar for isoprenoids, and the natural 2H-patterns permit their assignment to the mevalonate or non-mevalonate biosynthetic pathway. Generally, the correlations and regularities of the 2H-patterns of organic compounds found are a new reliable tool for the elucidation of biosynthetic pathways and origin assignments.
Article
The ability to quantify dietary inputs using stable isotope data depends on accurate estimates of isotopic differences between a consumer (c) and its diet (d), commonly referred to as trophic discrimination factors (TDFs) and denoted by Δ c-d . At present, TDFs are available for only a few mammals and are usually derived in captive settings. The magnitude of TDFs and the degree to which they vary in wild populations is unknown. We determined δ¹³C and δ¹⁵N TDFs for vibrissae (i.e., whiskers), a tissue that is rapidly becoming an informative isotopic substrate for ecologists, of a wild population of sea otters for which individual diet has been quantified through extensive observational study. This is one of the very few studies that report TDFs for free-living wild animals feeding on natural diets. Trophic discrimination factors of 2.2‰ ± 0.7‰ for δ¹³C and 3.5‰ ± 0.6‰ for δ¹⁵N (mean ± SD) were similar to those reported for captive carnivores, and variation in individual δ¹³C TDFs was negatively but significantly related to sea urchin consumption. This pattern may relate to the lipid-rich diet consumed by most sea otters in this population and suggests that it may not be appropriate to lipid-extract prey samples when using the isotopic composition of keratinaceous tissues to examine diet in consumers that frequently consume lipid-rich foods, such as many marine mammals and seabirds. We suggest that inherent variation in TDFs should be included in isotopically based estimates of trophic level, food chain length, and mixing models used to quantify dietary inputs in wild populations; this practice will further define the capabilities and limitations of isotopic approaches in ecological studies.
Article
Determination of amino acid enantiomers in environmental samples is difficult, because metals and organic impurities interfere with the analyses. We developed a new gas chromatographic method to assess amino acid enantiomer concentrations in complex soil matrices following hot HCl hydrolysis (6M, 12h, 105°C). The crucial focus was the establishment of a simple, reliable sample clean-up procedure. The presented method involved the adsorption of the enantiomers on a Dowex 50 W X8 cation exchange resin and the removal of interfering compounds with 0.1M oxalic acid prior to amino acid elution with 2.5M NH4OH. After conversion to N-pentafluoropropionyl-amino acid iso-propyl esters, the diastereomers were separated by a Chirasil l-Val capillary column and quantified by a flame ionization or mass selective detector. The lower limit of quantification tested here was ≤1pg injection amount. The recovery of amino acid enantiomers averaged 99±11% for pure standards and 97±11% for spiked soil hydrolysates. The general applicability of the method was demonstrated by determination of amino acid enantiomers in taxonomically different soils from different geographic regions. The coefficients of variation presented by d/l ratios were 12% for alanine and
Article
This review is concerned with the isotopic relationships between organic compounds produced by a single organism, specifically their enrichments or depletions in 13C relative to total-biomass carbon. These relationships are biogeochemically significant because 1. An understanding of biosynthetically controlled, between-compound isotopic contrasts is required in order to judge whether plausibly related carbon skeletons found in a natural mixture might come from a single source or instead require multiple sources. 2. An understanding of compound-to-biomass differences must underlie the interpretation of isotopic differences between individual compounds and total organic matter in a natural mixture. My approach is pedagogic. The coverage is meant to be thorough, but the emphases and presentation have been chosen for readers approaching this subject as students rather than as research specialists. In common with the geochemists in my classes, many readers of this paper may not be very familiar with biochemistry and microbiology. I have not tried to explain every concept from those subjects and I have not inserted references for points that appear in standard texts in biochemistry or microbiology. Among such books, I particularly recommend the biochemistry text by Garrett and Grisham (1999) and the microbiology text by Madigan et al. (2000). The biochemistry text edited by Zubay (1998) is also particularly elegant and detailed. White (1999) has written a superb but condensed text on the physiology and biochemistry of prokaryotes. A schematic overview of the relevant processes is shown in Figure 1⇓. Plants and other autotrophs fix CO2. Animals and other heterotrophs utilize organic compounds. If the assimilated carbon is a small molecule (like CO2, CH4, or acetate), significant isotopic fractionation is likely to accompany the fixation or assimilation of C. Such fractionations establish the isotopic relationship between an organism and its carbon source. Those associated …
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We present a new 3-dimensional 1° × 1° gridded data set for the annual mean seawater oxygen isotope ratio (δ18O) to use in oceanographic and paleoceanographic applications. It is constructed from a large set of observations made over the last 50 years combined with estimates from regional δ18O to salinity relationships in areas of sparse data. We use ocean fronts and water mass tracer concentrations to help define distinct water masses over which consistent local relationships are valid. The resulting data set compares well to the GEOSECS data (where available); however, in certain regions, particularly where sea ice is present, significant seasonality may bias the results. As an example application of this data set, we use the resulting surface δ18O as a boundary condition for isotope-enabled GISS ModelE to yield a more realistic comparison to the isotopic composition of precipitation data, thus quantifying the `source effect' of δ18O on the isotopic composition of precipitation.
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Stable isotope analysis (SIA) has proven to be a useful tool in reconstructing diets, characterizing trophic relationships, elucidating patterns of resource allocation, and constructing food webs. Consequently, the number of studies using SIA in trophic ecology has increased exponentially over the past decade. Several subdisciplines have developed, including isotope mixing models, incorporation dynamics models, lipid-extraction and correction methods, isotopic routing models, and compound-specific isotopic analysis. As with all tools, there are limitations to SIA. Chief among these are multiple sources of variation in isotopic signatures, unequal taxonomic and ecosystem coverage, over-reliance on literature values for key parameters, lack of canonical models, untested or unrealistic assumptions, low predictive power, and a paucity of experimental studies. We anticipate progress in SIA resulting from standardization of methods and models, calibration of model parameters through experimentation, and continued development of several recent approaches such as isotopic routing models and compound-specific isotopic analysis.
Article
We examine inherent variation in carbon and nitrogen stable isotope values of multiple soft tissues from a population of captive green turtles Chelonia mydas to determine the extent of isotopic variation due to individual differences in physiology. We compare the measured inherent variation in the captive population with the isotopic variation observed in a wild population of juvenile green turtles. Additionally, we measure diet-tissue discrimination factors to determine the offset that occurs between isotope values of the food source and four green turtle tissues. Tissue samples (epidermis, dermis, serum, and red blood cells) were collected from captive green turtles in two life stages (40 large juveniles and 30 adults) at the Cayman Turtle Farm, Grand Cayman, and analyzed for carbon and nitrogen stable isotopes. Multivariate normal models were fit to the isotope data, and the Bayesian Information Criterion was used for model selection. Inherent variation and discrimination factors differed among tissues and life stages. Inherent variation was found to make up a small portion of the isotopic variation measured in a wild population. Discrimination factors not only are tissue and life stage dependent but also appear to vary with diet and sea turtle species, thus highlighting the need for appropriate discrimination factors in dietary reconstructions and trophic-level estimations. Our measures of inherent variation will also be informative in field studies employing stable isotope analysis so that differences in diet or habitat are more accurately identified.
Article
1. Use of the natural ratios of carbon and nitrogen stable isotopes as tracers of trophic interactions has some clear advantages over alternative methods for food web analyses, yet is limited to situations where organic materials of interest have adequate isotopic separation between potential sources. This constrains the use of natural abundance stable isotope approaches to a subset of ecosystems with biogeochemical conditions favourable to source separation. 2. Recent studies suggest that stable hydrogen isotopes (δD) could provide a robust tracer to distinguish contributions of aquatic and terrestrial production in food webs, but variation in δD of consumers and their organic food sources are poorly known. To explore the utility of the stable hydrogen isotope approach, we examined variation in δD in stream food webs in a forested catchment where variation in δ13C has been described previously. 3. Although algal δD varied by taxa and, to a small degree, between sites, we found consistent and clear separation (by an average of 67‰) from terrestrial carbon sources. Environmental conditions known to affect algal δ13C, such as water velocity and stream productivity did not greatly influence algal δD, and there was no evidence of seasonal variation. In contrast, algal δ13C was strongly affected by environmental factors both within and across sites, was seasonally variable at all sites, and partially overlapped with terrestrial δ13C in all streams with catchment areas larger than 10 km2. 4. While knowledge of isotopic exchange with water and trophic fractionation of δD for aquatic consumers is limited, consistent source separation in streams suggests that δD may provide a complementary food web tracer to δ13C in aquatic food webs. Lack of significant seasonal or spatial variation in δD is a distinct advantage over δ13C for applications in many aquatic ecosystems.
Article
Summary • Carbon and nitrogen isotope ratios in consumer tissues are known to correlate with diet isotope composition, and nitrogen isotope ratios are observed to increase with increasing trophic level. • We analysed nitrogen and hydrogen isotope ratios of collagen from 19 species of British fish, birds and mammals to investigate how D also correlated with trophic level and with feeding environment (terrestrial or aquatic). • A strong relationship between trophic level and D was discovered for both terrestrial and aquatic consumers. • The correlation between trophic level and 15N was apparent for terrestrial consumers, but less so for aquatic consumers. • No differentiation was found between D of aquatic and terrestrial consumers at the same trophic level. • This observation should provide an additional tool in the study of current and ancient animal and human food web ecology. Journal of Animal Ecology (2005) 74, 877–881 doi: 10.1111/j.1365-2656.2005.00979.x
Article
Linking foraging and breeding habitats is key to the understanding of behaviour, ecology and demography of migratory species. Establishing such connections has long been hampered by the logistical problems of following individuals between foraging and breeding areas, especially in the marine realm. We used variation in nitrogen stable isotope patterns between 2 foraging regions of loggerhead sea turtles Caretta caretta determined from samples of satellite-tracked individuals to assign untracked turtles to a foraging region. We sought to enhance determination of the relative importance of geographically separated foraging regions and to investigate the relationship between fitness correlates and inferred migratory strategies. Of 18 turtles followed by satellite tracking from Zakynthos (Greece), 10 moved north to foraging areas in the Adriatic Sea and the Gulf of Amvrakikos and 8 moved south to foraging areas off the coast of North Africa. Of 51 untracked individuals sampled for stable isotope analysis, we considered the stable isotope signature of 47 to qualify for assignment to foraging areas in the north (n = 22) and south (n = 25). Females foraging north were significantly larger (curved carapace length), and the former group laid larger clutches (even after correction for body length) than turtles foraging south, a fact that can be interpreted as a carry-over effect. Combining satellite tracking with stable isotope signatures in marine turtles opens new perspectives into how forensic tracking methodologies may be used to scale up knowledge from electronic tracking of a limited number of individuals to sample sizes that are more meaningful from a population perspective.
Article
Extracting nutrients is of upmost importance to the survival of any individual or species. One of the distinguishing characteristics of the order Primates is the vast range of nutritional adaptations it exhibits. Within our own species all manner of adaptations are practiced and it has been a major focus of research to determine when and where these various patterns originated. We present one method based on stable isotope analysis in human tissues and discuss its contributions. The ratios of 13C/12C and 15N/14N vary among various pools (i.e., the atmosphere, the oceans, plant communities, trophic levels). These differences are transferred to humans via the foods they eat. The major differences in carbon occur between two photosynthetic pathways (C3 and C4), which in the New World permits tracing the introduction of maize (a C4 plant) and in Asia permits tracing the introduction of millet (also a C4 plant). The marine and terrestrial systems have distinctive isotope ratios of both carbon and nitrogen. Thus, the dependence on marine resources has been traced throughout several areas of the New and Old Worlds. We discuss several potential sources of variation including sex, age, nutritional status, among others. We conclude with some suggestions for future research.
Article
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 δ¹⁸O and δ²H (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 δ¹⁸O and δ²H, 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 δ²H=8·11δ¹⁸O+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 δ²H=8·17δ¹⁸O+10·35.
Article
The apparent digestibility coefficients for 4 size classes of the green turtle Chelonia mydas feeding on the seagrass Thalassia testudinum were measured in Union Creek, Great Inagua, Bahamas, from September 1975 to August 1976. The values ranged from 32.6 to 73.9% for organic matter; from 21.5 to 70.7% for energy; from 71.5 to 93.7% for cellulose; from 40.3 to 90.8% for hemicellulose; and from 14.4 to 56.6% for protein. Digestive efficiency increased with increases in water temperature and body size. There was no seasonal variation in the nutrient composition of T. testudinum blades. Grazing on T. testudinum may be limited by its low quality as a forage, a result of its high fiber content and possible low protein availability. Turtles did not graze at random over the extensive beds of T. testudinum, but maintained grazing plots of young leaves by consistent recropping. They thus consumed a more digestible forage-higher in protein and lower in lignin-than the ungrazed, older leaves of T. testudinum. The selectivity of green turtles for either a seagrass or algal diet may reflect the specificity of their intestinal microflora.
Article
A new method for the simultaneous, quantitative determination of neutral and acidic sugars liberated from non-cellulosic soil carbohydrates is described. A single hydrolytic step with 4 M trifluoroacetic acid (TFA) at 105°C for 4 h is suggested to be more effective in releasing sugar monomers from soil than other recommended hydrolysis procedures (2.5 M H2SO4 reflux for 20 min followed by 12 M H2SO4 for total sugar hydrolysis; 1 M HCl for 5 h at 105°C or for 7 h at 100°C; 4 M TFA at 125°C for 1 h or 2 M TFA at 105°C for 2 h). Different materials were tested for purification (cation exchange resin, activated carbon, C-18, XAD-4, XAD-7), and the combination of the cation exchange resin and of XAD-7 is recommended. Analysis of purified monomers involved separation of O-methyl-oxime-trimethylsilyl derivatives by capillary gas-liquid chromatography in 21 min. Results were compared with those obtained from spiked soil samples, resulting in a high recovery of standard mixtures (70–109% for individual sugars) processed in the way proposed here. The method was sensitive and characterized by simple, accurate and rapid work-up and, therefore, is suitable for routine use.
Article
Hydrogen-isotopic data are often interpreted using mathematical approximations originally intended for other isotopes. One of the most common, apparent in literature over the last several decades, assumes that delta values of reactants and products are separated by a constant fractionation factor: δp = δr + εp/r. Because of the large fractionations that affect hydrogen isotopes, such approximations can lead to substantial errors. Here we review and develop general equations for isotopic mass balances that include the differential fractionation of each component in a mixture and discuss their use in three geochemical applications. For the fractionation of a single component, the reactant and product are related by δp = αp/rδr + εp/r, where α and ε refer to the same fractionation. Regression of δp on δr should give equivalent fractionations based on the intercept and slope, but this has not generally been recognized in studies of D/H fractionation. In a mixture of two components, each of which is fractionated during mixing, there is no unique solution for the three unknown variables (two fractionation factors and the elemental mixing ratio of the two hydrogen sources). The flow of H from CH4 and H2O to bacterial lipids in the metabolism of Methylococcus capsulatus provides an example of such a case. Data and conclusions from an earlier study of that system (Sessions et al., 2002) are reexamined here. Several constraints on the variables are available based on plausible ranges for fractionation factors. A possible refinement to current experimental procedures is the measurement of three different isotopes, which would allow unique determination of all variables.
Article
The influence of diet on the distribution of carbon isotopes in animals was investigated by analyzing animals grown in the laboratory on diets of constant carbon isotopic composition. The isotopic composition of the whole body of an animal reflects the isotopic composition of its diet, but the animal is on average enriched in δ^(13)C by about 1‰ relative to the diet. In three of the four cases examined, the ^(13)C enrichment of the whole body relative to the diet is balanced by a ^(13)C depletion of the respired CO_2. The isotopic relationships between the whole bodies of animals and their diets are similar for different species raised on the same diet and for the same species raised on different diets. However, the δ^(13)C values of whole bodies of individuals of a species raised on the same diet may differ by up to 2‰. The relationship between the ^(13)C/^(12)C ratio of a tissue and the ^(13)C/^(12)C ratio of the diet depends both on the type of tissue and on the nature of the diet. Many of the isotopic relationships among the major biochemical fractions, namely the lipid, carbohydrate and protein fractions, are qualitatively preserved as diet carbon is incorporated into the animal. However, the difference between the δ^(13)C values of a biochemical fraction in an animal and in its diet may be as large as 3‰. The δ^(13)C values of the biochemical components collagen, chitin and the insoluble organic fraction of shells, all of which are often preserved in fossil material, are related to the isotopic composition of the diet. These results indicate that it will be possible to perform dietary analysis based on the determination of the ^(13)C/^(12)C ratio of animal carbon. Analysis of the total animal carbon will in most cases provide a better measure of diet than the analysis of individual tissues, biochemical fractions, or biochemical components. The limits of accuracy of this method will generally restrict its application to situations in which the diet is derived from sources with relatively large differences in their δ^(13)C values, such as terrestrial vs aquatic organisms or C_3 vs C_4 plants. The method should be applicable to fossil as well as to living material.
Article
Explains isotope terminology and fractionation, and summarises isotopic distributions in the C, N and S biogeochemical cycles. Five case studies (delta 15N measures of N2 fixation; the global carbon cycle and the CO2 problem; sulphur and acid deposition; use in archaeology; and detrital organic matter in saltmarshes) show how stable isotope measurements can provide crucial information for ecosystem analysis.-P.J.Jarvis
Article
The study of long-distance dispersal (LDD) in animals may be advanced by recent applications of stable isotope analyses designed to track migratory organisms and to link populations throughout their annual cycle. This approach depends on there being enough isotopic difference in tissues among potential source populations such that individuals can be unequivocally assigned to their source. The isotopic mapping of such populations will be feasible only for species occurring in relatively few disjunct populations. However, the identification of isotopic outliers within known populations will be an extremely useful first step in the forensic application of stable isotopes to identify dispersal in general, and LDD in particular. The use of deuterium isotope analysis (deltaD) of tissues that can be assigned to its source (e.g. feather moult or its hair growth location) has provided a recent breakthrough in our ability to associate individuals with geographical origins at continental scales. The combination of this stable isotope with others and the ultimate combination of a variety of techniques, including the measurement of trace elements and molecular genetics markers, will undoubtedly improve resolution. The isotopic cataloguing of known history (i.e. philopatric) individuals within populations will be an important step in applying isotope techniques to evaluating LDD in any species. For aquatic insects, the isotopic marking of large numbers of individuals is possible through isotopic enrichment of local food webs using labelled compounds.
Article
Because there are no internationally distributed stable hydrogen and oxygen isotopic reference materials of human hair, the U.S. Geological Survey (USGS) has prepared two such materials, USGS42 and USGS43. These reference materials span values commonly encountered in human hair stable isotope analysis and are isotopically homogeneous at sample sizes larger than 0.2 mg. USGS42 and USGS43 human-hair isotopic reference materials are intended for calibration of δ(2)H and δ(18)O measurements of unknown human hair by quantifying (1) drift with time, (2) mass-dependent isotopic fractionation, and (3) isotope-ratio-scale contraction. While they are intended for measurements of the stable isotopes of hydrogen and oxygen, they also are suitable for measurements of the stable isotopes of carbon, nitrogen, and sulfur in human and mammalian hair. Preliminary isotopic compositions of the non-exchangeable fractions of these materials are USGS42(Tibetan hair)δ(2)H(VSMOW-SLAP) = -78.5 ± 2.3‰ (n = 62) and δ(18)O(VSMOW-SLAP) = +8.56 ± 0.10‰ (n = 18) USGS42(Indian hair)δ(2)H(VSMOW-SLAP) = -50.3 ± 2.8‰ (n = 64) and δ(18)O(VSMOW-SLAP) = +14.11 ± 0.10‰ (n = 18). Using recommended analytical protocols presented herein for δ(2)H(VSMOW-SLAP) and δ(18)O(VSMOW-SLAP) measurements, the least squares fit regression of 11 human hair reference materials is δ(2)H(VSMOW-SLAP) = 6.085δ(2)O(VSMOW-SLAP) - 136.0‰ with an R-square value of 0.95. The δ(2)H difference between the calibrated results of human hair in this investigation and a commonly accepted human-hair relationship is a remarkable 34‰. It is critical that readers pay attention to the δ(2)H(VSMOW-SLAP) and δ(18)O(VSMOW-SLAP) of isotopic reference materials in publications, and they need to adjust the δ(2)H(VSMOW-SLAP) and δ(18)O(VSMOW-SLAP) measurement results of human hair in previous publications, as needed, to ensure all results on are on the same scales.