Climate at the onset of western Mediterranean agriculture expansion: Evidence from stable isotopes of sub-fossil oak tree rings in Spain

Climate conditions during the early Holocene may have contributed to the spread of Old World agriculture from its area of birth, in the Fertile Crescent (Near East), west through the Mediterranean Basin. Reconstructing the specific environmental conditions existing in early agricultural settlements of the western Mediterranean may help to elucidate this point. The aim of this work was to gain information on past climate of one of the earliest agricultural settlements of the Iberian Peninsula, La Draga, a lacustrine site dating back to the 2nd half of the 8th millennium BP, in which post fragments of deciduous oak have been exceptionally well preserved in an anaerobic environment (sub-fossil wood). We studied the relationship between climate factors and carbon isotope discrimination (Δ¹³C) and oxygen isotope composition (δ¹⁸O) in wood α-cellulose from modern tree-ring series of Quercus humilis Mill. and Quercus petraea (Matt) Liebl. recovered from the area. Climatic responses observed in extant material were used to interpret the isotopic signal of wood α-cellulose extracted from sub-fossil wood. Results showed that water availability of late spring, early summer and September influences Δ¹³C and δ¹⁸O, which allows their use as proxies for palaeoclimatic reconstruction. Differences between sub-fossil and extant samples in Δ¹³C (19.35 vs. 18.02‰) and δ¹⁸O (26.32 vs. 29.28‰) records suggest slightly lower temperatures and higher plant water availability than at present during the establishment of agriculture at the site. These results seem to disprove the existence of an environmental limitation preventing continuation of hunting and gathering activities as a cause for the adoption of agriculture in this early agricultural site on the Iberian Peninsula.     

A simple and rapid preparation of pure cellulose confirmed by monosaccharide compositions, δ13C,yields and C%

A two-step method to extract pure cellulose from tree rings is presented in this study. The method employs the reagents diglyme (diethylene glycol dimethyl ether), hydrochloric acid and sodium chlorite in a 6-h reaction using ordinary equipment. Through separation of the chemical compositions (monosaccharides) using liquid chromatography, the product of this method is shown to contain only glucose from pure cellulose. In contrast, the chemical composition of the “α-cellulose” extract obtained by the traditional Jayme–Wise method is primarily glucose and mannose, which may come from hemicelluloses. These results are also verified by comparing the stable carbon isotope ratio (δ¹³C), yields and percentage of carbon (C%) of the product extracted by this method and “α-cellulose” for different tree species. The results suggest that this method is simple, rapid and appropriate for processing large numbers of small wood samples to develop tree-ring isotope series for high-resolution paleoclimatic research with softwood trees.     

The environmental water of the middle Eocene Arctic: Evidence from δD, δ18O and δ13C within specific compounds

The extensive vertical exposure (> 150 m) of terrestrial sediments on Axel Heiberg Island, which contain thick fossiliferous lignites, presents an exceptional opportunity to follow the establishment and re-establishment of Arctic Metasequoia forests during the middle Eocene. We compared δD values in n-alkanes of chain length 23, 25, 27 and 29 with δ¹⁸O values in phenylglucosazone (P-G) derived from α-cellulose; we also analyzed %-abundance of ferns, gymnosperms and angiosperms using pollen and spores isolated from each lignite. Our results showed that forest composition was altered upon uplift, as gymnosperms became more abundant within the relatively well-drained upland sediments. This was also reflected in the small (1‰), but significant, increase in the δ¹³C value of TOM from lowland to upland environments. However, neither the δD values of n-alkanes nor the δ¹⁸O in P-G were statistically different in the upland sediments, as compared to the lowland sediments; from this we inferred that the oxygen isotope signature of environmental water available to the forests for plant growth was relatively uniform throughout the time of the fossil forests. The δD value of environmental water implied by both n-alkanes and P-G ranged from ? 168 to ? 131% and was considerably enriched compared to all environmental water samples available from the modern Arctic region (< ? 180%). In addition to indicating a warmer Eocene Arctic, subject to meteoric transport patterns different from today's, these results argue against the presence of an Eocene polar ice cap.     

Stable isotope composition of Late Cretaceous benthic foraminifera from the southern South Atlantic: Biological and environmental effects

The stable carbon and oxygen isotope composition of different benthic foraminiferal species of the latest Campanian and earliest Maastrichtian from Ocean Drilling Project Hole 690C (Weddell Sea, southern South Atlantic, ∼1800 m paleowater depth) have been investigated. The total range of measured isotope values of all samples exceeds ∼4‰ for δ¹³C and 1.1‰ for δ¹⁸O. Carbon isotope values of proposed deep infaunal species are generally similar or only slightly lower when compared to proposed epifaunal to shallow infaunal species. Interspecific differences vary between samples probably reflecting temporal changes in organic carbon fluxes to the sea floor. Constantly lower δ¹³C values for Pullenia marssoni and Pullenia reussi suggest the deepest habitat for these species. The strong depletion of δ¹³C values by up to 3‰ within lenticulinids may be attributed to a deep infaunal microhabitat, strong vital effects, or different feeding strategy when compared to other species or modern lenticulinids. The mean δ¹⁸O values reveal a strong separation of epifaunal to shallow infaunal and deep infaunal species. Epifaunal to shallow infaunal species are characterized by low δ¹⁸O values, deep infaunal species by higher values. This result possibly reflects lower metabolic rates and longer life cycles of deep infaunal species or the operating of a pore water [CO₃²⁻] effect on the benthic foraminiferal stable isotopes.Pyramidina szajnochae shows an enrichment of oxygen isotopes with test size comprising a total of 0.6‰ between 250 and 1250 μm shell size. Although δ¹³C lacks a corresponding trend these data likely represent the presence of changes in metabolic rates during ontogenesis. These results demonstrate the general applicability of multi-species stable isotope measurements of pristine Cretaceous benthic foraminifera to reconstruct past microhabitats and to evaluate biological and environmental effects on the stable isotope composition.     

树木年轮年内高分辨率稳定同位素记录:方法、进展和展望

树轮稳定同位素比率能有效地记录树木生长过程中气候环境变化信息及树木的生理响应机制。年内高分辨率树轮稳定同位素比率则能够提供更为详细的气候环境信息,揭示树木对季节尺度气候环境变化的生理生态响应机制,在古气候和全球变化生态学研究方面显示出巨大的潜力。本文收集了1990年以来发表的树轮年内高分辨率稳定同位素比率研究论文,从样品剥离方法、α-纤维素化学提取方法以及应用等方面综述了其研究进展,展望了年内高分辨率树轮稳定同位素记录研究的潜力和未来发展方向。     

Evaluation of the non-climatic (age-related) trends of stable oxygen and carbon isotopes in Swiss stone pine (Pinus cembra L.) tree rings from the Eastern Carpathians,Romania

The presence, magnitude, and duration of age and/or size-related trends in the stable isotopes in tree-ring cellulose time series has been a subject of scientific debate. Where present, their evaluation and removal are key to the development of robust climate calibrations and reconstructions, especially in the low-frequency domain and where sample replication levels are low. Where reported, results suggest that the age/size/height-related trend of stable oxygen (δ¹⁸O) and carbon (δ¹³C) isotope compositions in tree-ring cellulose may vary according to the tree species and the individual tree location. For this reason, it is important when developing long palaeoclimate reconstructions for a new species, geographical region or ecological setting to perform studies to investigate non-climatic (age-related) trends. This study evaluates the ontogenetic pattern in the δ¹⁸O and δ¹³C data in Swiss stone pine trees from the Eastern Carpathians, Romania. For this, we used a new multi-centennial dataset consisting of 5 living and 10 relict Swiss stone pine samples collected from the Eastern Carpathians, which were annually resolved for δ¹⁸O and δ¹³C composition. Age-related trends were evaluated using the conventional “slope of the mean” approach as well as the “mean of the slope” method. The results suggest that neither the traditional “slope-of-the mean” nor the “mean-of-the-slope” approach indicates a persistent linear trend in the Swiss stone pine δ¹⁸O data, while for the δ¹³C data a systematic enrichment in ¹³C was observed over a < 40 year-long period after germination. Despite the limited sample size of this developing dataset these findings help to inform more detailed analyses and future sampling strategies in the Eastern Carpathian Swiss stone pine stands.     

Introducing fecal stable isotope analysis in primate weaning studies

This research investigates the potential of a new, noninvasive method for determining age of weaning among primates using stable carbon and nitrogen isotope ratios in feces. Analysis of stable isotope ratios in body tissues is a well‐established method in archeology and ecology for reconstructing diet. This is the first study to investigate weaning in primates using fecal stable isotope ratios. Diets of a single François’ langur (Trachypithecus francoisi) mother–infant pair at the Toledo Zoo are reconstructed using this technique to track changes in infant suckling behavior over the weaning period. Stable isotope ratios in feces are sampled instead of more traditional samples such as bone or hair to enable daily, noninvasive snapshots of weaning status. Isotopic assessments of weaning status are compared to visual assessments to identify any discordance between the two. Three measurements documented the transition from breast milk to solid foods: stable carbon isotope ratios (δ¹³C), stable nitrogen isotope ratios (δ¹⁵N), and nitrogen content of feces (%N). It appears that solid foods were introduced at approximately 2 months of infant age, but that nursing continued into the 12th month, when sample collection ceased. Stable isotope data exposed a much longer weaning period than what was expected based on previously published data for captive langurs, and clarified visual estimates of weaning status. This reflects the method's sensitivity to suckling at night and ability to distinguish actual nursing from comfort nursing. After testing this method with zoo animals, it can readily be applied among wild populations. An isotopic approach to weaning provides a new, accurate, and biologically meaningful assessment of interbirth intervals, and facilitates a better understanding of mother–infant interactions. Both of these outcomes are critical for developing successful conservation strategies for captive and wild primates. Am. J. Primatol. 74:926‐939, 2012. © 2012 Wiley Periodicals, Inc.     

Effects of decomposition and storage conditions on the δ13C and δ15N isotope values of killer whale (Orcinus orca) skin and blubber tissues

Several different factors in the collection and preservation of whale skin and blubber samples were examined to determine their effect on the results obtained by stable nitrogen and carbon isotope (δ¹⁵N and δ¹³C) analysis. Samples of wet killer whale skin retained their original stable isotope values for up to 14 d at 4°C or lower. However, decomposition significantly changed the δ¹⁵N value within 3 d at 20°C. Storage at ?20°C was as effective as ?80°C for the preservation of skin and blubber samples for stable isotope analysis for at least a year. By contrast, once a skin sample had been freeze‐dried and lipid extracted, the stable isotope values did not change significantly when it was stored dry at room temperature for at least 12 mo. Preservation of whale skin samples for a month in DMSO‐salt solution, frozen or at room temperature, did not significantly change the δ¹⁵N and δ¹³C values of lipid extracted tissues, although the slight changes seen could influence results of a study if only small changes are expected.     

Effect of lipid removal on carbon and nitrogen stable isotope ratios in crustacean tissues

The analysis of tissue's naturally occurring stable carbon and nitrogen isotope ratios is a useful tool to delineate trophic relationships. However, the interpretation of δ¹³C and δ¹⁵N is complicated by the influence of multiple factors such as the tissue-specific lipid content. The aim of this work was to evaluate the effects of lipid extraction on δ¹³C and δ¹⁵N compositions in muscle, hepatopancreas and gonads of a marine decapod crustacean, the spider crab Maja brachydactyla. Samples were analyzed for stable isotopes before and after lipid removal, using a derived Soxhlet extraction method. Differences in δ¹³C and δ¹⁵N were measured among tissues before and after treatment. Lipid extraction of muscle did not have a significant effect on either δ¹³C or δ¹⁵N. By contrast, ecologically significant shifts for both carbon and nitrogen stable isotopes ratios (+ 2.9 ± 0.8‰ for δ¹³C, and + 1.2 ± 0.7‰ for δ¹⁵N) were noticed in the hepatopancreas. In regard to gonads, lipid extraction led to a shift only on δ¹³C (+ 1.3 ± 0.3‰). Finally, the derived Soxhlet extraction method removed the lipid influence for δ¹³C, and had an effect on δ¹⁵N composition for lipid-rich samples. We recommend this treatment for carbon stable isotope studies on decapod crustacean lipid-rich tissues.     

The effect of acidification and the combined effects of acidification/lipid extraction on carbon stable isotope ratios for sub-arctic and arctic marine zooplankton species

Stable isotope ratios of carbon (δ ¹³C) in zooplankton are widely applied in ecosystem-level studies of energy flux and trophic interactions. Carbonate (CaCO₃) and lipid content are highly variable both among and within zooplankton species. Such variability can arise from the δ ¹³C-depleted nature of lipids as well as differences in carbon incorporation among tissues (e.g., relative amount of carbonate). Critically, the impact of the common lipid- and carbonate-normalization steps of extraction and acidification is poorly understood and applied in an inconsistent manner. Here, we investigated the effect of lipid extraction and sample acidification (CaCO₃ removal) on δ ¹³C in sub-arctic and arctic marine zooplankton species. Our results indicate that, with the exception of the shelled mollusc Limacina helicina, acidification of samples can be omitted for all other marine zooplankton considered in this study. In the case of L. helicina, δ ¹³C can be corrected for carbonate content using the linear equation developed in this study. In contrast, the δ ¹³C for all species was significantly enriched by a combination of lipid extraction and acidification (up to +4.9 ‰) prior to stable isotope analysis. Our data were used to develop simple, predictive species-specific correction models for lipid-acid-normalized δ ¹³C using C:N and/or untreated δ ¹³C values. Our results indicate that the δ ¹³C value for all species, including those with lower C:N ratios (~3–4), should be corrected for lipid content. We recommend lipid extraction whenever possible, or else the use of species/taxon-specific δ ¹³C lipid-normalization models for accurate determination of carbon sources and dynamics for arctic and sub-arctic marine zooplankton.     

Estimation of growth and food consumption in juvenile Japanese flounder Paralichthys olivaceus using carbon stable isotope ratio δC under laboratory conditions

To estimate the accumulated food consumption and growth of juvenile Japanese flounder Paralichthys olivaceus, we investigated the relationships between individual food consumption and growth, and the change in the stable carbon isotope ratio (δ¹³C). Japanese flounder juveniles were individually reared and their diet was switched from one formulated feed EP1 (δ¹³C = − 19.47‰) to another EP3 (δ¹³C = − 17.21‰) and fed at different feeding regimes. After the switch, the δ¹³C content of the dorsal muscle was exponentially shifted to a different level in proportion to the feeding and growth rates. Therefore, measuring the carbon stable isotope ratio is a useful tool for estimating the food consumption and growth rate of juveniles. In addition, since the velocity of change and the asymptotic value of the carbon stable isotope ratio varied in muscle, caudal fin and liver tissue, different tissues can be used for different time scale estimations.     

Metabolism of dulcitol in Euonymus japonica

Dulcitol-1(6)-¹⁴C was administered to leaves of E. japonica and samples were taken for time periods ranging from 0·5 to 24 hr. For each time period the absolute activity of the glucose, galactose and dulcitol pools was determined. Such studies demonstrated that dulcitol is converted to glucose and galactose. The initial product was glucose, some of which was converted to galactose, glacturonic acid and glucuronic acid. Fractionation of a leaf sample into its pectin, lignin, hemicellulose and α-cellulose components, with subsequent hydrolysis, showed that the dulcitol pool is used in the synthesis of structural carbohydrates. The activity of these fractions was shown to reside in dulcitol, glucose, galactose, galacturonic acid and glucuronic acid residues.     

Quinolinic Acid Amyloid-like Fibrillar Assemblies Seed α-Synuclein Aggregation

Quinolinic acid (QA), a downstream neurometabolite in the kynurenine pathway, the biosynthetic pathway of tryptophan, is associated with neurodegenerative diseases pathology. Mutations in genes encoding kynurenine pathway enzymes, which control the level of QA production, are linked with elevated risk of developing Parkinson's disease. Recent findings have revealed the accumulation and deposition of QA in post-mortem samples, as well as in cellular models of Alzheimer's disease and related disorders. Furthermore, intrastriatal inoculation of mice with QA results in increased levels of phosphorylated α-synuclein and neurodegenerative pathological and behavioral characteristics. However, the cellular and molecular mechanisms underlying the involvement of QA accumulation in protein aggregation and neurodegeneration remain elusive. We recently established that self-assembled ordered structures are formed by various metabolites and hypothesized that these “metabolite amyloids” may seed amyloidogenic proteins. Here we demonstrate the formation of QA amyloid-like fibrillar assemblies and seeding of α-synuclein aggregation by these nanostructures both in vitro and in cell culture. Notably, α-synuclein aggregation kinetics was accelerated by an order of magnitude. Additional amyloid-like properties of QA assemblies were demonstrated using thioflavin T assay, powder X-ray diffraction and cell apoptosis analysis. Moreover, fluorescently labeled QA assemblies were internalized by neuronal cells and co-localized with α-synuclein aggregates. In addition, we observed cell-to-cell propagation of fluorescently labeled QA assemblies in a co-culture of treated and untreated cells. Our findings suggest that excess QA levels, due to mutations in the kynurenine pathway, for example, may lead to the formation of metabolite assemblies that seed α-synuclein aggregation, resulting in neuronal toxicity and induction of Parkinson's disease.     

Sample size under inverse negative binomial group testing for accuracy in parameter estimation

Background

The group testing method has been proposed for the detection and estimation of genetically modified plants (adventitious presence of unwanted transgenic plants, AP). For binary response variables (presence or absence), group testing is efficient when the prevalence is low, so that estimation, detection, and sample size methods have been developed under the binomial model. However, when the event is rare (low prevalence <0.1), and testing occurs sequentially, inverse (negative) binomial pooled sampling may be preferred.

Methodology/Principal Findings

This research proposes three sample size procedures (two computational and one analytic) for estimating prevalence using group testing under inverse (negative) binomial sampling. These methods provide the required number of positive pools (), given a pool size (k), for estimating the proportion of AP plants using the Dorfman model and inverse (negative) binomial sampling. We give real and simulated examples to show how to apply these methods and the proposed sample-size formula. The Monte Carlo method was used to study the coverage and level of assurance achieved by the proposed sample sizes. An R program to create other scenarios is given in Appendix S2.

Conclusions

The three methods ensure precision in the estimated proportion of AP because they guarantee that the width (W) of the confidence interval (CI) will be equal to, or narrower than, the desired width (), with a probability of . With the Monte Carlo study we found that the computational Wald procedure (method 2) produces the more precise sample size (with coverage and assurance levels very close to nominal values) and that the samples size based on the Clopper-Pearson CI (method 1) is conservative (overestimates the sample size); the analytic Wald sample size method we developed (method 3) sometimes underestimated the optimum number of pools.     

Tissue turnover and stable isotope clocks to quantify resource shifts in anadromous rainbow trout

Stable isotopes can illuminate resource usage by organisms, but effective interpretation is predicated on laboratory validation. Here we develop stable isotope clocks to track resource shifts in anadromous rainbow trout (Oncorhynchus mykiss). We used a diet-switch experiment and model fitting to quantify N stable isotope (δ¹⁵N) turnover rates and discrimination factors for seven tissues: plasma, liver, fin, mucus, red blood cells, muscle, and scales. Among tissues, diet-tissue δ¹⁵N discrimination factors ranged from 1.3 to 3.4 ‰. Model-supported tissue turnover half-lives ranged from 9.0 (fin) to 27.7 (scale) days. We evaluated six tissue turnover models using Akaike’s information criterion corrected for small sample sizes. The use of equilibrium tissue values was supported in all tissues and two-compartment models were supported in plasma, liver, and mucus. Using parameter estimates and their uncertainty we developed stable isotope clocks to estimate the time since resource shifts. Longer turnover tissues provided accurate estimates of time since resource switch for durations approximately twice their half-life. Faster turnover tissues provided even higher precision estimates, but only within their half-life post-switch. Averaging estimates of time since resource shift from multiple tissues provided the highest precision estimates of time since resource shift for the longest duration (up to 64 days). This study therefore provides insight into physiological processes that underpin stable isotope patterns, explicitly tests alternative models, and quantifies key parameters that are the foundation of field-based stable isotope analysis.     

A negative carbon isotope excursion defines the boundary from Cambrian Series 2 to Cambrian Series 3 on the Yangtze Platform,South China

A globally recorded negative carbon isotope excursion characterizes the transition from Cambrian Series 2 to Cambrian Series 3. This transition is also well exposed in sedimentary successions on the Yangtze Platform, and the Wuliu–Zengjiayan section, Guizhou Province, South China has been proposed as a potential Global Stratotype Section and Point (GSSP) for this boundary. Here, we report δ¹³C_carb values for the Jianshan and the Wuliu–Zengjiayan sections. Both sections display a progressive decrease in δ¹³C from values around + 3‰ upwards in stratigraphy to a pronounced δ¹³C minimum with values as low as ? 6.9‰ at the proposed boundary level, and a return to δ¹³C values between 0 and + 1‰ in the upper part of the sections. The δ¹³C minimum is thought to be caused by a transgressive event, flooding the shelf area with ¹³C depleted basinal anoxic bottom water. Our δ¹³C data are in good agreement with carbon isotope profiles recorded elsewhere. These define the so called ROECE event (Redlichiid–Oleneliid Extinction Carbon Isotope Excursion, cf. Zhu et al., 2006, 2007) and may reflect the perturbation of the global carbon cycle during the Cambrian Series 2 to Cambrian Series 3 transition.     

Developing tree ring chronologies from New Zealand matai (Prumnopitys taxifolia (D.Don) Laub.) for archaeological dating: Stable isotope dendrochronology

Aotearoa New Zealand has a rich cultural heritage but dating wooden objects using classic dendrochronology is challenging due to a paucity of master tree-ring width chronologies for species commonly identified in the archaeological record. This paper explores the potential for using a stable isotopic approach to dendrochronology for matai (Prumnopitys taxifolia (D.Don) Laub.). A total of six annual stable oxygen isotope (δ¹⁸O) series for matai growing in the Pureora Forest Reserve, were analysed and their oxygen isotopes found to exhibit a level of coherence that enabled cross-dating. A provisional chronology covering the period 1930–2018 CE was developed and tested against a sample of matai from the Kauaeranga Valley (200 km to the north of Pureora) and a sample of miro (Prumnopitys ferruginea (D.Don) Laub.). Miro is another species common in the cultural archive, but also difficult to date using ring-width dendrochronology. Both the Kaueranga matai and Pureora miro samples cross matched against the site chronology at the correct position, but with weaker dating statistics (Student’s t: 4.26 and 4.11 respectively) compared to the results obtained for two (non-2019) sampled Pureora matai analysed in this study (Student's t: 6.80 and 5.66). These results nevertheless demonstrate the presence of a regional-scale dating signal which may be developed for future use as a precision dating tool.     

A simplified procedure for GC/C/IRMS analysis of underivatized 19-norandrosterone in urine following HPLC purification

Nandrolone and/or its precursors are included in the World Anti-doping Agency (WADA) list of forbidden substances and methods and as such their use is banned in sport. 19-Norandrosterone (19-NA) the main metabolite of these compounds can also be produced endogenously. The need to establish the origin of 19-NA in human urine samples obliges the antidoping laboratories to use isotope ratio mass spectrometry (IRMS) coupled to gas chromatography (GC/C/IRMS). In this work a simple liquid chromatographic method without any additional derivatization step is proposed, allowing to drastically simplify the urine pretreatment procedure, leading to extracts free of interferences permitting precise and accurate IRMS analysis. The purity of the extracts was verified by parallel analysis by gas chromatography coupled to mass spectrometry with GC conditions identical to those of the GC/C/IRMS assay. The method has been validated according to ISO17025 requirements (within assay precision of ±0.3‰ and between assay precision of ±0.4‰). The method has been tested with samples obtained after the administration of synthetic 19-norandrostenediol and samples collected during pregnancy where 19-NA is known to be produced endogenously. Twelve drugs and synthetic standards able to produce through metabolism 19-NA have shown to present δ¹³C values around −29‰ being quite homogeneous (−28.8 ± 1.5; mean ± standard deviation) while endogenously produced 19-NA has shown values comparable to other endogenous produced steroids in the range −21 to −24‰ as already reported. The efficacy of the method was tested on real samples from routine antidoping analyses.     

Oxygen isotope fractionation between crocodilian phosphate and water

Oxygen isotope compositions of phosphate (δ¹⁸O_p) were measured in tooth enamel from captive and wild individuals of 8 crocodilian species. A rough linear correlation is observed between the δ¹⁸O_p of all the studied species and the oxygen isotope composition of ambient water (δ¹⁸O_w). Differences in mean air temperature, diet and physiology could contribute significantly to the large scatter of δ¹⁸O_p values. The combination of these parameters results in a fractionation equation for which the slope (0.82) is lower than that expected (≥ 1) from predictive model equations that assume temperature and diet as fixed parameters. Taking into account large uncertainties, the observed oxygen isotope fractionation between phosphate and ambient water does not statistically differ from that formerly established for aquatic turtles. Case studies show that δ¹⁸O_p values of fossil crocodile tooth enamel can be used to discriminate between marine and freshwater living environments within a precision of about ± 2‰ only.     

Within-lake variability in carbon and nitrogen stable isotope signatures

1. We assessed spatial and temporal variation in carbon and nitrogen isotopic signatures in different compartments of a single lake ecosystem. Stable isotope analyses were made on samples of particulate organic matter (POM), zooplankton, periphyton, macrophytes, macroinvertebrates and fish collected from several locations throughout the ice‐free period. 2. No spatial variation in δ¹³C or δ¹⁵N values was found for pelagic samples of POM and zooplankton. However, pelagic δ¹⁵N signatures increased steadily through the summer resulting in an almost 6‰ average increase in POM and zooplankton. A concurrent decrease in epilimnetic nitrate concentrations suggested that the increase in δ¹⁵N of POM and zooplankton could have resulted from a progressive ¹⁵N‐enrichment of the available inorganic nitrogen pool as the size of this pool was reduced. 3. Significant spatial variation in isotopic ratios was observed within littoral and profundal communities. Some spatial differences were likely related to lake‐specific characteristics, such as a major inlet and a small harbour area and some were interconnected with temporal events. 4. Marked differences between spring and autumn δ¹⁵N and δ¹³C values of fish at one site probably reflected a spring spawning immigration from a larger downstream lake and also indicated limited dispersal of these immigrants. 5. Our results indicate that restricted sampling of ecosystem components from lakes may provide misleading single values for the isotope end members needed for quantitative uses of stable isotopes in mixing models and for estimating trophic position. Hence we strongly advise that studies of individual lakes, or multiple lake comparisons, that utilise stable isotope analyses should pay more attention to potential within lake spatial and temporal variability of isotope ratios.