Interpretation Of Stable Isotope Data Research Articles

Ontogenetic niche shifts and resource partitioning of lake trout morphotypes

Resource polymorphisms are widely observed in fishes; however, ontogenetic contributions to morphological and ecological differences are poorly understood. This study examined whether ontogenetic changes in niche partitioning could explain morphological and buoyancy differences between lake trout ( Salvelinus namaycush ) morphotypes in Great Slave Lake (Northwest Territories, Canada). Morphometric analysis, buoyancy, capture depth, diet, and stable isotope data were used in concert to determine whether (i) differences occur in small, as well as large, lake trout, (ii) ontogenetic changes in morphology and buoyancy correlate with shifts in depth or diet, and (iii) a subset of small trout, putatively identified as “humpers”, are distinct from other morphotypes. Ontogenetic changes in lake trout morphology were associated with an ecological shift between benthic and pelagic feeding. Resource partitioning between lean and siscowet-like trout occurred within benthic (small trout) and pelagic (large trout) habitats. The humper subset did not differ from small siscowet-like trout. By combining multiple methods and an ontogenetic perspective, our study provides novel perspectives on resource polymorphisms in large, deep lakes and on existing interpretations of stable isotope data from large lakes in general.

Dogs, divers, deer and diet. Stable isotope results from Star Carr and a response to Dark

We here respond to Dark's [Dark, P., 2003. Dogs, a crane (not duck) and diet at Star Carr: a response to Schulting and Richards. Journal of Archaeological Science 30, 1353–1356] criticisms of our previous paper [Schulting, R.J., Richards, M.P., 2002. Dogs, ducks, deer and diet: a reappraisal of the stable isotope evidence on early domestic dogs from the Vale of Pickering, north-east England. Journal of Archaeological Science 29, 327–333] in which we supported the previous interpretation of stable isotope data from the Seamer Carr dog [Clutton-Brock, J., Noe-Nygaard, N., 1990. New osteological and C-isotope evidence on Mesolithic dogs: companions to hunters and fishers at Star Carr, Seamer Carr and Kongemose. Journal of Archaeological Science 17, 643–653] as indicating a marine-influenced diet. Additional isotopic data are presented on fauna from Star Carr, most importantly from two piscivorous diving birds. These new data further support the original contention that marine protein is the most likely source of the elevated carbon and nitrogen isotope values seen in the Seamer Carr dog. In addition, cutmarks on the humerus of a Brent goose, and the presence of an additional element (right femur) attributable to the previously known adult dog from Star Carr, are noted for the first time. Two new AMS determinations are presented for Star Carr, placing an adult dog in the period 9680 ± 55 BP (9270–8840 cal BC) and a subadult dog somewhat later at 9342±41 BP (8735–8475 cal BC). Unlike Seamer Carr, both dogs show entirely terrestrial (non-marine) diets.

Interannual variability of Greenland winter precipitation sources: Lagrangian moisture diagnostic and North Atlantic Oscillation influence

We present a new Lagrangian diagnostic for identifying the sources of water vapor for precipitation. Unlike previous studies, the method allows for a quantitative demarcation of evaporative moisture sources. This is achieved by taking into account the temporal sequence of evaporation into and precipitation from an air parcel during transport, as well as information on its proximity to the boundary layer. The moisture source region diagnostic was applied to trace the origin of water vapor for winter precipitation over the Greenland ice sheet for 30 selected months with pronounced positive, negative, and neutral North Atlantic Oscillation (NAO) index, using the European Centre for Medium‐Range Weather Forecasts' ERA‐40 reanalysis data. The North Atlantic and the Nordic seas proved to be the by far dominant moisture sources for Greenland. The location of the identified moisture sources in the North Atlantic basin strongly varied with the NAO phase. More specifically, the method diagnosed a shift from sources north of Iceland during NAO positive months to a maximum in the southeastern North Atlantic for NAO negative months, qualitatively consistent with changes in the concurrent large‐scale mean flow. More long‐range moisture transport was identified during the NAO negative phase, leading to the advection of moisture from more southerly locations. Different regions of the Greenland ice sheet experience differing changes in the average moisture source locations; variability was largest in the north and west of Greenland. The strong moisture source variability for Greenland winter precipitation with the NAO found here can have a large impact on the stable isotope composition of Greenland precipitation and hence can be important for the interpretation of stable isotope data from ice cores. In a companion paper, the implications of the present results are further explored in that respect.

Interspecific and nutrient-dependent variations in stable isotope fractionation: experimental studies simulating pelagic multitrophic systems

Stable isotope signatures of primary producers display high inter- and intraspecific variation. This is assigned to species-specific differences in isotope fractionation and variable abiotic conditions, e.g., temperature, and nutrient and light availability. As consumers reflect the isotopic signature of their food source, such variations have direct impacts on the ecological interpretation of stable isotope data. To elucidate the variability of isotope fractionation at the primary producer level and the transfer of the signal through food webs, we used a standardised marine tri-trophic system in which the primary producers were manipulated while the two consumer levels were kept constant. These manipulations were (1) different algal species grown under identical conditions to address interspecific variability and (2) a single algal species cultivated under different nutrient regimes to address nutrient-dependent variability. Our experiments resulted in strong interspecific variation between different algal species (Thalassiosira weissflogii, Dunaliella salina, and Rhodomonas salina) and nutrient-dependent shifts in stable isotope signatures in response to nutrient limitation of R. salina. The trophic enrichment in (15)N and (13)C of primary and secondary consumers (nauplii of Acartia tonsa and larval herring) showed strong deviations from the postulated degree of 1.0 per thousand enrichment in delta(13)C and 3.4 per thousand enrichment in delta(15)N. Surprisingly, nauplii of A. tonsa tended to keep "isotopic homeostasis" in terms of delta(15)N, a pattern not described in the literature so far. Our results suggest that the diets' nutritional composition and food quality as well as the stoichiometric needs of consumers significantly affect the degree of trophic enrichment and that these mechanisms must be considered in ecological studies, especially when lower trophic levels, where variability is highest, are concerned.

Isotopic evidence of distinct feeding ecologies and movement patterns in two migratory predators (yellowfin tuna and swordfish) of the western Indian Ocean

Ecologists primarily use δ15N values to estimate the trophic level of organisms, while δ13C, and even recently δ15N, are utilized to delineate feeding habitats. However, many factors can influence the stable isotopic composition of consumers, e.g. age, starvation or isotopic signature of primary producers. Such sources of variability make the interpretation of stable isotope data rather complex. To examine these potential sources of variability, muscle tissues of yellowfin tuna (Thunnus albacares) and swordfish (Xiphias gladius) of various body lengths were sampled between 2001 and 2004 in the western Indian Ocean during different seasons and along a latitudinal gradient (23°S to 5°N). Body length and latitude effects on δ15N and δ13C were investigated using linear models. Both latitude and body length significantly affect the stable isotope values of the studied species but variations were much more pronounced for δ15N. We explain the latitudinal effect by differences in nitrogen dynamics existing at the base of the food web and propagating along the food chain up to top predators. This spatial pattern suggests that yellowfin and swordfish populations exhibit a relatively unexpected resident behaviour at the temporal scale of their muscle tissue turnover. The body length effect is significant for both species but this effect is more pronounced in swordfish as a consequence of their different feeding strategies, reflecting specific physiological abilities. Swordfish adults are able to reach very deep water and have access to a larger size range of prey than yellowfin tuna. In contrast, yellowfin juveniles and adults spend most of their time in the surface waters and large yellowfin tuna continue to prey on small organisms. Consequently, nitrogen isotopic signatures of swordfish tissues are higher than those of yellowfin tuna and provide evidence for different trophic levels between these species. Thus, in contrast to δ13C, δ15N analyses of tropical Indian Ocean marine predators allow the investigation of complex vertical and spatial segregation, both within and between species, even in the case of highly opportunistic feeding behaviours. The linear models developed in this study allow us to make predictions of δ15N values and to correct for any body length or latitude differences in future food web studies.

Evaluating 15N/ 14N and 13C/ 12C isotope ratio analysis to investigate trophic relationships of elaterid larvae (Coleoptera: Elateridae)

Carbon and nitrogen isotope ratios in consumer tissues can be used to analyse the diet and trophic level of soil animals. However, life history traits may significantly influence stable isotope patterns. We evaluated in a series of experiments how stable isotope ratios of carbon ( 13C/ 12C) and nitrogen ( 15N/ 14N) at natural abundance can be used to study the diet and trophic position of long-lived macro-invertebrates, elaterid larvae, which are major below-ground herbivores. Small, but significant differences in δ 13C signatures were found between the larvaes’ anterior and posterior body segments, whereas exuvia reflected the body's overall isotopic composition. The species-specific trophic shift (±SE) in δ 15N for Agriotes obscurus and Agriotes sputator (1.62±0.24‰ and 1.08±0.27‰, respectively) was significantly lower than “mean enrichment estimates” reported in the literature, showing the limited applicability of such generalised estimates in studies of invertebrate trophic ecology. To avoid false-positive assignments to two trophic levels due to variation in δ 15N values, a minimum sample size of three and five individuals for A. obscurus and A. sputator, respectively, was needed to reduce this risk to below α=5%. Keeping elaterid larvae for up to 128 days without food did not affect their isotopic signatures, in contrast to previous studies on starving animals. Switching wireworms to isotopically different diets induced changes in their isotopic signatures within 2 weeks. Changes, however, were significant only when the isotopic difference between diets was large. We conclude that experimental studies evaluating how specific life history traits affect stable isotope signatures in consumers have to precede any interpretation of stable isotope data gathered in the field.

Rates and Mechanisms of Isotopic Exchange

### Background The proliferation of stable isotope laboratories in recent years has led to a marked increase in the routine use of stable isotope analyses in geochemical studies. This trend is likely to continue because of the development of micro-analytical techniques (e.g. laser ablation, ion microprobe, femtomole-carrier gas methods, etc.) for stable isotope analysis and the potential of these techniques to reveal the detailed sequence of thermal and fluid histories preserved in the rock record (see McKibben et al. 1998). Equilibrium isotope fractionation factors and rates of isotopic exchange form the cornerstones for the interpretation of stable isotope data from natural systems. Indeed, determination of the distribution of the stable isotopes of C-O-H-S in gases, fluids, and minerals has become a standard and extremely powerful approach in elucidating the temperatures, material fluxes, rock and fluid origins, and time scales associated with ancient and active fluid-rock interaction processes in the Earth’s crust (see Valley et al. 1986). Although the thermodynamic principles underlying isotopic fractionation behavior were developed more than fifty years ago by Urey (1947), the calibration of fractionation factors has proven to be difficult. During the last four decades, a great deal of progress has been made in determining the equilibrium fractionations of 18O/16O, D/H, 13C/12C, and 34S/32S among fluid and mineral components at high temperatures (≥400°C), both from experimental exchange studies, theoretical calculations (using vibrational spectra), semi-empirical calculations (using bond-type considerations), and empirical calibrations based on natural assemblages (see references in O’Neil 1986; Kyser 1987; Criss 1991, 1999; Chacko et al., this volume). Of these, the experimental method is the most direct because it involves the least number of a priori assumptions. Despite the utility of the equilibrium approach in quantifying the isotopic behavior in select water-rock …

SIMS stable isotope measurement: counting statistics and analytical precision

AbstractAnalytical precision is vital in the interpretation of stable isotope data collected by secondary ion mass spectrometry (SIMS) given the small analysis volumes and the small magnitude of natural isotopic variations. The observed precision of a set of measurements is represented by the standard deviation(precision of an individual measurement) or the standard error of the mean (precision of the mean value). The SIMS data show both systematic variations with time and random Poisson variability, but the former largely cancel out when data for two different isotopes are expressed as a ratio. The precision of a SIMS isotope ratio routinely matches that predicted by Poisson counting statistics and can approach that of conventional bulk analysis techniques for counting times of several hours. All sample analyse must be calibrated for instrumental mass fractionation using SIMS analyses of a standard material. There is often a gradual drift in the mass fractionation with time, but this can be modelled by least-squares regression of the standard isotope ratios. Drift in the sample analyses is eliminated by using the relevant point on this regression line to calibrate each sample. The final precision of a corrected isotope ratio must take into account the scatter in both the sample and the standard data.

Distribution and Metabolic Diversity of Microorganisms in Deep Igneous Rock Aquifers of Finland

Groundwater samples from 200- to 950-m depths in four igneous rock sites in Finland were investigated for different metabolic groups of microorganisms, and the data were compared with the available geochemical record. Samples were collected with a pressurized groundwater sampling system developed for gas and microbiological sampling. Two of the sites had groundwater that was fresh, with < 0.2 g/l dissolved solids, whereas that at the two other sites was much more saline, reaching a maximum of 24 g/l dissolved solids. The groundwater contained gases, 33 to 340 ml/l, with nitrogen or methane dominating. Total cell numbers were 10 5 to 10 6 cells/ml, which is typical for deep igneous rock aquifers. Growth media were designed to mimic the actual groundwater chemistry at each sampling point and used for most probable number enumeration of methanogens, acetogens, sulfate-reducing bacteria (SRB), and iron-reducing bacteria (IRB). SRB predominated in sites where iron sulfide fracture-filling minerals are common. IRB were the main population in one site where iron sulfide fracture minerals are not present, but iron hydroxide fracture minerals predominate. Fracture-filling minerals were a better indicator of microbial populations than was groundwater chemistry. Low numbers of autotrophic methanogens were cultured. One of several possible interpretations of stable isotope data suggested that most of the detected methane is thermogenic, which would correlate with few active methanogens. However, we concluded other interpretations were also possible.

Palaeoclimatic interpretation of stable isotope data from Holocene speleothems of the Waitomo district, North Island, New Zealand

One straw stalactite and three stalagmites from the Waitomo district of North Island, New Zealand, were examined for stable isotopes of oxygen and carbon with a view to interpreting their palaeoclimate signal. Dating was by uranium series and AMS 14C for the stalagmites and by gamma-ray spectrometry for the straw. Records were thus established for about 100 years for the straw and 3.9, 10.1 and 10.2 ka for the stalagmites. The range of variability in d18Oc and d13Cc this century is about two-thirds of that experienced over the entire Holocene, and is most simply explained in terms of the oceanic source area of rain. Stable isotope variations in three stalagmites show some general similarities, but have significant differences in detail, which underlines the necessity to base palaeoclimatic interpretations on more than one speleothem record. The d18Oc of each stalagmite varies positively with temperature, indicating the dominance of the ocean source of evaporation in determining the isotopic composition of precipitation and hence speleothem calcite in the Holocene. This conclusion is contrary to that of other authors working in New Zealand, who identified a negative relationship between d18Oc and temperature, while examining time periods extending across the Last Glacial Maximum. It is concluded here that, whereas the ice volume effect dominates the large climatic shifts of glacial-interglacial amplitude, the oceanic source effect becomes more important during the period of relatively stable sea level during the Holocene. Results also indicate a late-Holocene altitudinal effect of 0.2‰ d18Oc per 100 m and an associated temperature relationship of about 0.26‰ per°C. The average of two records identifies the postglacial climatic optimum to lie in the interval from prior to 10 ka BP to 7.5 ka BP, when d18Oc values were up to 0.6‰ less negative than present, implying an average annual mean temperature that was up to 2.3°C warmer. The average of three speleothem records for the last 3900 years reveals the coldest period of the Holocene to have occurred about 3 to 2 ka BP, when d18Oc values were typically 0.4‰ more negative than present and average temperatures may have been 1.5°C cooler. Mean annual temperature variability of about 2°C was sometimes experienced in little more than 100 years.

A late Weichselian stable isotope and Molluscan Stratigraphy from Southern Sweden

Late Weichselian and early Holocene partly carbonate-rich sediments from a small lacustrine deposit in Skane, southern Sweden, were analysed for stable carbon and oxygen isotopes and mollusc assemblages. The data from these analyses reveal a pronounced climatic warming at c. 12,400 BP, accompanied by a general change from continental to maritime conditions. Two significant climatic oscillations were identified, dated ate. 12,200-12,000 BP ande. 11,000-10,200 BP. The climatic cooling of the latter, corresponding to the Younger Dryas stadial, may have been initiated well before 11,000 BP. A climatic oscillation of minor amplitude may have occurred around 11,500 BP. Distinct variations in delta(13)C of bulk organic material and shell carbonate are discussed in terms of climatically-induced changes in lake-water chemistry. The general assumption of a positive correlation between delta(18)O of limnic carbonates and mean annual air temperature is questioned. A climatic warming may be associated with a depletion of (18)O in limnic carbonates. Comparisons of palaeoclimatic information from the stable isotope data with that provided by molluscs allow the identification of aspects of climatic and environmental change such as water temperature, water depth, and aquatic vegetation density. Changes in the mollusc assemblage reflect changes in the limnic environment and facilitates the interpretation of stable isotope data. (Less)

A Late Weichselian stable isotope stratigraphy compared with biostratigraphical data: A case study from southern Sweden

AbstractLate Weichselian lake sediments from a site in southern Sweden, were analysed for stable carbon and oxygen isotopes, as well as plant macrofossils and insect remains. By comparison of independent data sets, general climatic changes were demonstrated. Lithological, chemical and stable isotope data reveal two significant climatic oscillations at ca. 12 200–12 000 and ca. 11 000–10 200 yr BP respectively. Continental climatic conditions, indicated by evaporative enrichment of 18O in lake marl, characterise parts of the early lake history, including the Older Dryas Stadial. Distinct variations of δ13C in organic material is discussed in terms of climatically induced changes in lake‐water chemistry. Different types of photosynthetic assimilation of dissolved inorganic carbon is proposed as a contributing factor influencing lake marl δ13C. The universal application of a positive correlation between lake marl δ18O and mean annual air temperature is questioned.Quantifications of mean summer and winter temperatures based on beetle analysis show a climatic optimum around 12 000 yr BP, a marked cooling around 11 000 yr BP and a strong amelioration at ca. 10 200 yr BP. These climatic events were accompanied by distinct changes in aquatic vegetation. Plant macrofossil and insect analyses indicate an open vegetation during the entire period studied.Biostratigraphical data reflecting local limnic and terrestrial vegetation and regional climate facilitate the interpretation of stable isotope data.

Origin of Natural Gas in Sacramento Basin, California

The remnant of a late Mesozoic-early Cenozoic fore-arch basin, the Sacramento Valley is principally a dry gas province. Complex stratigraphic relations, widespread deformation and faulting, and the presence of igneous intrusives testify to an active tectonic history. Sediments are organically immature down to an average of 10,000 ft (3,000 m) and are geopressured throughout much of the basin. Small fields with nitrogen concentrations as high as 88% are found along the shallow eastern margin. Gases were sampled from 94 producing wells. Methane ^dgr13C and ^dgrD values range from -61 to -15 ^pmil and -200 to -96 ^pmil, respectively, indicating a wide range of source rock maturities. Wet gases in the western delta area (^SgrC2-C5/^SgrC1-C5 <= 12.5%) have ethane ^dgr13C values between -31 and -20 ^pmil and propane ^dgr13C values between -27 and -12 ^pmil. Dry gases elsewhere in the basin (^SgrC2-C5/^SgrC1-C5 <= 1.6%) have ethane ^dgr13C values as light as -58 ^pmil. The chemical and stable isotope data are consistent with four principal sources. Gases in the western delta area were generated from mature source rocks within the oil and condensate window. Gases in the northern and southeastern basin are mixtures of indigenous gas (mostly microbial methane with small quantities of diagenetic ethane), dry thermogenic gas migrated from post-mature source rocks in the basin deep, and nitrogen-rich gases thought to originate in metasedimentary rocks deep beneath the sediment. These results place logical constraints on the location of productive traps with high-Btu gas and suggest caution in the interpretation of stable isotope data from other tectonically active basins due to dynamic migration and mixing processes.