Chapter 3: Stable Isotope Background

2011

We determined both carbon and nitrogen isotope ratios of ten organic reference materials (CERKU-01 to CERKU-10) in the Center for Ecological Research (CER), Kyoto University, and three organic reference materials (BG-A, BG-P and BG-T) in the Institute of Biogeosciences (BioGeos), JAMSTEC, using an internationally recommended calibration method of two-point anchoring. The reference materials cover ! 13 CVPDB range of-34.92 to-9.45‰ and ! 15 NAir range of-5.22 to 22.71‰, and can be used to measure isotope ratios of naturally occurring substances.

International Journal of Mass Spectrometry and Ion Physics, 1973

In order to determine isotopic abundance ratios such as ' 3C/'2C, 180/160 Or "N/14N, a mass spectrometer reads the ratio of the voltages which are generated by the isotopic ion beams, trapped by a double collector system. The effect of the gas iniet pressure on the voltage ratio in the case of nitrogen and carbon dioxide is determined experimentally, using a Varian M86_ A knowledge of this pressure dependence provides a method of determining the voltage ratio, which is independent of the inlet pressure. This paper partly reviews the work of Craig and gives a detailed treatment of the measuring procedure, of the corrections to be applied to the voltage ratios, viz. for pressure effects, of the way in which the voltage ratios are transformed into isotopic abundances, and of disturbing factors ground and sample contamination. The corrections will be discussed successively are to be applied. In every case the exact equation approximation is given for use with natural samples. The major contributions to the technique of such as spectrometer backin the order in which they is deduced, while often an mass-spectrometric analysis of isotopic abundance ratios are due to Craig (1957) and Dansgaard (1961). The original choice of the latter to use absolute differences between a sample and standard, rather than relative, has been abandoned_ Craig made in his correction formula approximations applicable to samples deviating not more than a few percent from the standard and based on an accuracy of 0.1 o/Go, u.-bile nowadays the precision is often better than that. Consequently, we thought it necessary to review-the whole procedure and make revisions, where necessary. Although the stable isotopes of carbon, oxygen and nitrogen in CO2 and 274 N2 respectively are specifically dealt with, the general equations also apply to isotopic analyses on other gases, such as argon (36Ar/40Ar) an+~S0,'(34S/32S).

Geochimica Et Cosmochimica Acta, 1953

Several hundred samples of carbon from various geologic sources have been analyzed in a new survey of the variation of the ratio C13/C12 in nature. Mass spectrometric determinations were made on the instruments developed by H. C. Urey and his co-workers utilizing two complete feed systems with magnetic switching to determine small differences in isotope ratios between samples and a standard gas. With this procedure variations of the ratio C13/C12 can be determined with an accuracy of ±0.01% of the ratio.The results confirm previous work with a few exceptions. The range of variation in the ratio is 4.5%. Terrestrial organic carbon and carbonate rocks constitute two well defined groups, the carbonates being richer in C13 by some 2%. Marine organic carbon lies in a range intermediate between these groups. Atmospheric CO2 is richer in C13 than was formerly believed. Fossil wood, coal and limestones show no correlation of C13/C12 ratio with age. If petroleum is of marine organic origin a considerable change in isotopic composition has probably occurred. Such a change seems to have occurred in carbon from black shales and carbonaceous schists. Samples of graphites, diamonds, igneous rocks and gases from Yellowstone Park have been analyzed. The origin of graphite cannot be determined from C13/C12 ratios. The terrestrial distribution of carbon isotopes between igneous rocks and sediments is discussed with reference to the available meteoritic determinations. Isotopic fractionation between iron carbide and graphite in meteorites may indicate the mechanism by which early fractionation between deep seated and surface terrestrial carbon may have occurred.

Marine Ecology Progress Series, 2006

We compared effects of different sample preparation techniques on the stable carbon and nitrogen isotope values of fish muscle, whole crustaceans and particulate organic matter (POM). Comparisons were also made to untreated (i.e. dried and homogenised) samples. Relatively carbonate-rich samples treated with weak acid (0.1 N HCl), either by quickly wetting or acid dampening, were on average 1.3 ‰ more enriched in 13 C than duplicates soaked in 2 N HCl for 5 min, indicating incomplete carbonate removal with the weaker acid. In comparison, no differences in δ 15 N values were found between acid treatments, and a following water rinse had no effect on the δ 13 C or δ 15 N values. Chloroform-methanol (2:1 by volume) extraction overnight removed less lipids than Soxhlet extraction with 7% methanol in dichloromethane for 2 h, resulting in ~1.2 ‰ difference in δ 13 C values between treatments of lipid-rich duplicates. Different lipid-extraction methods did not lead to consistent differences in δ 15 N values, however. Depending on the lipid and carbonate content, untreated samples were depleted in 13 C by 0.8 to 4.4 ‰ and in 15 N by 0.6 to 1.4 ‰ compared to treated duplicates. We conclude that δ 13 C and δ 15 N values of samples with low lipid and carbonate content are highly comparable among studies regardless of pre-treatment methods, whereas the δ 13 C values of relatively lipid-and/or carbonate-rich samples must be carefully considered based on the pretreatment applied to samples. In comparison, δ 15 N values are relatively robust to differences in carbonate and lipid-removal methods, and δ 15 N values of untreated vs. carbonate-and lipid-treated samples are comparable within ±1.0 ‰.

Stable isotopes and changing paradigms on soil nitrogen and carbon biogeochemistry. Ecosistemas 19(3):14-23. Many perceptions on ecosystem biogeochemistry are based on flow diagrams in which element pools (boxes) are interconnected by abiotic and biotic mechanisms controlling transformations of chemical species and flows among the pools (arrows). Because of the ability of stable isotopes to integrate such processes over time and space, they have played a central role in our current understanding of nutrient cycling, particularly in the cases of N and C. Most fluxes and transformations involved in terrestrial nutrient cycling cross over or take place in soil compartments. We here review the development of new paradigms in soil nitrogen (and carbon) cycling research, to which stables isotopes contributed through three main approaches: (i) as integrators of nutrient input/output budgets from broad ecosystems compartments or "black-boxes", (ii) as tracers to unravel specific processes and end-member pools operating within these black-boxes, and (iii) as markers or indicators of nutrient use, availability and deficiency to plants. New challenges and future perspective to that respect are also discussed.

Functional Ecology, 2007

Stable isotope data are widely used to track the origins and transformations of materials in food webs. Reliable interpretation of these data requires knowledge of the factors influencing isotopic fractionation between diet and consumer. For practical reasons, isotopic fractionation is often assumed to be constant but, in reality, a range of factors may affect fractionation.

Journal of Animal Ecology, 2008

Lipids have more negative δ 13 C values relative to other major biochemical compounds in plant and animal tissues. Although variable lipid content in biological tissues alters results and conclusions of δ 13 C analyses in aquatic food web and migration studies, no standard correction protocol exists. 2. We compared chemical extraction and mathematical correction methods for freshwater and marine fishes and aquatic invertebrates to better understand impacts of correction approaches on carbon ( δ 13 C) and nitrogen ( δ 15 N) stable isotope data. 3. Fish and aquatic invertebrate tissue δ 13 C values increased significantly following extraction for almost all species and tissue types relative to nonextracted samples. In contrast, δ 15 N was affected for muscle and whole body samples from only a few freshwater and marine species and had a limited effect for the entire data set. 4. Lipid normalization models, using C : N as a proxy for lipid content, predicted lipid-corrected δ 13 C for paired data sets more closely with parameters specific to the tissue type and species to which they were applied. 5. We present species-and tissue-specific models based on bulk C : N as a reliable alternative to chemical extraction corrections. By analysing a subset of samples before and after lipid extraction, models can be applied to the species and tissues of interest that will improve estimates of dietary sources using stable isotopes.

Many organisms experience fasting in their life time, and this physiological process has the potential to alter stable isotope values of organisms, and confound interpretation of food web studies. However, previous studies on the effects of fasting and starvation on stable isotopes show disparate results, and have never been quantitatively synthesized. We performed a laboratory experiment and meta-analysis to determine how stable isotopes of d 15 N and d 13 C change with fasting, and we tested whether moderators such as taxa and tissue explain residual variation. We collected literature data from a wide variety of taxa and tissues. We surveyed over 2000 papers, and of these, 26 met our selection criteria, resulting in 51 data points for d 15 N, and 43 data points for d 13 C. We determine that fasting causes an average increase in the isotopic value of organisms of 0.5& for d 15 N and that the only significant moderator is tissue type. We find that the overall effect size for d 13 C is not significant, but when the significant moderator of tissue is considered, significant increases in blood and whole organisms are seen with fasting. Our results show that across tissues and taxa, the nutritional status of an organism must be considered when interpreting stable isotope data, as fasting can cause large differences in stable isotope values that would be otherwise attributed to other factors.

Oecologia, 2005

The increasing popularity of stable isotope analysis (SIA) as an ecological research tool and the ease of automated analysis have created a knowledge gap between ecologists using SIA and the operators of isotope ratio mass spectrometry (IRMS) equipment. This has led to deterioration in the understanding of IRMS methodology and its proper dissemination in the ecological literature. Of 330 ecological research papers surveyed, 63 (19%) failed to report any form of analytical error associated with IRMS. Of the 267 papers that reported analytical error, there was considerable variation both in the terminology and approach used to quantify and describe error. Internal laboratory standards were often used to determine the analytical error associated with IRMS, so chosen because they are homogenous and have isotopic signatures that do not vary over time. We argue that true ecological samples collected in the field are complex bulk mixtures and often fail to adhere to these two criteria. Hence the analytical error associated with samples is potentially greater than that of standards. A set of standard data run over time with a precision typically reported in the ecological literature (1 standard deviation: 1SD=0.26&) was simulated to determine the likelihood of spurious treatment effects depending on timing of analysis. There was a 90% likelihood of detecting a significant difference in the stable nitrogen ratio of a single sample (homogenized bovine liver) run in two time periods when n>30. Minor protocol adjustments, including the submission of blind replicates by researchers, random assignment of sample repeats within a run by analytical labs, and reporting 1SD of a single sample analyzed both within and between runs, will only serve to strengthen the interpretation of true ecological processes by both researchers and reviewers.