Geoff Koehler

An On-Line Technique for the Determination of the δ18O and δ17O of Gaseous and Dissolved Oxygen

Few studies have used the stable isotopic composition of O(2) as a tracer of gas transport or biogeochemical processes in environmental research. Here we demonstrate field sampling techniques for gaseous and dissolved O(2) and describe an analytical method for measuring δ(18)O and δ(17)O values of O(2) in air, soil gas, and water samples using continuous-flow isotope-ratio mass spectrometry (CF-IRMS). A Micromass CF-IRMS was altered to accommodate a sample gas injection port prior to a CO(2) and H(2)O trap and GC column. The GC column was a 1-m, 5-Å molecular sieve column held at 35 °C. The resolved sample O(2) was introduced to the IRMS via an open split. δ(18)O and δ(17)O values were determined by measurement of O(2) isotopes at m/z 34/32 and 33/32 and comparison to a reference pulse of O(2). Repeated injections of atmospheric oxygen yielded a repeatability (±SD) of ±0.17‰ for δ(18)O and ±0.5‰ for δ(17)O. IRMS source linearity was excellent for O(2) over a sample size range of 60-400 μL. The smallest sample for routine δ(18)O and δ(17)O determinations was ∼80 μL of O(2), with a sample analysis time of 180 s. Preliminary results from a riverine and soil gas study illustrate natural oxygen isotope fractionation processes.

Realtime stable isotope monitoring of natural waters by parallel-flow laser spectroscopy.

A parallel-flow H(2)O(liquid)-H(2)O(vapor) equilibration and laser spectroscopy method provides a new way to monitor the hydrogen and oxygen stable isotopic composition of water from rivers or lakes or in hydrologic tracer tests in real time. Two custom-built equilibrator devices and one commercial membrane device were tested to determine if they could be used to convert natural water samples (lakes, rivers, groundwater) to a H(2)O gas phase for continuous online δ(18)O and δD isotopic analysis by laser spectroscopy. Both the commercial minimodule device and the marble-filled equilibrator produced water vapor in isotopic equilibrium with the flowing liquid water, suggesting that unattended field measurement using these devices is possible. Oxygen isotope disequilibrium was indicated using the minimodule device at low temperatures.

On the use of stable oxygen isotope (δ18O) measurements for tracking avian movements in North America

Tracking migratory animals has benefitted using measurements of naturally occurring stable isotopes of hydrogen (δ2H) in keratinous tissues such as hair and feathers to link animal origins to continental patterns or isoscapes of δ2H in precipitation. However, for most taxa, much less information exists on the use of stable oxygen isotope ratios (δ18O) despite the fact that δ2H and δ18O are strongly linked in environmental waters through the meteoric relationship and the possibility of using both isotopes to infer greater information on origins and climatic conditions where tissues are grown. A fundamental requirement of using stable isotopes to assign individuals and populations to origins is the development of a rescaling function linking environmental food web signals to the tissue of interest and for birds, this has not been carried out. Here, we derived the relationship between H and O isotopes in known source feathers of 104 individuals representing 11 species of insectivorous passerines sampled across the strong precipitation isoscape of North America. We determined again a strong expected relationship between feather δ2H (δ2Hf) and long-term amount-weighted precipitation δ2H (δ2Hp; r2 = 0.77), but the corresponding relationship between δ18Of and δ18Op was poor (r2 = 0.32) for the same samples. This suggests that δ2H measurements are currently more useful for assignment of insectivorous songbirds to precipitation isoscapes but does not preclude other uses of the δ18Of data. Currently, mechanisms responsible for the decoupling of H and O isotopes in food webs is poorly known, and we advocate a much broader sampling of both isotopes in the same keratinous tissues across precipitation isotope gradients and across taxa to resolve this issue and to increase the power of using water isotopes to track migratory animals.

Measurement of stable isotope activities in saline aqueous solutions using optical spectroscopy methods

The requirement to measure the stable isotopic compositions of saline pore fluids by optical methods has prompted a re-evaluation of the isotopic salt effect for common salts. Hydrogen and oxygen isotopic salt effects were measured at room temperature (21°C) by optical methods. For hydrogen isotopes, our results agree well with those of previous studies and better define these effects at low temperatures. In contrast, measured oxygen isotope salt effects disagree within error for NaCl and CaCl2 solutions from those reported previously. Subtle differences between measurement methods may account for the discrepancy. In studies that involve highly saline fluids, the isotopic salt effect must be taken into account because modern methods that measure stable isotopic compositions as activities or concentrations may be not directly comparable to historical data sets.

Determination of the Hydrogen Isotopic Compositions of Organic Materials and Hydrous Minerals Using Thermal Combustion Laser Spectroscopy

Hydrogen isotopic compositions of hydrous minerals and organic materials were measured by combustion to water, followed by optical isotopic analysis of the water vapor by off-axis integrated cavity output spectroscopy. Hydrogen and oxygen isotopic compositions were calculated by numerical integration of the individual isotopologue concentrations measured by the optical spectrometer. Rapid oxygen isotope exchange occurs within the combustion reactor between water vapor and molecular oxygen so that only hydrogen isotope compositions may be determined. Over a wide range in sample sizes, precisions were ±3-4 per mil. This is comparable but worse than continuous flow-isotope ratio mass spectroscopy (CF-IRMS) methods owing to memory effects inherent in water vapor transfer. Nevertheless, the simplicity and reduced cost of this analysis compared to classical IRMS or CF-IRMS methods make this an attractive option to determine the hydrogen isotopic composition of organic materials where the utmost precision or small sample sizes are not needed.

Combining Denitrifying Bacteria and Laser Spectroscopy for Isotopic Analyses (δ 15 N, δ 18 O) of Dissolved Nitrate

We present a novel approach for nitrogen (δ(15)N) and oxygen (δ(18)O) isotopic analysis of nitrate in water based on the isotopic analysis of N2O produced from the conversion of NO3(-) by cultured denitrifying bacteria and off-axis integrated cavity output spectroscopy (OA-ICOS). The headspace N2O was manually injected into an OA-ICOS isotopic N2O laser analyzer through a syringe septum port. Sample analysis time was ∼300 s. The use of OA-ICOS technology yields accurate and precise δ(15)N and δ(18)O results for dissolved nitrate samples when nonlinearity issues are considered. This new isotope analytical technique thus improves the isotopic analysis of nitrates by (i) providing accurate measurements of δ(15)N and δ(18)O without preconcentration, (ii) eliminating interferences by other gas substances (i.e., H2O and CO2), and (iii) reducing extensive maintenance and costs of isotope ratio mass spectrometers (IRMS). This approach will greatly streamline the identification and quantification of nitrate sources in aquatic systems.

Spatio-temporal variation of nitrate sources to Lake Winnipeg using N and O isotope (δ15N, δ18O) analyses

Anthropogenic nitrogen inputs into Lake Winnipeg, Canada, from watershed sources have increased during the last decades, contributing to eutrophication. These nutrient N inputs include loadings from agriculture (inorganic fertilizer and animal waste) and urban sources (wastewater discharge from sewage treatment plants). The aim of this study was to evaluate the sources and seasonal patterns of dissolved nitrates in two major contributors to Lake Winnipeg; the Assiniboine and Red rivers. The relative contribution of nitrate sources was estimated using Bayesian isotope mixing models incorporating δ15N and δ18O values of dissolved nitrate. Overall, δ15N values of nitrate in the rivers ranged from -2 ‰ to +20 ‰, and δ18O values ranged from -20 ‰ to +20 ‰, which indicated variable contribution of nitrate sources, depending on the river reach and seasonal period of sampling. The results indicated that nitrate in the Assiniboine River originated up to 62 % from waste or municipal sources (i.e. manure and/or waste water discharge), whereas ca. 40 % of nitrate in the Red River originated predominantly from inorganic agricultural fertilizers. These different source contributions were temporally variable, with a decrease in fertilizer loading following spring snowmelt. We found higher proportions of inorganic fertilizers in the Assiniboine River watershed during flooding, which has relevant implications for water nutrient management in response to stochastic flooding events.

Effects of tanning on the stable isotopic compositions of hair

We investigated the effect of tanning on the stable isotopic compositions (CNHOS) of hair keratin. Samples of hair from polar bear (Ursus maritimus) hides were collected before and after the tanning process from a commercial tannery. With the exception of sulphur isotopes, tanning did not alter the stable isotopic compositions of hair. δ34S values were slightly more positive (1 per mil) in hair that had gone through the tanning process, likely as a result of the tanning agent, alum (Al2(SO4)3) or exposure to sulphuric acid (H2SO4). This indicates that, with the exception of sulphur isotopes, hair keratin is resistant to subsequent isotopic alteration by the techniques used in tanning of animal hides and thus the original stable isotopic information is likely to be preserved in archived samples, such as taxidermic mounts and museum specimens. This is an important consideration when dealing with ecological and forensic applications to wildlife, such as evaluating provenance or migratory reconstructions, and so will assist in conservation efforts and investigations of trafficking and poaching.