An automated technique for measuring deltaD and delta18O values of porewater by direct CO2 and H2 equilibration

An evaluation of materials and methods for vapour measurement of the isotopic composition of pore water in deep, unsaturated zones

The development of in situ vapour sampling methods to measure δ(2)H and δ(18)O in pore water of deep, unsaturated soil profiles, including mine tailings and waste rock, is required to improve our ability to track water migration through these deposits. To develop appropriate field sampling methods, a laboratory study was first undertaken to evaluate potential materials and sampling methods to collect and analyse vapour samples from unsaturated mine waste. Field methods were developed based on these findings and tested at two mine sites using either on-site analyses with a portable isotope laser spectrometer or sample collection and storage prior to laboratory analyses. The field sites included a series of deep (>50 m) multiport profiles within a coal waste rock dump and open wells installed in a sand tailings dyke at an oil sands mine. Laboratory results show that memory effects in sample bags and tubing require 3-5 pore volumes of vapour flushing prior to sample collection and sample storage times are limited to 24 h. Field sampling highlighted a number of challenges including the need to correct for sample humidity and in situ temperature. Best results were obtained when a portable laser spectrometer was used to measure vapour samples in situ.

An inexpensive, fast, and reliable method for vacuum extraction of soil and plant water for stable isotope analyses by mass spectrometry

The stable isotopes of water (hydrogen and oxygen isotopes) are of utmost interest in ecology and the geosciences. In many cases water has to be extracted directly from a matrix such as soil or plant tissue before isotopes can be analyzed by mass spectrometry. Currently, the most widely used technique for water is cryogenic vacuum extraction. We present a simple and inexpensive modification of this method and document tests conducted with soils of various grain size and tree core replicates taken on four occasions during 2010. The accuracies for sandy soils are between 0.4‰ and 3‰ over a range of 21‰ and 165‰ for δ(18)O and δ(2)H, respectively. Spiking tests with water of known isotope composition were conducted with soil and tree core samples; they indicate reliable precision after an extraction time of 15 min for sandy soils. For clayey soils and tree cores, the deviations were up to 0.63‰ and 4.7‰ for δ(18)O and δ(2)H, respectively. This indicates either that the extraction time should be extended or that mechanisms different from Rayleigh fractionation play a role. The modified protocol allows a fast and reliable extraction of large numbers of water samples from soil and plant material in preparation for stable isotope analyses.

High resolution pore water delta2H and delta18O measurements by H2O(liquid)-H2O(vapor) equilibration laser spectroscopy

A new H2O(liquid)-H2O(vapor) pore water equilibration and laser spectroscopy method provides a fast way to obtain accurate high resolution deltaD and delta18O profiles from single core samples from saturated and unsaturated geologic media. The precision and accuracy of the H2O(liquid)-H2O(vapor) equilibration method was comparable to or better than conventional IRMS-based methods, and it can be conducted on geologic cores that contain volumetric water contents as low as 5%. Significant advantages of the H2O(liquid)-H2O(vapor) pore water equilibration method and laser isotopic analysis method include dual hydrogen- and oxygen-isotope assays on single small core samples, low consumable and instrumentation costs, and the potential for field-based hydrogeologic profiling. A single core is sufficient to obtain detailed vertical isotopic depth profiles in geologic, soil, and lacustrine pore water, dramatically reducing the cost of obtaining pore water by conventional wells or physical water extraction methods. In addition, other inherent problems like contamination of wells by leakage and drilling fluids can be eliminated.

Oxygen isotope fractionation between human phosphate and water revisited

The oxygen isotope composition of human phosphatic tissues (delta18OP) has great potential for reconstructing climate and population migration, but this technique has not been applied to early human evolution. To facilitate this application we analyzed delta18OP values of modern human teeth collected at 12 sites located at latitudes ranging from 4 degrees N to 70 degrees N together with the corresponding oxygen composition of tap waters (delta18OW) from these areas. In addition, the delta18O of some raw and boiled foods were determined and simple mass balance calculations were performed to investigate the impact of solid food consumption on the oxygen isotope composition of the total ingested water (drinking water+solid food water). The results, along with those from three, smaller published data sets, can be considered as random estimates of a unique delta18OW/delta18OP linear relationship: delta18OW=1.54(+/-0.09)xdelta18OP-33.72(+/-1.51)(R2=0.87: p [H0:R2=0]=2x10(-19)). The delta18O of cooked food is higher than that of the drinking water. As a consequence, in a modern diet the delta18O of ingested water is +1.05 to 1.2 per thousand higher than that of drinking water in the area. In meat-dominated and cereal-free diets, which may have been the diets of some of our early ancestors, the shift is a little higher and the application of the regression equation would slightly overestimate delta18OW in these cases.

Rapid 18O analysis of small water and CO2 samples using a continuous-flow isotope ratio mass spectrometer

High-frequency throughput is often needed in isotopic studies in biological and medical fields. Here we report that high-precision oxygen isotope ratio measurements of water (+/-0.13 per thousand) were rapidly and routinely made on small samples (40-100 microL) using an isotope ratio mass spectrometer operated in continuous-flow mode. Simple modifications to existing instrumentation allow for rapid manual analyses of dilute CO2 (10% CO2/90% N2), including the addition of a septum port and water trap prior to the gas chromatography (GC) column (elemental analyzer column in this study) and the extension of fused-silica capillary tubing between the mass spectrometer source and the effluent tubing from the GC column (located within the CONFLO unit on Finnigan mass spectrometers). We routinely analyzed 20 small-volume samples per hour using this technique, without sacrificing precision of the oxygen isotope ratio measurement.