Greenland Ice Sheet Precipitation Research Articles

Comparison of mass spectrometry and spectroscopy methods applied to measurements of water triple oxygen isotopic compositions

<p indent="0mm">High-precision measurements of oxygen isotopic compositions in water are of great importance for understanding regional and global hydrological cycles and historical hydroclimate change. Several methods have been developed to determine the triple oxygen isotopic proxy (¹⁷O-excess) in water. However, these methods are not applicable because of poor precision or poisonous compound utilisation. Recently, a method based on CO₂-H₂O equilibration followed by an isotopic exchange between CO₂ and O₂ catalysed by hot platinum, was developed to measure water ¹⁷O-excess with high precision (less than 10 per meg, 1 per meg=10⁻⁶). It has also been demonstrated that this method has the capability to obtain sufficiently precise water ¹⁷O-excess values, by performing more injections per sample and/or giving longer integration time per injection. Here, we describe the water ¹⁷O-excess measurement method using cavity ring-down spectroscopy (Picarro L2140-i) and mass spectrometry (MAT 253) devices, the latter of which comprises CO₂-H₂O equilibrium and CO₂-O₂ equilibration systems that produce pretreated O₂ for measurement. The excellent agreement between the oxygen isotopic compositions of international standard waters, derived from both methods, and the recommended or previously reported values, confirms the reliability and accuracy of the two methods. Each method has advantages and disadvantages in terms of efficiency, sensitivity to environmental disturbance, sample quantity demand, and so on. Repeated measurements of Greenland Ice Sheet Precipitation (GISP) were performed to check the reliability of the data normalisation method to the Vienna Standard Mean Ocean Water-Standard Light Antarctic Precipitation (VSMOW-SLAP) scale. For the Picarro method, <italic>δ</italic>¹⁸O_VSMOW-SLAP and <italic>δ</italic>¹⁷O_VSMOW-SLAP values equalled −24.80‰ ± 0.03‰ and −13.15‰ ± 0.02‰, respectively (1<italic>σ</italic>), while for the mass spectrometry method, <italic>δ</italic>¹⁸O_VSMOW-SLAP and <italic>δ</italic>¹⁷O_VSMOW-SLAP values equalled −24.82‰ ± 0.04‰ and −13.16‰ ± 0.03‰, respectively (1<italic>σ</italic>). The International Atomic Energy Agency (IAEA) reported a GISP <italic>δ</italic>¹⁸O_VSMOW-SLAP value of −24.76‰ ± 0.09‰ (1<italic>σ</italic>), and the average GISP<italic> δ</italic>¹⁷O_VSMOW-SLAP value from previous studies is −13.12‰ ± 0.06‰. ¹⁷O-excess values were calculated from oxygen isotope ratios, and we obtained an average value of 26 ± 8 and 21 ± 6 per meg (1<italic>σ</italic>) for the Picarro and mass spectrometry methods, respectively, both of which are close to the average of previously reported values (22 ± 11 per meg). Collectively, these results demonstrate that both methods can be used to obtain accurate water triple oxygen isotopes with high precision. Moreover, both methods reproduce the previously reported 26 per meg difference between USGS 45 and USGS 47 ¹⁷O-excess values. This demonstrates the ability of the methods to distinguish the differences in precipitation, atmospheric vapour, cave drip water, and ice core water samples. Triple oxygen isotope measurement methods were applied to determine summer and winter precipitation ¹⁷O-excess values from Nanjing, Xi’an, and Urumqi, which exhibit distinct spatiotemporal differences. Although the precipitation <italic>δ</italic>¹⁸O values in Nanjing and Xi’an were significantly greater in winter and lower in summer, seasonal changes in ¹⁷O-excess appear complex, and thus more data are required. The precipitation <italic>δ</italic>¹⁸O values from Urumqi show the opposite pattern, with higher ¹⁷O-excess values in winter. Additional measurements are required to confirm this pattern.

Present-day and future Greenland Ice Sheet precipitation frequency from CloudSat observations and the Community Earth System Model

Abstract. The dominant mass input component of the Greenland Ice Sheet (GrIS) is precipitation, whose amounts and phase are poorly constrained by observations. Here we use spaceborne radar observations from CloudSat to map the precipitation frequency and phase on the GrIS, and we use those observations, in combination with a satellite simulator to enable direct comparison between observations and model, to evaluate present-day precipitation frequency in the Community Earth System Model (CESM). The observations show that substantial variability of snowfall frequency over the GrIS exists, that snowfall occurs throughout the year, and that snowfall frequency peaks in spring and fall. Rainfall is rare over the GrIS and only occurs in regions under 2000 m elevation and in the peak summer season. Although CESM overestimates the rainfall frequency, it reproduces the spatial and seasonal variability of precipitation frequency reasonably well. Driven by the high-emission, worst-case Representative Concentration Pathway (RCP) 8.5 scenario, CESM indicates that rainfall frequency will increase considerably across the GrIS, and will occur at higher elevations, potentially exposing a much larger GrIS area to rain and associated meltwater refreezing, firn warming, and reduced storage capacity. This technique can be applied to evaluate precipitation frequency in other climate models and can aid in planning future satellite campaigns.

A statistical analysis of IRMS and CRDS methods in isotopic ratios of 2H/1H and 18O/16O in water

Quantitative information about the variation in natural isotopic abundances in water is of great importance in a variety of fields. Due to the wide range of applications and types of samples, it is necessary that isotopic analyses have precision, accuracy and reproducibility. The present study compares the techniques of cavity ring-down spectroscopy (CRDS) and isotope-ratio mass spectrometry (IRMS) for the determination of the isotopic ratios of 2H/1H and 18O/16O in water in the two secondary standards, denoted PB3 and PB4, and in a certified material, GISP, Greenland Ice Sheet Precipitation, used as a quality tester of such measurements. The traditional method for measuring isotopic ratios is IRMS. Because of the nature of the molecule, the samples are not introduced directly into the mass spectrometer. Instead, the water is chemically converted to CO2 and H2. The other technique, CRDS, is a system of laser absorption that has great potential for the detection of atomic and molecular species with high sensitivity by measuring the light absorption ratio as a function of time, confined within an optical cavity of high finesse. In this technique, the water sample is converted into steam without undergoing conversion processes. Parametric (test T) and nonparametric (Wilcoxon) statistical tests were performed to compare the results obtained in the system, and CRDS and IRMS are from the same population. The values of the isotopic abundances of the two secondary standards [PB3, δD = − 1.9 ± 0.4 (‰) and δ18O = − 2.19 ± 0.24 (‰) and PB4, δ2H = − 71.4 ± 0.4 (‰) and δ18O = − 10.08 ± 0.19 (‰)] were determined with accuracy. For the certified standard GISP, values of δ2H = − 189.3 ± 0.5 (‰) and δ18O = − 24.69 ± 0.20 (‰) were obtained. Both techniques have factors that interfere with the accuracy of the measurements and require corrections. Comparing the results revealed that there was a greater accuracy for measurements with CRDS and greater precision for IRMS. However, the results are within the tolerance range of 0.2‰ for δ18O and 2.0‰ for δ2H in isotope hydrology.

Multipoint normalization of δ18O of water against the VSMOW2-SLAP2 scale with an uncertainty assessment

In this study, we measured the oxygen stable isotope ratio of drinking water using gas chromatography isotope ratio mass spectrometry. The δ18O value of drinking water was normalized based on the Vienna Standard Mean Ocean Water 2 (VSMOW2), Standard Light Antarctic Precipitation 2 (SLAP2), and Greenland Ice Sheet Precipitation (GISP) scale by CO2 equilibrium for 24 h. The isotope ratio responses of a dummy sample drifted as much as 0.145‰ due to a significant decrease in the amount of injected sample. The autodilution technique improved measurement precision of the δ18O of dummy sample two-fold compared to that without autodilution to 0.025‰. The autodilution of an injected concentration of equilibrated CO2 also helped improve the measurement precision of the isotope ratio response. The drift of the ratio responses was tested using linear model regression to validate linearity within the sample concentration and isotope ratio ranges. Measurement reliability was assessed using various statistical approaches. One-way analysis of variance verified non-reproducible results of individual measurements. Normalization uncertainties were then assessed by various normalization schemes including two-point and three-point values consisting of the VSMOW2, SLAP2, and GISP standards, showing equivalent results associated with similar extent of normalization uncertainties among various normalization methods. In particular, the uncertainty of the GISP (0.09‰) contributed to one-third of the total normalization uncertainty, implying that the three-point normalization can be improved by a potential standard of which uncertainty is equivalent to the bracketing standards, VSMOW2 (0.02‰) and SLAP2 (0.02‰).

Arctic cyclone water vapor isotopes support past sea ice retreat recorded in Greenland ice.

Rapid Arctic warming is associated with important water cycle changes: sea ice loss, increasing atmospheric humidity, permafrost thaw, and water-induced ecosystem changes. Understanding these complex modern processes is critical to interpreting past hydrologic changes preserved in paleoclimate records and predicting future Arctic changes. Cyclones are a prevalent Arctic feature and water vapor isotope ratios during these events provide insights into modern hydrologic processes that help explain past changes to the Arctic water cycle. Here we present continuous measurements of water vapor isotope ratios (δ18O, δ2H, d-excess) in Arctic Alaska from a 2013 cyclone. This cyclone resulted in a sharp d-excess decrease and disproportional δ18O enrichment, indicative of a higher humidity open Arctic Ocean water vapor source. Past transitions to warmer climates inferred from Greenland ice core records also reveal sharp decreases in d-excess, hypothesized to represent reduced sea ice extent and an increase in oceanic moisture source to Greenland Ice Sheet precipitation. Thus, measurements of water vapor isotope ratios during an Arctic cyclone provide a critical processed-based explanation, and the first direct confirmation, of relationships previously assumed to govern water isotope ratios during sea ice retreat and increased input of northern ocean moisture into the Arctic water cycle.

Measurement of δ18O, δ17O, and 17O-excess in Water by Off-Axis Integrated Cavity Output Spectroscopy and Isotope Ratio Mass Spectrometry

Stable isotopes of water have long been used to improve understanding of the hydrological cycle, catchment hydrology, and polar climate. Recently, there has been increasing interest in measurement and use of the less-abundant (17)O isotope in addition to (2)H and (18)O. Off-axis integrated cavity output spectroscopy (OA-ICOS) is demonstrated for accurate and precise measurements δ(18)O, δ(17)O, and (17)O-excess in liquid water. OA-ICOS involves no sample conversion and has a small footprint, allowing measurements to be made by researchers collecting the samples. Repeated (514) high-throughput measurements of the international isotopic reference water standard Greenland Ice Sheet Precipitation (GISP) demonstrate the precision and accuracy of OA-ICOS: δ(18)OVSMOW-SLAP = -24.74 ± 0.07‰ (1σ) and δ(17)OVSMOW-SLAP = -13.12 ± 0.05‰ (1σ). For comparison, the International Atomic Energy Agency (IAEA) value for δ(18)OVSMOW-SLAP is -24.76 ± 0.09‰ (1σ) and an average of previously reported values for δ(17)OVSMOW-SLAP is -13.12 ± 0.06‰ (1σ). Multiple (26) high-precision measurements of GISP provide a (17)O-excessVSMOW-SLAP of 23 ± 10 per meg (1σ); an average of previously reported values for (17)O-excessVSMOW-SLAP is 22 ± 11 per meg (1σ). For all these OA-ICOS measurements, precision can be further enhanced by additional averaging. OA-ICOS measurements were compared with two independent isotope ratio mass spectrometry (IRMS) laboratories and shown to have comparable accuracy and precision as the current fluorination-IRMS techniques in δ(18)O, δ(17)O, and (17)O-excess. The ability to measure accurately δ(18)O, δ(17)O, and (17)O-excess in liquid water inexpensively and without sample conversion is expected to increase vastly the application of δ(17)O and (17)O-excess measurements for scientific understanding of the water cycle, atmospheric convection, and climate modeling among others.

Simplified method for microlitre deuterium measurements in water and urine by gas chromatography–high‐temperature conversion–isotope ratio mass spectrometry

Deuterium (2H) in water and urine can be measured by off-line and, more recently, on-line techniques using isotope ratio mass spectrometry (IRMS). We describe a new simple on-line pyrolysis method for the analysis of 2H/1H in water and urine samples by continuous flow IRMS, normally used for 2H/1H measurements in organic compounds. A deactivated column connected the split injector to a high-temperature conversion reactor (TC HD), and 0.5 microL of sample was injected. Accuracy and precision were determined with Vienna Standard Mean Ocean Water (VSMOW), Standard Light Antarctic Precipitation (SLAP), and Greenland Ice Sheet Precipitation (GISP). The range of linearity was measured with a calibration curve of enriched water from 0 up to 0.1 atom percent excess (APE) (i.e. -72 up to 6323 delta per mil (deltaD per thousand)) with a precision of <5 per thousand and accuracy ranging between 1 and 55 per thousand. Blinded reanalysis of urine samples by an equilibration device (Gas Bench) and by a dedicated pyrolysis system (TC/EA) was performed and results compared by the Bland-Altman test. Enrichments ranged between 600 and 2400 per thousand deltaD(VSMOW) with a precision of +/-5 per thousand. Urine enrichments described by our method were strongly correlated with values obtained by Gas Bench and TC/EA (p < 0.0001). There was a significant memory effect that was reduced by injecting the sample 15 times and discarding the first 10 injections, together with accurate furnace conditioning and appropriate cleaning of the syringe. Data indicate that the method is accurate, and that it can be used for water and urine deuterium determination when a Gas Bench or TC/EA instrument is not available and the amount of sample is limited.

High-throughput simultaneous determination of plasma water deuterium and 18-oxygen enrichment using a high-temperature conversion elemental analyzer with isotope ratio mass spectrometry.

This paper presents a high-throughput method for the simultaneous determination of deuterium and oxygen-18 (18O) enrichment of water samples isolated from blood. This analytical method enables rapid and simple determination of these enrichments of microgram quantities of water. Water is converted into hydrogen and carbon monoxide gases by the use of a high-temperature conversion elemental analyzer (TC-EA), that are then transferred on-line into the isotope ratio mass spectrometer. Accuracy determined with the standard light Antartic precipitation (SLAP) and Greenland ice sheet precipitation (GISP) is reliable for deuterium and 18O enrichments. The range of linearity is from 0 up to 0.09 atom percent excess (APE, i.e. -78 up to 5725 delta per mil (dpm)) for deuterium enrichment and from 0 up to 0.17 APE (-11 up to 890 dpm) for 18O enrichment. Memory effects do exist but can be avoided by analyzing the biological samples in quintuplet. This method allows the determination of 1440 samples per week, i.e. 288 biological samples per week.

Isotope ratio mass spectrometric method for the on-line determination of oxygen-18 in organic matter.

A method for the on-line determination of oxygen-18, at a naturally occurring level, in organic material is presented. After pyrolysis of the samples to form carbon monoxide, which is performed at 1300 degrees C in a vitreous carbon tube, the pyrolysis products are transported by a stream of helium gas. Using an open split, a small part of the effluent is transferred to the ion source of an isotope ratio mass spectrometer. The ratio is obtained from a measurement of the ion current intensities at m/z 30 and 28 (12C18O and 12C16O). The method was tested with the secondary water standard GISP (Greenland Ice Sheet Precipitation) and the carbonate standard NBS 19. The values obtained were -24.8/1000 and 27.3/1000 vs. VSMOW (Vienna Standard Mean Ocean Water) (LAEA reference values are -24.8/1000 and 28.7/1000 vs. VSMOW). The potential of the method was demonstrated by measuring the 18O content of samples of beet and cane sucrose and also samples of vanillin extracted from vanilla pods or of synthetic origin.