Measurement of δ18O and δ2H values of fluid inclusion water in speleothems using cavity ring-down spectroscopy compared with isotope ratio mass spectrometry

Physical, chemical, and microbial feedbacks controlling brine geochemistry and lake morphology in polyextreme salar environments

Despite the harsh environmental conditions in the world's oldest and driest desert, some salt flat or 'salar' environments in the Atacama Desert host standing bodies of water known as saline lakes. Evaporite minerals deposited within saline lakes result from the equilibrium of environmental, sedimentological, and biogeochemical processes that occur in the salar; consequently, these minerals are sensitive records of human activities and ecological, evolutionary, and geological changes. The objective of this study was to evaluate feedbacks between physical, chemical, and microbial processes that culminate in distinct trends in brine chemistry, saline lake morphology, and associated evaporite sediments. Using samples from the Puquios of the Salar de Llamara, Atacama Desert, northern Chile, an analysis of spatial gradients and vertical stratification of lake elemental chemistry and mineral saturation indices were integrated with a comprehensive analysis of lake morphology, including depth, slope gradient, substrate type, and mineralogy. Lake waters ranged from saline to hypersaline, and exhibited normal, well mixed and inverse stratification patterns, and results suggest a correlation with lake morphology in the Salar de Llamara. Saline to hypersaline lakes (>150 mS/cm) with stratified brines tended to have crystalline substrate and deep (>35 cm) and steep-sided lake morphologies, while unstratified lakes with lower electrical conductivity (<90 mS/cm and microbial substrates had gentle slopes and characteristically shallow depths (<30 cm). Differences in minor element chemistry (Mn and Sr) between saline lakes were observed on scales of meters to kilometers, and result in different accessory mineral assemblages. Quantification of the physical, chemical, and microbial feedbacks that produce the observed heterogeneity in these ecosystems provides key insight into the geochemical composition and lake morphology of saline lakes in extreme environments around the world.

Last glacial temperature reconstructions using coupled isotopic analyses of fossil snails and stalagmites from archaeological caves in Okinawa, Japan

We applied a new geoarchaeological method with two carbonate archives, which are fossil snails from Sakitari Cave and stalagmites from Gyokusen Cave, on Okinawa Island, Japan, to reconstruct surface air temperature changes over the northwestern Pacific since the last glacial period. Oxygen isotope ratios (δ¹⁸O) of modern and fossil freshwater snail shells were determined to infer seasonal temperature variations. The observational and analytical data confirm that δ¹⁸O values of fluid inclusion waters in the stalagmite can be regarded as those of spring waters at the sites where snails lived. Our results indicate that the annual mean, summer, and winter air temperatures were lower by 6–7 °C at ca. 23 thousand years ago (ka) and 4–5 °C at ca. 16–13 ka than those of the present day. Our reconstruction implies that surface air cooling was possibly two times greater than that of seawater around the Ryukyu Islands during the Last Glacial Maximum, which potentially enhanced the development of the East Asian summer monsoon during the last deglaciation. Considering the potential uncertainties in the temperature estimations, the climatic interpretations of this study are not necessarily definitive due to the limited number of samples. Nevertheless, our new geoarchaeological approach using coupled δ¹⁸O determinations of fossil snails and stalagmite fluid inclusion waters will be useful for reconstructing snapshots of seasonally resolved time series of air temperatures during the Quaternary.

A comparison of isotope ratio mass spectrometry and cavity ring-down spectroscopy techniques for isotope analysis of fluid inclusion water

RATIONALE

Online oxygen (δ¹⁸ O) and hydrogen (δ² H) isotope analysis of fluid inclusion water entrapped in minerals is widely applied in paleo-fluid studies. In the state of the art of fluid inclusion isotope research, however, there is a scarcity of reported inter-technique comparisons to account for possible analytical offsets. Along with improving analytical precisions and sample size limitations, interlaboratory comparisons can lead to a more robust application of fluid inclusion isotope records.

METHODS

Mineral samples-including speleothem, travertine, and vein material-were analyzed on two newly setup systems for fluid inclusion isotope analysis to provide an inter-platform comparison. One setup uses a crusher unit connected online to a continuous-flow pyrolysis furnace and an isotope ratio mass spectrometry (IRMS) instrument. In the other setup, a crusher unit is lined up with a cavity ring-down spectroscopy (CRDS) system, and water samples are analyzed on a continuous standard water background to achieve precisions on water injections better than 0.1‰ for δ¹⁸ O values and 0.4‰ for δ² H values for amounts down to 0.2 μL.

RESULTS

Fluid inclusion isotope analyses on the IRMS setup have an average 1σ reproducibility of 0.4‰ and 2.0‰ for δ¹⁸ O and δ² H values, respectively. The CRDS setup has a better 1σ reproducibility (0.3‰ for δ¹⁸ O values and 1.1‰ for δ² H values) and also a more rapid sample throughput (<30 min per sample). Fluid inclusion isotope analyses are reproducible at these uncertainties for water amounts down to 0.1 μL on both setups. Fluid inclusion isotope data show no systematic offsets between the setups.

CONCLUSIONS

The close match in fluid inclusion isotope results between the two setups demonstrates the high accuracy of the presented continuous-flow techniques for fluid inclusion isotope analysis. Ideally, experiments such as the one presented in this study will lead to further interlaboratory comparison efforts and the selection of suitable reference materials for fluid inclusion isotopes studies.

Analysis of the Stable Isotope Ratios (18O/16O, 17O/16O, and D/H) in Glacier Water by Laser Spectrometry

A compact isotope ratio sensor based on laser absorption spectroscopy at 2.7 μm was developed for high precision and simultaneous measurements of the D/H, ¹⁸O/¹⁶O and ¹⁷O/¹⁶O isotope ratios in glacier water. Measurements of the oxygen and hydrogen isotope ratios in glacier water demonstrate a 1σ precision of 0.3‰ for δ¹⁸O, 0.2‰ for δ¹⁷O, and 0.5‰ for δ²H, respectively. The δ values of the working standard glacier water obtained by the calibrated sensor system is basically identical to the IRMS measurement results with a very high calibration accuracy from 0.17‰ to 0.75‰. Preliminary results on the reproducibility measurements display a standard deviation of 0.13‰ for δ¹⁸O, 0.13‰ for δ¹⁷O, and 0.64‰ for δ²H, respectively.

Online Differential Thermal Isotope Analysis of Hydration Water in Minerals by Cavity Ringdown Laser Spectroscopy

We have developed a new method for measuring the isotopic composition (δ¹⁸O and δD) of different types of bonded water (e.g., molecular water, hydroxyl) contained in hydrated minerals by coupling a thermal gravimeter (TG) and a cavity ringdown laser spectrometer (CRDS). The method involves precisely step-heating a mineral sample, allowing the separation of the different types of waters that are released at different temperatures. Simultaneously, the water vapor evolved from the mineral sample is analyzed for oxygen and hydrogen isotopes by CRDS. Isotopic values for the separate peaks are calculated by integrating the product of the water amounts and its isotopic values, after correcting for background. We provide examples of the application of the differential thermal isotope analysis (DTIA) method to a variety of hydrous minerals and mineraloids including gypsum, clays, and amorphous silica (opal). The isotopic compositions of the total water evolved from a set of natural gypsum samples by DTIA are compared with the results of a conventional offline water extraction method followed by CRDS analysis. The results from both methods are in excellent agreement, and precisions (1σ) for δ¹⁸O (±0.12‰) and δD (±0.8‰) of the total gypsum hydration water from the DTIA method are comparable to that obtained by the offline method. A range of analytical challenges and solutions (e.g., spectroscopic interferences produced by VOCs in natural samples, isotopic exchange with structural oxygen, etc.) are discussed. The DTIA method has wide ranging applications for addressing fundamental problems across many disciplines in earth and planetary sciences, including paleoclimatology, sedimentology, volcanology, water exchange between the solid earth and hydrosphere, and water on Mars and other planetary bodies.

Measurement of the D/H, ¹⁸O/¹⁶O, and ¹⁷O/¹⁶O isotope ratios in water by laser absorption spectroscopy at 2.73 μm

A compact isotope ratio laser spectrometry (IRLS) instrument was developed for simultaneous measurements of the D/H, 18O/16O and 17O/16O isotope ratios in water by laser absorption spectroscopy at 2.73 μm. Special attention is paid to the spectral data processing and implementation of a Kalman adaptive filtering to improve the measurement precision. Reduction of up to 3-fold in standard deviation in isotope ratio determination was obtained by the use of a Fourier filtering to remove undulation structure from spectrum baseline. Application of Kalman filtering enables isotope ratio measurement at 1 s time intervals with a precision (<1‰) better than that obtained by conventional 30 s averaging, while maintaining a fast system response. The implementation of the filter is described in detail and its effects on the accuracy and the precision of the isotope ratio measurements are investigated.