δ18O anchoring to VPDB: calcite digestion with 18O-adjusted ortho-phosphoric acid

Improving the routine analysis of siderite for δ13 C and δ18 O in environmental change research

RATIONALE

The carbon (δ¹³ C) and oxygen (δ¹⁸ O) isotope composition of siderite (FeCO₃ ) is used widely to understand and quantify geochemical processes to reconstruct past climate and environmental change. However, few laboratories follow precisely the same protocol for the preparation and analysis of siderite-bearing materials, which combined with the absence of international reference materials and mineral-specific acid fractionation factors, leads potentially to significant differences in isotope data generated by individual laboratories. Here we examine procedures for the isotope analysis of siderite and discuss factors potentially contributing to inconsistencies in sample measurement data.

METHODS

Isotope analysis of siderite is first assessed using similar versions of the classical off-line, sealed vessel acid digestion method by comparing data sets obtained from intercomparison materials measured at two participating laboratories. We then compare data from the classical method against those generated using an automated preparation technique using data produced from an independent set of test materials.

RESULTS

Measurement of siderite δ¹³ C is generally both repeatable and reproducible, but measurement of δ¹⁸ O may be subject to large (~1‰), method-dependent bias for siderite reacted at differing temperatures (70°C and 100°C) under classical and automated CO₂ preparation conditions. The potential for poor oxygen isotope measurement reproducibility is amplified by local differences in sample preparation protocols and procedures used to calibrate measurement data to international reference scales.

CONCLUSIONS

We offer suggestions for improving the repeatability and reproducibility of δ¹³ C and δ¹⁸ O analysis on siderite. The challenge of producing consistent isotope data from siderite can only be resolved by ensuring the availability of siderite reference materials to facilitate identical treatment as a basis for minimising method-dependent contributions to data inconsistency between laboratories.

Floating boat method for carbonate stable isotopic ratio determination in a GasBench II peripheral

RATIONALE

Efficient high-precision stable isotope ratio determination using the GasBench II peripheral for carbonates involves loading of a reaction vial with carbonate powder and injection of phosphoric acid of high density for carbonate digestion. Herein, we present an alternative method, which bypasses the need for acid dosing with an automated pump. The advantages of the new method include minimization of clogging within capillaries caused by the acid, acid spillage, and diffusive fractionation due to repeated piercing of the septa.

METHODS

The alternative method involves the use of low-cost boats preloaded with carbonate powder introduced into an Exetainer vial preinjected with phosphoric acid and placed within the compartment of a heated block maintained at a constant temperature in the GasBench II.

RESULTS

The new method yielded an improvement in precision for δ¹³ C_VPDB and δ¹⁸ O_VPDB values during replicate analyses of NBS 19, with an overall precision of ±0.04‰ and ±0.06‰, respectively. The accuracy and precision of analysis using the conventional method and the floating boat method were statistically re-evaluated using a bootstrap error analysis and Monte Carlo simulation methods.

CONCLUSIONS

The proposed floating boat method of acid digestion showed significant improvement in analytical procedure and overall precision. This method is easily adoptable in other laboratories and is free from frequent issues of needle clogging and irregular fractionation due to diffusion facilitated by repeated puncturing of septa, and can serve as an alternative method for high-precision carbonate stable isotope analysis.

USGS44, a new high-purity calcium carbonate reference material for δ13 C measurements

RATIONALE

The stable carbon isotopic (δ¹³ C) reference material (RM) LSVEC Li₂ CO₃ has been found to be unsuitable for δ¹³ C standardization work because its δ¹³ C value increases with exposure to atmospheric CO₂ . A new CaCO₃ RM, USGS44, has been prepared to alleviate this situation.

METHODS

USGS44 was prepared from 8 kg of Merck high-purity CaCO₃ . Two sets of δ¹³ C values of USGS44 were determined. The first set of values was determined by online combustion, continuous-flow (CF) isotope-ratio mass spectrometry (IRMS) of NBS 19 CaCO₃ (δ¹³ C_VPDB  = +1.95 milliurey (mUr) exactly, where mUr = 0.001 = 1‰), and LSVEC Li₂ CO₃ (δ¹³ C_VPDB  = -46.6 mUr exactly), and normalized to the two-anchor δ¹³ C_VPDB-LSVEC isotope-delta scale. The second set of values was obtained by dual-inlet (DI)-IRMS of CO₂ evolved by reaction of H₃ PO₄ with carbonates, corrected for cross contamination, and normalized to the single-anchor δ¹³ C_VPDB scale.

RESULTS

USGS44 is stable and isotopically homogeneous to within 0.02 mUr in 100-μg amounts. It has a δ¹³ C_VPDB-LSVEC value of -42.21 ± 0.05 mUr. Single-anchor δ¹³ C_VPDB values of -42.08 ± 0.01 and -41.99 ± 0.02 mUr were determined by DI-IRMS with corrections for cross contamination.

CONCLUSIONS

The new high-purity, well-homogenized calcium carbonate isotopic reference material USGS44 is stable and has a δ¹³ C_VPDB-LSVEC value of -42.21 ± 0.05 mUr for both EA/IRMS and DI-IRMS measurements. As a carbonate relatively depleted in ¹³ C, it is intended for daily use as a secondary isotopic reference material to normalize stable carbon isotope delta measurements to the δ¹³ C_VPDB-LSVEC scale. It is useful in quantifying drift with time, determining mass-dependent isotopic fractionation (linearity correction), and adjusting isotope-ratio-scale contraction. Due to its fine grain size (smaller than 63 μm), it is not suitable as a δ¹⁸ O reference material. A δ¹³ C_VPDB-LSVEC value of -29.99 ± 0.05 mUr was determined for NBS 22 oil.

Preparation and characterisation of IAEA-603, a new primary reference material aimed at the VPDB scale realisation for δ13 C and δ18 O determination

RATIONALE

NBS19 carbonate, a primary reference material (RM) for the Vienna Pee Dee Belemnite (VPDB) scale realisation introduced in 1987, was exhausted in 2009, and no primary RM was available for several years. This study describes the preparation and characterisation of a new RM, IAEA-603 (Ca-carbonate, calcite of marble origin), which shall serve as a new primary RM (replacement for NBS19) or primary calibrator aimed at the highest realisation of the VPDB scale for δ¹³ C and δ¹⁸ O values, including the VPDB-CO₂ δ¹⁸ O scale.

METHODS

IAEA-603 preparation and characterisation (value transfer) against NBS19 were performed by addressing the major modern technical requirements for the production and characterisation of RMs (ISO Guide 35). IAEA-603 was produced in a large quantity, and the first batch was sealed into ampoules (0.5 g) to ensure RM integrity during storage; four other batches were sealed for long-term storage. The most accurate method of CO₂ preparation for isotope mass spectrometry was used, namely carbonate-H₃ PO₄ reaction under controlled conditions.

RESULTS

The assigned values of δ¹³ C = +2.460 ± 0.010‰ and δ¹⁸ O = -2.370 ± 0.040‰ (k = 1) are based on a large number of analyses (~10 mg aliquots) performed at IAEA and address all the known uncertainty components. For aliquots down to 120 μg, the δ¹⁸ O uncertainty remains unchanged but shall be doubled for δ¹³ C. The uncertainty components considered are as follows: (a) material homogeneity (within and between the 5200 ampoules produced), (b) value assignment against NBS19, (c) storage effects and (d) effect of the ¹⁷ O correction.

CONCLUSIONS

The new primary RM IAEA-603 replaces NBS19 in its use as the highest calibrator for the VPDB δ¹³ C and δ¹⁸ O scale, including the VPDB-CO₂ δ¹⁸ O scale. The use of IAEA-603 will allow laboratories worldwide to establish consistent realisation of the scales for δ¹³ C and δ¹⁸ O values and metrological comparability of measurement results for decades. The VPDB scale definition based on NBS19 stays valid.

Isotopic fractionation during acid digestion of calcite: A combined ab initio quantum chemical simulation and experimental study

RATIONALE

Carbonate clumped isotope analysis involves the reaction of carbonate minerals with phosphoric acid to release CO₂ for measurement in a gas-source isotope ratio mass spectrometer. Although the clumped isotope proxy is based on the temperature dependence of ¹³ C-¹⁸ O bonding preference in the mineral lattice, which is captured in the product CO₂ , there is limited information on the phosphoric acid reaction mechanism and the magnitude of clumped isotopic fractionation (mass 63 in CO₃ ^2- to mass 47 in CO₂ ) during the acid digestion.

METHODS

We studied the reaction mechanism for the phosphoric acid digestion of calcite using first-principles density functional theory. We identified the transition state structures for each reaction involving different isotopologues and used the corresponding vibrational frequencies in reduced partition function theory to estimate the Δ₄₇ acid fractionation. Experimental Δ₄₇ data were acquired by processing the sample CO₂ gas through the dual-inlet peripheral of a ThermoFinnigan MAT253 isotope ratio mass spectrometer.

RESULTS

We showed that the acid digestion reaction, which results in the formation of CO₂ enriched with ¹³ C-¹⁸ O bonds, began with the protonation of calcium carbonate in the presence of water. Our simulations yielded a relationship between the Δ₄₇ acid fractionation and reaction temperature as Δ₄₇ = -0.30175 + 0.57700 × (10⁵ /T² ) - 0.10791 × (10⁵ /T² )² , with T varying between 298.15 and 383.15 K.

CONCLUSIONS

We propose a reaction mechanism that shows a higher slope (Δ₄₇ acid fractionation vs. 1/T² curve) for the phosphoric acid digestion of calcite than in previous studies. The theoretical estimates from the present and earlier studies encapsulate experimental observations from both "sealed vessel" and "common acid bath" acid digestion methods.

Acid digestion of carbonates using break seal method for clumped isotope analysis

RATIONALE

Acid digestion of carbonates to release CO₂ is a crucial and sensitive step in sample preparation for clumped isotope analysis. In addition to data reduction and instrumental artefacts, many other uncertainties in the clumped isotope analysis of carbonates arise from the method used for the preparation of CO₂ . We describe here an in-house-designed reaction vessel that circumvents degassing and contamination problems commonly associated with the McCrea-type digestion protocols.

METHODS

We designed a leak-free break seal reaction vessel (made of Pyrex™) suitable for prolonged acid digestion at 25°C. Using this new vessel, several carbonate reference materials widely used in the clumped isotope community and other in-house laboratory standards were acid-digested and analysed for their δ¹³ C, δ¹⁸ O and Δ₄₇ values with a dual inlet MAT 253 isotope ratio mass spectrometer following standard gas chromatography purification and data evaluation protocols.

RESULTS

Long-term reproducibility in Δ₄₇ determination was established using international references and in-house working standards as follows (mean and SE): Carrara-1 (0.395 ± 0.002‰, n = 43), Carrara-2 (0.441 ± 0.003‰, n = 22), OMC (0.587 ± 0.004‰, n = 16), NBS 19 (0.393 ± 0.005‰, n = 10), NBS 18 (0.473 ± 0.003‰, n = 5), ETH 1 (0.271 ± 0.005‰, n = 7), ETH 3 (0.698 ± 0.005‰, n = 3), MZ (0.715 ± 0.002‰, n = 3) and several others.

CONCLUSIONS

A new method using a break seal tube was found to be efficient for the clumped isotope analysis of carbonates that require longer reaction time at 25°C. This method yields good precision in Δ₄₇ analysis and was found to be suitable for acid digestions at any desired temperature.

Potential improvements aimed at high precision δ13C isotopic ratio determinations in CO2 mixtures using optical absorption spectrometry

The manuscript explores some advantages and limitations of laser based optical spectroscopy, aimed at achieving robust, high-reproducibility ¹³C¹⁶O₂ and ¹²C¹⁶O₂ ratio determinations on the VPDB-CO₂ δ¹³C scale by measuring the absorbance of line pairs of ¹³C¹⁶O₂ and ¹²C¹⁶O₂. In particular, the sensitivities of spectroscopic lines to both pressure (P) and temperature (T) are discussed. Based on the considerations and estimations presented, a level of reproducibility of the ¹³C¹⁶O₂/¹²C¹⁶O₂ ratio determinations may be achieved of about 10^-6. Thus one may establish an optical spectroscopic measurement technique for robust, high-precision ¹³C¹⁶O₂ and ¹²C¹⁶O₂ ratio measurements aimed at very low uncertainty. (Notably, creating such an optical instrument and developing technical solutions is beyond the scope of this paper.) The total combined uncertainty will also include the uncertainty component(s) related to the accuracy of calibration on the VPDB-CO₂ δ¹³C scale. Addressing high-accuracy calibrations is presently not straightforward - absolute numerical values of ¹³C/¹²C for the VPDB-CO₂ scale are not well known. Traditional stable isotope mass-spectrometry uses calibrations vs CO₂ evolved from the primary carbonate reference materials; which can hardly be used for calibrating commercial optical stable isotope analysers. In contrast to mass-spectrometry, the major advantage of the laser-based spectrometric technique detailed in this paper is its high robustness. Therefore one can introduce a new spectrometric δ¹³C characterisation method which, being once well-calibrated on the VPDB-CO₂ scale, may not require any further (re-)calibrations. This can be used for characterisation of δ¹³C in CO₂-in-air mixtures with high precision and also with high accuracy. If this technique can be realised with the estimated long-term reproducibility (order of 10^-6), it could potentially serve as a more convenient Optical Transfer Standard (OTS), characterising large amounts of CO₂ gas mixtures on the VPDB-CO₂ δ¹³C scale without having to compare to carbonate-evolved CO₂. Furthermore, if the OTS method proves to be successful, it might be considered for re-defining the VPDB-CO₂ δ¹³C-scale as the ratio of selected CO₂ spectroscopic absorbance lines measured at pre-defined T & P conditions. The approach can also be expanded to δ¹⁸O characterisation (using ¹⁶O¹²C¹⁸O and ¹⁶O¹²C¹⁶O absorbance lines) of CO₂ gas mixtures and potentially to other isotope ratios of other gases.

Isotopic disequilibrium in Globigerina bulloides and carbon isotope response to productivity increase in Southern Ocean

Oxygen and carbon isotope ratios in planktonic foraminifera Globigerina bulloides collected from tow samples along a transect from the equatorial Indian ocean to the Southern Ocean (45°E and 80°E and 10°N to 53°S) were analysed and compared with the equilibrium δ¹⁸O and δ¹³C values of calcite calculated using the temperature and isotopic composition of the water column. The results agree within ~0.25‰ for the region between 10°N and 40°S and 75–200 m water depth which is considered to be the habitat of Globigerina bulloides. Further south (from 40°S to 55°S), however, the measured δ¹⁸O and δ¹³C values are higher than the expected values by ~2‰ and ~1‰ respectively. These enrichments can be attributed to either a ‘vital effect’ or a higher calcification rate. An interesting pattern of increase in the δ¹³C(DIC) value of the surface water with latitude is observed between 35°S and~ 60°S, with a peak at~ 42°S. This can be caused by increased organic matter production and associated removal. A simple model accounting for the increase in the δ¹³C(DIC) values is proposed which fits well with the observed chlorophyll abundance as a function of latitude.

Clumped isotope analysis of carbonates: comparison of two different acid digestion techniques

RATIONALE

The kinetic nature of the phosphoric acid digestion reaction enables clumped isotope analysis of carbonates using gas source isotope ratio mass spectrometry (IRMS). In most laboratories acid digestions are performed at 25°C in sealed vessels or at 90°C in a common acid bath. Here we show that different Δ47 results are obtained depending on the digestion technique employed.

METHODS

Several replicates of a biogenic aragonite and NBS 19 were reacted with 104% H3 PO4 in sealed vessels at 25°C and at 90°C using a common acid bath. The sample size varied between 4 mg and 14 mg. Purification methods that are standard for clumped isotope analyses were applied to the evolved CO2 before measuring the abundances of masses 44 to 49 relative to a reference gas by IRMS.

RESULTS

A systematic trend to lower and more consistent Δ47 values is observed for reactions at 25°C if the sample size is increased. We suggest that secondary re-equilibration of evolved CO2 or reaction intermediates with free water molecules preferentially occurs for relatively small samples (4-7 mg), finally yielding elevated Δ47 values compared with >7 mg aliquots. In contrast, no such sample size effect on Δ47 values is observed for carbonates that are digested at 90°C using the common acid bath.

CONCLUSIONS

The determination of Δ47 values of carbonate samples smaller than 7 mg becomes more precise and accurate if digestions are performed at 90°C. Based on our results we propose that the difference in phosphoric acid fractionation factor between 25°C and 90°C is 0.07‰ for both calcite and aragonite.