How well do we know VPDB? Variability of δ¹³C and δ¹⁸O in CO₂ generated from NBS19‐calcite

Abstract

In order to generate a local daughter scale from the material defining the international delta13C and delta18O stable isotope ratio scales (NBS19-calcite),1,2 the carbon and oxygen must be liberated to the gas phase, usually as CO2, using acid digestion of the calcite with H3PO4. It is during this conversion step that systematic errors can occur, giving rise to commonly observed discrepancies in isotopic measurements between different stable isotope laboratories. Scale consistency is of particular importance for air-CO2 isotope records where very small differences in isotopic composition have to be reliably compared between different laboratories and quantified over long time periods.3 The information is vital for estimating carbon budgets on regional and global scales and for understanding their variability under the conditions of climate change. Starting from this requirement a number of CO2 preparations from NBS19 were made at Environment Canada (EC) and analyzed in our laboratories together with Narcis II, a set of well-characterized CO2 samples in sealed tubes available from the National Institute for Environmental Studies (NIES).4,5 Narcis II is very homogeneous in delta13C and delta18O with the isotopic composition close to NBS19-CO2. Among our laboratories the results for delta13C agreed to within +/-0.004 per thousand. The same level of agreement in delta13C was obtained when CO2 was generated from NBS19-calcite using different experimental procedures and conditions in the other two laboratories. For delta18O, the corresponding data were +/-0.011 per thousand when using NBS19-CO2 produced at EC, but discrepancies were enhanced by almost one order of magnitude when NBS19-CO2 was prepared by the other laboratories using slightly different reaction conditions (range=0.13 per thousand).In a second series of experiments, larger amounts of CO2 prepared from NBS19 at the Max-Planck-Institut für Biogeochemie (MPI-BGC) were analyzed together with Narcis II and then mixed into CO2-free air. The resulting artificial air samples then were measured by the same three laboratories for the stable isotopic composition of CO2 using locally established extraction and evaluation procedures. Comparison of the results with the prior CO2 values and between the laboratories revealed additional systematic differences owing to the local CO2 extraction processes and standardization procedures. For delta13C the results showed a narrow range of discrepancies of about 0.02 per thousand; for delta18O cumulative disagreements in the range of 0.1 per thousand were observed. From these results the following conclusions are inferred: NBS19-CO2 is a reliable primary anchor to the VPDB delta13C scale. Although prepared by different methods an accuracy of better than +/-0.003 per thousand has been reached. This applies to sample amounts of 5 mg calcite or more.NBS19-CO2 can be used as a general anchor to the VPDB delta18O scale only for accuracy requirements of +/-0.1 per thousand. For a higher scale resolution additional agreements regardin...

Jena Reference Air Set (JRAS): a multi-point scale anchor for isotope measurements of CO<sub>2</sub> in air

“…The system prepares CO 2 by digestion of the calcites in highly concentrated phosphoric acid and mixes it into CO 2 -free air in batches large enough to provide three 5 L flasks (p = 1.5 bar) at a time. The δ 13 C scale at the BGC Isolab has been established by repeated analysis of preparations of the primary calcites, NBS 19 and LSVEC, using the automated preparation system (Ghosh et al, 2005;Brand et al, 2009). This scale, JRAS-06, which is firmly anchored at +1.95 ‰ and …”

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

“…For example, the d 13 C value is independent of the reaction temperature. [14] For the d 18 O of CO 2 this is not the case; oxygen is not quantitatively transferred to the CO 2 . Two of the three oxygen atoms remain attached to the carbon and end up in the produced CO 2 , while the third is transferred to the outgoing water molecule.…”

“…Further recent information about H 2 CO 3 and the reactions involved between water, bicarbonate and dissolved CO 2 can be found in Stirling and Papai [21] and Wang et al [22] Since the dissociation rate is temperature dependent, the oxygen isotopic composition of the resulting CO 2 will vary with temperature. [14,[23][24][25][26][27][28] Oxygen-16 transfer is kinetically favored and, hence, 16 O preferentially ends up in the water molecule. The remaining oxygen in the CO 2 is left depleted in 16 O, i.e.…”

Prerequisites for application of hyperbolic relaxed eddy accumulation on managed grasslands and alternative net ecosystem exchange flux partitioning

“…This, as well as the CO 2 mixing ratio analysis, were accomplished in the isotope and trace gas laboratory of the Max Planck Institute in Jena, Germany. All 13 C isotopic signatures in this study were analyzed in relation to 13 C isotopic abundances in the international standards VPDB (Vienna Pee Dee Belemnite; Brand et al, 2009;Wendeberg et al, 2011;JRAS scale;Ghosh et al, 2005;Wendeberg et al, 2011). The precision in the laboratory of 0.012 ‰ for δ 13 C (for more detailed information about the laboratory analysis see Werner et al, 2001), the application of a hyperbolic deadband (hyperbolic relaxed eddy accumulation, HREA; Bowling et al, 1999b) and comprehensive REA system and component laboratory tests made the resolution of up-and downdraft isotope ratio and mixing ratio differences possible, and consequently the determination of δ 13 C isofluxes (Wichura, 2009;Ruppert et al, 2012).…”