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Rationale: Measurements of oxygen-17 excess ((17)Oexcess) in carbonates have become of great importance. However, to compare results obtained by different laboratories, it is necessary to normalize them to international standards. With this purpose in mind, we measured accurate and high precision δ(17)O and (17)Oexcess values for NBS19 and NBS18, two international standards, for which δ(18)O values are already widely used as the references for carbonates., Methods: The measurements are based on isotopic exchange at steady state between O2 and CO2 over hot platinum sponge at 750°C. Dual-inlet isotope ratio mass spectrometry (IRMS) measurements of the δ(17)O and δ(18)O values of this O2 allow δ(17)O values of CO2 to be obtained with very high precision (0.01 to 0.03‰) and, correspondingly, accurate (17)Oexcess values with a precision of 5 per meg., Results: We measured, for the first time, the δ(17)O values and (17)Oexcess of CO2 liberated from NBS19 at 25°C (39.196 ± 0.026; 20.276 ± 0.015‰ and -227 ± 4 per meg, respectively) and NBS18 (17.591 ± 0.041‰; 9.253 ± 0.018‰ and 3 ± 5 per meg, respectively). The values are given versus VSMOW., Conclusions: Accurate values of δ(17)O and (17)Oexcess of the international standards NBS19-CO2 and NBS18-CO2 are now available. The new values should be used for normalization of measured oxygen isotope ratios of carbonates to allow meaningful comparison of results among different laboratories., (Copyright © 2015 John Wiley & Sons, Ltd.)

Carbonates are common paleoclimate archives in a variety of environments with land carbonates recording both temperatures and hydrological conditions. Whereas δ18O has long been used for that purpose, triple oxygen isotopes (given as 17O excess) has only recently been analyzed in CaCO 3. Carbonate 17O excess is expected to reflect the temperature, in which the carbonate formed, and the triple oxygen isotopic composition of its parent water. The reconstruction of past water 17O excess values from carbonate records requires characterization of the 17O fractionation between water and CaCO 3. Here, we use freshwater mollusk shells grown in well constrained spring environments, at a temperature range of 15–28 °C, to derive the fractionation factor 17α between CO 2 extracted from CaCO 3 and the carbonate habitat water. The spring water temperature and isotopic composition are constant year-round, avoiding uncertainty associated with seasonality. The derived 18α and 17α between CO 2 extracted from mollusk aragonite and spring water are used to calculate the fractionation slope θ (=ln17α/ln18α), resulting in 0.5231 ± 0.0003, with no discernible temperature dependence. Using this value, we evaluated the fractionation slope of the acid digestion of CaCO 3 as 0.5170 ± 0.0004. The minor difference between the fractionation slope between aragonite and parent water and the reference slope of 0.528, implies that 17O excess in carbonates is only weakly temperature dependent. On the other hand, carbonate 17O excess values directly record variability in 17O excess of the parent water, making it a useful paleo-hydrological proxy. [ABSTRACT FROM AUTHOR]

Rationale Measurements of oxygen-17 excess (17Oexcess) in carbonates have become of great importance. However, to compare results obtained by different laboratories, it is necessary to normalize them to international standards. With this purpose in mind, we measured accurate and high precision δ17O and 17Oexcess values for NBS19 and NBS18, two international standards, for which δ18O values are already widely used as the references for carbonates. Methods The measurements are based on isotopic exchange at steady state between O2 and CO2 over hot platinum sponge at 750°C. Dual-inlet isotope ratio mass spectrometry (IRMS) measurements of the δ17O and δ18O values of this O2 allow δ17O values of CO2 to be obtained with very high precision (0.01 to 0.03 ‰) and, correspondingly, accurate 17Oexcess values with a precision of 5 per meg. Results We measured, for the first time, the δ17O values and 17Oexcess of CO2 liberated from NBS19 at 25°C (39.196 ± 0.026; 20.276 ± 0.015 ‰ and −227 ± 4 per meg, respectively) and NBS18 (17.591 ± 0.041 ‰; 9.253 ± 0.018 ‰ and 3 ± 5 per meg, respectively). The values are given versus VSMOW. Conclusions Accurate values of δ17O and 17Oexcess of the international standards NBS19-CO2 and NBS18-CO2 are now available. The new values should be used for normalization of measured oxygen isotope ratios of carbonates to allow meaningful comparison of results among different laboratories. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Carbonate bearing materials, such as foraminifera, mollusks shells, or speleothems, have the potential to preserve geochemical and isotopic signatures reflecting the environmental conditions at the time they formed. Beyond the conventional δ18O, information from triple oxygen isotopes (reported as 17O excess , which is defined as (ln (δ17O/1000 + 1) − λ ref ln (δ18O/1000 + 1)) × 106) in these archives promises to be a valuable tool to reconstruct past hydrological processes. The goal of this study is to determine the triple oxygen isotope fractionation between CaCO 3 and H 2 O under well-constrained laboratory conditions. We performed laboratory experiments to precipitate CaCO 3 polymorphs, either calcite or aragonite, at temperatures between 10 and 35 °C. We then evaluated the effect of polymorphism, temperature, and solution concentration on the 17O excess of CO 2 extracted from these carbonates and the 17O isotopic fractionation (17α) between water and CaCO 3. The obtained values of 18α and 17α between CO 2 extracted from CaCO 3 and parent water allow us to calculate the fractionation slope θ (=ln17α/ln18α). Our observations suggest that θ is indistinguishable at temperatures of 10 and 27 °C, but is slightly lower at 35 °C. The lower value at 35 °C may be related to disequilibrium during these experiments. We found that θ is independent of polymorph and of solution concentration, indicating that 17O excess is less sensitive than δ18O to these geochemical parameters and can thus be a robust proxy for reconstructing 17O excess of parent water. • The 17O fractionation between CaCO 3 and its parent water is examined experimentally. • The triple oxygen isotope fractionation slope, (ln17α/ln18α), is similar in calcite and aragonite. • The fractionation slope is independent of temperature at 10, 27 °C but somewhat lower at 35 °C. [ABSTRACT FROM AUTHOR]