Letter to the Editor Received: 1 October 2013
Revised: 26 November 2013
Accepted: 2 December 2013
Published online in Wiley Online Library
Rapid Commun. Mass Spectrom. 2014, 28, 563–567 (wileyonlinelibrary.com) DOI: 10.1002/rcm.6799
Dear Editor, Carbon and oxygen isotope ratios and their temperature dependence in carbonate and tooth enamel using a GasBench II preparation device
Rapid Commun. Mass Spectrom. 2014, 28, 563–567
Copyright © 2014 John Wiley & Sons, Ltd.
563
The oxygen isotope ratios of carbonates that are widely distributed in various environments are a function of temperature, and are therefore a useful paleothermometer.[1] After the report by McCrea[2] on the dissolution method of carbonate using 100% phosphoric acid, carbon and oxygen isotope analysis has been used in the geological and environmental sciences.[3–7] The oxygen isotopes of marine carbonates accumulated over geological time are used for research in paleooceanography and paleoclimate studies, and the carbon and oxygen isotopes of hydrothermal carbonates are used to investigate the origin of hydrothermal ore deposits.[8–13] Thus, automated and high-precision isotope ratio mass spectrometers have been developed for isotopic studies of carbonates. Hydroxyapatite, which constitutes the bone and teeth of mammals (bioapatite), contains 3% structural carbonate. The carbon and oxygen isotope ratios of bioapatite from skeletal remains of fossil mammals can be used as excellent geochemical proxies for the reconstruction of paleoenvironmental and paleoclimatic conditions during the lifetime of the studied sample.[14–20] Recently, the analysis of tooth enamel from human and hominine fossils has attracted significant interest.[21–23] Thus, carbon and oxygen isotope analysis of bioapatite can be applied to environmental science, ecology, and anthropology, as well as to geochemistry. The oxygen isotope analysis of smaller samples is required because the teeth of humans and small mammals are smaller than those of large mammals, and the rarity of fossil samples means that destructive analysis is unsuitable.[24] The development of a fast and appropriate method is also required for the collection of the data required to infer a paleoenvironment. To use oxygen isotope ratios of bioapatite as a paleoclimatic proxy requires consideration of the effect of reaction temperature on the fractionation during the reaction of bioapatite with phosphoric acid.[25] Passey and co-workers reported that the temperature dependence of oxygen isotope fractionation was different between enamel and carbonate, and that this should be investigated in each preparation method, such as the sealed vessel method, the common acid bath method, and the Kiel device method.[26] A multi-loop gas injection system, such as the GasBench II preparation device (Thermo Fisher Scientific, Waltham, MA, USA), coupled with a continuous-flow isotope ratio mass spectrometer is now widely used.[27,28] By reducing the size of the sample vial, the carbon and oxygen isotope ratios in small amounts of carbonate samples can be measured.[29] However, there have been no investigations of the temperature dependence of oxygen isotope fractionation from bioapatite using the GasBench II. We have developed a fast measuring procedure with an isotope
ratio mass spectrometer that uses a 4.5 mL sample vial with a silver (Ag) cup. The purpose of this short communication is to reveal the temperature dependence of oxygen isotope ratios for calcite and bioapatite using the GasBench II device. Four carbonates and two types of bioapatite were used for this study. Two international isotopic standard materials, NBS19 (δ13C = 1.95‰, δ18O = 2.2‰) and LSVEC (δ13C = 46.6‰, δ18O = 26.7‰), were used for the experiments. Limestone and synthesized carbonate, whose carbon and oxygen isotope ratios are unknown, were also used. Limestone (JLs-1) is a rock standard material distributed by the Geological Survey of Japan. Synthesized carbonate (RIHN-MC) is a reagent material of calcium carbonate supplied by Merck (Suprapure, calcium carbonate, Lot. B0391759 034, Merck, Darmstadt, Germany). Modern and archaeological human teeth were used as bioapatite samples. The enamel sample of a modern human tooth (RIHN-TE, 0.6 g) was drilled and crushed using a mill (CryoMill, MM400, Retsch, Haan, Germany). The enamel was then soaked in 36% H2O2 for 2 days to digest the organic components. Archaeological tooth enamels (Archaeological TE) were from the human skeletal remains of four individuals excavated from the Inariyama shell mound of the Jomon period (Holocene). The archaeological enamels were soaked in 10% H2O2 for 24 h, and washed in 0.1 M buffered acetic acid for 10 min to dissolve diagenetic carbonates. Carbon and oxygen isotope ratios were measured with a DELTA V Plus isotope ratio mass spectrometer (Thermo Fisher Scientific) coupled with a PAL autosampler (GC PAL, CTC Analytics, Zwingen, Switzerland) and a GasBench II preparation device. Instead of the standard 12 mL vials,[29] 4.5 mL round-bottomed borosilicate vials (No. 948W, Labco, Ceredigion, UK) were used. All samples were weighed in Ag cups using an ultramicrobalance (UMX2, Mettler Toledo, Greifensee, Switzerland), after which the Ag cup with the sample was placed into a 4.5 mL vial. The use of Ag cups enables the samples to be more easily weighed, and avoids the problem of sample loss caused by static electricity when the samples are weighed and placed in the sample vials. The Ag cups do not react with phosphoric acid, but are used as a precautionary means of trapping trace amounts of sulfur in the samples that produce SO2 during acid digestion.[15] The sample vials were flushed with He for 240 s at a flow rate of 80 mL/min. Phosphoric acid (orthophosphoric acid 99%, Merck) was dried with a liquid nitrogen trap before use. Five to eight drops of phosphoric acid were manually added to the sample vial using a syringe. The reaction conditions were 24 h at 25 °C, 3 h at 50 °C, and 1 h at 70 °C. A single measurement gives four reference CO2 gas peaks and four sample gas peaks, and takes 440 s. A 0.025 mg carbonate sample and a 0.5 mg tooth sample gave ca 4 V peak intensity (mass 44 amplitude). The internal standard deviations of the four peaks for all the samples were in the range of 0.01–0.11‰ for δ13C values and 0.01–0.15‰ for δ18O values. The isotope ratios of the sample CO2 gas peaks were calculated against the values of the reference CO2 gas
Letter to the Editor peaks that were calibrated relative to the VPDB (Vienna PeeDee Belemnite) scale. These values were used to calculate the fractionation factor for the oxygen isotope ratios. The carbon and oxygen isotope ratios of JLs-1, RIHN-MC, and RIHN-TE were then normalized, based on two-point calibration with NBS19 and LSVEC.[30] Isotopic fractionation of CO2 by the reaction of carbonate and phosphoric acid is expressed by: 18 O=16 O CO2 α ¼ 18 16 (1) ð O= OÞCarbonate This indicates that the isotope ratios of all the oxygen in the carbonate should be measured to calculate the fractionation factor using a method such as the thermal decomposition method or the BrF5 technique.[31] However, the isotopic composition of evolved CO2 was measured using acid digestion; therefore, the fractionation factors of the oxygen isotope ratios were calculated by comparing the values at 25 °C, according to the following equation[32]: 18 δ OT 1000 þ 1 α25 αT ¼ (2) δ18 O25°C 1000 þ 1 where α25 is the fractionation factor of carbonate reacted at 25 °C (=1.01025[31–33]), and αT is that for the sample reacted at T (°C). The δ18O25°C and δ18OT values are the oxygen isotope ratios of the sample reacted at 25 °C and T (°C), respectively. It was assumed that the α25 value for bioapatite was the same as that for calcite.[26,32]
The results for the isotope measurements of CO2 are shown in Table 1. The external standard deviations of the carbon and oxygen isotope ratios are 0.05–0.13‰ and 0.08–0.21‰, respectively. There was no difference in these errors for the three reaction temperatures. Each sample had characteristic isotope ratios dependent on their origin. The mean value of the carbon isotope ratios varied from 0.1 to 0.5‰, whereas the mean value of the oxygen isotope ratios in the carbonates and tooth enamels varied from 0.5 to 3‰. There was no relationship between the carbon isotope ratios and temperature; however, the oxygen isotope ratios decreased at higher temperature, which indicates that consideration of the α-values for the oxygen isotope ratios of carbonates is important. Fractionation factors at each temperature were calculated using Eqn. (2) and are shown in Table 1. The standard error of the α-values for NBS19 and LSVEC was 0.0001, which is equal to a variance of 0.2‰ for δ18O values. Although the oxygen isotope ratios of NBS19 and LSVEC were widely different, there was no difference between the α-values for NBS19 and LSVEC. Although the α-values of RIHN-MC were somewhat lower than those of NBS19 and LSVEC (by 0.0001), the values of all carbonate samples were within a small range of 1.00955– 1.00977 at 50 °C and 1.00872–1.00898 at 70 °C. The linear regression between the α-values of all carbonates combined measured using the GasBench method and the reaction temperatures (K) is expressed by: αT ¼ 1:00480 ð±0:00038Þ þ
490 ð±38Þ T2
(3)
Table 1. Carbon and oxygen isotope analysis results (δ13C and δ18O values) for carbonates and tooth enamels using the GasBench II T (°C)
Reaction time (min)
n
δ13C (‰)
564
NBS19 (Limestone) 25 1440 8 1.91 50 180 6 2.39 70 60 6 1.90 LSVEC (Lithium carbonate) 25 1440 8 –45.85 50 180 6 –45.64 70 60 5 –45.83 JLs-1 (Limestone) 25 1440 8 1.95 50 180 6 2.35 70 60 6 1.82 RIHN-MC (Calcium carbonate) 25 1440 8 –46.60 50 180 6 –46.29 70 60 6 –46.56 RIHN-TE (Modern tooth enamel) 25 1440 8 –11.18 50 180 5 –10.91 70 60 4 –11.40 Archaeological TE (Archaeological tooth enamel) 25 1440 4 –11.13 50 180 4 –11.10 70 60 4 –10.88
wileyonlinelibrary.com/journal/rcm
1σ
δ18O (‰) of CO2
1σ
αT
1SE
0.11 0.06 0.07
7.89 7.41 6.53
0.21 0.08 0.10
1.01025 1.00977 1.00889
0.00003 0.00004
0.13 0.09 0.10
–16.01 –16.61 –17.25
0.11 0.11 0.20
1.01025 1.00963 1.00898
0.00005 0.00009
0.10 0.08 0.10
5.58 5.03 4.06
0.16 0.19 0.16
1.01025 1.00970 1.00872
0.00008 0.00006
0.05 0.08 0.06
–3.06 –3.75 –4.43
0.11 0.14 0.14
1.01025 1.00955 1.00886
0.00006 0.00006
0.08 0.11 0.10
3.78 2.70 2.03
0.11 0.21 0.14
1.01025 1.00916 1.00849
0.00009 0.00007
1.47 1.37 1.51
4.95 3.79 1.95
0.21 0.31 0.45
1.01025 1.00908 1.00723
0.00009 0.00024
Copyright © 2014 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2014, 28, 563–567
Letter to the Editor The α-values for RIHN-TE at 50 and 70 °C are almost the same as those determined for the modern tooth enamel using a sealed vessel method (α50 = 1.00920 and α70 = 1.00851[26]). The equation for the fractionation factor of modern tooth enamel measured using the GasBench method is: 639 ð±10Þ (4) T2 The α-values for archaeological enamel were higher by 0.00021 at 50 °C and lower by 0.00075 at 70 °C than those measured using the sealed vessel method (α50 = 1.00887 and α70 = 1.00799[26]). These differences were much larger than the variation for the carbonates, but within the variation of the fossil samples measured by the sealed vessel method. The equation for the fractionation factor of archaeological tooth enamel measured using the GasBench method is: αT ¼ 1:00305 ð±0:00010Þ þ
αT ¼ 0:99853 ð±0:00110Þ þ
1057 ð±112Þ T2
(5)
Figure 1 shows the oxygen isotope fractionation factors for carbonate and enamel as a function of the reaction temperature. The results clearly indicate that the fractionation factors for the carbonates are higher than those for bioapatite at each temperature.
The lower fractionation factor and larger variation for the archaeological teeth than those for modern teeth indicate that the chemical composition has been changed by a diagenetic effect. This could change the carbonate content of bioapatite, involve the inclusion of fluoride and other external elements, and change the mineral structure.[26] Environmental conditions (temperature, humidity, soil pH) may influence the degree of a diagenetic effect,[34] and these factors may be different at each site; however, we do not have detailed evidence on the relationship between the environmental conditions and diagenetic effects. Such diagenetic alterations of bioapatite cause large variation of the α-value, which decreases the reproducibility of the oxygen isotope ratios. Therefore, a large variation of the fractionation factor should be taken into account, even in Holocene teeth. The fractionation factors for the carbonates are higher than those reported by Swart and co-workers (Table 2).[32] This difference could be due to the difference between the GasBench and sealed vessel preparation methods. In the GasBench method, the reaction temperature may be lower than the set temperature of the sample tray, because phosphoric acid is added manually with a syringe. However, this possibility does not account for the difference in the fractionation factor for
Figure 1. Temperature dependence of α-values for carbonates and tooth enamel. One standard error for the α-values of tooth enamel is shown, and that for the carbonate samples is smaller than the symbol size. Curve fits obtained using Eqn. (3) for carbonates (solid line), Eqn. (4) for modern enamel (dashed line), and Eqn. (5) for archaeological enamel (dashed bold line) are also plotted. The α-values for calcite calculated by Swart et al.[32] and those for modern and fossil tooth enamel calculated by Passey et al.[26] are shown for comparison. Table 2. Comparison of α-values obtained using different methods Method Sealed vessel Sealed vessel GasBench
α50
α70
Ref.
Swart calcite Passey modern TE Passey fossil TE Calcite Modern TE Archaeological TE
1.00931 1.00920 1.00887 1.00949 1.00917 1.00865
1.00870 1.00851 1.00799 1.00896 1.00848 1.00751
Swart et al.[32] Passey et al.[26] Passey et al.[26] This study, Eqn. (3) This study, Eqn. (4) This study, Eqn. (5)
Rapid Commun. Mass Spectrom. 2014, 28, 563–567
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Sample
Letter to the Editor Table 3. Carbon and oxygen isotope ratios (δ13C and δ18O values) calibrated against the VPDB scale
N
δ13C (‰) vs VPDB
1σ
JLs-1 (Limestone) 8 6 6 Mean RIHN-MC (Calcium carbonate) 8 6 6 Mean RIHN-TE (Modern tooth enamel) 8 6 4 Mean
1σ
αT
T (°C)
1.98 1.91 1.87 1.92
0.10 0.08 0.10 0.09
–4.57 –4.63 –4.75 –4.65
0.16 0.19 0.15 0.17
1.01025 1.00949 1.00896
25 50 70
–47.37 –47.26 –47.34 –47.32
0.05 0.08 0.06 0.07
–13.43 –13.59 –13.50 –13.51
0.11 0.14 0.14 0.13
1.01025 1.00949 1.00896
25 50 70
–11.36 –11.50 –11.58 –11.48
0.08 0.11 0.10 0.10
–6.42 –6.68 –6.34 –6.48
0.11 0.21 0.14 0.15
1.01025 1.00917 1.00848
25 50 70
566
calcite, whereas the fractionation factor for bioapatite did not differ between the two methods. In addition, other reasons cannot be ruled out, such as the exchange between evolved CO2 and water produced or in the acid, the dissolution of CO2 in the acid, and the acid viscosity strength.[32] The carbon and oxygen isotope ratios of the carbonates and modern enamels were calculated with two-point anchoring of the NBS19 and LSVEC international standards, after calculation using the α-value from Eqns. (3) and (4) at each temperature (Table 3). JLs-1 showed mean δ13C and δ18O values of 1.92 ± 0.06‰ and 4.65 ± 0.09‰, respectively, while those of RIHN-MC were δ13C = 47.32 ± 0.06‰ and δ18O = 13.51 ± 0.08‰. After normalization the ranges of carbon and oxygen isotope ratios for JLs-1 decreased from 0.53‰ to 0.11‰, and from 1.52‰ to 0.17‰, respectively. The ranges of RIHN-MC also decreased from 0.31‰ to 0.11‰ for δ13C values, and from 1.37‰ to 0.16‰ for δ18O values. RIHN-TE showed mean δ13C and δ18O values of 11.48 ± 0.11‰ and 6.48 ± 0.18‰, respectively. The range of difference between the three temperatures decreased from 0.49‰ to 0.21‰ for δ13C values and from 1.75‰ to 0.34‰ for δ18O values. This decrease in the ranges of the carbon and oxygen isotope ratios suggests that normalization based on the standard materials is essential to enhance the analytical precision. In summary, a procedure was developed that employs a small sample vial with an Ag cup to measure small sample amounts using the GasBench II. The temperature dependence of the oxygen isotope ratios for carbonates and tooth enamel using the GasBench II was different from that obtained with other methods (Table 2). The variation of the fractionation factor for archaeological teeth was larger at higher temperature; therefore, lower temperature reaction should be employed to enhance the analytical precision. We recommend measurement of the oxygen isotope ratios of bioapatite at 25 °C with the GasBench method under the assumption that the fractionation factor is 1.01025 for bioapatite. The study proposes carbon and oxygen isotope ratios for carbonates and modern enamel standard materials. Furthermore, the preparation of a large amount of bioapatite standard materials is crucial for the measurement and
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δ18O (‰) vs VPDB
comparison of isotope ratios between different laboratories. Inter-laboratory comparison would enhance the reproducibility of oxygen isotope ratios of bioapatite for paleoenvironmental and paleoclimatic studies.
Acknowledgements The authors thank Kevin T. Uno (Columbia University) for helpful advice, and Maki Morimoto (Nagoya University) and the members of the Center for Research Promotion of the Research Institute for Humanity and Nature, for guidance with experimental techniques. This study was supported by a Research Fellowship from the Japan Society for the Promotion of Science (JSPS, #12J02772).
Soichiro Kusaka* and Takanori Nakano Research Institute for Humanity and Nature, Kyoto 603-8047, Japan *Correspondence to: S. Kusaka, Research Institute for Humanity and Nature, Kyoto 603-8047, Japan. E-mail: [email protected]
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Copyright © 2014 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2014, 28, 563–567
Letter to the Editor [7] J. Veizer, D. Ala, K. Azmy, P. Bruckschen, D. Buhl, F. Bruhn, G. A. F. Carden, A. Diener, S. Ebneth, Y. Godderis, T. Jasper, C. Korte, F. Pawellek, O. G. Podlaha, H. Strauss. 87Sr/86Sr, δ13C and δ18O evolution of Phanerozoic seawater. Chem. Geol. 1999, 161, 59. [8] C. Emiliani. Pleistocene temperatures. J. Geol. 1955, 538. [9] N. J. Shackleton. Oxygen isotope analyses and Pleistocene temperatures re-assessed. Nature 1967, 215, 15. [10] J. Zachos, M. Pagani, L. Sloan, E. Thomas, K. Billups. Trends, rhythms, and aberrations in global climate 65 Ma to present. Science 2001, 292, 686. [11] H. Ohmoto. Systematics of sulfur and carbon isotopes in hydrothermal ore deposits. Econ. Geol. 1972, 67, 551. [12] R. O. Rye, H. Ohmoto. Sulfur and carbon isotopes and ore genesis: a review. Econ. Geol. 1974, 69, 826. [13] J. M. Peter, W. C. Shanks III. Sulfur, carbon, and oxygen isotope variations in submarine hydrothermal deposits of Guaymas Basin, Gulf of California, USA. Geochim. Cosmochim. Acta 1992, 56, 2025. [14] T. E. Cerling, N. E. Levin, B. H. Passey. Stable isotope ecology in the Omo-Turkana Basin. Evol. Anthrop. 2011, 20, 228. [15] K. T. Uno, T. E. Cerling, J. M. Harris, Y. Kunimatsu, M. G. Leakey, M. Nakatsukasa, H. Nakaya. Late Miocene to Pliocene carbon isotope record of differential diet change among East African herbivores. Proc. Natl. Acad. Sci. USA 2011, 108, 6509. [16] T. D. White, S. H. Ambrose, G. Suwa, D. F. Su, D. DeGusta, R. L. Bernor, J.-R. Boisserie, M. Brunet, E. Delson, S. Frost, N. Garcia, I. X. Giaourtsakis, Y. Haile-Selassie, F. C. Howell, T. Lehmann, A. Likius, C. Pehlevan, H. Saegusa, G. Semprebon, M. Teaford, E. Vrba. Macrovertebrate paleontology and the Pliocene habitat of Ardipithecus ramidus. Science 2009, 326, 67. [17] L. K. Ayliffe, A. R. Chivas. Oxygen isotope composition of the bone phosphate of Australian kangaroos: Potential as a palaeoenvironmental recorder. Geochim. Cosmochim. Acta 1990, 54, 2603. [18] M. J. Kohn, T. E. Cerling, in Reviews in Mineralogy and Geochemistry Phosphates Geochemical, Geological, and Materials Importance, vol. 48, (Eds: M. J. Kohn, J. Rakovan, J. M. Hughes). Mineralogical Society of America, Washington, DC, 2002, pp. 455–488. [19] A. Longinelli. Oxygen isotopes in mammal bone phosphate: A new tool for paleohydrological and paleoclimatological research? Geochim. Cosmochim. Acta 1984, 48, 385. [20] A. Gehler, T. Tütken, A. Pack. Oxygen and carbon isotope variations in a modern rodent community – Implications for palaeoenvironmental reconstructions. PLoS ONE 2012, 7, e49531. [21] T. E. Cerling, F. K. Manthi, E. N. Mbua, L. N. Leakey, M. G. Leakey, R. E. Leakey, F. H. Brown, F. E. Grine, J. A. Hart,
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567
Rapid Commun. Mass Spectrom. 2014, 28, 563–567
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/rcm
Revised: 26 November 2013
Accepted: 2 December 2013
Published online in Wiley Online Library
Rapid Commun. Mass Spectrom. 2014, 28, 563–567 (wileyonlinelibrary.com) DOI: 10.1002/rcm.6799
Dear Editor, Carbon and oxygen isotope ratios and their temperature dependence in carbonate and tooth enamel using a GasBench II preparation device
Rapid Commun. Mass Spectrom. 2014, 28, 563–567
Copyright © 2014 John Wiley & Sons, Ltd.
563
The oxygen isotope ratios of carbonates that are widely distributed in various environments are a function of temperature, and are therefore a useful paleothermometer.[1] After the report by McCrea[2] on the dissolution method of carbonate using 100% phosphoric acid, carbon and oxygen isotope analysis has been used in the geological and environmental sciences.[3–7] The oxygen isotopes of marine carbonates accumulated over geological time are used for research in paleooceanography and paleoclimate studies, and the carbon and oxygen isotopes of hydrothermal carbonates are used to investigate the origin of hydrothermal ore deposits.[8–13] Thus, automated and high-precision isotope ratio mass spectrometers have been developed for isotopic studies of carbonates. Hydroxyapatite, which constitutes the bone and teeth of mammals (bioapatite), contains 3% structural carbonate. The carbon and oxygen isotope ratios of bioapatite from skeletal remains of fossil mammals can be used as excellent geochemical proxies for the reconstruction of paleoenvironmental and paleoclimatic conditions during the lifetime of the studied sample.[14–20] Recently, the analysis of tooth enamel from human and hominine fossils has attracted significant interest.[21–23] Thus, carbon and oxygen isotope analysis of bioapatite can be applied to environmental science, ecology, and anthropology, as well as to geochemistry. The oxygen isotope analysis of smaller samples is required because the teeth of humans and small mammals are smaller than those of large mammals, and the rarity of fossil samples means that destructive analysis is unsuitable.[24] The development of a fast and appropriate method is also required for the collection of the data required to infer a paleoenvironment. To use oxygen isotope ratios of bioapatite as a paleoclimatic proxy requires consideration of the effect of reaction temperature on the fractionation during the reaction of bioapatite with phosphoric acid.[25] Passey and co-workers reported that the temperature dependence of oxygen isotope fractionation was different between enamel and carbonate, and that this should be investigated in each preparation method, such as the sealed vessel method, the common acid bath method, and the Kiel device method.[26] A multi-loop gas injection system, such as the GasBench II preparation device (Thermo Fisher Scientific, Waltham, MA, USA), coupled with a continuous-flow isotope ratio mass spectrometer is now widely used.[27,28] By reducing the size of the sample vial, the carbon and oxygen isotope ratios in small amounts of carbonate samples can be measured.[29] However, there have been no investigations of the temperature dependence of oxygen isotope fractionation from bioapatite using the GasBench II. We have developed a fast measuring procedure with an isotope
ratio mass spectrometer that uses a 4.5 mL sample vial with a silver (Ag) cup. The purpose of this short communication is to reveal the temperature dependence of oxygen isotope ratios for calcite and bioapatite using the GasBench II device. Four carbonates and two types of bioapatite were used for this study. Two international isotopic standard materials, NBS19 (δ13C = 1.95‰, δ18O = 2.2‰) and LSVEC (δ13C = 46.6‰, δ18O = 26.7‰), were used for the experiments. Limestone and synthesized carbonate, whose carbon and oxygen isotope ratios are unknown, were also used. Limestone (JLs-1) is a rock standard material distributed by the Geological Survey of Japan. Synthesized carbonate (RIHN-MC) is a reagent material of calcium carbonate supplied by Merck (Suprapure, calcium carbonate, Lot. B0391759 034, Merck, Darmstadt, Germany). Modern and archaeological human teeth were used as bioapatite samples. The enamel sample of a modern human tooth (RIHN-TE, 0.6 g) was drilled and crushed using a mill (CryoMill, MM400, Retsch, Haan, Germany). The enamel was then soaked in 36% H2O2 for 2 days to digest the organic components. Archaeological tooth enamels (Archaeological TE) were from the human skeletal remains of four individuals excavated from the Inariyama shell mound of the Jomon period (Holocene). The archaeological enamels were soaked in 10% H2O2 for 24 h, and washed in 0.1 M buffered acetic acid for 10 min to dissolve diagenetic carbonates. Carbon and oxygen isotope ratios were measured with a DELTA V Plus isotope ratio mass spectrometer (Thermo Fisher Scientific) coupled with a PAL autosampler (GC PAL, CTC Analytics, Zwingen, Switzerland) and a GasBench II preparation device. Instead of the standard 12 mL vials,[29] 4.5 mL round-bottomed borosilicate vials (No. 948W, Labco, Ceredigion, UK) were used. All samples were weighed in Ag cups using an ultramicrobalance (UMX2, Mettler Toledo, Greifensee, Switzerland), after which the Ag cup with the sample was placed into a 4.5 mL vial. The use of Ag cups enables the samples to be more easily weighed, and avoids the problem of sample loss caused by static electricity when the samples are weighed and placed in the sample vials. The Ag cups do not react with phosphoric acid, but are used as a precautionary means of trapping trace amounts of sulfur in the samples that produce SO2 during acid digestion.[15] The sample vials were flushed with He for 240 s at a flow rate of 80 mL/min. Phosphoric acid (orthophosphoric acid 99%, Merck) was dried with a liquid nitrogen trap before use. Five to eight drops of phosphoric acid were manually added to the sample vial using a syringe. The reaction conditions were 24 h at 25 °C, 3 h at 50 °C, and 1 h at 70 °C. A single measurement gives four reference CO2 gas peaks and four sample gas peaks, and takes 440 s. A 0.025 mg carbonate sample and a 0.5 mg tooth sample gave ca 4 V peak intensity (mass 44 amplitude). The internal standard deviations of the four peaks for all the samples were in the range of 0.01–0.11‰ for δ13C values and 0.01–0.15‰ for δ18O values. The isotope ratios of the sample CO2 gas peaks were calculated against the values of the reference CO2 gas
Letter to the Editor peaks that were calibrated relative to the VPDB (Vienna PeeDee Belemnite) scale. These values were used to calculate the fractionation factor for the oxygen isotope ratios. The carbon and oxygen isotope ratios of JLs-1, RIHN-MC, and RIHN-TE were then normalized, based on two-point calibration with NBS19 and LSVEC.[30] Isotopic fractionation of CO2 by the reaction of carbonate and phosphoric acid is expressed by: 18 O=16 O CO2 α ¼ 18 16 (1) ð O= OÞCarbonate This indicates that the isotope ratios of all the oxygen in the carbonate should be measured to calculate the fractionation factor using a method such as the thermal decomposition method or the BrF5 technique.[31] However, the isotopic composition of evolved CO2 was measured using acid digestion; therefore, the fractionation factors of the oxygen isotope ratios were calculated by comparing the values at 25 °C, according to the following equation[32]: 18 δ OT 1000 þ 1 α25 αT ¼ (2) δ18 O25°C 1000 þ 1 where α25 is the fractionation factor of carbonate reacted at 25 °C (=1.01025[31–33]), and αT is that for the sample reacted at T (°C). The δ18O25°C and δ18OT values are the oxygen isotope ratios of the sample reacted at 25 °C and T (°C), respectively. It was assumed that the α25 value for bioapatite was the same as that for calcite.[26,32]
The results for the isotope measurements of CO2 are shown in Table 1. The external standard deviations of the carbon and oxygen isotope ratios are 0.05–0.13‰ and 0.08–0.21‰, respectively. There was no difference in these errors for the three reaction temperatures. Each sample had characteristic isotope ratios dependent on their origin. The mean value of the carbon isotope ratios varied from 0.1 to 0.5‰, whereas the mean value of the oxygen isotope ratios in the carbonates and tooth enamels varied from 0.5 to 3‰. There was no relationship between the carbon isotope ratios and temperature; however, the oxygen isotope ratios decreased at higher temperature, which indicates that consideration of the α-values for the oxygen isotope ratios of carbonates is important. Fractionation factors at each temperature were calculated using Eqn. (2) and are shown in Table 1. The standard error of the α-values for NBS19 and LSVEC was 0.0001, which is equal to a variance of 0.2‰ for δ18O values. Although the oxygen isotope ratios of NBS19 and LSVEC were widely different, there was no difference between the α-values for NBS19 and LSVEC. Although the α-values of RIHN-MC were somewhat lower than those of NBS19 and LSVEC (by 0.0001), the values of all carbonate samples were within a small range of 1.00955– 1.00977 at 50 °C and 1.00872–1.00898 at 70 °C. The linear regression between the α-values of all carbonates combined measured using the GasBench method and the reaction temperatures (K) is expressed by: αT ¼ 1:00480 ð±0:00038Þ þ
490 ð±38Þ T2
(3)
Table 1. Carbon and oxygen isotope analysis results (δ13C and δ18O values) for carbonates and tooth enamels using the GasBench II T (°C)
Reaction time (min)
n
δ13C (‰)
564
NBS19 (Limestone) 25 1440 8 1.91 50 180 6 2.39 70 60 6 1.90 LSVEC (Lithium carbonate) 25 1440 8 –45.85 50 180 6 –45.64 70 60 5 –45.83 JLs-1 (Limestone) 25 1440 8 1.95 50 180 6 2.35 70 60 6 1.82 RIHN-MC (Calcium carbonate) 25 1440 8 –46.60 50 180 6 –46.29 70 60 6 –46.56 RIHN-TE (Modern tooth enamel) 25 1440 8 –11.18 50 180 5 –10.91 70 60 4 –11.40 Archaeological TE (Archaeological tooth enamel) 25 1440 4 –11.13 50 180 4 –11.10 70 60 4 –10.88
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1σ
δ18O (‰) of CO2
1σ
αT
1SE
0.11 0.06 0.07
7.89 7.41 6.53
0.21 0.08 0.10
1.01025 1.00977 1.00889
0.00003 0.00004
0.13 0.09 0.10
–16.01 –16.61 –17.25
0.11 0.11 0.20
1.01025 1.00963 1.00898
0.00005 0.00009
0.10 0.08 0.10
5.58 5.03 4.06
0.16 0.19 0.16
1.01025 1.00970 1.00872
0.00008 0.00006
0.05 0.08 0.06
–3.06 –3.75 –4.43
0.11 0.14 0.14
1.01025 1.00955 1.00886
0.00006 0.00006
0.08 0.11 0.10
3.78 2.70 2.03
0.11 0.21 0.14
1.01025 1.00916 1.00849
0.00009 0.00007
1.47 1.37 1.51
4.95 3.79 1.95
0.21 0.31 0.45
1.01025 1.00908 1.00723
0.00009 0.00024
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Letter to the Editor The α-values for RIHN-TE at 50 and 70 °C are almost the same as those determined for the modern tooth enamel using a sealed vessel method (α50 = 1.00920 and α70 = 1.00851[26]). The equation for the fractionation factor of modern tooth enamel measured using the GasBench method is: 639 ð±10Þ (4) T2 The α-values for archaeological enamel were higher by 0.00021 at 50 °C and lower by 0.00075 at 70 °C than those measured using the sealed vessel method (α50 = 1.00887 and α70 = 1.00799[26]). These differences were much larger than the variation for the carbonates, but within the variation of the fossil samples measured by the sealed vessel method. The equation for the fractionation factor of archaeological tooth enamel measured using the GasBench method is: αT ¼ 1:00305 ð±0:00010Þ þ
αT ¼ 0:99853 ð±0:00110Þ þ
1057 ð±112Þ T2
(5)
Figure 1 shows the oxygen isotope fractionation factors for carbonate and enamel as a function of the reaction temperature. The results clearly indicate that the fractionation factors for the carbonates are higher than those for bioapatite at each temperature.
The lower fractionation factor and larger variation for the archaeological teeth than those for modern teeth indicate that the chemical composition has been changed by a diagenetic effect. This could change the carbonate content of bioapatite, involve the inclusion of fluoride and other external elements, and change the mineral structure.[26] Environmental conditions (temperature, humidity, soil pH) may influence the degree of a diagenetic effect,[34] and these factors may be different at each site; however, we do not have detailed evidence on the relationship between the environmental conditions and diagenetic effects. Such diagenetic alterations of bioapatite cause large variation of the α-value, which decreases the reproducibility of the oxygen isotope ratios. Therefore, a large variation of the fractionation factor should be taken into account, even in Holocene teeth. The fractionation factors for the carbonates are higher than those reported by Swart and co-workers (Table 2).[32] This difference could be due to the difference between the GasBench and sealed vessel preparation methods. In the GasBench method, the reaction temperature may be lower than the set temperature of the sample tray, because phosphoric acid is added manually with a syringe. However, this possibility does not account for the difference in the fractionation factor for
Figure 1. Temperature dependence of α-values for carbonates and tooth enamel. One standard error for the α-values of tooth enamel is shown, and that for the carbonate samples is smaller than the symbol size. Curve fits obtained using Eqn. (3) for carbonates (solid line), Eqn. (4) for modern enamel (dashed line), and Eqn. (5) for archaeological enamel (dashed bold line) are also plotted. The α-values for calcite calculated by Swart et al.[32] and those for modern and fossil tooth enamel calculated by Passey et al.[26] are shown for comparison. Table 2. Comparison of α-values obtained using different methods Method Sealed vessel Sealed vessel GasBench
α50
α70
Ref.
Swart calcite Passey modern TE Passey fossil TE Calcite Modern TE Archaeological TE
1.00931 1.00920 1.00887 1.00949 1.00917 1.00865
1.00870 1.00851 1.00799 1.00896 1.00848 1.00751
Swart et al.[32] Passey et al.[26] Passey et al.[26] This study, Eqn. (3) This study, Eqn. (4) This study, Eqn. (5)
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Sample
Letter to the Editor Table 3. Carbon and oxygen isotope ratios (δ13C and δ18O values) calibrated against the VPDB scale
N
δ13C (‰) vs VPDB
1σ
JLs-1 (Limestone) 8 6 6 Mean RIHN-MC (Calcium carbonate) 8 6 6 Mean RIHN-TE (Modern tooth enamel) 8 6 4 Mean
1σ
αT
T (°C)
1.98 1.91 1.87 1.92
0.10 0.08 0.10 0.09
–4.57 –4.63 –4.75 –4.65
0.16 0.19 0.15 0.17
1.01025 1.00949 1.00896
25 50 70
–47.37 –47.26 –47.34 –47.32
0.05 0.08 0.06 0.07
–13.43 –13.59 –13.50 –13.51
0.11 0.14 0.14 0.13
1.01025 1.00949 1.00896
25 50 70
–11.36 –11.50 –11.58 –11.48
0.08 0.11 0.10 0.10
–6.42 –6.68 –6.34 –6.48
0.11 0.21 0.14 0.15
1.01025 1.00917 1.00848
25 50 70
566
calcite, whereas the fractionation factor for bioapatite did not differ between the two methods. In addition, other reasons cannot be ruled out, such as the exchange between evolved CO2 and water produced or in the acid, the dissolution of CO2 in the acid, and the acid viscosity strength.[32] The carbon and oxygen isotope ratios of the carbonates and modern enamels were calculated with two-point anchoring of the NBS19 and LSVEC international standards, after calculation using the α-value from Eqns. (3) and (4) at each temperature (Table 3). JLs-1 showed mean δ13C and δ18O values of 1.92 ± 0.06‰ and 4.65 ± 0.09‰, respectively, while those of RIHN-MC were δ13C = 47.32 ± 0.06‰ and δ18O = 13.51 ± 0.08‰. After normalization the ranges of carbon and oxygen isotope ratios for JLs-1 decreased from 0.53‰ to 0.11‰, and from 1.52‰ to 0.17‰, respectively. The ranges of RIHN-MC also decreased from 0.31‰ to 0.11‰ for δ13C values, and from 1.37‰ to 0.16‰ for δ18O values. RIHN-TE showed mean δ13C and δ18O values of 11.48 ± 0.11‰ and 6.48 ± 0.18‰, respectively. The range of difference between the three temperatures decreased from 0.49‰ to 0.21‰ for δ13C values and from 1.75‰ to 0.34‰ for δ18O values. This decrease in the ranges of the carbon and oxygen isotope ratios suggests that normalization based on the standard materials is essential to enhance the analytical precision. In summary, a procedure was developed that employs a small sample vial with an Ag cup to measure small sample amounts using the GasBench II. The temperature dependence of the oxygen isotope ratios for carbonates and tooth enamel using the GasBench II was different from that obtained with other methods (Table 2). The variation of the fractionation factor for archaeological teeth was larger at higher temperature; therefore, lower temperature reaction should be employed to enhance the analytical precision. We recommend measurement of the oxygen isotope ratios of bioapatite at 25 °C with the GasBench method under the assumption that the fractionation factor is 1.01025 for bioapatite. The study proposes carbon and oxygen isotope ratios for carbonates and modern enamel standard materials. Furthermore, the preparation of a large amount of bioapatite standard materials is crucial for the measurement and
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δ18O (‰) vs VPDB
comparison of isotope ratios between different laboratories. Inter-laboratory comparison would enhance the reproducibility of oxygen isotope ratios of bioapatite for paleoenvironmental and paleoclimatic studies.
Acknowledgements The authors thank Kevin T. Uno (Columbia University) for helpful advice, and Maki Morimoto (Nagoya University) and the members of the Center for Research Promotion of the Research Institute for Humanity and Nature, for guidance with experimental techniques. This study was supported by a Research Fellowship from the Japan Society for the Promotion of Science (JSPS, #12J02772).
Soichiro Kusaka* and Takanori Nakano Research Institute for Humanity and Nature, Kyoto 603-8047, Japan *Correspondence to: S. Kusaka, Research Institute for Humanity and Nature, Kyoto 603-8047, Japan. E-mail: [email protected]
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