13:45 〜 15:15
[SGL21-P10] High-precision Δ'17O measurements of phosphate in modern and fossil bioapatite: application for reconstructing paleo-environments
キーワード：三酸素同位体、バイオアパタイト、リン酸、古環境
Since the empirical study on temperature dependency of the 18O/16O ratios (the δ18O values) between bioapatite [Ca5(PO4)3OH], the main component of bone and tooth of vertebrates, and ambient H2O, the δ18O value of phosphate in bioapatite has been proposed to use as a proxy to reconstruct paleo-environment of ancient earth (Longinelli and Nuti, 1973). Because phosphate in bioapatite is under the oxygen isotope exchange equilibrium with the body water through the enzymatic activity of pyrophosphatase (PPase), the δ18O value of phosphate in bioapatite reflects that of ambient H2O where they lived. Additionally, phosphate is resistant to oxygen isotope exchange with H2O in the absence of PPase owing to the strong chemical bonds between phosphorus and oxygen in phosphate. Therefore, if we can assume that the δ18O value of the body water corresponded with that of ambient H2O, fossil bioapatite can be a robust proxy to reconstruct paleo-environment where and when they lived.
The body water of multicellular organisms, however, is a mixture of both ambient H2O ingested as foods/water and metabolic H2O of which the oxygen atoms are derived from atmospheric O2. As a result, the δ18O value of body water is often deviated from that of ambient H2O and the estimation of paleo-environment only by using δ18O of bioapatite as a proxy is often inaccurate without estimating an accurate mixing ratio of metabolic H2O within body water.
In this study, we used the Δ'17O value (≈ δ17O – 0.528 × δ18O) of phosphate in bioapatite as an additional tracer to quantify the metabolic H2O incorporated into body water. Because the variations in the Δ'17O values are negligible during the “mass-dependent” isotope fractionations on the δ17O and δ18O values such as those during the progress of the enzymatic reactions, only the mixing between a pair of the same oxygen compounds with different Δ'17O values causes a variation in the Δ'17O values. While the Δ'17O values of seawater and meteoric water reported in previous studies ranged from −5 × 10−6 to +37 × 10−6, that of atmospheric O2 was reported to be −443 × 10−6. Thus, the metabolic H2O of which the oxygen atoms are derived from atmospheric O2 can be distinguished from ambient H2O by using the Δ'17O values as a tracer.
To test this hypothesis, we determined the δ18O and Δ'17O values of modern and fossil (from 0.01 to 120 Ma) bioapatite of fish, mammals, dinosaur, and guano. Phosphate extracted from bioapatite was converted into Ag3PO4 as an analyte for the oxygen isotope measurements. The decomposition of Ag3PO4 to extract oxygen atoms as O2 was performed with a fluorination technique using BrF5 at 250℃. Extracted and purified O2 was analyzed using the dual inlet mode of a mass spectrometer with a cup configuration of m/z = 32, 33, and 34. The SD during multiple measurements of in-house Ag3PO4 standards showed 0.6 ‰ for δ18O, and 0.02 ‰ for Δ'17O.
We found that the Δ'17O values of phosphate in bioapatite showed the 17O-depleted Δ'17O values, ranging from −181 × 10−6 to −42 × 10−6, irrespective of the sample types. These Δ'17O values were significantly lower than those of seawater and meteoric water. Thus, we conclude that the incorporation of the metabolic H2O derived from atmospheric O2 was responsible for this depletion in 17O. Based on the isotopic mass balance using the Δ'17O values, the mixing ratios of the metabolic H2O within the body water were estimated to be 28 ± 6 % (1SD). We conclude that the Δ'17O values of phosphate in bioapatite preserve the physiological information and the mixing ratios of the metabolic H2O are almost stable irrespective of habitats and ages. In this presentation, we will discuss the δ18O values of bioapatite as well.
The body water of multicellular organisms, however, is a mixture of both ambient H2O ingested as foods/water and metabolic H2O of which the oxygen atoms are derived from atmospheric O2. As a result, the δ18O value of body water is often deviated from that of ambient H2O and the estimation of paleo-environment only by using δ18O of bioapatite as a proxy is often inaccurate without estimating an accurate mixing ratio of metabolic H2O within body water.
In this study, we used the Δ'17O value (≈ δ17O – 0.528 × δ18O) of phosphate in bioapatite as an additional tracer to quantify the metabolic H2O incorporated into body water. Because the variations in the Δ'17O values are negligible during the “mass-dependent” isotope fractionations on the δ17O and δ18O values such as those during the progress of the enzymatic reactions, only the mixing between a pair of the same oxygen compounds with different Δ'17O values causes a variation in the Δ'17O values. While the Δ'17O values of seawater and meteoric water reported in previous studies ranged from −5 × 10−6 to +37 × 10−6, that of atmospheric O2 was reported to be −443 × 10−6. Thus, the metabolic H2O of which the oxygen atoms are derived from atmospheric O2 can be distinguished from ambient H2O by using the Δ'17O values as a tracer.
To test this hypothesis, we determined the δ18O and Δ'17O values of modern and fossil (from 0.01 to 120 Ma) bioapatite of fish, mammals, dinosaur, and guano. Phosphate extracted from bioapatite was converted into Ag3PO4 as an analyte for the oxygen isotope measurements. The decomposition of Ag3PO4 to extract oxygen atoms as O2 was performed with a fluorination technique using BrF5 at 250℃. Extracted and purified O2 was analyzed using the dual inlet mode of a mass spectrometer with a cup configuration of m/z = 32, 33, and 34. The SD during multiple measurements of in-house Ag3PO4 standards showed 0.6 ‰ for δ18O, and 0.02 ‰ for Δ'17O.
We found that the Δ'17O values of phosphate in bioapatite showed the 17O-depleted Δ'17O values, ranging from −181 × 10−6 to −42 × 10−6, irrespective of the sample types. These Δ'17O values were significantly lower than those of seawater and meteoric water. Thus, we conclude that the incorporation of the metabolic H2O derived from atmospheric O2 was responsible for this depletion in 17O. Based on the isotopic mass balance using the Δ'17O values, the mixing ratios of the metabolic H2O within the body water were estimated to be 28 ± 6 % (1SD). We conclude that the Δ'17O values of phosphate in bioapatite preserve the physiological information and the mixing ratios of the metabolic H2O are almost stable irrespective of habitats and ages. In this presentation, we will discuss the δ18O values of bioapatite as well.