09:15 〜 09:30
[MIS12-12] Accurate estimation on body temperature of extinct marine vertebrates by using the triple oxygen isotope compositions of phosphate in bioapatite
キーワード：三酸素同位体組成、リン酸、バイオアパタイト、古環境、酸素同位体交換平衡
The body temperature of extinct vertebrates provides us information on their physiology and the paleo-environments where and when they lived. Because phosphate attains oxygen isotope exchange equilibrium with surrounding water in the coexistence of the enzyme pyrophosphatase (PPase), the 18O/16O ratios (the δ18O values) of phosphate in bioapatite, the main component of bone and tooth of vertebrates, are the function of both the δ18O values of body water and body temperature. Additionally, phosphate is resistant to oxygen isotope exchange with H2O in the absence of PPase due to the strong chemical bonds between phosphorus and oxygen atoms in phosphate. Therefore, the body temperature of extinct vertebrates can be reconstructed by using the δ18O values of phosphate in bioapatite as a proxy, if the δ18O value of body water can be estimated accurately.
The body water of multicellular organisms, however, consists not only of ambient H2O ingested as foods/water, but also of metabolic H2O, of which the oxygen atoms are derived from inhaled atmospheric O2. Thus, it is difficult to constrain the δ18O values of body water without correcting the contribution of metabolic H2O within body water.
A few previous studies showed that the contribution of metabolic H2O within body water can be quantified by using the triple oxygen isotope compositions [Δ'17O = ln(δ17O+1) – 0.528 × ln(δ18O+1)] of carbonate and phosphate in bioapatite as a tracer because the Δ'17O value of atmospheric O2 (−443 × 10−6) is significantly different from that of ambient H2O (−5 × 10−6 for seawater and +40 × 10−6 for meteoric water). Because the variations in Δ'17O should be negligible during the "mass-dependent" isotope fractionations on the δ17O and δ18O values, only the mixing between a pair of the same oxygen compounds with different Δ'17O values primarily causes the variation in Δ'17O. However, the recent study showed the significant variation in Δ'17O of carbonate under the oxygen isotope exchange equilibrium with H2O as a function of temperature, implying that the Δ'17O value of carbonate does not directly reflect that of surrounding H2O. Yet, the variation in Δ'17O of phosphate under the oxygen isotope exchange equilibrium with H2O has not been quantified.
In this study, at first, we conducted the in vitro experiments using PPase to quantify the variation in Δ'17O of phosphate under the oxygen isotope exchange equilibrium with H2O. Then, we determined the δ18O and Δ'17O values of phosphate in bioapatite of both extant and extinct marine vertebrates and were compared with those of phosphate equilibrated with seawater. The δ18O and Δ'17O values of phosphate were analyzed by using the fluorination method introduced by Sambuichi et al. (2023).
We found that the Δ'17O values of phosphate under the oxygen isotope exchange equilibrium with surrounding H2O were approximately 100 × 10−6 lower than that of surrounding H2O. Additionally, comparing with the Δ'17O values of phosphate under the oxygen isotope exchange equilibrium with the modern seawater, the Δ'17O values of phosphate in bioapatite of marine vertebrates, ranging from −109 × 10−6 to −41 × 10−6, can be explained by the oxygen isotope exchange equilibrium with body water. This implies that the contribution of atmospheric O2 within the body water of marine vertebrates was negligibly minor and the δ18O and Δ'17O values of seawater in the past, at least during the Mesozoic era, were similar to those of modern seawater. Therefore, we conclude that the Δ'17O value of phosphate in bioapatite is effective as an additional tracer for the accurate estimations of body temperature as well as paleo-environments.
The body water of multicellular organisms, however, consists not only of ambient H2O ingested as foods/water, but also of metabolic H2O, of which the oxygen atoms are derived from inhaled atmospheric O2. Thus, it is difficult to constrain the δ18O values of body water without correcting the contribution of metabolic H2O within body water.
A few previous studies showed that the contribution of metabolic H2O within body water can be quantified by using the triple oxygen isotope compositions [Δ'17O = ln(δ17O+1) – 0.528 × ln(δ18O+1)] of carbonate and phosphate in bioapatite as a tracer because the Δ'17O value of atmospheric O2 (−443 × 10−6) is significantly different from that of ambient H2O (−5 × 10−6 for seawater and +40 × 10−6 for meteoric water). Because the variations in Δ'17O should be negligible during the "mass-dependent" isotope fractionations on the δ17O and δ18O values, only the mixing between a pair of the same oxygen compounds with different Δ'17O values primarily causes the variation in Δ'17O. However, the recent study showed the significant variation in Δ'17O of carbonate under the oxygen isotope exchange equilibrium with H2O as a function of temperature, implying that the Δ'17O value of carbonate does not directly reflect that of surrounding H2O. Yet, the variation in Δ'17O of phosphate under the oxygen isotope exchange equilibrium with H2O has not been quantified.
In this study, at first, we conducted the in vitro experiments using PPase to quantify the variation in Δ'17O of phosphate under the oxygen isotope exchange equilibrium with H2O. Then, we determined the δ18O and Δ'17O values of phosphate in bioapatite of both extant and extinct marine vertebrates and were compared with those of phosphate equilibrated with seawater. The δ18O and Δ'17O values of phosphate were analyzed by using the fluorination method introduced by Sambuichi et al. (2023).
We found that the Δ'17O values of phosphate under the oxygen isotope exchange equilibrium with surrounding H2O were approximately 100 × 10−6 lower than that of surrounding H2O. Additionally, comparing with the Δ'17O values of phosphate under the oxygen isotope exchange equilibrium with the modern seawater, the Δ'17O values of phosphate in bioapatite of marine vertebrates, ranging from −109 × 10−6 to −41 × 10−6, can be explained by the oxygen isotope exchange equilibrium with body water. This implies that the contribution of atmospheric O2 within the body water of marine vertebrates was negligibly minor and the δ18O and Δ'17O values of seawater in the past, at least during the Mesozoic era, were similar to those of modern seawater. Therefore, we conclude that the Δ'17O value of phosphate in bioapatite is effective as an additional tracer for the accurate estimations of body temperature as well as paleo-environments.