5:15 PM - 6:45 PM
[MIS12-P04] A challenge in paleowater temperature reconstruction using oxygen isotope ratio of biogenic carbonate: quantitative evaluation of species-specificity due to vital effect
Keywords:Biogenic carbonate, Oxygen stable isotope, Vital effect, Paleowater temperature restoration
Stable oxygen isotope ratio (δ18O) of biogenic carbonates is the most widely used proxy as water temperature. Although temperature conversion equations between the δ18O and water temperature in biogenic carbonates from various taxa have been reported, it is known that there are species-specific differences between these equations and those obtained from inorganic precipitation experiments. This difference is believed to be due to a vital effect caused by various processes in the organism (Gilbert et al., 2022). Therefore, to reconstruct more reliable paleotemperatures, it is necessary to quantitatively evaluate and correct for the species-specificity of the oxygen isotope fractionation.
In this study, we aimed to evaluate the species-specificity of oxygen isotope fractionation of biogenic carbonates precisely and quantitatively, by minimizing uncertainty derived from differences in (micro) environments of carbonate precipitation and analytical methodological discrepancy. For this purpose, bivalve shells and fish otoliths are obtained from the rearing experiment conducted in the same tanks under controlled temperatures (22°C, 19°C, and 16°C). By rearing them in the same tank, we eliminated the influence of differences in calcification conditions. Furthermore, systematic analytical errors were eliminated by analyzing all samples using the same equipment and methods. The temperature conversion equations for each organism were then calculated from the obtained oxygen isotope ratio of carbonate (δ18OC), oxygen isotope ratio of rearing water (δ18OW), and water temperature.
Differences between the temperature conversion equations for Anadara broughtonii and Ruditapes philippinarum were 0.28 ± 0.26 ‰ at 19°C and 0.72 ± 0.39 ‰ at 22°C, with deviations from isotopic equilibrium conditions being particularly pronounced for R. philippinarum. In comparison between fish otoliths and bivalve shells, the difference between Engraulis japonicus and A.broughtonii was 0.11 ± 0.35 ‰ and that between E. japonicus and R. philippinarum was 0.61 ± 0.35 ‰. These results indicate that species-specificity exists even among taxa that have been believed to form carbonates in isotopic equilibrium, such as bivalves. Furthermore, our results suggest that the effects of vital effects may exist across taxonomic groups.
On the two bivalve species, we also examined the factors that cause the offset of shell oxygen isotope ratio from oxygen isotope values in equilibrium with seawater (δ18OC-δ18Oeq). The results were compared with the metabolic carbon contribution MC(%) from bivalve mollusks, and it was found that δ18OC-δ18Oeq tended to increase as MC(%) increased. Therefore, it is possible that the oxygen isotope ratios of bivalve shells reflect the metabolic rate, but further study is needed.
In this study, we aimed to evaluate the species-specificity of oxygen isotope fractionation of biogenic carbonates precisely and quantitatively, by minimizing uncertainty derived from differences in (micro) environments of carbonate precipitation and analytical methodological discrepancy. For this purpose, bivalve shells and fish otoliths are obtained from the rearing experiment conducted in the same tanks under controlled temperatures (22°C, 19°C, and 16°C). By rearing them in the same tank, we eliminated the influence of differences in calcification conditions. Furthermore, systematic analytical errors were eliminated by analyzing all samples using the same equipment and methods. The temperature conversion equations for each organism were then calculated from the obtained oxygen isotope ratio of carbonate (δ18OC), oxygen isotope ratio of rearing water (δ18OW), and water temperature.
Differences between the temperature conversion equations for Anadara broughtonii and Ruditapes philippinarum were 0.28 ± 0.26 ‰ at 19°C and 0.72 ± 0.39 ‰ at 22°C, with deviations from isotopic equilibrium conditions being particularly pronounced for R. philippinarum. In comparison between fish otoliths and bivalve shells, the difference between Engraulis japonicus and A.broughtonii was 0.11 ± 0.35 ‰ and that between E. japonicus and R. philippinarum was 0.61 ± 0.35 ‰. These results indicate that species-specificity exists even among taxa that have been believed to form carbonates in isotopic equilibrium, such as bivalves. Furthermore, our results suggest that the effects of vital effects may exist across taxonomic groups.
On the two bivalve species, we also examined the factors that cause the offset of shell oxygen isotope ratio from oxygen isotope values in equilibrium with seawater (δ18OC-δ18Oeq). The results were compared with the metabolic carbon contribution MC(%) from bivalve mollusks, and it was found that δ18OC-δ18Oeq tended to increase as MC(%) increased. Therefore, it is possible that the oxygen isotope ratios of bivalve shells reflect the metabolic rate, but further study is needed.