Quantitative analysis of intraspecific variations in the carbon and oxygen isotope compositions of the modern cool-temperate brachiopod Terebratulina crossei

This study unravels intraspecific variations in the carbon isotope (δ¹³C) and oxygen isotope (δ¹⁸O) compositions of shells of the modern cool-temperate brachiopod Terebratulina crossei collected at a water depth of 70m in Otsuchi Bay, northeastern Honshu, Japan. Brachiopod shells have been used as proxies of the δ¹³C values of dissolved inorganic carbon (DIC) (δ¹³C_DIC) and seawater temperature/δ¹⁸O (δ¹⁸O_SW) values to reconstruct the evolution of Phanerozoic oceans. To identify more reliable shell portions as the proxies, we conducted a rigorous time-series comparison of δ¹³C and δ¹⁸O values between the brachiopod shells and calcite precipitated in isotopic equilibrium with ambient seawater (equilibrium calcite) (δ¹³C_EC and δ¹⁸O_EC values, respectively). Samples were collected from the outer and inner surfaces of the secondary shell layer along the maximum growth axis (ontogenetic-series and inner-series samples, respectively). The ontogenetic-series δ¹³C values, which showed regular annual and irregular non-annual cycles, partly fell in but were mostly less than the range of the δ¹³C_EC values. The δ¹³C cycles were often associated with one or two minor negative peaks. The peaks were likely resulted from an increased incorporation of respiration-derived ¹²C due to elevated metabolic activity during spawning. The ontogenetic-series δ¹⁸O values showed distinct seasonal variations and were mostly within the range of δ¹⁸O_EC values. The amplitude of the δ¹⁸O profiles was relatively large during the younger fast-growth stage, and decreased during the senescent slow-growth stage. The inner-series δ¹³C and δ¹⁸O values of individual shells varied within narrow ranges. The inner-series δ¹³C values were close to the minimum δ¹³C_EC values. The inner-series δ¹⁸O values were in the upper range of the δ¹⁸O_EC values. Kinetic isotope fractionation effects were evident, but its degree varied among different shells. We identified the shell portions reliably recording past ocean environments. The best estimates of annual average δ¹³C_DIC values were obtained from ontogenetic-series δ¹³C values near the anterior shell edge and from inner-series δ¹³C values; those of the annual average seawater temperature were obtained from ontogenetic-series δ¹⁸O values from the entire transect and from the fast-growth stage.