High-resolution carbon and oxygen isotope record in modern brachiopod Pictothyris picta collected off Okinoshima, Japan

Carbon (δ¹³C) and oxygen (δ¹⁸O) isotope composition of Rhynchonelliformea brachiopods (hereafter, called ‘brachiopods’) have been regarded as useful paleoenvironmental indicators throughout the Phanerozoic. However, recent studies have revealed that the isotopic composition in modern brachiopod shells records not only environmental changes in ambient seawater but also is influenced by biological controls such as the chemical/isotopic composition of calcifying fluids and physiological processes (e.g., growth rates, metabolism). The latter is known as biological isotope fractionation effects, such as kinetic, metabolic, and pH effects. Recently, a new calcification mechanism in brachiopod shell formation, ion transport mechanism, was proposed. In this study, we measured δ¹³C and δ¹⁸O values of the primary (PL) and secondary (SL) shell layers of three Pictothyris picta (one male and two female specimens) collected at a water depth of~61 m off Okinoshima to improve our understanding of biological isotope fractionation effects during their shell secretion. We obtained ontogenetic-series δ¹³C and δ¹⁸O profiles from the PL (PL-Ont) and the uppermost SL (SL-Ont) at the sampling resolution of 3 days to 8 months per sample. We obtained inner-series δ¹³C and δ¹⁸O profiles from the innermost SL (SL-In) as well. The variations in the δ¹³C and δ¹⁸O profiles of the PL-Ont showed similar trends to those of the SL-Ont. However, the PL-Ont values mostly exhibited relatively lower δ¹⁸O values than those of the SL-Ont. Cross plots between the δ¹³C and δ¹⁸O values of the PL-Ont indicated a strong positive correlation and were lower than those of calcite precipitated in isotopic equilibrium with ambient seawater at the fast growth stage, suggesting the significant influence of the kinetic isotope fractionation effect. The SL was precipitated in oxygen isotopic equilibrium with ambient seawater regardless of the growth stage and/or the seasonal changes in living environments. Furthermore, the PL-Ont, SL-Ont, and SL-Inshowed similar δ¹⁸O values during the cold season, indicating negligible influences of the kinetic, pH, and magnesium effects on δ¹⁸O composition. The δ¹³C values of the PL-Ont formed at the cold season (= micro-portion formed under the least kinetic isotope fractionation effect) were lower than those of the SL, indicating the stronger metabolic effect on the PL secretion. Our isotopic data showed that the time lag of the PL and the SL formation varies among specimens.