[B-PT01] Biomineralization and Geochemistry of Proxies

[BPT01-08] High-resolution minor element concentration records in modern brachiopod Pictothyris picta shells and their controlling factors

*Kazuma Oikawa¹, Hideko Takayanagi¹, Ryuichi Shinjo², Kazuyoshi Endo³, Masa-aki Yoshida⁴, Yasufumi Iryu¹ (1. Institute of Geology and Paleontology Graduate School of Science Tohoku University, 2.Department of Physics and Earth Sciences, University of the Ryukyus, 3.Department of Earth and Planetary Science, Tokyo University, 4.Shimane University Faculty of Life and Environmental Science)

Keywords:brachiopod, carbon and oxygen isotope, minor element concentrations, calcification, vital effect

Carbon (δ¹³C) and oxygen (δ¹⁸O) isotope composition of Rhynchonelliformea brachiopods (hereafter, called ‘brachiopods’) have been regarded as useful paleoenvironmental indicators throughout the Phanerozoic. However, variations of minor element concentrations in modern brachiopod shells and their relationships with environmental factors (e.g., temperature, salinity, and pH) and biological controls are not well understood. In this study, we measured δ¹³C and δ¹⁸O values and minor element concentrations of the primary (PL) and secondary (SL) layers of three Pictothyris picta shells (one male and two female individuals) collected at a water depth of 61 m off Okinoshima to clarify the factors controlling the isotopic and elemental variations. We obtained ontogenetic-series δ¹³C, δ¹⁸O, and minor element (i.e., Mg/Ca, Mn/Ca, Fe/Ca, and Sr/Ca) 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 also obtained minor element concentrations as well as δ¹³C and δ¹⁸O from the innermost SL (SL-In). The SL-Ont was precipitated in oxygen isotopic equilibrium with ambient seawater regardless of the growth stage and the seasonal changes at the growth site. Mg/Ca profiles of the PL- and SL-Ont showed that the Mg uptake into the P. picta shells is related to the shell growth stages. This relationship likely indicates that Mg removal mechanism works during brachiopod shell precipitation, and the extent of the Mg removal varies with the growth stages. Consequently, we identified three growth stages (sectors 1 to 3) based on differences in Mg/Ca values between PL- and SL-Ont. A significant positive correlation between Sr/Ca and δ¹³C values was found in the sectors 1 and 2 of PL- and SL-Ont profiles of the three shells. This correlation implies that the shell precipitation rate is likely to be influenced by the metabolic effect affecting the δ¹³C values. The shell Mn and Fe concentrations were low and constant throughout the shells, respectively, except for Mn/Ca values from sectors 2 and 3 of the PL-Ont.