Keywords:brachiopod, carbon and oxygen isotope, trace metal element concentrations
The carbon (δ13C) and oxygen (δ18O) isotope composition of Rhynchonelliform brachiopods (hereafter, called ‘brachiopods’) have been regarded as useful for paleoceanographic conditions throughout the Phanerozoic. However, recent studies have revealed that the isotopic composition and minor element concentrations in modern brachiopod shells are not only influenced by environmental changes in ambient seawater but also by biological effects such as the chemical composition of calcification fluid and the growth process (e.g., growth rates, metabolism). In order to improve our knowledge on the calcification mechanism of brachiopod shells and to make the isotopic composition and the chemical composition more reliable paleoenvironmental proxies, it is necessary to accumulate data on them from various modern brachiopod species. In this study, we measured δ13C and δ18O values and minor element concentrations of the primary (PL) and secondary (SL) shell layers of three (one male and two female) specimens of Pictothyris picta collected at a water depth of ~61 m off Oki Island. We performed a high-resolution micrometer-scale sampling along the maximum growth axis, corresponding to time resolution of 3 days to 8 months. Samples were also collected from the innermost shell surface. We examined whether there are differences in isotopic composition and minor element concentrations between male and female specimens. The δ13C and δ18O values of calcite precipitated in equilibrium with surrounding seawater (δ13CEC and δ18OEC) were calculated using oceanographic data (e.g., temperature, salinity) and isotopic composition of dissolved inorganic carbon (δ13CDIC) and seawater (δ18OSW) at the brachiopod growth site to compare with the shell δ13C and δ18O values.
The δ18O values of three P. picta of the SL are mostly close to oxygen isotope equilibrium with ambient seawater. The δ13C values of the PL and SL show clear seasonal variations. There are no differences in shell δ13C and δ18O values between male and female specimens. Whereas, intraspecific (inter-specimen) differences in δ13C and δ18O values are not negligible. There is no significant correlation between Mg/Ca and δ18O values. Whereas Sr/Ca and δ13C values are negatively correlated.