[B-PT04] Biomineralization and Geochemistry of Proxies

[BPT04-06] Variations in carbon and oxygen isotope composition and trace element concentrations of the modern brachiopod shells collected off Okinoshima

*及川 一真¹、高柳 栄子¹、新城 竜一²、遠藤 一佳³、吉田 真明⁴、井龍 康文¹ (1.東北大学大学院理学研究科地学専攻、2.琉球大学理学部物質地球科学科、3.東京大学大学院理学系研究科 、4.島根大学生物資源科学部)

キーワード：腕足動物、炭素酸素同位体、微量金属元素濃度

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 (δ¹³C_EC and δ¹⁸O_EC) were calculated using oceanographic data (e.g., temperature, salinity) and isotopic composition of dissolved inorganic carbon (δ¹³C_DIC) and seawater (δ¹⁸O_SW) 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.