ABSTRACT

We present carbon- and oxygen-isotope compositions in a shell (δ¹³C_S and δ¹⁸O_S, respectively) of a modern brachiopod Campagea japonica collected at a water depth of 606 m off Aguni-jima, Central Ryukyu Islands, southwestern Japan. The δ¹³C_S and δ¹⁸O_S profiles of the outermost portions of the secondary shell layer along the axis of shell growth do not display significant seasonal variations. The δ¹³C_S values display a gradual decrease with growth, whereas the δ¹⁸O_S values are nearly constant. The δ¹³C_S and δ¹⁸O_S values along a particular growth line and from the inner shell surface are rather constant. The δ¹³C_S values near the posterior shell edge of the ontogenetic series and the δ¹⁸O_S values near the anterior shell edge of the inner series fall within the ranges of δ¹³C and δ¹⁸O values of calcite in equilibrium with ambient seawater (equilibrium calcite) (δ¹³C_EC and δ¹⁸O_EC, respectively). Therefore, they are the best portions to use as proxies of δ¹³C values of dissolved inorganic carbon (δ¹³O_DIC) and temperature and/or δ¹⁸O values of seawater (δ¹⁸O_SW), respectively. The δ¹³C_S values of the ontogenetic series near the anterior shell edge and the entire isochronous and inner series display the smallest decrease (less than ~0.2‰) compared with the δ¹³C_EC values. The δ¹⁸O_S values of the entire ontogenetic series and an interior part of the inner series are slightly greater (less than ~0.3‰) than the δ¹⁸O_EC values. Thus, these portions can be used as paleoenvironmental proxies if appropriate corrections are made. A significant positive correlation is recognized between the δ¹³C_S and δ¹⁸O_S values for the ontogenetic-series samples, which can be ascribed to a kinetic isotope effect. A metabolic isotope effect is not evident for the δ¹⁸O_S values in these samples. Our study provides useful information for the appropriate selection of brachiopod taxa and shell portions that reflect isotopic composition of ambient seawater for paleoenvironmental studies.