Carbon and oxygen isotopic compositions of modern brachiopod shells from a warm-temperate shelf environment, Sagami Bay, central Japan

This study presents carbon (delta C-13) and oxygen (delta O-18) isotopic compositions of shells of an articulated brachiopod (Laqueus rubellus) living in a warm-temperate shelf environment in Japan. High-resolution, three-dimensional samplings were conducted (1) to clarify variations in the isotopic compositions within a single shell, (2) to evaluate offset of the compositions from those of the calcite precipitated in isotopic equilibrium with ambient seawater (equilibrium calcite), and (3) to specify shell portions that reflect the isotopic compositions of ambient seawater. The delta C-13 and delta O-18 values of samples from the secondary shell layer, which constitutes the main body of the brachiopod shell, are in and/or out of those of the equilibrium calcite. The isotopic compositions of samples from the outermost part of the secondary shell layer (outer surface of the secondary shell layer) correlated well with growth rates. Positive correlations are recognized between the delta C-13 and delta O-18 values of these samples, which can be ascribed to a kinetic fractionation effect. Although limited data on delta C-13 of total dissolved inorganic carbon (DIC) and delta O-18 of seawater at the brachiopod sample site enable only approximate estimates of the delta C-13 and delta O-18 of the equilibrium calcite, they are comparable with those of the brachiopod shell. The delta O-18 values of samples from the outermost part of the secondary layer along a growth axis of Laqueus rubellus are a reliable environmental proxy, because they mostly fall in the delta O-18 range for the equilibrium calcite. Although samples from the inner series of L. rubellus are depleted (partially enriched) in C-13 (O-18) by <0.8 parts per thousand. (<0.3 parts per thousand) relative to the equilibrium calcite, both isotopic variations are quite small. Therefore, the isotopic compositions in this series would also be useful if the offsets are corrected. This study clearly illustrates that, although the delta C-13 and delta O-18 of modern brachiopod shells are influenced by the kinetic fractionation effect, appropriate selection of species and shell portions reflecting the isotopic compositions of ambient seawater enables the reconstruction of secular variations in delta C-13 or delta O-18 in the oceans throughout geologic time. (C) 2010 Elsevier B.V. All rights reserved.