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

This study presents carbon (δ13C) and oxygen (δ18O) 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 δ13C and δ18O 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 δ13C and δ18O values of these samples, which can be ascribed to a kinetic fractionation effect.Although limited data on δ13C of total dissolved inorganic carbon (DIC) and δ18O of seawater at the brachiopod sample site enable only approximate estimates of the δ13C and δ18O of the equilibrium calcite, they are comparable with those of the brachiopod shell. The δ18O 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 δ18O range for the equilibrium calcite. Although samples from the inner series of L. rubellus are depleted (partially enriched) in 13C (18O) by < 0.8‰ (< 0.3‰) 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 δ13C and δ18O 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 δ13C or δ18O in the oceans throughout geologic time.