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

Abstract

This study presents carbon ( δ 13C) and oxygen ( δ 18O) 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 δ 13C and δ 18O 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 δ 13C and δ 18O values of these samples, which can be ascribed to a kinetic fractionation effect. Although limited data on δ 13C of total dissolved inorganic carbon (DIC) and δ 18O of seawater at the brachiopod sample site enable only approximate estimates of the δ 13C and δ 18O of the equilibrium calcite, they are comparable with those of the brachiopod shell. The δ 18O 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 δ 18O 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 δ 13C and δ 18O 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 δ 13C or δ 18O in the oceans throughout geologic time.