Intraspecific variations in carbon-isotope and oxygen-isotope compositions of a brachiopod Basiliola lucida collected off Okinawa-jima, southwestern Japan

This study presents intraspecific variations in carbon-isotope (delta C-13) and oxygen-isotope (delta O-18) compositions of nine specimens of a subtropical brachiopod, Basiliola lucida, collected west of Okinawa-jima, Ryukyu Islands, southwestern Japan. The delta C-13 values of samples collected along the maximum growth axis (ontogenetic samples) from two modern and seven older (pre-1945 cal AD) shells show no seasonal changes. The modern shells, which were collected from comparable depths, have similar delta C-13 values that fall within the range of calcite precipitated in isotopic equilibrium with ambient seawater (equilibrium calcite) (delta C-13(EC) values), and their mean d delta C-13 values are similar to 1.1-1.6 parts per thousand less than those from the older shells. This decrease in d delta C-13 values is similar in magnitude to the decreases in atmospheric CO2 and the oceanic dissolved inorganic carbon at the sea surface in recent years (C-13 Suess effect), suggesting that the effect can even be detected at water depths of 200-300 m in the subtropical northwestern Pacific Ocean. The delta O-18 values fluctuate within a narrow range (0.26-0.41 parts per thousand) with no seasonal changes, and they exhibit small (0.14-0.51 parts per thousand) offsets from those of equilibrium calcite (delta O-18(EC) values). A statistically significant negative linear relationship is established between seawater temperature and mean delta O-18 values of the nine shells, but the slope (-0.31 parts per thousand/degrees C) is steeper than those of equilibrium calcite (-0.23 parts per thousand/degrees C) and other calcareous organisms (-0.15 parts per thousand to -0.26 parts per thousand/degrees C). The cross-plots of the delta C-13 and delta O-18 values suggest that the degree of the vital effect varies among individuals in this species. The delta C-13 and delta O-18 values of B. lucida are potentially useful for reconstructing the delta C-13 and delta O-18 evolution of ancient oceans, because both values show small intraspecific variations, the former is identical to the delta C-13(EC) values, and the latter shows small within-shell variations and small, nearly constant offsets from the delta O-18(EC) values. (C) 2013 Elsevier Ltd. All rights reserved.