δ18O and δ13C values of modern brachiopod shells

Abstract

Researchers have not rigorously tested the hypothesis that calcite from modern brachiopod shells is precipitated in oxygen isotope equilibrium with ambient seawater. Isotopic variability at the intraspecimen and intertaxon levels has not been examined. Without such data for modern brachiopods, similar data from ancient brachiopods cannot be accurately interpreted. In this study, a survey is made of δ18O and δ13C values of Terebratulid, Rhynchonellid, Thecideidine, and Craniacean brachiopods from Antarctica, the Bay of Fundy, Curacao, Japan, New Zealand, Norway, Puget Sound, Palau, Sicily, and South Africa. This suite of samples provides a wide range of taxonomic levels, temperatures, salinities, and depositional environments for evaluating the degree of isotopic equilibrium attained during precipitation of brachiopod calcite. New data indicate that modem brachiopod calcite is not always precipitated in oxygen and carbon isotope equilibrium with ambient seawater.Calcite from the primary layer and specialized shell structures (hinge, brachidium, foramen, interarea, muscle scars) are depleted in both 18O and 13C, a characteristic of biological fractionation or “vital” effects often found in other calcerous, marine organisms. Our findings suggest that these portions of the brachiopod shell should be avoided during sampling of ancient brachiopods. Secondary layer calcite, the material most often analyzed in ancient brachiopods, has higher δ18O and δ13C values which approach and sometimes correspond with predicted equilibrium values. Therefore, secondary layer calcite is the most suitable portion of the brachiopod shell for use as an ancient seawater proxy.Although near equilibrium precipitation in secondary layer calcite is encouraging to those studying the isotopic composition of ancient oceans, these data come with caveats. Large intraspecimen variability in the δ18O values of secondary layer calcite (±1‰ in some samples) limits the use of brachiopods as precise indicators of the δ18O value or temperature of ancient seawater. The δ18O and δ13C values of secondary layer calcite deviate from predicted equilibrium values toward the lower values of the primary layer and specialized shell structures, indicating a measurable “vital” effect. In most cases, measured δ18O values are lower than the calculated equilibrium values dictated by seasonal variations in the δ18O value and temperature of seawater. Ontogenetic variations in the δ13C values of secondary layer calcite have also been measured and must be assessed when interpreting ancient data.Modern brachiopods occur in a wide range of seawater salinities and temperatures. Therefore, the relation between salinity and the δ18O value of seawater must be known to correctly calculate the temperature of calcite precipitation from secondary layer δ18O values. Errors of up to 15°C are found if ambient seawater is assumed to have a constant δ18O value (e.g., 0‰ SMOW) and the δ18O values of secondary layer calcite are used to calculate temperature. Coupled with “vital” effects and intraspecimen variability, these errors can obscure the resolution of secular variations in the rock record.