Shell structure, patterns and trends of oxygen and carbon stable isotopes in modern brachiopod shells

Abstract

This study investigates δ13C and δ18O variations in the shells of modern brachiopods representing all extant groups of calcite-precipitating brachiopods, collected live from 8 locations. SEM examinations determined the ultrastructural characteristics of each species prior to isotope analyses. δ13C and δ18O analyses of shell carbonate were carried out with samples representing disparate morphological features and ultrastructural shell layers of both ventral and dorsal valves. Generally, δ18O values from the fibrous secondary or prismatic tertiary shell layers of the articulated Terebratulida and Rhynchonellida species were in oxygen isotopic equilibrium with ambient seawater. Isotopic temperatures extrapolated from these values are close to measured annual mean seawater temperatures. δ18O values were relatively unaffected by shell specialisation. The only exception was Antarctic species Liothyrella uva, which did not have a complete tertiary shell layer typical of this genus and had δ18O values of the innermost layer strongly correlated with δ13C and mostly not in oxygen isotopic equilibrium with ambient seawater. With the exception of the rhynchonellid Notosaria nigricans, the outer primary layer material was depleted in δ13C and δ18O and highly variable. Inclusion of this material even as part of a whole shell sample could lead to misinterpretation of seawater temperature, therefore only fossil secondary layer material should be used. The anomalous articulated thecideidine brachiopod Thecidellina barretti is composed of mainly primary shell material and was not in oxygen isotope equilibrium. δ18O values from the laminar secondary layer material of the inarticulated Craniida are highly variable. Therefore, these species are not recommended for use as palaeoenvironmental proxies. This study suggests caution when employing fossil brachiopod shells with similar ultrastructures to modern craniid and thecideidine brachiopods. The carbon isotope composition is highly variable in all of the brachiopods studied. Analysis of samples from specialised regions of the secondary shell layer show a pattern of depletion in 13C relative to non-specialised secondary material. The carbon isotope variability is independent of δ18O and is repeated in most of the articulated species regardless of geographical location. This is possibly a vital effect produced by metabolic prioritisation. © 2005 Elsevier B.V. All rights reserved.