The use of stable isotope determinations from brachiopod shells in environmental reconstruction
This study investigates d13C and d18O variations in the shells of modern brachiopods that represent all extant groups of calcite-precipitating brachiopods, and were collected live from 8 locations. Protocols and methods of sample preparation are presented that can produce good estimations of annual mean temperatures of ambient seawaters from some brachiopod groups. SEM examinations determined the ultrastructural characteristics of each species prior to isotope analyses. d13C and d18O analyses of shell carbonate were carried out with samples representing different morphological features and ultrastructural shell layers of both ventral and dorsal valves. Generally, d18O 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. d18O 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 d18O values of the innermost layer strongly correlated with d13C 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 d13C and d18C 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.