Brachiopods recording seawater temperature—A matter of class or maturation?

Calcite fibres of the innermost secondary layer of low magnesium-calcite brachiopod shells are in oxygen isotope equilibrium with ambient seawater. Previous work on Terebratalia transversa indicates that the first formed calcite fibres of the secondary layer are not in isotopic equilibrium while the later fibres of mature valves are in oxygen isotope equilibrium with seawater. Here, secondary ion mass spectrometry (SIMS) analyses reveal that this trend of oxygen isotope equilibrium, being attained towards the innermost shell rather than being a feature throughout the secondary layer, occurs in brachiopods from two Orders and two sub-orders. This suggests a general pattern among brachiopods with low magnesium-calcite shells and leads to the recommendation that only the innermost fibres of mature valves be included in proxy calculations of seawater temperature. Although the trend is common in the three species studied here, the extent of the isotopic range is much less marked in the species that lacks punctae.