Oxygen isotope composition, magnesium distribution and crystallography of Terebratulina retusa

Citation

Cusack M, England J, Parkinson D, Dalbeck P, Lee MR, Curry GB & Fallick AE (2008) Oxygen isotope composition, magnesium distribution and crystallography of Terebratulina retusa. In: Harper D, Long S & Nielsen C (eds.) Brachiopoda: Fossil and Recent. Fossils and strata monograph series, 54. Chichester: Wiley-Blackwell, pp. 259-267. http://eu.wiley.com/WileyCDA/WileyTitle/productCd-140518664X.html

Abstract
Calcite-shelled brachiopods have been used extensively as palaeothermometers via the δ18O seawater temperature proxy. The secondary shell layer is precipitated in isotopic equilibrium with seawater and is an accurate recorder of temperature. Recent work has demonstrated that, in Terebratalia transversa, the innermost part of the secondary layer is closest to oxygen isotope equilibrium with seawater. The Mg/Ca ratio of several marine carbonate biominerals has been exploited in palaeothermometry, although this proxy has not been applied to brachiopods. In this study, the oxygen isotope composition of Terebratulina retusa has been compared with the concentration of magnesium and its distribution through the shell and these data have been interpreted in the context of shell microstructure characterized by electron backscatter diffraction. The oxygen isotope composition of the primary and secondary layers of T. retusa is within the predicted equilibrium values for seawater although the secondary layer is closer to the recorded mean temperature. Magnesium concentrations are higher in the primary than in the secondary layer. Within the secondary layer, magnesium concentrations are lowest in the innermost, most recently secreted part. This may result from kinetics with slower crystal growth succeeding in the exclusion of elements that may otherwise substitute for calcium. No information is yet available concerning the isotopic composition of the outer and inner secondary layer of T. retusa. A change in crystallography occurs within the secondary layer of T. retusa such that the fibres of the outer secondary layer have orientations closer to their neighbours than those fibres in the innermost secondary layer, suggesting a relationship between oxygen isotope composition, magnesium concentrations and crystallography. This relationship should be understood fully to ensure accurate application of both proxies. © 2008 The Lethaia Foundation.

Journal
Fossils and Strata, Issue 54