Assessing kinetic fractionation in brachiopod calcite using clumped isotopes

David Bajnai,Sara Milner Garcia,Jens Fiebig,Adam Tomašových,Uwe Brand,Niklas Löffler,Claire Rollion-Bard,Jacek Raddatz,Cristina Primo-Ramos

doi:10.1038/s41598-017-17353-7

Abstract

Brachiopod shells are the most widely used geological archive for the reconstruction of the temperature and the oxygen isotope composition of Phanerozoic seawater. However, it is not conclusive whether brachiopods precipitate their shells in thermodynamic equilibrium. In this study, we investigated the potential impact of kinetic controls on the isotope composition of modern brachiopods by measuring the oxygen and clumped isotope compositions of their shells. Our results show that clumped and oxygen isotope compositions depart from thermodynamic equilibrium due to growth rate-induced kinetic effects. These departures are in line with incomplete hydration and hydroxylation of dissolved CO2. These findings imply that the determination of taxon-specific growth rates alongside clumped and bulk oxygen isotope analyses is essential to ensure accurate estimates of past ocean temperatures and seawater oxygen isotope compositions from brachiopods.

Highlights

Biomineralising marine organisms serve as important geochemical archives of past climate conditions
The δ18OVPDB values of the modern brachiopod shells analysed in this study range between −2.20(±0.02)‰ and 3.92(±0.02)‰, while the δ13CVPDB values range between −0.88(±0.02)‰ and 2.44(±0.01)‰
If the offset seen in the modern brachiopod δ18O values would arise solely from the varying Mg-content of the analysed shells, one would expect that this offset would disappear if the equilibrium values were calculated using the equation of Brand et al.[20] instead of Kim and O’Neil[35], since the former includes a correction for the Mg-effect

Summary

Introduction

Biomineralising marine organisms serve as important geochemical archives of past climate conditions. Brachiopods constitute one group of calcifying invertebrates that have great potential for palaeoenvironmental reconstructions due to their common occurrences in Phanerozoic sediments since the Cambrian[1] Their high abundance in Palaeozoic sediments makes them valuable for deep-time seawater temperature reconstructions based on shell oxygen isotope compositions[2]. It has been postulated that brachiopods incorporate oxygen isotopes into shell calcite (secondary and tertiary layers) in equilibrium with ambient seawater, certain parts of the shell (i.e., primary layer, uppermost part of the secondary layer, umbo and muscle scar areas) yield depleted δ18O values[17,18,19,20,21] In these shell areas, the observed 18O-depletion has been linked to growth-rate-driven kinetic isotope fractionation[18,22,23,24,25,26]

Methods

Results

Conclusion