The clumped (13C-18O) isotope composition of echinoid calcite: Further evidence for vital effects in the clumped isotope proxy

Carbonate clumped isotope thermometry is based on the thermodynamically dependent relative abundance of C-13 - O-18 bonds (quantified as Delta(47)) within the carbonate crystal lattice. The clumping of C-13 - O-18 in carbonates is based on a self-reaction of isotope exchange that occurs rapidly at near neutral pH. Similar Delta(47)-temperature relationships between biogenic and inorganically precipitated carbonate in initial studies led to the promise of a proxy free of biologically driven disequilibrium effects, commonly referred to as vital effects. This has been largely the case for most organisms investigated. Biologically mediated disequilibrium precipitation has however been reported in corals and cephalopods and brachiopod molluscs. Echinoderms, despite their complex inter-cellular bio-mineralization strategy, large inter-skeletal fractionation of delta O-18, delta C-13 and rapid calcite precipitation have however not been previously investigated with regards to their clumped isotope composition. We present clumped isotopic composition ( Delta(47)) of 25 inter-skeletal elements of 5 echinoid species with varying growth temperatures. We found no statistically significant inter-skeletal variation in Delta(47) in all echinoid species measured, a surprising find given the important inter-skeletal variability reported for delta C-13 and delta O-18. Our echinoid Delta(47)-temperature calibration however shows a statistically significant positive offset from Delta(47) temperature calibration for inorganic calcite of 0.014 parts per thousand. The pattern of isotopic fractionation in delta O-18 and Delta(47) of echinoderms is not consistent with CO2 hydration or hydroxylation, diffusion or the high-Mg calcite composition of echinoid calcite. Positive offsets in the Delta(47) of echinoid calcite may however relate to deviations in the pH of the calcifying fluid from the pH at which equilibrium calcite is precipitated. (C) 2018 The Author(s). Published by Elsevier Ltd.