Carbonate clumped isotope thermometry is based on the
thermodynamically dependent relative abundance of 13C-18O bonds
(quantified as Δ47) within the carbonate crystal lattice. The clumping of
13C-18O in carbonates is based on a self-reaction of isotope exchange
that occurs rapidly at near neutral pH. Similar Δ47-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 δ18O, δ13C and rapid calcite
precipitation have however not been previously investigated with regards
to their clumped isotope composition. We present clumped isotopic
composition (Δ47) of 25 inter-skeletal elements of 5 echinoid species
with varying growth temperatures. We found no statistically significant
inter-skeletal variation in Δ47 in all echinoid species measured, a
surprising find given the inter-skeletal variability reported for δ13C
and δ18O. Our echinoid Δ47-temperature calibration however shows a
statistically significant positive offset from Δ47-temperature
calibration for inorganic calcite of 0.015‰. The pattern of isotopic
fractionation in δ18O and Δ47 of echinoderms is not consistent with CO2
hydration or hydroxylation, diffusion or high-Mg composition of echinoid
calcite. Positive offsets in the Δ47 of echinoid calcite may however
relate to deviations in the pH of the calcifying fluid from the pH at
which equilibrium calcite is precipitated.