4.1 Isotope ordering in aragonitic mollusks
Clumped isotope values of our temperature-controlled A. islandicasamples consistently plot on a linear relationship with other
low-temperature aragonite datapoints (Fig. 1 and 2 ; seesection 4.2 ). The absence of a consistent offset betweenA. islandica datapoints and other aragonites (+0.003 ± 0.004‰;
95% CL, see Fig. 2 and S8 ) and agreement between the
linear ∆47-\(\frac{1}{T^{2}}\) dependence of the
aragonitic mollusk data in this study and the regression through the
complete low-temperature aragonite dataset (Fig. 1 andsection 3.1 ) strongly supports a common temperature dependence
for all aragonites in this study, biogenic or inorganic, and argues
against disequilibrium fractionation in aragonite precipitated
inorganically or by bivalves or foraminifera (see section 3.1 ;Fig. 1 and 2 ). Our highly temperature-controlled growth
experiments uniquely allow us to exclude variability in the growth
environment between specimens from the same growth treatment as a driver
of shell composition. Strong similarity of Δ47 values
between individual A. islandica specimens grown at the same
temperature thus rules out significant (95% CL) specimen-specific vital
effects on the clumped isotope composition aragonitic bivalve shells
(see section 3.1 ; Fig. 1 , Table 1 andS6 ). These findings corroborate measurements in calcitic
mollusks showing that clumped isotope values in mollusk carbonates
adhere to the same temperature relationship as other carbonates
precipitated in equilibrium (except for juvenile oyster shells; Huyghe
et al., 2022). Clumped isotope analyses in (fossil) mollusk shells thus
provide an independent temperature proxy, allowing paleoclimatologists
to disentangle the effects of variability in temperature and the
hydrological cycle (as measured in
δ18Ow) throughout geological history
down to the seasonal timescale (e.g. Caldarescu et al., 2021; de Winter
et al., 2021; Letulle et al., 2022).