Evidence of taxonomic non-equilibrium effects in the clumped isotope composition of modern cephalopod carbonate

The stable oxygen isotope composition (818O) of ammonites and belemnites is a common proxy for Jurassic and Cretaceous sea temperatures. The challenges and uncertainties associated with cephalopod 818O and other proxy based paleotemperature reconstructions make cephalopods such as ammonites and belemnites a favourable target for carbonate clumped isotope paleothermometry. Our measurements of the clumped isotopic composition of modern cephalopods (Nautilus pompilius, Nautilus macromphalus, Nautilus belauensis and Sepia officianalis), however, confirm significant vital effects in clumped isotopic of cephalopod aragonite. We present the first intra-shell measurements of clumped isotopic composition of nautilus demonstrating that clumped isotope composition (447) is correlated with 818O shell carbonate. We observe a decrease in 447 from juvenile to adult septa, which does not correspond with changes in surface seawater temperature or temperature variation on vertical migration of the nautilus within the water column. The clumped isotope composition of juvenile septa yield temperatures that are within error of growth temperature while most recently formed septa reflect growth temperatures 3-8 degrees C above the upper temperature limit of survival of the organism (27 degrees C). Nonequilibrium effects observed in the 447 of modern nautilus comprise up to a 29 degrees C overestimate in shell formation temperature. Combined 818O and 447 measurements are used to differentiate between potential mechanisms that may generate non-equilibrium signatures observed. Non-equilibrium clumped isotope signatures may result from variations in the salinity of cameral fluid within nautilus chambers, dehydration of HCO3- and/or isotopic fractionation during CO2 diffusion across cell walls. The inter-skeletally variable non-equilibrium offsets in 447 of nautili aragonite observed in this study suggest that care must be taken when using the clumped isotope composition of cephalopod carbonate, in particular ammonite aragonite, as a paleotemperature proxy. Belemnite calcite and ontogenetically early septa of aragonitic ammonites may be feasible targets for paleotemperature reconstruction by carbonate clumped isotope thermometry.

Automated summary

The stable oxygen isotope composition (818O) of ammonites and belemnites is commonly used as proxy for Jurassic and Cretaceous sea temperatures, while carbon clumped isotope thermometry has been proposed as an alternative method due to challenges with stable isotopes. Nonetheless, the clumped isotopic composition of cephalopods shows significant vital effects and non-equilibrium signatures, which may result in temperature overestimations. Further research is needed to understand the mechanisms behind these non-equilibrium signals and to differentiate between potential factors influencing the clumped isotope composition of cephalopod carbonate.