Carbon and oxygen isotope composition of Nautilus macromphalus: a record of thermocline waters off New Caledonia

This study investigates the controls on the stable isotope composition of shell nacreous layer of a modern Nautilus macromphalus from New Caledonia. Closely spaced samples were taken both along growth lines (isochrons) and growth axis (time series). Carbon isotope compositions (δ13C=−0.3‰ to 1‰ PDB) of the shell decrease in the domains of maximal growth (−0.3‰) and muscle insertion (−0.4‰). Maximal δ13C variations of about 1 ‰ are recorded in time series. A global ontogenetic decrease of δ13C values is observed for both dorsal and ventral areas (0.4‰ to 0.8‰) and can be ascribed to a metabolic effect with age or to changes in the food source. By contrast, oxygen isotope compositions are quite constant (1.1<δ18O<1.3) and reflect the mean temperature of shell secretion, corresponding to a depth of about 250 m. Comparison of fossil cephalopod shells with co-existing fish scales from the Lower Cretaceous of Russia suggests that most cephalopods precipitate their shells close to oxygen isotope equilibrium with seawater.