New isotopic analyses of exceptionally well-preserved Early Cretaceous ammonoids from Madagascar reveal that δ¹⁸O and δ¹³C values in septa and simultaneously formed outer shells are not consistent. Septal δ¹⁸O values are consistently more negative (−4.58 to −1.57‰) than those of the outer shells (−1.71 to 0.28‰), with δ¹³C showing a similar trend (−13.59 to −6.86‰ for septa and −1.77 to 0.93‰ for outer shells). These differences cannot be fully accounted for by variations in salinity or temperature. Instead, we propose that kinetic isotope effects, particularly those occurring during rapid septal formation in juveniles, significantly influence these values. This suggests that δ¹⁸O values in the examined ammonoid septa do not reliably represent ambient seawater temperatures. Septal isotope data should be interpreted more cautiously for palaeotemperature reconstructions, as they likely reflect a combination of kinetic, thermal, and salinity influences, at least in some ammonoid species.
Nakagawa et al. (Thu,) studied this question.