Inconsistent oxygen isotopic values between contemporary secreted septa and outer shell walls in modern Nautilus

Abstract

The isotopic analyses of modern Nautilus pompilius from the Philippines demonstrated that the δ18O values are not always consistent between contemporary secreted septa and outer shell walls. This discrepancy is hypothesized as being related to the osmotic emptying through the siphuncle. The local osmosis in water at depths >240 m, which enhances salt concentration in siphuncular intercellular spaces, possibly makes the more positive salinity of extrapallial fluid in the interspace between the rear of soft parts and the newly forming septum, from which the calcium carbonate of the septum is precipitated. Therefore, the δ18O values of septa become more positive than those in the outer shell walls. Actually, the δ18O values of outer shell walls fit more reasonably the actual seawater temperature at the study area, than those of septa. Furthermore, the δ18O values are not constant even in a single septum with a discrepancy equivalent to ~1–4 °C of the estimated water temperature. These facts suggest that the δ18O values from the outer shell walls are more appropriate to be analysed for estimating their habitats in modern nautili. This scenario is supposed to be more significant in the shells that lived in water at greater depths, so that the discrepancy of δ18O values between contemporary secreted septa and outer shell wall might be useful as a criterion to recognize the deeper habitat depth in both modern and fossil cephalopods.