Isotope systematics of subfossil, historical, and modern Nautilus macromphalus from New Caledonia

by Benjamin J. Linzmeier, Andrew D. Jacobson, Bradley B. Sageman, Matthew T. Hurtgen, Meagan E. Ankney, Andrew L. Masterson, Neil H. Landman

Cephalopod carbonate geochemistry underpins studies ranging from Phanerozoic, global-scale change to outcrop-scale paleoecological reconstructions. Interpreting these data hinges on assumed similarity to model organisms, such as Nautilus, and generalization from other molluscan biomineralization processes. Aquarium rearing and capture of wild Nautilus suggest shell carbonate precipitates quickly (35 μm/day) in oxygen isotope equilibrium with seawater. Other components of Nautilus shell chemistry are less well-studied but have potential to serve as proxies for paleobiology and paleoceanography. To calibrate the geochemical response of cephalopod δ¹⁵N_org, δ¹³C_org, δ¹³C_carb, δ¹⁸O_carb, and δ^44/40Ca_carb to modern anthropogenic environmental change, we analyzed modern, historical, and subfossil Nautilus macromphalus from New Caledonia. Samples span initial human habitation, colonialization, and industrial pCO₂ increase. This sampling strategy is advantageous because it avoids the shock response that can affect geochemical change in aquarium experiments. Given the range of living depths and more complex ecology of Nautilus, however, some anthropogenic signals, such as ocean acidification, may not have propagated to their living depths. Our data suggest some environmental changes are more easily preserved than others given variability in cephalopod average living depth. Calculation of the percent respired carbon incorporated into the shell using δ¹³C_org, δ¹³C_carb, and Suess-effect corrected δ¹³C_DIC suggests an increase in the last 130 years that may have been caused by increasing carbon dioxide concentration or decreasing oxygen concentration at the depths these individuals inhabited. This pattern is consistent with increasing atmospheric CO₂ and/or eutrophication offshore of New Caledonia. We find that δ^44/40Ca remains stable across the last 130 years. The subfossil shell from a cenote may exhibit early δ^44/40Ca diagenesis. Questions remain about the proportion of dietary vs ambient seawater calcium incorporation into the Nautilus shell. Values of δ¹⁵N do not indicate trophic level change in the last 130 years, and the subfossil shell may show diagenetic alteration of δ¹⁵N toward lower values. Future work using historical collections of Sepia and Spirula may provide additional calibration of fossil cephalopod geochemistry.