CALIBRATING CEPHALOPOD ISOTOPE RESPONSES USING NAUTILUS MACROMPHALUS FROM NEW CALEDONIA (Invited Presentation)

Cephalopod carbonate geochemistry underpins studies ranging from Phanerozoic-scale global change to outcrop-scale paleoecological reconstruction. Interpretation of these data hinges on assumed similarities to model organisms, such as Nautilus, and generalizations from other examples of molluscan biomineralization. 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 are potentially 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, colonization, and industrial pCO₂ increase. This sampling strategy is advantageous because it avoids the shock response that can dominate geochemical change in aquarium experiments. Anthropogenic CO₂ emission—although geologically rapid—is slow compared to individual lifespans, and thus may not cause shock.

Our data suggest some environmental changes are more easily preserved than others given variability in cephalopod average living depth. The percent respired carbon incorporated into the shell increases over the last 130 years based on calculations using δ¹³C_org, δ¹³C_carb, and Suess-effect corrected δ¹³C_DIC. Anthropogenic environmental pressures that raise metabolic rate could cause respired carbon to increase in the shell. 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 digenetic alteration of δ¹⁵N toward lower values. Future work using historical collections of Sepia and Spirula may provide additional calibration of fossil cephalopod geochemistry.