GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 171-10
Presentation Time: 10:30 AM


LINZMEIER, Benjamin1, JACOBSON, Andrew D.1, SAGEMAN, Bradley B.1, HURTGEN, Matthew T.1, ANKNEY, Meagan E.1, MASTERSON, Andrew L.1 and LANDMAN, Neil H.2, (1)Earth and Planetary Sciences, Northwestern University, 2145 Sheridan Rd, Evanston, IL 60208, (2)Division of Paleontology (Invertebrates), American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024-5192

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 δ15Norg, δ13Corg, δ13Ccarb, δ18Ocarb, and δ44/40Cacarb 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 δ13Corg, δ13Ccarb, and Suess-effect corrected δ13CDIC. Anthropogenic environmental pressures that raise metabolic rate could cause respired carbon to increase in the shell. We find that δ44/40Ca remains stable across the last 130 years. The subfossil shell from a cenote may exhibit early δ44/40Ca 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.