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

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 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, 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 δ13Corg, δ13Ccarb, and Suess-effect corrected δ13CDIC 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 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 diagenetic alteration of δ15N toward lower values. Future work using historical collections of Sepia and Spirula may provide additional calibration of fossil cephalopod geochemistry.

Cryogenian glaciation and the onset of carbon-isotope decoupling.

Global carbon cycle perturbations throughout Earth history are frequently linked to changing paleogeography, glaciation, ocean oxygenation, and biological innovation. A pronounced carbonate carbon-isotope excursion during the Ediacaran Period (635 to 542 million years ago), accompanied by invariant or decoupled organic carbon-isotope values, has been explained with a model that relies on a large oceanic reservoir of organic carbon. We present carbonate and organic matter carbon-isotope data that demonstrate no decoupling from approximately 820 to 760 million years ago and complete decoupling between the Sturtian and Marinoan glacial events of the Cryogenian Period (approximately 720 to 635 million years ago). Growth of the organic carbon pool may be related to iron-rich and sulfate-poor deep-ocean conditions facilitated by an increase in the Fe:S ratio of the riverine flux after Sturtian glacial removal of a long-lived continental regolith.