THE NAUTILUS DEATH CENOTE

Exploration of a cenote on Lifou (Loyalty Islands, South Pacific) revealed more than 35 empty shells of the cephalopod Nautilus macromphalus in saltwater on the cenote floor, ~35-40 meters below the piezometric surface. This is the first known occurrence of modern Nautilus shells in a karstic system. The shells are scattered and oriented randomly on the sloping scree of the cenote floor. Most are mature individuals and are unbroken with faded brown stripes. Some have cemented carbonate mud partly filling the umbilical opening and body chambers. Seven shells were collected for analysis. These shells have a chalky outer surface but no mineral precipitates. No other organisms, living or dead, were observed on the Nautilus shell surfaces, attached to the limestone rubble, or anywhere in the cenote. Radiocarbon dating of the shells indicated ages of 6380 ± 30 to 7095 ± 30 y BP, making these the oldest Nautilus shells known since the Pleistocene. The ²³⁸U series radionuclides ²¹⁰Pb (half-life = 22.3 y) and ²²⁶Ra (half-life = 1600 y) were also measured and generally showed radioactive equilibrium between these nuclides, consistent with their old radiocarbon ages. At least one specimen showed excess ²¹⁰Pb, however, suggesting an age of <100 y. Correcting the ²²⁶Ra activities for decay yielded ²²⁶Ra activities much greater than those found in living Nautilus. Exposure to high activities of ²²²Rn and ²²⁶Ra in the salty groundwater of the cenote likely altered the activities originally incorporated into the shells. The taphonomic pathway of this Nautilus death assemblage is only partly understood at this time. Human placement of the shells in the cave is rejected based on the radiocarbon age and geometry of the cenote. The restricted radiocarbon ages of the shells suggest that a connection to the adjacent marine waters existed for ~700 years and Nautilus occasionally entered from the seaward site through a flooded karstic system. Unable to find the exit, they were trapped and died. After ~6400 y BP, the connection with the adjacent ocean was lost. This unique occurrence provides a minimum age for the appearance of Nautilus in the Loyalty Islands and provides insight into fossil cephalopod occurrences in karstic environments.