RESOLVING DEPTH MIGRATION IN NAUTILUS MACROMPHALUS BY ION MICROPROBE ANALYSIS OF δ18O

Nautilus is one of two surviving genera of externally shelled cephalopods and provides an analogue for biology of extinct cephalopods. Remote tracking of wild nautilids has shown daily depth migration ranging up to 200 m, a change of 10 ºC (Ward et al., 1984). Previous studies using δ¹⁸O as a proxy for ambient seawater temperature have reported significant depth changes through ontogeny, but were unable to resolve daily depth changes due to sample size (Zakharov et al., 2006). We have imaged light and dark bands by Scanning Electron Microscopy (SEM) that average ~100 mm thick and interpret them as growth records for Nautilus macromphalus. Recent advances in Secondary Ion Mass Spectrometry (SIMS) allow measurements of δ¹⁸O with 10μm beam spot sizes and a spot-to-spot precision of 0.3 ‰ (2 SD), thus sub-daily δ¹⁸O records in nautilid shell can be precisely measured.

The final four centimeters, corresponding to ~13 months of growth, from a single Nautilus macromphalus shell was prepared for SIMS analysis. The specimen was live collected in July 2002, 20 km south of Nouméa, New Caledonia. Intrashell growth banding was imaged by combining SEM techniques of Cathodoluminescence, Secondary Electron (SE) and Backscattered Electron imaging. Preliminary results show δ¹⁸O values from 0.13 to -3.13 ‰ [PDB]. Transects perpendicular to growth banding (~100 μm/day from SE images) with ~20 μm spacing of SIMS analyses resolve daily depth migration with a resolution of one δ¹⁸O value averaging 2.4 hours. Within a 24h period, δ¹⁸O values vary by up to ~2‰ (average ~1‰), corresponding to a temperature change of 10 ºC and a depth migration of ~200m, in agreement with maximum day-night depth changes of Ward et al., 1984. With the advent of high spatial resolution analysis with high accuracy and precision, the daily behavior of fossil cephalopods and the thermal structure of the surrounding ocean can be reliably studied.