REFINING THE INTERPRETATION OF OXYGEN ISOTOPE VARIABILITY IN CEPHALOPODS

Serially sampled δ¹⁸O from fossil and modern cephalopods may provide new insight into the behavior and longevity of individuals. Interpretation of these data generally more difficult than similar data from bivalves or brachiopods because the measured δ¹⁸O signal combines both seasonal change and depth change over the life of an individual. Here I present a simple null model combining the three fundamental controls on a measured δ¹⁸O profile in a free-swimming organism: swimming behavior, seasonal water column change, and time averaging in sampling. Model results indicate that seasonal variability in δ¹⁸O can occasionally be interpreted in a free-swimming organism but is complicated by swimming velocity and is sometimes best expressed by changes in variance rather than simple sinusoidal patterns. In other locations, no variability defined by seasonality would be expected. Furthermore, large ranges of δ¹⁸O (~4‰) are possible within and between individuals in some settings, depending on the implosive depth limit. These results suggest that future interpretation of serially sampled should consider seasonal water column variation from either modern or modeling sources in addition to comparison to co-occurring benthic and planktic organisms. Additionally, this modeling casts doubt on the promise of isotope sclerochronology alone as a growth chronometer in ammonites and highlights the need for other methods of quantitatively determining age.