New insights from isotopic ecology

By S. David Webb

Field excavations in the Aucilla River provide the rich records of Florida prehistory upon which the ARPP program is predicated. Recovery of such records, however, is only the first of three stages through which our adventurous enterprise passes. The second stage involves a variety of processes in the museum; these include conservation of the samples, computerization of field records, cataloguing of specimens, and other curatorial procedures. The third stage includes all the analysis and interpretations that ultimately enrich our understanding of our prehistoric heritage. These are now beginning to unfold, and will be most evident when the ARPP produces its first full-fledged book, now contracted with Plenum Press, but still about two years in the future.

One of the advanced studies that is providing new insights into the prehistory of the Aucilla River region is known as isotopic ecology. Dr. Paul L. Koch and Ms. Kathryn A. Hoppe, an advanced graduate student, both in the Department of Earth Sciences at the University of California in Santa Cruz, have analyzed carbon and oxygen isotopes in tooth enamel of five species in late Pleistocene levels at the Page/Ladson Site. The oxygen isotope ratios (between atomic weight 16 O and 18 O), microsampled in one mammoth tooth, reflect temperature fluctuations, and the lower temperatures are correlated with slow (winter) growth enamel bands. The stable isotopes of carbon (13 C and 12 C, not 14 C which is unstable, i.e. radioactive) generally indicate the percentage of grass vs. leaves, or, in other words, whether an animal was a grazer, mixed-feeder, or browser. Aucilla mammoth diets, according to these results, consisted of about 90% tropical grasses. Mastodons, on the other hand, were almost purely browsers. In this respect the carbon isotope data confirm the direct evidence, previously reported, from presumed mastodon digesta.

A surprising result was that horses teeth had a mixed diet, about half browse and half tropical grasses. Although this seems surprising, the Santa Cruz team obtained similar results from horse teeth from three other late Pleistocene sites in the Florida peninsula. Tapirs and deer were even purer browsers than the mastodon.

In another study, just begun, late Pleistocene deer diets will be compared with those from the early Holocene, when, according to the Aucilla River site records, deer became the dominant large mammal and was much more extensively hunted by early humans. Our hypothesis is that deer diet will reflect a broader spectrum of fodder, including occasional grazing, after the extinction of the rest of the megafauna.