TOWARDS QUANTITATIVE RECONSTRUCTIONS OF DIET AND ENVIRONMENT USING STABLE ISOTOPE PROFILES IN TISSUES: EXAMPLES FROM ELEPHANTS

Stable isotope analysis of modern and fossil tissues from terrestrial fauna is a widely used proxy for diet and environmental reconstruction. Tissues such as tusks, molars, and hair can be serially sampled along their growth axes to produce isotope profiles. The profiles provide time series of isotope data with unparalleled resolution among terrestrial proxies that span weekly to multi-year time scales. Isotope profiles in fossil tissues therefore have the potential to address questions pertaining to seasonality of diet and climate (e.g., precipitation) in the past.

Quantitative reconstructions of diet and environment from teeth and tusks requires characterizing variables that include but are not limited to growth rate, sampling geometry, and mineralization. In tooth enamel, these variables blur the original diet or environmental input signal such that it may differ significantly from the measured isotope profile. To mitigate blurring some researchers have refined sampling location, size (i.e., micro-sampling), and geometry, while others have developed inverse methods to correct for blurring.

We present isotope profiles from modern elephant (Loxodonta africana) tail hair, tusks, and molars that illustrate the importance of determining tissue variables. Bomb-curve radiocarbon is used to measure growth rates in tusks and individual molar plates of ca. 4 to 5 cm/yr and ca.1.5 cm/yr, respectively. We use micro-CT and histological methods to characterize enamel mineralization in molars. This permits inverse modeling of isotope data from conventionally drilled profiles. We then compare inverse model results from conventional profiles with isotope profiles generated by micro-sampling techniques (Micromill and laser ablation). Inverse model results more closely match the known input signal than those from micro-sampling techniques. Characterizing growth rates and enamel mineralization in molars from additional extant mammalian taxa will expand the utility of inverse methods and lead to more quantitative diet and environmental reconstructions. Comparing enamel histology of related modern and fossil taxa will minimize uncertainty in applying growth rates and mineralization parameters from modern to fossil teeth.