COMPARING THREE GEOCHEMICAL METHODS OF ASSESSING TROPHIC DIVERSITY IN ANCIENT MARINE ECOSYSTEMS

Trophic diversity is an important character of marine ecosystems, yet it is often difficult to quantify from evidence preserved in the fossil record. Recent studies of modern foodwebs have increasingly relied on the use of biological fractionation and purification of isotopes as an important tracer of trophic level; this has the potential to yield similar information for paleontological research. Three potential geochemical proxies for trophic level are: elemental ratios (e.g., Sr/Ca, Ba/Ca); nitrogen isotopes (δ¹⁵N); and calcium isotopes (δ⁴⁴Ca) in bone or tooth material. In order for these proxies to be useful in the study of fossil materials, it is necessary to first establish their effectiveness in modern ecosystems. The aim of this project is to assess the relative effectiveness of these proxies for trophic level in modern marine mammals and to determine which proxy (or combination of proxies) would be the most effective for the study of trophic diversity in ancient marine ecosystems.

Tooth and bone material was collected from multiple individuals (n≥3, where possible) of all major groups of marine mammals (Pinnipedia, Sirenia, Cetacea, and Mustellidae). Samples were taken from collections at the Smithsonian in Washington D. C. and the Los Angeles County Natural History Museum in Los Angeles, California and primarily represent populations from the North Pacific and North Atlantic, with a few species that range globally. For each individual, data was collected on the Sr/Ca, Ba/Ca, δ¹⁵N, and δ⁴⁴Ca values of bone or tooth material.

Each of these methods of assessing trophic diversity has drawbacks that must be considered before applying them to paleontological research. Some of these drawbacks include the preservation potential over long timescales (>100 kyrs), spatiotemporal variation in baseline elemental and isotope values for foodwebs, and high analytical costs for large numbers of samples. Preliminary results indicate that δ¹⁵N is the most effective proxy when looking at recent communities (<100 kyrs) from a small geographic area; Sr/Ca and Ba/Ca can be used over the same timescales but over larger geographic areas; and δ⁴⁴Ca analysis is most effective for older material (>100 kyrs) collected over large geographic areas.