SSIDIATE · The significance of stable isotopes as dietary indicators in ancient terrestrial ecosystems

The significance of stable isotopes as dietary indicators in ancient terrestrial ecosystems

1. Publishable Summary New and barely explored isotopic systems can provide hard evidence for reconstructing food webs among extinct animals. Concerns about modern biodiversity and conservation require intimate understanding of ecosystem function, and ultimately, a deep time perspective. Understanding trophic organization of species is important because the parameters building an ecosystem are not static, but driven by changing factors, including climate. The fossil record bears witness to such changes and it is now timely to understand how species in ancient communities interacted with each other. Major niche shifts have been important in the history of some groups, and the aim here is to apply an isotopic perspective to palaeoecology. Isotopic systems of carbon (C) and nitrogen (N) preserved in bone collagen can give direct evidence on original food intake. However, because of the rapid decay of organic matter, these trophic inferences are limited to the recent past and together with other approaches such as Strontium-Calcium and Barium-Calcium ratios, they remain affected by diagenesis. Pristine preservation is very rare and tackling deep past trophic reconstruction remains a real challenge. There is therefore a need to implement more robust proxies to infer diets in extinct organisms by measuring those intimate elemental constituents of tooth apatite that demonstrate no diagenesis and indicate fractionation along the food chain: isotopes of calcium and magnesium. 1.1. Description of the work performed since the beginning of the project The main objective of this project is to identify new stable isotope proxies that could yield information on the trophic structures of vertebrates populating terrestrial ecosystems. My project began with setting up protocols of extraction of magnesium from tooth and bone apatite in extant ecosystems. I received training on chemical separation in the clean lab as well as training on mass spectrometry (now working independently on these instruments). 1.2. Main results achieved so far I am now fully able to work from the field down to the treatment and interpretation of results obtained on mass spectrometers. The first year of the project (September 2011-September 2012) has led to the acquisition of the first extant magnesium isotope database on two modern mammal faunas from South Africa, the results of which have been submitted for publication in June 2013. The second year of the project was dedicated to protocol of extraction of plant remains and the magnesium measurement of tooth samples from a modern ecosystem from equatorial Africa sampled in August 2011, prior to the start of the MC project. Traditional isotopic approaches were also implemented for this ecosystem by measuring carbon and oxygen isotopes. In parallel, fossil ecosystems were selected and investigated for the variation of magnesium isotopes together with a comparison of traditional isotopes as a comparative baseline (collaborations R. Amiot, V. Balter and C. Lécuyer, Lyon) for assessing the significance of the newly developed proxy (δ26Mg and δ44Ca). Finally, the period from December 2012 to July 2013 was dedicated to protocols of extraction and setting up analytical techniques for measuring calcium isotopes in osseous tissues from the modern equatorial forest samples (collaboration B. Dhuime, and J. Lewis, Bristol). 1.3. Expected final results Altogether, this project recently led to the accumulation of a large database of new isotopic values for calcium, magnesium, oxygen, carbon and strontium. The expected final results are the implementation of a new isotopic proxy for diet in modern ecosystem: magnesium. Its application to the fossil record remains to be further tested with the addition of new fossil specimens spanning deep time ages. This is beyond the time allowed by the project.

Data: CORDIS, © European Union