SGER: An Amino Acid-based Isotopic Method for Studying the Dynamics of Ancient Ecosystems

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An amino acid-based isotopic method for studying the dynamics of ancient ecosystemsPI: David E. Fastovsky; Co-PIs: Rebecca S. Robinson; Steven L. D¡¦HondtThe idea behind this proposal is to develop a method by which we can measure the nitrogen isotopic composition of amino acids from fossil bones and teeth. It has been shown that the amount of 15N/14N (which we measure as ?Ô15N) increases each trophic step in a food chain. This means in effect that the 15N/14N ratio is lowest among primary producers (in the terrestrial realm, this generally means plants), and increases in those organisms that eat the primary producers, and increases even further in those animals that eat the animals that eat the primary producers. So the ?Ô15N signature is actually a way to measure at which levels long-extinct organisms functioned in long-extinct ecosystems.The mechanics of the development of this technique, which is in fact the point of this proposal, is fraught with problems. Most significant among these are contamination and volume. The contamination problem is the same one that plagues all studies of ancient molecules: there is a very real possibility that the ?Ô15N numbers that we obtain could come from younger organisms ¡V possibly even those living today ¡V and if so their ?Ô15N signatures would obviously be wrong. To avoid this possibility, we would like to obtain our nitrogen from known molecules. We have chosen to use an amino acid signature profile for each tooth, which would tell us that we are at least looking at ancient, preserved molecules. This takes us to the volume problem, because if we are looking at the ?Ô15N signature for just a particular series of identifiable amino acids, then we are obviously required to deal with much smaller samples than if we were looking at the bulk, or total, ?Ô15N signature of a particular tooth or bone. Moreover, during the degradation that takes place as molecules age, certain amino acids are preferentially maintained while others are lost. By targeting of specific amino acids (preferably more than one) and tracing their ?Ô15N signatures through a food web, we will provide a much more reliable picture of dietary relationships than traditional bulk-organic methods allow.The fractionation of nitrogen isotopes as a record of trophic position has been repeatedly demonstrated for both marine and terrestrial ecosystems. With the development of the technique this proposal supports, we anticipate having a precise isotopic tool that identifies (and avoids) problems due to alteration or contamination of fossil material. For this reason, the technique has broad applications for studying ancient ecosystems.