Graduation Year

2009

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Geology

Major Professor

Gregory Herbert, Ph.D.

Keywords

Amino acids, Stable isotopes, Food web, Trophic, Mollusk shell

Abstract

Shell organic matrix proteins in fossils are valuable geochemical archives for studying ancient environments and food webs. Compound-specific studies of stable carbon isotope ratios offer particularly good resolution of trophic level of consumers and the identities of primary producers and can be used to detect diagenetic alteration of isotopic ratios. To interpret compound specific isotope data, however, controlled diet studies in the laboratory are needed to reveal trophic enrichment patterns of 13C in tissues and shell organic matter. This study examines the relationship between d 13C of 11 amino acids in diet, soft tissues, and shell organic matter in laboratory-cultured Strombus alatus, an herbivorous marine gastropod. The d 13C values of amino acids in this animal's foot and mantle tissues are consistently enriched in 13C relative to the diet.

Phenylalanine (+1.8 ppm) and alanine (+3.8 ppm) showed the least fractionation between diet and tissues, while aspartic acid (+10.7 ppm) and glutamic acid (+14.6 ppm) showed the greatest enrichment. On average, nonessential amino acids exhibited greater enrichment than did essential amino acids (+7.1 ppm vs. + 4.1 ppm). Shell organic matter amino acids showed a very similar pattern, with aspartic and glutamic acids again showing the greatest enrichment (+7.2 ppm and +11.1 ppm respectively). Nonessential amino acids in shell (+4.9 ppm) were also more enriched than the essential amino acids (+3.5 ppm). Overall, the carbon isotopic compositions of amino acids in shell organic matrix appear to parallel those in animal tissue, validating the utility of employing this material as a surrogate for animal tissue in fossil samples.

Interpreting trophic position information in consumers is difficult, however, as the variation in the magnitude of trophic enrichments for glutamic and aspartic acids between species, tissue types and diet is still poorly understood. As phenylalanine has the most consistent diet-consumer enrichments, the most suitable application for d 13C isotope analysis at this time is the reconstruction of base food sources.

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