Stable sulfur isotope composition of organic matter: Cambrian to Recent

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ABSTRACTStable carbon isotopes have proven to be a powerful probe for investigations of the complex interplay between the evolution of the Earth's crust and biosphere throughout the Phanerozoic. However, detailed studies of this interrelationship for other light elements that comprise organic matter (N, O, S, H) in terms of global evolutionary change are lacking. Previous studies have focused on inorganic sulfur in minerals (sulfates, sulfides) and in shell carbonate. Thus, how the organic sulfur content of marine organisms varied through time and its relationship to and impact on the inorganic sulfur isotope record is based largely on circumstantial evidence provided by this inorganic sulfur record rather than by a direct probe. A hypothesis will be tested with respect to documenting changes in the stable sulfur isotope composition of organic matter during the Phanerozoic. The stable sulfur isotope composition of protein amino acids (methionine and cysteine) in fossil shells of pelagic organisms can be used as a new proxy for reconstructing secular changes in the sulfur isotope composition of the oceans. Broader Impacts: Stable carbon isotope geochemistry has played an essential role in improving our understanding of the complex interplay of the biosphere, hydrosphere and lithosphere as they have evolved through time. The proposed detailed study of the stable sulfur isotope composition of organic matter will provide a new approach for assessment of this interplay. To date, this information has only been acquired indirectly by the study of minerals thought to reflect biological processes. The stable sulfur isotope compositions of sulfur-containing amino acid constituents of shell protein in well-preserved fossils is likely to reflect original sea water chemistry and is less likely to have been altered by diagenetic processes than evaporites. This approach will provide the first independent test of the validity of the inorganic sulfur isotope curves that have been proposed for the Phanerozoic. The requested funds will be used to support the research of Louise Totten, a new graduate student in our program. Louise has an undergraduate degree in chemistry and is thus well suited for this type of project. Students from Engel's undergraduate courses in Geology and Oceanography are regular participants in on going research projects in his stable isotope and amino acid laboratories. It is anticipated that two or more undergraduates will be involved in this project for their senior theses in science education.