HR: 09:30h
AN: B31D-06    [Abstracts]
TI: Reconstruction Of Ancient Food-Chain Dynamics From the Isotopic Composition of Molecular Fossils
AU: * Rothman, D H
EM: dan@segovia.mit.edu
AF: Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 United States
AB: Changes in the isotopic compositions of bulk sedimentary carbon are commonly used to infer past changes in the global carbon cycle. When both inorganic and organic carbon are analyzed, coevolving changes have revealed aspects of the dynamical interactions between these two bulk reservoirs. Of at least as much interest, however, are the interactions {\it within} the organic reservoir. Here we show how such interactions can be inferred from the changing isotopic compositions of specific organic compounds (i.e., ``molecular fossils'' or ``biomarkers''). Previous work has identified particular compounds whose isotopic compositions covary with primary (i.e., photosynthetically derived) and secondary (i.e., not primary) organic carbon as a whole. We model the evolution of these isotopic compositions as carbon flow on an elementary food chain. Two results are obtained. First, the prediction of a particular steady-state representation of the data provides a method for estimating the relative burial fluxes of primary and secondary sedimentary organic carbon. Second, the temporal fluctuations of the difference between primary and secondary isotopic compositions is related to community-level metabolism. Changes in the accumulation of such ``metabolic isotope effects'' have previously been related to changing food-chain lengths. A quantitative analysis suggests, however, that changes in the metabolic isotope effect itself---due, for example, to changing oxygen concentrations---provide a more plausible cause of such fluctuations. Preliminary studies of biomarkers traversing the Permian-Triassic extinction and the Cenomanian-Turonian oceanic anoxic event appear to confirm these predictions.
DE: 4805 Biogeochemical cycles (1615)
DE: 4806 Carbon cycling
DE: 1040 Isotopic composition/chemistry
DE: 1055 Organic geochemistry
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting