HR: 14:10h
AN: PP53D-03 INVITED     [Abstracts]
TI: A Cenozoic terrestrial isotope record and the evolution of C$_{4}$ photosynthesis
AU: * Gr”cke, D R
EM: grocke@mcmaster.ca
AF: McMaster University, 1280 Main St. W., Hamilton, ON L8S 4K1 Canada
AU: Tipple, B J
EM: brett.tipple@yale.edu
AF: Yale University, 210 Whitney Ave., New Haven, CT 06511 United States
AU: Pagani, M
EM: mark.pagani@yale.edu
AF: Yale University, 210 Whitney Ave., New Haven, CT 06511 United States
AB: Our understanding of C$_{4}$ plant evolution and expansion has predominantly relied on site-specific fossil teeth, paleosols, and pedogenic carbonates carbon-isotope records and suggests a global dominance between 15-6 Ma. However, more recent techniques using bulk and compound-specific carbon-isotope ratios from terrestrial organic matter and other biomarker evidence suggest C$_{4}$ plants may have evolved multiple times. Furthermore, C$_{4}$ plants may have been present in terrestrial environments much earlier than the late Miocene expansion, but owing to their environmental preference and low preservation potential may not have been preserved in the terrestrial sedimentary record, and/or such latitudinal sites have not been fully explored. An additional implication is that the carbon-isotope composition of CO$_{2}$ (\delta $^{13}$C$_{CO2}$) has changed through time and paleoecologic reconstructions based on teeth and carbonate isotopic signatures may not reflect accurate floral contributions. Thus, terrestrial and atmospheric carbon-isotope signatures must be integrated in order to assess the Cenozoic history of C$_{4}$ photosynthesis. Presently, we are constructing a carbon-isotope record of long-chain {\it n}-alkanes with high carbon preference indices (indicative of higher plant input) from a globally distributed set of oligotropic and marginal DSDP/ODP marine sediments. As mentioned above, an estimate of the C$_{4}$ plant proportion of total land-plant biomass requires an understanding of changes in \delta $^{13}$C$_{CO2}$ through time. Accordingly, we have constrained this parameter by establishing the carbon-isotope composition of C$_{3}$ plant organic matter from Paleogene-age shallow marine shelf and lagoonal sediments from the Isle of Wight, UK, by assuming constant carbon-isotope discrimination between CO$_{2}$ and C$_{3}$ photosynthesis. Such \delta $^{13}$C$_{CO2}$ records can be directly compared with alkenone-based {\it p}CO$_{2}$ and {\it n}-alkane based floral contribution estimates. Using integrated isotopic proxies, our preliminary data suggest that the C$_{4}$ photosynthetic pathway evolved prior to the late Miocene and was possibly driven by paleoenvironmental pressures.
DE: 9604 Cenozoic
DE: 4853 Photosynthesis
DE: 4870 Stable isotopes
DE: 1851 Plant ecology
DE: 1055 Organic geochemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting