2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 10
Presentation Time: 4:00 PM


TIPPLE, Brett J., Department of Biology, University of Utah, EHLERINGER LABORATORY 522 ALINE SKAGGS BIOLOGY BUILDING, Salt Lake City, UT 84112 and PAGANI, Mark, Department of Geology & Geophysics, Yale Univ, P.O. Box 208109, New Haven, CT 06520, brett.tipple@utah.edu

The evolution and expansion of the C4 photosynthetic pathway was driven by specific environmental conditions and thus reflects a record of global environmental change. Given the distinct stable carbon isotopic compositions of C3 and C4 flora, terrestrial δ13Corg can be applied to assess the relative proportion of C4 plant input with time. Estimates of the C4 plant biomass by this approach require an understanding of changes in the δ13C of atmospheric carbon dioxide (δ13CCO2) and atmospheric partial pressure of carbon dioxide (pCO2) with time. In this study, we evaluate the history of C4 photosynthesis by measuring the δ13C of terrestrially-derived n-alkanes from a globally distributed set of oligotropic and marginal DSDP/ODP marine sediments. Estimates of paleo-δ13CCO2 are established from (1) the δ13C of C3 plant organic matter from Paleogene-age sediments from the Isle of Wight, UK, and (2) from published δ13C records of planktonic foraminifera. From our data and recently developed Paleogene pCO2 records, we model C3 and C4 δ13Calkane values and calculate the percent abundance of C4 plant input for a given sedimentary δ13Calkane composition. Our preliminary results from the Atlantic and Indian Ocean basins indicate terrestrial C4 photosynthesis contributed little-to-no (≤ 5%) eolian-derived organic material during the Late Eocene. However, a sharp increase in C4 plant input from less than 5% to greater than 15% is associated with the early Oligocene decrease in atmospheric pCO2. The sharp decrease in pCO2 levels would have enhanced rates of photorespiration in C3 plants, selecting for the evolution and expansion of the C4 pathway. Following the initial decrease in pCO2, C4 plant input remained persistent throughout the later Oligocene and Miocene, constituting ~10 to 30% of eolian-derived organic material delivered to marine sediments.