2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 6
Presentation Time: 9:15 AM


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 of the C4 photosynthetic pathway was likely driven by specific environmental conditions and thus reflects a proxy record of terrestrial environmental change.  In order to assess the Cenozoic history of C4 photosynthesis, we are developing stable carbon isotopic records of long-chain n-alkanes (d13Calk) with high carbon preference indices, indicative of higher plant input.  In this study, n-alkanes extracted from a globally distributed set of DSDP/ODP marine sediments are used to assess eolian-derived higher plant organic matter.  This sampling scheme allows for evaluation of terrestrial plant material from a wide geographic range and thus allows an interpretation of the integrated regional, and perhaps global, importance of C4 photosynthesis through time.  In addition, estimates of the C4 plant proportion of total land plant biomass by this approach require an understanding of changes in the d13C of paleo-atmospheric carbon dioxide (d13CCO2).  Accordingly, we have constrained d13CCO2 by (1) establishing the carbon isotopic composition of C3 plant matter from Paleogene-age sediments and (2) from published d13C records of Miocene-age planktonic foraminifera. Given this record of paleo-d13CCO2 we modeled expected C3 and C4 d13Calk values in order to predict the percent abundance of C4 plant input for a given sedimentary d13Calk composition.

Presently, a limited number of Miocene-age and Eocene-Oligocene-age d13Calk records have been established.  Our preliminary data from the N. Atlantic, S. Atlantic, and N.W. Indian Oceans suggest that early to middle Miocene pelagic sediments maintained an approximately 20 to 30% C4 plant input.  Wind trajectories suggest C4 plant inputs derive from N. America, S. America, and E. Africa.

We continue to establish d13CCO2 records and higher plant ecology trends for the Paleogene.  Nonetheless, our preliminary results suggest C4 flora likely contributed a substantial percentage of higher plant ecology prior to the late Miocene C4 grass expansion.  If our preliminary results prove robust, then the C4 pathway may have evolved prior to the Miocene and was possibly driven by environmental pressures such as a significant decrease in the partial pressure of atmospheric carbon dioxide (pCO2) during the Oligocene as suggested by recent alkenone-based pCO2 estimates.