LONG-TERM REGIONAL CONTRIBUTION OF CENOZOIC C4 PLANTS TO MARINE SEDIMENTS

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 (d¹³C_alk) 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 (d¹³C_CO2).  Accordingly, we have constrained d¹³C_CO2 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-d¹³C_CO2 we modeled expected C₃ and C₄ d¹³C_alk values in order to predict the percent abundance of C₄ plant input for a given sedimentary d¹³C_alk composition.

Presently, a limited number of Miocene-age and Eocene-Oligocene-age d¹³C_alk 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 d¹³C_CO2 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.