A BIOMARKER RECORD OF THE PALEOGENE EVOLUTION AND EXPANSION OF C4 PLANTS

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 δ¹³C_org 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 (δ¹³C_CO2) 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-δ¹³C_CO2 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 δ¹³C_alkane values and calculate the percent abundance of C₄ plant input for a given sedimentary δ¹³C_alkane 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.