The evolution and expansion of the C (sub 4) 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 C (sub 3) and C (sub 4) flora, terrestrial delta (super 13) C (sub org) can be applied to assess the relative proportion of C (sub 4) plant input with time. Estimates of the C (sub 4) plant biomass by this approach require an understanding of changes in the delta (super 13) C of atmospheric carbon dioxide (delta (super 13) C (sub CO2) ) and atmospheric partial pressure of carbon dioxide (pCO (sub 2) ) with time. In this study, we evaluate the history of C (sub 4) photosynthesis by measuring the delta (super 13) C of terrestrially-derived n-alkanes from a globally distributed set of oligotropic and marginal DSDP/ODP marine sediments. Estimates of paleo-delta (super 13) C (sub CO2) are established from (1) the delta (super 13) C of C (sub 3) plant organic matter from Paleogene-age sediments from the Isle of Wight, UK, and (2) from published delta (super 13) C records of planktonic foraminifera. From our data and recently developed Paleogene pCO (sub 2) records, we model C (sub 3) and C (sub 4) delta (super 13) C (sub alkane) values and calculate the percent abundance of C (sub 4) plant input for a given sedimentary delta (super 13) C (sub alkane) composition. Our preliminary results from the Atlantic and Indian Ocean basins indicate terrestrial C (sub 4) photosynthesis contributed little-to-no (< or = 5%) eolian-derived organic material during the Late Eocene. However, a sharp increase in C (sub 4) plant input from less than 5% to greater than 15% is associated with the early Oligocene decrease in atmospheric pCO (sub 2) . The sharp decrease in pCO (sub 2) levels would have enhanced rates of photorespiration in C (sub 3) plants, selecting for the evolution and expansion of the C (sub 4) pathway. Following the initial decrease in pCO (sub 2) , C (sub 4) plant input remained persistent throughout the later Oligocene and Miocene, constituting approximately 10 to 30% of eolian-derived organic material delivered to marine sediments.