The Early Origins of Terrestrial C4 Photosynthesis

The C4 photosynthetic pathway is a series of structural and biochemical modifications around the more primitive C3 pathway that improve the photosynthetic efficiency under specific climatic conditions. Hence, the origin and subsequent geographical expansion of the C4 plants likely reflects a record of climate change. Multiple paleoatmospheric pCO2 proxies indicate a critical CO2 threshold was breached ∼30 Ma, that potentially selected for CO2-concentrating mechanisms to overcome photorespiratory stresses imposed on the basic C3 pathway. Details of the C4 pathway's earliest origins remain enigmatic given the paucity of the geologic record. Nonetheless, δ¹³C proxy records from paleosol carbonates, ungulate teeth, and plant-derived compounds indicate C4 plants likely represented an important component of plant biomass by the Early Miocene. Low CO2 levels appear to be a precondition for the development of the C4 photosynthetic pathway; however, comparisons of δ¹³C proxy records indicate that the timing of C4 geographical expansion was not globally synchronous, and thus point toward more regional controls on the development of C4-dominated ecosystems. Terrestrial and marine records indicate that continental aridity and wind strength increased during the Late Miocene. These conditions would have likely increased fire occurrence and fire intensity leading to the clearing of large tree stands and the expansion of C4 grasses in warm-season precipitation regimes.