WAS THERE C4 PHOTOSYNTHESIS IN THE LATE CRETACEOUS?

The evolution of C₄ plants is thought to have occurred several times during the Miocene when atmospheric carbon dioxide levels were low. Transition from C₃ to C₄ dominated communities during the Miocene are inferred from the ¹³C enrichment of paleosols and fossils. Earlier appearance of C₄-like CO₂ concentrating mechanisms have only been found in CAM plants which are confined to aquatic areas. A positive ¹³C excursion measured in land plant biomarkers at 10ºN paleolatitude from sediments deposited during the Cenomanian-Turonian (Late Cretaceous) ocean anoxic event (OAE II) may be evidence of early expression of C₄ metabolism (Kyupers et al., 1999; Nature 399, p342). We have confirmed these observations at 35ºN but not at 55ºN in the Western Interior Seaway (USA). However, at 35ºN, this positive excursion occurs significantly later than the excursion recorded at 10ºN. In a period of angiosperm expansion, burial of OM during OAE II resulted in pCO₂ draw-down and pO₂ increase. These conditions may have prompted the expression of C₄-type photosynthetic pathways that momentarily dominated the ecosystem in Late Cretaceous intertropical regions. However, C₄ metabolism is not confined to Kranz anatomy (Vosznesenskaya et al., 2001; Nature 414, p543) which likely evolved in the Miocene. Additionally, C₄ metabolism has been identified in cells of stems and petioles of a C₃ plant, suggesting that genes for expression of C₄ photosynthesis are present in C₃ plants, possibly explaining why C₄ photosynthesis has evolved independently many times (Hibberd and Quick, 2002; Nature 415, p451) and might have evolved as early at the Late Cretaceous.