A large and abrupt fall in atmospheric CO2 concentration during Cretaceoustimes

Marine carbonates and organic matter show a sharp increase in their C-13/C- 12 isotope ratio at the Cenomanian/Turonian (C/T) boundary(1,2) in the Cret aceous period. This isotopic shift resulted from an increase in the rate of sedimentary burial of C-13-depleted organic carbon in response to the C/T 'oceanic anoxic event'(2), The enhanced burial rate should have led to a si gnificant drop in the atmospheric CO2 concentration, which could explain th e apparent climate cooling of early Turonian times(2-4). Here we present st able carbon isotope data for specific compounds from terrestrial leaves and marine phytoplankton, and quantify the abruptness and magnitude of the atm ospheric CO2 concentration change. Isotope shifts in leaf-wax components ex tracted from abyssal sediments in the northeastern tropical Atlantic Ocean- the components are wind-delivered from Africa-indicate a sudden change in p lant communities of the north African continent. Specifically, the data sug gest that plants using the C-3-type photosynthetic pathway were succeeded b y plants using the C-4-type pathway. If C-4 plants can outcompete C-3 plant s only at atmospheric CO2 concentrations below 500 p.p.m.v. (ref. 5), the o bserved vegetation change indicates a far larger reduction in C/T CO2 conce ntration-some 40-80%-than previously suggested(6). The isotopic excursion i n the marine phytoplankton compounds is consistent with this estimate. We i nfer that this dramatic fall in the atmospheric CO2 concentration was abrup t, occurring in just 60,000 years.