DIFFERENTIAL LATITUDINAL RESPONSE OF LAND PLANT ECOSYSTEMS TO THE CENOMANIAN/TURONIAN OCEANIC ANOXIC EVENT

The positive carbon isotope excursion associated with the Cenomanian-Turonian (C/T) boundary (~93 Ma BP) reflects the increased burial rate of 13C depleted organic carbon at the onset and during an oceanic anoxic event (OAE). A concomitant drop in atmospheric pCO2 of 20% to 40-80% was estimated previously. We present stable carbon isotope data on long-chain n-alkanes derived from terrestrial plants, isolated from marine sediments of two Cenomanian-Turonian sections of the Western Interior Seaway at paleolatitudes of 35ºN and 55ºN. Our results are compared to results obtained by Kyupers et al. (1999; Nature 399, p342) at 10ºN paleolatitude. Our results deliver compelling evidence that the land plant ecosystem responded differentially with latitude to the C/T OAE. A positive excursion in the stable isotopic composition of land-plant derived biomarkers was observed at 35ºN. This excursion occurred significantly later than at 10ºN. Such an isotopic excursion is not observed at 55ºN paleolatitude. At 35ºN we observe that, towards the end of the OAE, the isotopic composition of land plant derived biomarkers returns progressively to pre-OAE values. The differential response of the isotopic composition of land plant biomarkers with latitude suggests that the land-plant ecosystem responded to the OAE at low latitudes first and higher latitudes later. This observed shift in ecosystem bares characteristics of the evolution of a temporarily dominant photosynthetic pathway driven by considerable environmental change, perhaps similar to the C3/C4 crossover function hypothesized for the Miocene/Pliocene boundary by Cerling et al. (1997; Nature 389, p153).