| Summary: | International audience Understanding how and why global climate tipped from greenhouse to icehouse conditions remains a major challenge.This critical shift is well documented in the marine realm characterized by a steady decline in global temperatureuntil a large and rapid cooling step at the Eocene-Oligocene Transition (EOT). However, the chronology andmechanisms of cooling on land remain unclear. To reconstruct Paleogene climate conditions for the Tibetan Plateauand the Asian continental interior, clumped isotope thermometry and palynology in accurately-dated continentalrecords from northeastern Tibet, are here combined with climate and vegetation simulations. Our results show twosuccessive dramatic (>9 C) drops in soil carbonate temperature, at 37 Ma and at 33.5 Ma associated respectivelywith the appearance and dominance of high altitude conifer forests. Such large temperature decreases associatedwith ecological reorganisations cannot result from regional cooling alone. They require shifting of the rainy seasonto cooler months, which we interpret to reflect a decline of monsoonal intensity. Our results suggest that theresponse of Asian temperatures, monsoonal rainfall and vegetation to the decline of atmospheric CO2 and globaltemperature through the late Eocene occurred in two steps separated by a period of climatic instability. Our resultssupport the onset of the Antarctic Circumpolar Current coeval to the Oligocene isotope event 1 (Oi-1) glaciationat 33.5 Ma, reshaping the distribution of surface heat worldwide; however, the origin of the 37 Ma cooling eventremains to be determined.
|
|---|