Late Miocene decoupling of oceanic warmth and atmospheric carbon dioxide forcing

Measurements from several ocean cores reveal that ocean warmth persisted throughout the late Miocene epoch despite CO 2 levels of only 200–350 p.p.m.v., probably driven by a deep thermocline that isolated climate responses from CO 2 variations. When ocean temperature and atmospheric carbon dioxide delinked Geological records of past climate provide the only opportunity to investigate Earth's response to large variations in atmospheric carbon dioxide concentrations. During the late Miocene, between 12 million and 5 million years ago, atmospheric CO 2 levels of only 200 to 350 parts per million by volume were accompanied by almost ice-free conditions in the Northern Hemisphere and the continents were warmer than they are at present, raising questions about the persistence of CO 2 –climate couplings. Using measurements from several ocean cores, LaRiviere et al . show that ocean warmth persisted throughout the late Miocene, probably driven by a thick thermocline that isolated climate responses from CO 2 variations. As the depth of the thermocline declined during the late Miocene, the more familiar relationship between CO 2 and climate was established. Deep-time palaeoclimate studies are vitally important for developing a complete understanding of climate responses to changes in the atmospheric carbon dioxide concentration (that is, the atmospheric partial pressure of CO 2 , p CO 2 ) 1 . Although past studies have explored these responses during portions of the Cenozoic era (the most recent 65.5 million years (Myr) of Earth history), comparatively little is known about the climate of the late Miocene (∼12–5 Myr ago), an interval with p CO 2 values of only 200–350 parts per million by volume but nearly ice-free conditions in the Northern Hemisphere 2 , 3 and warmer-than-modern temperatures on the continents 4 . Here we present quantitative geochemical sea surface temperature estimates from the Miocene mid-latitude North Pacific Ocean, and show that oceanic warmth persisted throughout the interval of low p CO 2 ∼12–5 Myr ago. We also present new stable isotope measurements from the western equatorial Pacific that, in conjunction with previously published data 5 , 6 , 7 , 8 , 9 , 10 , reveal a long-term trend of thermocline shoaling in the equatorial Pacific since ∼13 Myr ago. We propose that a relatively deep global thermocline, reductions in low-latitude gradients in sea surface temperature, and cloud and water vapour feedbacks may help to explain the warmth of