The Miocene is characterized by a series of key climatic events that led to the founding of the late Cenozoic icehouse mode and the dawn of modern biota. The processes that caused these developments, and particularly the role of atmospheric CO 2 as a forcing factor, are poorly understood. Here we present a CO 2 record based on stomatal frequency data from multiple tree species. Our data show striking CO 2 fluctuations of Ϸ600 -300 parts per million by volume (ppmv). Periods of low CO 2 are contemporaneous with major glaciations, whereas elevated CO 2 of 500 ppmv coincides with the climatic optimum in the Miocene. Our data point to a long-term coupling between atmospheric CO 2 and climate. Major changes in Miocene terrestrial ecosystems, such as the expansion of grasslands and radiations among terrestrial herbivores such as horses, can be linked to these marked fluctuations in CO 2.atmospheric CO2 ͉ fossil plants ͉ paleoclimates ͉ stomata ͉ C4 plants T he Miocene is distinguished by extreme climatic optima alternating with major long-term climatic cooling, which together mark the founding of the modern late Cenozoic cold mode and the evolution of modern terrestrial biomes (1). Grass-dominated ecosystems became established in the low and middle latitudes of many parts of the world, such as North America, Eurasia, Africa, and Australia (2). Major radiations in large mammalian herbivores have been attributed to changes in the distribution of vegetation and terrestrial primary productivity (3-5). A significant change in dental morphology from lowto high-crowned toothed horses occurs during the middle Miocene, whereas a transition from a C 3 plant to C 4 plant diet did not take place before the late Miocene (6).Both Cenozoic climate trends and changes in terrestrial ecosystems have been thought to be influenced by long-term CO 2 fluctuations (6-8). Before marine pCO 2 proxy records were available, Cenozoic CO 2 trends were inferred from carbonisotope records of paleosols (9) and from carbon cycling models (10), which indicated a long-term decrease from Ϸ1,000 to Ͻ500 parts per million by volume (ppmv) throughout the Cenozoic. Approximately a decade later, CO 2 reconstructions based on marine geochemical proxies indicated consistently low late Pleistocene (glacial-like) CO 2 values of Ϸ200-280 ppmv (11, 12). Consequently, the Miocene has been regarded as a geological period in which climate and the carbon cycle were essentially decoupled. Because of this alleged decoupling, the role of atmospheric CO 2 as a climate forcing factor has been disputed (13-15). However, a permanently low CO 2 scenario has been challenged because photosynthetic models predict that plant life would not have thrived under such conditions (16). Climate models showed the importance of atmospheric CO 2 as a fundamental boundary condition for Cenozoic climate change (17). In fact, a coupling between atmospheric CO 2 and glacialinterglacial cycles over the past 600,000 years is well documented by ice core analysis (18). Understanding the long-term ...

No abstract

No abstract

No abstract