The impact of Miocene atmospheric carbon dioxide fluctuations on climate and the evolution of terrestrial ecosystems

Abstract

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 ...

Boundary conditions for the Middle Miocene Climate Transition (MMCT v1.0)

“…Ma according to Tripati et al (2009), ~740 ppmv between ~15.6 and ~14.6 Ma or ~420 ppmv between ~15.6 and ~12.8 Ma according to Retallack (2009), and over ~120-220 ppmv between ~15.5 and ~14 Ma or over ~60-160 ppmv between ~15.5 and ~12 Ma according to Kuerschner et al (2008).…”

“…Retallack (2009), based on carbon isotopes of pedogenic carbonate, suggests atmospheric pCO 2 levels reaching a maximum of ~850 ppmv at ~15.6 Ma, dropping down to ~115 ppmv by ~14.6 Ma and increasing to ~430 ppmv by ~12.8 Ma. Kuerschner et al (2008) used stomatal-density data from fossil leaves to support pCO 2 values reaching a maximum over ~400-500 ppmv at ~15.5 Ma, decreasing to ~280 ppmv by ~14 Ma and increasing to ~340 ppmv by ~12 Ma.…”

Equatorial convergence of India and early Cenozoic climate trends

“…This may have been the case especially once pCO 2 decreased to around modern levels in the Oligocene and long-term changes in pCO 2 and global climate became less obviously coupled (14,74; but see ref. 75). …”

Mid-Miocene cooling and the extinction of tundra in continental Antarctica

“…The ultimate cause of the climate change is uncertain. A decline in global pCO 2 , based on changes in stomatal density on fossil leaves, has been proposed as the primary driver (29), although marine geochemical evidence suggests that pCO 2 declined several million years earlier than the MMCT (30). Alternative explanations focus on tectonically driven changes to oceanic circulation resulting in a reorganization of the oceanatmosphere-cryosphere system (2,31,32).…”