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 ...

During the end-Permian ecological crisis, terrestrial ecosystems experienced preferential dieback of woody vegetation. Across the world, surviving herbaceous lycopsids played a pioneering role in repopulating deforested terrain. We document that the microspores of these lycopsids were regularly released in unseparated tetrads indicative of failure to complete the normal process of spore development. Although involvement of mutation has long been hinted at or proposed in theory, this finding provides concrete evidence for chronic environmental mutagenesis at the time of global ecological crisis. Prolonged exposure to enhanced UV radiation could account satisfactorily for a worldwide increase in land plant mutation. At the end of the Permian, a period of raised UV stress may have been the consequence of severe disruption of the stratospheric ozone balance by excessive emission of hydrothermal organohalogens in the vast area of Siberian Traps volcanism.

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The Miocene epoch (23.03-5.33 Ma) was a time interval of global warmth, relative to today.Continental configurations and mountain topography transitioned toward modern conditions, and many flora and fauna evolved into the same taxa that exist today. Miocene climate was dynamic: long periods of early and late glaciation bracketed a ∼2 Myr greenhouse interval-the Miocene Climatic Optimum (MCO). Floras, faunas, ice sheets, precipitation, pCO 2 , and ocean and atmospheric circulation mostly (but not ubiquitously) covaried with these large changes in climate. With higher temperatures and moderately higher pCO 2 (∼400-600 ppm), the MCO has been suggested as a particularly appropriate analog for future climate scenarios, and for assessing the predictive accuracy of numerical climate models-the same models that are used to simulate future climate. Yet, Miocene conditions have proved difficult to reconcile with models. This implies either missing positive feedbacks in the models, a lack of knowledge of past climate forcings, or the need for re-interpretation of proxies, which might mitigate the model-data discrepancy. Our understanding of Miocene climatic, biogeochemical, and oceanic changes on broad spatial and temporal scales is still developing. New records documenting the physical, chemical, and biotic aspects of the Earth system are emerging, and together provide a more comprehensive understanding of this important time interval. Here, we review the state-of-the-art in Miocene climate, ocean circulation, biogeochemical cycling, ice sheet dynamics, and biotic adaptation research as inferred through proxy observations and modeling studies. Plain Language Summary During the Miocene time period (∼23-5 million years ago),Planet Earth looked similar to today, with some important differences: the climate was generally warmer and highly variable, while atmospheric CO 2 was not much higher. Continental-sized ice sheets were only present on Antarctica, but not in the northern hemisphere. The continents drifted to near their modernday positions, and plants and animals evolved into the many (near) modern species. Scientists study the Miocene because present-day and projected future CO 2 levels are in the same range as those reconstructed for the Miocene. Therefore, if we can understand climate changes and their biotic responses from the Miocene past, we are able to better predict current and future global changes. By comparing Miocene climate reconstructions from fossil and chemical data to climate simulations produced by computer models, scientists are able to test their understanding of the Earth system under higher CO 2 and warmer conditions than those of today. This helps in constraining future warming scenarios for the coming STEINTHORSDOTTIR ET AL.

Psiloceras spelae tirolicum. The "Golden Spike" was fixed at Kuhjoch East. The section displays a high and continuous sedimentation rate with a constant facies trend across the boundary level. Other bio-events include the FO of the aragonitic foraminifer Praegubkinella turgescens, and of diverse ostracod species 1.0-3.40 cm below the FO of P. spelae and 3.2 m below P. spelae occurs the continental palynomorph Cerebropollenites thiergartii. Because of the lack of other terrestrial microfloral events this is yet the FO event closest to the FO of P. spelae and allows a correlation with nonmarine sediments. The δ 13 C org record shows a strong initial negative excursion at the boundary between the Kössen and Kendlbach formations, 5.8 m (Kuhjoch W) below the T-J boundary, a shift to more positive δ 13 C org in the Schattwald Beds and a gradual decline to more negative values at the transition of the Schattwald Beds to the proximate Tiefengraben Mb. The stratotype point lies within a zone of smaller negative and positive δ 13 C org peaks, which is superimposed on a longer lasting main negative shift. According to recent investigations, the radiometric age of the T-J boundary is about 201,3 Ma.

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Macrofossils (mostly leaves) and sporomorphs (pollen and spores) preserve conflicting records of plant biodiversity during the endPermian (P-Tr), Triassic-Jurassic (Tr-J), and end-Cretaceous (K-T) mass extinctions. Estimates of diversity loss based on macrofossils are typically much higher than estimates of diversity loss based on sporomorphs. Macrofossils from the Tr-J of East Greenland indicate that standing species richness declined by as much as 85% in the Late Triassic, whereas sporomorph records from the same region, and from elsewhere in Europe, reveal little evidence of such catastrophic diversity loss. To understand this major discrepancy, we have used a new high-resolution dataset of sporomorph assemblages from Astartekløft, East Greenland, to directly compare the macrofossil and sporomorph records of Tr-J plant biodiversity. Our results show that sporomorph assemblages from the Tr-J boundary interval are 10-12% less taxonomically diverse than sporomorph assemblages from the Late Triassic, and that vegetation composition changed rapidly in the boundary interval as a result of emigration and/or extirpation of taxa rather than immigration and/or origination of taxa. An analysis of the representation of different plant groups in the macrofossil and sporomorph records at Astartekløft reveals that reproductively specialized plants, including cycads, bennettites and the seed-fern Lepidopteris are almost absent from the sporomorph record. These results provide a means of reconciling the macrofossil and sporomorph records of Tr-J vegetation change, and may help to understand vegetation change during the P-Tr and K-T mass extinctions and around the PaleoceneEocene Thermal Maximum.ompilations of stratigraphic ranges of land plants through geological time do not show abrupt declines in taxonomic diversity (1-3). This contrasts sharply with the history of animal life, which is marked by five geologically rapid decreases in global taxonomic diversity, known as mass extinctions (4, 5). This fundamental difference between the evolutionary histories of plants and animals may be due to the persistence of higher plant taxa, and has led to the suggestion that plants are more resistant to mass extinction than animals (1, 6-9). Despite this, studies of fossil plants during times of faunal mass extinction have revealed extensive ecological disruption and decreased plant genus/species diversity on local and regional scales (8, 9), suggesting that plants are not immune to the myriad environmental changes accompanying mass extinctions.However, plant fossils preserve conflicting records of diversity loss during these critical intervals in Earth history. Estimates of diversity loss based on macrofossils (mostly leaves) are typically much higher than estimates of diversity loss based on sporomorphs (pollen and spores). The end-Permian mass extinction [P-Tr; ∼251 million years ago (Ma)] in Australia saw a 97% regional diversity loss of macrofossils but a 19% loss of sporomorph diversity (8, 10), and the end-Cretaceous mass extinct...