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| Miocene climatic optimum | |
|---|---|
| Name | Miocene climatic optimum |
| Period | Miocene |
| Epoch | Neogene |
| Time start | ~17 million years ago |
| Time end | ~14 million years ago |
| Primary lithology | marine sediments, terrestrial deposits |
| Location | global |
Miocene climatic optimum The Miocene climatic optimum was a prolonged interval of global warmth during the Miocene Epoch characterized by elevated temperatures, reduced polar ice, and major biotic turnover. Paleoclimatologists, paleobotanists, stratigraphers, and marine geologists study this interval using deep-sea cores, terrestrial fossil floras, and isotopic records from institutions such as the Smithsonian Institution, Max Planck Society, United States Geological Survey, and the Geological Society of America. The event influenced the evolution and distribution of mammals, angiosperms, and marine faunas and is central to debates among researchers at universities like Harvard University, University of Cambridge, University of California, Berkeley, and Stanford University.
The interval traditionally called the Miocene climatic optimum occurred in the middle Miocene, roughly between 17 and 14 million years ago, bracketed by stratigraphic markers used by the International Commission on Stratigraphy and correlated in marine stages such as the Langhian and Badenian. Chronologies derive from magnetostratigraphy tied to geomagnetic polarity timescales established by the International Union of Geological Sciences and radiometric calibration from laboratories at the California Institute of Technology and ETH Zurich. Global correlation employs records from the North Atlantic Ocean, Mediterranean Sea, Southern Ocean, and continental basins like the Siwalik Group and the Petrified Forest National Park stratigraphic sections.
Several interacting forcings are invoked to explain warming during the Miocene climatic optimum: changes in atmospheric greenhouse gas concentrations, notably carbon dioxide reconstructed by geochemists at the Scripps Institution of Oceanography and the British Antarctic Survey; variations in oceanic circulation linked to gateways such as the Tethys Sea closure, the narrowing of the Central American Seaway, and uplift of the Tibetan Plateau and Andes; and shifts in orbital parameters computed with models developed at the Jet Propulsion Laboratory and NASA Goddard Institute for Space Studies. Climate modeling experiments by groups at Princeton University, MIT, and the National Center for Atmospheric Research indicate feedbacks involving albedo changes from reduced Antarctic and Greenland ice sheets, altered cloud regimes studied by teams at the Lawrence Berkeley National Laboratory, and enhanced poleward heat transport in the Atlantic Meridional Overturning Circulation. Volcanism associated with provinces like the Columbia River Basalt Group and the Deccan Traps has been discussed by researchers at the Geological Survey of India as potential transient forcings affecting greenhouse gas budgets.
During the optimum, sea-surface temperatures inferred from foraminiferal assemblages collected by the International Ocean Discovery Program and the Ocean Drilling Program were higher across the Pacific Ocean, Indian Ocean, and Southern Ocean, with expanded subtropical and tropical biomes recorded in the Mediterranean Basin and Caribbean Sea. Reduced Antarctic ice extent documented in sediment cores from the Weddell Sea and Ross Sea contrasts with later Neogene glacials recognized in cores analyzed at the Alfred Wegener Institute. Marine transgressions reshaped continental shelves including those of the North Sea and the Gulf of Mexico and influenced depositional regimes in basins such as the Paraná Basin and North China Basin. Atmospheric circulation shifts inferred by paleoweathering studies at the Max-Planck Institute for Chemistry and pollen records from the Royal Botanic Gardens, Kew altered precipitation patterns across regions including the Mediterranean Region, East Africa, and South America.
The Miocene climatic optimum drove range expansions and extinctions evident in vertebrate faunas from the Siwalik Hills, Fayum Depression, and Valley of Lakes and in marine faunas from the Calvert Cliffs and Pisco Formation. Mammalian groups such as early Hominoidea, Cervidae, and Equidae exhibited diversification or dispersal events recorded by paleontologists at institutions like the American Museum of Natural History and the Natural History Museum, London. Plant macrofossils and palynological assemblages curated by the New York Botanical Garden and the Muséum national d'Histoire naturelle document expansion of subtropical forests, shifts in Laurasia–Gondwana-derived floras, and later replacement by grassland-adapted taxa leading to the evolution of C4 photosynthesis studied by biochemists at the University of Illinois Urbana-Champaign and Michigan State University. Marine ecosystems saw proliferation of diatoms investigated at the Alfred Wegener Institute and diversification of cetaceans cataloged by researchers at the Smithsonian Institution.
Reconstruction of the optimum relies on multi-proxy datasets: oxygen isotope ratios from benthic and planktonic foraminifera in cores from the Deep Sea Drilling Project and International Ocean Discovery Program; alkenone paleothermometry developed by chemists at the Woods Hole Oceanographic Institution; leaf physiognomy and stomatal indices from floras in the Florissant Fossil Beds and Riversleigh; and paleosol geochemistry analyzed by staff at the United States Geological Survey. Stable carbon isotopes from soil carbonates, boron isotopes measured at the University of California, Santa Cruz, and trace element proxies assembled by teams at Lamont–Doherty Earth Observatory enable reconstruction of pCO2 and salinity. High-resolution records from the Mediterranean Sapropel sequences, the Equatorial Pacific sections, and the Sea of Japan demonstrate regional variability in timing and magnitude.
The termination of the Miocene climatic optimum was followed by progressive cooling through the middle to late Miocene and into the Pliocene, driven by declining atmospheric CO2 reconstructed by investigators at the National Oceanography Centre and tectonic reorganizations such as continued uplift of the Himalayas and continued closure of the Tethys Sea. The expansion of Antarctic ice sheets around the Late Miocene and intensification of glaciation in the Northern Hemisphere correlate with isotopic shifts in benthic foraminifera catalogued in the International Chronostratigraphic Chart. This transition set the stage for later Pleistocene glacial–interglacial cycles studied in ice cores from Vostok Station, EPICA Dome C, and Greenland Ice Sheet Project sites and synthesized by climate research centers such as the Intergovernmental Panel on Climate Change.