The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell and comes from Ancient Greek μείων (meíōn), , and καινός (kainós), ,See: Letter from William Whewell to Charles Lyell dated 31 January 1831 in: and thus means "less recent", because it has 18% fewer modern marine invertebrates than the Pliocene has. The Miocene followed the Oligocene and preceded the Pliocene.
As Earth went from the Oligocene through the Miocene and into the Pliocene, the climate slowly cooled towards a series of ice ages. The Miocene boundaries are not marked by distinct global events but by regionally defined transitions from the warmer Oligocene to the cooler Pliocene Epoch.
During the Early Miocene, Afro-Arabia collided with Eurasia, severing the connection between the Mediterranean and Indian Oceans and enabling the interchange of fauna between the continents, including the dispersal of proboscideans and hominoids into Eurasia. During the late Miocene, the connections between the Atlantic and Mediterranean closed, causing the Mediterranean Sea to almost completely evaporate. This event is referred to as the "Messinian salinity crisis". Then, at the Miocene–Pliocene boundary, the Strait of Gibraltar opened, and the Mediterranean refilled. That event is referred to as the "Zanclean flood".
Also during the early Miocene (specifically the Aquitanian and Burdigalian Stages), the apes first evolved, began diversifying, and became widespread throughout the Old World. Around the end of this epoch, the ancestors of humans had split away from the ancestors of the chimpanzees and had begun following their own evolutionary path during the final Messinian Stage (7.5–5.3 Ma) of the Miocene. As in the Oligocene before it, grasslands continued to expand, and forests to dwindle. In the seas of the Miocene, kelp forests made their first appearance and soon became one of Earth's most productive ecosystems.
The plants and animals of the Miocene were recognizably modern. Mammals and birds were well established. Whales, pinnipeds, and kelp spread.
The Miocene is of particular interest to geologists and palaeoclimatologists because major phases of the geology of the Himalaya occurred during that epoch, affecting monsoonal patterns in Asia, which were interlinked with glacial periods in the northern hemisphere.
Subdivisions
The Miocene faunal stages from youngest to oldest are typically named according to the International Commission on Stratigraphy: Sub-epoch Faunal stage Time range Late Miocene Messinian 7.246–5.333 Ma Tortonian 11.63–7.246 Ma Middle Miocene Serravallian 13.82–11.63 Ma Langhian 15.97–13.82 Ma Early Miocene Burdigalian 20.44–15.97 Ma Aquitanian 23.03–20.44 Ma
Land mammal ages Cenozoic mammal faunas are often described using biochronological or biostratigraphic systems, which arrange rock strata or intervals of time based on distinctive fossil assemblages or the evolution of new mammal species. Apart from Antarctica, each continent has a unique series of Cenozoic land mammal ages (LMAs) overprinted on the global geological time scale. These different land mammal ages are listed in the following table, which shows rough correlations to each other and the six global ages/stages of the Miocene: Global ageNALMA(North America)SALMA(South America)ELMA / MN(Europe)ALMA(Asia)AFLMA(Africa)(Australia)MessinianHemphillianMontehermosanTurolian (MN 11-13)BaodeanBaringianWaiteanHuayquerianSugutanTortonianBaheanClarendonianChasicoanVallesian (MN 9-10)MayoanTugenianAstaracian (MN 6-8)TunggurianSerravallianLaventanCamfieldianBarstovianTinderetianLanghianColloncuranOrleanian (MN 3-5)FriasianBurdigalianHemingfordianSantacrucianShanwangianKisingirianWipajirianArikareeanLegetetianXiejianAquitanianColhuehuapianAgenian (MN 1-2)Turkwelian
Paleogeography
Continents continued to drift toward their present positions. Of the modern geologic features, only the land bridge between South America and North America was absent, although South America was approaching the western subduction zone in the Pacific Ocean, causing both the rise of the Andes and a southward extension of the Meso-American peninsula.
Mountain building took place in western North America, Europe, and East Asia. Both continental and marine Miocene deposits are common worldwide with marine outcrops common near modern shorelines. Well studied continental exposures occur in the North American Great Plains and in Argentina.
The global trend was towards increasing aridity caused primarily by global cooling reducing the ability of the atmosphere to absorb moisture, particularly after 7 to 8 million years ago. Uplift of East Africa in the late Miocene was partly responsible for the shrinking of tropical rain forests in that region, and Australia got drier as it entered a zone of low rainfall in the Late Miocene.
Eurasia The Indian Plate continued to collide with the Eurasian Plate, creating new mountain ranges and uplifting the Tibetan Plateau, resulting in the rain shadowing and aridification of the Asian interior. The Tian Shan experienced significant uplift in the Late Miocene, blocking westerlies from coming into the Tarim Basin and drying it as a result.
At the beginning of the Miocene, the northern margin of the Arabian plate, then part of the African landmass, collided with Eurasia; as a result, the Tethys seaway continued to shrink and then disappeared as Africa collided with Eurasia in the Turkish–Arabian region. The first step of this closure occurred 20 Ma, reducing water mass exchange by 90%, while the second step occurred around 13.8 Ma, coincident with a major expansion of Antarctic glaciers. This severed the connection between the Indian Ocean and the Mediterranean Sea and formed the present land connection between Afro-Arabia and Eurasia. The subsequent uplift of mountains in the western Mediterranean region and a global fall in sea levels combined to cause a temporary drying up of the Mediterranean Sea (known as the Messinian salinity crisis) near the end of the Miocene. The Paratethys underwent a significant transgression during the early Middle Miocene. Around 13.8 Ma, during a global sea level drop, the Eastern Paratethys was cut off from the global ocean by the closure of the Bârlad Strait, effectively turning it into a saltwater lake. From 13.8 to 13.36 Ma, an evaporite period similar to the later Messinian salinity crisis in the Mediterranean ensued in the Central Paratethys, cut off from sources of freshwater input by its separation from the Eastern Paratethys. From 13.36 to 12.65 Ma, the Central Paratethys was characterised by open marine conditions, before the reopening of the Bârlad Strait resulted in a shift to brackish-marine conditions in the Central Paratethys, causing the Badenian-Sarmatian Extinction Event. As a result of the Bârlad Strait's reopening, the lake levels of the Eastern Paratethys dropped as it once again became a sea.
The Fram Strait opened during the Miocene and acted as the only throughflow for Atlantic Water into the Arctic Ocean until the Quaternary period. Due to regional uplift of the continental shelf, this water could not move through the Barents Seaway in the Miocene.
The modern day Mekong Delta took shape after 8 Ma. Geochemistry of the Qiongdongnan Basin in the northern South China Sea indicates the Pearl River was a major source of sediment flux into the sea during the Early Miocene and was a major fluvial system as in the present.
South America During the Oligocene and Early Miocene, the coast of northern Brazil, Colombia, south-central Peru, central Chile and large swathes of inland Patagonia were subject to a marine transgression. The transgressions in the west coast of South America are thought to be caused by a regional phenomenon while the steadily rising central segment of the Andes represents an exception. While there are numerous registers of Oligocene–Miocene transgressions around the world it is doubtful that these correlate.
It is thought that the Oligocene–Miocene transgression in Patagonia could have temporarily linked the Pacific and Atlantic Oceans, as inferred from the findings of marine invertebrate fossils of both Atlantic and Pacific affinity in La Cascada Formation. Connection would have occurred through narrow epicontinental seaways that formed channels in a dissected topography.
The Antarctic Plate started to subduct beneath South America 14 million years ago in the Miocene, forming the Chile Triple Junction. At first the Antarctic Plate subducted only in the southernmost tip of Patagonia, meaning that the Chile Triple Junction lay near the Strait of Magellan. As the southern part of Nazca Plate and the Chile Rise became consumed by subduction the more northerly regions of the Antarctic Plate begun to subduct beneath Patagonia so that the Chile Triple Junction advanced to the north over time. The asthenospheric window associated to the triple junction disturbed previous patterns of mantle convection beneath Patagonia inducing an uplift of ca. 1 km that reversed the Oligocene–Miocene transgression.
As the southern Andes rose in the Middle Miocene (14–12 million years ago) the resulting rain shadow originated the Patagonian Desert to the east.
Australia Far northern Australia was monsoonal during the Miocene. Although northern Australia is often believed to have been much wetter during the Miocene, this interpretation may be an artefact of preservation bias of riparian and lacustrine plants; this finding has itself been challenged by other papers. Western Australia, like today, was arid, particularly so during the Middle Miocene.
Climate
Climates remained moderately warm, although the slow global cooling that eventually led to the Pleistocene glaciations continued. Although a long-term cooling trend was well under way, there is evidence of a warm period during the Miocene when the global climate rivalled that of the Oligocene. The climate of the Miocene has been suggested as a good analogue for future warmer climates caused by anthropogenic global warming, with this being especially true of the global climate during the Middle Miocene Climatic Optimum (MMCO), because that was the last time CO2 levels were comparable to projected future atmospheric CO2 levels resulting from anthropogenic climate change. The Ross Sea margin of the East Antarctic Ice Sheet (EAIS) was highly dynamic during the Early Miocene.
The Miocene began with the Early Miocene Cool Event (Mi-1) around 23 million years ago, which marked the start of the Early Miocene Cool Interval (EMCI). This cool event occurred immediately after the Oligocene-Miocene Transition (OMT) during a major expansion of Antarctica's ice sheets, but was not associated with a significant drop in atmospheric CO2 levels. Both continental and oceanic thermal gradients in mid-latitudes during the Early Miocene were very similar to those at present. Global cooling caused the East Asian Summer Monsoon (EASM) to begin to take on its modern form during the Early Miocene. From 22.1 to 19.7 Ma, the Xining Basin experienced relative warmth and humidity amidst a broader aridification trend.