The Miocene Epoch, the first chapter of the Neogene Period, unfurls from approximately 23.04 to 5.333 million years ago (Ma). It’s a span of time that, while ancient, feels remarkably… familiar. A geologist named Charles Lyell, a man who clearly had a penchant for the dramatic, christened it from the Ancient Greek words "meíōn" (less) and "kainós" (new, recent). This nomenclature, "less recent," hints at the subtle shift occurring in the fossil record – a gradual decrease in the proportion of modern marine invertebrates compared to the subsequent Pliocene Epoch. The Miocene epoch followed the Oligocene Epoch and, in turn, preceded the Pliocene.
As the Earth transitioned from the Oligocene, through the Miocene, and into the Pliocene, a palpable cooling trend began to take hold, ushering in the ice ages. These epochal boundaries aren't marked by cataclysmic, planet-shattering events, but rather by more nuanced, regionally defined shifts, like the slow exhale from the warmth of the Oligocene into the chill of the Pliocene.
The early Miocene witnessed a significant tectonic dance: Afro-Arabia, a force of nature in itself, collided with Eurasia. This wasn't just a geological reshuffle; it effectively sealed off the connection between the Mediterranean and the Indian Oceans. This continental embrace allowed for a fascinating exchange of lifeforms, a biological diaspora where creatures like proboscideans and hominoids ventured into Eurasia. Later in the Miocene, the connection between the Atlantic and the Mediterranean was severed, leading to a dramatic event: the near-total evaporation of the Mediterranean Sea, an episode grimly known as the "Messinian salinity crisis". The sea, however, wouldn't remain dry forever; at the cusp of the Miocene and Pliocene, the Strait of Gibraltar reopened, unleashing the "Zanclean flood" and refilling the basin.
This epoch also marks a pivotal moment in our own lineage. During the Early Miocene, specifically in the Aquitanian and Burdigalian Stages, the apes began their evolutionary journey, diversifying and spreading across the Old World. By the epoch's close, the ancestors of humans had diverged from those of chimpanzees), embarking on their distinct evolutionary path during the final Messinian Stage (7.5–5.3 Ma). The story of the land mirrored the cooling climate: grasslands continued their relentless expansion, while forests receded. Yet, in the vastness of the oceans, a new and vibrant ecosystem emerged: kelp forests, which quickly became some of the most productive environments on the planet.
The flora and fauna of the Miocene were, for the most part, recognizably modern. Mammals and birds had firmly established their place. The seas teemed with new life: whales, pinnipeds, and the now ubiquitous kelp.
For geologists and paleoclimatologists, the Miocene is a period of intense interest. Major phases in the geology of the Himalaya unfolded during this epoch, profoundly influencing monsoonal patterns across Asia, which in turn were intricately linked to the glacial periods that gripped the Northern Hemisphere.
Subdivisions
The Miocene is meticulously divided into distinct periods, each with its own character and story. These subdivisions are typically defined by the International Commission on Stratigraphy based on characteristic fossil assemblages, particularly marine fauna.
Sub-epoch and Faunal Stages
The Miocene is segmented into three major sub-epochs: Late, Middle, and Early. Within these, the faunal stages provide a finer resolution:
- 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
These formal subdivisions, based on marine stratigraphy, are complemented by regional systems, particularly those focusing on land mammals. These "Land Mammal Ages" offer a different lens through which to view the epoch's temporal progression, often overlapping with the preceding and succeeding epochs:
| Land Mammal Ages | European | North American | South American |
|---|---|---|---|
| • Turolian (9.0 to 5.3 Ma) | • Hemphillian (10.3 to 4.9 Ma) | • Montehermosan (6.8 to 4.0 Ma) | |
| • Vallesian (11.6 to 9.0 Ma) | • Clarendonian (13.6 to 10.3 Ma) | • Huayquerian (9.0 to 6.8 Ma) | |
| • Astaracian (16.0 to 11.6 Ma) | • Barstovian (16.3 to 13.6 Ma) | • Mayoan (11.8 to 9.0 Ma) | |
| • Orleanian (20.0 to 16.0 Ma) | • Hemingfordian (20.6 to 16.3 Ma) | • Laventan (13.8 to 11.8 Ma) | |
| • Agenian (23.8 to 20.0 Ma) | • Arikareean (30.6 to 20.6 Ma) | • Colloncuran (15.5 to 13.8 Ma) | |
| • Friasian (16.3 to 15.5 Ma) | |||
| • Santacrucian (17.5 to 16.3 Ma) | |||
| • Colhuehuapian (21.0 to 17.5 Ma) |
Paleogeography
The Earth’s continents continued their stately march across the globe during the Miocene, drifting ever closer to their present-day configurations. The most striking absent feature was the land bridge connecting South America and North America. South America, however, was drawing nearer to the intense subduction zone of the Pacific Ocean, a cosmic embrace that spurred the rise of the Andes and extended the Meso-American peninsula southward.
The Miocene was a period of significant geological upheaval. Mountain building was a global phenomenon, sculpting the landscapes of western North America, Europe, and East Asia. Evidence of both continental and marine Miocene deposits is found worldwide, with marine outcrops frequently gracing modern coastlines. Particularly well-studied continental exposures can be found in the North American Great Plains and in Argentina.
A prevailing global trend was the increasing aridity of the climate. This was primarily driven by a gradual global cooling, which diminished the atmosphere’s capacity to hold moisture, a trend that intensified significantly after 7 to 8 million years ago. The uplift of East Africa in the Late Miocene played a role in the shrinking of its tropical rain forests, while Australia became progressively drier as it entered a zone of low rainfall during the Late Miocene.
Eurasia
The relentless collision of the Indian Plate with the Eurasian Plate continued, giving rise to new mountain ranges and elevating the Tibetan Plateau. This immense geological activity created a rain shadow effect, leading to the aridification of the Asian interior. The Tian Shan mountains experienced significant uplift in the Late Miocene, effectively blocking the moisture-laden westerlies from reaching the Tarim Basin and contributing to its desiccation.
At the dawn of the Miocene, the northern edge of the Arabian plate, then still part of the African landmass, made contact with Eurasia. This collision caused the Tethys seaway to shrink progressively, eventually disappearing as Africa and Eurasia became a single landmass in the Turkish–Arabian region. The initial phase of this closure, occurring approximately 20 million years ago, reduced water mass exchange by a staggering 90%. A second, more significant closure transpired around 13.8 million years ago, coinciding with a major expansion of Antarctic glaciers. This event definitively severed the connection between the Indian Ocean and the Mediterranean Sea, establishing the present-day land bridge between Afro-Arabia and Eurasia. Subsequently, the uplift of mountains in the western Mediterranean region, coupled with a global drop in sea levels, culminated in the temporary desiccation of the Mediterranean Sea, known as the Messinian salinity crisis, near the epoch's end.
The Paratethys, a vast ancient sea, experienced a notable transgression during the early Middle Miocene. Around 13.8 Ma, during a global sea level decline, the Eastern Paratethys was isolated from the global ocean by the closure of the Bârlad Strait, transforming it into a large saltwater lake. From 13.8 to 13.36 Ma, an evaporite period, eerily similar to the later Messinian salinity crisis in the Mediterranean, took hold in the Central Paratethys, which had been cut off from freshwater sources by its separation from the Eastern Paratethys. From 13.36 to 12.65 Ma, the Central Paratethys reverted to open marine conditions, only for the reopening of the Bârlad Strait to usher in brackish-marine conditions, triggering the Badenian-Sarmatian Extinction Event. This reopening also led to a drop in the lake levels of the Eastern Paratethys, as it once again became a sea.
The Fram Strait emerged during the Miocene, serving as the sole conduit for Atlantic Water to enter the Arctic Ocean until the Quaternary period. Due to the regional uplift of the continental shelf, this water pathway was obstructed in the Barents Seaway during the Miocene.
The distinctive landscape of the modern-day Mekong Delta began to take shape after 8 Ma. Geochemistry studies of the Qiongdongnan Basin in the northern South China Sea reveal that the Pearl River was a significant source of sediment flux into the sea during the Early Miocene, functioning as a major fluvial system much like it does today.
South America
Throughout the Oligocene and Early Miocene, the coastlines of northern Brazil, south-central Peru, central Chile, Colombia, and vast inland areas of Patagonia experienced a marine transgression. The transgressions along the western coast of South America are believed to have been driven by regional geological processes, with the steadily rising central segment of the Andes representing a notable exception. While numerous accounts of Oligocene–Miocene transgressions exist globally, it remains uncertain whether these events were directly correlated.
It is theorized that the Oligocene–Miocene transgression in Patagonia may have temporarily connected the Pacific and Atlantic Oceans, as evidenced by the discovery of marine invertebrate fossils of both Atlantic and Pacific affinities within the La Cascada Formation. This connection would have been facilitated by narrow epicontinental seaways) that carved channels through a dissected topography.
The Antarctic Plate began to subduct beneath South America approximately 14 million years ago, during the Miocene, leading to the formation of the Chile Triple Junction. Initially, the Antarctic Plate subducted only at the southernmost tip of Patagonia, placing the Chile Triple Junction near the Strait of Magellan. As the southern portion of the Nazca Plate and the Chile Rise were progressively consumed by subduction, more northerly regions of the Antarctic Plate began to subduct beneath Patagonia. Consequently, the Chile Triple Junction migrated northward over time. The asthenospheric window associated with this triple junction disrupted the prevailing patterns of mantle convection beneath Patagonia, inducing an uplift of approximately 1 km that effectively reversed the Oligocene–Miocene transgression.
As the southern Andes rose during the Middle Miocene (14–12 million years ago), the resultant rain shadow to the east gave rise to the Patagonian Desert.
Australia
Far northern Australia experienced a monsoonal climate during the Miocene. While it's often assumed that northern Australia was significantly wetter during this epoch, this perception might be an artifact of preservation bias, favoring riparian and lacustrine plant fossils. This interpretation, however, has been contested by other research. Western Australia, much like today, was an arid region, a condition that intensified during the Middle Miocene.
Climate
The Miocene climate remained moderately warm, although the gradual global cooling that would eventually lead to the Pleistocene glaciations continued its inexorable progression. Despite this overarching cooling trend, evidence points to a period of significant warmth within the Miocene, when global temperatures rivaled those of the Oligocene. The climate of the Miocene has been posited as a valuable analogue for future warmer climates anticipated due to anthropogenic global warming. This analogy is particularly strong when considering the global climate during the Middle Miocene Climatic Optimum (MMCO), as atmospheric carbon dioxide levels during that time were comparable to projected future levels resulting from human-induced climate change. The margin of the East Antarctic Ice Sheet (EAIS) along the Ross Sea displayed considerable dynamism during the Early Miocene.
The Miocene commenced with the Early Miocene Cool Event (Mi-1) approximately 23 million years ago, marking the onset of the Early Miocene Cool Interval (EMCI). This cooling event followed closely on the heels of the Oligocene-Miocene Transition (OMT), coinciding with a significant expansion of Antarctica's ice sheets, yet it was not associated with a substantial decrease in atmospheric carbon dioxide levels. Thermal gradients, both continental and oceanic, in the mid-latitudes during the Early Miocene were remarkably similar to present-day conditions. The global cooling influenced the East Asian Summer Monsoon (EASM), which began to assume its modern characteristics during the Early Miocene. Between 22.1 and 19.7 Ma, the Xining Basin experienced periods of relative warmth and humidity, amidst a broader trend of aridification.
The EMCI concluded around 18 million years ago, giving way to the Middle Miocene Warm Interval (MMWI), the peak of which was the MMCO, beginning approximately 16 million years ago. As the world entered the MMCO, carbon dioxide concentrations fluctuated between 300 and 500 parts per million (ppm). The global annual mean surface temperature during the MMCO is estimated to have been around 18.4 °C. The warmth of the MMCO was partly fueled by the intense volcanic activity of the Columbia River Basalts and amplified by a reduced albedo resulting from the contraction of deserts and the expansion of forests. Climate modeling suggests that other, as yet unidentified, factors also contributed to the warm conditions of the MMCO. The MMCO witnessed a significant expansion of the tropical climatic zone, far exceeding its current extent. The Intertropical Convergence Zone (ITCZ), the region of maximum monsoonal rainfall, shifted northward, leading to increased precipitation over southern China while simultaneously reducing it over Indochina within the EASM. Western Australia, during this period, was characterized by extreme aridity. In Antarctica, average summer temperatures on land reached approximately 10 °C. In the oceans, the lysocline, the depth at which calcium carbonate begins to dissolve, shoaled by roughly half a kilometer during warm phases that corresponded to periods of maximum orbital eccentricity. The MMCO concluded around 14 million years ago, with global temperatures declining during the Middle Miocene Climate Transition (MMCT). Abrupt increases in opal deposition suggest this cooling was driven by an enhanced drawdown of atmospheric carbon dioxide through silicate weathering. The MMCT resulted in a drop in sea surface temperature (SST) of approximately 6 °C in the North Atlantic. The decrease in benthic foraminiferal δ 18 O values was most pronounced in the waters surrounding Antarctica, indicating that the cooling was most intense in that region. Around this time, the Mi3b glacial event, signifying a substantial expansion of Antarctic glaciers, occurred. The East Antarctic Ice Sheet (EAIS) exhibited marked stabilization following the MMCT. The intensification of glaciation led to a decoupling of sediment deposition patterns from the 405 kyr eccentricity cycle.
The Middle Miocene Warm Interval eventually ended around 11 Ma, giving way to the Late Miocene Cool Interval (LMCI). A significant, though transient, warming event occurred around 10.8–10.7 Ma. During the Late Miocene, the Earth's climate began to exhibit a high degree of similarity to present-day patterns. The 173 kyr obliquity modulation cycle, influenced by Earth's gravitational interactions with Saturn, became detectable in the Late Miocene. By 12 Ma, Oregon had transformed into a savanna environment, reminiscent of the western margins of the Sierra Nevada in northern California. Central Australia grew progressively drier, although southwestern Australia experienced a notable increase in wetness between approximately 12 and 8 Ma. The South Asian Winter Monsoon (SAWM) strengthened around 9.2–8.5 Ma. From 7.9 to 5.8 Ma, the East Asian Winter Monsoon (EAWM) intensified concurrently with a southward shift of the subarctic front. Greenland may have begun to host large glaciers as early as 8 to 7 Ma, although the climate, for the most part, remained warm enough to sustain forests there well into the Pliocene. Zhejiang, China, was demonstrably more humid than it is today. In the Great Rift Valley of Kenya, a gradual and progressive trend of increasing aridification was observed, though this trend was not unidirectional, and episodes of wet humidity continued to occur. Between 7 and 5.3 Ma, temperatures experienced another sharp decline during the Late Miocene Cooling (LMC), most likely attributable to a decrease in atmospheric carbon dioxide and a reduction in the amplitude of Earth's obliquity. During this period, the Antarctic ice sheet approached its present-day size and thickness. Ocean temperatures plummeted to near-modern values during the LMC; extratropical sea surface temperatures dropped substantially, by approximately 7–9 °C. The 41 kyr obliquity cycles became the dominant orbital control on climate around 7.7 Ma, and this dominance strengthened by 6.4 Ma. Benthic δ 18 O values indicate significant glaciation occurred from 6.26 to 5.50 Ma, during which glacial-interglacial cycles were governed by the 41 kyr obliquity cycle. A major reorganization of the carbon cycle occurred approximately 6 Ma, leading to a situation where continental carbon reservoirs no longer expanded during cold spells, as they had done during colder periods in the Oligocene and most of the Miocene. At the close of the Miocene, global temperatures rose again as the amplitude of Earth's obliquity increased, which in turn led to increased aridity in Central Asia. Around 5.5 Ma, the EAWM underwent a period of rapid intensification.
Life
Life during the Miocene Epoch was significantly shaped by the emergence of two new major biomes: kelp forests and grasslands. The expansion of grasslands provided new niches for a burgeoning population of grazers, including early forms of horses, rhinoceroses, and hippos. By the end of this epoch, an impressive ninety-five percent of modern plant species had already come into existence. In the oceans, modern bony fish genera were well established. A modern-style latitudinal biodiversity gradient began to emerge around 15 Ma.
Flora
The dragon blood tree, a striking and ancient plant, is thought to be a relic of the Mio-Pliocene Laurasian subtropical forests, which have since largely vanished from North Africa.
A crucial evolutionary development was the coevolution of gritty, fibrous, and fire-tolerant grasses with long-legged, gregarious ungulates possessing high-crowned teeth. This intricate relationship fueled a significant expansion of grassland-grazer ecosystems. Herds of large, swift grazers became the prey of formidable predators across vast expanses of open grasslands, displacing desert and woodland habitats and favoring browsers less.
The increased organic content and superior water retention of the deeper, richer grassland soils, coupled with the long-term burial of carbon in sediments, created a significant carbon and water vapor sink. This, in combination with the higher surface albedo and reduced evapotranspiration characteristic of grasslands, contributed to a cooler, drier global climate. C4 grasses, which possess a more efficient method of assimilating carbon dioxide and water compared to C3 grasses, began to gain ecological significance towards the end of the Miocene, between 6 and 7 million years ago, although their northward expansion was limited during the Late Miocene. The expansion of grasslands and the subsequent radiations among terrestrial herbivores are strongly correlated with fluctuations in atmospheric CO2 levels. However, one study attributes the expansion of grasslands not solely to a CO2 drop but also to increasing seasonality and aridity, exacerbated by a monsoon climate that promoted frequent wildfires. The Late Miocene expansion of grasslands left a discernible imprint on global carbon isotope records, indicating cascading effects on the global carbon cycle.
Cycads, ancient seed plants, began to re-diversify between 11.5 and 5 million years ago, following periods of decline due to climatic shifts. This suggests that modern cycads are not simply static "living fossils." Fossil leaves of Eucalyptus have been found in Miocene deposits in New Zealand, a location where the genus is not native today, indicating introduction from Australia.
Fauna
Both marine and terrestrial fauna during the Miocene exhibited a largely modern appearance, although marine mammals were less diverse than they are today. Only in isolated continents like South America and Australia did vastly different fauna persist.
In Eurasia, the genus richness of large mammals shifted southward towards lower latitudes from the Early to the Middle Miocene. Europe's large mammal diversity experienced a significant decline during the Late Miocene.
During the Early Miocene, several groups that had thrived in the Oligocene, such as nimravids (false saber-toothed cats), entelodonts (extinct pig-like mammals), and three-toed equids, remained diverse. As in the preceding Oligocene, oreodonts were still abundant, only to disappear at the very beginning of the Pliocene. The mammals of the later Miocene were more recognizable, featuring easily identifiable canids (dog family), bears, red pandas, procyonids (raccoon family), equids (horses and their relatives), beavers, deer, camelids, and whales. Alongside these modern groups, extinct lineages also flourished, including borophagine canids (bone-crushing dogs), certain gomphotheres (extinct elephant relatives), three-toed horses (a distinct lineage of early horses), and hornless rhinoceroses like Teleoceras and Aphelops. The Late Miocene also marks the extinction of the last surviving members of the hyaenodonts, a group of predatory mammals. Islands began to form between South and North America in the Late Miocene, facilitating the dispersal of ground sloths, such as Thinobadistes, to North America via oceanic dispersal. The expansion of silica-rich C4 grasses contributed to worldwide extinctions of herbivorous species that lacked high-crowned teeth. The mustelids (weasel family) diversified into their largest forms, with terrestrial predators like Ekorus, Eomellivora, and Megalictis, as well as bunodont otters such as Enhydriodon and Sivaonyx, appearing during this time. Eulipotyphlans (shrews, moles, and hedgehogs) were widespread in Europe, though less diverse in Southern Europe compared to regions further north due to increased aridity.
Birds also evolved significantly. Unequivocally recognizable species of dabbling ducks, plovers, typical owls, cockatoos, and crows make their appearance in the Miocene fossil record. By the end of the epoch, it is believed that all or nearly all modern bird groups were present; any post-Miocene bird fossils that cannot be confidently placed within the evolutionary tree are typically too poorly preserved rather than lacking definitive characteristics. Marine birds achieved their highest diversity ever during the course of this epoch.
The youngest known representatives of Choristodera, an extinct order of aquatic reptiles that first appeared in the Middle Jurassic, are found in the Miocene of Europe, belonging to the genus Lazarussuchus. This genus had been the only known surviving member of the group since the beginning of the Eocene.
The last known representatives of the archaic primitive mammal order Meridiolestida, which dominated South America during the Late Cretaceous, are known from the Miocene of Patagonia, represented by the mole-like Necrolestes.
The most recent known representatives of metatherians (the broader group to which marsupials belong) in Europe, Asia, and Africa are found in the Miocene. These include the European herpetotheriid Amphiperatherium, the Asian peradectids Siamoperadectes and Sinoperadectes, and the possible herpetotheriid Morotodon from the late Early Miocene of Uganda.
Approximately 100 species of apes inhabited the Earth during this time, ranging across Africa, Asia, and Europe, and exhibiting considerable variation in size, diet, and anatomy. Due to the scarcity of fossil evidence, it remains unclear which specific ape or apes contributed to the modern hominid clade, although molecular data suggests this divergence occurred between 18 and 13 million years ago. The first hominins (bipedal apes belonging to the human lineage) emerged in Africa at the very end of the Miocene, including species like Sahelanthropus, Orrorin, and an early form of Ardipithecus (A. kadabba). The chimpanzee–human divergence is thought to have occurred around this time. The evolution of bipedalism in apes at the close of the Miocene initiated an accelerated rate of faunal turnover in Africa. In contrast, European apes faced extinction at the end of the Miocene, likely due to increased habitat uniformity.
The expansion of grasslands in North America also triggered a remarkable evolutionary radiation among snakes. Previously, snakes were a relatively minor component of the North American fauna, but during the Miocene, the number of species and their prevalence increased dramatically. This period saw the first appearances of vipers and elapids in North America, alongside a significant diversification of Colubridae, including the origin of many modern genera such as Nerodia, Lampropeltis, Pituophis, and Pantherophis.
Arthropods were abundant during the Miocene, even in regions like Tibet, where they were previously thought to have been less diverse. Neoisopterans (a group of termites) diversified and expanded their range into areas they had not previously inhabited, such as Madagascar and Australia.
Oceanic
In the oceans, brown algae, commonly known as kelp, proliferated, forming vast forests that supported a wealth of new marine life, including various species of otters, fish, and numerous invertebrates.
Corals experienced a significant local decline along the northeastern coast of Australia during the Tortonian, likely a consequence of warming seawater temperatures.
Cetaceans (whales, dolphins, and porpoises) achieved their greatest diversity during the Miocene, with over 20 recognized genera of baleen whales compared to only six living genera today. This diversification is closely linked to the emergence of gigantic macro-predators, such as the megatoothed sharks and raptorial sperm whales. Prominent among these apex predators were Otodus megalodon and Livyatan melvillei. Other notable large sharks from this era included Otodus chubutensis, Isurus hastalis, and Hemipristis serra.
Crocodilians also displayed signs of diversification during the Miocene. The largest among them was a colossal caiman, Purussaurus, which inhabited South America. Another gigantic form was a false gharial, Rhamphosuchus, found in what is now modern-day India. A peculiar species, Mourasuchus, thrived alongside Purussaurus. This species had evolved a specialized filter-feeding mechanism and likely preyed on small fauna despite its immense size.
The youngest members of the Sebecidae, a clade of large terrestrial predatory crocodyliformes distantly related to modern crocodilians (from which they likely diverged over 180 million years ago), are known from the Miocene of South America.
The last known Desmostylians, a unique group of extinct marine mammals, thrived during this period before their ultimate extinction, leaving them as the only extinct order of marine mammals.
The pinnipeds, which first appeared near the end of the Oligocene, became increasingly adapted to an aquatic lifestyle. A prominent genus from this era was Allodesmus. A formidable walrus, Pelagiarctos, may have preyed upon other pinniped species, including Allodesmus.
Furthermore, the waters of South America witnessed the arrival of Megapiranha paranensis, a species of piranha considerably larger than modern-day piranhas.
New Zealand's Miocene fossil record is exceptionally rich. Marine deposits reveal a diverse array of cetaceans and penguins, illustrating the evolutionary pathways that led to their modern representatives. The Early Miocene Saint Bathans Fauna represents the only terrestrial fossil record from the Cenozoic era on the landmass. It showcases a remarkable variety of bird species, including early members of clades such as moa, kiwi), and adzebills. It also yields a diverse herpetofauna of sphenodontians, crocodiles, and turtles, as well as a rich terrestrial mammal fauna comprising various species of bats and the enigmatic Saint Bathans Mammal.
Microbiota
Microbial life within the igneous crust of the Fennoscandian Shield underwent a significant shift. Previously dominated by methanogens, it became primarily composed of sulphate-reducing prokaryotes. This change was triggered by fracture reactivation during the Pyrenean-Alpine orogeny, which facilitated the percolation of sulphate-reducing microbes into the Fennoscandian Shield via descending surface waters.
The diversity of diatoms during the Miocene exhibited an inverse correlation with carbon dioxide levels and global temperatures. Most modern lineages of diatoms had emerged by the Late Miocene.
Oceanic
Evidence derived from oxygen isotopes at Deep Sea Drilling Program sites suggests that ice began to accumulate in Antarctica around 36 Ma, during the Eocene. Further significant decreases in temperature during the Middle Miocene, occurring around 15 Ma, likely reflect increased ice growth in Antarctica. It can therefore be inferred that East Antarctica possessed some glaciers during the early to mid-Miocene (23–15 Ma). The cooling of the oceans was partly attributed to the formation of the Antarctic Circumpolar Current, and approximately 15 million years ago, the ice cap in the Southern Hemisphere began to develop into its present form. The Greenland ice cap, in contrast, formed later, during the Middle Pliocene, approximately 3 million years ago.
Middle Miocene disruption
The "Middle Miocene disruption" refers to a wave of extinctions affecting both terrestrial and aquatic life forms. This event occurred in the wake of the Miocene Climatic Optimum (18 to 16 Ma), around 14.8 to 14.5 million years ago, during the Langhian Stage of the mid-Miocene. A significant and permanent cooling step transpired between 14.8 and 14.1 Ma, associated with increased production of cold Antarctic deep waters and a major expansion of the East Antarctic ice sheet. The closure of the Indonesian Throughflow, which led to an accumulation of warm water in the western Pacific that then spread eastward, potentially reducing upwelling in the eastern Pacific, may also have contributed to this event. A Middle Miocene increase in δ 18 O values, indicating a relative enrichment of the heavier oxygen isotope, has been observed in the Pacific, the Southern Ocean, and the South Atlantic. Barium and uranium became enriched in seafloor sediments during this period.
Impact event
A substantial impact event is believed to have occurred either during the Miocene (23–5.3 Ma) or the Pliocene (5.3–2.6 Ma). This event is responsible for the formation of the Karakul crater) (52 km diameter) located in Tajikistan. The estimated age of this crater is less than 23 Ma, or potentially less than 5 Ma.
See also
- Geologic time scale
- List of fossil sites
- Category:Miocene animals