ThePliocene (/ˈplaɪ.əsiːn,ˈplaɪ.oʊ-/PLY-ə-seen,PLY-oh-;[6][7] alsoPleiocene)[8] is theepoch in thegeologic time scale that extends from 5.33 to 2.58[9] million years ago (Ma). It is the second and most recent epoch of theNeogene Period in theCenozoic Era. The Pliocene follows theMiocene Epoch and is followed by thePleistocene Epoch. Prior to the 2009 revision of the geologic time scale, which placed the four most recent major glaciations entirely within the Pleistocene, the Pliocene also included theGelasian Stage, which lasted from 2.59 to 1.81 Ma, and is now included in the Pleistocene.[10] The name comes fromAncient Greekπλείων (pleíōn), meaning "most", andκαινός (kainós), meaning "new, recent".
As with other older geologic periods, thegeological strata that define the start and end are well-identified but the exact dates of the start and end of the epoch are slightly uncertain. The boundaries defining the Pliocene are not set at an easily identified worldwide event but rather at regional boundaries between the warmer Miocene and the relatively cooler Pleistocene. The upper boundary was set at the start of the Pleistocene glaciations.
Charles Lyell (later Sir Charles) gave the Pliocene its name inPrinciples of Geology (volume 3, 1833).[11]
The name "Pliocene" comes fromAncient Greekπλείων (pleíōn), meaning "most", andκαινός (kainós), meaning "new",[12] and means roughly "continuation of the recent", referring to the essentially modern marinemollusc fauna.
In theParatethys area (centralEurope and parts of western Asia) the Pliocene contains the Dacian (roughly equal to the Zanclean) and Romanian (roughly equal to the Piacenzian and Gelasian together) stages. As usual in stratigraphy, there are many other regional and local subdivisions in use.
InBritain, the Pliocene is divided into the following stages (old to young): Gedgravian,Waltonian, Pre-Ludhamian, Ludhamian, Thurnian,Bramertonian or Antian,Pre-Pastonian or Baventian,Pastonian andBeestonian. In theNetherlands the Pliocene is divided into these stages (old to young): Brunssumian C, Reuverian A, Reuverian B, Reuverian C,Praetiglian,Tiglian A, Tiglian B, Tiglian C1-4b, Tiglian C4c, Tiglian C5, Tiglian C6 andEburonian. The exact correlations between these local stages and theInternational Commission on Stratigraphy (ICS) stages is not established.[18]
Mid-Pliocene reconstructed annual sea surface temperature anomaly
During the Pliocene epoch (5.3 to 2.6 million years ago (Ma)), the Earth's climate became cooler and drier, as well as more seasonal, marking a transition between the relatively warmMiocene to the coolerPleistocene.[19] However, the beginning of the Pliocene was marked by an increase in global temperatures relative to the coolerMessinian. This increase was related to the 1.2 million yearobliquity amplitude modulation cycle.[20] By 3.3–3.0 Ma, during theMid-Piacenzian Warm Period (mPWP), global average temperature was 2–3 °C higher than today,[21] while carbon dioxide levels were the same as today (400 ppm).[22] Global sea level was about 25 m higher,[23] though its exact value is uncertain.[24][25] The northern hemisphere ice sheet was ephemeral before the onset of extensiveglaciation overGreenland that occurred in the late Pliocene around 3 Ma.[26] The formation of an Arcticice cap is signaled by an abrupt shift inoxygenisotope ratios andice-rafted cobbles in theNorth Atlantic andNorth Pacific Ocean beds.[27] Mid-latitude glaciation was probably underway before the end of the epoch. The global cooling that occurred during the Pliocene may have accelerated on the disappearance of forests and the spread of grasslands and savannas.[28]
During the Pliocene the earthclimate system response shifted from a period of high frequency-low amplitude oscillation dominated by the 41,000-year period of Earth'sobliquity to one of low-frequency, high-amplitude oscillation dominated by the 100,000-year period of theorbital eccentricity characteristic of the Pleistocene glacial-interglacial cycles.[29]
During the late Pliocene and early Pleistocene, 3.6 to 2.6 Ma, the Arctic was much warmer than it is at the present day (with summer temperatures some 8 °C warmer than today). That is a key finding of research into a lake-sediment core obtained in Eastern Siberia, which is of exceptional importance because it has provided the longest continuous late Cenozoic land-based sedimentary record thus far.[30]
During the late Zanclean, Italy remained relatively warm and humid.[31]Central Asia became more seasonal during the Pliocene, with colder, drier winters and wetter summers, which contributed to an increase in the abundance of C4 plants across the region.[32] In theLoess Plateau,δ13C values of occluded organic matter increased by 2.5% while those of pedogenic carbonate increased by 5% over the course of the Late Miocene and Pliocene, indicating increased aridification.[33] Further aridification of Central Asia was caused by the development of Northern Hemisphere glaciation during the Late Pliocene.[34] A sediment core from the northern South China Sea shows an increase in dust storm activity during the middle Pliocene.[35] The South Asian Summer Monsoon (SASM) increased in intensity after 2.95 Ma, likely because of enhanced cross-equatorial pressure caused by the reorganisation of theIndonesian Throughflow.[36]
In the south-centralAndes, an arid period occurred from 6.1 to 5.2 Ma, with another occurring from 3.6 to 3.3 Ma. These arid periods are coincident with global cold periods, during which the position of theSouthern Hemispherewesterlies shifted northward and disrupted the South American Low Level Jet, which brings moisture to southeastern South America.[37]
From around 3.8 Ma to about 3.3 Ma, North Africa experienced an extended humid period.[38] In northwestern Africa, tropical forests extended up to Cape Blanc during the Zanclean until around 3.5 Ma. During the Piacenzian, from about 3.5 to 2.6 Ma, the region was forested at irregular intervals and contained a significant Saharan palaeoriver until 3.35 Ma, when trade winds began to dominate over fluvial transport of pollen. Around 3.26 Ma, a strong aridification event that was followed by a return to more humid conditions, which was itself followed by another aridification around 2.7 Ma. From 2.6 to 2.4 Ma, vegetation zones began repeatedly shifting latitudinally in response to glacial-interglacial cycles.[39]
The climate of eastern Africa was very similar to what it is today. Unexpectedly, the expansion of grasslands in eastern Africa during this epoch appears to have been decoupled from aridification and not caused by it, as evidenced by their asynchrony.[40]
Southwestern Australia hostedheathlands,shrublands, andwoodlands with a greater species diversity compared to today during the Middle and Late Pliocene. Three different aridification events occurred around 2.90, 2.59, and 2.56 Ma, and may have been linked to the onset of continental glaciation in the Arctic, suggesting that vegetation changes in Australia during the Pliocene behaved similarly to during the Late Pleistocene and were likely characterised by comparable cycles of aridity and humidity.[41]
The equatorial Pacific Oceansea surface temperature gradient was considerably lower than it is today. Mean sea surface temperatures in the east were substantially warmer than today but similar in the west. This condition has been described as a permanentEl Niño state, or "El Padre".[42] Several mechanisms have been proposed for this pattern, includingincreased tropical cyclone activity.[43]
The extent of theWest Antarctic Ice Sheet oscillated at the 40kyr period of Earth's obliquity. Ice sheet collapse occurred when the global average temperature was 3 °C warmer than today and carbon dioxide concentration was at 400 ppmv. This resulted in open waters in theRoss Sea.[44] Global sea-level fluctuation associated with ice-sheet collapse was probably up to 7 meters for the west Antarctic and 3 meters for the east Antarctic. Model simulations are consistent with reconstructed ice-sheet oscillations and suggest a progression from a smaller to a larger West Antarctic ice sheet in the last 5 million years. Intervals of ice sheet collapse were much more common in the early-mid Pliocene (5 Ma – 3 Ma), after three-million-year intervals with modern or glacial ice volume became longer and collapse occurs only at times when warmer global temperature coincide with strong austral summer insolation anomalies.[45]
Examples of migrant species in the Americas after the formation of the Isthmus of Panama. Olive green silhouettes denote North American species with South American ancestors; blue silhouettes denote South American species of North American origin.
Continents continued todrift, moving from positions possibly as far as 250 km from their present locations to positions only 70 km from their current locations.South America became linked to North America through theIsthmus of Panama during the Pliocene, making possible theGreat American Interchange and bringing a nearly complete end to South America's distinctivenative ungulate fauna,[46] though other South American lineages like itspredatory mammals were already extinct by this point and others likexenarthrans continued to do well afterwards. The formation of the Isthmus had major consequences on global temperatures, since warm equatorial ocean currents were cut off and an Atlantic cooling cycle began, with cold Arctic and Antarctic waters decreasing temperatures in the now-separated Atlantic Ocean.[47]
During the Late Pliocene, the Himalayas became less active in their uplift, as evidenced by sedimentation changes in theBengal Fan.[50]
Theland bridge betweenAlaska andSiberia (Beringia) was first flooded near the start of the Pliocene, allowing marine organisms to spread between the Arctic and Pacific Oceans. The bridge would continue to be periodically flooded and restored thereafter.[51]
During the Pliocene parts of southern Norway and southern Sweden that had been near sea level rose. In Norway this rise elevated theHardangervidda plateau to 1200 m in the Early Pliocene.[56] In Southern Sweden similar movements elevated theSouth Swedish highlands leading to a deflection of the ancientEridanos river from its original path across south-central Sweden into a course south of Sweden.[57]
The Pliocene is bookended by two significant events in the evolution of human ancestors. The first is the appearance of thehomininAustralopithecus anamensis in the early Pliocene, around 4.2 million years ago.[58][59][60] The second is the appearance ofHomo, the genus that includesmodern humans and their closest extinct relatives, near the end of the Pliocene at 2.6 million years ago.[61] Key traits that evolved among hominins during the Pliocene include terrestrial bipedality and, by the end of the Pliocene,encephalized brains (brains with a largeneocortex relative to body mass[62][a] and stone tool manufacture.[63]
Improvements indating methods and in the use ofclimate proxies have provided scientists with the means to test hypotheses of the evolution of human ancestors.[63][64] Early hypotheses of the evolution of human traits emphasized the selective pressures produced by particular habitats. For example, many scientists have long favored thesavannah hypothesis. This proposes that the evolution of terrestrial bipedality and other traits was an adaptive response to Pliocene climate change that transformed forests into more opensavannah. This was championed byGrafton Elliot Smith in his 1924 book,The Evolution of Man, as "the unknown world beyond the trees", and was further elaborated byRaymond Dart as thekiller ape theory.[65] Other scientists, such asSherwood L. Washburn, emphasized an intrinsic model of hominin evolution. According to this model, early evolutionary developments triggered later developments. The model placed little emphasis on the surrounding environment.[66] Anthropologists tended to focus on intrinsic models while geologists and vertebrate paleontologists tended to put greater emphasis on habitats.[67]
Alternatives to the savanna hypothesis include the woodland/forest hypothesis, which emphasizes the evolution of hominins in closed habitats, or hypotheses emphasizing the influence of colder habitats at higher latitudes or the influence of seasonal variation. More recent research has emphasized the variability selection hypothesis, which proposes that variability in climate fostered development of hominin traits.[63] Improved climate proxies show that the Pliocene climate of east Africa was highly variable, suggesting that adaptability to varying conditions was more important in driving hominin evolution than the steady pressure of a particular habitat.[62]
The change to a cooler, drier, more seasonal climate had considerable impacts on Pliocene vegetation, reducing tropical species worldwide.Deciduous forests proliferated,coniferous forests andtundra covered much of the north, andgrasslands spread on all continents (except Antarctica). Eastern Africa in particular saw a huge expansion ofC4 grasslands.[68] Tropical forests were limited to a tight band around the equator, and in addition to drysavannahs,deserts appeared in Asia and Africa.[69][failed verification]
Both marine and continental faunas were essentially modern, although continental faunas were a bit more primitive than today.
The land mass collisions meant great migration and mixing of previously isolated species, such as in theGreat American Interchange.Herbivores got bigger, as did specialized predators.
InEurasia rodents did well, whileprimate distribution declined.Elephants,gomphotheres andstegodonts were successful in Asia (the largest land mammals of the Pliocene were such proboscideans asDeinotherium,Anancus, andMammut borsoni,[71]) though proboscidean diversity declined significantly during the Late Pliocene.[72]Hyraxes migrated north from Africa.Horse diversity declined, while tapirs and rhinos did fairly well.Bovines andantelopes were successful; some camel species crossed into Asia from North America.Hyenas and earlysaber-toothed cats appeared, joining other predators including dogs, bears, and weasels.
In Africa, climatic variability played little role in mammalian extinction and speciation rates.[73] Africa was dominated by hoofed animals, and primates continued their evolution, withaustralopithecines (some of the firsthominins) andbaboon-like monkeys such as theDinopithecus appearing in the late Pliocene. Rodents were successful, and elephant populations increased. Cows and antelopes continued diversification and overtookpigs in numbers of species. Earlygiraffes appeared. Horses and modern rhinos came onto the scene. Bears, dogs and weasels (originally from North America) joined cats, hyenas andcivets as the African predators, forcing hyenas to adapt as specialized scavengers. Most mustelids in Africa declined as a result of increased competition from the new predators, althoughEnhydriodon omoensis remained an unusually successful terrestrial predator.
Themarsupials remained the dominant Australian mammals, with herbivore forms includingwombats andkangaroos, and the hugeDiprotodon. Carnivorous marsupials continued hunting in the Pliocene, includingdasyurids, the dog-likethylacine and cat-likeThylacoleo. The first rodents arrived in Australia. The modernplatypus, amonotreme, appeared.
At either the boundary between the Zanclean and Piacenzian or the end of the Pliocene, a massive avifaunal turnover took place inCentral Asia.[74] The predatory South Americanphorusrhacids were rare in this time; among the last wasTitanis, a large phorusrhacid that migrated to North America and rivaled mammals as top predator. Other birds probably evolved at this time, some modern (such as the generaCygnus,Bubo,Struthio andCorvus), some now extinct.
Alligators andcrocodiles died out in Europe as the climate cooled.Venomous snake genera continued to increase as more rodents and birds evolved.Rattlesnakes first appeared in the Pliocene. The modern speciesAlligator mississippiensis, having evolved in the Miocene, continued into the Pliocene, except with a more northern range; specimens have been found in very late Miocene deposits ofTennessee.Giant tortoises still thrived in North America, with genera likeHesperotestudo.Madtsoidsnakes were still present in Australia. The amphibian orderAllocaudata became extinct.
In the Western Atlantic, assemblages ofbivalves exhibited remarkable stasis with regards to theirbasal metabolic rates throughout the various climatic changes of the Pliocene.[75]
The Pliocene was a high water mark for species diversity among Caribbean corals. From 5 to 2 Ma, coral species origination rates were relatively high in the Caribbean, although a noticeable extinction event and drop in diversity occurred at the end of this interval.[76]
Oceans continued to be relatively warm during the Pliocene, though they continued cooling. TheArctic ice cap formed, drying the climate and increasing cool shallow currents in the North Atlantic. Deep cold currents flowed from the Antarctic.
The formation of the Isthmus of Panama about 3.5 million years ago[77] cut off the final remnant of what was once essentially a circum-equatorial current that had existed since the Cretaceous and the earlyCenozoic. This may have contributed to further cooling of the oceans worldwide.
In 2002, Narciso Benítezet al. calculated that roughly 2 million years ago, around the end of the Pliocene Epoch, a group of brightO and B stars called theScorpius–CentaurusOB association passed within 130 light-years of Earth and that one or moresupernova explosions gave rise to a feature known as theLocal Bubble.[78] Such a close explosion could have damaged the Earth'sozone layer and caused the extinction of some ocean life (at its peak, a supernova of this size could have the sameabsolute magnitude as an entire galaxy of 200 billion stars).[79][80] Radioactiveiron-60 isotopes that have been found in ancient seabed deposits further back this finding, as there are no natural sources for this radioactive isotope on Earth, but they can be produced in supernovae.[81] Furthermore, iron-60 residues point to a huge spike 2.6 million years ago, but an excess scattered over 10 million years can also be found, suggesting that there may have been multiple, relatively close supernovae.[81]
In 2019, researchers found more of these interstellar iron-60 isotopes in Antarctica, which have been associated with theLocal Interstellar Cloud.[82]
^Because of the 2009 reassignment of the Pliocene-Pleistocene boundary from 1.8 to 2.6 million years ago, older papers on Pliocene hominin evolution sometimes include events that would now be regarded as taking place in the early Pleistocene.
Lyell, Charles (1833).Principles of Geology, …. Vol. 3. London, England: John Murray. p. 53. From p. 53: "We derive the term Pliocene from πλειων, major, and χαινος, recens, as the major part of the fossil testacea of this epoch are referrible to recent species*."
^Tedford, Richard H.; Albright, L. Barry; Barnosky, Anthony D.; Ferrusquia-Villafranca, Ismael; Hunt, Robert M.; Storer, John E.; Swisher, Carl C.; Voorhies, Michael R.; Webb, S. David; Whistler, David P. (31 December 2004). "6. Mammalian Biochronology of the Arikareean Through Hemphillian Interval (Late Oligocene Through Early Pliocene Epochs)".Late Cretaceous and Cenozoic Mammals of North America:169–231.doi:10.7312/wood13040-008.ISBN978-0-231-13040-0.
^Flynn, J.; Swisher, C.C. III (1995). "Cenozoic South American Land Mammal Ages: correlation to global geochronology". In William A. Berggren; Dennis V. Kent; Marie-Pierre Aubry; Jan Hardenbol (eds.).Geochronology Time Scales and Global Stratigraphic Correlation. Society for Sedimentary Geology. pp. 317–333.doi:10.2110/pec.95.04.0317.
^Robinson, M.; Dowsett, H. J.; Chandler, M. A. (2008). "Pliocene role in assessing future climate impacts".Eos, Transactions, American Geophysical Union.89 (49):501–502.Bibcode:2008EOSTr..89..501R.doi:10.1029/2008eo490001.
^Gautier, F., Clauzon, G., Suc, J.P., Cravatte, J., Violanti, D., 1994. Age and duration of the Messinian salinity crisis. C.R. Acad. Sci., Paris (IIA) 318, 1103–1109.
^Deméré, Thomas A. (1983). "The Neogene San Diego basin: a review of the marine Pliocene San Diego formation".Cenozoic Marine Sedimentation, Pacific Margin. Pacific Section,m Society for Sedimentary Geology. pp. 187–195. Retrieved7 June 2021.
^Selli, Raimondo (September 1965). "The Pliocene-Pleistocene boundary in Italian marine sections and its relationship to continental stratigraphies".Progress in Oceanography.4:67–86.Bibcode:1965PrOce...4...67S.doi:10.1016/0079-6611(65)90041-8.
^Lewis, Barry; et al. (2013).Understanding Humans: Introduction to Physical Anthropology and Archaeology (11th ed.). Belmont, CA: Wadsworth Publishing.{{cite book}}: CS1 maint: location missing publisher (link)
^Stringer CB (1994). "Evolution of early humans". In Jones S, Martin R, Pilbeam D (eds.).The Cambridge Encyclopedia of Human Evolution. Cambridge: Cambridge University Press. p. 242.
^abPotts, R. (2007), Bobe, René; Alemseged, Zeresenay; Behrensmeyer, Anna K. (eds.), "Environmental hypotheses of Pliocene human evolution",Hominin Environments in the East African Pliocene: An Assessment of the Faunal Evidence, Vertebrate Paleobiology and Paleoanthropology Series, Dordrecht: Springer Netherlands, pp. 25–49,doi:10.1007/978-1-4020-3098-7_2,ISBN978-1-4020-3098-7
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