View source for Oligocene

==Climate==
[[Image:65 Myr Climate Change.png|300px|thumb|Climate change during the last 65 million years<ref>{{Cite journal| doi = 10.1126/science.1059412| pmid = 11326091| year = 2001| last1 = Zachos | first1 = J.| last2 = Pagani | first2 = M.| last3 = Sloan | first3 = L.| last4 = Thomas | first4 = E.| last5 = Billups | first5 = K.| title = Trends, rhythms, and aberrations in global climate 65 Ma to present| volume = 292| issue = 5517| pages = 686–693| journal = Science|bibcode = 2001Sci...292..686Z | s2cid = 2365991| url = http://doc.rero.ch/record/13508/files/PAL_E304.pdf}}</ref>]]
Climate during the Oligocene reflected a general cooling trend following the [[Early Eocene Climatic Optimum]]. This transformed the Earth's climate from a greenhouse to an icehouse climate.{{sfn|Prothero|2005|p=473}}

===Eocene-Oligocene transition and Oi1 event===
The Eocene-Oligocene transition was a major cooling event and reorganization of the biosphere,<ref>{{cite book |last1=Berggren |first1=William A. |last2=Prothero |first2=Donald R. |title=Eocene-Oligocene Climatic and Biotic Evolution |chapter=Eocene-Oligocene climatic and biotic evolution: an overview |date=1992 |doi=10.1515/9781400862924.1 |publisher=Princeton University Press |page=1|isbn=978-1-4008-6292-4 }}</ref><ref>{{cite book |last1=Coxall |first1=H.K. |last2=Pearson |first2=P.N. |year=2007 |chapter=The Eocene–Oligocene Transition |editor-last1=Williams |editor-first1=M. |editor-last2=Haywood |editor-first2=A.M. |editor-last3=Gregory |editor-first3=F.J. |editor-last4=Schmidt |editor-first4=D.N. |title=Deep-Time Perspectives on Climate Change: Marrying the Signal from Computer Models and Biological Proxies |series=The Micropalaeontological Society, Special Publications |publisher=The Geological Society |location=London |pages=351–387}}</ref> being part of a broader trend of global cooling lasting from the [[Bartonian]] to the Rupelian.<ref name="LauretanoEtAl2021">{{cite journal |last1=Lauretano |first1=Vittoria |last2=Kennedy-Asser |first2=Alan T. |last3=Korasidis |first3=Vera A. |last4=Wallace |first4=Malcolm W. |last5=Valdes |first5=Paul J. |last6=Lunt |first6=Daniel J. |last7=Pancost |first7=Richard T. |last8=Naafs |first8=B. David A. |date=2 August 2021 |title=Eocene to Oligocene terrestrial Southern Hemisphere cooling caused by declining pCO2 |url=https://www.nature.com/articles/s41561-021-00788-z |journal=[[Nature Geoscience]] |volume=14 |issue=9 |pages=659–664 |doi=10.1038/s41561-021-00788-z |bibcode=2021NatGe..14..659L |s2cid=236781214 |access-date=8 December 2022|hdl=1983/45dea1c1-704b-469d-9fce-7d760100a309 |hdl-access=free |url-access=subscription }}</ref><ref>{{cite journal |last1=Gutiérrez |first1=Néstor M. |last2=Pino |first2=Juan Pablo |last3=Le Roux |first3=Jacobus P. |last4=Petroza |first4=Viviana |last5=Orayzun |first5=José Luis |last6=Hinojosa |first6=Luis Felipe |date=15 August 2019 |title=An Oligocene microthermal forest dominated by Nothofagus in Sierra Baguales, Chilean Patagonia: Response to global cooling and tectonic events |url=https://www.sciencedirect.com/science/article/pii/S0031018218303961 |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=528 |pages=1–13 |doi=10.1016/j.palaeo.2019.04.006 |bibcode=2019PPP...528....1G |s2cid=149478504 |access-date=4 December 2022|url-access=subscription }}</ref> The transition is marked by the Oi1 event, an oxygen isotope excursion occurring approximately 33.55 million years ago,<ref name="JovaneEtAl2006GGG">{{cite journal |last1=Jovane |first1=Luigi |last2=Florindo |first2=Fabio |last3=Sprovieri |first3=Mario |last4=Pälike |first4=Heiko |date=27 July 2006 |title=Astronomic calibration of the late Eocene/early Oligocene Massignano section (central Italy) |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2005GC001195 |journal=[[Geochemistry, Geophysics, Geosystems]] |volume=7 |issue=7 |pages=1–10 |article-number=2005GC001195 |doi=10.1029/2005GC001195 |bibcode=2006GGG.....7.7012J |s2cid=127299427 |access-date=6 December 2022|url-access=subscription }}</ref> during which [[Marine isotope stages|oxygen isotope ratios]] decreased by 1.3{{char|‰}}. About 0.3–0.4{{char|‰}} of this is estimated to be due to major expansion of Antarctic ice sheets. The remaining 0.9 to 1.0{{char|‰}} was due to about {{convert|5 to 6|C-change|sigfig=1|sp=us}} of [[global cooling]].{{sfn|Prothero|2005|p=473}} The transition likely took place in three closely spaced steps over the period from 33.8 to 33.5 mya. By the end of the transition, sea levels had dropped by {{convert|105|m||sp=us}}, and ice sheets were 25% greater in extent than in the modern world.<ref>{{cite journal |last1=Katz |first1=Miriam E. |last2=Miller |first2=Kenneth G. |last3=Wright |first3=James D. |last4=Wade |first4=Bridget S. |last5=Browning |first5=James V. |last6=Cramer |first6=Benjamin S. |last7=Rosenthal |first7=Yair |title=Stepwise transition from the Eocene greenhouse to the Oligocene icehouse |journal=Nature Geoscience |date=May 2008 |volume=1 |issue=5 |pages=329–334 |doi=10.1038/ngeo179|bibcode=2008NatGe...1..329K }}</ref>

The effects of the transition can be seen in the geological record at many locations around the world. Ice volumes rose as temperature and sea levels dropped.<ref>{{cite journal |last1=Miller |first1=K. G. |last2=Browning |first2=J. V. |last3=Aubry |first3=M.-P. |last4=Wade |first4=B. S. |last5=Katz |first5=M. E. |last6=Kulpecz |first6=A. A. |last7=Wright |first7=J. D. |title=Eocene-Oligocene global climate and sea-level changes: St. Stephens Quarry, Alabama |journal=Geological Society of America Bulletin |date=1 January 2008 |volume=120 |issue=1–2 |pages=34–53 |doi=10.1130/B26105.1|bibcode=2008GSAB..120...34M }}</ref> [[Endorheic basin|Playa]] lakes of the Tibetan Plateau disappeared at the transition, pointing to cooling and aridification of central Asia.<ref>{{cite journal |last1=Dupont-Nivet |first1=Guillaume |last2=Krijgsman |first2=Wout |last3=Langereis |first3=Cor G. |last4=Abels |first4=Hemmo A. |last5=Dai |first5=Shuang |last6=Fang |first6=Xiaomin |title=Tibetan plateau aridification linked to global cooling at the Eocene–Oligocene transition |journal=Nature |date=February 2007 |volume=445 |issue=7128 |pages=635–638 |doi=10.1038/nature05516|pmid=17287807 |s2cid=2039611 }}</ref> Pollen and spore counts in marine sediments of the Norwegian-Greenland Sea indicate a drop in winter temperatures at high latitudes of about {{convert|5|C-change||sp=us}} just prior to the Oi1 event.<ref>{{cite journal |last1=Eldrett |first1=James S. |last2=Greenwood |first2=David R. |last3=Harding |first3=Ian C. |last4=Huber |first4=Matthew |title=Increased seasonality through the Eocene to Oligocene transition in northern high latitudes |journal=Nature |date=June 2009 |volume=459 |issue=7249 |pages=969–973 |doi=10.1038/nature08069|pmid=19536261 |bibcode=2009Natur.459..969E |s2cid=4365115 }}</ref> Borehole dating from the Southeast Faroes drift indicates that deep-ocean circulation from the Arctic Ocean to the North Atlantic Ocean began in the early Oligocene.<ref>{{cite journal |last1=Davies |first1=Richard |last2=Cartwright |first2=Joseph |last3=Pike |first3=Jennifer |last4=Line |first4=Charles |title=Early Oligocene initiation of North Atlantic Deep Water formation |journal=Nature |date=April 2001 |volume=410 |issue=6831 |pages=917–920 |doi=10.1038/35073551|pmid=11309613 |bibcode=2001Natur.410..917D |s2cid=4429436 |url=https://durham-repository.worktribe.com/output/1554932 }}</ref>

The best terrestrial record of Oligocene climate comes from North America, where temperatures dropped by {{convert|7 to 11|C-change||sp=us}} in the earliest Oligocene. This change is seen from Alaska to the Gulf Coast. Upper Eocene [[paleosols]] reflect annual precipitation of over a meter of rain, but early Oligocene precipitation was less than half this.{{sfn|Prothero|2005|p=475}}<ref name="Late Eocene and Oligocene paleosols">{{cite journal|last1=Retallack |first1=G.J. |year=1983 |title=Late Eocene and Oligocene paleosols from Badlands National Park, South Dakota |volume=193 |journal=Geological Society of America Special Paper |series=Geological Society of America Special Papers |page=1 |doi=10.1130/SPE193-p1 |bibcode=1983GSASP.193....1. |isbn=978-0-8137-2193-4}}</ref> In central North America, the cooling was by 8.2 ± 3.1&nbsp;°C over a period of 400,000 years, though there is little indication of significant increase in aridity during this interval.<ref name="Large temperature drop across the E">{{cite journal |last1=Zanazzi |first1=Alessandro |last2=Kohn |first2=Matthew J. |last3=MacFadden |first3=Bruce J. |last4=Terry |first4=Dennis O. |title=Large temperature drop across the Eocene–Oligocene transition in central North America |journal=Nature |date=February 2007 |volume=445 |issue=7128 |pages=639–642 |doi=10.1038/nature05551|pmid=17287808 |s2cid=4301193 }}</ref> Ice-rafted debris in the Norwegian-Greenland Sea indicated that glaciers had appeared in Greenland by the start of the Oligocene.<ref>{{cite journal |last1=Eldrett |first1=James S. |last2=Harding |first2=Ian C. |last3=Wilson |first3=Paul A. |last4=Butler |first4=Emily |last5=Roberts |first5=Andrew P. |title=Continental ice in Greenland during the Eocene and Oligocene |journal=Nature |date=March 2007 |volume=446 |issue=7132 |pages=176–179 |doi=10.1038/nature05591|pmid=17287724 |bibcode=2007Natur.446..176E |s2cid=4372596 }}</ref>

Continental ice sheets in Antarctica reached sea level during the transition.{{sfn|Coxall|Pearson|2007|pages=351–387}}{{sfn|Berggren|Prothero|1992|p=1}}<ref>{{cite journal |last1=O'Brien |first1=Charlotte L. |last2=Huber |first2=Matthew |last3=Thomas |first3=Ellen |author3-link=Ellen Thomas (scientist) |last4=Pagani |first4=Mark |last5=Super |first5=James R. |last6=Elder |first6=Leanne E. |last7=Hull |first7=Pincelli M. |title=The enigma of Oligocene climate and global surface temperature evolution |journal=Proceedings of the National Academy of Sciences |date=13 October 2020 |volume=117 |issue=41 |pages=25302–25309 |doi=10.1073/pnas.2003914117|pmid=32989142 |pmc=7568263 |bibcode=2020PNAS..11725302O |doi-access=free }}</ref> Glacially rafted debris of early Oligocene age in the [[Weddell Sea]] and [[Kerguelen Plateau]], in combination with Oi1 isotope shift, provides unambiguous evidence of a continental ice sheet on Antarctica by the early Oligocene.{{sfn|Berggren|Prothero|1992|pp=5–6}}

The causes of the Eocene-Oligocene transition are not yet fully understood.{{sfn|O'Brien|Huber|Thomas|Pagani|2020|p=25302}}
The timing is wrong for this to be caused either by known [[impact event]]s or by the volcanic activity on the Ethiopean Plateau.{{sfn|Prothero|2005|p=474}} Two other possible drivers of climate change, not mutually exclusive, have been proposed.{{sfn|O'Brien|Huber|Thomas|Pagani|2020|p=25302}} The first is thermal isolation of the continent of Antarctica by development of the [[Antarctic Circumpolar Current]].{{sfn|Prothero|2005|pp=474, 476}}{{sfn|Berggren|Prothero|1992|p=1}}<ref name=Torsvik/> Deep sea cores from south of New Zealand suggest that cold deep-sea currents were present by the early Oligocene.{{sfn|Prothero|2005|p=474}} However, the timing of this event remains controversial.<ref>{{cite journal |last1=Lyle |first1=Mitchell |last2=Gibbs |first2=Samantha |last3=Moore |first3=Theodore C. |last4=Rea |first4=David K. |title=Late Oligocene initiation of the Antarctic Circumpolar Current: Evidence from the South Pacific |journal=Geology |date=2007 |volume=35 |issue=8 |page=691 |doi=10.1130/G23806A.1|bibcode=2007Geo....35..691L }}</ref> The other possibility, for which there is considerable evidence, is a drop in atmospheric [[carbon dioxide]] levels ([[pCO2]]) during the transition.{{sfn|O'Brien|Huber|Thomas|Pagani|2020|p=25302}}<ref>{{cite journal |last1=Francis |first1=J.E. |last2=Marenssi |first2=S. |last3=Levy |first3=R. |last4=Hambrey |first4=M. |last5=Thorn |first5=V.C. |last6=Mohr |first6=B. |last7=Brinkhuis |first7=H. |last8=Warnaar |first8=J. |last9=Zachos |first9=J. |last10=Bohaty |first10=S. |last11=DeConto |first11=R. |title=Chapter 8 From Greenhouse to Icehouse – The Eocene/Oligocene in Antarctica |journal=Developments in Earth and Environmental Sciences |date=2008 |volume=8 |pages=309–368 |doi=10.1016/S1571-9197(08)00008-6|bibcode=2008DvEES...8..309F |isbn=978-0-444-52847-6 }}</ref><ref name="LauretanoEtAl2021" /> The pCO2 is estimated to have dropped just before the transition, to 760 ppm at the peak of ice sheet growth, then rebounded slightly before resuming a more gradual fall.<ref>{{cite journal |last1=Pearson |first1=Paul N. |last2=Foster |first2=Gavin L. |last3=Wade |first3=Bridget S. |title=Atmospheric carbon dioxide through the Eocene–Oligocene climate transition |journal=Nature |date=October 2009 |volume=461 |issue=7267 |pages=1110–1113 |doi=10.1038/nature08447|pmid=19749741 |bibcode=2009Natur.461.1110P |s2cid=205218274 }}</ref> Climate modeling suggests that glaciation of Antarctica took place only when pCO2 dropped below a critical threshold value.{{sfn|Francis|Marenssi|Levy|Hambrey|2008}}

[[Brachiopod]] oxygen isotope ratios from New Zealand suggest that a proto-Subtropical Convergence developed during the Early Oligocene, with northern New Zealand being subtropical and southern and eastern New Zealand being cooled by cold, subantarctic water.<ref>{{Cite journal |last1=Buening |first1=Nancy |last2=Carlson |first2=Sandra J. |last3=Spero |first3=Howard J. |last4=Lee |first4=Daphne E. |date=April 1998 |title=Evidence for the Early Oligocene formation of a proto-Subtropical Convergence from oxygen isotope records of New Zealand Paleogene brachiopods |url=https://linkinghub.elsevier.com/retrieve/pii/S0031018297001132 |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |language=en |volume=138 |issue=1–4 |pages=43–68 |doi=10.1016/S0031-0182(97)00113-2 |bibcode=1998PPP...138...43B |access-date=4 May 2024 |via=Elsevier Science Direct|url-access=subscription }}</ref>

===Middle Oligocene climate and the Oi2 event===
Oligocene climate following the Eocene-Oligocene event is poorly known.{{sfn|O'Brien|Huber|Thomas|Pagani|2020|pages=25302–25309}} There were several pulses of glaciation in middle Oligocene, about the time of the Oi2 oxygen isotope shift. This led to the largest drop of sea level in past 100 million years, by about {{convert|75|m|sp=us}}. This is reflected in a mid-Oligocene incision of continental shelves and unconformities in marine rocks around the world.{{sfn|Prothero|2005|p=475}}

Some evidence suggests that the climate remained warm at high latitudes{{sfn|O'Brien|Huber|Thomas|Pagani|2020|pages=25302–25309}}<ref>{{cite journal |last1=Lear |first1=C. H. |title=Cenozoic Deep-Sea Temperatures and Global Ice Volumes from Mg/Ca in Benthic Foraminiferal Calcite |journal=Science |date=14 January 2000 |volume=287 |issue=5451 |pages=269–272 |doi=10.1126/science.287.5451.269|pmid=10634774 |bibcode=2000Sci...287..269L }}</ref> even as ice sheets experienced cyclical growth and retreat in response to [[orbital forcing]] and other climate drivers.<ref>{{cite journal |last1=Pälike |first1=Heiko |last2=Norris |first2=Richard D. |last3=Herrle |first3=Jens O. |last4=Wilson |first4=Paul A. |last5=Coxall |first5=Helen K. |last6=Lear |first6=Caroline H. |last7=Shackleton |first7=Nicholas J. |last8=Tripati |first8=Aradhna K. |last9=Wade |first9=Bridget S. |title=The Heartbeat of the Oligocene Climate System |journal=Science |date=22 December 2006 |volume=314 |issue=5807 |pages=1894–1898 |doi=10.1126/science.1133822|pmid=17185595 |bibcode=2006Sci...314.1894P |s2cid=32334205 |url=https://eprints.soton.ac.uk/42935/2/Paelike_etal_Science2006_supplMat.pdf }}</ref> Other evidence indicates significant cooling at high latitudes.{{sfn|Coxall|Pearson|2007|pages=351–387}}<ref>{{cite journal |last1=Liu |first1=Z. |last2=Pagani |first2=M. |last3=Zinniker |first3=D. |last4=DeConto |first4=R. |last5=Huber |first5=M. |last6=Brinkhuis |first6=H. |last7=Shah |first7=S. R. |last8=Leckie |first8=R. M. |last9=Pearson |first9=A. |title=Global Cooling During the Eocene-Oligocene Climate Transition |journal=Science |date=27 February 2009 |volume=323 |issue=5918 |pages=1187–1190 |doi=10.1126/science.1166368|pmid=19251622 |bibcode=2009Sci...323.1187L |s2cid=46623205 |url=http://doc.rero.ch/record/210392/files/PAL_E4399.pdf }}</ref> Part of the difficulty may be that there were strong regional variations in the response to climate shifts. Evidence of a relatively warm Oligocene suggests an enigmatic climate state, neither hothouse nor icehouse.{{sfn|O'Brien|Huber|Thomas|Pagani|2020|pages=25303}}

===Late Oligocene warming===
The late Oligocene (26.5 to 24 mya) likely saw a warming trend in spite of low pCO2 levels, though this appears to vary by region.{{sfn|O'Brien|Huber|Thomas|Pagani|2020|pages=25302-25303}} However, Antarctica remained heavily glaciated during this warming period.<ref>{{cite journal |last1=Pekar |first1=Stephen F. |last2=DeConto |first2=Robert M. |last3=Harwood |first3=David M. |title=Resolving a late Oligocene conundrum: Deep-sea warming and Antarctic glaciation |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=February 2006 |volume=231 |issue=1–2 |pages=29–40 |doi=10.1016/j.palaeo.2005.07.024|bibcode=2006PPP...231...29P |url=http://doc.rero.ch/record/13512/files/PAL_E309.pdf }}</ref><ref>{{cite journal |last1=Hauptvogel |first1=D. W. |last2=Pekar |first2=S. F. |last3=Pincay |first3=V. |title=Evidence for a heavily glaciated Antarctica during the late Oligocene "warming" (27.8–24.5 Ma): Stable isotope records from ODP Site 690: LATE OLIGOCENE STABLE ISOTOPE RECORD |journal=Paleoceanography |date=April 2017 |volume=32 |issue=4 |pages=384–396 |doi=10.1002/2016PA002972|doi-access=free }}</ref> The late Oligocene warming is discernible in pollen counts from the Tibetan Plateau, which also show that the [[south Asian monsoon|South Asian Monsoon]] had already developed by the late Oligocene.<ref>{{cite journal |last1=Wu |first1=Fuli |last2=Miao |first2=Yunfa |last3=Meng |first3=Qingquan |last4=Fang |first4=Xiaomin |last5=Sun |first5=Jimin |title=Late Oligocene Tibetan Plateau Warming and Humidity: Evidence From a Sporopollen Record |journal=Geochemistry, Geophysics, Geosystems |date=January 2019 |volume=20 |issue=1 |pages=434–441 |doi=10.1029/2018GC007775|bibcode=2019GGG....20..434W |doi-access=free }}</ref> Around 25.8 Ma, the South Asian Monsoon underwent an episode of major intensification brought on by the uplift of the Tibetan Plateau.<ref>{{Cite journal |last1=Jin |first1=Chun-Sheng |last2=Xu |first2=Deke |last3=Li |first3=Mingsong |last4=Hu |first4=Pengxiang |last5=Jiang |first5=Zhaoxia |last6=Liu |first6=Jianxing |last7=Miao |first7=Yunfa |last8=Wu |first8=Fuli |last9=Liang |first9=Wentian |last10=Zhang |first10=Qiang |last11=Su |first11=Bai |last12=Liu |first12=Qingsong |last13=Zhang |first13=Ran |last14=Sun |first14=Jimin |date=11 April 2023 |title=Tectonic and orbital forcing of the South Asian monsoon in central Tibet during the late Oligocene |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |language=en |volume=120 |issue=15 |article-number=e2214558120 |doi=10.1073/pnas.2214558120 |doi-access=free |issn=0027-8424 |pmc=10104490 |pmid=37011203 |bibcode=2023PNAS..12014558J }}</ref>

A deep 400,000-year glaciated Oligocene-Miocene boundary event is recorded at [[McMurdo Sound]] and [[King George Island (South Shetland Islands)|King George Island]].<ref>{{cite journal |last1=Wilson |first1=G.S. |last2=Pekar |first2=S.F. |last3=Naish |first3=T.R. |last4=Passchier |first4=S. |last5=DeConto |first5=R. |title=Chapter 9 The Oligocene–Miocene Boundary – Antarctic Climate Response to Orbital Forcing |journal=Developments in Earth and Environmental Sciences |date=2008 |volume=8 |pages=369–400 |doi=10.1016/S1571-9197(08)00009-8|bibcode=2008DvEES...8..369W |isbn=978-0-444-52847-6 }}</ref>