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== Climate ==
== Climate ==
Following some [[climate model]]s, the EECO was marked by an extremely high [[Global surface temperature|global mean surface temperature]],<ref name="ChristopherRobertScotese" /> which has been estimated to be anywhere between 23.2 and 29.7 °C, with the mean estimate being around 27.0 °C.<ref>{{Cite journal |last1=Inglis |first1=Gordon N. |last2=Bragg |first2=Fran |last3=Burls |first3=Natalie J. |last4=Cramwinckel |first4=Margot J. |last5=Evans |first5=David |last6=Foster |first6=Gavin L. |last7=Huber |first7=Matthew |last8=Lunt |first8=Daniel J. |last9=Siler |first9=Nicholas |last10=Steinig |first10=Sebastian |last11=Tierney |first11=Jessica E. |last12=Wilkinson |first12=Richard |last13=Anagnostou |first13=Eleni |last14=de Boer |first14=Agatha M. |last15=Dunkley Jones |first15=Tom |last16=Edgar |first16=Kirsty M. |last17=Hollis |first17=Christopher J. |last18=Hutchinson |first18=David K. |last19=Pancost |first19=Richard D. |date=26 October 2020 |title=Global mean surface temperature and climate sensitivity of the early Eocene Climatic Optimum (EECO), Paleocene–Eocene Thermal Maximum (PETM), and latest Paleocene |url=https://cp.copernicus.org/articles/16/1953/2020/ |journal=[[Climate of the Past]] |language=en |volume=16 |issue=5 |pages=1953–1968 |doi=10.5194/cp-16-1953-2020 |bibcode=2020CliPa..16.1953I |issn=1814-9332 |access-date=24 December 2023 |doi-access=free |hdl=1983/24a30f12-51a6-4544-9db8-b2009e33c02a |hdl-access=free }}</ref> In [[North America]], the mean annual temperature was 23.0 °C, while the continent's overall mean annual precipitation (MAP) was about 1500 mm.<ref name="WoodburneGunnellStucky2009" /> The mean annual temperature range (MATR) of North America may have been as low as 47 °C or as high as 61 °C, while the MATR of [[Asia]] was anywhere from 51 to 60 °C.<ref>{{Cite journal |last1=Sloan |first1=L.Cirbus |last2=Morrill |first2=C |date=15 November 1998 |title=Orbital forcing and Eocene continental temperatures |url=https://linkinghub.elsevier.com/retrieve/pii/S0031018298000911 |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |language=en |volume=144 |issue=1–2 |pages=21–35 |doi=10.1016/S0031-0182(98)00091-1 |bibcode=1998PPP...144...21S |access-date=24 December 2023 |via=Elsevier Science Direct}}</ref> The Okanagan Highlands had a moist mesothermal climate, with bioclimatic analysis of the region yielding estimates of a mean annual temperature (MAT) of 12.7-16.6 °C, a cold month mean temperature (CMMT) of 3.5-7.9 °C, and a MAP of 103-157 cm.<ref>{{Cite journal |last1=Mathewes |first1=Rolf W. |last2=Greenwood |first2=David R. |last3=Archibald |first3=S. Bruce |date=14 April 2016 |editor-last=Pigg |editor-first=Kathleen B. |title=Paleoenvironment of the Quilchena flora, British Columbia, during the Early Eocene Climatic Optimum |url=http://www.nrcresearchpress.com/doi/10.1139/cjes-2015-0163 |journal=[[Canadian Journal of Earth Sciences]] |language=en |volume=53 |issue=6 |pages=574–590 |doi=10.1139/cjes-2015-0163 |bibcode=2016CaJES..53..574M |hdl=1807/71979 |issn=0008-4077 |access-date=5 July 2024 |via=Canadian Science Publishing|hdl-access=free }}</ref> [[Clumped isotopes|Clumped isotope]] measurements from the [[Green River Basin]] confirm a high seasonality of temperature, contradicting climatological predictions of an equable climate under greenhouse conditions.<ref>{{Cite journal |last1=Hyland |first1=Ethan G. |last2=Huntington |first2=Katharine W. |last3=Sheldon |first3=Nathan D. |last4=Reichgelt |first4=Tammo |date=4 October 2018 |title=Temperature seasonality in the North American continental interior during the Early Eocene Climatic Optimum |url=https://cp.copernicus.org/articles/14/1391/2018/ |journal=[[Climate of the Past]] |language=en |volume=14 |issue=10 |pages=1391–1404 |doi=10.5194/cp-14-1391-2018 |doi-access=free |bibcode=2018CliPa..14.1391H |issn=1814-9332 |access-date=3 February 2024|hdl=2027.42/148644 |hdl-access=free }}</ref> Lake temperatures in the Green River Formation ranged from 28 °C to 35 °C.<ref>{{Cite journal |last=Frantz |first=Carie M. |last2=Petryshyn |first2=Victoria A. |last3=Marenco |first3=Pedro J. |last4=Tripati |first4=Aradhna |last5=Berelson |first5=William M. |last6=Corsetti |first6=Frank A. |date=1 July 2014 |title=Dramatic local environmental change during the Early Eocene Climatic Optimum detected using high resolution chemical analyses of Green River Formation stromatolites |url=https://linkinghub.elsevier.com/retrieve/pii/S0031018214001849 |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |language=en |volume=405 |pages=1–15 |doi=10.1016/j.palaeo.2014.04.001 |access-date=22 August 2024 |via=Elsevier Science Direct}}</ref> Sediments from [[San Diego County, California]] record a MAP of 1100 ± 299 mm, notably drier than the region was during the [[Paleocene–Eocene Thermal Maximum|Palaeocene-Eocene Thermal Maximum]].<ref>{{Cite journal |last1=Broz |first1=Adrian P. |last2=Pritchard-Peterson |first2=Devin |last3=Spinola |first3=Diogo |last4=Schneider |first4=Sarah |last5=Retallack |first5=Gregory |last6=Silva |first6=Lucas C. R. |date=31 January 2024 |title=Eocene (50–55 Ma) greenhouse climate recorded in nonmarine rocks of San Diego, CA, USA |journal=[[Scientific Reports]] |language=en |volume=14 |issue=1 |pages=2613 |doi=10.1038/s41598-024-53210-0 |pmid=38297060 |pmc=10830502 |bibcode=2024NatSR..14.2613B |issn=2045-2322 }}</ref> [[Sea surface temperature]]s (SSTs) off of Seymour Island were ~15 °C.<ref>{{Cite journal |last1=Ivany |first1=L. C. |last2=Lohmann |first2=K. C. |last3=Hasiuk |first3=F. |last4=Blake |first4=D. B. |last5=Glass |first5=A. |last6=Aronson |first6=R. B. |last7=Moody |first7=R. M. |date=1 May 2008 |title=Eocene climate record of a high southern latitude continental shelf: Seymour Island, Antarctica |url=https://pubs.geoscienceworld.org/gsabulletin/article/120/5-6/659-678/2280 |journal=[[Geological Society of America Bulletin]] |language=en |volume=120 |issue=5–6 |pages=659–678 |doi=10.1130/B26269.1 |bibcode=2008GSAB..120..659I |issn=0016-7606 |via=GeoScienceWorld}}</ref> The high elevation areas of Asia, [[Africa]], and [[Antarctica]] saw elevation dependent warming (EDW), while those in North America and [[Indian subcontinent|India]] saw elevation dependent cooling (EDC).<ref>{{Cite journal |last1=Kad |first1=Pratik |last2=Blau |first2=Manuel Tobias |last3=Ha |first3=Kyung-Ja |author-link3=Kyung-Ja Ha |last4=Zhu |first4=Jiang |date=1 November 2022 |title=Elevation-dependent temperature response in early Eocene using paleoclimate model experiment |journal=[[Environmental Research Letters]] |volume=17 |issue=11 |pages=114038 |bibcode=2022ERL....17k4038K |doi=10.1088/1748-9326/ac9c74 |issn=1748-9326 |doi-access=free}}</ref>
Following some [[climate model]]s, the EECO was marked by an extremely high [[Global surface temperature|global mean surface temperature]],<ref name="ChristopherRobertScotese" /> which has been estimated to be anywhere between 23.2 and 29.7 °C, with the mean estimate being around 27.0 °C.<ref>{{Cite journal |last1=Inglis |first1=Gordon N. |last2=Bragg |first2=Fran |last3=Burls |first3=Natalie J. |last4=Cramwinckel |first4=Margot J. |last5=Evans |first5=David |last6=Foster |first6=Gavin L. |last7=Huber |first7=Matthew |last8=Lunt |first8=Daniel J. |last9=Siler |first9=Nicholas |last10=Steinig |first10=Sebastian |last11=Tierney |first11=Jessica E. |last12=Wilkinson |first12=Richard |last13=Anagnostou |first13=Eleni |last14=de Boer |first14=Agatha M. |last15=Dunkley Jones |first15=Tom |last16=Edgar |first16=Kirsty M. |last17=Hollis |first17=Christopher J. |last18=Hutchinson |first18=David K. |last19=Pancost |first19=Richard D. |date=26 October 2020 |title=Global mean surface temperature and climate sensitivity of the early Eocene Climatic Optimum (EECO), Paleocene–Eocene Thermal Maximum (PETM), and latest Paleocene |url=https://cp.copernicus.org/articles/16/1953/2020/ |journal=[[Climate of the Past]] |language=en |volume=16 |issue=5 |pages=1953–1968 |doi=10.5194/cp-16-1953-2020 |bibcode=2020CliPa..16.1953I |issn=1814-9332 |access-date=24 December 2023 |doi-access=free |hdl=1983/24a30f12-51a6-4544-9db8-b2009e33c02a |hdl-access=free }}</ref> In [[North America]], the mean annual temperature was 23.0 °C, while the continent's overall mean annual precipitation (MAP) was about 1500 mm.<ref name="WoodburneGunnellStucky2009" /> The mean annual temperature range (MATR) of North America may have been as low as 47 °C or as high as 61 °C, while the MATR of [[Asia]] was anywhere from 51 to 60 °C.<ref>{{Cite journal |last1=Sloan |first1=L.Cirbus |last2=Morrill |first2=C |date=15 November 1998 |title=Orbital forcing and Eocene continental temperatures |url=https://linkinghub.elsevier.com/retrieve/pii/S0031018298000911 |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |language=en |volume=144 |issue=1–2 |pages=21–35 |doi=10.1016/S0031-0182(98)00091-1 |bibcode=1998PPP...144...21S |access-date=24 December 2023 |via=Elsevier Science Direct}}</ref> The Okanagan Highlands had a moist mesothermal climate, with bioclimatic analysis of the region yielding estimates of a mean annual temperature (MAT) of 12.7-16.6 °C, a cold month mean temperature (CMMT) of 3.5-7.9 °C, and a MAP of 103-157 cm.<ref>{{Cite journal |last1=Mathewes |first1=Rolf W. |last2=Greenwood |first2=David R. |last3=Archibald |first3=S. Bruce |date=14 April 2016 |editor-last=Pigg |editor-first=Kathleen B. |title=Paleoenvironment of the Quilchena flora, British Columbia, during the Early Eocene Climatic Optimum |url=http://www.nrcresearchpress.com/doi/10.1139/cjes-2015-0163 |journal=[[Canadian Journal of Earth Sciences]] |language=en |volume=53 |issue=6 |pages=574–590 |doi=10.1139/cjes-2015-0163 |bibcode=2016CaJES..53..574M |hdl=1807/71979 |issn=0008-4077 |access-date=5 July 2024 |via=Canadian Science Publishing|hdl-access=free }}</ref> [[Clumped isotopes|Clumped isotope]] measurements from the [[Green River Basin]] and the [[Bighorn Basin]] confirm a high seasonality of temperature, contradicting climatological predictions of an equable climate under greenhouse conditions.<ref>{{Cite journal |last1=Hyland |first1=Ethan G. |last2=Huntington |first2=Katharine W. |last3=Sheldon |first3=Nathan D. |last4=Reichgelt |first4=Tammo |date=4 October 2018 |title=Temperature seasonality in the North American continental interior during the Early Eocene Climatic Optimum |url=https://cp.copernicus.org/articles/14/1391/2018/ |journal=[[Climate of the Past]] |language=en |volume=14 |issue=10 |pages=1391–1404 |doi=10.5194/cp-14-1391-2018 |doi-access=free |bibcode=2018CliPa..14.1391H |issn=1814-9332 |access-date=3 February 2024|hdl=2027.42/148644 |hdl-access=free }}</ref><ref>{{Cite journal |last=Snell |first=Kathryn E. |last2=Thrasher |first2=Bridget L. |last3=Eiler |first3=John M. |last4=Koch |first4=Paul L. |last5=Sloan |first5=Lisa C. |last6=Tabor |first6=Neil J. |date=1 January 2013 |title=Hot summers in the Bighorn Basin during the early Paleogene |url=https://pubs.geoscienceworld.org/gsa/geology/article-abstract/41/1/55/131031/Hot-summers-in-the-Bighorn-Basin-during-the-early?redirectedFrom=fulltext |journal=[[Geology (journal)|Geology]] |language=en |volume=41 |issue=1 |pages=55–58 |doi=10.1130/G33567.1 |issn=0091-7613 |access-date=23 October 2024 |via=GeoScienceWorld}}</ref> Lake temperatures in the Green River Formation ranged from 28 °C to 35 °C,<ref>{{Cite journal |last=Frantz |first=Carie M. |last2=Petryshyn |first2=Victoria A. |last3=Marenco |first3=Pedro J. |last4=Tripati |first4=Aradhna |last5=Berelson |first5=William M. |last6=Corsetti |first6=Frank A. |date=1 July 2014 |title=Dramatic local environmental change during the Early Eocene Climatic Optimum detected using high resolution chemical analyses of Green River Formation stromatolites |url=https://linkinghub.elsevier.com/retrieve/pii/S0031018214001849 |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |language=en |volume=405 |pages=1–15 |doi=10.1016/j.palaeo.2014.04.001 |access-date=22 August 2024 |via=Elsevier Science Direct}}</ref> with lacustrine photic zone euxinia being prevalent.<ref>{{Cite journal |last=Elson |first=Amy L. |last2=Schwark |first2=Lorenz |last3=Whiteside |first3=Jessica H. |last4=Hopper |first4=Peter |last5=Poropat |first5=Stephen F. |last6=Holman |first6=Alex I. |last7=Grice |first7=Kliti |date=September 2024 |title=A paleoenvironmental and ecological analysis of biomarkers from the Eocene Fossil Basin, Green River Formation, U.S.A. |url=https://www.sciencedirect.com/science/article/pii/S0146638024000950 |journal=[[Organic Geochemistry]] |language=en |volume=195 |pages=104830 |doi=10.1016/j.orggeochem.2024.104830 |access-date=23 October 2024 |via=Elsevier Science Direct}}</ref> Sediments from [[San Diego County, California]] record a MAP of 1100 ± 299 mm, notably drier than the region was during the [[Paleocene–Eocene Thermal Maximum|Palaeocene-Eocene Thermal Maximum]].<ref>{{Cite journal |last1=Broz |first1=Adrian P. |last2=Pritchard-Peterson |first2=Devin |last3=Spinola |first3=Diogo |last4=Schneider |first4=Sarah |last5=Retallack |first5=Gregory |last6=Silva |first6=Lucas C. R. |date=31 January 2024 |title=Eocene (50–55 Ma) greenhouse climate recorded in nonmarine rocks of San Diego, CA, USA |journal=[[Scientific Reports]] |language=en |volume=14 |issue=1 |pages=2613 |doi=10.1038/s41598-024-53210-0 |pmid=38297060 |pmc=10830502 |bibcode=2024NatSR..14.2613B |issn=2045-2322 }}</ref> [[Sea surface temperature]]s (SSTs) off of Seymour Island were ~15 °C.<ref>{{Cite journal |last1=Ivany |first1=L. C. |last2=Lohmann |first2=K. C. |last3=Hasiuk |first3=F. |last4=Blake |first4=D. B. |last5=Glass |first5=A. |last6=Aronson |first6=R. B. |last7=Moody |first7=R. M. |date=1 May 2008 |title=Eocene climate record of a high southern latitude continental shelf: Seymour Island, Antarctica |url=https://pubs.geoscienceworld.org/gsabulletin/article/120/5-6/659-678/2280 |journal=[[Geological Society of America Bulletin]] |language=en |volume=120 |issue=5–6 |pages=659–678 |doi=10.1130/B26269.1 |bibcode=2008GSAB..120..659I |issn=0016-7606 |via=GeoScienceWorld}}</ref> The high elevation areas of Asia, [[Africa]], and [[Antarctica]] saw elevation dependent warming (EDW), while those in North America and [[Indian subcontinent|India]] saw elevation dependent cooling (EDC).<ref>{{Cite journal |last1=Kad |first1=Pratik |last2=Blau |first2=Manuel Tobias |last3=Ha |first3=Kyung-Ja |author-link3=Kyung-Ja Ha |last4=Zhu |first4=Jiang |date=1 November 2022 |title=Elevation-dependent temperature response in early Eocene using paleoclimate model experiment |journal=[[Environmental Research Letters]] |volume=17 |issue=11 |pages=114038 |bibcode=2022ERL....17k4038K |doi=10.1088/1748-9326/ac9c74 |issn=1748-9326 |doi-access=free}}</ref>


The latitudinal climate gradient is generally believed to have been smaller, which was mainly the result of a decrease in albedo differences across Earth's surface.<ref>{{Cite journal |last1=Lunt |first1=Daniel J. |last2=Bragg |first2=Fran |last3=Chan |first3=Wing-Le |last4=Hutchinson |first4=David K. |last5=Ladant |first5=Jean-Baptiste |last6=Morozova |first6=Polina |last7=Niezgodzki |first7=Igor |last8=Steinig |first8=Sebastian |last9=Zhang |first9=Zhongshi |last10=Zhu |first10=Jiang |last11=Abe-Ouchi |first11=Ayako |last12=Anagnostou |first12=Eleni |last13=de Boer |first13=Agatha M. |last14=Coxall |first14=Helen K. |last15=Donnadieu |first15=Yannick |last16=Foster |first16=Gavin |last17=Inglis |first17=Gordon N. |last18=Knorr |first18=Gregor |last19=Langebroek |first19=Petra M. |last20=Lear |first20=Caroline H. |last21=Lohmann |first21=Gerrit |last22=Poulsen |first22=Christopher J. |last23=Sepulchre |first23=Pierre |last24=Tierney |first24=Jessica E. |last25=Valdes |first25=Paul J. |last26=Volodin |first26=Evgeny M. |last27=Jones |first27=Tom Dunkley |last28=Hollis |first28=Christopher J. |last29=Huber |first29=Matthew |last30=Otto-Bliesner |first30=Bette L. |date=15 January 2021 |title=DeepMIP: model intercomparison of early Eocene climatic optimum (EECO) large-scale climate features and comparison with proxy data |url=https://cp.copernicus.org/articles/17/203/2021/ |journal=[[Climate of the Past]] |language=en |volume=17 |issue=1 |pages=203–227 |doi=10.5194/cp-17-203-2021 |doi-access=free |bibcode=2021CliPa..17..203L |issn=1814-9332 |access-date=25 June 2024|hdl=1983/22ea9a7d-eccc-4eca-b04d-7f003e8d1d2e |hdl-access=free }}</ref> Although SSTs are often believed to have had a shallow latitudinal temperature gradient, this is likely to be an artefact of burial-induced oxygen isotope reequilibration in [[fossil]]ised benthic foraminifera.<ref>{{Cite journal |last1=Bernard |first1=S. |last2=Daval |first2=D. |last3=Ackerer |first3=P. |last4=Pont |first4=S. |last5=Meibom |first5=A. |date=26 October 2017 |title=Burial-induced oxygen-isotope re-equilibration of fossil foraminifera explains ocean paleotemperature paradoxes |journal=[[Nature Communications]] |language=en |volume=8 |issue=1 |pages=1134 |doi=10.1038/s41467-017-01225-9 |issn=2041-1723 |doi-access=free |pmid=29070888 |pmc=5656689 |bibcode=2017NatCo...8.1134B }}</ref>
The latitudinal climate gradient is generally believed to have been smaller, which was mainly the result of a decrease in albedo differences across Earth's surface.<ref>{{Cite journal |last1=Lunt |first1=Daniel J. |last2=Bragg |first2=Fran |last3=Chan |first3=Wing-Le |last4=Hutchinson |first4=David K. |last5=Ladant |first5=Jean-Baptiste |last6=Morozova |first6=Polina |last7=Niezgodzki |first7=Igor |last8=Steinig |first8=Sebastian |last9=Zhang |first9=Zhongshi |last10=Zhu |first10=Jiang |last11=Abe-Ouchi |first11=Ayako |last12=Anagnostou |first12=Eleni |last13=de Boer |first13=Agatha M. |last14=Coxall |first14=Helen K. |last15=Donnadieu |first15=Yannick |last16=Foster |first16=Gavin |last17=Inglis |first17=Gordon N. |last18=Knorr |first18=Gregor |last19=Langebroek |first19=Petra M. |last20=Lear |first20=Caroline H. |last21=Lohmann |first21=Gerrit |last22=Poulsen |first22=Christopher J. |last23=Sepulchre |first23=Pierre |last24=Tierney |first24=Jessica E. |last25=Valdes |first25=Paul J. |last26=Volodin |first26=Evgeny M. |last27=Jones |first27=Tom Dunkley |last28=Hollis |first28=Christopher J. |last29=Huber |first29=Matthew |last30=Otto-Bliesner |first30=Bette L. |date=15 January 2021 |title=DeepMIP: model intercomparison of early Eocene climatic optimum (EECO) large-scale climate features and comparison with proxy data |url=https://cp.copernicus.org/articles/17/203/2021/ |journal=[[Climate of the Past]] |language=en |volume=17 |issue=1 |pages=203–227 |doi=10.5194/cp-17-203-2021 |doi-access=free |bibcode=2021CliPa..17..203L |issn=1814-9332 |access-date=25 June 2024|hdl=1983/22ea9a7d-eccc-4eca-b04d-7f003e8d1d2e |hdl-access=free }}</ref> Although SSTs are often believed to have had a shallow latitudinal temperature gradient, this is likely to be an artefact of burial-induced oxygen isotope reequilibration in [[fossil]]ised benthic foraminifera.<ref>{{Cite journal |last1=Bernard |first1=S. |last2=Daval |first2=D. |last3=Ackerer |first3=P. |last4=Pont |first4=S. |last5=Meibom |first5=A. |date=26 October 2017 |title=Burial-induced oxygen-isotope re-equilibration of fossil foraminifera explains ocean paleotemperature paradoxes |journal=[[Nature Communications]] |language=en |volume=8 |issue=1 |pages=1134 |doi=10.1038/s41467-017-01225-9 |issn=2041-1723 |doi-access=free |pmid=29070888 |pmc=5656689 |bibcode=2017NatCo...8.1134B }}</ref>
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The final phase of the [[Cretaceous Terrestrial Revolution|Angiosperm Terrestrial Revolution]] occurred during the EECO.<ref name="Benton2021">{{cite journal |last1=Benton |first1=Michael James |last2=Wilf |first2=Peter |last3=Sauquet |first3=Hervé |date=26 October 2021 |title=The Angiosperm Terrestrial Revolution and the origins of modern biodiversity |url=https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.17822 |journal=[[New Phytologist]] |volume=233 |issue=5 |pages=2017–2035 |doi=10.1111/nph.17822 |pmid=34699613 |s2cid=240000207 |access-date=24 November 2022 |hdl-access=free |hdl=1983/82a09075-31f4-423e-98b9-3bb2c215e04b}}</ref> The supergreenhouse climate of the EECO fostered extensive floral diversification and increased habitat complexity in North American terrestrial biomes.<ref name="WoodburneGunnellStucky2009" /> The hot, humid conditions of the EECO may have facilitated the evolution of epiphytic bryophytes, with the oldest member of Lejeuneaceae being described from fossils from the Cambay amber dating back to the EECO.<ref>{{Cite journal |last1=Heinrichs |first1=Jochen |last2=Scheben |first2=Armin |last3=Bechteler |first3=Julia |last4=Lee |first4=Gaik Ee |last5=Schäfer-Verwimp |first5=Alfons |last6=Hedenäs |first6=Lars |last7=Singh |first7=Hukam |last8=Pócs |first8=Tamás |last9=Nascimbene |first9=Paul C. |last10=Peralta |first10=Denilson F. |last11=Renner |first11=Matt |last12=Schmidt |first12=Alexander R. |date=31 May 2016 |editor-last=Wong |editor-first=William Oki |title=Crown Group Lejeuneaceae and Pleurocarpous Mosses in Early Eocene (Ypresian) Indian Amber |journal=[[PLOS ONE]] |language=en |volume=11 |issue=5 |pages=e0156301 |bibcode=2016PLoSO..1156301H |doi=10.1371/journal.pone.0156301 |issn=1932-6203 |pmc=4887038 |pmid=27244582 |doi-access=free}}</ref> The Okanagan Highlands in British Columbia and Washington became a biodiversity hotspot from which newly evolved lineages of temperate-adapted plants radiated from following the end of the EECO.<ref>{{Cite journal |last1=Smith |first1=Robin Y. |last2=Basinger |first2=James F. |last3=Greenwood |first3=David R. |date=21 October 2011 |title=Early Eocene plant diversity and dynamics in the Falkland flora, Okanagan Highlands, British Columbia, Canada |url=http://link.springer.com/10.1007/s12549-011-0061-5 |journal=Palaeobiodiversity and Palaeoenvironments |language=en |volume=92 |issue=3 |pages=309–328 |doi=10.1007/s12549-011-0061-5 |issn=1867-1594 |access-date=25 June 2024 |via=Springer Link}}</ref>
The final phase of the [[Cretaceous Terrestrial Revolution|Angiosperm Terrestrial Revolution]] occurred during the EECO.<ref name="Benton2021">{{cite journal |last1=Benton |first1=Michael James |last2=Wilf |first2=Peter |last3=Sauquet |first3=Hervé |date=26 October 2021 |title=The Angiosperm Terrestrial Revolution and the origins of modern biodiversity |url=https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.17822 |journal=[[New Phytologist]] |volume=233 |issue=5 |pages=2017–2035 |doi=10.1111/nph.17822 |pmid=34699613 |s2cid=240000207 |access-date=24 November 2022 |hdl-access=free |hdl=1983/82a09075-31f4-423e-98b9-3bb2c215e04b}}</ref> The supergreenhouse climate of the EECO fostered extensive floral diversification and increased habitat complexity in North American terrestrial biomes.<ref name="WoodburneGunnellStucky2009" /> The hot, humid conditions of the EECO may have facilitated the evolution of epiphytic bryophytes, with the oldest member of Lejeuneaceae being described from fossils from the Cambay amber dating back to the EECO.<ref>{{Cite journal |last1=Heinrichs |first1=Jochen |last2=Scheben |first2=Armin |last3=Bechteler |first3=Julia |last4=Lee |first4=Gaik Ee |last5=Schäfer-Verwimp |first5=Alfons |last6=Hedenäs |first6=Lars |last7=Singh |first7=Hukam |last8=Pócs |first8=Tamás |last9=Nascimbene |first9=Paul C. |last10=Peralta |first10=Denilson F. |last11=Renner |first11=Matt |last12=Schmidt |first12=Alexander R. |date=31 May 2016 |editor-last=Wong |editor-first=William Oki |title=Crown Group Lejeuneaceae and Pleurocarpous Mosses in Early Eocene (Ypresian) Indian Amber |journal=[[PLOS ONE]] |language=en |volume=11 |issue=5 |pages=e0156301 |bibcode=2016PLoSO..1156301H |doi=10.1371/journal.pone.0156301 |issn=1932-6203 |pmc=4887038 |pmid=27244582 |doi-access=free}}</ref> The Okanagan Highlands in British Columbia and Washington became a biodiversity hotspot from which newly evolved lineages of temperate-adapted plants radiated from following the end of the EECO.<ref>{{Cite journal |last1=Smith |first1=Robin Y. |last2=Basinger |first2=James F. |last3=Greenwood |first3=David R. |date=21 October 2011 |title=Early Eocene plant diversity and dynamics in the Falkland flora, Okanagan Highlands, British Columbia, Canada |url=http://link.springer.com/10.1007/s12549-011-0061-5 |journal=Palaeobiodiversity and Palaeoenvironments |language=en |volume=92 |issue=3 |pages=309–328 |doi=10.1007/s12549-011-0061-5 |issn=1867-1594 |access-date=25 June 2024 |via=Springer Link}}</ref>


The climate was warm enough to allow palms and palm beetles to inhabit upland regions of British Columbia and Washington.<ref>{{Cite journal |last1=Archibald |first1=S. Bruce |last2=Morse |first2=Geoffrey E. |last3=Greenwood |first3=David R. |last4=Mathewes |first4=Rolf W. |date=12 May 2014 |title=Fossil palm beetles refine upland winter temperatures in the Early Eocene Climatic Optimum |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |language=en |volume=111 |issue=22 |pages=8095–8100 |doi=10.1073/pnas.1323269111 |doi-access=free |issn=0027-8424 |pmc=4050627 |pmid=24821798 |bibcode=2014PNAS..111.8095A }}</ref> Ellesmere Island became inhabited by basal primatomorphs.<ref>{{Cite journal |last1=Miller |first1=Kristen |last2=Tietjen |first2=Kristen |last3=Beard |first3=K. Christopher |date=25 January 2023 |editor-last=Meloro |editor-first=Carlo |title=Basal Primatomorpha colonized Ellesmere Island (Arctic Canada) during the hyperthermal conditions of the early Eocene climatic optimum |journal=[[PLOS ONE]] |language=en |volume=18 |issue=1 |pages=e0280114 |doi=10.1371/journal.pone.0280114 |doi-access=free |issn=1932-6203 |pmc=9876366 |pmid=36696373 |bibcode=2023PLoSO..1880114M }}</ref>
The climate was warm enough to allow palms and palm beetles to inhabit upland regions of British Columbia and Washington.<ref>{{Cite journal |last1=Archibald |first1=S. Bruce |last2=Morse |first2=Geoffrey E. |last3=Greenwood |first3=David R. |last4=Mathewes |first4=Rolf W. |date=12 May 2014 |title=Fossil palm beetles refine upland winter temperatures in the Early Eocene Climatic Optimum |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |language=en |volume=111 |issue=22 |pages=8095–8100 |doi=10.1073/pnas.1323269111 |doi-access=free |issn=0027-8424 |pmc=4050627 |pmid=24821798 |bibcode=2014PNAS..111.8095A }}</ref> Ellesmere Island became inhabited by basal primatomorphs.<ref>{{Cite journal |last1=Miller |first1=Kristen |last2=Tietjen |first2=Kristen |last3=Beard |first3=K. Christopher |date=25 January 2023 |editor-last=Meloro |editor-first=Carlo |title=Basal Primatomorpha colonized Ellesmere Island (Arctic Canada) during the hyperthermal conditions of the early Eocene climatic optimum |journal=[[PLOS ONE]] |language=en |volume=18 |issue=1 |pages=e0280114 |doi=10.1371/journal.pone.0280114 |doi-access=free |issn=1932-6203 |pmc=9876366 |pmid=36696373 |bibcode=2023PLoSO..1880114M }}</ref> The leadup to the EECO was marked by an increase in mammal diversity in Wyoming's Bighorn Basin.<ref>{{Cite journal |last=Chew |first=Amy E. |last2=Oheim |first2=Kathryn B. |date=1 January 2013 |title=Diversity and climate change in the middle-late Wasatchian (early Eocene) Willwood Formation, central Bighorn Basin, Wyoming |url=https://www.sciencedirect.com/science/article/abs/pii/S003101821200569X |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |language=en |volume=369 |pages=67–78 |doi=10.1016/j.palaeo.2012.10.004 |access-date=23 October 2024 |via=Elsevier Science Direct}}</ref>


Northern Yakutia was covered in mangroves.<ref>{{Cite journal |last1=Bondarenko |first1=Olesya V. |last2=Utescher |first2=Torsten |date=19 May 2022 |title=Late early to early middle Eocene climate and vegetation change at Tastakh Lake (northern Yakutia, eastern Siberia) |url=https://link.springer.com/10.1007/s12549-022-00530-6 |journal=Palaeobiodiversity and Palaeoenvironments |language=en |volume=103 |issue=2 |pages=277–301 |doi=10.1007/s12549-022-00530-6 |issn=1867-1594 |access-date=5 July 2024 |via=Springer Link|doi-access=free }}</ref> Mongolia witnessed a humidification event that transformed it from a shrubland into a forest and significantly reducing local wildfire incidence.<ref>{{Cite journal |last1=Zhou |first1=Xinying |last2=Wang |first2=Jian |last3=Li |first3=Qian |last4=Bai |first4=Bin |last5=Mao |first5=Fangyuan |last6=Li |first6=Xiaoqiang |last7=Wang |first7=Yuan-Qing |date=29 June 2023 |title=Late Paleocene to early Oligocene fire ecology of the south Mongolian highland |journal=[[Frontiers in Earth Science]] |volume=11 |doi=10.3389/feart.2023.1171452 |doi-access=free |bibcode=2023FrEaS..1171452Z |issn=2296-6463 }}</ref>
Northern Yakutia was covered in mangroves.<ref>{{Cite journal |last1=Bondarenko |first1=Olesya V. |last2=Utescher |first2=Torsten |date=19 May 2022 |title=Late early to early middle Eocene climate and vegetation change at Tastakh Lake (northern Yakutia, eastern Siberia) |url=https://link.springer.com/10.1007/s12549-022-00530-6 |journal=Palaeobiodiversity and Palaeoenvironments |language=en |volume=103 |issue=2 |pages=277–301 |doi=10.1007/s12549-022-00530-6 |issn=1867-1594 |access-date=5 July 2024 |via=Springer Link|doi-access=free }}</ref> Mongolia witnessed a humidification event that transformed it from a shrubland into a forest and significantly reducing local wildfire incidence.<ref>{{Cite journal |last1=Zhou |first1=Xinying |last2=Wang |first2=Jian |last3=Li |first3=Qian |last4=Bai |first4=Bin |last5=Mao |first5=Fangyuan |last6=Li |first6=Xiaoqiang |last7=Wang |first7=Yuan-Qing |date=29 June 2023 |title=Late Paleocene to early Oligocene fire ecology of the south Mongolian highland |journal=[[Frontiers in Earth Science]] |volume=11 |doi=10.3389/feart.2023.1171452 |doi-access=free |bibcode=2023FrEaS..1171452Z |issn=2296-6463 }}</ref>
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In [[South America]], the EECO coincided with the [[Itaboraian]] [[South American land mammal age|South American Land Mammal Age]].<ref>{{Cite journal |last1=Woodburne |first1=Michael O. |last2=Goin |first2=Francisco J. |last3=Raigemborn |first3=Maria Sol |last4=Heizler |first4=Matt |last5=Gelfo |first5=Javier N. |last6=Oliveira |first6=Edison V. |date=October 2014 |title=Revised timing of the South American early Paleogene land mammal ages |url=https://linkinghub.elsevier.com/retrieve/pii/S0895981114000649 |journal=[[Journal of South American Earth Sciences]] |language=en |volume=54 |pages=109–119 |doi=10.1016/j.jsames.2014.05.003 |bibcode=2014JSAES..54..109W |hdl=11336/79162 |access-date=3 February 2024 |via=Elsevier Science Direct|hdl-access=free }}</ref> [[Cingulata|Cingulates]] diversified over the course of the EECO.<ref>{{Cite journal |last1=Fernicola |first1=Juan Carlos |last2=Zimicz |first2=Ana N. |last3=Chornogubsky |first3=Laura |last4=Ducea |first4=Mihai |last5=Cruz |first5=Laura E. |last6=Bond |first6=Mariano |last7=Arnal |first7=Michelle |last8=Cárdenas |first8=Magalí |last9=Fernández |first9=Mercedes |date=10 May 2021 |title=The Early Eocene Climatic Optimum at the Lower Section of the Lumbrera Formation (Ypresian, Salta Province, Northwestern Argentina): Origin and Early Diversification of the Cingulata |url=https://link.springer.com/10.1007/s10914-021-09545-w |journal=[[Journal of Mammalian Evolution]] |language=en |volume=28 |issue=3 |pages=621–633 |doi=10.1007/s10914-021-09545-w |s2cid=236602601 |issn=1064-7554 |access-date=3 February 2024 |via=Springer}}</ref>
In [[South America]], the EECO coincided with the [[Itaboraian]] [[South American land mammal age|South American Land Mammal Age]].<ref>{{Cite journal |last1=Woodburne |first1=Michael O. |last2=Goin |first2=Francisco J. |last3=Raigemborn |first3=Maria Sol |last4=Heizler |first4=Matt |last5=Gelfo |first5=Javier N. |last6=Oliveira |first6=Edison V. |date=October 2014 |title=Revised timing of the South American early Paleogene land mammal ages |url=https://linkinghub.elsevier.com/retrieve/pii/S0895981114000649 |journal=[[Journal of South American Earth Sciences]] |language=en |volume=54 |pages=109–119 |doi=10.1016/j.jsames.2014.05.003 |bibcode=2014JSAES..54..109W |hdl=11336/79162 |access-date=3 February 2024 |via=Elsevier Science Direct|hdl-access=free }}</ref> [[Cingulata|Cingulates]] diversified over the course of the EECO.<ref>{{Cite journal |last1=Fernicola |first1=Juan Carlos |last2=Zimicz |first2=Ana N. |last3=Chornogubsky |first3=Laura |last4=Ducea |first4=Mihai |last5=Cruz |first5=Laura E. |last6=Bond |first6=Mariano |last7=Arnal |first7=Michelle |last8=Cárdenas |first8=Magalí |last9=Fernández |first9=Mercedes |date=10 May 2021 |title=The Early Eocene Climatic Optimum at the Lower Section of the Lumbrera Formation (Ypresian, Salta Province, Northwestern Argentina): Origin and Early Diversification of the Cingulata |url=https://link.springer.com/10.1007/s10914-021-09545-w |journal=[[Journal of Mammalian Evolution]] |language=en |volume=28 |issue=3 |pages=621–633 |doi=10.1007/s10914-021-09545-w |s2cid=236602601 |issn=1064-7554 |access-date=3 February 2024 |via=Springer}}</ref>


The northern margins of the Australo-Antarctic Gulf, then located at 60-65 °S, were covered in wet-tropical lowland vegetation.<ref>{{Cite journal |last1=McGowran |first1=Brian |last2=Hill |first2=Robert S. |date=9 June 2015 |title=Cenozoic climatic shifts in southern Australia |url=https://www.tandfonline.com/doi/full/10.1080/03721426.2015.1035215 |journal=[[Transactions of the Royal Society of South Australia]] |language=en |volume=139 |issue=1 |pages=19–37 |doi=10.1080/03721426.2015.1035215 |bibcode=2015TRSAu.139...19M |issn=0372-1426 |access-date=5 July 2024 |via=Taylor and Francis Online}}</ref>
The northern margins of the Australo-Antarctic Gulf, then located at 60-65 °S, were covered in wet-tropical lowland vegetation.<ref>{{Cite journal |last1=McGowran |first1=Brian |last2=Hill |first2=Robert S. |date=9 June 2015 |title=Cenozoic climatic shifts in southern Australia |url=https://www.tandfonline.com/doi/full/10.1080/03721426.2015.1035215 |journal=[[Transactions of the Royal Society of South Australia]] |language=en |volume=139 |issue=1 |pages=19–37 |doi=10.1080/03721426.2015.1035215 |bibcode=2015TRSAu.139...19M |issn=0372-1426 |access-date=5 July 2024 |via=Taylor and Francis Online}}</ref> ''[[Nypa (genus)|Nypa]]'' pollen is recorded in southeastern Australian sediments.<ref>{{Cite journal |last=Holdgate |first=Guy R. |last2=Sluiter |first2=Ian R.K. |last3=Clowes |first3=Chris D. |last4=Reichgelt |first4=Tammo |last5=Frieling |first5=Joost |date=1 September 2024 |title=The Paleocene - Eocene mangroves of southeastern Australia: spatial and temporal occurrences across four geological basins |url=https://linkinghub.elsevier.com/retrieve/pii/S0031018224003067 |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |language=en |volume=649 |pages=112317 |doi=10.1016/j.palaeo.2024.112317 |access-date=23 October 2024 |via=Elsevier Science Direct}}</ref>


At Shatsky Rise, the planktonic foraminifera ''Morozovella'' and ''Chiloguembelina'' declined in abundance. ''[[Acarinina]]'' became the dominant planktonic foraminifer in this locality.<ref>{{Cite journal |last=Filippi |first=Giulia |last2=Barrett |first2=Ruby |last3=Schmidt |first3=Daniela N. |last4=D'Onofrio |first4=Roberta |last5=Westerhold |first5=Thomas |last6=Brombin |first6=Valentina |last7=Luciani |first7=Valeria |date=8 August 2024 |title=Impacts of the Early Eocene Climatic Optimum (EECO, ∼53‐49 Ma) on Planktic Foraminiferal Resilience |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023PA004820 |journal=[[Paleoceanography and Paleoclimatology]] |language=en |volume=39 |issue=8 |doi=10.1029/2023PA004820 |issn=2572-4517 |access-date=22 August 2024|doi-access=free }}</ref> ''Morozovella'' underwent a switch from dextral to sinistral coiling across the EECO.<ref>{{Cite journal |last=Luciani |first=Valeria |last2=D'Onofrio |first2=Roberta |last3=Dickens |first3=Gerald R. |last4=Wade |first4=Bridget S. |date=November 2021 |title=Dextral to sinistral coiling switch in planktic foraminifer Morozovella during the Early Eocene Climatic Optimum |url=https://www.sciencedirect.com/science/article/pii/S0921818121002198 |journal=[[Global and Planetary Change]] |language=en |volume=206 |pages=103634 |doi=10.1016/j.gloplacha.2021.103634 |access-date=6 September 2024 |via=Elsevier Science Direct|hdl=11392/2465676 |hdl-access=free }}</ref> The [[euryhaline]] [[dinoflagellate]] ''Homotryblium'' became superabundant at the site of Waipara in [[New Zealand]] during the early and middle EECO, reflecting the occurrence of significant stratification of surficial waters as well as increased salinity.<ref>{{Cite journal |last1=Crouch |first1=E. M. |last2=Shepherd |first2=C. L. |last3=Morgans |first3=H. E. G. |last4=Naafs |first4=B. D. A. |last5=Dallanave |first5=E. |last6=Phillips |first6=A. |last7=Hollis |first7=C. J. |last8=Pancost |first8=R. D. |date=1 January 2020 |title=Climatic and environmental changes across the early Eocene climatic optimum at mid-Waipara River, Canterbury Basin, New Zealand |url=https://www.sciencedirect.com/science/article/pii/S0012825219302892 |journal=[[Earth-Science Reviews]] |volume=200 |pages=102961 |doi=10.1016/j.earscirev.2019.102961 |bibcode=2020ESRv..20002961C |hdl=1983/aedc04cc-bba8-44c6-8f9d-ba398bb24607 |s2cid=210618370 |issn=0012-8252 |access-date=11 September 2023|hdl-access=free }}</ref>
The central Tethys in what is now northeastern Italy was a hotspot of coral diversity, with its mesophotic deltaic environment acting as a refugium.<ref>{{Cite journal |last=Bosellini |first=Francesca R. |last2=Benedetti |first2=Andrea |last3=Budd |first3=Ann F. |last4=Papazzoni |first4=Cesare A. |date=1 December 2022 |title=A coral hotspot from a hot past: The EECO and post-EECO rich reef coral fauna from Friuli (Eocene, NE Italy) |url=https://www.sciencedirect.com/science/article/pii/S0031018222004552 |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |language=en |volume=607 |pages=111284 |doi=10.1016/j.palaeo.2022.111284 |access-date=23 October 2024 |via=Elsevier Science Direct}}</ref> At Shatsky Rise, the planktonic foraminifera ''Morozovella'' and ''Chiloguembelina'' declined in abundance. ''[[Acarinina]]'' became the dominant planktonic foraminifer in this locality.<ref>{{Cite journal |last=Filippi |first=Giulia |last2=Barrett |first2=Ruby |last3=Schmidt |first3=Daniela N. |last4=D'Onofrio |first4=Roberta |last5=Westerhold |first5=Thomas |last6=Brombin |first6=Valentina |last7=Luciani |first7=Valeria |date=8 August 2024 |title=Impacts of the Early Eocene Climatic Optimum (EECO, ∼53‐49 Ma) on Planktic Foraminiferal Resilience |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023PA004820 |journal=[[Paleoceanography and Paleoclimatology]] |language=en |volume=39 |issue=8 |doi=10.1029/2023PA004820 |issn=2572-4517 |access-date=22 August 2024|doi-access=free }}</ref> ''Morozovella'' underwent a switch from dextral to sinistral coiling across the EECO.<ref>{{Cite journal |last=Luciani |first=Valeria |last2=D'Onofrio |first2=Roberta |last3=Dickens |first3=Gerald R. |last4=Wade |first4=Bridget S. |date=November 2021 |title=Dextral to sinistral coiling switch in planktic foraminifer Morozovella during the Early Eocene Climatic Optimum |url=https://www.sciencedirect.com/science/article/pii/S0921818121002198 |journal=[[Global and Planetary Change]] |language=en |volume=206 |pages=103634 |doi=10.1016/j.gloplacha.2021.103634 |access-date=6 September 2024 |via=Elsevier Science Direct|hdl=11392/2465676 |hdl-access=free }}</ref> The [[euryhaline]] [[dinoflagellate]] ''Homotryblium'' became superabundant at the site of Waipara in [[New Zealand]] during the early and middle EECO, reflecting the occurrence of significant stratification of surficial waters as well as increased salinity.<ref>{{Cite journal |last1=Crouch |first1=E. M. |last2=Shepherd |first2=C. L. |last3=Morgans |first3=H. E. G. |last4=Naafs |first4=B. D. A. |last5=Dallanave |first5=E. |last6=Phillips |first6=A. |last7=Hollis |first7=C. J. |last8=Pancost |first8=R. D. |date=1 January 2020 |title=Climatic and environmental changes across the early Eocene climatic optimum at mid-Waipara River, Canterbury Basin, New Zealand |url=https://www.sciencedirect.com/science/article/pii/S0012825219302892 |journal=[[Earth-Science Reviews]] |volume=200 |pages=102961 |doi=10.1016/j.earscirev.2019.102961 |bibcode=2020ESRv..20002961C |hdl=1983/aedc04cc-bba8-44c6-8f9d-ba398bb24607 |s2cid=210618370 |issn=0012-8252 |access-date=11 September 2023|hdl-access=free }}</ref>


== Geologic effects ==
== Geologic effects ==

Revision as of 05:51, 24 October 2024

The Early Eocene Climatic Optimum (EECO), also referred to as the Early Eocene Thermal Maximum (EETM),[1] was a period of extremely warm greenhouse climatic conditions during the Eocene epoch. The EECO represented the hottest sustained interval of the Cenozoic era and one of the hottest periods in all of Earth's history.[2]

Duration

The EECO lasted from about 54 to 49 Ma.[1] The EECO's onset is signified by a major geochemical enrichment in isotopically light carbon, commonly known as a negative δ13C excursion, that demarcates the hyperthermal Eocene Thermal Maximum 3 (ETM3).[3]

Climate

Following some climate models, the EECO was marked by an extremely high global mean surface temperature,[1] which has been estimated to be anywhere between 23.2 and 29.7 °C, with the mean estimate being around 27.0 °C.[4] In North America, the mean annual temperature was 23.0 °C, while the continent's overall mean annual precipitation (MAP) was about 1500 mm.[2] The mean annual temperature range (MATR) of North America may have been as low as 47 °C or as high as 61 °C, while the MATR of Asia was anywhere from 51 to 60 °C.[5] The Okanagan Highlands had a moist mesothermal climate, with bioclimatic analysis of the region yielding estimates of a mean annual temperature (MAT) of 12.7-16.6 °C, a cold month mean temperature (CMMT) of 3.5-7.9 °C, and a MAP of 103-157 cm.[6] Clumped isotope measurements from the Green River Basin and the Bighorn Basin confirm a high seasonality of temperature, contradicting climatological predictions of an equable climate under greenhouse conditions.[7][8] Lake temperatures in the Green River Formation ranged from 28 °C to 35 °C,[9] with lacustrine photic zone euxinia being prevalent.[10] Sediments from San Diego County, California record a MAP of 1100 ± 299 mm, notably drier than the region was during the Palaeocene-Eocene Thermal Maximum.[11] Sea surface temperatures (SSTs) off of Seymour Island were ~15 °C.[12] The high elevation areas of Asia, Africa, and Antarctica saw elevation dependent warming (EDW), while those in North America and India saw elevation dependent cooling (EDC).[13]

The latitudinal climate gradient is generally believed to have been smaller, which was mainly the result of a decrease in albedo differences across Earth's surface.[14] Although SSTs are often believed to have had a shallow latitudinal temperature gradient, this is likely to be an artefact of burial-induced oxygen isotope reequilibration in fossilised benthic foraminifera.[15]

Climate modelling simulations point to a carbon dioxide concentration in the atmosphere of about 1,680 ppm to reproduce the observed hothouse conditions of the EECO,[16] although geochemical proxies suggest only 700-900 ppm.[17] Stomatal density in Gingko leaves suggests pCO2 was over twice that of preindustrial levels.[18] Additionally, methane concentrations in the Early Eocene may have been significantly higher than in the present day.[19]

The nature of the hydrological cycle during the EECO is controversial. Evidence from German peat bogs suggests that it was highly variable, with alternations between aridity and humidity.[20] Hydroclimatic variability in the Gonjo Basin was predominantly controlled by orbital eccentricity cycles.[21] Evidence from North America, in contrast, suggests that the hydrological cycle was enhanced during the EECO, although it remained relatively stable, unlike during the earlier hyperthermals, and that the stable hydroclimate may ultimately have ended the EECO by enabling high rates of organic carbon burial in lacustrine settings.[22]

Causes

The EECO was preceded by a major long-term warming trend in the Late Palaeocene and Early Eocene.[23] It was initiated by a series of intense hyperthermal events in the Early Eocene, including Eocene Thermal Maximum 2 (ETM2) and ETM3.[24]

The emplacement of the Pana Formation, a volcanic rock formation in southern Tibet that may represent the product of a supereruption, has also been proposed as a source of excess carbon flux into the atmosphere that drove the EECO.[25] Other research attributes the elevated greenhouse gas levels to increased generation of petroleum in sedimentary basins and enhanced ventilation of marine carbon.[26]

Biotic effects

The final phase of the Angiosperm Terrestrial Revolution occurred during the EECO.[27] The supergreenhouse climate of the EECO fostered extensive floral diversification and increased habitat complexity in North American terrestrial biomes.[2] The hot, humid conditions of the EECO may have facilitated the evolution of epiphytic bryophytes, with the oldest member of Lejeuneaceae being described from fossils from the Cambay amber dating back to the EECO.[28] The Okanagan Highlands in British Columbia and Washington became a biodiversity hotspot from which newly evolved lineages of temperate-adapted plants radiated from following the end of the EECO.[29]

The climate was warm enough to allow palms and palm beetles to inhabit upland regions of British Columbia and Washington.[30] Ellesmere Island became inhabited by basal primatomorphs.[31] The leadup to the EECO was marked by an increase in mammal diversity in Wyoming's Bighorn Basin.[32]

Northern Yakutia was covered in mangroves.[33] Mongolia witnessed a humidification event that transformed it from a shrubland into a forest and significantly reducing local wildfire incidence.[34]

In South America, the EECO coincided with the Itaboraian South American Land Mammal Age.[35] Cingulates diversified over the course of the EECO.[36]

The northern margins of the Australo-Antarctic Gulf, then located at 60-65 °S, were covered in wet-tropical lowland vegetation.[37] Nypa pollen is recorded in southeastern Australian sediments.[38]

The central Tethys in what is now northeastern Italy was a hotspot of coral diversity, with its mesophotic deltaic environment acting as a refugium.[39] At Shatsky Rise, the planktonic foraminifera Morozovella and Chiloguembelina declined in abundance. Acarinina became the dominant planktonic foraminifer in this locality.[40] Morozovella underwent a switch from dextral to sinistral coiling across the EECO.[41] The euryhaline dinoflagellate Homotryblium became superabundant at the site of Waipara in New Zealand during the early and middle EECO, reflecting the occurrence of significant stratification of surficial waters as well as increased salinity.[42]

Geologic effects

The EECO caused an increase in chert deposition by way of basin–basin fractionation by deep-sea circulation, causing increased silica concentrations in the North Atlantic which in turn resulted in direct precipitation of silica as well as its absorption by clay minerals.[43] The Equatorial Pacific displays extensive chert deposits laid down during the EECO.[44] The EECO was also marked by enhanced glauconite deposition.[45]

Comparison to present global warming

Because the pCO2 values of the EECO could potentially be reached if anthropogenic greenhouse gas emissions continue unabated for three centuries, the EECO has been used as an analogue for high-end projections of the Earth's future climate that would result from humanity's burning of fossil fuels.[46] Based on the Representative Concentration Pathway 8.5 (RCP8.5) emission scenario, by 2150 CE, the climates across much of the world would resemble conditions during the EECO.[47] One scenario of Lee et. al. (2021) suggests that conditions comparable to EECO could occur by 2300 CE.[48]

See also

References

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