Editing Eocene (section)

=== Middle Eocene ===

The Eocene is not only known for containing the warmest period during the Cenozoic; it also marked the decline into an icehouse climate and the rapid expansion of the [[Antarctic ice sheet]]. The transition from a warming climate into a cooling climate began at around 49 Ma. Isotopes of carbon and oxygen indicate a shift to a global cooling climate.<ref name="Speelman7" /> The cause of the cooling has been attributed to a significant decrease of >2,000&nbsp;ppm in atmospheric carbon dioxide concentrations.<ref name="Pearson4" /> One proposed cause of the reduction in carbon dioxide during the warming to cooling transition was the [[azolla event]]. With the equable climate during the early Eocene, warm temperatures in the arctic allowed for the growth of [[azolla]], which is a floating aquatic fern, on the [[Arctic Ocean]]. The significantly high amounts of carbon dioxide also acted to facilitate [[azolla]] blooms across the Arctic Ocean. Compared to current carbon dioxide levels, these azolla grew rapidly in the enhanced carbon dioxide levels found in the early Eocene.<ref name="Speelman7" /> The isolation of the Arctic Ocean, evidenced by euxinia that occurred at this time,<ref>{{cite journal |last1=Ogawa |first1=Yusuke |last2=Takahashi |first2=Kozo |last3=Yamanaka |first3=Toshiro |last4=Onodera |first4=Jonaotaro |date=30 July 2009 |title=Significance of euxinic condition in the middle Eocene paleo-Arctic basin: A geochemical study on the IODP Arctic Coring Expedition 302 sediments |url=https://www.sciencedirect.com/science/article/abs/pii/S0012821X09003446 |journal=[[Earth and Planetary Science Letters]] |volume=285 |issue=1–2 |pages=190–197 |doi=10.1016/j.epsl.2009.06.011 |bibcode=2009E&PSL.285..190O |access-date=6 April 2023}}</ref> led to stagnant waters and as the azolla sank to the sea floor, they became part of the sediments on the seabed and effectively sequestered the carbon by locking it out of the atmosphere for good. The ability for the azolla to sequester carbon is exceptional, and the enhanced burial of azolla could have had a significant effect on the world atmospheric carbon content and may have been the event to begin the transition into an ice house climate.<ref name="Speelman7" /> The azolla event could have led to a draw down of atmospheric carbon dioxide of up to 470 ppm. Assuming the carbon dioxide concentrations were at 900 ppmv prior to the [[Azolla Event]] they would have dropped to 430 ppmv, or 30 ppmv more than they are today, after the Azolla Event.<ref name="Speelman7" /> This cooling trend at the end of the EECO has also been proposed to have been caused by increased siliceous plankton productivity and marine carbon burial, which also helped draw carbon dioxide out of the atmosphere.<ref name="Pearson4" /> Cooling after this event, part of a trend known as the Middle-Late Eocene Cooling (MLEC),<ref name="ChristopherRobertScotese">{{Cite journal |last1=Scotese |first1=Christopher Robert |last2=Song |first2=Haijun |last3=Mills |first3=Benjamin J.W. |last4=van der Meer |first4=Douwe G. |date=April 2021 |title=Phanerozoic paleotemperatures: The earth's changing climate during the last 540 million years |url=https://linkinghub.elsevier.com/retrieve/pii/S0012825221000027 |journal=[[Earth-Science Reviews]] |language=en |volume=215 |pages=103503 |doi=10.1016/j.earscirev.2021.103503 |bibcode=2021ESRv..21503503S |access-date=24 September 2023}}</ref> continued due to continual decrease in atmospheric carbon dioxide from organic productivity and [[weathering]] from [[mountain building]].<ref name="Bohaty8" /> Many regions of the world became more arid and cold over the course of the stage, such as the Fushun Basin.<ref name="FushunBasinJijuntunLakes" /> In East Asia, lake level changes were in sync with global sea level changes over the course of the MLEC.<ref name="LakeLevelEoceneEastAsia">{{Cite journal |last1=Ma |first1=Yiquan |last2=Fan |first2=Majie |last3=Li |first3=Mingsong |last4=Ogg |first4=James G. |last5=Zhang |first5=Chen |last6=Feng |first6=Jun |last7=Zhou |first7=Chunhua |last8=Liu |first8=Xiaofeng |last9=Lu |first9=Yongchao |last10=Liu |first10=Huimin |last11=Eldrett |first11=James S. |last12=Ma |first12=Chao |date=15 January 2023 |title=East Asian lake hydrology modulated by global sea-level variations in the Eocene warmhouse |url=https://www.sciencedirect.com/science/article/pii/S0012821X22005611 |journal=[[Earth and Planetary Science Letters]] |volume=602 |pages=117925 |doi=10.1016/j.epsl.2022.117925 |bibcode=2023E&PSL.60217925M |issn=0012-821X |access-date=24 September 2023}}</ref>

Global cooling continued until there was a major reversal from cooling to warming in the Bartonian. This warming event, signifying a sudden and temporary reversal of the cooling conditions, is known as the [[Middle Eocene Climatic Optimum]] (MECO).<ref name="ItalyMECO" /><ref>{{cite journal |last1=Shi |first1=Juye |last2=Jin |first2=Zhijun |last3=Liu |first3=Quanyou |last4=Zhang |first4=Rui |last5=Huang |first5=Zhenkai |date=March 2019 |title=Cyclostratigraphy and astronomical tuning of the middle eocene terrestrial successions in the Bohai Bay Basin, Eastern China |url=https://www.sciencedirect.com/science/article/abs/pii/S0921818118300729 |journal=[[Global and Planetary Change]] |volume=174 |pages=115–126 |bibcode=2019GPC...174..115S |doi=10.1016/j.gloplacha.2019.01.001 |s2cid=135265513 |access-date=3 January 2023}}</ref> At around 41.5 Ma, stable isotopic analysis of samples from [[Southern Ocean]] drilling sites indicated a warming event for 600,000 years.<ref name="Bohaty8" /> A similar shift in carbon isotopes is known from the Northern Hemisphere in the Scaglia Limestones of Italy.<ref name="ItalyMECO">{{cite journal |last1=Jovane |first1=Luigi |last2=Florindo |first2=Fabio |last3=Coccioni |first3=Rodolfo |last4=Marsili |first4=Andrea |last5=Monechi |first5=Simonetta |last6=Roberts |first6=Andrew P. |last7=Sprovieri |first7=Mario |date=1 March 2007 |title=The middle Eocene climatic optimum event in the Contessa Highway section, Umbrian Apennines, Italy |url=https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/119/3-4/413/125390/The-middle-Eocene-climatic-optimum-event-in-the |journal=[[Geological Society of America Bulletin]] |volume=119 |issue=3–4 |pages=413–427 |doi=10.1130/B25917.1 |bibcode=2007GSAB..119..413J |access-date=18 May 2023}}</ref> [[Oxygen isotope]] analysis showed a large negative change in the proportion of heavier oxygen isotopes to lighter oxygen isotopes, which indicates an increase in global temperatures.<ref>{{cite journal |last1=Edgar |first1=Kirsty M. |last2=Wilson |first2=P. A. |last3=Sexton |first3=P. F. |last4=Gibbs |first4=S. J. |last5=Roberts |first5=Andrew P. |last6=Norris |first6=R. D. |date=20 November 2010 |title=New biostratigraphic, magnetostratigraphic and isotopic insights into the Middle Eocene Climatic Optimum in low latitudes |url=https://www.sciencedirect.com/science/article/abs/pii/S0031018210005869 |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=297 |issue=3–4 |pages=670–682 |doi=10.1016/j.palaeo.2010.09.016 |bibcode=2010PPP...297..670E |access-date=18 May 2023}}</ref> The warming is considered to be primarily due to carbon dioxide increases, because carbon isotope signatures rule out major methane release during this short-term warming.<ref name="Bohaty8" /> A sharp increase in atmospheric carbon dioxide was observed with a maximum of 4,000 ppm: the highest amount of atmospheric carbon dioxide detected during the Eocene.<ref name="Pearson9" /> Other studies suggest a more modest rise in carbon dioxide levels.<ref>{{cite journal |last1=Henehan |first1=Michael J. |last2=Edgar |first2=Kirsty M. |last3=Foster |first3=Gavin L. |last4=Penman |first4=Donald E. |last5=Hull |first5=Pincelli M. |last6=Greenop |first6=Rosanna |last7=Anagnostou |first7=Eleni |last8=Pearson |first8=Paul N. |date=9 March 2020 |title=Revisiting the Middle Eocene Climatic Optimum "Carbon Cycle Conundrum" With New Estimates of Atmospheric pCO2 From Boron Isotopes |url=https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019PA003713 |journal=[[Paleoceanography and Paleoclimatology]] |volume=35 |issue=6 |doi=10.1029/2019PA003713 |bibcode=2020PaPa...35.3713H |s2cid=216309293 |access-date=18 May 2023}}</ref> The increase in atmospheric carbon dioxide has also been hypothesised to have been driven by increased [[seafloor spreading]] rates and metamorphic decarbonation reactions between Australia and Antarctica and increased amounts of [[volcanism]] in the region. One possible cause of atmospheric carbon dioxide increase could have been a sudden increase due to metamorphic release due to continental drift and collision of India with Asia and the resulting formation of the [[Himalayas]]; however, data on the exact timing of metamorphic release of atmospheric carbon dioxide is not well resolved in the data.<ref name="Bohaty8" /> Recent studies have mentioned, however, that the removal of the ocean between Asia and India could have released significant amounts of carbon dioxide.<ref name="Pearson9" /> Another hypothesis still implicates a diminished negative feedback of silicate weathering as a result of continental rocks having become less weatherable during the warm Early and Middle Eocene, allowing volcanically released carbon dioxide to persist in the atmosphere for longer.<ref>{{cite journal |last1=Van der Ploeg |first1=Robin |last2=Selby |first2=David |last3=Cramwinckel |first3=Margot J. |last4=Li |first4=Yang |last5=Bohaty |first5=Steven M. |last6=Middelburg |first6=Jack J. |last7=Sluijs |first7=Appy |date=23 July 2018 |title=Middle Eocene greenhouse warming facilitated by diminished weathering feedback |journal=[[Nature Communications]] |volume=9 |issue=1 |page=2877 |bibcode=2018NatCo...9.2877V |doi=10.1038/s41467-018-05104-9 |pmc=6056486 |pmid=30038400}}</ref> Yet another explanation hypothesises that MECO warming was caused by the simultaneous occurrence of minima in both the 400 kyr and 2.4 Myr eccentricity cycles.<ref>{{cite journal |last1=Giorgioni |first1=Martino |last2=Jovane |first2=Luigi |last3=Rego |first3=Eric S. |last4=Rodelli |first4=Daniel |last5=Frontalini |first5=Fabrizio |last6=Coccioni |first6=Rodolfo |last7=Catanzariti |first7=Rita |last8=Özcan |first8=Ercan |date=27 June 2019 |title=Carbon cycle instability and orbital forcing during the Middle Eocene Climatic Optimum |journal=[[Scientific Reports]] |volume=9 |issue=1 |page=9357 |doi=10.1038/s41598-019-45763-2 |pmid=31249387 |pmc=6597698 |bibcode=2019NatSR...9.9357G }}</ref> During the MECO, sea surface temperatures in the Tethys Ocean jumped to 32–36 °C,<ref>{{cite journal |last1=Cramwinckel |first1=Margot J. |last2=Van der Ploeg |first2=Robin |last3=Van Helmond |first3=Niels A. G. M. |last4=Waarlo |first4=Niels |last5=Agnini |first5=Claudia |last6=Bijl |first6=Peter K. |last7=Van der Boon |first7=Annique |last8=Brinkhuis |first8=Henk |last9=Frieling |first9=Joost |last10=Krijgsman |first10=Wout |last11=Mather |first11=Tamsin A. |last12=Middelburg |first12=Jack J. |last13=Peterse |first13=Francien |last14=Slomp |first14=Caroline P. |last15=Sluijs |first15=Appy |display-authors=5 |date=1 September 2022 |title=Deoxygenation and organic carbon sequestration in the Tethyan realm associated with the middle Eocene climatic optimum |url=https://pubs.geoscienceworld.org/gsa/gsabulletin/article/135/5-6/1280/616563/Deoxygenation-and-organic-carbon-sequestration-in |journal=[[Geological Society of America Bulletin]] |volume=135 |issue=5–6 |pages=1280–1296 |doi=10.1130/B36280.1 |s2cid=252033074 |access-date=18 May 2023}}</ref> and Tethyan seawater became more dysoxic.<ref>{{cite journal |last1=Spofforth |first1=D. J. A. |last2=Agnini |first2=C. |last3=Pälike |first3=H. |last4=Rio |first4=D. |last5=Fornaciari |first5=E. |last6=Giusberi |first6=L. |last7=Luciani |first7=V. |last8=Lanci |first8=L. |last9=Muttoni |first9=G. |date=24 August 2010 |title=Organic carbon burial following the middle Eocene climatic optimum in the central western Tethys |url=https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2009PA001738 |journal=[[Paleoceanography and Paleoclimatology]] |volume=25 |issue=3 |pages=1–11 |doi=10.1029/2009PA001738 |bibcode=2010PalOc..25.3210S |access-date=18 May 2023}}</ref> A decline in carbonate accumulation at ocean depths of greater than three kilometres took place synchronously with the peak of the MECO, signifying [[ocean acidification]] took place in the deep ocean.<ref>{{cite journal |last1=Bohaty |first1=Steven M. |last2=Zachos |first2=James C. |last3=Florindo |first3=Fabio |last4=Delaney |first4=Margaret L. |date=9 May 2009 |title=Coupled greenhouse warming and deep-sea acidification in the middle Eocene |url=https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2008PA001676 |journal=[[Paleoceanography and Paleoclimatology]] |volume=24 |issue=2 |pages=1–16 |doi=10.1029/2008PA001676 |bibcode=2009PalOc..24.2207B |access-date=20 May 2023}}</ref> On top of that, MECO warming caused an increase in the respiration rates of [[Pelagic zone|pelagic]] [[Heterotroph|heterotrophs]], leading to a decreased proportion of [[Primary production|primary productivity]] making its way down to the seafloor and causing a corresponding decline in populations of benthic foraminifera.<ref>{{Cite journal |last1=Boscolo Galazzo |first1=Flavia |last2=Thomas |first2=Ellen |last3=Giusberti |first3=Luca |date=1 January 2015 |title=Benthic foraminiferal response to the Middle Eocene Climatic Optimum (MECO) in the South-Eastern Atlantic (ODP Site 1263) |url=https://www.sciencedirect.com/science/article/abs/pii/S003101821400491X |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |language=en |volume=417 |pages=432–444 |doi=10.1016/j.palaeo.2014.10.004 |bibcode=2015PPP...417..432B |access-date=19 November 2023}}</ref> An abrupt decrease in lakewater salinity in western North America occurred during this warming interval.<ref>{{cite journal |last1=Mulch |first1=Andreas |last2=Chamberlain |first2=C. P. |last3=Cosca |first3=Michael A. |last4=Teyssier |first4=Christian |last5=Methner |first5=Katharina |last6=Hren |first6=Michael T. |last7=Graham |first7=Stephan A. |display-authors=5 |date=April 2015 |title=Rapid change in high-elevation precipitation patterns of western North America during the Middle Eocene Climatic Optimum (MECO) |url=https://www.ajsonline.org/content/315/4/317.short |journal=[[American Journal of Science]] |volume=315 |issue=4 |pages=317–336 |doi=10.2475/04.2015.02 |bibcode=2015AmJS..315..317M |s2cid=129918182 |access-date=18 May 2023 |archive-date=19 May 2023 |archive-url=https://web.archive.org/web/20230519020516/https://www.ajsonline.org/content/315/4/317.short |url-status=dead }}</ref> This warming is short lived, as benthic oxygen isotope records indicate a return to cooling at ~40 Ma.<ref name="Pagani10" />