Cretaceous Thermal Maximum
{{short description|Period of climatic warming that reached its peak approximately 90 million years ago}}
The Cretaceous Thermal Maximum (CTM), also known as Cretaceous Thermal Optimum, was a period of climatic warming that reached its peak approximately 90 million years ago (90 Ma) during the Turonian age of the Late Cretaceous epoch. The CTM is notable for its dramatic increase in global temperatures characterized by high carbon dioxide levels.
Characteristics
During the Cretaceous Thermal Maximum (CTM), atmospheric carbon dioxide levels rose to over 1,000 parts per million (ppm) compared to the pre-industrial average of 280 ppm. Rising carbon dioxide resulted in a significant increase in the greenhouse effect, leading to elevated global temperatures.{{Cite journal|last=Rothman|first=Daniel H.|date=2002-04-02|title=Atmospheric carbon dioxide levels for the last 500 million years|journal=Proceedings of the National Academy of Sciences of the United States of America|language=en|volume=99|issue=7|pages=4167–4171|doi=10.1073/pnas.022055499|issn=0027-8424|pmc=123620|pmid=11904360|bibcode=2002PNAS...99.4167R|doi-access=free}} In the seas, crystalline or "glassy" foraminifera predominated, a key indicator of higher temperatures. The CTM began during the Cenomanian/Turonian transition and was associated with a major disruption in global climate as well as global anoxia during Oceanic Anoxic Event 2 (OAE-2).Norris, Richard (2018). "Cretaceous Thermal Maximum ~85-90 Ma." Scripps Institution of Oceanography. Accessed 20 September 2018. http://scrippsscholars.ucsd.edu/rnorris/book/cretaceous-thermal-maximum-85-90-ma {{Webarchive|url=https://web.archive.org/web/20180920235002/http://scrippsscholars.ucsd.edu/rnorris/book/cretaceous-thermal-maximum-85-90-ma |date=2018-09-20 }} The CTM was one of the most extreme disruptions of the carbon cycle in the past 100 million years.Foster, A., et al. "The Cretaceous Thermal Maximum and Oceanic Anoxic Event 2 in the Tropics: Sea- Surface Temperature and Stable Organic Carbon Isotopic Records from the Equatorial Atlantic." American Geophysical Union, Fall Meeting 2006. The Smithsonian/NASA Astrophysics Data System. Web. 20 Oct. 2009.
Geological causes
From 250 to 150 Ma, Pangaea covered the Earth's surface, forming one super continent and one gargantuan ocean. During the breakup of Pangaea from 150 to 130 Ma, the Atlantic Ocean began to form the "Atlantic Gateway".{{Cite journal|last1=Pucéat|first1=Emmanuelle|last2=Lécuyer|first2=Christophe|last3=Sheppard|first3=Simon M. F.|last4=Dromart|first4=Gilles|last5=Reboulet|first5=Stéphane|last6=Grandjean|first6=Patricia|date=2003-05-03|title=Thermal evolution of Cretaceous Tethyan marine waters inferred from oxygen isotope composition of fish tooth enamels|journal=Paleoceanography|language=en|volume=18|issue=2|pages=1029|doi=10.1029/2002pa000823|issn=0883-8305|bibcode=2003PalOc..18.1029P|doi-access=free}} Geological records from both the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP) support the enhancement of the CTM by the rifting of the Atlantic Ocean. Rising atmospheric carbon dioxide is thought to have been enhanced by the changing geography of the oceans. While rising carbon dioxide levels caused increased global warming, the climate models of the Cretaceous period do not show such elevated global temperatures due to the Earth's carbon dioxide variations. Geologic records show evidence of dissociation of methane clathrates, which causes a rise in carbon dioxide, as the oxygen gas in the atmosphere will oxidize the released methane.{{Cite journal|last1=Jahren|first1=A. Hope|last2=Arens|first2=Nan Crystal|last3=Sarmiento|first3=Gustavo|last4=Guerrero|first4=Javier|last5=Amundson|first5=Ronald|date=2001|title=Terrestrial record of methane hydrate dissociation in the Early Cretaceous|url=https://pubs.geoscienceworld.org/gsa/geology/article/29/2/159/192003/terrestrial-record-of-methane-hydrate-dissociation|journal=Geology|language=en|volume=29|issue=2|pages=159–162|doi=10.1130/0091-7613(2001)029<0159:TROMHD>2.0.CO;2|issn=0091-7613|bibcode=2001Geo....29..159J}}
Progression with time
Measurements of the ratio of stable oxygen isotopes in samples of calcite from foraminifera from sediment cores show gradual warming starting in the Albian period and leading to the interval of peak warmth in the Turonian{{Cite journal|last1=Clarke|first1=Leon J.|last2=Jenkyns|first2=Hugh C.|date=1999|title=New oxygen isotope evidence for long-term Cretaceous climatic change in the Southern Hemisphere|url=https://pubs.geoscienceworld.org/gsa/geology/article/27/8/699/198010/new-oxygen-isotope-evidence-for-long-term|journal=Geology|language=en|volume=27|issue=8|pages=699–702|doi=10.1130/0091-7613(1999)027<0699:NOIEFL>2.3.CO;2|issn=0091-7613|bibcode=1999Geo....27..699C}} followed by a gradual cooling of surface temperatures to the end of the Maastrichitan age.{{Cite journal|last1=Huber|first1=Brian T.|last2=Hodell|first2=David A.|last3=Hamilton|first3=Christopher P.|date=October 1995|title=Middle–Late Cretaceous climate of the southern high latitudes: Stable isotopic evidence for minimal equator-to-pole thermal gradients|url=https://pubs.geoscienceworld.org/gsa/gsabulletin/article/107/10/1164/182984/middle-late-Cretaceous-climate-of-the-southern|journal=Geological Society of America Bulletin|language=en|volume=107|issue=10|pages=1164–1191|doi=10.1130/0016-7606(1995)107<1164:MLCCOT>2.3.CO;2|issn=0016-7606|bibcode=1995GSAB..107.1164H}} During the Turonian, several pronounced but relatively short-lived cooler intervals punctuate the otherwise remarkably stable interval of extreme warmth.
Impact
Late Cenomanian sea surface temperatures (SSTs) in the equatorial Atlantic Ocean were substantially warmer than today (~27-29 °C). Turonian equatorial SSTs are conservatively estimated based on δ18O and high pCO2 estimates to have been ~32 °C, but may have been as high as 36 °C.Wilson, Paul A., Richard D. Norris, and Matthew J. Cooper. "Testing the Cretaceous greenhouse hypothesis using glassy foraminiferal calcite from the core of the Turonian tropics on Demerara Rise." Geology 30.7 (2002):607-610. Web. Oct.2009.
