Cenomanian-Turonian boundary event

The Cenomanian-Turonian boundary event, or the Cenomanian-Turonian extinction event, the Cenomanian-Turonian anoxic event (OAE 2), and referred also as the Bonarelli event,[2] was one of two anoxic extinction events in the Cretaceous period. (The other being the earlier Selli event, or OAE 1a, in the Aptian.[3]) Selby et al. in 2009 concluded the OAE 2 occurred approximately 91.5 ± 8.6 Ma,[4] though estimates published by Leckie et al. (2002) are given as 93–94 Ma.[5] The Cenomanian-Turonian boundary has been refined in 2012 to 93.9 ± 0.15 Ma[6] There was a large carbon disturbance during this time period. However, apart from the carbon cycle disturbance, there were also large disturbances in the oxygen and sulfur cycles of the ocean.

System/
Period		 Series/
Epoch		 Stage/
Age		 Age (Ma)
Paleogene Paleocene Danian younger
Cretaceous Upper/
Late		 Maastrichtian 66.0 72.1
Campanian 72.1 83.6
Santonian 83.6 86.3
Coniacian 86.3 89.8
Turonian 89.8 93.9
Cenomanian 93.9 100.5
Lower/
Early		 Albian 100.5 ~113.0
Aptian ~113.0 ~125.0
Barremian ~125.0 ~129.4
Hauterivian ~129.4 ~132.9
Valanginian ~132.9 ~139.8
Berriasian ~139.8 ~145.0
Jurassic Upper/
Late		 Tithonian older
Subdivision of the Cretaceous system
according to the ICS, as of 2017.[1]

Background

The Cenomanian and Turonian stages were first noted by D'Orbigny between 1843 and 1852. The global type section for this boundary is located in the Bridge Creek Limestone Member of the Greenhorn Formation near Pueblo, Colorado, which are bedded with the Milankovitch orbital signature. Here, a positive carbon-isotope event is clearly shown, although none of the characteristic, organic-rich black shale is present. It has been estimated that the isotope shift lasted approximately 850,000 years longer than the black shale event, which may be the cause of this anomaly in the Colorado type section.[7] A significantly expanded OAE2 interval from southern Tibet documents a complete, more detailed, and finer-scale structures of the positive carbon isotope excursion that contains multiple shorter-term carbon isotope stages amounting to a total duration of 820 ±25 ka.[8]

The boundary is also known as the Bonarelli event because of 1-to-2-metre (3 ft 3 in to 6 ft 7 in) layer of thick, black shale that marks the boundary and was first studied by Guido Bonarelli in 1891.[9] It is characterized by interbedded black shales, chert and radiolarian sands and is estimated to span a 400,000-year interval. Planktonic foraminifera do not exist in this Bonarelli level, and the presence of radiolarians in this section indicates relatively high productivity and an availability of nutrients.

Cenomanian-Turonian event

Causes

One possible cause of this event is sub-oceanic volcanism, possibly the Caribbean large igneous province, with increased activity approximately 500,000 years earlier. During that period, the rate of crustal production reached its highest level for 100 million years. This was largely caused by the widespread melting of hot mantle plumes under the ocean crust, at the base of the lithosphere. This may have resulted in the thickening of the oceanic crust in the Pacific and Indian Oceans. The resulting volcanism would have sent large quantities of carbon dioxide into the atmosphere, leading to an increase in global temperatures. Within the oceans, the emission of SO2, H2S, CO2, and halogens would have increased the acidity of the water, causing the dissolution of carbonate, and a further release of carbon dioxide. When the volcanic activity declined, this run-away greenhouse effect would have likely been put into reverse. The increased CO2 content of the oceans could have increased organic productivity in the ocean surface waters. The consumption of this newly abundant organic life by aerobic bacteria would produce anoxia and mass extinction.[10] The resulting elevated levels of carbon burial would account for the black shale deposition in the ocean basins.[11]

Large igneous provinces and their possible contribution

Several independent events related to large igneous provinces (LIP) occurred around the time of OAE2. Within the time period from about 95 to 90 million years ago, two separate LIP events occurred; the Madagascar and the Caribbean-Colombian. Trace metals such as chromium (Cr), scandium (Sc), copper (Cu) and cobalt (Co) have been found at the Cenomanian-Turonian boundary, which suggests that an LIP could have been one of the main basic causes involved in the contribution of the event.[12]

The timing of the peak in trace metal concentration coincides with the middle of the anoxic event, suggesting that the effects of the LIPs may have occurred during the event, but may not have initiated the event. Other studies linked the lead (Pb) isotopes of OAE-2 to the Caribbean-Colombian and the Madagascar LIPs.[13]

A modeling study performed in 2011 confirmed that it is possible that a LIP may have initiated the event, as the model revealed that the peak amount of carbon dioxide degassing from volcanic LIP degassing could have resulted in more than 90 percent global deep-ocean anoxia.[14]

Effects

The event brought about the extinction of the pliosaurs, and most ichthyosaurs. Coracoids of Maastrichtian age were once interpreted by some authors as belonging to ichthyosaurs, but these have since been interpreted as plesiosaur elements instead.[15] Although the cause is still uncertain, the result starved the Earth's oceans of oxygen for nearly half a million years, causing the extinction of approximately 27 percent of marine invertebrates, including certain planktic and benthic foraminifera, mollusks, bivalves, dinoflagellates and calcareous nannofossils.[10] The global environmental disturbance that resulted in these conditions increased atmospheric and oceanic temperatures. Boundary sediments show an enrichment of trace elements, and contain elevated δ13C values.[11]

The δ13C isotope excursion

The positive δ13C isotope excursion found at the Cenomanian-Turonian boundary is one of the main carbon isotope events of the Mesozoic. It represents one of the largest disturbances in the global carbon cycle from the past 110 million years. This δ13C isotope excursion indicates a significant increase in the burial rate of organic carbon, indicating the widespread deposition and preservation of organic carbon-rich sediments and that the ocean was depleted of oxygen at the time.[16][17][18] Within the positive carbon isotope excursion, short eccentricity scale carbon isotope variability is documented in a significantly expanded OAE2 interval from southern Tibet.[8]

Changes in oceanic biodiversity and its implications

The alterations in diversity of various marine invertebrate species such as calcareous nannofossils indicate a time when the oceans were warm and oligotrophic, in an environment with short spikes of productivity followed by long periods of low fertility. A study performed in the Cenomanian-Turonian boundary of Wunstorf, Germany, reveal the uncharacteristic dominance of a calcareous nannofossil species, Watznaueria, present during the event. Unlike the Biscutum species, which prefer mesotrophic conditions and were generally the dominant species before and after the C/T boundary event; Watznaueria species prefer warm, oligotrophic conditions.[19]

At the time, there were also peak abundances of green algal groups Botryococcus and prasinophytes, coincident with pelagic sedimentation. The abundances of these algal groups are strongly related to the increase of both the oxygen deficiency in the water column and the total organic carbon content. The evidence from these algal groups suggest that there were episodes of halocline stratification of the water column during the time. A species of freshwater dinocyst—the Bosedinia was also found in the rocks dated to the time and these suggest that the oceans had reduced salinity.[20][21]

See also

References
  1. Super User. "ICS - Chart/Time Scale". www.stratigraphy.org.
  2. Cetean, Claudia G.; Balc, Ramona; Kaminski, Michael A.; Filipescu, Sorin (August 2008). "Biostratigraphy of the Cenomanian-Turonian boundary in the Eastern Carpathians (Dâmboviţa Valley): preliminary observations". Studia Universitatis Babeş-Bolyai, Geologia. 53 (1): 11–23. doi:10.5038/1937-8602.53.1.2.
  3. Li, Yong-Xiang; Bralower, Timothy J.; Montañez, Isabel P.; Osleger, David A.; Arthur, Michael A.; Bice, David M.; Herbert, Timothy D.; Erba, Elisabetta; Premoli Silva, Isabella (2008-07-15). "Toward an orbital chronology for the early Aptian Oceanic Anoxic Event (OAE1a, ≈120 Ma)". Earth and Planetary Science Letters. 271 (1–4): 88–100. Bibcode:2008E&PSL.271...88L. doi:10.1016/j.epsl.2008.03.055.
  4. Selby, David; Mutterlose, Jörg; Condon, Daniel J. (July 2009). "U–Pb and Re–Os geochronology of the Aptian/Albian and Cenomanian/Turonian stage boundaries: Implications for timescale calibration, osmium isotope seawater composition and Re–Os systematics in organic-rich sediments". Chemical Geology. 265 (3–4): 394–409. Bibcode:2009ChGeo.265..394S. doi:10.1016/j.chemgeo.2009.05.005.
  5. Leckie, R; Bralower, T.; Cashman, R. (2002). "Oceanic anoxic events and plankton evolution: Biotic response to tectonic forcing during the mid-Cretaceous" (PDF). Paleoceanography. 17 (3): 1–29. Bibcode:2002PalOc..17.1041L. doi:10.1029/2001pa000623.
  6. Meyers, Stephen R.; Siewert, Sarah E.; Singer, Brad S.; Sageman, Bradley B.; Condon, Daniel J.; Obradovich, John D.; Jicha, Brian R.; Sawyer, David A. (January 2012). "Intercalibration of radioisotopic and astrochronologic time scales for the Cenomanian-Turonian boundary interval, Western Interior Basin, USA". Geology. 40 (1): 7–10. Bibcode:2012Geo....40....7M. doi:10.1130/g32261.1. ISSN 1943-2682.
  7. Sageman, Bradley B.; Meyers, Stephen R.; Arthur, Michael A. (2006). "Orbital time scale and new C-isotope record for Cenomanian-Turonian boundary stratotype". Geology. 34 (2): 125. Bibcode:2006Geo....34..125S. doi:10.1130/G22074.1. S2CID 16899894.
  8. Li, Yong-Xiang; Montañez, Isabel P.; Liu, Zhonghui; Ma, Lifeng (March 2017). "Astronomical constraints on global carbon-cycle perturbation during Oceanic Anoxic Event 2 (OAE2)". Earth and Planetary Science Letters. 462: 35–46. Bibcode:2017E&PSL.462...35L. doi:10.1016/j.epsl.2017.01.007. ISSN 0012-821X.
  9. G. Bonarelli, Il territorio di Gubbio - Notizie geologiche, Roma 1891
  10. "Submarine eruption bled Earth's oceans of oxygen". New Scientist. 16 July 2008. Retrieved 2018-05-09.(subscription required)
  11. Kerr, Andrew C. (July 1998). "Oceanic plateau formation: a cause of mass extinction and black shale deposition around the Cenomanian–Turonian boundary?". Journal of the Geological Society. 155 (4): 619–626. Bibcode:1998JGSoc.155..619K. doi:10.1144/gsjgs.155.4.0619.
  12. Ernst, Richard E.; Youbi, Nasrrddine (July 2017). "How Large Igneous Provinces affect global climate, sometimes cause mass extinctions, and represent natural markers in the geological record". Palaeogeography, Palaeoclimatology, Palaeoecology. 478: 30–52. Bibcode:2017PPP...478...30E. doi:10.1016/j.palaeo.2017.03.014.
  13. Kuroda, J; Ogawa, N; Tanimizu, M; Coffin, M; Tokuyama, H; Kitazato, H; Ohkouchi, N (15 April 2007). "Contemporaneous massive subaerial volcanism and late cretaceous Oceanic Anoxic Event 2". Earth and Planetary Science Letters. 256 (1–2): 211–223. Bibcode:2007E&PSL.256..211K. doi:10.1016/j.epsl.2007.01.027. ISSN 0012-821X.
  14. Flögel, S.; Wallmann, K.; Poulsen, C.J.; Zhou, J.; Oschlies, A.; Voigt, S.; Kuhnt, W. (May 2011). "Simulating the biogeochemical effects of volcanic CO2 degassing on the oxygen-state of the deep ocean during the Cenomanian/Turonian Anoxic Event (OAE2)". Earth and Planetary Science Letters. 305 (3–4): 371–384. Bibcode:2011E&PSL.305..371F. doi:10.1016/j.epsl.2011.03.018. ISSN 0012-821X.
  15. Sachs, Sven; Grant‐Mackie, Jack A. (March 2003). "An ichthyosaur fragment from the Cretaceous of Northland, New Zealand". Journal of the Royal Society of New Zealand. 33 (1): 307–314. doi:10.1080/03014223.2003.9517732.
  16. Nagm, Emad; El-Qot, Gamal; Wilmsen, Markus (December 2014). "Stable-isotope stratigraphy of the Cenomanian–Turonian (Upper Cretaceous) boundary event (CTBE) in Wadi Qena, Eastern Desert, Egypt". Journal of African Earth Sciences. 100: 524–531. Bibcode:2014JAfES.100..524N. doi:10.1016/j.jafrearsci.2014.07.023. ISSN 1464-343X.
  17. Jenkyns, Hugh C. (March 2010). "Geochemistry of oceanic anoxic events: REVIEW". Geochemistry, Geophysics, Geosystems. 11 (3): n/a. Bibcode:2010GGG....11.3004J. doi:10.1029/2009GC002788.
  18. Schlanger, S. O.; Arthur, M. A.; Jenkyns, H. C.; Scholle, P. A. (1987). "The Cenomanian-Turonian Oceanic Anoxic Event, I. Stratigraphy and distribution of organic carbon-rich beds and the marine δ 13 C excursion". Geological Society, London, Special Publications. 26 (1): 371–399. doi:10.1144/GSL.SP.1987.026.01.24. ISSN 0305-8719.
  19. Linnert, Christian; Mutterlose, Jörg; Erbacher, Jochen (February 2010). "Calcareous nannofossils of the Cenomanian/Turonian boundary interval from the Boreal Realm (Wunstorf, northwest Germany)". Marine Micropaleontology. 74 (1–2): 38–58. Bibcode:2010MarMP..74...38L. doi:10.1016/j.marmicro.2009.12.002. ISSN 0377-8398.
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  21. Fonseca, Carolina; Mendonça Filho, João Graciano; Lézin, Carine; de Oliveira, António Donizeti; Duarte, Luís V. (December 2019). "Organic matter deposition and paleoenvironmental implications across the Cenomanian-Turonian boundary of the Subalpine Basin (SE France): Local and global controls". International Journal of Coal Geology. 218: 103364. doi:10.1016/j.coal.2019.103364.

Further reading
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