Cenomanian–Turonian boundary

Cenomanian–Turonian boundary
Name	Cenomanian–Turonian boundary
Period	Late Cretaceous
Age ma	~93.9
Location	Global
Type	Chronostratigraphic boundary

Cenomanian–Turonian boundary is the formal chronostratigraphic boundary between the Cenomanian and Turonian stages of the Late Cretaceous that marks a major turnover in Earth's stratigraphy and paleoclimate near 93.9 million years ago; it is associated with the globally recognized Oceanic Anoxic Event 2 (OAE2), pronounced geochemical excursions, and substantial biotic shifts recorded in marine and terrestrial sections.

Definition and Chronostratigraphy

The boundary defines the base of the Turonian Stage and the top of the Cenomanian Stage within the Cretaceous Series and is tied to a suite of chronostratigraphic markers including ammonite zonation, calcareous nannofossil assemblages, and planktonic foraminiferal turnovers documented across classic sections such as those in GSSP candidates; it anchors the International Commission on Stratigraphy timescale used by institutions like the International Union of Geological Sciences, the Geological Society of America, and the Royal Society for regional and global correlation.

Global Stratotype Section and Point (GSSP) and Correlation

The formal Global Boundary Stratotype Section and Point is defined at a GSSP locality that integrates biostratigraphic, chemostratigraphic, and magnetostratigraphic data and is compared with reference sections in regions such as the Chalk Group, the Western Interior Seaway, the Bulotina exposures, and classic European and African type sections; correlations employ datum ties to ammonite biozones used by researchers affiliated with the Paleontological Society, the European Geosciences Union, and national geological surveys including the United States Geological Survey and the British Geological Survey.

Oceanic Anoxic Event 2 and Geochemical Signatures

OAE2 is recorded across the boundary by prominent negative and positive excursions in carbon isotope ratios (δ13C), authigenic enrichments in trace metals such as molybdenum and uranium, widespread deposition of organic-rich black shales, and mercury anomalies that have been studied by teams from universities including Harvard University, University of Oxford, and ETH Zurich; geochemical fingerprints also include shifts in strontium isotope ratios (87Sr/86Sr), osmium isotopes, and evidence for volcanic input linked in some studies to magmatism from provinces analogous to the Kerguelen Plateau and the Carlsberg Ridge as discussed in publications by the Geological Society of London.

Paleoclimate, Sea-Level and Environmental Changes

Paleoclimate reconstructions across the boundary indicate peak greenhouse conditions with elevated sea-surface temperatures, expansion of epeiric seas such as the Western Interior Seaway, and major sea-level highstands recorded in carbonate platforms like the Book Cliffs and Sierra Madre, with climate modeling undertaken by groups at NASA, NOAA, and the Max Planck Society linking CO2-driven warming, ocean stratification, and changes in precipitation patterns that impacted terrestrial floras documented in floristic assemblages from the Iberian Peninsula and Kazakhstan.

Biotic Responses and Extinction/Turnover Events

Marine biota experienced selective extinctions and turnovers at the boundary including declines in benthic foraminifera, radiations among planktonic foraminifera and calcareous nannoplankton, ammonite faunal restructuring, and perturbations in marine reptile and fish assemblages recorded in Lagerstätten comparable to Chorhankhta, with paleontological studies by curators at the Smithsonian Institution, the Natural History Museum, London, and the Muséum national d'Histoire naturelle documenting taxonomic shifts, biogeographic provincialism, and subsequent evolutionary radiations.

Regional Stratigraphy and Key Type Sections

Prominent regional stratigraphic records across the boundary include the pelagic chalks of England's White Chalk Group, hemipelagic successions in the Aptian–Turonian basins of France, the black shale horizons in Jordan and Israel, and siliciclastic sequences in the Gabon and South Atlantic margins; type sections and regional correlations have been the focus of field campaigns supported by national academies such as the French National Centre for Scientific Research and the National Autonomous University of Mexico.

Methods of Study and Chronology (Biostratigraphy, Isotope Stratigraphy, Radiometric Dating)

Chronology across the boundary integrates biostratigraphy using ammonites, planktonic foraminifera, and calcareous nannofossils as established by experts from the International Paleontological Association and the International Nannoplankton Association, high-resolution isotope stratigraphy employing δ13C and δ18O curves generated in laboratories at Caltech and Scripps Institution of Oceanography, and radiometric dating (U-Pb zircon and Ar-Ar) of intercalated ash beds tied to analytical facilities at Lamont–Doherty Earth Observatory and the Geological Survey of Canada; Bayesian age modeling and cyclostratigraphic tuning using orbital solutions from teams at the University of Bergen and the University of Vienna refine the boundary age to the current standard adopted by the International Commission on Stratigraphy.

Category:Geologic boundaries Category:Late Cretaceous