Oceanic Anoxic Event 2

Oceanic Anoxic Event 2
Name	Oceanic Anoxic Event 2
Date	Cenomanian–Turonian boundary (~94.5 Ma)
Location	Global (notably Tethys, Atlantic, Pacific margins)
Type	Marine anoxia, black shale deposition

Oceanic Anoxic Event 2 was a major mid-Cretaceous interval of widespread marine anoxia centered on the Cenomanian–Turonian boundary (~94.5 million years ago) associated with organic-rich black shale deposition, a global positive carbon isotope excursion, and significant biotic and environmental change. It links to contemporaneous processes recorded in the Cretaceous stratigraphic record and has been studied through correlations among sections tied to chronostratigraphy, isotopic stratigraphy, and paleontological turnover. Research on this event integrates data from International Ocean Discovery Program, classic localities such as the GSSP-equivalent sections, and work by investigators affiliated with institutions like Smithsonian Institution, University of Cambridge, and ETH Zurich.

Background and Timing

The event is temporally constrained to the Cenomanian–Turonian boundary and the early Turonian, overlapping biozones defined by cephalopod and foraminiferal turnovers documented in the chronostratigraphy of the Geological Time Scale and magnetostratigraphic frameworks developed by teams from Lamont–Doherty Earth Observatory, Caltech, and USGS. Age models combine radiometric dates from zircon-bearing tuffs in sites studied by researchers at University of California, Berkeley and correlation with the global δ13C excursion recognized in cores collected by the Deep Sea Drilling Project and the Ocean Drilling Program.

Causes and Mechanisms

Multiple interacting drivers are invoked, including enhanced volcanic CO2 release from the Caribbean Large Igneous Province and plume activity related to the Ontong Java Plateau, nutrient loading tied to intensified weathering on continents such as Laurasia and Gondwana, and changes in ocean circulation influenced by paleogeography reconstructed by groups at University of Texas at Austin and Paleomap Project. Hypothesized mechanisms also involve feedbacks among greenhouse warming, stratification of water columns studied in models by National Center for Atmospheric Research and Princeton University, and euxinic conditions mediated by sulfur cycling researched by teams at Lamont–Doherty Earth Observatory and Woods Hole Oceanographic Institution.

Geological and Geochemical Evidence

The signature includes deposition of organic-rich black shales preserved in basins like the Bohemia-region, Western Interior Seaway, and Tethyan shelves documented by fieldwork from University of Vienna and Natural History Museum, London. Geochemical markers include a pronounced global positive δ13C excursion recorded in carbonate and organic carbon measured by laboratories at Max Planck Institute for Chemistry and Scripps Institution of Oceanography, elevated total organic carbon (TOC) concentrations, enrichments of redox-sensitive trace metals such as molybdenum and uranium investigated by researchers at ETH Zurich and University of Southern California, and biomarkers indicating photic-zone euxinia reported by analysts at Shell Oil Company and academic groups at University of Bristol.

Biological and Ecological Impacts

Biotic responses include extinction and turnover patterns in pelagic and benthic faunas: significant losses among rudist bivalves documented in collections of the Natural History Museum, Paris, planktonic foraminifera shifts recorded by teams at University of Tokyo, and radiations within certain ammonite lineages curated at Muséum national d'Histoire naturelle. Phytoplankton productivity changes are inferred from coccolithophore microfossil assemblages studied by investigators at Plymouth University and dinoflagellate cyst records compiled by researchers at University of Oslo. Ecosystem restructuring recorded in the Western Interior Seaway influenced vertebrate assemblages preserved in museums like the Royal Tyrrell Museum.

Spatial Distribution and Stratigraphy

Stratigraphic expression varies across depositional settings: expanded black shale intervals in epicontinental seas such as the Western Interior Seaway contrast with condensed records on passive margins like the Cretaceous Atlantic documented by the Bureau of Economic Geology and the GeoForschungsZentrum Potsdam. Correlation uses multiple proxies—δ13C, plankton biostratigraphy, and magnetostratigraphy—integrated by teams from International Commission on Stratigraphy and regional geological surveys including British Geological Survey and Geological Survey of Japan to map the lateral extent and facies architecture of the OAE-associated strata.

Climatic Consequences and Carbon Cycle

The interval is linked to global warmth recorded in oxygen isotope studies conducted at ETH Zurich and Paleoceanography and Paleoclimatology-affiliated groups, with estimates of elevated pCO2 derived from stomatal proxy work at University of Göttingen and boron isotope analyses performed by laboratories at University of Washington. The positive carbon isotope excursion implies large-scale redistribution of carbon between reservoirs, modeled by carbon cycle studies from University of Cambridge and the National Oceanography Centre, and featuring feedbacks with ocean anoxia and organic carbon burial quantified by researchers at Princeton University.

Research History and Debates

The event was first recognized through black shale correlations described by investigators publishing in venues associated with Geological Society of America and later refined through international drilling campaigns by the Deep Sea Drilling Project and International Ocean Discovery Program. Debates persist over the relative roles of volcanic forcing linked to the Caribbean Large Igneous Province versus orbital pacing hypotheses advanced by scientists at University of Edinburgh and MIT, the importance of nutrient versus circulation controls argued by groups at Woods Hole Oceanographic Institution and Columbia University, and the degree to which local versus global drivers explain heterogeneity highlighted by researchers at University of Barcelona.

Category:Cretaceous events