Global enhancement of ocean anoxia during oceanic anoxic event 2: A quantitative approach using U isotopes

Carolina Montoya-Pino, Stefan Weyer, Ariel D. Anbar, Jörg Pross, Wolfgang Oschmann, Bas van de Schootbrugge, Helge W. Arz

Research output: Contribution to journalArticlepeer-review

161 Scopus citations


During the Mesozoic greenhouse world, the oceans underwent several oceanic anoxic events (OAEs) characterized by intervals during which organic-rich black shales were deposited, indicating strong oxygen depletion in the marine realm. The Cenomanian-Turonian OAE2 (ca. 93 Ma) represents one of the most prominent events of the Cretaceous, with significant perturbations of the global carbon cycle. Although OAE2 likely reached a global scale, the spatial extent of seawater anoxia during this OAE is poorly constrained. Here we demonstrate that variations in the 238U/235U isotope ratio (δ238U), a newly developed paleoredox proxy, can be used to quantify the extent of marine anoxia. For black shales from the mid-Cretaceous OAE2 we find a systematic shift toward lighter δ238U and lower U concentrations as compared to modern equivalent organic-rich sediments from the Black Sea. This shift translates to a global increase of oceanic anoxia during OAE2 by at least a factor of three as compared to the present day or to periods before and after OAE2. The constant offset in U concentrations and isotope compositions of black shales throughout OAE2 compared to modern Black Sea sediments indicates an enhancement of oceanic anoxic conditions already prior to the onset of OAE2.

Original languageEnglish (US)
Pages (from-to)315-318
Number of pages4
Issue number4
StatePublished - Apr 2010

ASJC Scopus subject areas

  • Geology


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