• journal_title：Geology
• Contributor：Carolina Montoya-Pino ; Stefan Weyer ; Ariel D. Anbar ; Jörg Pross ; Wolfgang Oschmann ; Bas van de Schootbrugge ; Helge W. Arz
• Publisher：Geological Society of America
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.1130/G30652.1
• journal_abbrev：Geology
• issn：0091-7613
• volume：38
• issue：4
• firstpage：315
• section：Articles

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.