ELSEVIER Earth and Planetary Science Letters 158 (1998) 165–173 A euxinic southern North Atlantic Ocean during the Cenomanian=Turonian oceanic anoxic event Jaap S. Sinninghe Damste ´ Ł , Ju ¨rgen Ko ¨ster Department of Marine Biogeochemistry and Toxicology, Netherlands Institute for Sea Research (NIOZ), P.O.Box 59, 1790 AB Den Burg, Texel, Netherlands Received 16 December 1997; accepted 7 March 1998 Abstract During mid-Cretaceous times large amounts of organic carbon (C org ) became sequestered in ‘black shales’, possibly due to ‘oceanic anoxic events’ characterized by the development of an extended oxygen minimum zone (OMZ). Here, we provide the first direct evidence for an open ocean OMZ in the Cenomanian=Turonian (C=T) southern North Atlantic and in fact show that the base of the photic zone was euxinic as revealed by molecular fossils from photosynthetic green sulfur bacteria in C=T black shales. This, together with evidence for bottom water anoxia and accumulation of redox-sensitive trace metals and hydrogen-rich organic matter, indicates a continuously euxinic water column. Concurrent with the high C org accumulation rates, which are 15–150 times greater in the southern than in the northern North Atlantic, and the low biological productivity, this suggests that preservation controlled the accumulation of C org in C=T black shales.  1998 Elsevier Science B.V. All rights reserved. Keywords: anaerobic environment; Cenomanian; Turonian; black shale; paleo-oceanography; southern North Atlantic 1. Introduction One of the most striking results of the Deep Sea Drilling Project during the 1970s was the discov- ery that during the Middle Cretaceous the oceans were sites of deposition of sediments anomalously enriched in C org (‘black shales’). It has been pro- posed that during black shale deposition the oceans were locally or regionally deficient of oxygen pos- sibly caused by intensification and expansion of the OMZ and such events have been termed oceanic anoxic events (OAE) [1–3]. During the widespread Ł Corresponding author. Tel.: C31 222 369550; Fax: C31 222 369569; E-mail: damste@nioz.nl Cenomanian=Turonian (C=T) OAE the burial of iso- topically light C org was even so extensive that the oceanic and atmospheric CO 2 reservoirs became rel- atively enriched, which has been recorded in sedi- mentary biogenic carbonates and C org as a positive carbon isotope excursion [2–4]. Some authors con- sider the C=T OAE as a global phenomenon and assume that “significant parts of the world ocean were periodically oxygen deficient” [2,3] whilst oth- ers (e.g. [5]) believe that there is no evidence for ‘global anoxia’. The C=T ‘black shales’ were de- posited in epicontinental seas, on the continental slope and in abyssal sites of the oceans [1–9] but were generally deposited in water depths of >100– 200 m. Black shales are best developed on the east- 0012-821X/98/$19.00  1998 Elsevier Science B.V. All rights reserved. PII S0012-821X(98)00052-1