Sulfidic (or euxinic) conditions, which exist today in many water bodies from ponds to various land-surrounded mediterranean seas such as the Black Sea, were particularly prevalent in the Cretaceous Atlantic but also characterised other parts of the world ocean. In an ice-free sea of these supposed super-greenhouse worlds, oceanic waters were as much as  higher, in some eras. During the timespans in question, the continental plates are believed to have been well separated, and the mountains as they are known today were (mostly) future tectonic events—meaning the overall landscapes were generally much lower— and even the half super-greenhouse climates would have been eras of highly expedited water erosion carrying massive amounts of nutrients into the world oceans fuelling an overall explosive population of microorganisms and their predator species in the oxygenated upper layers.
Detailed stratigraphic studies of Cretaceous black shales from many parts of the world have indicated that two oceanic anoxic events (OAEs) were particularly significant in terms of their impact on the chemistry of the oceans, one in the early Aptian (~120 Ma), sometimes called the Selli Event (or OAE 1a) after the Italian geologist Raimondo Selli (1916–1983), and another at the Cenomanian–Turonian boundary (~93 Ma), also called the Bonarelli Event (or OAE2) after the Italian geologist Guido Bonarelli (1871–1951). OAE1a lasted for ~1.0 to 1.3 Myr. The duration of OAE2 is estimated to be ~820 kyr based on a high-resolution study of the significantly expanded OAE2 interval in southern Tibet, China.
* Insofar as the Cretaceous OAEs can be represented by type localities, it is the striking outcrops of laminated black shales within the vari-coloured claystones and pink and white limestones near the town of Gubbio in the Italian Apennines that are the best candidates.
* The 1-metre thick black shale at the Cenomanian–Turonian boundary that crops out near Gubbio is termed the Livello Bonarelli after the scientist who first described it in 1891.
More minor oceanic anoxic events have been proposed for other intervals in the Cretaceous (in the Valanginian, Hauterivian, Albian and Coniacian–Santonian stages), but their sedimentary record, as represented by organic-rich black shales, appears more parochial, being dominantly represented in the Atlantic and neighbouring areas, and some researchers relate them to particular local conditions rather than being forced by global change.