text: The concept of the oceanic anoxic event (OAE) was first proposed in 1976 by Seymour Schlanger (1927–1990) and geologist Hugh Jenkyns and arose from discoveries made by the Deep Sea Drilling Project (DSDP) in the Pacific Ocean. The finding of black, carbon-rich shales in Cretaceous sediments that had accumulated on submarine volcanic plateaus (e.g. Shatsky Rise, Manihiki Plateau), coupled with their identical age to similar, cored deposits from the Atlantic Ocean and known outcrops in Europe—particularly in the geological record of the otherwise limestone-dominated Apennines chain in Italy—led to the observation that these widespread, similarly distinct strata recorded very unusual, oxygen-depleted conditions in the world's oceans spanning several discrete periods of geological time.
  Modern sedimentological investigations of these organic-rich sediments typically reveal the presence of fine laminations undisturbed by bottom-dwelling fauna, indicating anoxic conditions on the seafloor believed to coincide with a low-lying poisonous layer of hydrogen sulfide, HS. Furthermore, detailed organic geochemical studies have recently revealed the presence of molecules (so-called biomarkers) that derive from both purple sulfur bacteria and green sulfur bacteria—organisms that required both light and free hydrogen sulfide (HS), illustrating that anoxic conditions extended high into the photic upper-water column.
  This is a recent understanding, the puzzle having been pieced slowly together in the last three decades. The handful of known and suspected anoxic events have been tied geologically to large-scale production of the world's oil reserves in worldwide bands of black shale in the geologic record.