THE CENOMANIAN TURONIAN ANOXIC EVENT IN EUROPE - AN ORGANIC GEOCHEMICAL STUDY

Thirty-two sediment samples, mainly black shales from Cenomanian/Turonian organic-rich sequences in Europe and Tunisia, were examined by organic geochemical techniques. The results of molecular analyses indicate that all the black shales are dominated by marine-derived organic matter from both algal and bacterial sources, with only a minor terrestrial component. Oxygen-deficient conditions prevailed during the deposition of the black shales, although interbedded non-laminated, organic-lean horizons indicate that oxygen deficiency was intermittent at some sites. The presence of 17α(H), 18α(H), 21β(H)-28, 30-bisnorhopane in some black shale samples from shallow shelf areas may be evidence for severely oxygen-depleted conditions, possibly in association with upwelling and an oxygen-minimum layer. Overall, the organic geochemical data are in agreement with a depositional model whereby an oxygen-minimum layer developed in response to high surface productivity. This oxygen-minimum layer may have extended over a considerable thickness of the water column (>1000 m) in some areas. We report the first observation of an A-ring methylated C29 hopane, namely methyl-17α(H), 18α(H), 21β(H)-28, 30-bisnorhopane. The environmental significance of this compound cannot yet be assessed, although in the sample in which it occurs, the direct parallel between the carbon number distribution of the hopanes and that of the methylhopanes suggests either that they have the same biological origin or that methylation of hopanoids may have occurred, possibly during diagenesis. © 1990.