Identification of isorenieratane and isorenieratane derivatives from Leg 207 samples is consistent with previous work (Kuypers et al., 2002) that shows intermittent events of photic zone anoxia must have taken place during the deposition of the Cretaceous black shales at the Demerara Rise, even though the timing and extent of these events cannot be resolved. The repeated recurrence of organic matter–rich black shales suggests that excellent conditions for organic matter preservation occurred during Albian–Santonian time.
The low sedimentation rates appear to contradict high-productivity conditions as a major driving factor for anoxia, which would be needed to explain the genesis of black shales on the Demerara Rise. However, sedimentation could have been affected by postdepositional reworking of material (erosion or winnowing processes) on the seafloor or, at least partially, by dissolution of the carbonate phase in the sediments, both leading in consequence to lowered sedimentation rates. An alternative way of maintaining oxygen-depleted conditions in the water column over such a long timespan is to assume permanent stratification (e.g., of the silled basin type) or to import anoxia from a more distal and deeper location in the proto-Atlantic Ocean (stagnating ocean model for Cretaceous black shales) by upwelling of strongly O2-depleted oceanic water masses. This latter interpretation is consistent with the idea of stagnant Cretaceous oceans and would explain how large amounts of sulfur could be provided for its incorporation into the organic matter. However, higher sedimentation rates would favor the buildup of an expanded and permanent OMZ mainly driven by elevated surface water bioproductivity as a likely alternative scenario for the Cretaceous black shale sedimentation on the Demerara Rise.