CONFLICTING DATA FROM SEVERAL INDICES OF WATER COLUMN OXYGENATION IN THE EARLY TOARCIAN (JURASSIC) OF NORTH YORKSHIRE, UK: EVIDENCE FOR CYCLIC ANOXIA?

The Grey Shales lie below the well known Jet Rock and record a transition from shallow-water, well-oxygenated, oolitic ironstones to the laminated deeper-water organic-rich sediments of the Jet Rock itself. The Grey Shales are dominated by bioturbated siltstones but also contain three thin laminated black shales (here termed the Sulphur Bands). Samples were analysed for Fe, C and S geochemistry, framboid size distribution, and palaeoecology. Degree of Pyritization and Indicator of Anoxicity Fe ratios and oxygen-restricted biofacies data from the Sulphur Bands indicate that the water column was anoxic, and probably euxinic at the time of their deposition. In contrast, the framboid size distributions show characteristics that are similar to samples from upper dysaerobic facies. This apparent contradiction can be resolved by an interpretation of cyclic anoxia, perhaps developed on an annual basis. In anoxic basins framboids form in the water column and their size is controlled by the rate at which they sink out of the zone of formation at the chemocline. There are no such mass limitations in sediments deposited under a dysoxic or normally oxygenated water column. Hence, in the anoxic stage of Sulphur Band formation, small framboids formed in the water column and the proportion of Fe bound as pyrite increased. The abundant larger framboids record formation in conditions where the redox boundary lay within the sediment and the lower water column was oxygenated. The presence of broad storm scour-and-fill structures shows that the Sulphur Bands were deposited close to storm wave base. The inferred shallow depth of water (and the concomitant reduced reservoir of dissolved oxygen) would facilitate frequent transitions between anoxic and oxygenated conditions.