VARIABLE FORCING OF ANOXIC-OXIC CONDITIONS IN THE CRETACEUOS OCEAN

Three major changes in carbon reservoir occurred between 200–65 Ma in the western Tethys and are recorders of major changes in the Earth Systems. Driving forces for such changes are poorly understood and frequently are contraversional.

To achieve anoxia in deep ocean waters, replenishment of O2 from the surface must be slower than the rate of decomposition of organic matter. This implies sluggish vertical circulation. In contrast a larger supply of organic matter requires high bioproductivity in surface waters. This implies upwelling of nutrients and vigorous ocean circulation. Thus we arrive to Cretaceous conundrum. Change from “dysoxic” to “oxic” deep ocean occurring during Turonian time is not a synchronous event around western Tethys, therefore it is difficult to explain it by such hypothesis as the CO₂ drawdown, which further documents complexity of the interaction of the Earth Systems.

Hypothesis to be discussed in a view of new geochemical studies conclude that the deep water composition during the middle Cretaceous “ anoxic- black shale “ ocean period and late Cretaceous “oxic –pelagic red bed” ocean period, was not much different. The major difference between deep sea black shale facies and pelagic red beds is in that they record differences in redox conditions in the deposited sediments during early diagenesis, which is controlled by 3 main factors - the amount of dissolved oxygen in the deep ocean water, organic matter deposited at the sea floor and the sedimentation rate. Interplay between these 3 major factors however could be very complex as they respond to changes in climate, ocean geography, ocean circulation, plate motions and tectonics.

Furthermore, the Cretaceus Ocean was not a homogenous medium as indicated by the presence of middle Cretaceous organic carbon enriched black shale deposits in the deep basin, while at the some time on adjacent continental slope of the Blake-Bahama platform were deposited pelagic and hemipelagic red beds. The oxic bottom conditions result from the area being located within Horse paleolatitude, therefore calm sea and low surface mixing, thus low bioproductivity, higher salinity all leading to an extensive downwelling. Inhomogeniety in oceanic waters chemistry is seldom considered in marine oxygen isotope studies.