Accumulation of organic-carbon-rich sediments in the Late Jurassic and Cretaceous Atlantic Ocean — A synthesis

Abstract

Organic-carbon-rich sediments (“black shales”) were deposited at some 40 Deep Sea Drilling Project (DSDP) sites in the Atlantic Ocean during Late Jurassic and Cretaceous times. In order to reconstruct the depositional environments of black shale formation, we have studied in detail changes in the occurrence of black shales as well as the type and amount of organic matter in relation to changes in paleogeography, paleooceanic circulation and paleoclimate. We have calculated accumulation rates of marine and terrigenous organic carbon and combined these with organic carbon content—sedimentation rate relationships to obtain information about paleoenvironments. Main factors causing the preservation and accumulation of organic carbon are: (1) the supply of terrestrial organic matter; (2) the supply of marine organic matter; (3) the oxygenation of the deep water; and (4) the bulk sedimentation rate. Changes in the importance of these different factors and changes in the interrelationships among them determine the complex system of black shale formation and their cyclic variations. Increased supply of terrestrial organic matter due to increased river discharge and humid climate in Northeast America, Southwest Europe and South Africa were important in the Hauterivian—Albian Northwest and northern Northeast Atlantic and in the Late Jurassic—Aptian South Atlantic. In the latter, anoxic deep-water conditions due to restricted circulation in the nascent largely isolated basin as well as higher oceanic productivity, may have reinforced the high preservation rates of marine organic matter during that time. Off Northwest Africa, higher flux rates of marine organic matter were recorded in areas of higher productivity which may have been caused by fluvial nutrient supply during the Hauterivian and by intensified upwelling during Albian/Turonian times. In general, the depositional environment was oxygenated and the content of marine organic matter was a function of the sedimentation rate. Furthermore, at some places redeposition by turbidity currents or slumps may have been important for the accumulation of organic matter in the deep-sea basins. The only episode of widespread anoxia in the western and eastern basins of the North Atlantic may have occurred in the Cenomanian—Turonian when high amounts of marine organic matter were also preserved in areas of low productivity and low sedimentation rates.