Reappraisal of Anoxia and Organic Richness, with Emphasis on Cretaceous of North Atlantic

Abstract

Accumulation of anoxic sediments in the North Atlantic during Barremian through Turonian time (Early to middle Cretaceous) was controlled mainly by local conditions, although global climatic and oceanographic factors played a supporting role. Evidence does not support the popular hypothesis developed over the past few years of global or oceanwide anoxia. Data which contradict this hypothesis are based on (1) a reevaluation of criteria for identifying sediments that accumulated in anoxic water, (2) a critical reappraisal of reported worldwide occurrences of anoxic sediments of these ages, and (3) an examination of the spatial and temporal distribution of anoxic sediments in the North Atlantic and elsewhere. A model is proposed in which the earliest anoxic sediments of Cretaceous age in the North Atlantic were deposited in deep, restricted basins. By middle Cenomanian time (about 97 m.y. ago), sluggish circulation had led to a gradual expansion of the oxygen-minimum layer, permitting deposition of anoxic sediment in nonbasinal settings as well. Expansion of the oxygen-minimum layer caused the calcite compensation depth to rise, promoting oxidation of organic carbon and causing contraction of the oxygen-minimum layer. Development of anoxia was thus self-damping. Anoxia effectively disappeared by early Senonian time (88 m.y. ago), when improved circulation created oceans more like those of the present. Some evidence indicates that enhanced biologic productivity and upwelling may have been local factors in fostering anoxia, but the Early and middle Cretaceous were generally not highly productive times.