ABSTRACT

The prevailing explanation for the origin of organic-rich sediments and rocks invokes deposition under conditions of anoxia. However, recent research suggests that high primary production and not water-column anoxia provides the first-order control on the accumulation of organic-rich facies in the modern oceans. Oxygen minima do not appear to have any direct effect on carbon accumulation in continental margin or marginal sea environments. Sediments accumulating in the modern Black Sea, the type euxinic basin, are not particularly enriched in organic matter despite the presence of an anoxic water column, although a sapropel containing extremely high carbon concentrations was deposited during the Holocene at a time when the basin was oxic. Results of a recently published coupled ocean-atmosphere model indicate that during the Cretaceous, thermohaline and surface circulation in the oceans was similar to or more intense than modern conditions, despite the overall equable climate. Such conditions confound the idea that circulation in the Cretaceous Atlantic, for example, was punctuated by oceanic anoxic events brought about by more sluggish circulation.

Sporadic temporal and spatial increases in primary production, reflecting changes in the behavior and/or state of the ocean-atmosphere system, constitute a more tenable explanation for the occurrence of modern and Quaternary carbon-rich sediments and Cretaceous black shales. Consequently, the fundamental control on the accumulation of carbon-rich facies in the oceans and marginal seas is not the presence or absence of anoxia.

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