ABSTRACT: Accumulation and Preservation of Organic Carbon in Marine Sediments: The Roles of Anoxia vs. Production

S. E. Calvert, T. F. Pedersen

Organic carbon enrichments in marine sediments and sedimentary rocks commonly are explained by the preferential preservation of the deposited organic matter under anoxic conditions; the role of primary organic (plankton) production is seldom considered. A review of the available information shows that modern marine sediment accumulating in oxic and anoxic basins in similar topographic and sedimentary settings have very similar carbon contents. On continental slopes, carbon maxima are apparently produced by the complex interplay between the supply of carbon to the sea floor, the texture of the sediment, the dilution of carbon by other sediment components, and the decreasing settling flux of carbon in the deeper waters of the open ocean. Contrary to contemporary thought, th re is no causal relationship between such maxima and the position of the oxygen minimum.

The degradation of sedimentary organic matter by aerobes and by sulfate reducers is very similar where the supply of fresh organic matter to the sea floor is similar. Hence, there is no evidence for the preferential preservation of organic matter under anoxic conditions. Terrestrial organic matter, however, appears to be degraded to a lesser extent by sulfate reducers. The burial of carbon below the surficial, oxygenated horizons of a sediment removes the easily oxidized fractions leaving material that may be less susceptible to attack by sulfate reducers.

Sedimentary carbon maxima in Pleistocene glacial horizons are due to the increased settling flux of organic matter brought about by climatically induced increases in upwelling in the equatorial and marginal areas of the ocean. Changes in bottom water oxygen levels during these periods plays a minor role in producing these signals.

Previous work that claimed that anoxic bottom waters were prevalent during the accumulation of organic-rich black shales in the geological record should be reevaluated. Such facies more likely reflect marked changes in ocean circulation and fertility and can provide important information on the feedbacks involved in the evolution of the Earth's climate system.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990