Predicting Locations of Carbon Preservation in Cretaceous

John Southam

The widespread deposition of organic-rich, black shales during the Cretaceous indicates that significant parts of the global ocean were periodically anoxic. Enhanced accumulation of organic carbon is usually attributed to increased primary production in regions of upwelling and/or increased input of carbon from terrestrial sources. An alternative hypothesis is that the enhanced accumulation was due to oxygen-depleted depositional environments.

The difficulty associated with increasing the carbon flux to the deep oceans is appreciated by consideration of the present-day fluxes. The carbon production from upwelling regions and from terrestrial sources is only 0.1% and 2-4% respectively of the total net carbon from the euphotic zone. Therefore, to achieve Cretaceous accumulation rates would require dramatic increases in these carbon sources.

To produce anoxic deep water that preserves organic matter requires an initial source of dense surface water, depleted in oxygen. As this dense water sinks and spreads, its oxygen will be further depleted by oxidation of settling organic matter. Two settings that can produce the initially dense water are marginal seas like the Mediterranean and seas with wind-induced trapping like the Wedell Sea. The trajectory of the sinking water will be determined by Coriolis effects, dilution by entrainment of interior water, and the local bathymetry.

The keys to predicting source rocks formed by this proposed mechanism are, together with paleogeographic maps, simple models of Cretaceous oceanic atmospheric circulation.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.