Quantitative studies of the preservation and biogeographic distribution of calcareous nannofossils in Mid-Cretaceous sediments from the Atlantic are combined with observations on the temporal and spatial distribution of major facies patterns to put constraints on speculations about the paleoceanographic evolution of the Atlantic about 110 to 90 m.y.BP. Organic-rich sediments in deep ocean and continental margin settings are indicative of organic carbon influx from terrestrial and marine sources and considerable carbonate dissolution largely caused by carbon dioxide production during catabolic breakdown of organic matter. Preservation of organic carbon in sediments is the result of rapid burial, especially along continental margins, and/or low concentrations of oxygen in warm and saline bottom waters derived largely from marginal evaporite basins. In the eastern basin of the Atlantic perhaps only the interstitial waters within the sediments were anoxic. There is an overall negative correlation between calcium carbonate and organic carbon content in organic-rich sediments. Coccolith preservation is generally poorest in the most organic carbon-rich shales. Semiquantitative preservation estimates are as sensitive as quantitative estimates using dissolution indices.

Paleogeographic patterns of coccolith distribution show weak latitudinal gradients and more pronounced neritic- oceanic gradients. Boreal and austral assemblages first observed in the later half of the Mid-Cretaceous are restricted to latitudes greater than 40°. Rapid spatial and temporal fluctuation in coccolith abundance, including monospecific blooms, are typical of higher latitudes and restricted basins (e.g., Angola Basin). Neritic assemblages are found along the continental margin of the eastern Atlantic and over the Walvis-Rio Grande Ridge and are possibly indicative of increased advection of nutrient-rich waters. Oceanic assemblages characterize areas far removed from continents such as deep basin and Mid-Atlantic Ridge sites. At the beginning of the Upper Cretaceous, better deepwater connections, the rise in sea-level, and the development of stronger temperature gradients resulted in more vigorous mixing of the deep and surface waters and reduced detrital organic carbon input. Stagnation of deep basins ended, and surface-water temperature-gradients became stronger.