PLANKTONIC FORAMINIFERAL POROSITY NEAR A CENOMANIAN OCEAN FRONT

Foraminifera construct more porous shells in warmer waters and less saline waters (Bé, 1968; Scott, 1972; Frerichs et al., 1972; Bé et al., 1973, 1976; Kennett, 1976; Caron et al., 1987). Bijma et al. (1990) found shells grown in waters of 28oC were 2.8 times as porous as those grown at 19.5oC. Bijma et al. (1990) also found that shells grown at the same temperature but different salinities were 1.3 to 1.7 times more porous in the less saline waters. Warmer and less saline waters result in higher porosity shells and also produce lower density seawater. Because of the relationship between temperature, salinity, and porosity and temperature, salinity and water density, planktic foraminiferal porosity can be used as a proxy for relative seawater densities. Using the present as a model for the past we have expanded on the work of Frerichs et. al. (1978) by using porosity as a paleoceanographic tool.

An Upper Cenomanian facies change is exposed in southeast Montana and northeast Wyoming. Dominantly non-calcareous shales in the northwest, intercalcate with dominantly calcareous shales to the southeast. Fisher (1991) interpreted this facies change as an ocean front. We have analyzed specimens of the common planktic foraminifera Hedbergella delrioensis near the proposed front. In the Calycoceras canitarurinum through Metoicoceras mosbyense ammonite zones Hedbergella delrioensis porosity is low and exhibits little variability. The low porosity suggests water of greater density (cooler/more saline). Up-section, in the Sciponoceras gracile and Neocardioceras juddii zones, porosity increases. The increase in porosity suggest water density decreased (warmer/less saline). The increase in porosity coincides with sea-level rise and Fisher's (1991) proposed northwestward migration of the front.