EXAMINING THE RELATIONSHIP BETWEEN CENOMANIAN PLANKTONIC FORAMINIFERAL SPECIES POROSITIES AND THEIR MORPHOTYPE-INFERRED DEPTH DISTRIBUTIONS

The Cretaceous Cenomanian-Turonian was a time of dynamic oceanographic conditions in the Western Interior Seaway of North America. As sea level rose planktonic foraminifera occurrence expanded laterally to the east and west and to the north. Planktonic foraminiferal diversity also increased as the depth of the Seaway increased. It is generally assumed that the increase in diversity was the result of an increase in vertical or depth niches. The senior author has shown that the porosity of the widely distributed, relatively shallow-dwelling species Hedbergella delrioensis, is useful in reconstructing the geographic distribution of relative water mass density. We are now investigating the relationship between the porosity of various species of planktonic foraminifera and their previously inferred depth habitats. In order to maximize our depth coverage we have selected a variety of planktonic morphotypes. Porosities are compared to the relatively shallow-dwelling species Hedbergella delrioensis. Studied taxa, listed in order of increasing inferred-depth below the Hedbergella delrioensis, include species of Globigerinelloides, Clavihedbergella, and Rotalipora. Modern investigations provide the basis for our method. It has been documented that modern planktonic foraminifera porosity can prove an effective proxy for ocean water temperature and salinity, the combination of which determine water density. It has also been shown that in the Modern Ocean porosity varies with latitude. Tests grown at low latitudes display greater porosity than those from progressively higher latitudes.