| Abstract: | Modern studies support the hypothesis that shell porosity in planktic foraminifera reflect the temperature and salinity of the water in which they grow. Foraminifera construct more porous shells in warmer waters and less saline waters. Bijma et al. (1990) found shells grown in waters of 28°C were up to 2.8 times as porous as those grown at 19.5°C. They 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. Modern planktic foraminifera are less diverse at lower salinities and do not inhabit modern waters <30.5 psu (Boltovskoy, 1976), so shell porosity of diverse ancient planktic foraminiferal assemblages must be primarily in response to paleo-temperature. Both properties are largely responsible for water density, and therefore porosity may be a proxy for relative water mass density, which is useful in interpreting paleo-ocean circulation.
Application of the planktic foraminiferal porosity method to the mid-Cretaceous has produced promising results. Specimens of the widely distributed mid-Cretaceous species Hedbergella delrioensis have been temporally studied through several sections and spatially studied along time-lines from many geographic localities across the Greenhorn Seaway. Stratigraphic studies show that porosity increases from the upper Cenomanian Metoicoceras mosbyense Zone into the lower Turonian Watinoceras Zone. The porosity increase is coincident with previously proposed sea-level rise. The porosity increase is interpreted as an increase in the influence of warm water that entered from the Tethyan Ocean. Geographic studies along time-lines reveal high porosities in the central seaway, with porosity decreasing shoreward and northward. The distributions are interpreted as a stratified seaway, consisting of a warm central near-surface water underlain by deeper waters that surfaced shoreward and northward. As sea-level rose the warmer central core expanded north and shoreward. If porosities are used as relative density proxy data, contour maps of porosity can be used to reconstruct geostrophic flow. |