PLANKTONIC FORAMINIFERAL POROSITY: A PROXY FOR SEA SURFACE TEMPERATURE AND RELATIVE DENSITY

Prior field and laboratory work conducted on planktonic foraminifera indicates that temperature and salinity affect shell porosity. 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 28^oC were 2.8 times as porous as those grown at 19.5^oC. Early field researchers suggested that planktonic foraminifera construct less dense (more porous) shells as a buoyancy response (Bé, 1968; Scott, 1972; Frerichs et al., 1972; and Bé et al., 1973, 1976; and Kennett, 1976). Bijma et al. (1990) proposed that in warmer waters there is a corresponding increase in metabolism or growth rates, which might require higher oxygen consumption and therefore more porous shell construction. The link between pore size and gas exchange is extrapolated from work conducted on benthic foraminifera respiration (Berthold et al., 1976). Bijma (1990) 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. The cause of increased porosity with decreased salinity is unclear because oxygen is more soluble in lower salinity waters; but may be the result of higher metabolic rates associated with increased osmotic pressure (Bijma et al., 1990).

Laboratory work shows that temperature may produce up to twice the pore size variability of salinity. Modern planktonic foraminifera are less diverse at lower salinities and do not inhabit modern waters <30.5 psu (Boltovskoy, 1976), so shell porosity of diverse ancient planktonic foraminiferal assemblages must be primarily in response to paleo-temperature. Warmer and less saline waters result in higher porosity shells and both produce lower density seawater. Therefore, regardless of the actual physiological cause, porosity is a proxy for relative seawater densities. Using the present as a model for the past I have expanded on the work of Frerichs et. al. (1978) by using porosity as a paleoceanographic tool. Relative densities of water masses are represented by porosity data plotted on late Cenomanian time lines. The maps are contoured, used to reconstruct geostrophic flow, and compared to population analyses of foraminifera and nannofossils.