New analytical approaches have made possible the high quality, reproducible 18O/16O analysis of diatom silica. These techniques could be applied to diatomaceous sediments from oceanic and lacustrine environments to obtain detailed information on paleoclimate and climate change. However, some evidence suggest that the fractionation of oxygen isotopes between biogenic opal and water may be kinetically controlled. If so, diatom 18O may depend partly on the silicic acid content of the water from which diatoms extract their silica. Because the silica concentration of ancient lakes and seas would seldom be known, a silicic-acid dependence--if on exists--could diminish the precision of an otherwise promising paleothermometer. In this investigation, marine and lacustrine diatoms will be cultured to learn whether their 18O depends on the silicic acid content of the environmental water. A kinetic fractionation model will be developed if a dependence is found. Diatom silica is valuable source of information having application to paleohydrologic studies.
