Oxygen-isotope fractionation between marine biogenic silica and seawater

A stepwise fluorination technique has been used to selectively react away the water component of hydrous silica in order to better investigate the oxygen-isotope fractionation between biogenic opal and seawater, and to determine whether all taxa produce opal which is suitable for oxygen isotope paleothermometry. δ¹⁸O of the tetrahedrally coordinated silicate oxygen of siliceous sponge spicules grown at a wide variety of temperatures varies independently of temperature. δ¹⁸O from an Eocene radiolarian ooze sample is much more enriched than would be expected from any reasonable isotopic temperature curve, given the probable growing temperature of the sample. δ¹⁸O of diatom samples seems to vary systematically with temperature and to conform approximately to the isotopic temperature curve for diatom frustules obtained by Labeyrie and coworkers using an entirely different analytical technique. Sponges appear to precipitate silica in isotopic disequilibrium with seawater oxygen, and old radiolarian silica may exchange readily with cold oceanic bottom water. Neither will apparently be useful for paleo-climate reconstructions. Diatoms may be useful in deducing ancient surface-water temperatures, but the systematic variation of α with temperature for diatoms may not be related to the quartz-H₂O equilibrium isotope fractionation.