The oxygen isotopic composition of marine diatom frustules (silica-bound oxygen only) is thought to reflect the (paleo-) ocean surface water conditions and the precipitation temperature of the opaline material. Isotope analysis of water-containing opal-A, however, is difficult to perform as secondary isotope exchange reactions might alter the primary oxygen-18 content of the silica. In order to study the paleoceanographic potential of sedimentary diatom material we analysed recent diatom blooms from the Antarctic and the Norwegian-Greenland Sea where both the water temperature and the oxygen isotopic composition of the surface water were known. In addition, we also measured laboratory diatom cultures which grew under defined water and temperature conditions.
Oxygen from diatom samples was extracted by two independant methods. For the first time we show on the same diatom material that both techniques, progressive fluorination (the only direct method of silica-bound oxygen isotope analysis) and
the equilibration technique (which is the only applicable method when the amount of sample material is less than 40 mg) give reliable results of the (silica-bound) oxygen isotopic composition. The latter method requires replacement of the opaline water by water which is strongly enriched in 18-O (as it is the oxygen of the silica structure) prior to dehydration and fluorination.
Our results show that recent diatom blooms and laboratory cultures do not fractionate oxygen isotopes in equilibrium with sea- (and laboratory-) water in which they grew. Assuming isotopic equilibrium conditions under low growing temperatures
(0 to 4°C) a larger fractionation by some 8 between silica-bound oxygen and water should be established compared to what we measured. Moreover, a comparison with d18O values of sedimentary diatoms from the Antarctic indicates "later" isotope exchange between silica and sea- (pore-) water to occur. Isotope analyses from diatoms collected from both sediment traps and surface sediments focusses on the problem of isotope exchange reactions during sedimentation of diatoms and, thus, will enable us to evaluate the paleoceanographic potential of diatom isotope stratigraphy.