| Summary: | The oxygen isotopic composition (δ 18 O) of diatom frustules is potentially a quantitative paleoenvironmental proxy. However, δ 18 O analysis has some uncertainties because of difficulties in removing clay-mineral contamination and taxon-specific separation due to the small size of diatom frustules. The clay-mineral contamination seriously affects δ 18 O measurements, and the δ 18 O of bulk diatoms contains a mixture of information from diatoms living in various seasons and water depths. To resolve such problems, we established a new method for purifying the frustules of disc-shaped diatoms using a cell sorter and succeeded in removing the clay-mineral contamination and restricting the number of species for the δ 18 O analysis. We developed this method using samples from a sediment core retrieved from the Conrad Rise in the Indian Ocean sector of the Southern Ocean. First, the disc-shaped diatoms were enriched and cleaned by sieving, elutriation, and heavy-liquid separation. The samples were then further purified by a cell sorter. Regardless of the diatom assemblage or the sediment type in the core sample, this method generated samples consisting of more than 95% disc-shaped diatoms, dominated by Thalassiosira lentiginosa (Janisch) Fryxell, which occurs ubiquitously throughout the Southern Ocean. The δ 18 O profile for the sediment core obtained from purified disc-shaped diatoms showed the last glacial cycle since ca. 40 ka, which can be correlated with the LR04 benthic δ 18 O stack record and the temperature record in the EPICA Dome C ice core, indicating that this method can contribute to obtaining an accurate δ 18 O record of diatoms associated with glacial–interglacial cycles.
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