A sediment core from Jane Basin, northern Weddell Sea, contains distinct bands, up to 3 cm thick, of well preserved diatom frustules, contrasting with the generally poor preservation of adjacent material. Most notably, the fragile diatom genus Corethron is abundant in the layers and found as whole valves, while in the remainder of the core is rare and valves are broken and corroded. Extrapolation from contemporary data on phytoplankton production and standing crop suggests that the silicon content of the diatom ooze layers is very much greater than could be expected from a single year's phytoplankton production sedimenting from a corresponding area of the water column. Assuming direct, quantitative sedimentation, it would have taken decades to centuries to accumulate 3 cm of diatom ooze. The preservation of these layer indicates that unusual conditions prevailed, so that diatom valves settled through the water column more-or-less unaltered and remained preserved in this state in the sediment. Mass sedimentation of a diatom bloom followed by rapid burial seems the most likely combination of circumstances that could have caused this. It is unlikely that this would have occurred consistently over a long period. Horizontal concentration must have taken place, and this is most likely to have been associated with a physical feature of the water column such as an eddy or “chimney”, which entrained phytoplankton biomass from a wide area of the surface mixed layer and caused it to sediment to a smaller area of the seabed.