The paleo-depositional environment of inner Prydz Bay, East Antarctica, has been reconstructed for the past 21,320 14C yr B.P., using diatom assemblages and sediment facies from a short, 352-cm-long gravity core. Between 21,320 and 11,650 14C yr B.P., compact tillite and diamicton are present in the core, and diatom frustules are rare to absent. These data suggest that an ice sheet grounded over the site during the last glacial maximum. Following glacial retreat, siliceous muddy ooze was deposited, from 11,650 to 2600 14C yr B.P., in an open marine setting. During this stage, diatom frustules are abundant and well preserved, and Thalassiosira antarctica resting spores and Fragilariopsis curta dominate the assemblage. This assemblage suggests open marine deposition in an environment where the spatial and temporal distribution of sea ice is less than today. Since 2600 14C yr B.P., sea-ice and ice-edge diatom species have become more abundant, and neoglacial cooling is inferred. The assemblage is similar to that forming currently in Prydz Bay, where sea ice is absent (<10% cover) for 2–3 months of the year and permanent ice edge and/or multiyear sea ice remains in close proximity to the site.

Fifty four surface sediment samples from the western and central Ross Sea were examined to determine relationships between modern oceanographic conditions and surface sediment diatom assemblages. A centered R mode principal components analysis demonstrates four geographically distinct assemblages. The assemblage just north of the Ross Ice Shelf in the central Ross Sea is most closely associated with Stephanopyxis spp. (a heavily silicified diatom abundant during the Pliocene), and may result from a combination of winnowing/reworking, and modern flux of primarily non-siliceous algae. The algal assemblage in the western part of the central Ross Sea is most closely associated with Thalassiosira gracilis (an open water diatom), and reflects early seasonal pack ice break up during the late spring inception of the Ross Sea polynya. The algal assemblage north of Drygalski Ice Tongue, in the western Ross Sea, is most closely associated with Fragilariopsis curta (a diatom common in stratified ice edge zones), suggesting that water column seeding by species melting out of coastal sea ice is important in this area. The assemblage south of Drygalski Ice Tongue is most closely associated with resting spores of Thalassiosira antarctica (a diatom associated with coastal waters). Although the habitat of T. antarctica requires future research, we speculate that sea ice conditions unique to area B support an autumnal T. antarctica bloom.