(Figure 2) Diatom relative abundance of sediment core PS58/254-1

The goal of this study is to assess the changes that have occurred during the Mid-Pleistocene Transition, an important transitional period in cryosphere evolution, by examining the siliceous microfossil record of sediments collected proximal to a major ice drainage outlet for the West Antarctic Ice Sheet. Core PS58/254 was collected from a sediment drift on the upper continental rise in the Amundsen Sea, directly offshore from Pine Island Bay, one of the three main discharge areas for the West Antarctic Ice Sheet (WAIS). Published data on physical properties, geochemical composition, grain size and clay mineral assemblages are complemented here by a high-resolution record (sample spacing 10 cm) of the siliceous microfossil assemblages (diatoms and silicoflagellates). Between 1200 ka and 621 ka, the assemblage is relatively diverse, with Actinocyclus ingens, Thalassiothrix antarctica and Fragilariopsis kerguelensis dominating the assemblages, but diatom abundance is variable from low to barren. Additionally, the occurrence of A. ingens, Thalassiosira elliptipora and Thalassiosira fasciculata is used to confirm and further refine the existing age model and extend it back to 1200 ka. Species composition during the last ca. 621 ka is dominated by F. kerguelensis, which consistently comprises 80-90% of the assemblage. A clear relationship between diatom abundance and glacial/interglacial variability is apparent after 621 ka, which resembles the glacial-interglacial variability previously observed in other proxy data.