Evidence for Pliocene reduction of the East Antarctic Ice Sheet (EAIS) has been inferred from the presence of Pliocene planktic marine diatoms in tills of the Sirius Group now exposed along the slopes of the Transantarctic Mountains. We highlight deficiencies in this diatom evidence for a Pliocene reduction of the EAIS. First, we examine what diatom habitats are left behind by a retreating ice sheet, in this case the Fennoscandian Ice Sheet (FIS), and find that diatoms should occur in planktic and benthic marine, brackish water, freshwater, and terrestrial habitats. We expect that during former reexpansions, these diverse diatom assemblages were picked up by the advancing FIS and deposited in tills. This is shown to be the case, and diatoms found in tills from Sweden and Finland reflect the many habitats over which the FIS passed. If there was a reduction of the EAIS during the Pliocene followed by renewed glaciation, diatoms from many different habitats should be found in the resulting tills of the Sirius Group. This is not the case. Only marine planktic diatoms and a few freshwater forms are reported. Similarly, vertical (downsection) distribution of diatoms in Sirius Group and Fennoscandian tills differ. While in the Sirius Group, diatoms appear restricted to surface and near-surface sediments, in Fennoscandia they are found throughout the tills. Such data do not support a Pliocene reduction model for the EAIS.
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