| Summary: | The recently recovered ANDRILL McMurdo Ice Shelf AND-1B drillcore provides an opportunity to address the work presented herein. A new diatom biostratigraphic zonation is introduced for this near-shore Antarctic region. Unique diatom assemblages present in this core reflect a distinct coastal marine setting; open ocean marker species are more common in the early Pliocene with the younger part of the core containing a more endemic assemblage. Differences in assemblages between the open ocean and this coastal setting result in four new zones being proposed, one in the early Pliocene, three in the late Pliocene to early Pleistocene. The presence of an increasingly endemic assemblage is interpreted to result from regional cooling and the early beginnings of the modern sea-ice dominated environment. However, the establishment of this modern environment is not recorded in AND-1B due to the lack of diatomaceous sediment and pronounced glacial activity in the late Pleistocene interval of this core. The biostratigraphic levels of two new species, Fragilariopsis tigris and Thalassiosira teres, are introduced with this new zonation. The new zonation and early to mid-Pliocene diatom assemblages of the AND-1B drillcore are compared to other drillcores in the McMurdo Sound region. The preservation and richness of these assemblages in AND-1B document an open marine environment not present in other cores recovered from sites on the continental margin. The new ANDRILL record expands our knowledge of the early to mid-Pliocene, allowing a better understanding of the Pliocene environment of McMurdo Sound and the expression of this in assemblages from sites at the paleo-coast and open marine locations. The AND-1B biostratigraphic framework, which shares some species in common with Southern Ocean sites, indicates some intervals of the coastal cores are younger than previously interpreted. Southern Ocean marker species in the early to mid-Pliocene are observed to have incomplete biostratigraphic ranges in McMurdo Sound. Comparison of ...
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