ABSTRACT: In extended lower to mid-Pliocene stratigraphic record of near-shore environmental variation in the southwestern Ross Sea, Antarctica is preserved in ANDRILL’s McMurdo Ice Shelf Project AND-1B drillcore.A94meter-thick sequence of three diatomite and diatomaceous mudstone intervals represents the highest latitude record (78° S) of persistent open-marine conditions on the Antarctic continental shelf, in response to a protracted period of Pliocene warmth. This core provides new chronostratigraphic control for Antarctic paleoclimatic events, and describes a sequence of distinct diatom assemblages comprised of both extant and extinct species that are unknown on the Antarctic shelf today. Common taxa include those with modern distributions close to the Polar Frontal Zone (e.g. Shionodiscus tetraoestrupii, Stellarima stellaris, Thalassionema nitzschioides), suggesting marine temperatures in the southwestern Ross Sea warmer than at present. Representatives of the modern sea-ice assemblage (e.g. Fragilariopsis curta, Stellarima microtrias) and cold open-marine species associated with the retreating sea-ice front (e.g. Eucampia antarctica var. recta) are also present, but in much lower abundance than in the Ross Sea today as suggested by core top and surface sample data. The co-occurrence of species from these three distinct modern environments in Pliocene sediments, in close proximity to the continent, suggests that modern ecological zonal concepts cannot be applied directly in Pliocene paleoenvironmental analyses. Species present in the 4 m-thick lowest diatomite unit (~4.5 Ma) indicate a cold openmarine environment,with the period of deposition likely comprising the length of one interglacial period. The middle diatomite unit (~4.1-4.3 Ma; 14m) records warmer and perhaps seasonally mixed conditions with often abundant Thalassionema nitzschioides and Shionodiscus tetraoestrupii and little evidence of sea-ice associated species. Documentation of changes in diatom assemblages through the upper 75m-thick diatomite interval (~3.3-3.6Ma) offers the opportunity to resolve environmental limits of Pliocene diatom species thatwill serve as a guide to diatom-based paleoenvironmental interpretations for other Pliocene deposits. This upper unit records deposition during several glacial/interglacial cycles indicating an extended interval with open marine productivity during the mid-Pliocene. In addition, environmental associations for extinct species are inferred by their co-occurrence with extant taxa. Persistent intervals of pure diatomite suggest high productivity in an open-marine setting and minimal to no influence of sea ice. Commonly occurring species can be placed in five ecological cateories; Neritic/Stratified, Warm, Cold/Sea Ice, Heavy/Wind Mixed and Mixed. Comparison of this long diatom-bearing interval with coeval Antarctic deposits presents a regional and continental picture of Pliocene warmth, suggesting East Antarctic Ice Sheet was smaller with sub-polar marginal conditions.