The Antarctic Polar Front, which is characterized in the modern ocean by SST's of ca. 4-5°C (February) and 1-2°C (August), apparently was shifted by about 6° of latitude to the south in the eastern South Atlantic and Indian Ocean at 3 Ma. The Sub-Antarctic Front (SAF), which forms the northern boundary of the Polar Frontal Zone (PFZ) and which separates diatom-rich sediments in the south from diatom-poor, coccolith-rich sediments in the north, seems to have experienced little (ca. 2°) if any southward displacement in the mid-Pliocene, resulting in an expanded PFZ. In the modern ocean the SAF is characterized by SST's of ca. 7-10°C (February) and 4-5°C (February). South of the 3 Ma PFZ (ca. 56°S), mid- Pliocene Antarctic diatom assemblages were dominated by provincial diatom taxa (Nitzschia barronii, N. interfrigidaria, Rouxia spp., Thalassiosira lentiginosa, and T. oliverana).
Evidence for very much reduced sea-ice in the high-latitude Southern Ocean below the upper part of the Gauss normal-polarity chron (>2.9 Ma) includes a greatly reduced number of sea ice-related diatoms (Eucampia antarctica, Nitzschia curta, and N. cylindra), as well as the presence of well-preserved diatomaceous sediments in the high-latitude Weddell Sea. Mid-Pliocene diatom assemblages from drill holes adjacent or near to the Antarctic continent - DVDP10 and DSDP 274- contain only rare to absent E. antarctica and N. curta, and are suggestive of very minimal sea ice. Consequently, only minimal sea-ice is inferred in PRISM reconstructions for the Antarctic coast for the Antarctic summer. Unfortunately, diatoms cannot provide evidence for the Antarctic winter extent of sea-ice. For PRISM reconstructions, winter sea-ice was arbitrarily expanded to about 71°S to conform to the northern limits of the East Antarctic in the mid-Pliocene. Because sea-ice has an SST of -1.6°C, areas predicted to have been covered with sea ice at 3 Ma were assigned an SST of -1.6°C.