Evidence for warm periods in Antarctica during the Neogene has produced considerable debate (e.g., Webb et al., 1984; Warnke et al., 1996). The Prydz Bay region and Prince Charles Mountains have outcrops of late Miocene and early Pliocene deposits that probably formed under relatively warm conditions (Whitehead et al., 2001; Whitehead and McKelvey, 2001), and the Leg 188 drill sections also provide fragmentary evidence of warm episodes.
Pospichal (this volume) notes the sporadic presence of warmer-water nannoplankton and thin chalk horizons throughout the Miocene at Site 1165. Whitehead and Bohaty (2003) report Pliocene diatom assemblages at Sites 1165 (16–22 mbsf) and 1166 indicative of sea-ice concentrations less than those of today on the continental rise and on the shelf. Samples from these sites cover the range of ~1.95–3.2 Ma. The youngest evidence of warmer conditions is a few warmer-water foraminifers in the early Pleistocene section of Site 1165 (Quilty, this volume) and nannoplankton in mud units at Site 1167 at 37.4 mbsf (~780 ka) and 218 mbsf (~1.13 Ma) (M. Lavelle, pers. comm., 2001). The IRD signal at Site 1165 inferred from the >250-µm size fraction also suggests warmer conditions in the early Pliocene (Grützner, this volume; Warnke et al., this volume).
These observations suggest that oceanic temperatures in the Miocene and Pliocene were warmer than at present and that brief intrusions of warm water in the early Pleistocene were accompanied by nannoplankton and foraminifers. Warmer conditions are indicated by open-water facies deposited in the Prince Charles Mountains during the Pliocene–Pleistocene 250 km from the present Amery Ice Shelf edge (~3.1–1.0 Ma) (Whitehead and McKelvey, 2001). Lower Pliocene (4.5–4.1 Ma) diatomaceous deposits in the Vestfold Hills contain diatom assemblages that suggest summer temperatures were 1.6°–3.0°C warmer than today (Quilty, 1993; Whitehead et al., 2001). The low sea-ice concentrations derived from diatom assemblages (J.M. Whitehead and Wotherspoon, pers. comm., 2003) and the warm-water periods inferred from high abundances of silicoflagellates (Dictyocha) at Site 1165 (Whitehead and Bohaty, 2003) are consistent with isotopic estimates of warmer conditions than present during the early Pliocene (Hodell and Venz, 1992).