In comparing these four sites, it is evident that discoasters occur in the upper Pliocene sediments of the northern sites (1168 and 1172) but are noticeably absent at Sites 1170 and 1171 (Shipboard Scientific Party, 2001b, 2001d, 2001e, 2001f). Discoasters have been traditionally used as indicators of warm-water environments (Perch-Nielsen, 1985), whereas more recent analysis suggests that they are indicators of oligotrophic conditions (Aubry, 1993). As discoasters are extinct and warm waters are often oligotrophic, it is difficult to tease the two factors apart. Nevertheless, it is clear that an oceanographic boundary separated the northern sites from the southern sites in the latest Pliocene. This oceanographic boundary is inferred to be a paleo-Subtropical Front. The modern STF is a surficial expression of the 12°C thermobar (Findlay and Flores, 2000).
The presence of D. brouweri in Hole 1172A but not Hole 1168A suggests that there was also a temperature or nutrient difference between the western and eastern coasts of Tasmania in the latest Pliocene. The modern EAC, which bathes the ETR, is warmer than the Leeuwin Current, which runs down the west coast of Australia and across the Great Australian Bight toward Tasmania (Fig. F1). The apparent warmer surface water conditions at Site 1172 were also noted during the late Miocene (McGonigal, submitted [N1]). Surface water conditions at Site 1172 diverged from those at Sites 1168, 1170, and 1171 in the late Miocene, based on the ratio of Reticulofenestra perplexa to C. pelagicus. This difference was attributed to the beginning of EAC activity in the East Tasman Plateau region.
The hiatus observed at Sites 1168 and 1172 is similar to hiatuses found around the Southern Ocean. Edwards and Perch-Nielsen (1975) indicate a hiatus at DSDP Site 280 (Fig. F1) that eliminates the entire lower Pleistocene as well as a hiatus at DSDP Site 282 that extends from the upper Miocene LO of Triquetrorhabdulus rugosus (5.5 Ma) to the middle Pleistocene LO of P. lacunosa. No hiatus was recorded at DSDP Site 281, and a middle Pleistocene hiatus at DSDP Site 283 (Fig. F1) was later discounted by Hornibrook (1982). Lohmann (1986) reported the LOs of C. macintyrei and H. sellii as co-occurring at DSDP Site 593, Challenger Plateau. The reliability of the LO of C. macintyrei was called into question, however, based on sedimentation rate issues and disagreement with the magnetostratigraphic interpretation (Nelson et al., 1986). At shallow-water ODP Site 1120, Campbell Plateau, a hiatus below the small Gephyrocapsa Zone was noted with an associated hardground, separating Pleistocene and Miocene sediments (Shipboard Scientific Party, 1999). ODP Site 1127 on the Great Australian Bight records an early Pleistocene hiatus eliminating the Helicosphaera sellii Zone (Ladner, 2002). Brunner et al. (2002) attributed a regional hiatus at Sites 1126, 1127, 1130, 1132, and 1134 to a major fall in sea level associated with third-order sequence boundary events. Flores and Marino (2002) observed a Pleistocene hiatus at ODP Site 1088 located in the South Atlantic Ocean south of the STF. This hiatus is marked by the closely spaced FO of R. asanoi and the LO of C. macintyrei, encompassing approximately the same interval as the Helicosphaera sellii Zone. Helicosphaera sellii was not recorded at any ODP Leg 177 sites. The other carbonate-rich sites (1089 and 1090) do not reveal this hiatus, although Site 1090 does reveal a condensed Helicosphaera sellii Zone. Fontanesi and Villa (2002) identified a hiatus in ODP Hole 1165B (Prydz Bay, Antarctica) in SubChron C2r.2r, a magnetostratigraphic zone that correlates with the Helicosphaera sellii Zone in age.
Helicosphaera sellii is generally rare and sporadic at the top of its range and is not regarded as a consistent pelagic bioevent. Issues with the possible diachroneity of C. macintyrei have also arisen and contribute to the tentative identification of this hiatus. The work by Brunner et al. (2002) and the tightly constrained bioevents at Site 1171 demonstrate the LOs of H. sellii and C. macintyrei to be reliable events from the Great Australian Bight, south of the STF (48°S) in the Tasmanian Gateway.
The distribution of the sites recording this hiatus around the Southern Ocean may indicate a potential circum-Antarctic hiatus. Brunner et al. (2002) suggest that this is only a local manifestation of third-order sequence boundary events. The identification of a hiatus at Site 1165 in Prydz Bay and at Site 1088 in the South Atlantic, in sediments of roughly similar age, disputes the restriction of this event to the Southern Ocean sector around Australia. The suspect nature of H. sellii and C. macintyrei may have contributed to lack of identification of this event based solely on nannofossil biostratigraphy at other locations. Further research and quantitative analysis of previously drilled sites could reveal the extent of this event. The prominence of this event at shallower sites (1168 and 1172) from this study supports the idea that this event is associated with a lowering of sea level and not enhanced current activity.