Site 1169 is located in present-day upper abyssal water depths (~3570 m) on the western part of the STR near its western margin at ~47°S in northern subantarctic waters (Fig. F3 in the "Leg 189 Summary" chapter). The site is located in thick, almost flat-lying sediments accumulated in a shallow basin on a gentle structural low formed on the Cretaceous surface (Exon et al., 1997). The younger part of the Neogene section is seismically transparent and has accumulated in the lee of the Tasman Fracture Zone, which forms a ridge to the west. The older Neogene section is well bedded but hummocky in character and probably consists of chalks with some hiatuses. The lowest part of the Neogene may be absent. The seismically similar underlying Eocene sequence is probably largely mudstone.
Site 1169 was selected at the intersection of multichannel seismic profiles Tasmante 125-4 (Fig. F1) and -9. This shallow-penetration site was designed to fully recover a thick, young Neogene sequence. We planned to penetrate 0.23 s (195 m) of Pliocene to Holocene transparent ooze that apparently onlapped 0.22 s (185 m) of upper Oligocene to Miocene ooze and chalk above an inferred Oligocene unconformity. Below the Oligocene unconformity is 0.1 s (110 m) of presumed upper Eocene to lower Oligocene marine mudstone, marl, and chalk, above 0.08 s (90 m) of presumed middle Eocene mudstone. The base of the site at 246.3 mbsf was shown to be middle Miocene, and the upper onlapped surface was just ~30 m above the base.
The primary objective of Site 1169 was to core a complete late Neogene sequence of high sedimentation rate in northern subantarctic waters for high-resolution biostratigraphic and paleoclimatic studies. The depths are sufficiently shallow to provide a high-quality carbonate biogenic sequence and at a high enough latitude for preservation of both calcareous and siliceous microfossils as proxies for paleoclimatic change. This location was specifically selected because of seismic stratigraphic data indicating a sequence of especially high sedimentation rate that would allow high-resolution paleoclimatic investigations. Site 1169 would also provide basic biostratigraphic records for several microfossil groups of late Neogene age from the northern subantarctic of the Pacific/Indian Ocean region. These would complement such records obtained from the subantarctic region south of South Africa (Shipboard Scientific Party, 1999).
To the north of Site 1169 is the Subtropical Front (Subtropical Convergence), which marks the northern limit of subantarctic surface waters and the Southern Ocean and is marked by a strong surface-water temperature gradient of ~4°C (from 14° to 18°C in summer) and a temperature of 11°C at 150 m (Tchernia, 1980; Rintoul et al., 1997; Sikes 1995). Site 1169 lies north of the Subantarctic Front marked by a surface-water temperature gradient of 6°-8°C (Rintoul et al., 1997). This forms the boundary between the Australasian surface water and Circumpolar Subantarctic surface water (Heath, 1985). Subantarctic waters are driven to the east by often strong prevailing westerlies as part of the Circumpolar Current. The subantarctic water mass is bounded to the south by the Antarctic Polar Front or Convergence, a major boundary marked by the 2°C vertical isotherm in winter (Barrows et al., 2000) and considered the northern limit of the antarctic zone.
Site 1169 is well located to have recorded meridional movement of surface-water masses including the oceanic fronts separating major water masses (Sikes, 1995). Today, the Subtropical Convergence overlies the STR, and the Polar Front is only a few degrees to the south. At 3500 m, Site 1169 lies within Circumpolar Deep Water and has the potential to record changes in this water mass that may be linked with oscillations in the strength of the so-called oceanic conveyor belt circulation.
The Southern Ocean is considered to be very important in Quaternary global climate change as it certainly was earlier in the Cenozoic. Very high biological productivity of the Southern Ocean in combination with major regional production of deep and intermediate waters strongly influence atmospheric CO2 levels and, hence, global climate (Broecker et al., 1980; Mortlock et al., 1991; Howard and Prell, 1992). Quaternary climatic changes in the Southern Ocean have been large and on Milankovitch scales, as documented by changes in the extent of sea ice, sea-surface temperatures, water-mass boundaries, deep- and intermediate-water production, and biological productivity of surface waters. The relative importance of North Atlantic and Southern Ocean sources of deep waters to the ocean has oscillated during the late Quaternary with major implications for climate change (Boyle and Keigwin, 1985; Charles et al., 1996). The subantarctic region is a repository of paleoclimatic information critical for better understanding of late Neogene climatic processes and variability. Site 1169 was designed to obtain records for such investigations.