DRILLING STRATEGY

Sites have been selected along two depth transects and one linking latitudinal transect 12 longitude. The latitudinal transect extends from near the estimated glacial position of the SAF (51S) to north of the modern position of the STC (39S). Within a single leg of drilling, a tradeoff is required between providing either (1) fewer and shorter extremely high-quality (triple advanced hydraulic piston corer [APC]) records through mainly younger Neogene sediment; or (2) a greater number of high-quality (double APC) records that cover a wider geographic range, and some of which penetrate back to the critical 35-20 Ma interval over which the South Tasman and Drake Passage gateways opened. We have approached this tradeoff by scheduling triple-APC coring on the primary Chatham Rise transect, and double APC at most other sites.

1. The Chatham Rise Transect (Sites SWPAC-2B, 5B, 9B)
This transect comprises three holes in water depths between 615 m and 3308 m located in a northeast band across major sediment drifts of the Chatham Rise. All sites are above the modern CCD, recently determined to be at ~4750 m in this area (McCave and Carter, 1997). Sections span an inferred age of early Miocene to Holocene, within a thickness of 400-750 m. Two sites (Sites SWPAC-2B and 4A) terminate in presumed Oligocene sediment, which marks the regional inception of strong bottom-water flow into the Pacific.

2. Campbell Plateau Transect (Sites SWPAC-1C, DSDP 594, SWPAC-6B, 7B)
This transect commences with Site SWPAC-1C in 315 m of water on the eastern South Island shelf (Canterbury Drifts; to sample upper AAIW and basal thermocline water), then passes southeast close to DSDP 594 to link to two sites near the eastern edge of the Campbell Plateau, Sites SWPAC-6B and 7B. Site SWPAC-6B (543 m water depth) is targeted to sample AAIW. Biopelagite is present at the surface and expected to extend downhole to the target depth. Sedimentation rate in a nearby core (F-121) is <1 cm/k.y., but from the thickening of seismic intervals toward Site SWPAC-6B, significantly higher rates are expected there. Site SWPAC-7B is situated on the crest of the large Campbell Drift, and at 4505 m lies close to the regional CCD. However, monospecific benthic and planktonic isotope records from a site to the north at 4802 m and pilot studies of bulk carbonates (Shackleton et al., 1993) suggest that an isotope record may be determinable with paleomagnetics and perhaps tephrochonology as additional means of age control.

The Campbell Plateau transect will yield a terrigenous silt record of late Miocene to Holocene fluctuations in current strength near the AAIW/thermocline transition (Site SWPAC-1C), a mainly carbonate record of the Neogene AAIW paleohydrography of the Campbell Plateau (Site SWPAC 6B), and a mixed carbonate-abyssal mud record of Neogene paleohydrography of the DWBC (Site SWPAC-7B).

3. Linking Latitudinal Transect (Sites SWPAC-7B, 8A, 2B, 9B)
This transect spans a latitudinal range from 51S to 39S, commencing on the Campbell Drift (Site SWPAC-7B, Campbell Plateau transect) and passing through the North Chatham Drift (Site SWPAC-2B, Chatham Rise transect). The addition of two other sites makes up the transect: Site SWPAC-8A near 47S on the north levee of the Bounty Fan, and Site SWPAC-9B at 39S on the Rekohu Drift.

The latitudinal transect has been inserted to better track movements of fronts (STC, SAF, and possibly the AAC) during glacial and interglacial cycles. Penetration is therefore limited to the Pliocene-Pleistocene (last 3 m.y.) at Site SWPAC-8A. This site is also deliberately located on a fan levee to retrieve sedimentary process information, particularly on the frequency of turbidity currents and their potential as a proxy for paleoseismic events and the glacial/interglacial switching of sediment supply into the DWBC system.

Alternate Sites
Drilling plans for the alternate sites are discussed in the Site Summary Section (pages 62 and 65).

To 181 Logging Plan/Sampling Strategy

To 181 Table of Contents