All sites have an alternate in case pack ice covers the primary sites (although this is most unlikely).
In addition, lower priority sites (each with an alternate) have been selected in the mid-shelf basin
where pre-collision sediments occur (APSHE-11A), and in the South Shetland trench where a
high-resolution paleoclimate record is preserved within a trench turbidite succession (APSST
01A). Also, the Planning Committee (PCOM) recommended that an originally-specified Plio
Pleistocene paleoclimate site in Bransfield Strait (APBRS-01A) be removed from the drilling plan
because of a shortage of time. In fact, only the primary sites listed above will be drilled as planned
if the leg proceeds without massive disruption by ice.
Drilling on the rise will involve double advanced hydraulic piston corer (APC) and extended core
barrel (XCB) coring at the proximal site (to about 700 m), followed possibly by rotary core barrel
(RCB) coring to 1450 mbsf, or most probably by offset to the distal APC/XCB site where the
same full section is accessible within 600 m. The depth limit of drilling at Site APRIS-01A will
depend on the effects of silica diagenesis on the biostratigraphy and on time constraints. These
sites are in fine-grained alternating biosiliceous and barren muds and mudstones. The 50-m ultra
high-resolution site (APSHE-13A), assumed to be essentially a diatom ooze, will be sampled by
triple APC. The other shelf sites pose problems, in that the unsorted diamictons are unsuitable for
APC/XCB sampling. Therefore, all five sites (that are between 500 and 800 m deep) will probably
(in the absence of advice to the contrary) be rotary-drilled from the seabed down. Recovery will be
reduced because of their lithology (with the possible exception of Site APSHE-05A), but we shall
try for as complete a section as possible.
We plan to log shelf sites APSHE-01A to 05A and rise sites APRIS-01A and 02A with the
following tool strings: the Triple Combination (litho-density, porosity, resistivity, and natural
gamma tools); Formation MicroScanner (FMS)-Sonic (resistivity imaging, sonic velocity, and
natural gamma tools); and Geological High-sensitivity Magnetometer (GHMT; magnetic field,
susceptibility, and natural gamma tools). Also, we plan a vertical incidence vertical seismic profile
(VSP) survey, using the well seismic tool (WST; downhole geophone), at one of the rise sites and
at one or more of the shelf sites.
Thus, an extensive logging program is planned. Briefly, geophysical and geochemical logs will
provide lithological information to complement core data and to help offset possible low recovery
on the shelf; the FMS images can be used to investigate fine-scale bedding and fabric, including
orientation; the GHMT will provide a magnetic stratigraphy; and the WST and synthetic
seismograms will be used to correlate hole depths with travel times on the seismic section. The
good quality logging results obtained on previous Antarctic Ocean Drilling Program (ODP) legs
(113 and 119) in similar sediments, bode well for successful logging on this leg.UNDERWAY GEOPHYSICS
Standard ODP practice is to collect 3.5- and 12-kHz echo-sounder data on approach to each site. In
addition, long magnetic and 3.5-kHz profiles will be acquired on the transits from Punta Arenas to
the Antarctic Peninsula and from there to Cape Town.
For Leg 178, almost all sampling will probably be accomplished during the cruise; no postcruise
sample party is currently planned. Most sites consist of a single hole, where fairly standard
sampling is envisaged. At other sites, which will be double (Site APRIS-01A) and triple (Site
APSHE-13A) cored, a composite section will be constructed for each site and then a sampling
strategy will be developed. Nonstandard sampling at some of the sites might include high
resolution whole-round sampling for physical properties and geochemical studies, U-channel
sampling for paleomagnetism, and microbiological sampling.
All sampling to be conducted on Leg 178 must be approved by the sampling allocation committee
(SAC), consisting of the Co-chiefs, Staff Scientist, and Curatorial Representative. Sampling plans
are subject to modification depending upon the actual material recovered and collaborations that
may evolve between scientists during the leg.
To 178 References
To 178 Table of Contents