172 Scientific Prospectus
The goals of Leg 172 can best be achieved by advanced hydraulic piston coring/extended core barrel coring(APC/XCB) one site on the Bermuda Rise, coring sites at many depths on the BBOR (seven primary sites and one secondary site), and coring two sites on the CS. The CS and BBOR sites will form the first such ODP transect in the North Atlantic. The original drilling plan called for APC coring at all the sites and only coring with the XCB if it was necessary to achieve 300-m depth penetration on the BR. At the annual Planning Committee (PCOM) meeting in December 1995, PCOM considered this proposal in the light of results from the Leg 162 North Atlantic
Arctic Gateways II and decided that ODP's long-term planning for climate and ocean circulation studies would be best served by extending the penetration of some of the sites in the transect.
Although we have extended the penetration of some sites (below), it is difficult to predict how successful Leg 172 will be in recovering high deposition rate Pliocene sediment. Judging from the experience on Leg 164, APC coring in the presence of sediments containing gas hydrate will not be successful below ~150 m. According to shipboard scientists, gas from the sediment charged the tool, which interfered with operation of the piston corer. It should also be noted that hydraulic piston coring was abandoned at about the same depth at Site 533, several hundred meters deeper on the seaward flank of the Blake Outer Ridge. The presence of gas also contributed to poor recovery of XCB cores on Leg 164 (Fig. 9), as well as to sediment disturbance.
There are a few important constraints on the overall drilling plan. First, for efficiency, sites must be occupied in the BBOR/CS region from shallow to deep water depths (Table 1). Second, of 56 days overall, 15 are allotted to transit, and 1.1 to seismic and 3.5 kHz surveys. About 2.2 days have been budgeted for logging, which will help provide continuous stratigraphic coverage where coring may be incomplete and in situ measurements of the sediments including gas hydrate. This leaves about 37.7 days for drilling and coring operations (Table 1). Third, it is hoped that gas hydrate will be less of a problem with increasing water depth, so we have chosen Site BBOR-4B on the Blake Outer Ridge for deep penetration. If the rate of sedimentation is significantly overestimated from the site survey cores, and if some combination of APC and XCB coring is successful to 350 mbsf, then that site could provide a high resolution record of the late Pliocene.
To 172 Seismic Profiling
172 Table of Contents