Operations plan/Drilling strategy

The drilling program targets seven primary and two alternate sites, all located in either Portuguese or Spanish territorial waters (Tables T1, T2, T3), two of which are on the southwest Portuguese continental margin and the remaining five in the Gulf of Cádiz (Fig. F2). However, the final operations plan and number of sites to be cored is contingent upon changes to the R/V JOIDES Resolution operations schedule, operational risks (see below), review by the Environmental Protection and Safety Panel, and the outcome of a request for permission to occupy these sites.

The coring strategy will consist of advanced piston coring (APC) in two holes (A and B) at each site to refusal, except at proposed Site SHACK-04 where four holes are planned. Multiple holes will allow us to build a composite section at each site. If a complete composite record of the APC section cannot be achieved by double coring, a third hole (C) may be spot-cored to complete the splice or fill any desirable gaps. The "drillover" technique will be employed to maximize APC penetration where desirable. For planning purposes, the APC refusal depth for most sites is estimated at 100 meters below seafloor (mbsf), although we anticipate that this may be exceeded at some of the more mud rich sites. APC refusal will be followed by extended core barrel (XCB) coring at each site to ~350 mbsf (150 mbsf at proposed Site SHACK-04) and then by rotary core barrel (RCB) coring to total depth.

Proposed Site SHACK-04 will be drilled at a depth of 2578 mbsl. The water depth range is mostly below the influence of MOW, which is confined to depths shallower than 1600 mbsl during interglacials and shallower than ~2200 mbsl during glacial periods (Schönfeld and Zahn, 2000). Other Expedition 339 sites are located in the core of MOW and thus, combined with proposed Site SHACK-04, will provide a detailed history of both intermediate and deep water. Proposed Site SHACK-04 was chosen to obtain a continuous Pleistocene sequence with the highest resolution while avoiding disturbance by downslope transport (i.e., turbidites and mass flow deposits). The site is located at the crossing point of seismic lines Steam 9406 and BS-09 (Fig. F4) at the same position as Core MD01-2444, which has been studied extensively as part of the Pole-Ocean-Pole (POP) project (e.g., Fig. F3; Skinner and Shackleton, 2006; Vautravers et al., 2006; Martrat et al., 2007; Margari et al., 2010).

The drilling program at proposed Site SHACK-04 will consist of APC coring using nonmagnetic core barrels until refusal depth (if less than 150 m) and then, if necessary, using magnetic core barrels to the target depth of 150 mbsf (Table T1). Emphasis will be placed on recovering complete composite sections in the depth range achievable using APC drilling techniques (generally as deep as 200–300 mbsf) because high-quality cores will be vital for establishing high-resolution stratigraphies. In anticipation of the high sample demand for these Quaternary reference sections, four holes are required to produce multiple copies of the composite sections to meet immediate and future sampling needs.

In accordance with routine drilling procedures, downhole temperature measurements will be obtained with the advanced piston corer temperature tool (APCT-3) and, where warranted, with the Sediment Temperature Tool (SET). Temperature measurements will allow reconstruction of the thermal gradient at each site. This information will help constrain the history of burial diagenesis of the sediments encountered.