The (high resolution) stratigraphic goals require high sedimentation rates (>5 cm/k.y.) at the chosen sites, as well as complete and undisturbed recovery of the stratigraphic sequence. The drilling strategy will consist of advanced piston coring (APC) in three or more holes at each site to ensure complete and undisturbed recovery of the stratigraphic section (Tables T1, T2). We propose to use the "drillover" strategy employed during ODP Leg 202 to maximize APC penetration. Traditionally, the depth limit of APC coring is controlled by the overpull required to retrieve the core barrel. In cases where the full APC stroke is achieved but excessive force is required to retrieve the core barrel (often the limit of APC penetration), the drillover strategy entails advance of the rotary bit to free the APC barrel. APC coring will terminate when the pressure gauge on the rig floor indicates that full APC stroke can no longer be achieved. Because of the pivotal role of magnetic studies in the objectives of the proposal, nonmagnetic core barrels will be used throughout.
Two factors influenced the decision to terminate holes at the limit of the APC, and therefore not to utilize extended core barrel (XCB) technique: The increase in drilling disturbance associated with the XCB, particularly in the upper part of the XCB section, has not been conducive to the generation of high-resolution PAC chronologies. Poor recovery and "biscuiting" are common in poorly consolidated lithologies recovered by XCB.
Penetration depths (using APC methods) will not exceed 400 meters below seafloor (mbsf). At North Atlantic sites with similar lithologies, the maximum penetration depth has been 250 mbsf. APC drilling over may increase this penetration to >300 mbsf. In the seismic sections accompanying this report, we mark 400 ms two-way traveltime (TWT) (~320 mbsf) as the maximum possible penetration depth. Note that several days for each expedition are not scheduled for operations to provide for appropriate weather contingency.
Drilling in the North Atlantic outside the optimal weather window (JulyAugust) will require flexibility in the operations plan, which may be dictated by weather conditions. If Expedition 303 has to be modified due to poor weather conditions, sites from Expedition 306 may be substituted in the 303 operations program. Any Expedition 303 sites not drilled during that expedition will be transferred to the operations plan for Expedition 306.
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