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 consisted of advanced piston coring (APC) in three or more holes at each site to ensure complete and undisturbed recovery of the stratigraphic section. The "fast track" magnetic susceptibility core logger (MSCL) allowed near real-time drilling decisions to aid complete recovery of the stratigraphic section. We used the "drillover" strategy employed during ODP Leg 202 to maximize APC recovery and penetration. The depth limit of APC coring is traditionally controlled by the overpull required to retrieve the core barrel. In cases where the full APC stroke was achieved but excessive force was required to retrieve the core barrel (often the limit of APC penetration), the drillover strategy entailed advance of the rotary bit to free the APC barrel. APC coring was generally terminated when the full APC stroke could no longer be achieved. Because of the pivotal role of magnetic studies in the objectives of the proposal, nonmagnetic core barrels were generally used. However, due to the relative fragility and high cost of nonmagnetic core barrels, the normal steel magnetic barrels were used after the initiation of drillover. This proved to be a prudent policy, as four steel core barrels were bent during drillover at Site U1308.
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.
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