LOGGING PLANThe proposed leg aims to investigate the development of the Antarctic Circum-Polar Current (ACC) by coring five sites near Tasmania. Downhole measurements provide a means of investigating the compositional and physical variability in situ, thereby providing continuous records of climate driven cyclicity.
The triple combo and Formation MicroScanner (FMS)/Sonic tool strings as well as the geological high-resolution magnetometer (GHMT) will be run at all sites drilled deeper than 250 m and will be particularly important when dealing with cyclic changes in sediment lithology. The sedimentation rates are high enough at most sites for standard logs such as density and gamma ray to continuously resolve climate-related changes in sediment lithology (e.g., percent CaCO3) on orbital timescales. The addition of the new high-resolution gamma tool will allow approximately three times better vertical resolution than the current tool for minimal time costs. The FMS images will allow suborbital variability to be resolved in addition to allowing observation of bedding, turbidites, faults, clasts, nodules, and bioturbated beds. The extremely high resolution of the FMS tool provides the potential for fine-scale (centimeter) core-log integration. In addition, ice-rafted detritus rich intervals are identifiable by their physical properties, notably gamma ray and magnetic susceptibility. Logs also provide a key link between core and seismic data: sonic velocity logs and synthetic seismograms may be directly compared to the seismic section.
The total magnetic field, in tandem with the magnetic susceptibility, provides a downhole magnetic polarity stratigraphy. The present Earth's field at the sites will be strong enough to be in the range of the magnetic field sensor, and there should be a strong borehole anomaly (sites are well clear of the +35° inclination zone). The sites have moved about 20° northward since the middle Eocene, but this should not prevent a full polarity stratigraphy from being obtained using the GHMT. In addition, previous logging in the high-latitude Southern Ocean (Leg 177) and the North Atlantic (Leg 162) shows that magnetic susceptibility is particularly useful for core-log integration, allowing the recovered core to be mapped back to its true stratigraphic depth.
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