Shipboard Scientific Party
The primary objective of ODP Leg 177 was to document the paleoceanographic history of the Southern Ocean using both core and log data. The positions of all sites drilled during Leg 177 relative to important oceanographic fronts are shown in Figure 1.
Figure 1: Sites drilled during Leg 177
Wireline logging was conducted at site 1093 using the Triple Combination (including natural gamma, porosity, bulk density, and resistivity tools) and the Geological High-resolution Magnetic Tool (measuring magnetic susceptibility and total magnetic field strength). The natural gamma tool was also run with the GHMT for purposes of intercalibration. Although wide hole conditions prevented the recovery of reliable density and porosity logs for much of the hole there were no significant obstructions to logging and continuous, high quality, natural gamma, magnetic susceptibility and resistivity logs from near the hole bottom to the drill pipe were recovered.
Site 1093 was drilled in order to recover a millennial scale records of conditions near the modern day polar front. The high sedimentation rates at Site 1093 (~25cm/ka in the Quaternary and 8 cm/ka below 434 mbsf estimated from shipboard magnetic and biostratigraphy) allows the examination of both orbital and millennial scale variability in the downhole logs. The natural gamma and magnetic susceptibility logs both show clear and coherent downhole variability associated with lithological changes related to oceanographic and climatic reorganizations (Figure 2).
Figure 2: The natural gamma and magnetic susceptibility logs for Hole 1093
The high values of natural gamma and magnetic susceptibility correspond to depths where IRD rich muddy diatom oozes dominate the lithology. In contrast, the diatom mats and foraminiferal diatom oozes both have low natural gamma and magnetic susceptibility values. A similar relationship is observed in the discrete measurements made on cores allowing the integration of core and log data for improved stratigraphic control and sediment rate estimates. This approach also provides the examination of sediment physical properties in intervals where little or no sediment was recovered by coring. In addition, the core-log integration allows "floating" sections where core recovery was good (e.g. 460-520 mcd) to be placed into their proper stratigraphic position.
Ulysses Ninnemann, Borehole Research Group, Lamont-Doherty Earth Observatory