Figure F12 compares results from downhole measurement of magnetic susceptibility obtained during Leg 118 (Pariso et al., 1991) with the edited MST measurements on Leg 176 rocks, both transformed to SI units. There is a small data gap between the deepest downhole measurement at 490 mbsf and the first MST measurement at 504 mbsf. The chief difficulty in making this comparison is that the logging tool and the Bartington sensor sampled different volumes of rock in different geometrical relationships. The downhole log looked outward, sensing rock in the wall of the hole. It was sensitive, among other things, to borehole diameter. The Bartington sensor looked inward, toward a column of core 6.6 cm in diameter. Its measurements were very refined by comparison. To ameliorate this a bit, I coarsened the line thickness for the plot of MST measurements in order to merge oscillations in high-magnetic susceptibility spikes to a scale of ~2 m. My intention, however, is primarily to show where there are concentrations of high-susceptibility spikes and to derive some sense of their amplitudes. On this basis, the entire core can be divided into several distinctive "susceptibility regions," (Fig. F12) with lithologic inferences based on core descriptions, as follows:
Gabbros with the highest magnetic susceptibility are concentrated in the upper 500 m of the core drilled during Leg 118. These include the two very strong, but narrow, spikes detected by the downhole log and the entire interval of oxide gabbros constituting lithologic Unit IV, from 220 to 280 mbsf. For comparison, I have also plotted the three highest minicore measurements of magnetic susceptibility obtained during each of Legs 118 (Kikawa and Pariso, 1991) and 176 (Shipboard Scientific Party, 1999b). These all exceed the logged measurement of magnetic susceptibility at their corresponding intervals, as might be expected when oxide-rich rock can be directly sampled by a paleomagnetist drilling a small minicore. On the other hand, corrected background measurements by MST and on minicores are very similar. Below 700 mbsf, however, almost all minicore measurements were obtained on olivine gabbros and troctolites; only two of the large spikes in MST magnetic susceptibility have corresponding minicores, indicating the preference of shipboard paleomagnetists to sample representative core rather than narrow intervals of unusual material.
Figure F13 compares the susceptibility measurements in 20-m intervals, whether on minicores above 500 mbsf or using the MST below that. Proportions of measurements both above background and above 2000 x 10-6 MU are given in the right panel, with the proportion of core in the same intervals described as oxide gabbro in the shipboard descriptions in the left. Bear in mind that only 6-12 measurements of magnetic susceptibility on minicores were made in each 20 m of the upper 500 m of core, whereas there are usually >200 measurements per 20 m from the MST. There is a strong correspondence between the curves for strong peaks (>2000 x 10-6 MU) and proportions of oxide gabbros, indicating the approximate equivalence of the human eye and the MST at detecting obvious seams of oxide gabbro. The proportions of oxide gabbro fluctuate strongly throughout the core but are clearly higher upward in each 500-m portion of the core, with the substantial zone of oxide gabbros (lithologic Unit IV) flaring out at 250 mbsf. The fault at 1100 mbsf is not evident in the distribution of strong spikes in magnetic susceptibility, but it is apparent as an abrupt drop in the sum of measurements above background, given by the red curve. There is no way as yet to extend this curve to the upper 500 m of the core.