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doi:10.2204/iodp.proc.322.103.2010

Paleomagnetism

Because of the unavailability of the Chikyu's cryogenic magnetometer (see "Paleomagnetism" in the "Methods" chapter), regular whole-round core pass-through magnetic measurements of core sections could not be carried out for Expedition 322. Thus, shipboard paleomagnetic studies for Site C0011 consisted of only natural remanent magnetization (NRM) and discrete sample progressive demagnetization measurements with Chikyu's spinner magnetometer and alternating-field (AF) demagnetizer. Paleomagnetic cubes or minicores were stepwise AF demagnetized to evaluate the directional stability and coercivity spectrum of each sample. We analyzed the results in Zijderveld diagrams (Zijderveld, 1967) and calculated the characteristic remanent magnetization (ChRM) direction using principal component analysis (Kirschvink, 1980). Volume magnetic susceptibility of these discrete samples was measured after AF demagnetization measurements with the Kappabridge  KLY 3. The Königsberger ratio, which is defined as the ratio of remanent magnetization to induced magnetization in Earth's magnetic field, was also determined for the measured samples. Two samples were taken next to the samples used for AF demagnetization and subjected to progressive thermal demagnetization to determine their unblocking temperatures and magnetic carriers, as well as to compare whether thermal demagnetization is better than the AF technique in removing drilling-induced remanent magnetization (DIRM) and viscous remanent magnetization (VRM) and isolating primary components. In addition, four specimens, which went through AF demagnetization experiments, were given an isothermal remanent magnetization (IRM) in the laboratory with the pulse magnetizer MMPM 10 before being subjected to thermal demagnetization to identify magnetic minerals.

NRM, magnetic susceptibility, and Königsberger ratio

NRM and magnetic susceptibility

Paleomagnetic data obtained at Site C0011 exhibit significant variations in demagnetization behavior among the various lithologies recovered. The most significant variations in NRM intensity and susceptibility for discrete samples from Site C0011 are well correlated with lithology (Fig. F36). For example, tuffaceous sandstone and volcaniclastic sandstone layers in lithologic Unit II and the upper part of lithologic Unit III (from ~340 to 410 m CSF) have the highest NRM intensity (with a mean of ~270 mA/m). From 410 to 510 m CSF (middle part of Unit II and upper part of Unit III), NRM intensity averages ~4 mA/m except for a few discrete peaks of higher NRM values in some depth intervals (e.g., at around 413, 435, 447, 464, and 477 m CSF), which can be tied directly to a visible presence of volcaniclastic and tuffaceous sandstone in these regions (see "Lithology"). In the middle part of Unit III, paleomagnetic measurements also indicate that intensity tends to increase at 530 m CSF and then gradually decrease at 570 m CSF, forming a broad NRM intensity high peak (averaging ~30 mA/m). This intensity peak appears to coincide with changes in the hemipelagic sedimentation rate and P-wave velocity trend at this depth interval (see "Lithology" and "Physical properties"). From 570 m CSF to the bottom of lithologic Unit IV, NRM intensity consistently exhibits lower values (~4 mA/m). On the other hand, tuff and tuffaceous silt/claystone throughout lithologic Unit V have higher NRM intensity values (mean of ~30 mA/m).

Magnetic susceptibility variations generally parallel NRM intensity variations (Fig. F36). Magnetic susceptibility values are generally ~3.3 × 10^–4 SI for muddy turbidites but significantly higher (>1 × 10^–3 SI) for volcaniclastic sandstone layers. Four sharp increases in magnetic susceptibility are present at approximately 361, 377, 407, and 480 m CSF (base of lithologic Unit II). The variation in susceptibility is most likely caused by variations in the magnetic mineral type or magnetic mineral content in the depth ranges where volcanic ashes were observed.

Paleomagnetic stability tests

Remanent magnetization of discrete samples was investigated using stepwise AF or thermal demagnetization techniques in order to extract the primary component of magnetizations acquired at the time of deposition. Figure F37 illustrates the magnetic stability behavior of several representative samples from various lithologic units. Figure F37A (Sample 322-C0011B-4R-1, 72–74 cm) and F37B (Sample 7R-1, 118–120 cm) shows examples of normal polarity and reversed polarity intervals, giving confidential results for polarity determination. The behavior demonstrates the removal of nearly vertical downward DIRM after AF demagnetization at 10 mT and the isolation of a stable component of magnetization that univectorially decays toward the origin of the vector plots (Zijderveld, 1967).

An example of the demagnetization behavior of a sample (322-C0011B-15R-4, 96–98 cm) during thermal demagnetization is illustrated in Figure F37C. A secondary component of magnetization was removed at low temperatures (200°C), and the ChRM component, having higher unblocking temperatures, could be identified by heating the sample up to 400°C. Figure F37D illustrates the AF demagnetization behavior of a sample (322-C0011B-15R-4, 99–101 cm) just below the sample demagnetized by the thermal technique mentioned above. The same drilling overprint and ChRM components are easily revealed by AF demagnetization above 10 mT. The response of ChRM to AF and thermal demagnetization suggests that ChRM in most samples might be carried by fine (i.e., single-domain to pseudosingle-domain), low-Ti titanomagnetite grains. Several muddy turbidite samples from lithologic Unit IV (Sample 322-C0011B-49R-7, 74–76 cm, 735.67 m CSF), however, exhibit very high coercivity (>180 mT) during AF demagnetization (Fig. F37E), suggesting that hematite might exist in these samples as well. The inclination values of ChRM components in each unit are slightly higher than the expected inclination for the site (52.2° for normal polarity or –52.2° for reversed polarity). This higher inclination may be an indication of the incomplete removal of DIRM by AF demagnetization. Although the ChRM inclination is higher, it suggests that ChRM might be the primary magnetization acquired when sediments were deposited. ChRM components were extracted from most of the samples by fitting linear regression lines with least-squares minimization of Kirschvink (1980).

Drilling-induced remanent magnetization

The most common property during demagnetization experiments on Site C0011 samples is a pervasive remagnetization (overprint) imparted by the coring process, which is characterized by NRM inclinations that are strongly biased toward vertical (toward +90°) in all cores (Fig. F36; black circles). In most cases, this steep downward component of magnetization imparted by the coring process can be easily removed by 10 mT AF demagnetization (Fig. F38). Thus, the vector components of DIRM were calculated by subtracting NRM after AF demagnetization at 10 mT (NRM_10 mT) from NRM before demagnetization (NRM_0 mT) and projected on an equal area plot (Fig. F38). Indeed, the component calculated for 100 samples taken from lithologic Unit II has a very steep mean inclination (declination = –176°, inclination = 87.5°, and α₉₅ = 1.1°). Although inclination is very steep and difficult to distinguish from the vertical direction, declination close to 180° may indicate that the magnetization is slightly inward of the core.

To further clarify the nature of DIRM and try to increase the quality of paleomagnetic results by removing the outer part of the core, a standard paleomagnetic minicore sample (322-C0011B-7R6, 11–13 cm, 398.38 m CSF) was sliced into three specimens, and each specimen was AF demagnetized separately (Fig. F39). It is expected that this remagnetization affects the external portions of the cores most severely, presumably because the outside of the core is physically closer to the magnetized core barrel. Figure F39A, F39B, and F39C displays the demagnetization behavior for these three specimens. Although apparent differences in DIRM component inclinations cannot be readily observed on vector endpoint diagrams, the inclination after AF demagnetization at 10 mT was slightly improved from bottom to top (Fig. F39D; blue circles). However, inclination calculated by linear fitting (Fig. F39D; green circles) are mostly the same from top to bottom. DIRM intensity was reduced only slightly from bottom (outer part) to middle and top part (Fig. F39E; red circles). These results suggest that remagnetization may come mostly from the rotation/vibration of the core barrel in the presence of a strong magnetic field but not by friction between the core and drill bit and has more or less uniform effects on the core sections. Thus, the mechanism of DIRM for rotary coring is different from that for piston coring accompanying significant deformation of sediment, which can be reduced in the center of the core (Acton et al., 2002). To understand the source of the strong magnetic field, the magnetic field of the drill bit was measured after the termination of drilling in Hole C0011B, which was lower than Earth's magnetic field (less than ~0.5 G). This may indicate that the magnetic field produced by the core barrel and/or BHA is strong enough to magnetize the sediments/rocks during drilling.

Although this DIRM is not a welcome feature in isolating the characteristic component of magnetization, its steep positive inclinations do serve as a useful check for whether or not shipboard paleomagnetic samples were inverted by human errors during either collecting or measuring processes. Using this information, we have successfully identified and recovered several such errors for cores from Hole C0011B. This feature was also applied to several small blocks of rocks partially eroded by the drilling process where up/down directions were not completely sure and successfully identified the polarity of magnetization.

We also noticed that certain lithologies are more susceptible to this overprint, and neither AF nor thermal demagnetization can be effective in removing the DIRM. As shown in Figure F40, the DIRM persisted to at least 40 mT AF demagnetization on the silty sandstone Sample 322-C0011B-16R-1, 35–37 cm, and to as high as 600°C temperature treatment on the accompanying Sample 16R-1, 38–40 cm. No ChRM component could be identified from these two samples or from an additional sample in this core. Shipboard smear slide observations on this core (interval 322-C0011B-16R-3, 4 cm) revealed an abundance of pyroxene in these silty sandstone samples (see "Site C0011 smear slides" in "Core descriptions"). In addition, considerable abundance of barite was observed for the same smear slide, which is very unusual throughout the site. It is not clear whether the mineral assemblage in these samples facilitated the complete remagnetization or if a different core barrel with stronger magnetization was used to retrieve Core 322-C0011B-16R.

Königsberger ratio

The Königsberger (Q) ratio is defined as the ratio of remanent magnetization to the induced magnetization in Earth's magnetic field. In general, the Königsberger ratio is used as a measure of stability to indicate a rock's capability of maintaining a stable remanence relative to the induced magnetization. The magnetic field value of 50,000 nT (39.79 A/m), which is close to the current total field value of the International Geomagnetic Reference Field at Site C0011 (45,351 nT), was used to calculate the Königsberger ratio for rock samples at Site C0011:

where

• H = local geomagnetic field (A/m),

• K_m = bulk susceptibility (SI), and

• J_nrm = NRM intensity (A/m).

In order to detect the effect of the DIRM to Q ratio determination, we calculate Q ratio in the following two ways: (1) using DIRM, which can be estimated by subtracting NRM_10 mT from NRM_0 mT assuming most of the DIRM was essentially removed after AF demagnetization to 10 mT, and (2) NRM_10 mT. As shown in Figure F36 (last column), the large decrease in Q ratios between NRM_0 mT – NRM_10 mT (blue symbols) and NRM_10 mT (red symbols) demonstrates the existence of the pervasive DIRM imparted to these cores. In general, results show that Q ratios of the majority of samples after AF demagnetization at 10 mT (red symbols) are less than unity, suggesting that in situ total magnetization of the sediments contains dominantly induced magnetization. Furthermore, the large Q ratio values of various sandstone layers (triangles and rectangles) in Figure F36 are the results of DIRM, as evidenced by the same Q ratio values between these sandstone layers and mudstone after AF demagnetization to 10 mT (Fig. F36; last column, red symbols). The low-coercivity magnetic minerals in various sandstone layers carry an unstable remanence that is more susceptible to an external magnetic field, with exceptions for sandstone samples in Unit II (~360 m CSF) and Unit IV (~730 m CSF) that do not show this behavior. Instead, the Q ratio values after AF demagnetization at 10 mT for these sandstones are more scattered. It is also notable that the Q ratio for NRM_0 mT – NRM_10 mT (blue symbols) shows a broad peak between 520 and 560 m CSF where NRM intensity is high, indicating different magnetic mineralogy.

Polarity sequence and magnetostratigraphy

Because of the rotary technique used for drilling, relative rotation frequently occurs between different segments of sediment within the core. Consequently, the magnetic polarity has been assigned on the basis of the stable remanent magnetization inclination. As Site C0011 is situated at a moderate latitude in the Northern Hemisphere, positive (downward directed) inclinations are taken to signify normal polarity, and negative (upward directed) inclinations signify reversed polarity.

We used ChRM inclinations from discrete measurements to define magnetic polarity sequences for Site C0011. Because of incomplete recovery and extensive whole-round sampling of core sections, continuous magnetic polarity sequences from Hole C0011B could not be completed. Nevertheless, several important polarity boundaries were discerned on the basis of changes in the sign of inclinations of paleomagnetic samples, which constitute magnetostratigraphic records for Hole C0011B (Fig. F36; polarity column between inclination and intensity). Each of the major polarity zones is defined by continuity or dominance of the same polarity.

The most diagnostic feature in the paleomagnetic data obtained for Hole C0011B is that some changes in magnetic polarity can be correlated with changes in biostratigraphic zonations. For example, core sections between 394.69 and 452.54 m CSF (Sections 322-C0011B-7R-1, 118–120 cm, through 13R-2, 64 cm) show dominantly reversed polarity. Biostratigraphic Zone NN11a/NN10b with well-defined FO Discoaster berggrenii events (Zone NN11a/NN10 boundary) is also placed at this interval (see "Biostratigraphy"), suggesting the reversed polarity should correlate with Chron C4r (8.108–8.769 Ma). Shipboard biostratigraphic data also suggest that sediments within 495.49–557.73 m CSF are older than 9.61 Ma but younger than 10.81 Ma (see "Biostratigraphy"). This information suggests that the observed long normal polarity interval between 507.90 and 564.89 m CSF (Sections 322-C00011B-21R-7, 81 cm, through 28R-1, 40 cm) should correspond to the normal polarity Chron C5n.2n (9.99–11.04 Ma). This match is definite because Chron C5n.2n is the only long normal chron of this particular age. Furthermore, biostratigraphic data suggest that sediments between 653 and 667 m CSF in Unit IV may be 11.88–12.04 Ma in age (see "Biostratigraphy"). This information suggests that the relatively well defined record of the normal/reversed polarity boundary at 695 m CSF should correspond to the end of Chron C5An.2r (12.41 Ma).

An unexpected result generated from our preliminary study is the identification of what appears to be a magnetic excursion zone from 865.5 to 866.2 m CSF in lithologic Unit V (corresponding to Sections 322-C0011B-58R-8, 86 cm, through 59R-1, 78 cm). Magnetic polarity changes from normal to reversed and then back to normal within an interval of only 0.7 m (Fig. F41). NRM intensity also changes significantly in this zone. On the other hand, magnetic susceptibility does not show a major change through the depth interval, indicating that the magnetic mineral does not change significantly within the depth interval. In addition, this 0.7 m layer of tuffaceous sandstone displays contorted laminae because of soft-sediment folding. Thus, the apparent switch in magnetic polarity is probably an artifact of the folding.

Integrated age model and sedimentation rates for Hole C0011B

Paleomagnetic and biostratigraphic datum events are summarized in Table T11, and the main features of the magnetostratigraphic interpretation along with the inferred biostratigraphic zones at Site C0011 are presented in Figure F42. It is clear from this compilation that the paleomagnetic and paleontological age determinations for the late Miocene sequence in Hole C0011B are compatible, but the resolution of the paleomagnetic data is significantly greater. This allows a more precise determination of sedimentation rates and better definition of the times during which significant changes in sedimentation rate occurred.

Late Miocene sediments cored in Hole C0011B yielded a pilot magnetic polarity stratigraphy (Fig. F42A), from which preliminary sedimentation rates can be calculated. Major geomagnetic chrons and biostratigraphic zonal boundaries for Hole C0011B are shown in Figure F42B. If correct, these calibration points allow the determination of sedimentation accumulation rate values and assignment of "absolute" ages to the lithologic unit boundaries identified in Hole C0011B. Figure F42A summarizes the results inferred from magnetostratigraphic data alone: Unit II was deposited between 7.6 and 9.1 Ma. The paleomagnetically inferred mean sediment accumulation rate for Unit II is 9.5 cm/k.y. This average includes both hemipelagic setting and rapid gravity flow events. Unit III was deposited between 9.1 and 12.3 Ma. The magnetostratigraphic record infers a relatively slow sediment accumulation rate (2.7 cm/k.y.) for the upper part of Unit III (between 480 and 520 m CSF) and an increased rate (6.3 cm/k.y.) for the lower part of Unit III. Unit IV was deposited between 12.3 and 13.9 Ma with a mean sediment accumulation rate of 11.0 cm/k.y.

On the other hand, if we combine both biostratigraphic and paleomagnetic data to create an integrated age model, the results are slightly modified as shown in Figure F42B. The mean sediment accumulation rate is 9.4 cm/k.y. for Unit II and 9.5 cm/k.y. for Units III–V except for 480–540 m CSF, where the sedimentation rate is only 4.0 cm/k.y. The timescale of nannofossil biostratigraphy used for Expedition 322 and throughout the NanTroSEIZE is based on Raffi et al. (2006), which is calibrated relative to the astronomically tuned geomagnetic polarity timescale used for the magnetostratigraphy of Expedition 322 (ATNTS2004; Lourens et al., 2004). Thus, it is natural to conduct the combined analysis using both magnetostratigraphy and nannofossil stratigraphy on sedimentation rates and age models. The number of stratigraphic datum events are 11 for magnetostratigraphy and 7 for nannofossil stratigraphy, which are comparable. Thus, for the combined analysis, equal weights are put on each data point. Because three linearly aligned segments could readily be recognized on the diagram, the slope of each segment was calculated by least-squares fitting with a line. The robustness of the fitting was tested by moving the boundary between the second (~10 Ma) and third (oldest) segments for slope calculation one or two data points back and forth. The age showing change in sedimentation rates moves only between 10.7 and 11.3 Ma (centered at 11 Ma). Thus, it can be concluded that the change in sedimentation rate in the middle of Unit III happened at around 11 Ma with an uncertainty of ± ~0.3 Ma. Improvements to precision need to be conducted including the error of each datum in combination with the uncertainty of recognition of magnetostratigraphy and nannofossils.

Lithologic equivalents of Units II–V of the lower Shikoku Basin facies have been recovered in other drilling transects of the Nankai Trough area (Moore, Taira, Klaus, et al., 2001). For the Muroto transect, Sites 808 and 1174 (inside of the trench) and 1173 (outside of the trench) give sedimentation rates of 3.2, 3.5, and 2.7 cm/k.y., respectively for the lower Shikoku Basin facies deposits. These values are slightly lower than, but comparable to, the sedimentation rates we derived for the upper part of Unit III at Site C0011 (4.0 cm/k.y.). In general, they show significant changes in sedimentation rates at ~11 Ma (Sites 808 and 1174) or ~13 Ma (Site 1173). This can also be correlated with the change in sedimentation rate at 11 Ma for Site C0011. However, Site 1173, which is comparable to Site C0011, gives a slightly older age. On the other hand, Site 1177, situated outside of the trench of the Ashizuri transect, gives a comparable sedimentation rate of 2.8 cm/k.y. Sedimentation rate estimated from magnetostratigraphy at Site 1177 shows a slight downward increase at ~7 Ma, which is potentially associated with the change in the lower Shikoku Basin from hemipelagic facies to turbidite facies.

Paleomagnetic reorientation of the cores

Paleomagnetic declinations were used extensively for reorienting cored material. Table T7 lists the paleomagnetic directions used for reorienting coherent blocks cored by RCB for structural parameters relative to the geographic coordinate system (see "Structural geology"). Although clockwise rotation of ~30° is expected for the Shikoku Basin between 5 and 15 Ma according to Sdrolias et al. (2004), we assumed that cored material did not experience significant rotation relative to the geographic frame after deposition. Further analysis to detect reliable magnetization components by distinguishing both VRM and primary remanent magnetization could be conducted by using a superconducting quantum interference device magnetometer in combination with AF and thermal demagnetizations in shore-based studies.

Rock magnetic characterization at Site C0011

Self-reversal of IRM of tuffaceous sandstone

In an attempt to characterize magnetic minerals in tuffaceous sandstone and silty claystone samples, thermal demagnetization of composite IRM experiments were conducted according to procedures by Lowrie (1990). During the experiments, the unusual nature of self-reversal of IRM was observed for two samples taken from tuffaceous sandstones (Samples 322-C0011B-3R-4, 8–10 cm, and 6R-1, 34–36 cm) in lithologic Unit II. Figure F43A (blue) shows the behavior of Sample 322-C0011B-3R-4, 8–10 cm, during the IRM acquisition experiment. Magnetization was imparted in +z (1; + 2.9 T), +y (2; +0.4 T) and +x (3; +0.12 T) axes successively, but the acquired magnetization directions are always in the opposite directions of the applied field. Directions for Sample 322-C0011B-6R-1, 34–36 cm (silty claystone), showed normal magnetization acquisition behavior in the directions of the magnetizing field (Fig. F43A [red]). Afterward, stepwise thermal demagnetization was conducted and the polarity of the x magnetization component (0.12 T: low-coercivity component) changed from negative to positive at temperatures between 400° and 450°C (Fig. F43B). Sample 322-C0011B-6R-1, 34–36 cm, taken from silty claystone, showed different but still anomalous behavior during heating. All three magnetization components for Sample 322-C0011B-6R-1, 34–36 cm, show shift from positive to negative between 450° and 500°C (Fig. F43C). Demagnetization behavior on heating lower than 450°C (Fig. F43D) shows unblocking temperatures of ~350°C for low- (0.12 T) and medium- (0.4 T) coercivity components.

Considering the source of the reversal of IRM for tuffaceous sandstones, we need to perform additional experiments on shore. However, we may possibly associate the phenomena with the self-reversal of thermoremanent magnetization for hemoilmenite-bearing pumices such as those from the Haruna Volcano (Nagata et al., 1951). There is also documentation of the reversal of IRM for a pyrrhotite crystal during the increase in magnetic field strength (Zapletal, 1992). Although we cannot tell what kind of magnetic mineral is showing the reversed IRM described above, exchange coupling such as that reported for lamellar magnetism of titanohematite with ilmenite exsolution (Fabian et al., 2008) may have played a role. The strange behavior observed for silty claystone samples may also be an indication of similar exchange coupling of the volcaniclastic material included in the sediment sample but in combination with other magnetic minerals contained in normal silty claystone.

Anisotropy of magnetic susceptibility

For limited samples taken from tuffaceous/volcaniclastic sandstone layers and sand/silty sandstone layers of Hole C0011B, anisotropy of magnetic susceptibility (AMS) was measured with the Kappabridge KLY 3S but only after AF demagnetization. The difference in AMS before and after AF demagnetization is checked with two representative samples from silty sandstone and silty claystone. Figure F44A shows the difference in AMS before (small symbols) and after (large symbols) AF demagnetization for Samples 322-C0011B-12R-5, 15–17 cm (blue; silty sandstone), and 12R-7, 9–11 cm (green; silty claystone). No significant difference is shown before and after AF demagnetization for K_min of silty sandstone. For the silty claystone sample, K_min does not show any significant changes, whereas K_int and K_max exhibit a detectable difference. However, considering the relatively large 95% confidence angles for K_int and K_min along this plane (25°~26°), the difference is actually not very significant. Both of these samples show K_min axes close to the vertical direction, indicating the preservation of primary fabric of sediments. Figure F44B shows the difference in shape parameter (T) and anisotropy degree (P′) for the two samples. The silty sandstone sample did not show significant change before and after AF demagnetization, whereas the silty claystone sample shows slight change after AF demagnetization toward less anisotropic fabric and a shift from oblate to prolate. Mean volume magnetic susceptibility showed only minor changes through demagnetization from 5.008 × 10^–4 to 5.040 × 10^–4 SI for silty sandstone and from 2.771 × 10^–4 to 2.700 × 10^–4 SI for silty claystone. Although it was recommended to perform AMS measurements before AF demagnetization (e.g., Jordanova et al., 2007), we conclude that we can use AMS after demagnetization on sandstone or silty sandstone layers encountered at this site.

Figure F44C shows the AMS results measured on tuffaceous/volcaniclastic sandstones (blue) and silty sandstones (red) for Unit II. K_min inclination is >60°; however, it is <60° for tuffaceous sandstones and volcaniclastic sandstones where magnetic susceptibility is high. These horizons correspond to the prolate shape of the ellipsoid (negative values of shape parameter). The anisotropy degree also changes with the shape parameter. This evidence suggests that there is a mechanism leading to the development of lineation for tuffaceous sandstones and volcaniclastic sandstones where high magnetic susceptibility is observed, which might be related to the formation of these layers. For sandstones, shape parameter is positive indicating the oblate shape of the ellipsoid and K_min inclination is steeper than 60°. These lines of fact delineate that the sedimentary fabric is mostly preserved for the measured sandstones.

Cross-check of magnetic susceptibility measurements

Along the MSCL track, whole-round magnetic susceptibility was measured continuously using a loop sensor with an 80 mm aperture diameter (see "Physical properties"). As we obtained reliable magnetic susceptibility by measurements on discrete shipboard paleomagnetic samples (cubes or minicores) with the Kappabridge, we conducted a cross-check of volume magnetic susceptibility measured with the MSCL and Kappabridge. Figure F45A shows volume magnetic susceptibility measured with the Kappabridge (blue circles) along with those measured with the MSCL loop sensor (solid line). The MSCL values show significantly lower values compared with those measured with the Kappabridge. The raw value (MS) of the MSCL is used to calculate the corrected volume magnetic susceptibility (CMS) value according to the following formula written in the instruction manual:

where

• t = X/LD,

• X = diameter of core (liner thickness subtracted), and

• LD = loop aperture diameter (80 mm).

Throughout Expedition 322, LD = 80 mm (aperture diameter of the loop sensor) was used for the calculation of volume magnetic susceptibility (CMS values). Figure F45B shows the correlation between CMS values and the magnetic susceptibility measured with the Kappabridge. CMS values are systematically lower than the values with the Kappabridge, which may indicate that there are significant spaces between liner and core materials. Although the discrepancy might be explained by the smaller diameter of the real core (~60 mm) compared with the inner diameter of the core liner (66 mm), which can be observed on the cross section of X-ray CT scan images, proper calibration with the reliable standard material might better be conducted in the future.

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