10-mT
magnetic fields), which are easily remagnetized by coring-induced magnetic
fields.A large variety of paleomagnetic and rock magnetic experiments were conducted on discrete samples in addition to the standard remanence measurements on the archive halves. The most significant conclusions to be drawn from these combined observations are
The details of the measurements and experiments upon which these conclusions are based are described below.
The magnetic remanence of all archive halves of APC (0-112.2 mbsf), XCB (112.2-722.6 mbsf), and RCB (703.3-1181.6 mbsf) cores recovered at Site 1150 was measured at 2- or 5-cm intervals. Each section was also stepwise AF demagnetized at 0 (natural remanent magnetization [NRM]), 10, 20, and 30 mT. Two sections, 186-1150A-43X-4 and 186-1150B-23R-4, were demagnetized at 5-mT steps from 0 to 60 mT. Magnetic susceptibility was measured on whole-core sections every 2 cm (Fig. F25). Both remanence and susceptibility data for these sections are available from the ODP (Janus) database.
Before demagnetization, the inclinations displayed a very strong tendency toward large positive values (~60° to 90°) over the entire cored interval, indicating the presence of a steep downward-directed drill-string overprint. In addition, the declinations are biased toward 0° in the interval cored with the APC and XCB systems or have a slight bias toward 180° in the interval cored with the RCB system. These biases in azimuthally unoriented cores indicate the presence of a radial overprint. The overprint is radially inward for the APC and XCB cores and radially outward for the RCB cores. Both the radial and vertical overprints are observed during most ODP legs and are artifacts of the drilling process.
We have further documented the presence of the drilling-related overprint by illustrating the amount of magnetization removed in the z direction (vertical component) relative to that in the x direction (horizontal component that is perpendicular to the archive-half split-core face) and y direction (horizontal component perpendicular to x-axis; see Fig. F7 in the "Explanatory Notes" chapter) following AF demagnetization at 10 mT (Fig. F26). The thin dark line in Fig. F26 shows that 10 to 180 times more magnetization is removed in the z direction than in the y direction. Thus, the vertical component of the drill-string overprint clearly contributes a larger low coercivity component than do other secondary or primary horizontal components. The z:y ratio is about six to eight times larger in the APC-cored interval than in the XCB- and RCB-cored intervals, illustrating that the vertical drill-string overprint is largest in the APC cores. The z:x ratio (the thick shaded line in Fig. F26) is much smaller than the z:y ratio in the APC-cored interval and slightly smaller in the other intervals. This illustrates that the radial component, which contributes no net magnetization in the y direction owing to the orientation system used, contributes a significant low-coercivity component in the x direction. The size of this component is still small relative to the vertical overprint as is evidenced by the ratio of removed magnetization (z:x in Fig. F26) being >5 and averaging ~15.
A large proportion of the
initial magnetization is removed by AF demagnetization, even with peak fields of
only 10 mT (Fig. F27).
For most of the APC and XCB cores, <40% of the magnetization remains after 10
mT demagnetization, whereas the RCB cores retain ~50%-60% of their
magnetization. From this and the above information, we conclude that ~40%-80% of
the remanence is carried by very low coercivity magnetic minerals (those
influenced by 10-mT
magnetic fields), which are easily remagnetized by coring-induced magnetic
fields.
The magnetization remaining after demagnetization at 20 or 30 mT reflects the characteristic remanent magnetization (ChRM), a term we use to refer to the high coercivity or high unblocking temperature component that records the ancient magnetization of the sediments. In some cases the ChRM is not a primary magnetization that was acquired at or near the time of deposition but is instead a secondary magnetization probably acquired during subsequent chemical and mechanical remagnetization.
After 30 mT demagnetization, the bias that was present in the declination appears to be removed for the APC and RCB cores, but a bias remains for the XCB cores (Fig. F28). The declinations for each APC core cluster at different mean declinations, as expected for azimuthally unoriented cores. Reorientation of the APC cores (Cores 186-1150A-3H to 8H) using the Tensor-tool data brings the mean declination to near 0° (Fig. F29), as would be expected for the sediments of Brunhes age (<780,000 yr). Cores 186-1150A-9H and 12H are not included in Fig. F29 because they are partly or totally reversed polarity, and Cores 186-1150A-10H and 11H are not included because their core liners were shattered by gas expansion while the cores were still on the catwalk. The large standard deviation for the corrected declinations (52.9°) reflects the uncertainty in the Tensor-tool correction along with possible rotation of the core barrels as they were shot into the sediments. The standard deviation may also be inflated because of the relatively low number of oriented cores collected.
Examination of inclinations after 30 mT AF demagnetization (Fig. F28) illustrates that both normal and reversed polarities are recorded in at least the upper 850 m, though with a considerable amount of noise. Below 850 m, however, virtually the entire section has very stable positive inclinations, with mean inclinations of 56° to 60° (Fig. F28). This would indicate that the sediments are of normal polarity, with the exception of a few short intervals (e.g., from 940 to 950 mbsf). This conflicts with biostratigraphic information, which instead indicates that about half of the section should be reversed polarity. Given the abundance and good preservation of diatoms, and the resulting high quality of the biostratigraphy (see "Biostratigraphy"), it seems unlikely that the sedimentary section below 850 mbsf has a primary remanence. Remagnetization appears to be required and is supported by other evidence. For example, middle Miocene sediments obtained from DSDP Site 584 in the Japan Trench were remagnetized as confirmed by a bedding-correction test (Niitsuma, 1986).
The remagnetization process that has replaced all or nearly all of the primary remanence in the lower 850 mbsf has probably also partially replaced the magnetization in the section above. The noise in the interval below ~70 mbsf we attribute to the low intensities and to partial remagnetization of the sedimentary section. Assuming that either chemical remagnetization, mechanical remagnetization, or both increase progressively downhole, then it is not surprising that below 850 mbsf there is relatively little evidence of reversed polarity and that in the interval from 850 to 70 mbsf the reversed polarity intervals are more poorly resolved than the normal. Given that the field has had a normal polarity for the past 780,000 yr, there would be ample opportunity for the net bias of the sedimentary section to be in a normal-polarity direction. An additional source of noise for the sediments from 700 to 70 mbsf comes from drilling disturbance generated by XCB coring, which results in drilling biscuits (coherent pieces of core) surrounded by slurry.
In contrast, the sediments above 70 mbsf likely contain a primary magnetization acquired at or near the time of deposition. The magnetization of this interval appears to be unaffected by reduction diagenesis as evidenced by the intensity and susceptibility data, which again are about an order of magnitude higher than for sediment lower in the section. The presence of pyrite in the upper cores may indicate that some level of reduction takes place shortly after deposition, but not enough to consume completely magnetic minerals carrying a primary remanence. Unlike the lower part of the sedimentary section, there has been little time for mechanical deformation to remagnetize the sediment in the upper 70 mbsf, and evidence from ash layers and burrows suggests deformation is absent. Finally, inclinations and declinations within this interval are very stable and consistent with a Brunhes normal polarity direction (Fig. F30).
The remanent magnetization directions and intensities obtained from measurements on discrete samples (7-cm3 cubes) agree well with split-core results (Figs. F30, F31). The only small systematic differences observed in the inclinations are in the APC-cored intervals, where the split-core inclinations are steeper by ~8° to 13°. Because the inclinations from the discrete samples agree with the expected geocentric axial dipole inclination and because the discrete samples come from the center of the core and are unaffected by sediment shearing that distorts the outer part of the APC cores, the discrete samples more accurately represent the paleomagnetic inclination. The sediment shear effect has been shown to bias the inclination of APC cores to both shallower and steeper values but generally produces steeper values for an expected inclination of ~60° (Keigwin, Rio, Acton, et al., 1998).
The demagnetization behavior of typical samples from the entire cored interval is illustrated in Figures F32, F33, F34, F35, F36, F37, F38, and F39. Thermal demagnetization of samples from the upper 6 mbsf (Figs. F32, F33) indicates that a large proportion (>60%) of the magnetization has unblocking temperatures of <210°C. By ~330°C, the samples had been demagnetized to the noise level of the cryogenic magnetometer. In comparison with AF demagnetization of samples in the same interval (Figs. F32, F33, F34), thermal demagnetization appears to be less effective at separating the steep drill-string overprint from the ChRM. Basically, the unblocking temperature spectrum for the overprint and the ChRM are similar, with both components being removed simultaneously during demagnetization.
For samples from the upper 70 mbsf (down to about the middle of Core 186-1150-8H), the ChRM direction can be resolved well with AF demagnetization (e.g., Fig. F34). In general, the steep drill-string overprint is removed by 8 to 10 mT, leaving a liner decay of the magnetization toward the origin of the orthogonal plots (Figs. F32, F33, F34). In some cases, the sample does not decay directly to the origin, which might be caused by the samples picking up a small ARM during AF demagnetization, a viscous component of magnetization following each thermal demagnetization step (as the samples are carried for the magnetically shielded oven to the sample tray), and/or an induced component within the magnetometer.
Below 70 mbsf, the ChRM is more difficult to resolve because of the chemical and mechanical overprinting (discussed above) and the decreased intensity. In general, it is possible to determine at least the polarity of the sample. For example, Sample 186-1150A-10H-1, 20 cm (83.90 mbsf), is normal polarity and Sample 186-1150A-12H-2, 39 cm (104.59 mbsf), is reversed polarity, but both samples give very noisy demagnetization paths on the orthogonal plots (Fig. F35). In other intervals, the magnetic directions can be resolved well (to better than 5°), particularly for some RCB cores (see Figs. F37, F38, F39).
Initially the noise level on the spinner magnetometer was high because of the continuous change in the ship's orientation. The noise in the electronics of the NP2 was reduced from 2 mV to 0.1 mV by placing a 3-layer µ-metal shield (35.5 cm outer diameter, 30.5 cm inner diameter, and 61 cm height) over the spinner magnetometer. After stacking repeat measurements 10 times, the noise level approached that of the cryogenic magnetometer (see "Paleomagnetism" in the "Explanatory Notes" chapter). Demagnetization of the NRM and ARM was conducted up to 40 mT in 10-mT steps (Tables T10, T11, both also available in ASCII format). The ARM was applied in a peak AF of 40 mT with a 29-µT direct current-biasing field (Table T10).
Measurements made by the NP2 agree well with those of discrete samples and split-core sections made by the cryogenic magnetometer. As noted above, for APC cores, the inclinations from discrete samples were on average 8° to 13° shallower than from the split-core sections. In addition, the inclinations from the NP2 are shallower than discrete samples measured by the cryogenic magnetometer by an insignificant 2.5° ± 3.4° (95% confidence limit). Thus, similar conclusions to those discussed above can be drawn from the NP2 data.
Some additional insights,
however, can be gained from the NP2 data because the NP2 provides measurements
beyond those commonly obtained by the cryogenic magnetometer. In particular,
demagnetization of the ARM shows an interesting contrast to the NRM
demagnetization (Figs. F40,
F41). The median
destructive field (MDF)—the AF demagnetization field at which half of the
magnetization is removed—for the NRM is typically <8 mT from 0 to 150 mbsf,
whereas it averages 20 mT for the ARM. This illustrates two main points. First,
there must be a significant proportion (
50%)
of magnetic minerals in the samples that have coercivities greater than 20 mT as
shown by the MDF for the ARMs. Second, the drill-string overprint, which applies
an IRM, is very effective at magnetizing the sediments with coercivities <8
mT. On the other hand, the processes by which the sediments in the upper part of
the sedimentary section acquire an ancient magnetization, presumably by a
depositional remanent magnetization (DRM) or a postdeposition remanent
magnetization (pDRM), must be fairly poor at orienting/magnetizing the minerals
in the direction of the ambient field, at least poor relative to an ARM. Had the
NRM been as effective as the ARM at magnetizing the magnetic minerals with
coercivities greater than 8 mT, then the MDF for the NRM would have been similar
to that for the ARM.
Below 150 mbsf, the MDF of the ARM and NRM are similar, both averaging ~20 mT, though the MDF for the NRM shows much greater variability. Below 650 mbsf, one-third of the samples have MDFs greater than 40 mT. Two primary processes contribute to the change in behavior with depth. First, the proportion of magnetic minerals with coercivities <~8 mT decreases below ~150 mbsf, a point that can also be drawn from the split-core data (e.g., Figs. F26, F27). Because larger magnetic grains commonly have lower coercivities, this indicates that there are more coarse-grained magnetic minerals above 150 mbsf than below. As discussed above, reduction diagenesis is likely responsible for the decrease in magnetization and in concentration of magnetic minerals as magnetite and titanomagnetite grains are converted to iron sulfide minerals. Second, the magnetic minerals with coercivities higher than ~10-20 mT are more effectively magnetized below ~150 mbsf. Thus, the magnetization imparted by the ARM is more similar to the NRM below 150 mbsf than above.
The only direct evidence
of the identity of magnetic minerals comes from visual examination of the cores
and from smear-slide analyses. Both of these confirm the presence of iron
sulfides in the form of pyrite patches on the surface of many split cores and
pyrite framboids in the smear slides (see "Lithostratigraphy,"
and the "Core
Descriptions" contents list). Pyrrhotite (e.g., Fe7S8)
and greigite (Fe3S4) might also be expected to be present
in the reducing environment in which the sediments have been deposited. Pyrite
is paramagnetic and so carries no remanent signal, though pyrrhotite and
greigite may contribute to the paleomagnetic signal (e.g., Chapter 3 of Opdyke
and Channell, 1996). Other minerals common to marine sediments include magnetite
(Fe3O4), titanomagnetite (Fe2+xTi1-xO4;
0 < x 1),
maghemite (
-Fe2O3),
and hematite (Fe2O3).
In an attempt to determine the identity of the magnetic minerals that are the most significant carriers of the remanent magnetization, we have conducted a variety of rock magnetic experiments. Each experiment provides only circumstantial evidence. Some of the characteristics we use include the unblocking temperature and coercivity spectra. Curie/Néel temperatures are 320°C for greigite, ~325°C for pyrrhotite, -150°C to 580°C for titanomagnetite, 580°C for magnetite, and 675°C for hematite. At temperatures >250°C maghemite inverts to hematite, and at a temperature >300°C iron sulfides oxidize to magnetite (e.g., Opdyke and Channell, 1996). The coercivities of these minerals are dependent on grain size and/or relative amount of titanium. In general, titanomagnetite's coercivity < magnetite ~ maghemite ~ greigite < pyrrhotite < hematite (e.g., Opdyke and Channell, 1996).
From the thermal demagnetization of the NRM, we know that most of the unblocking temperatures of the NRM are <300°C (Figs. F32, F33, F42). However, we were prevented from looking at the higher end of the temperature spectrum because the samples demagnetize to the noise level of the magnetometer above ~300°C. We circumvented this problem by thermal demagnetization of a composite three-axis IRM using the method of Lowrie (1990). Four samples from normally magnetized zones in Hole 1150B were selected. IRMs of 1 T, 0.3 T, and 0.1 T, respectively, were imparted along the three axes of each sample and then progressively demagnetized in 30°C steps. The results from all samples are similar (Fig. F43). First, demagnetization of the 1-T component (the high or "hard" coercivity component) can be separated into three segments: a 30° to 180°C segment with a gradual decay in magnetization, a 180° to 360°C segment with a sharp decrease, and a 360° to 600°C segment with a gradual decay of the remaining magnetization (~20%-25%). The 0.1-T ("soft") and 0.3-T ("medium") components showed similar behavior but were weak and difficult to monitor accurately. The retention of some magnetization for the hard component between 330° and 600°C indicates the presence of fine-grained magnetite and/or low-titanium titanomagnetite. Those minerals with unblocking temperatures <330°C could be pseudo-single or multidomain magnetite, titanomagnetite, maghemite, pyrrhotite, or greigite.
The sediments also contain a viscous remanent magnetization (VRM) component as shown by combined thermal and AF-demagnetization experiments (Fig. F39). In this experiment, samples were taken in stably magnetized intervals and then thermally demagnetized. After each thermal demagnetization step, a sample was measured in the cryogenic magnetometer (Fig. F39B) and then subjected to a weak AF demagnetization (5 mT) to remove soft coercivity components and remeasured (Fig. F39A). Soft components with intensities of 1-2 × 10-5 A/m appeared after samples were heated in the furnace. This can be seen by comparing the data before and after AF demagnetization. Before AF demagnetization, the sample displays a variable decay of magnetization and changes in directions (a noisy signal) on the orthogonal plot, whereas after demagnetization the sample has a very linear decay of magnetization. The noise is attributed to a VRM that is acquired during the time the sample cools in the oven or in the time it takes to carry the sample from the oven to the sample tray. Because VRMs are commonly associated with coarse-grained minerals, we can conclude that part of the magnetic mineralogy includes a viscous coarse-grained assembly of minerals.
AF demagnetization of the NRM, ARM, and IRM, along with IRM acquisition experiments for the upper three cores (Fig. F44), indicate that coercivities are also consistent with magnetite or titanomagnetites being the dominant magnetic minerals. Some intervals appear to have an additional magnetic mineral that has a significant susceptibility but contributes little to the paleomagnetic intensity. Evidence for this comes from the relative paleointensity records, which can be derived by normalizing the NRM intensity after 30 mT demagnetization by the susceptibility (NRM/k), and by the ARM (NRM/ARM) and IRM (NRM/IRM), both also after 30 mT demagnetization (Fig. F45). The normalization removes the effect of variations in the concentration of the magnetic minerals, assuming that the minerals present are those that carry the remanent signal. All three records would then give similar relative paleointensities. The NRM/ARM and NRM/IRM records give similar results but the NRM/k record differs from the other two in a couple of intervals. The presence of iron sulfides could explain this observation because the iron sulfides have high susceptibilities but often do not carry a paleomagnetic signal (e.g., Chapter 3 of Thompson and Oldfield, 1986). Alternatively, a viscous coarse-grained titanomagnetite assembly could be responsible for the differences since the viscous assembly of grains would be represented in the susceptibility but could be remagnetized in the time it takes to carry the samples across the lab following application of the ARM or IRM.
From the above information, we suggest that magnetite and titanomagnetite are present in fine- to coarse-grain sizes and are the dominant magnetic minerals. The low coercivity and low unblocking temperature component, that which acquires the drill-string overprint, is likely carried by the coarser grain sizes (multidomain grains) and by high-titanium titanomagnetites. The ChRM or paleomagnetic signal is likely carried by the finer grain sizes (pseudo-single and single-domain grains) and by magnetite and low-titanium titanomagnetite. Pyrrhotite, greigite, and maghemite could also be present and could contribute significantly to the remanent signal, though their presence is not required based on the observations.
Chemical treatments of seven samples from XCB and RCB intervals illustrate that some of the magnetic minerals, particularly part of the fine-grained fraction, may be bound to organic matter (OM) and carbonate. This could explain why the demagnetization spectrum for the NRM and ARM differ, since both DRM and pDRM processes would be less effective at orienting the fine-grained fraction if it was bound to other material.
The seven samples were selected from the core catchers. Three samples were from diatom dominant intervals (Cores 186-1150A-21X, 41X, and 61X). Four samples were taken from cyclic changing intervals where diatom dominant (Cores 186-1150B-36R and 48R) or terrigenous-matter dominant (Cores 186-1150B-28R and 40R). They were remagnetized to the present geomagnetic field.
The chemical treatments were conducted as follows: the selected samples were ground into a loose sediment. The sediments were washed with water and centrifuged for 10-20 min at 4000 rpm. Each sample was then divided into two specimens, which were placed in plastic sample cubes (the French cube). The first specimen had its NRM measured and then was demagnetized at 5, 10, 20, 30, and 40 mT. These and other measurements were made by N. Niitsuma in the NP2 magnetometer. Next, the sample was given an ARM and then demagnetized as with the NRM. The second specimen was subjected to various reagents (using ~30 mL). First, the sediment was removed from the cube, soaked in 5% hydrogen peroxide (H2O2) for 1 hr, washed with water, centrifuged, and packed back into the cube, and then the remanence was measured before and after 5 mT AF demagnetization. Second, the sediment was removed from the cube, soaked in 0.03-M di-sodium ethylenediamine-tetraacetate (EDTA) for 6 hr, washed with water, centrifuged, and packed back into the cube, and then the remanence was measured before and after 5 mT AF demagnetization. Third, the sediment was removed from the cube, soaked in 2-N hydrochloric acid (HCl) for 1.5 hr, washed with water, centrifuged, and packed back into the cube, and then the remanence was measured before and after AF demagnetization at 5, 10, 20, 30, and 40 mT. The sample was given an ARM and then demagnetized again up to 40 mT. The results are given in Table T12, also available in ASCII format.
Hydrogen peroxide breaks down OM. The overall magnetization showed a decrease only for Sample 186-1150A-21X-CC and increases for the three lowermost samples (Table T12). This is interpreted as an increase in the magnetic fraction, possibly by oxidation of iron or by a release of the magnetic fraction that was bound to OM. The released fraction would then be able to orient to the ambient field. In either case, the magnetic fraction released or created was most likely coarse grained because the MDF decreased from 10-23 mT to 2.3-5.5 mT for the four shallowest samples. This is interpreted as an overall loss of the fine-grained fraction.
EDTA removes Ca and Mg ions and decomposes carbonates. The changes in the intensities of remanent magnetization after the EDTA treatment varied with samples, as shown in Table T12. This variation may be explained by depth-dependent releases of different grain size particles. For example, the samples from depths above the salinity cline, determined from the interstitial water (<400 mbsf), showed increases where carbonates with finer grained particles were deposited on the seafloor. Below this, the finer grained particles may have been dissolved. The intensity increase and larger MDFs above 400 mbsf may therefore have occurred as a result of releases of finer grains.
Most of the remanent magnetization was destroyed by the HCl treatment. However, the ARM measurements suggest that the remaining ARM carrier is different from that of NRM.
The results from above-described chemical experiments, together with other results, are expected to help relate intensity changes to different depths and infer their causes among decomposition of OM, dissolution of carbonates, or precipitation of the diagenesis. For example, although further verification is necessary, Figure F46 constructed from Table T12 suggests that the above processes seem to be depth dependent.
Magnetostratigraphic interpretation is hampered by the large amount of overprinting caused by drilling, reduction diagenesis, coring disturbance, and structural deformation. Even determining the location of the Brunhes/Matuyama reversal was difficult because gas expansion caused core deformation and resulted in the destruction of core liners for Cores 186-1150A-10H and 11H and the loss of their azimuthal orientation. Unfortunately, the Brunhes/Matuyama boundary probably occurs in this region of core deformation, probably near the top of Core 186-1150A-10H (~84.18 mbsf). Below this there are a few prominent reversals, but identifying the geomagnetic chron to which they belong is impossible without additional age information.
Diatom datums provide the main age control for the sediments at Site 1150. A magnetostratigraphy generated from the diatom age constraints (see "Biostratigraphy" in the "Explanatory Notes" chapter) predicts the location of some of the best resolved reversals observed in the paleomagnetic inclination data (Fig. F47; Table T9). Given the overall age constraints provided by the diatoms, the prominent reversals can be used to make slight adjustments to the age estimates for the sediments, particularly between diatom datums. None of the adjustments violate the diatom datums, and together the biostratigraphic and magnetostratigraphic data provide good constraints on the sedimentation rates (see "Sedimentation Rates").
