In addition to remanent magnetization, magnetic susceptibility was measured on all cores as a result of established shipboard procedures. Those procedures include whole-core sections measured with the MST and archive-half sections measured by the archive multisensor track (AMST) (see "Instrumentation Used" in "Paleomagnetism" in the "Explanatory Notes" chapter). The redundancy is the consequence of the fact that routine AMST data gathering was done for the first time during this leg. AMST software now ensures that the susceptibility of unrecovered intervals in the core is not measured; therefore, sample volume is well known (as opposed to the merely relative values of the past), and reasonably accurate susceptibility values are now obtained. The greater accuracy of AMST relative to MST susceptibility measurements was demonstrated by an experimental test that included discrete sample, AMST, and MST susceptibility measurements; the results are reported in "Measurements and Procedures" in "Paleomagnetism" in the "Explanatory Notes" chapter. In the test, the AMST gave approximately the same values as obtained from discrete samples. The far greater accuracy of the values of susceptibility per volume measured on the AMST makes these data preferable to those of the MST. Discrete samples may not need to be measured in the future.
The data for the Hole 801C basaltic basement obtained from both instruments are plotted in interpolated form in Figure F57. No volume correction was applied to the MST data because the appropriate amount of correction is difficult to establish and because the greater accuracy of the AMST determinations makes correction of MST data redundant. The short intervals of significantly higher susceptibility values in Figure F57 correspond to the presence of thick flow units (e.g., Cores 185-801C-30R through 31R at ~745-765 mbsf, or Cores 37R through 38R between 810 and 830 mbsf), which possess coarse-grained magnetite. A large difference occurs between the two curves at depths of 710-900 mbsf. This depth range encompasses all of the thick flow units, which possess inherently greater susceptibility because of the presence of coarser grained magnetite. Both the presence of the higher susceptibility material and the lack of valid volume information for the MST instrument probably account for the discrepancy between the two instruments in this region.
All sections of the archive halves of the whole cores were measured and demagnetized with the shipboard automated long-core cryogenic magnetometer. Measurements on the recovered basement cores were made at 5-cm intervals through each core section. Prior to demagnetization, fragmented core pieces were carefully reconstructed to their original unfragmented form using Styrofoam spacers in the trays, and unoriented core pieces were removed. Thus, the continuous measurements obtained present as complete a representation of the remanent magnetization of the core as possible.
The nominal width of the sensor region of the magnetometer is >20 cm; thus, each measurement represents a weighted average of the remanent magnetization of at least 20 cm of basalt or of basalt plus empty spaces between core pieces. For this reason, discrete samples also were taken, so that more precise values of the remanent inclination and intensity could be obtained and so that vectors removed by demagnetization and various rock-magnetic properties could be examined in greater detail.
The natural remanent magnetization (NRM) measured by the shipboard magnetometer prior to any demagnetization generally possessed intensities of 2-4 A/m (Fig. F58). NRM intensities average ~4 A/m down to ~830 mbsf; below, values are a little lower, between 2 and 3 A/m. The higher intensities coincide with the presence of thick basalt flows. The last of the thick flows occurs in Core 185-801C-38R. This is also the level at which NRM intensities change from a dominance of values between 3 and 5 A/m to ones between 0.5 and 4 A/m (Fig. F58). The thick flows likely have coarser grained titanomagnetites than the pillow basalts; hence, the correlation of intensity and flow thickness suggests that a significant portion of the NRM intensity observed is an induced, rather than remanent, magnetization. The induced magnetization may result either from imposition of a drilling remanence, such as is nearly always observed in ODP cores or may be a remanence induced by the present geomagnetic field.
The NRM inclinations of the archive halves of all cores obtained from Hole 801C during Leg 185 are shown in Figure F59. These results are startling in that they display several polarity intervals, and the entire sequence forms a sinusoidal pattern of polarity changes. Particularly surprising are the numerous flows displaying intermediate directions between flows of opposite polarity directions. NRM inclinations were measured from two-thirds of the discrete samples collected, and their inclinations display the same sinusoidal pattern (Fig. F60) as the long-core sections. In addition, numerous flows possess intermediate magnetic directions.
Archive halves were routinely demagnetized at alternating fields (AFs) of 2, 5, 10, 15, 20, 25, 30, and 35 mT. Samples of normal, reversed, and transitional inclination flows show the same responses to AF demagnetization (see Fig. F61A, F61B, F61C, respectively). The sinusoidal signature remains through demagnetization; the results after demagnetization at 10 mT are shown in Figure F62. Above 10 mT, progressive acquisition of anhysteretic remanence (ARM) occurs with increasing field strength, which, as discussed more fully in "Instrumentation Used" in "Paleomagnetism" in the "Explanatory Notes" chapter, gradually obscures the remanent signature. The ARM acquisition is first apparent in the thick flows at ~15 mT but eventually (~25 mT) affects the pillow basalts as well. Thus, above 10 mT, the remanent signature becomes progressively more obscured by ARM acquisition. The sample in Figure F61B shows the beginning of a the typical trend away from the characteristic remanent magnetization direction of this reversed polarity sample and toward the direction of the field existing at the AF coils (the direction of that field is ~190° declination and 80° inclination). Figure F61C shows a sample of normal polarity magnetization with incipient acquisition of the ARM.
The data of Figure F62 indicate three changes of geomagnetic field polarity. Moreover, the upper portion of the tholeiite sequence obtained during Leg 129 (Wallick and Steiner, 1992) indicated two additional polarity intervals. The Leg 129 sequence ended in normal polarity, whereas that cored during Leg 185 begins with reversed polarity; ~10 m between the two sequences was not recovered. The Leg 185 sequence begins with Southern Hemisphere reversed polarity between 600 and 680 mbsf. The designations of normal and reversed polarity were determined from the results obtained during Legs 129 and 144. During Leg 129, the Southern Hemisphere location of the plate during the mid-Cretaceous was clearly established by the normal polarity signature in the sedimentary section of Site 801 during the 25-m.y.-long normal polarity of the geomagnetic field, the Cretaceous Normal Polarity Superchron (Steiner and Wallick, 1992). By extrapolation, the first tholeiites encountered were determined to have a Southern Hemisphere reversed polarity remanence (Wallick and Steiner, 1992). Downhole logging of Hole 801C with a three-axis magnetometer during Leg 144 confirmed that the tholeiite section has a Southern Hemisphere reversed polarity direction, that changed downhole to normal polarity (Ito et al., 1995).
The average inclination in the entirely reversed or normal polarity intervals is ~45°. Because several polarity intervals are encompassed by this section of basaltic crust, secular variations of the geomagnetic field definitely have been averaged; therefore, the 45° value is the true site inclination, corresponding to a site paleolatitude of 27° at the time of oceanic crust formation.
The section between 600 and 630 mbsf contains reversed polarity in the least-altered basalts. Much of Cores 185-801C-15R and 16R contain basalts and interflow sediments that have been extensively altered to green or tan colors, and the middle of Core 16R contains a zone of hydrothermal deposits. The magnetic signature associated with both the hydrothermal zone and the altered zones is one of very low inclinations. Both green or tan basalts and the altered interpillow and interflow sediments have exclusively very low (nearly zero) inclinations. The alteration overprinted the basalts because little-altered basalts in Cores 185-801C-14R and 17R on either side of this altered interval have notably steeper positive inclinations. Similar characteristics are associated with most zones of greenish alteration higher in the hole, cored during Leg 129. The similar magnetic directions suggest that the alteration and the emplacement of the hydrothermal deposit are related events. Presumably, these events occurred when the plate was near the equator and, perhaps, are related to the Oxfordian unconformity recorded in the overlying sedimentary sequence at a time when the site lay on the equator (Steiner and Wallick, 1992).
Below Core 185-801C-16R (~633 mbsf), basalts with visible large-scale alteration features, such as saponite replacement and carbonate vesicle and vein fillings, display the same remanences as those without such changes. Basalts with excessive celadonite or gray and brown halos also do not appear to have their remanence altered relative to nearby unaltered basalt. Thus, generally speaking, two remanent magnetic signatures were observed in Hole 801C tholeiites, one presumed to be associated with crystallization and the other with the hydrothermal alteration event. The remanent signature associated with the hydrothermal event is visible only as deep as Core 185-801C-16R.
Site 801 is a unique basement site in that the drilling penetrated nearly 400 m into oceanic crust that has very low amplitude, or an absence of magnetic anomalies. This is a characteristic of Middle Jurassic basement in all oceans and termed the "Jurassic Quiet Zone" (JQZ). Many hypotheses have been advanced in order to explain the absence of lineated anomalies: the mid-Jurassic JQZ occurred in a time (1) without geomagnetic field reversals, (2) of anomalously low geomagnetic field intensity, or (3) of very rapid reversals. The continuous measurements of remanent magnetization obtained from the Hole 801C cores with the shipboard magnetometer show gradual changes in the magnetic field direction from one polarity interval to the other, with flows between those of opposite polarities displaying intermediate inclination values. In combination with the previous results from Leg 129, the basement of Hole 801C shows as many as six polarity intervals over 400 m. The drilling results indicate that the igneous basement at Site 801 erupted in a period of rapid polarity alternations of the geomagnetic field. Hence, these data indicate that the explanation for the origin of a "quiet" magnetic signature in the JQZ lies in the existence of numerous superposed flows of opposite polarities of magnetization.
The presence of numerous flows with directions intermediate to those of the normal and reversed polarity flows suggests excessively rapid extrusion rates. The present-day East Pacific Rise is the fastest among oceanic spreading ridges, spreading at 10-15 cm/yr (whole rate). The geomagnetic field during relatively recent Earth history requires 2000-5000 yr to fully reverse its direction. The reversal highest in the section that is displayed in Figure F62 occupies ~100 m between the fully reversed to the fully normal direction. At the fastest EPR spreading rates and the minimum known reversal rate duration, one would expect a full reversal of the geomagnetic field direction to occupy ~160 m of extrusive basalt, assuming continuous extrusion. This example shows that the Site 801 magnetic observations of the highest reversal of Figure F62 certainly are generally consistent with known reversal and spreading rates. Since extrusion is probably not continuous, the observed 100 m for the full reversal appears reasonable. However, the lower reversal in Figure F62 (845-900 mbsf) appears less reasonable in terms of present knowledge. That reversal is found over 55 m of the basaltic column, but fewer transitional flows appear to exist. In particular, at 875-880 mbsf, the directional change between two consecutively cored intervals (Cores 185-801C-43R to 44R) is much larger than elsewhere, perhaps suggesting a less continuous extrusion rate during this reversal. However, a breccia unit that could be interpreted as a fault was present, thus 55 m may not represent the full thickness of the transition.
In summary, this remarkable record of geomagnetic field behavior in the basaltic column of Hole 801C requires one or a combination of two mechanisms, both of which are somewhat poorly known at present. The number of flows with intermediate directions suggests that the ridge was spreading extremely fast, equal to or at a greater rate than the fastest rates observed today. Alternatively, or in addition, the geomagnetic field may have had a somewhat different component composition than that at present and in the recent past. At present the dipole field component dominates the quadripole component, but in the Middle Jurassic, perhaps the dipole was subordinate and the quadripole dominant. This arrangement provides one possible mechanism for the prominence of nondipolar directions observed between the polarity intervals in Hole 801C.