Shipboard paleomagnetic measurements for Holes 1127A and 1127B consisted of long-core measurements at 5-cm intervals of the natural remanent magnetization (NRM) and the remanence after alternating field (AF) demagnetization of 20 mT, as described in "Paleomagnetism" in the "Explanatory Notes" chapter. Measurements were performed on archive halves of all APC and XCB cores, except for intervals affected by core disturbance consisting of reoriented biscuit and substantial interbiscuit reconstituted mud. Additional demagnetization steps up to 30 mT were applied to some cores. When orientation could be recovered, 10-cm3 discrete samples were cut from lithified fragments from intervals where core disturbance precluded long-core measurements. Discrete samples were also collected from representative core material and were subjected to progressive AF and thermal demagnetization up to 50 mT and 250°C, respectively. These samples were also used for rock magnetic analysis.
The intensity of initial remanence ranges between 0.025 and 5 × 10-5 A/m, with a median of ~6 × 10-3 A/m. The highest values occur as anomalous spikes observed at the top of several cores. Unlithified packstones and wackestones that predominate above 250 mbsf are characterized by higher intensities than similar but more lithified materials below that depth. This difference is possibly caused by the interference between the magnetization of reconstituted mud and the remanence of lithified core segments in the deeper material. The NRM is of moderate to shallow negative inclination (Fig. F7), changing to shallow to moderate positive inclination at a depth of ~300 mbsf. Progressive AF demagnetization results in moderate changes in inclination and reduces considerably the scatter in both declination and inclination. Inductions of 20 mT isolate magnetizations of moderate to steep inclination from which we derived a magnetic polarity stratigraphy.
Below 250 mbsf, lower recovery and persistent core disturbance in partially lithified materials results in intervals where the measurements produce unrealistically rapid changes in inclination, with both normal and reverse polarity. This ambiguity was resolved using data from discrete samples that yield excellent data and unequivocal polarity interpretations.
Measurements on discrete samples reveal a characteristic magnetization that is isolated after removing a spurious overprint, which we interpreted as a drilling-induced remanence probably acquired during passage of the core up the drill string. Median destructive fields determined from demagnetization of discrete samples range between 25 and 30 mT, and most of the remanence (>70%) is removed after heating to 250°C. Results from Sample 182-1127B-25X-1, 135-137 cm (Fig. F8A), show a soft component with a steep downward inclination and univectorial decay to the origin above 10 mT. Thermal demagnetization of Sample 182-1127B-41X-3, 63-65 cm (Fig. F8B), shows a single-component characteristic magnetization of distributed laboratory unblocking temperatures between 90° and 250°C. Line fits were used to define the direction of the magnetization. These yield low maximum angular deviation values of <5°. The mean inclination obtained from principal component analysis of 25 samples (-51°) is indistinguishable from the overall mean determined for long-core measurements for the Brunhes epoch (-52.6°; 34 determinations from an equal number of cores). Typical standard values from averaged inclinations range from 10 to 20. The average excludes data from Core 182-1127B-11H, a pelagic ooze in which intensities varied rapidly and inclinations are anomalously low.
The magnetic Tensor tool was used in APC Cores 182-1127B-3H to 7H and 11H to 16H. Within-core declinations are relatively well grouped, although between-core declinations are generally scattered. After correcting azimuths using the Tensor tool, declinations center around the expected field direction. Interestingly, for XCB cores for which azimuthal orientation is not available, declinations are preferentially along the core fiducial line, an observation attributed in other paleomagnetic studies of ocean sediments to acquisition of a radially inward-directed moment during drilling (Curry et al., 1995).
Normal polarity magnetizations are persistent in Cores 182-1127B-1H through 37X. Within this normal polarity, we observed intensity fluctuations in NRM after 20-mT demagnetization, which appeared to follow the pattern of changes in relative geomagnetic field intensities determined from earlier ocean sediment records (e.g., Valet and Meynadier, 1993) (Fig. F9). Although these NRMs have not been normalized, there are no obvious features in the downhole susceptibility log that would account for the intensity fluctuations, and the sedimentary package is homogenous throughout the entire interval of interest except for a thin interval of pelagic ooze in part of Cores 182-1127B-10H and 11H. There are clear similarities between the virtual axial geodipole moment and NRM intensity waveforms, and a sedimentation rate of ~400 m/m.y. is generally consistent with the periodicity of the oscillations observed in the composite paleointensity record.
Moderately to steeply positive inclinations in Section 182-1127B-37X-4 (343.4 mbsf) are interpreted as reversed magnetizations. A magnetostratigraphy was based upon this reversal and extended downhole. Below 400 mbsf, data are of lesser quality, although polarity interpretations derived from discrete samples are unambiguous. The quality of the record improved toward the bottom of the hole, where a normal-to-reverse transition is recorded at 467.2 mbsf.
Rock magnetism analysis consisted of measurements of weak-field susceptibility at two frequencies, progressive isothermal remanent magnetization (IRM) acquisition, and AF demagnetization of anhysteretic remanent magnetization (ARM). Decay of the NRM upon AF demagnetization is typical of single-domain magnetite, although laboratory unblocking temperatures are low (<300°C). This behavior, however, is typically observed in magnetic sulfides. Four representative samples were given a 300-mT IRM and subsequently measured over a period of three days. All samples displayed a systematic decrease in saturation isothermal remanent magnetization (SIRM). The SIRM decrease after 48 hr was ~30%, although one specimen from Core 182-1127B-18X lost as much as 60% of the initial IRM during that time. This decay is not associated with viscous decay of a magnetization in grains near the superparamagnetic single-domain threshold, because the same saturation field was used before each measurement. Furthermore, viscous decay at rapid time scales (10-104 s) was not observed, and susceptibility displayed only a weak frequency dependence. We attribute this "vanishing magnetization" to the oxidation of a metastable phase such as greigite, which is consistent with the low unblocking temperature and relatively high coercivity observed.
Rock magnetic results are presented in the form of modified Cisowski plots (Cisowski, 1981). In these plots (Fig. F10) the acquisition of IRM, and the demagnetization of NRM, ARM, and IRM are shown in absolute values. The NRM systematically demagnetizes above the noise level of the instrument; all samples display high ARM:IRM ratios that suggest a single-domain grain size.
The normal polarity magnetizations persistent in Cores 182-1127B-1H through 37X are interpreted to be the Brunhes, with the top of the Matuyama epoch (Chron C1r1r) (Fig. F7) corresponding to the positive inclinations in Core 182-1127B-37X at 343.4 mbsf. The top of Chron C1r1n (Jaramillo) is observed at 380.7 mbsf, and the base lies between 393 and 395 mbsf. If the intensity variations within the Brunhes are correlated with the estimates of variation in the mean dipole moment of the geomagnetic field for the same period, a higher resolution magnetostratigraphy becomes possible for this period of time. The more obvious peaks in the record would then provide an approximate time-depth relationship. The low in the NRM record at 10-14 mbsf then corresponds to the low centered at ~20,000 yr (Mono Lake excursion) in the magnetic field record. Similarly, the double peak between 60 and 70 mbsf corresponds to 220,000 to 250,000 yr, and the broad high with three peaks at 90 to 120 mbsf then corresponds to the field intensity feature between 320,000 and 400,000 yr.