We took pass-through magnetometer measurements on all split-core archive sections. Sediment cores were measured at 5-cm intervals. We measured at 1-cm intervals coherent basalt pieces that could be oriented unambiguously with respect to the top. Pass-through magnetic susceptibility measurements were taken on all unsplit core sections at 4-cm intervals.
In order to isolate the characteristic remanent magnetization (ChRM), cores were subjected to alternating-field (AF) cleaning. The number of AF demagnetization steps and peak-field intensity varied depending on lithology, the natural remanent magnetization (NRM) intensity, and the amount of time available. On average, sediment half-cores were demagnetized using three AF steps, in addition to the measurement of NRM. The basalt half-cores were demagnetized using a minimum of six AF steps. The maximum applied field ranged between 25 and 50 mT. We analyzed the results in Zijderveld and stereoplot diagrams; where possible, we calculated the ChRM direction using principal component analysis (Kirschvink, 1980). Examples of the AF demagnetization of sediment and basalt samples are shown in Figure F49.
Core recovery of sediments at Site 1186 was generally poor. Many of the recovered intervals were either too short or too disturbed for reliable pass-through magnetometer measurements. Sediment magnetizations vary considerably, with NRM intensities that range from 6 × 10-5 to 0.7 A/m. Magnetic susceptibilities are generally low and, in some intervals, negative. Five intervals with relatively high susceptibility were caused by magnetic contamination of the core liners (see "Physical Properties"). Because the contaminated intervals are short, they affect only one or two consecutive pass-through measurements; consequently, we were able to detect and discard these spurious values.
There is a progressive increase in NRM intensity downward through the sedimentary section, but NRM intensities of individual lithostratigraphic units (see "Lithostratigraphy") are quite variable. Unit II is weakly magnetic, with a mean NRM intensity of 1.6 × 10-3 A/m. Too few measurements were made to characterize variations within this unit. Subunit IIIA is more strongly magnetized (mean NRM intensity = 2.9 × 10-3 A/m) with a discernible increase in intensity in Core 192-1186A-21 at 882.5 mbsf and downward to the boundary with Subunit IIIB. Subunit IIIB is much more strongly magnetized, with a mean NRM intensity of 3.6 × 10-2 A/m. At the base of Subunit IIIB, the NRM intensity increases substantially to 0.7 A/m for the 36 cm of reddish yellow to pinkish white bioturbated limestone in Core 192-1186A-30R. The interval of dark brown claystone that immediately overlies the basement was too short to be measured.
The poor core recovery of sediments at Site 1186 meant that we could not confidently make magnetostratigraphic correlations for much of the cored interval. In particular, we were unable to obtain reliable magnetic polarity data for the Paleocene-Eocene sediments (Cores 192-1186A-2R through 13R). We have constructed a preliminary magnetic stratigraphy for the pre-Paleocene sediments (Cores 192-1186A-14R through 30R), where recovery was better, but the resulting correlations should be treated with caution because of low recovery and correspondingly large gaps in the paleomagnetic record.
We were generally able to identify the magnetic polarity of the Maastrichtian-Campanian and older sediments with confidence. The sequence of polarities obtained from Cores 192-1186A-16R through 29R was combined with biostratigraphic information (see "Biostratigraphy") to develop a correlation with the geomagnetic polarity timescale (GPTS) of Berggren et al. (1995) (Fig. F50). The geomagnetic field during this interval (i.e., 120-65 Ma) was characterized by dominantly normal polarity with a few distinct, reverse-polarity intervals.
Sediment in Core 192-1186A-16R is entirely reversely magnetized. We correlate it with Chron C30r, a short reverse-polarity interval in the middle to late Maastrichtian. This correlation agrees well with the presence, in this core, of early late Maastrichtian nannoplankton from Zone CC25B (see "Biostratigraphy"). Core 192-1186A-17R is normally magnetized, except in the lower 20 cm, where we observe positive inclinations that we interpret as the downward transition to reverse polarity. No pass-through measurements were possible on Core 192-1186A-18R, but sediments in Core 19R are entirely reversely magnetized; we view this as a continuation of the reverse polarity inferred at the base of Core 192-1186A-17R. The calcareous nannofossil Micula praemura first occurs in Chron C31r. Because this nannofossil is found in Core 192-1186A-18R (see "Biostratigraphy"), we interpret the magnetization in Core 192-1186A-17R as being associated with the transition from C31n to C31r. Core 192-1186A-19R is correlated directly with Chron C31r, an ~2.5-m.y. reverse-polarity interval of the early Maastrichtian (Fig. F50).
A possible unconformity between Cores 192-1186A-19R and 20R (see "Biostratigraphy" and Table T5) places Core 20R-CC below the Maastrichtian/Campanian boundary. Unfortunately, Core 192-1186A-20R was unsuitable for pass-through magnetic measurements. Core 192-1186A-21R has a complicated magnetic polarity character involving three polarity changes, including a complete cycle from normal to reverse and back to normal between 881.1 and 882.4 mbsf. Based on the presence of nannofossils from Zone CC22 in Core 192-1186A-21R, we correlate these magnetization changes with polarity transitions C32r.1r to C32n.2n and C32n.2n to C32r.2r in Chron 32 (Fig. F50). The lower 5.5 m of Core 192-1186A-21, and Cores 23R-26R, are all normally magnetized; we have correlated these cores with Chron C33n on the basis of available biostratigraphic evidence and the absence of reversely magnetized intervals in the recovered sediments.
At 930.55 mbsf in Section 192-1186A-26R-3, there is a clay-rich band that corresponds to a condensed interval where the age of the sediments abruptly increases from Campanian to Albian (see "Biostratigraphy" and Table T5). Sediments both above and below this condensed interval are normally magnetized; consequently, we did not detect the reverse polarity Chron C33r in the sediments. Our sampling interval (5 cm) and the size of the region sensed by the magnetometer, however, are too large to adequately examine any fine-scale magnetization changes present in the condensed interval. Shore-based studies of discrete samples may help in identifying smaller-scale features not discernible in our pass-through measurements.
The lower part of Core 192-1186A-26R (below 930.55 mbsf), Cores 192-1186A-28R and 29R, the sedimentary portion of Core 30R, and the basement rocks are all normally magnetized (Core 192-1186A-27R consists of many small chert pieces and was unsuitable for pass-through measurements). The normal polarity is consistent with magnetization acquired during the Cretaceous Normal Superchron (CNS). Although no reverse-polarity intervals were detected, a significant portion of the paleomagnetic record of this stratigraphic interval was not sampled because core recovery was poor.
The rock-magnetic properties of basalts recovered from Site 1186 are similar to those observed in basalts from other Leg 192 sites. We observed a distinct difference between the pillow lavas, with relatively low magnetic susceptibility and high median destructive field (MDF), and the more massive lava flows, with high susceptibility and low MDF (Fig. F51). This difference can be explained by either the finer grain size of the pillow lavas or their higher degree of low-temperature alteration (see "Alteration") or a combination of these factors. Particularly interesting is the variation in rock magnetic properties within the massive part of basement Unit 4, where very low MDFs (<5 mT) and high susceptibility values correlate with intervals of coarser grain size (see "Igneous Petrology"), probably indicating a higher concentration of large, multidomain titanomagnetite grains in these intervals.
Similar to previous Leg 192 sites, we observe a drilling-induced magnetization in all basalt cored at Site 1186. This drilling-induced overprint is particularly severe for the massive flows, probably because of their higher concentration of low coercivity, multidomain titanomagnetite. An example of a sample with a strong drilling-induced overprint is shown in Figure F52A. Although only a few percent of the NRM remains after 20 mT AF demagnetization (Fig. F52C), defining the ChRM direction using the 20 and 25 mT demagnetization steps (Fig. F52B) is still possible. Pillow lavas are generally less overprinted (e.g., Fig. F49B). The ChRM direction, NRM intensity, magnetic susceptibility, Koenigsberger ratio, and MDF for all coherent basalt pieces longer than 15 cm for which a reliable ChRM direction could be defined are listed in Table T9. For coherent pieces longer than 50 cm we list data for roughly every 25 cm. The ChRM inclinations obtained from different parts of long, coherent pieces generally agree to within a few degrees.
The magnetic inclination is negative for all 83 ChRM determinations (Table T9), indicating normal polarity for all basalt cores recovered. The normal-polarity magnetization is consistent with the biostratigraphic late early Aptian age of the limestone immediately overlying basalt basement (see "Biostratigraphy"), indicating lava emplacement during the CNS (Fig. F50). In order to define the downhole variation in the paleomagnetic inclination data, we combined the individual ChRM data into paleomagnetic units following the statistical method outlined for Site 1185 (see "Paleomagnetism" in the "Site 1185" chapter). Using this procedure, we divided the basement section at Hole 1186A into eight paleomagnetic units (Table T10). All basement unit boundaries correspond to paleomagnetic unit boundaries. However, we observe more than one paleomagnetic unit within basement Units 2 and 4. Shore-based studies on discrete samples are necessary for a more precise definition of the paleomagnetic units and their mean inclination.
Downhole logging indicates that Hole 1186A is vertical to within 1° and that the sedimentary beds are horizontal (see "Downhole Measurements"). Because of the disrupted nature of most sedimentary material sampled at Site 1186, obtaining consistent magnetic directions from the pass-through measurements was impossible for the majority of sedimentary cores. Exceptions were Cores 192-1186A-21R and 23R through 26R, in which magnetic inclinations were consistent over intervals ranging from 2 m to almost 6 m in thickness. We calculated mean inclination values and corresponding paleolatitudes for these cores (Table T11). Mean inclinations for Cores 192-1186A-23R through 26R are essentially equal (i.e., -35° ± 2°), yielding similar paleolatitudes (Table T11). Biostratigraphic data limit the age of the sediments in Cores 192-1186A-23R and 25R to the late Campanian, or ~74-78 Ma. Consistent but significantly lower inclinations were obtained from principal component analyses of magnetic data from Core 192-1186A-21R. These low inclinations might relate to slumping, which would also explain the abrupt change in the rock-magnetic properties in Core 21R described above. We will use shore-based measurements on discrete samples to better define the magnetic characteristics of Core 21R sediments.
The mean inclination for the eight paleomagnetic units within the basement was calculated using the statistics of Kono (1980) (Table T11). Unfortunately, eight units are too few to reliably estimate the recorded paleosecular variation (i.e., angular standard deviation).
The paleolatitudes obtained from sedimentary Cores 192-1186A-23R through 26R and the basalts compare very well with those obtained for similar-age sediments and basalts obtained from other Leg 192 sites (e.g., Fig. F92 in the "Site 1183" chapter).