All of the archive halves of the soft sedimentary sequence, lithologic Units I and II (see "Unit I" and "Unit II"), were measured and demagnetized with the shipboard-automated long-core cryogenic magnetometer. Recovery in lithologic Units III and IV was quite discontinuous; therefore, each chert, porcelanite, marl, or chalk piece was placed separately in the magnetometer with the bedding planes carefully oriented to geographic vertical (depositional orientation). In this manner, inclination was obtained, although the polarity of inclination was not known. These data allow paleolatitude to be obtained. Site 1149 is located on the same tectonic plate as the sites of Leg 129 (Sites 800, 801, and 802) for which paleolatitudes had been determined in detail (Steiner and Wallick, 1992). Projection of the paleolatitude records of Sites 800 and 801 to Site 1149 indicates that Site 1149 was located in southern latitudes during the deposition of lithologic Unit IV and most likely in low southerly paleolatitudes during deposition of Unit III. Measurement and demagnetization of bedding-plane oriented fragments of siliceous and carbonate sedimentary rocks allow a record of paleolatitude to be constructed. Finally, vertically oriented pieces of the basaltic basement also were measured and demagnetized with the shipboard magnetometer.
The natural remanent magnetization (NRM) inclinations of the archive halves of all cores obtained from Holes 1149A and 1149B are shown in Figure F63. Nearly all inclinations have values between +60° and +80°, whereas the present geomagnetic field inclination at this site is only 51°. This result was surprising because it indicates nearly complete overprinting of the original remanent magnetization by a drilling remanence.
The main objective in measuring the archive halves of the cores recovered at Site 1149 was to obtain a magnetostratigraphy for dating and correlation purposes in sedimentological and paleontological shipboard studies. Because of the volume of core obtained and the rate at which cores were recovered, a brief blanket demagnetization procedure was undertaken to apply to all cores of similar lithology. Detailed alternating-field (AF) demagnetization was performed on one section of the first core recovered. That section was demagnetized at field strengths of 10, 15, 20, 25, 30, 40, and 50 mT. A large part of the drilling remanence overprint was removed by 10 mT (Fig. F64); complete removal required at least 25 mT, and even 35 mT was insufficient in some strata to completely remove the secondary magnetization. Because of the anhysteretic remanence (see "Paleomagnetism" in the "Explanatory Notes" chapter) imposed by the AF demagnetization apparatus, the cores were not treated higher than 25-35 mT. The 25-mT step generally is a good representation of the true remanence, as it displays in detail the magnetic polarity reversal pattern. For the sake of speed in a high core volume setting, all cores were demagnetized at 25 mT; some also were demagnetized at 15 and 35 mT. Toward the bottom of the unlithified sedimentary section in Hole 1149B (lithologic Unit II, see "Unit II"), even 35 mT was unable to adequately remove the coring overprint.
The results from the upper 110 m of the sedimentary sequence (lithologic Unit I) (see "Unit I") were remarkable. A complete replica of the magnetic polarity time scale for 0-6.3 Ma was obtained after demagnetization to 25 mT. Cores 185-1149A-4H through 19H were oriented with the tensor tool; therefore, absolute declination as well as inclination was obtained in the paleomagnetic data. Figure F65 displays the declination (left) and inclination (right) for this sedimentary interval. Figure F60 in the "Site 801" chapter compares those data to the most recent Cenozoic magnetic polarity time scale (Berggren et al., 1995); a remarkable one-to-one match with the known behavior of the geomagnetic field is exhibited.
Through this magnetostratigraphic record, the upper 110 m of the sedimentary section (Cores 185-1149A-1H through 13H) can be very accurately dated (Fig. F66). Paleomagnetic and 40Ar/39Ar studies of volcanic rocks from the last 7 m.y. have determined the ages of the geomagnetic field reversal record to an accuracy of two decimal places (see time scale of Berggren et al., 1995). The well-known ages of these reversals allows sedimentation rates to be calculated. The magnetostratigraphy shows that sedimentation rates during the current Brunhes normal geomagnetic polarity interval were a remarkable 34 cm/k.y. A number of rate changes are obvious between late Miocene and the present (see "Sedimentation Rates").
An abrupt change in remanence is present between Sections 185-1149A-13H-1 and 13H-2. The change is obvious both in NRM and the demagnetized data. Beginning at Section 185-1149A-13H-2, the consistency of magnetic directions from centimeter to centimeter within the core decreases, resulting in a degraded magnetic polarity signature. However, some indications of polarity structure remain evident down through Core 185-1149A-20X (Fig. F65). Although not well defined, therefore not identifiable to the chron and subchron level, an overall pattern still is obvious. A large proportion of the strata between ~114 and 134 mbsf exhibits a dominance of normal polarity with a suggestion of frequent, poorly defined polarity changes. No information is available regarding hiatuses in this part of the section; therefore, the validity of matching lithologic Unit II to the polarity time scale immediately preceding the last identified chron in lithologic Unit I (late Miocene Chron C3Ar) may not be supportable. Nevertheless, the dominance of normal polarity and suggested abundant polarity changes in the data of lithologic Unit II bears a resemblance to a compressed version of some part of perhaps the late Oligocene through the early late Miocene geomagnetic polarity time scale. Considering the sedimentation rates, it is possible that the large amount of reversed polarity exhibited between ~150 and 166 mbsf represents the reversed polarity of latest Eocene-early Oligocene time; at present, however, these are merely speculations based on available data.
A major objective of the study of the paleomagnetism of Site 1149 was to obtain a paleolatitude tract for this more northern Pacific plate site to compare it to those produced at Sites 800, 801, and 802, 10°-15° to the south. Numerous paleolatitude data were accumulated. These data will be fully analyzed and reported in a later publication. At present, a few generalizations can be made. The site appears to have achieved approximately its present latitude by the Miocene or earlier. The inclination from the upper Miocene to the Pleistocene in lithologic Unit I appears to be constant and similar to the present inclination (50.6°) at this 31.3°N latitude site. The approximate mean inclination for Unit I is 47°-48°, corresponding to a paleolatitude of 28°-29°N. Lithologic Unit II, when magnetically well defined, has inclinations that are the same or slightly shallower (35°-50°) than at present. These inclinations suggest paleolatitudes of 20°-30°N and may correspond to Eocene-Miocene time. The top of Unit III (Cores 185-1149B-4R through 13R) has approximately zero inclinations, within ±5°-10°, suggesting that the site was on or near the equator. Because these core pieces are predominantly unoriented, and as yet undated, paleomagnetic results cannot define whether the site was north or south of the equator at the time of deposition of lithologic Unit III. The distribution of inclinations suggests that the site was located on the equator and that it remained on the equator for the entire time of deposition of this lithologic unit. Lithologic Units IV and V (within the basement) have shallow, but nonequatorial inclinations.
The basaltic basement displays a large amount of alteration and, perhaps consequently, displays a complicated magnetization. Oriented basaltic basement pieces were measured in all three holes that penetrated basement (Holes 1149B, 1149C, and 1149D). Normally, a multicomponent magnetization is observed, and the components are not adequately separated by the AF demagnetization performed shipboard. Some samples in all of the basement holes (Holes 1149B, 1149C, and 1149D) display a seemingly single-component magnetization, as judged by linear behavior during increasing AF treatment. Samples with this behavior become more abundant deeper in the basement (Hole 1149D). These samples all have positive inclination magnetizations, indicative of reversed polarity in the Southern Hemisphere, consistent with the predicted M11 age. A very preliminary estimate of the inclination value is +20°-30°, corresponding to a paleolatitude of 10°-16°S.
Three major results came from paleomagnetic measurement of samples from Site 1149. The magnetostratigraphic record contained in the upper strata (lithologic Unit I) replicates, in great detail, the magnetic polarity time scale between 0 and 6.3 Ma. The detail obtained places tight age constraints on the upper 110 m of the sedimentary record and demonstrates very high sedimentation rates (~30 m/m.y.) for the upper Neogene. A paleolatitude record for the site was obtained from Units I, III, IV, and the basement, which shows that plate motion was nonuniform as the site moved from ~10°-15°S in Early Cretaceous time to its present location of 31°N. The magnetization of the basaltic basement has reversed polarity, consistent with the location of Site 1149 on the reversed polarity magnetic Anomaly M11A.