Paleomagnetic investigations during Leg 199 focused mainly on measurements of the natural remanent magnetization (NRM) and alternating-field (AF) demagnetization of archive-half core, and on detailed measurements of discrete samples. Discrete samples were collected during the cruise from working halves of core sections from Hole A at each site in standard (8 cm3) cubic plastic boxes. The sampling frequency was generally two samples per section. About 30% of the samples were demagnetized on board; the rest of them will be analyzed on shore to help determine the mineralogy and grain size of the magnetic minerals, complement the shipboard magnetic stratigraphy, and confirm the paleolatitude of the studied sites.
Remanent magnetization was measured using the shipboard 2G Enterprises (model 760R) long-core cryogenic magnetometer equipped with direct-current superconducting quantum interference devices (DC SQUIDs) and an in-line, automated AF demagnetizer capable of reaching a peak field of 80 mT. Continuous core measurements were made at 5-cm intervals with 10-cm-long headers and trailers. The response curve from the sensor coils of the cryogenic magnetometer, however, is ~12 cm wide; therefore, measurements taken every 5 cm are not completely independent from each other. Measurements at core and section ends and within intervals of drilling-related core disturbance were not measured or were removed during data processing. The background noise of the instrument seems to be amplified by the ship's movement compared to shore-based instruments, and it was estimated to be ~5 x 10-5 A/m assuming a measured volume of ~100 cm3. The relatively large volume of core material within the sensing region compensates for the relatively high background noise, and with very few exceptions, the sediment magnetization was well above the instrumental noise level.
The standard ODP coordinate system was used, where +x is the vertical upward direction when the core is on its side, +y is the direction to the right along the split-core surface when looking upcore, and +z is the downcore direction. AF demagnetization on the archive halves was performed routinely with the in-line AF demagnetizer at typical fields of up to 20 mT in order to avoid compromising future shore-based paleomagnetic studies.
Magnetic field measurements taken in the AF coil region of the magnetometer have shown two prominent leaks in the magnetic shielding at the points where the magnetic shields are bolted together. This results in peak fields of ~30,000 nT directed along the z-axis of the magnetometer, which might produce a spurious anhysteretic remanent magnetization (ARM) during AF demagnetization. To check for potential problems, we tested the in-line demagnetizer by comparing the results with those obtained using the onboard D-tech apparatus (D-2000). The test that was conducted on discrete samples using a relatively high field (up to 80 mT) suggested that there are not any major differences in the results obtained with the two instruments; therefore, the faster in-line demagnetizer was used for all subsequent samples.
The Schonstedt Demagnetizer (model TSD-1) was used to test thermal demagnetization on a set of discrete samples from Hole 1215A, but this technique was not routinely used as it was more time consuming compared to use of the in-line AF demagnetizer.
The orientations of APC cores were recorded using the Tensor tool (Tensor Inc., Austin, Texas). The instrument has a three-axis fluxgate magnetometer that records the orientation of the double lines scribed on the core liner with respect to magnetic north. The critical parameters for core orientation are the inclination angle (typically <2°) and the angle between magnetic north and the double line on the core liner, known as the magnetic toolface (MTF) angle. The Tensor tool readings were recorded continuously at 30-s intervals, downloaded to a computer, and analyzed once the tool was back on deck. Orientation of cores is of particular importance in paleomagnetic studies of equatorial regions, where the paleomagnetic inclination is close to zero. Unfortunately we encountered some difficulties in using the MTF angle, and some cores had to be reoriented by aligning the paleomagnetic declination to north. Though the reference paleomagnetic direction for the Paleogene is a few degrees to the east, this assumption is acceptable for our magnetostratigraphic purposes.
Measurements of NRM and stepwise AF demagnetization were performed on all archive halves and discrete samples taken from the working halves. More than 10,000 runs were carried out on the shipboard magnetometer. After some tests at 5, 10, 15, 20, 25, and 30 mT, most core sections were demagnetized at 5, 10, 15, and 20 mT or 7, 15, and 20 mT depending on core flow in the laboratory. Because AF demagnetization of the archive half was conducted mostly to remove the soft magnetic overprint that was acquired during the drilling process, field strengths were limited to 20 mT to avoid erasing the entire primary NRM component. Most of the discrete samples were fully demagnetized using the in-line AF demagnetizer in fields of up to 80 mT.
Site paleolatitudes for the Neogene and Paleogene are important goals of Leg 199, and this makes the characteristic remanent magnetization (ChRM) inclinations an important target in the paleomagnetic study, especially in discrete samples. ChRM can be potentially contaminated by a spurious ARM acquired in the magnetometer due to the presence of a small DC field around the AF demagnetizing coils. Because this DC field is directed along the z axis (horizontal and parallel to the long axis of the magnetometer) the ARM would directly affect the inclination. To avoid this potential problem we chose to measure the discrete samples with the samples' -z axis directed along the magnetometer's x axis and the samples' +x axis directed along the magnetometer's z axis. Using this configuration, a possible ARM acquired along the magnetometer's z axis would mostly affect the declination but would preserve the inclination of the ChRM. The correct coordinate system of the sample magnetization was restored after the measurement using a simple Perl script or an Excel spreadsheet.
We have taken two different approaches to estimate site paleolatitude. When discrete samples were available, we used standard Fisher statistics to compute a mean declination and inclination and the corresponding paleolatitude assuming a dipole field. Limited time between sites reduced the number of discrete samples that could be analyzed, so the long-core data were also used to calculate mean magnetic inclination. From the core data we discarded all samples with virtual geomagnetic pole (VGP) latitudes smaller than ±45° to remove possible transitional, spurious, and partially isolated primary magnetization directions. We then grouped them by age in large data sets. Although less precise than discrete samples, the latitudes computed from core data comprise a very large data set (often larger than 1000 measurements), and we believe that such a redundancy compensates for the lower quality of the long-core data compared to discrete samples data.
The magnetic polarity was calculated using VGP latitudes. Whenever possible, we offer an interpretation of the magnetic polarity, with the naming convention following that of correlative anomaly numbers prefaced by the letter C (Tauxe et al., 1984). Normal polarity subchrons are referred to by adding suffixes (n1, n2) that increase with age. For the younger part of the timescale (Pliocene-Pleistocene) we often use the traditional names to refer to the various chrons and subchrons (e.g., Brunhes, Jaramillo). In general, polarity reversals occurring at core ends have been treated with extreme caution and always double-checked with overlapping records from parallel holes. Occasionally, the polarity of paleomagnetic directions for cores taken from higher-latitude holes (Sites 1215 and 1216) was assigned by using inclination-only data from continuous core measurements. At paleoequatorial sites (all the remaining sites) and, in general, for sediments older than late Oligocene, however, this was not possible, and we combined the data from parallel holes to find the best match of both polarity assignment and core orientation. This left a minor uncertainty in core orientation, which was typically resolved by using the orientation of the present-day field overprint as revealed by progressive demagnetization in discrete samples. The same procedure was used to retrieve the orientation of samples taken from nonoriented XCB cores.
The ages of the polarity intervals used in Leg 199 are a composite of four previous magnetic polarity timescales. We used Shackleton et al.'s (1990) astronomically tuned timescale for the interval between 0 and 2.6 Ma, Hilgen's (1991a, 1991b) astronomically tuned timescale for the interval between 2.6 and 5.23 Ma, Shackleton et al.'s (1995b, table 17) partially astronomically tuned timescale for the interval between 5.23 and 14.8 Ma, and Cande and Kent's (1995) seafloor spreading-based timescale for ages older than 14.8 Ma. These four separate timescales are compatible and should not introduce artificial discontinuities in the calculated sedimentation rates. This composite timescale approach was developed by Curry, Shackleton, Richter, et al. (1995) and subsequently adopted by Berggren et al. (1995b). The absolute ages that make up the Leg 199 magnetic polarity timescale are summarized in Table T4.