Samples were obtained as 2.5-cm (1 in) minicores drilled perpendicular to the split face of the rock cores. Samples were taken only from hemi-cylindrical pieces long enough to ensure that they remained oriented during rotary coring. An upcore orientation mark was scribed on each sample to provide vertical orientation. Samples were spaced at irregular intervals in the core, with the object of collecting at least two to three samples from each flow unit, where units have been defined. In those cores without any igneous unit definitions, the sampling rate was one to three samples per section.
Samples were measured with the shipboard cryogenic magnetometer aboard the JOIDES Resolution during Leg 191. All samples were demagnetized to isolate a characteristic remanent magnetization. Typically, AF demagnetization was attempted on a subset of samples and if this procedure seemed adequate, it was used for the remainder of the samples. AF demagnetization was conducted in two different demagnetization units. Demagnetization in low fields (up to 30 mT) was conducted using the inline AF demagnetization coils mated to the shipboard cryogenic magnetometer. Because these coils may produce a spurious magnetization with high AF field values, demagnetization at fields above 30 mT was done with a separate discrete sample demagnetization unit. Usually the AF demagnetization proceeded in 5-mT steps from 10 to 40 or 50 mT and 10-mT steps up to 70 mT, but a few experiments included smaller demagnetization steps.
For some cores, thermal demagnetization methods were used when AF demagnetization apparently worked poorly. Thermal demagnetization samples underwent stepwise demagnetization beginning at 150°C and typically continuing in 50°C steps up to 450°-625°C. On occasion, more detailed demagnetization experiments were conducted with 25°C steps above 400°-450°C. If the sample demagnetization appeared to isolate the characteristic remanence direction at lower-temperature steps, the demagnetization was sometimes curtailed at lower temperatures (e.g., ~450°-500°C).
Demagnetization results from each sample were plotted on an orthogonal vector diagram (Zijderveld plot) to find a characteristic magnetization direction. Using principal component analysis (Kirschvink, 1980), a least-squares line was fit to a straight segment of the demagnetization curve (if one was evident) anchored at the plot origin. The best least-squares magnetization vector was calculated, along with the maximum angle of deflection (MAD) (Kirshvink, 1980), which is a measure of the scatter in the points constraining the magnetization inversion.
Magnetization inclinations from the individual samples were combined to calculate a mean direction for the site in several steps. I used analysis methods described by Cox and Gordon (1984), which recommend working with colatitude, rather than inclination, but are otherwise analogous to inclination analysis. First, colatitude values for samples from each individual flow were averaged to determine a flow mean. Flow average colatitude values were tested against those of adjacent flows to see if they are statistically different at the 95% confidence level using the Z statistic (Kono, 1980). If adjacent means were not statistically distinct, they were combined and the procedure repeated with other adjacent flows. A few flows were sampled with a single sample, which precluded determining the standard deviation for that flow. In such cases, the average standard deviation for all flows was used for the single sample for testing the colatitude relative to surrounding flows. Using this method, a series of group means was calculated. Cox and Gordon (1984) recommend that group means representing an interval of time less than the coherency time of paleosecular variation should be combined and averaged so that oversampling of short time intervals does not bias the mean colatitude and uncertainty estimates. To accomplish this, group means were judged to be serially correlated if they did not show a large change between successive means (~8° to 10°) or if a group of colatitudes followed a smooth trend (Cox and Gordon, 1984). Although this procedure is subjective, the result is a more conservative estimate of the mean colatitude, the number of independent flow mean colatitudes, and the error limits.
Using the independent group mean colatitude values, the site mean value was computed, again following the methods of Cox and Gordon (1984). This procedure both provides a correction for bias caused by averaging inclination-only data and gives an estimate of the data errors and site mean colatitude. The mean colatitude is calculated by averaging the group means and applying a correction for bias from inclination-only data. An estimate of the random error is determined from between-group colatitude variations, and an estimate of the colatitude variance produced by secular variation is taken from a model of secular variation. The final error bounds are calculated including an assumption of 2° of systematic error possibly caused by off-vertical tilt of the borehole (a quantity not measured during most legs) and correcting for the number of independent group means (Cox and Gordon, 1984).