Natural remanent magnetization (NRM) measurements have a range of 1.4 to 32.9 A/m (median value = 7.5 A/m) (Table T1). These NRM values are in the range of values reported for ocean crustal basalts (Johnson et al., 1996), although they are on the strong side of average. On orthogonal vector diagrams, the NRM is often a steep, downward-pointing vector. This is commonly observed with ODP samples and is attributed to overprint by the drill string (Acton et al., 2002).
AF demagnetizations give low median destructive field (MDF) values in the range of 2.5 to 5.9 mT (Fig. F2; Table T1). Most samples had only negligible magnetization remaining after the 30-mT demagnetization step. This suggests that the magnetization has a low coercivity and is susceptible to acquiring an overprint. Because the magnetization weakens rapidly with demagnetization, we attribute much of the strong NRM values to this overprint. AF and thermal demagnetization results showed three types of behavior. A total of 18 samples (29%) gave scattered directions, making the calculation of a characteristic remanence direction inappropriate. Samples with poor demagnetization results were often physically close to one another in the cores, suggesting lithology as a factor. For example, the upper part of Unit 3 has a large number of inconsistent samples (Table T1). Of the samples that produced consistent demagnetization results, 7 gave positive inclinations (e.g., Samples 198-1213B-31R-6, 69–72 cm, and 31R-1, 45–47 cm) (Fig. F2) and 27 gave shallow, negative inclinations (e.g., Samples 31R-2, 36–38 cm, and 32R-5, 54–56 cm) (Fig. F2). We think that the negative inclinations, being more prevalent and consistent, likely represent the true characteristic remanence (see "Discussion and Conclusions"). The positive inclinations may come from samples from which the drill string overprint was incompletely removed.
Hysteresis measurements range from 3.6 to 13.6 mT in saturation coercivity (Fig. F3), and for remanence coercivity, low values are consistent with the MDF results (Table T1). Low coercivity and hysteresis curves that rapidly reach saturation may indicate that titanomagnetite is the predominant type of magnetic grain. On a Day plot (Mr/Ms vs. Bcr/Bc), Hole 1213B samples plot to the left of published curves for single-domain and multidomain grain mixtures (Fig. F4). The displacement from model curves is probably a result of the samples having grains with different shape and composition characteristics compared to those assumed for the simple mixing model. However, the trend of Hole 1213B samples is more or less parallel with the mixing curves, implying that a similar phenomenon occurs in these samples. The vertical position of Hole 1213B samples on this diagram suggests that the magnetic behavior results from a mixture of single-domain and multidomain grains with a large fraction of the latter. Multidomain grains often have magnetizations that are easily modified by applied magnetic fields, and this factor may be responsible for the inconsistent demagnetization behavior of many samples.