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doi:10.2204/iodp.proc.330.108.2012

Paleomagnetism

Holes U1377A and U1377B had only shallow penetration and, particularly for Hole U1377A, poor core recovery. Nonetheless, samples from both holes appear to have moderate to steep positive inclinations, indicating Southern Hemisphere reversed polarity. Shipboard sampling at these holes was limited because of the short time remaining for shipboard analysis by the end of the expedition.

Archive-half core remanent magnetization data

The remanent magnetization of the archive halves of Cores 330-U1377A-3R through 6R and 330-U1377B-2R, 4R, and 5R was measured at 2 cm intervals using the cryogenic magnetometer. All data acquired within 4.5 cm of either piece end were filtered out prior to further processing, and thus only pieces longer than 9 cm were considered. No core pieces in the sedimentary units were suitable for measurement, and no measurements were made for ghost Core 330-U1377B-3G.

The natural remanent magnetization (NRM) intensity at Site U1377 is lower than at previous guyots sampled during Expedition 330. The average NRM intensity is <1 A/m (Fig. F27B). These low NRM values may be related to the high degree of alteration (see “Alteration petrology”).

Best-fit principal component directions were calculated from alternating-field (AF) demagnetization data for each 2 cm interval of the archive halves using an automated routine that maximizes the percentage of remanence incorporated and minimizes the scatter about the best-fit direction and the deviation of this vector from the origin (see “Paleomagnetism” in the “Methods” chapter [Expedition 330 Scientists, 2012a]). Based on the distribution of misfit values, only directions with misfit values below 2.42 for Hole U1377A and 1.89 for Hole U1377B were considered; these represent the most reliable 40% of the total number of 2 cm interval principal component directions for each hole. These misfit values are lower than those used for Sites U1372, U1373, and U1374, where the misfit cutoff value was set at ~3.4. The resulting inclinations, intensities, and stability of remanent magnetization, as represented by the median destructive field (MDF′), are shown in Figure F27. Although Site U1377 is dominated by positive inclinations, indicative of Southern Hemisphere reversed polarity (Fig. F27D), the shallow penetration and low recovery limit recognition of any inclination trends with depth or lithology.

Discrete sample remanent magnetization data

The remanent magnetization of 17 discrete basalt samples (10 from Hole U1377A and 7 from Hole U1377B) was measured with the spinner magnetometer. NRM intensities range more widely in Hole U1377B (1.35 × 10^–2 to 3.99 A/m) than in Hole U1377A (0.13 to 1.86 A/m) (Fig. F28; Table T9). Königsberger ratio (Qn) values for Hole U1377A samples are typically >1, but those for Hole U1377B display a broader range, with a minimum of 0.18 and a maximum of 8.90.

Ten discrete samples from Hole U1377A were demagnetized, six by AF demagnetization and four by thermal demagnetization (Table T9; Fig. F29). Only AF demagnetization was attempted for discrete samples from Hole U1377B because of the limited time. Unfortunately, all seven discrete samples from Hole U1377B were remagnetized by a malfunction of the demagnetizer DTech 2000, and no characteristic remanent magnetization directions were obtained. The demagnetization results from basalt samples reveal relatively simple behavior and are generally, though not always (see below), consistent with the moderate to steep positive inclinations observed in the archive-half data. Although only limited thermal demagnetization data are available, results from thermal and AF demagnetization apparently recover the same magnetization component (Fig. F29B, F29C). The coercivity of the discrete samples from Site U1377 is generally lower than for previous sites (Fig. F29).

Anisotropy of magnetic susceptibility

The anisotropy of magnetic susceptibility was determined for all discrete samples (Table T10). The average degree of anisotropy (P′) is 1.01, and shape factors (T) range from –0.49 to 0.72 (where T = –1 if prolate and T = 1 if oblate; Jelinek, 1981).

Discussion

Inclinations at Site U1377 are uniformly positive (reversed polarity; Fig. F30; Table T11) but are more variable than those observed at some other sites (e.g., U1374 and U1376). This variability suggests that the pillows/lava lobes reflect accumulation over some significant amount of time. Discrete sample inclinations in some cases do not agree well with archive-half results (e.g., Cores 330-U1377A-4R and 5R). This discrepancy will be the subject of shore-based studies. Any inferences about the paleolatitude of Site U1377 should be undertaken with care, given the limited penetration and core recovery.

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