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At Site U1366, we measured natural remanent magnetization (NRM) of all archive-half sections from Holes U1366B–U1366F using the three-axis cryogenic magnetometer at 2.5 cm intervals before and after alternating-field demagnetization. The archive-half sections were demagnetized by alternating fields of 10 and 20 mT. The present-day normal field in this region, as expected from the geocentric axial dipole model at Site U1366, has a negative inclination (approximately –44.4°), so positive remanence inclinations indicate reversed polarity. Data from Holes U1366D and U1366F provide only a partial record because whole-round samples were taken for geochemical and microbiological analyses. From Hole U1366C, 18 discrete sediment samples (7 cm3 cubes) were additionally taken at an interval of one per section from the working halves, and compatibility of magnetization between archive half and working half was analyzed. Of these discrete samples, 12 were measured for NRM after demagnetization at peak fields of 10 mT and 20 mT using the pass-through magnetometer. The primary objective of the shipboard measurements for Site U1366 was to provide chronostratigraphic constraint by determining magnetic polarity stratigraphy. During coring operations at Site U1366, neither nonmagnetic core barrels nor the Flexit core orientation tool were used because of the shallow drilling depth of the sediment column (see “Operations”).


Paleomagnetic data for Holes U1366B–U1366F are presented in Figures F25, F26, F27, F28, and F29, together with the whole-core susceptibility data measured on the WRMSL. Magnetization directions are not interpretable throughout most of the Site U1366 cores, possibly because of a magnetic overprint acquired during coring (high negative inclination), viscous remanent magnetization, or diagenetic changes in the sediment. In addition, manganese nodules often hamper recognition of paleomagnetic events in the uppermost several meters of sediment (see “Lithostratigraphy”).

In Hole U1366B, the records of magnetic inclination and declination show polarity changes from 1.5 to 7.5 mbsf (Fig. F25). However, it is difficult to assign this record to any portion of the polarity timescale because

  • Similar reversal records are not clearly identified in the other holes of Site U1366,

  • This record does not clearly correspond to any specific portion of the polarity timescale,

  • No independent shipboard evidence of age can be used to constrain polarity assignments, and

  • The NRM record from 12 discrete samples from Hole U1366C significantly deviates from the half-core record at multiple horizons (Fig. F26).

Given the difficulty in determining sediment age by shipboard paleomagnetic studies, chronostratigraphy for Site U1366 must be determined by postexpedition studies, perhaps using other chronostratigraphic tools.