Previous   |    Next

doi:10.2204/iodp.proc.329.105.2011

Paleomagnetism

At Site U1367, we measured the natural remanent magnetization (NRM) of all archive-half sections from Holes U1367B–U1367E using the three-axis cryogenic magnetometer at 2.5 cm intervals before and after demagnetization. The archive-half sections were demagnetized by alternating fields (AF) of 10 and 20 mT. The present-day normal field in this region, as expected from the geocentric axial dipole model at Site U1367, has a negative inclination (approximately –44.9°), so positive remanence inclinations indicate reversed polarity. Data from Holes U1367C and U1367D provide only a partial record because whole-round core samples were taken from these holes for geochemical and microbiological analyses. From Hole U1367B, 20 discrete sediment samples (7 cm³ cubes) were taken from the working halves, and compatibility of magnetization between archive and working halves was analyzed. Of these discrete samples, 17 were measured for NRM after demagnetization at peak fields of 10 and 20 mT using the pass-through magnetometer. The primary objective of the shipboard measurements for Site U1367 was to provide chronostratigraphic constraints by determining magnetic polarity stratigraphy. During the coring operation at Site U1367, neither nonmagnetic core barrels nor the Flexit core orientation tool were used because of the shallow drilling depth of the sediment column (see “Operations”).

Results

Paleomagnetic data for Holes U1367B–U1367E are presented in Figures F29, F30, F31, and F32 together with the whole-core susceptibility data measured on the WRMSL. The lithology at Site U1367 changed from metalliferous clay (Unit I) at the top to nannofossil ooze (Unit II) at the bottom (see “Lithostratigraphy”). The metalliferous clay unit extends from 0–5.5 mbsf in Hole U1367B to 0–8.5 mbsf in Hole U1367E. Consistently, NRM intensities and magnetic susceptibilities in Unit I were 10^–1 to 10^–2 A/m (>100 × 10^–5 volume SI) and decreased to ~10^–2 to 10^–3 A/m (10 × 10^–5 to 50 × 10^–5 volume SI) in Unit II (nannofossil ooze) (Fig. F33). Using magnetic susceptibility data, it was possible to clearly correlate between Holes U1367B and U1367E. This correlation was applied to the magnetic intensity data and to the inclination and declination data (Fig. F34).

Magnetic directions are not clearly interpretable throughout most of the Site U1367 cores, possibly because of a magnetic overprint acquired during coring (high negative inclination), viscous remanent magnetization (VRM), or diagenetic changes in the sediment. However, the carbonate ooze in Unit II in Hole U1367E showed slightly less magnetic overprinting with intervals of reversed polarity recorded (Fig. F34). According to planktonic foraminiferal assemblages, the record at Site U1367 spans from the earliest Oligocene (slightly younger than 34 Ma) at the sediment/basement interface to ~28 Ma at 13.4 mbsf (see “Paleontology and biostratigraphy”). However, based on the correlation of magnetic susceptibility, the shipboard magnetic polarity record is inconsistent between Holes U1367B and U1367E (Fig. F34) and it is not possible to make any interpretation of the directional data in terms of polarity chrons.

The AF demagnetization record from 12 discrete samples from Hole U1367B deviates slightly from the half-core record (Fig. F29). Magnetic intensities of the discrete samples were lower than those for half cores, and it is likely that the influence of the magnetic overprint is not completely removed from the half-core record.

Given the difficulty in determining the age of the sediment section by shipboard paleomagnetic studies, chronostratigraphy for Site U1367 must be determined by postexpedition studies, including use of other chronostratigraphic tools and further magnetic cleaning by increased AF demagnetization.

Top of page   |    Previous   |    Next