Previous   |    Next

doi:10.2204/iodp.proc.329.106.2011

Paleomagnetism

At Site U1368, we measured natural remanent magnetization (NRM) of all archive-half sections from Holes U1368B–U1368E using the three-axis cryogenic magnetometer at 2.5 cm intervals before and after alternating-field (AF) 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 U1368, has a negative inclination (approximately –46.7°), so positive remanence inclinations indicate reversed polarity. Data from Holes U1368C and U1368D provide only a partial record because whole-round core samples were taken from these holes for geochemical and microbiological analyses. From Hole U1368B, 13 discrete sediment samples (7 cm³ cubes) were 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, eight 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 U1368 was to provide chronostratigraphic constraint by determining magnetic polarity stratigraphy. A discrete rock sample was also taken and measured for NRM from each of the RCB basement cores from Hole U1368F. With the basement samples, NRM was measured after AF demagnetization at peak fields of 10, 20, 30, 40, 50, and 60 mT using the pass-through magnetometer and the Agico spinner magnetometer. During coring operations at Site U1368, 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 U1368B–U1368F are presented in Figures F54, F55, F56, F57, and F58, together with the whole-core susceptibility data measured on the WRMSL (see “Physical properties”). The lithology at Site U1368 changes from clay-bearing nannofossil ooze (lithologic Unit I) at the top to nannofossil-bearing clay (Unit II) to metalliferous lithic sand and clay (Unit III) at the bottom (see “Lithostratigraphy”). The nannofossil ooze unit extends from 0 to ~12 mbsf in Hole U1368B. Using magnetic susceptibility data, it was possible to correlate between Holes U1368B and U1368E (Fig. F59). This correlation was applied to the magnetic intensity data and to the inclination and declination data (Fig. F60).

Magnetic directions at Site U1368 show both reversed and normal polarity. However, the records are not consistent between holes, possibly because of a magnetic overprint acquired during coring (high negative inclination), viscous remanent magnetization, or diagenetic changes in the sediment.

According to the shipboard interpretation of planktonic foraminiferal assemblages, the sedimentary record at Site U1368 spans from the late Miocene (~13 Ma) near the sediment/basement interface (~16 mbsf in Hole U1368C) to ~3.5 Ma at 3 mbsf in Hole U1368B (see “Paleontology and biostratigraphy”).

The AF demagnetization record from eight discrete samples from Hole U1368B deviates significantly from the half-core record (Fig. F54). It is likely that the influence of the magnetic overprint is not completely removed from the half-core record.

Given the difficulty in determining sediment age by shipboard paleomagnetic studies, chronostratigraphy for Site U1368 must be determined by postexpedition studies, including further magnetic cleaning by increased AF demagnetization and use of other chronostratigraphic tools.

Top of page   |    Previous   |    Next