Previous   |    Next

doi:10.2204/iodp.proc.329.108.2011

Paleomagnetism

At Site U1370, we measured natural remanent magnetization of all archive-half sections for Holes U1370B and U1370D–U1370F 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 U1370, has a negative inclination (approximately –60.8°), so positive remanence inclinations indicate reversed polarity. Data from Holes U1370E and U1370F provide only a partial record because whole-round core samples were taken from these holes for geochemical and microbiological analyses. The primary objective of the shipboard measurements for Site U1370 was to provide chronostratigraphic constraint by determining magnetic polarity stratigraphy. During coring operations at Site U1370, both nonmagnetic core barrels and the Flexit core orientation tool were used (see “Operations”).

Results

Paleomagnetic data for Holes U1370B and U1370D–U1370F are presented in Figures F24, F25, F26, and F27, together with the whole-core susceptibility data measured on the WRMSL (see “Physical properties”). The lithology at Site U1370 changed from zeolitic metalliferous pelagic clay (Unit I) at the top to nannofossil ooze in Unit II and metalliferous clay in Unit III, immediately above the basaltic basement (see “Lithostratigraphy”).

Using magnetic susceptibility data, it was possible to correlate between Holes U1370B and U1370D (Fig. F28). For the upper sediment column, this correlation was applied to the magnetic intensity data and to the inclination and declination data. The correlation between Holes U1370B and U1370D clearly shows that changes in magnetic polarity are consistent between the two holes in the uppermost ~10 mbsf. Reversals are seen in inclination and declination data throughout most of the ~70 m of Holes U1370D–U1370F. Correlation between Holes U1370D–U1370F for the entire ~70 m section using magnetic susceptibility and magnetic intensity is shown in Figure F29. This correlation was applied to the inclination and declination data (Fig. F30), showing that some reversals are consistent between the three holes in the deeper part of the section.

According to the shipboard interpretation of planktonic foraminiferal assemblages found in the nannofossil ooze in Unit II in Hole U1370D (at ~64 mbsf), the sedimentary record at Site U1370 has been assigned an estimated age of early Paleocene (~64.9 Ma) (see “Paleontology and biostratigraphy”). However, it is not possible to make any interpretation of the magnetic polarity stratigraphy at Site U1370. Given the difficulty in determining the age of the sediment section by shipboard paleomagnetic studies, chronostratigraphy for Site U1370 must be determined by postexpedition studies, including further magnetic cleaning by increased alternating-field demagnetization and use of other chronostratigraphic tools.

Top of page   |    Previous   |    Next