Diagenetic modification of magnetic properties observed in a piston core (MD01–2407) from the Oki Ridge, Japan Sea

Abstract

We investigated the magnetic properties of u-channel samples from a giant piston core (IMAGES MD01–2407), in order to obtain high-resolution records of past geomagnetic field and paleoenvironmental changes in the Japan Sea. Measurements of natural remanent magnetization (NRM) show occurrence of anomalous magnetic directions at several horizons, which may represent geomagnetic polarity excursions in the Brunhes Chron. However, most of them occur close to the lithological boundary including tephra layers, suggesting that these features might have been caused by deformation of core sediments. Intensity of NRM normalized by artificially induced remanence shows no significant correlation with a standard paleointensity record. This is probably caused by a variation of lithology including volcanic ash layers and effect of the diagenetic changes of magnetic minerals. Down-core variations of magnetic properties in the topmost part depict in detail the process of dissolution and alteration of magnetic minerals during reductive diagenesis, which is to be expected for hemipelagic sediments characterized by a high sedimentation rate and a high organic matter content. Magnetic concentration parameters including low-field susceptibility, anhysteretic remanent magnetization and isothermal remanent magnetization (IRM) indicate that a loss of fine-grained magnetite occurs from 0.6 to 1.2 mbsf. While remanence intensities below the level of the early diagenesis are one or two orders of magnitude lower than those in the topmost part, most of the thinly-laminated (TL) dark layers showed rather enhanced magnetic intensities. Typically, TL2 shows a marked peak at the top with a gradual decrease towards the bottom. Variation of S-ratio, which gives the proportion of a low-coercivity component in saturation IRM, suggests that low-coercivity and fine-grained ferromagnetic minerals are dominant. These data suggest new magnetic minerals formed during deposition of the TL layers, which may be analogous to the diagenetic magnetic enhancement known in sapropels of the Mediterranean Sea.