Magnetic Intensity Variations in Sediment Cores from Leg 128 1

Variations of the intensity of magnetization in the sediment cores from Sites 798 and 799 were examined. Closely correlated variations in the magnetic intensity enabled estimation of the relative change of the sedimentation rate between the two sites. A high sedimentation rate of more than 30 cm/1,000 yr was inferred for about 550,000 yr ago at Site 799. The sedimentation rate was used to compare the distribution of volcanic ash layers at two sites. INTRODUCTION During Ocean Drilling Program (ODP) Leg 128, sediment cores were obtained from Sites 798 and 799. At Site 798 (37.04°N, 134.80°E), Holes 798A, 798B and 798C were cored with the advanced hydraulic piston corer (APC) to depths of 142.5, 142.6, and 120.1 m below seafloor (mbsf), respectively. At Site 799 (39.22°N, 133.87°E), APC cores were taken to a depth of 184.1 mbsf (see Ingle, Suyehiro, von Breymann, et al., 1990). The APC cores possess a very stable remanence and reveal detailed records of the geomagnetic field during the Pliocene-Pleistocene period (Hamano et al., this volume). The sedimentation rate at these sites during the Pliocene-Pleistocene period can be estimated from the magnetic polarity boundaries in these cores and biostratigraphy. At Site 798, the sedimentation rate has been relatively constant at a high value of 110 m/m.y. for the last 2 m.y. On the other hand, the sedimentation rate at Site 799 abruptly increased at the Brunhes/Matuyama polarity boundary. The sedimentation rate before this polarity boundary was as low as 30 m/m.y. since 2 m.y. ago. The sedimentation rate increased to a high value of more than 120 m/m.y. after the Matuyama/Brunhes polarity transition. The sedimentation rate mentioned here has a kind of averaged nature, because the check points for the rate are generally separated in time by more than several hundreds of thousands of years. In the present paper, we attempt to obtain a more detailed record of variation in the relative sedimentation rate from a comparison of the intensity variations of the magnetization in the sediments obtained from the Leg 128 Japan Sea holes. Because of the depth limitation of the APC cores, we compare mainly the variations during the Brunhes Epoch (last 700,000 yr). ANALYSIS During Leg 128, the natural remanent magnetization (NRM) of the APC cores from Hole 798A was measured from the top of the hole to the bottom at 142.5 mbsf and that of the cores from Hole 798C was measured from the top to the total depth of 120.1 mbsf. Cores from Hole 798B were measured from 65 mbsf to the bottom of the hole. The measurements were made at several steps of alternating field (AF) demagnetization up to 15 mT. For all of the cores, an AF field of 10 mT was sufficient to clean up the secondary component of the magnetization. The entire APC cored section from Hole 799A was measured by using a pass-through cryogenic magnetometer with the several steps of the AF field. Although the magnetization of the cores from Hole 799A is less stable than that of the cores from Site 798, a Tamaki, K., Suyehiro, K., Allan, J., McWilliams, M., et al., 1992. Proc. ODP, Sci. Results, 127/128, Pt. 2: College Station, TX (Ocean Drilling Program). 2 Geophysical Institute, Faculty of Science, University of Tokyo, Tokyo 113, Japan. 3 Geologisches Institut, Universitat Köln, Köln, Germany. demagnetization field of 15 mT was sufficient to reveal the polarity sequences during the Pliocene-Pleistocene period. Variations of the intensity of magnetization during the Brunhes normal Epoch observed in the APC cores from Holes 798A, 798C, and 799 A are compared in Figure 1. Based on the mud-line measurements, the top of the cored sediment section from Hole 798A was estimated to be several meters below the mud line. Hence, the data for Hole 798 A were shifted by 4 m in Figure 1. The right-hand side of the plot in each hole corresponds to the depth of the Brunhes/ Matuyama polarity boundary observed from the inclination and declination data. In this depth scale, the intensity variations from the three holes are well correlated. Holes 798A and 798C, are within 40 m of each other, but the distance between Site 798 and Site 799 is about 300 km. Therefore, the similar pattern of the variation in intensity suggests that it is not a local effect. Figure 2 is a comparison of the variations of the magnetic susceptibility in the three holes as a function of depth, which indicates variations in the concentration of magnetic minerals in the sediments. The depth range in Figure 2 is the same as in Figure 1. As is evident in Figure 2, the amplitude of the susceptibility variations is much smaller than that of the variation in magnetic intensity. In addition, the various features of the depth variation of the magnetic intensity are not observed in the variation of the magnetic susceptibility. In Figure 1, short scale variations of the magnetic intensity with wavelengths of several meters to 10 m are observed, whereas such variation is not apparent in the plot in Figure 2. Sedimentary sequence of Unit I at Site 798 (top of the section to 220 mbsf) is characterized by a rhythmic alternation of darkand light-colored sediments on a size scale of decimeters to meters. On a larger scale, however, the sedimentation rate seems constant for the last 2 m.y.. On the other hand, the sedimentation rate at Site 799 seems to have fluctuated more. Sediments in some sections (10.0-11.2 and 40-79 mbsf) have been affected by soft-sediment deformation. Turbidity flow structures were also detected in various parts of the sedimentary section. Hence, we can assume that the sedimentary environment at Hole 798A was more stable than that at Site 799. We tried to observe the change in sedimentation conditions at Site 799 relative to that at Site 798, assuming that the sedimentation rate at Site 798 had been stable during the last 700,000 yr. The comparison of variations in the magnetic intensity at the three holes is on approximately the same time scale in Figure 1, because the overall sedimentation rate for these three holes is similar from the depth of the Brunhes/Matuyama boundary to the top of the sediment in each hole. Before this polarity boundary, the sedimentation rate at Site 799 was much slower than that at Site 798. Hence, the intensity variations before the Brunhes/Matuyama polarity boundary should be compared using different depth scales. An example of the comparison for a section immediately below the Brunhes/Matuyama boundary is shown in Figure 3, where the depth scale is adjusted by the onset and the termination of the Jaramillo normal event. The depth range