HR: 1340h
AN: GP13A-0024    [Abstracts]
TI: Diagenetic Effects on Magnetic Properties of Marine Core Sediments From the Southern Okhotsk Sea
AU: * Kawamura, N
EM: kwmr@gaia.h.kyoto-u.ac.jp
AF: Kumi Shioi, 1-1 Ridai-cho, Okayama, Japan, Okayama, 700-0005 Japan
AU: Oda, H
EM: hirokuni-oda@aist.go.jp
AF: Kumi Shioi, 1-1 Ridai-cho, Okayama, Japan, Okayama, 700-0005 Japan
AU: Yamazaki, T
EM: toshi-yamazaki@aist.go.jp
AF: Shisyo Hatakeyama, 1-1 Ridai-cho, Okayama, Japan, Okayama, 700-0005 Japan
AU: Ikehara, K
EM: k-ikehara@aist.go.jp
AF: Syunsuke Taga, 1-1 Ridai-cho, Okayama, Japan, Okayama, 700-0005 Japan
AU: Torii, M
EM: torii@big.ous.ac.jp
AF: Shisyo Hatakeyama, 1-1 Ridai-cho, Okayama, Japan, Okayama, 700-0005 Japan
AB: In order to clarify the impact of early diagenesis on magnetic records of marine sediments, eleven gravity cores from the continental shelf, southern Okhotsk Sea, were analyzed. According to some age models, the sediments deposited during Holocene. The rock magnetic parameters (k, kARM, SIRM, S-0.3T, HIRM, and kARM/k) have the peaks caused by the presence of volcanic ashes, basaltic granules and/or relatively coarser grain-sized layers. Except remarkable peaks caused by episodic material input such as volcanic ash, the concentration dependent parameters (k, kARM, and SIRM) show characteristic rapidly decrease in the middle part of cores. This zone of rapid decreases in magnetic concentration parameters, that we call as rock magnetic transition zone (RMTZ), appears in the homogeneous silty clay can be interpreted that the total amount of magnetic minerals decreased, caused by the early diagenesis. The results of low- and high-temperature magnetic properties, the IRM acquisition curves, and stepwise thermal demagnetization of composite three-axis IRMs show that the highest concentration of maghemized magnetite above RMTZ. In RMTZ, the dominant magnetic mineral is magnetite, but the present of hematite and greigite are presence. RMTZs are characterized by decreases the coercivity index (S-0.3T) and the magnetic granulometric index (kARM/k), which suggest selective dissolution of fine-grained magnetite and hematite. On the other hand, the new mineral such as framboidal pyrite starts to appear in RMTZ. k and SIRM monotonously decreased, while kARM/k and S-0.3 T start to increase toward the core bottom. Those results suggest the change of the ratio of magnetite and hematite. RTMZ are found in all the core, however, the thicknesses of zones are different. The depth of RMTZ is roughly proportional with the distance from the coast of Hokkaido, which implies that organic carbon in the detrital supply from the land area control bacterial activity in the sediments and the depth of RMTZ is raised in the area of high sedimentation in near shore area. Our results indicate that diagenetic effect on rock magnetic parameters dependent on sedimentary environment of each core site.
UR: http://www.gaia.h.kyoto-u.ac.jp/~kwmr/
DE: 1512 Environmental magnetism
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005