HR: 1340h
AN: PP23B-1431    [Abstracts]
TI: Biogenic Magnetite and Magnetostratigraphic Dating of Shallow-Water Carbonates from the Ryukyu Islands, Northwestern Pacific
AU: * SAkAI, S
EM: saburo@soc.shimane-u.ac.jp
AF: Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AB: Magnetostratigraphy in shallow-water carbonates has been used successfully for establishing polarity reversal sequences and determining chronologies. However, this has only rarely been attempted for the Pleistocene shallow-water carbonates (the Ryukyu Group) of the Ryukyu Islands in northwestern Pacific. Here I describe the magnetic grains contained in the Ryukyu Group and in Holocene unconsolidated surface sediments (the Kerama Islands; intertidal zone, 10m, 15.5m, 91m in water depth) using transmission electron microscope (TEM). I also show the successful magnetostratigraphic dating of a core from the Ryukyu Group on Irabu Island. Carbonate environments of the Ryukyu Islands are essentially isolated from detrital sources because the Kuroshio Current prevents the input of terrigenous material from the Asian continent. Sediments consist of pure carbonates, and only rarely contain detrital magnetic particles. Extracted magnetic particles reveal the presence of single-domain crystals of magnetite similar to those produced by magnetotactic bacteria. Widespread occurrence of single-domain magnetite in these pure carbonates suggests that bacterial magnetite is probably the major carrier of early in-situ depositional magnetization. On the basis of the remnant magnetism of 56 samples from a 61-m core taken from the Ryukyu Group, the polarity sequence can be correlated with the magnetic polarity time scale from the Matsuyama chron containing the Jaramillo subchron to the Brunhes chron. This age-estimate is fixed by biostratigraphic datums. Magnetic polarities were determined on the basis of relative up-down direction in the unoriented core. The results suggest that magnetostratigraphy, based presumably on bacterial magnetite, can be a powerful chronostratigraphic tool in the shallow-water carbonates of the Ryukyu Islands, which contain very little detrital magnetite.
DE: 9355 Pacific Ocean
DE: 9604 Cenozoic
DE: 4803 Bacteria
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1520 Magnetostratigraphy
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting