MAGNETIC-PROPERTIES OF LIGHT AND DARK SEDIMENT LAYERS FROM THE JAPAN SEA - DIAGENETIC AND PALEOCLIMATIC IMPLICATIONS

Rock magnetic/paleoclimatic/diagenetic relationships of sediments span ning the last 0.78 Ma have been investigated using samples collected f rom light and dark layers recovered at ODP Sites 794 (Yamato Basin) an d 795 (Japan Basin). Rock-magnetic parameters (K, K-fd, ARM, SIRM, S-r atio) are shown to reflect diagenetic processes and climate-related va riations in the concentration, mineralogy and grain-size of the magnet ic minerals contained within the sediments. The magnetic mineralogy is dominated by ferrimagnetic (magnetite-type) minerals with a small con tribution made by hematite and iron sulphides such as pyrrhotite and/o r greigite. Magnetic mineral concentration and grain size vary between light and dark layers with the former characterized by a higher magne tic content and a finer magnetic grain size. Magnetite dissolution, re lated to sulfate reduction due to bacterial degradation of organic mat ter, is the process responsible for the magnetic characteristics obser ved in the dark layers, testifying to the reducing conditions in the b asin. Variations in the rock magnetic properties of the sediments are strongly correlated with global oxygen isotope fluctuations, with glac ial stages characterized by a lower magnetic mineral content and a coa rser magnetic grain size relative to interglacial stages. Major downco re changes in the magnetic properties observed at Site 794 can be rela ted to changes in the oceanographic conditions of the basin associated with the flow of the warm Tsushima Current into the Japan Sea at abou t 0.35-0.40 Ma ago. (C) 1998 Elsevier Science Ltd. All rights reserved .