Pergamon Qurrrmcrn Scirm r KP\iew.v. Vol. 16. pp. 1093-l I 14, 1997. C 1998 Elsevier Science I-id. PII: SO277-3791(96)00118-7 All rights reserved. Printed in Great Britain. 0277-3791197 1332.00 MAGNETIC PROPERTIES OF LIGHT AND DARK SEDIMENT LAYERS FROM THE JAPAN SEA: DIAGENETIC AND PALEOCLIMATIC IMPLICATIONS L. VIGLIOTTI Istituto di Geologia Marina, CNR, Via P. Gohetti 101, 40129 Bologna, Italy {E-mail: M~f#@hoigm2.igm.ho.cnr.it) Abstract - Rock magnetic/paleoclimatic/diagenetic relationships of sediments spanning the last 0.78 Ma have been investigated using samples collected from light and dark layers recovered at ODP Sites 794 (Yamato Basin) and 795 (Japan Basin). Rock-magnetic parameters (K. Kfd, ARM. SIRM, S-ratio) are shown to reflect diagenetic processes and climate-related variations in the concentration, mineralogy and grain-size of the magnetic minerals contained within the sediments. The magnetic mineralogy is dominated by ferrimagnetic (magnetite-type) minerals with a small contribution made by hematite and iron sulphides such as pyrrhotite and/or greigite. Magnetic mineral concentration and grain size vary between light and dark layers with the former characterized by a higher magnetic content and a finer magnetic grain size. Magnetite dissolution, related to sulfate reduction due to bacterial degradation of organic matter, is the process responsible for the magnetic characteristics observed in the dark layers. testifying to the reducing conditions in the basin. Variations in the rock magnetic properties of the sediments are strongly correlated with global oxygen isotope fluctuations. with glacial stager, characterized by a lower magnetic tnineral content and a coarser magnetic grain size relative to interglacial stages. Major downcore changes in the magnetic properties observed at Site 794 can be related to changes in the oceanographic conditions of the basin associated with the flow of the warm Tsushima Current into the Japan Sea at about 0.35-0.40 Ma ago. Q 1998 Elsevier Science Ltd. All rightx reserved INTRODUCTION Magnetic parameters provide a rapid and effective tool for monitoring the supply of terrigenous sediment to the oceans in responseto climatic change, and for recon- structing the paleoenvironmental records of deep-sea sediments (Kent, 1982; Oldfield and Robinson, 1985; Doh et al., 1988; Bloemendal rf al., 1992; Sahota et ml., 1995). Rock-magnetic properties often vary with changes in the lithology of the sediments and these variations are in many cases controlled by climate. Thus changes in the concentration, mineralogy and grain size of magnetic mineral assemblages often reflect climatically-induced variations within the lithogenic fraction of the sediment (Robinson, 1986; Bloemendal el al., 1988; Hall and King, 1989; Sagerand Hall, 1990; Robinsonrf ul., 1995). Gross downcore changes in the magnetic properties reflect physical changes in the sedimentary environment that may indicate paleoceanographic variations and/or diage- netic processes. The aim of this paper is to investigate the rock magnetic/paleoclimatic/diagenetic relationships of the light-dark sedimentary cycles deposited during the BrunhesChron (Upper Quaternary, 0.78 Ma) in the Japan Sea at ODP Sites 794 and 795. QSR Tada et al. (1992) found that variations in sediment composition are closely related to glacial-interglacial cycles expressed in the standard oxygen isotope curve (Imbrie et crl., 1984). The detrital fraction of the sediment tends to be enriched and the maximum grain size tendsto be larger during the glacial stages. Consideringthat often it is the terrigenous fraction of the sediment which contains magnetic minerals, the magnetic properties of the bulk sediment should exhibit distinct differences between glacial and interglacial horizons of the sequence. GEOLOGIC SETTING The Japan Seais a semi-enclosed back-arc basinwith a distinctive physiographic configuration which is crucial to its oceanographicregime. Most of the basin exceedsa water depth of 2000-3000 m, but it is connected to outer seas only by shallow water sills: the Mamiya (I2 m), Soya (55 m). Tsugaru (130 m) and Tsushima (130 m) straits (Fig. I ). The oceanographic conditions of the basin are regulated by the precarious balance of waters flowing through theseshallow sills. Therefore, glacio-eustatic sea- level change should play an important role in the oceanographic conditions of the basin, and it is 1093