Abstract
Abstract It is well known that magnetic records of marine sediments are affected to a certain degree by early diagenesis, the extent and mode of which are controlled by depositional environments. In order to investigate variations in the effects of early diagenesis, we have analyzed 11 gravity cores estimated to be Holocene in age that constitute a depth transect off the southern coast of the Okhotsk Sea. The rock magnetic parameters have peaks produced by the presence of volcanic ashes, basaltic granules and/or relatively coarser-grained layers. The concentration-dependent parameters display characteristic trends of a rapid decrease within the homogeneous silty clay zone in the middle parts of the cores, assuming that the total amount of magnetic minerals decreases through early diagenesis at this interval. This decrease is found in all of the cores from the transect (from both near-shore and offshore sites). In near-shore sites where larger volumes of organic materials are supplied compared to offshore sites, the depth of the decrease in magnetic minerals is relatively shallow. These results imply that the presence of organic materials accelerates the pyritization through the bacterial reduction of ferric ion, thereby causing the decreases in magnetic minerals to become shallower.
Highlights
Rock magnetic analysis of marine sediments has been widely used in paleomagnetic, paleoenvironmental and paleoclimatological studies (e.g., Walden et al, 1999; Evans and Heller, 2003)
Common features are observed in the rock magnetic parameters for all sites: (1) prominent peaks associated with episodic changes in lithology and (2) a rapid but smooth downward decrease in the rock magnetic parameters
The sample containing basaltic rock displayed a high value of χ, χARM, SIRM and S−0.3 T, reflecting the generally acknowledged fact that basaltic rocks are abundant in magnetite (Fe3O4) (e.g., Hunt et al, 1995)
Summary
Rock magnetic analysis of marine sediments has been widely used in paleomagnetic, paleoenvironmental and paleoclimatological studies (e.g., Walden et al, 1999; Evans and Heller, 2003). In the case of marine sediments having high sedimentation rates, a straightforward interpretation has been hindered due to post-depositional diagenetic degradation of rock magnetic records. The evaluation of diagenetic effects on rock magnetic properties is essential in studies on marine sediments. The Okhotsk Sea is a semi-closed marginal sea located on the western rim of the Northwest Pacific Ocean. It has a generally cyclonic current system and exchanges water with the Pacific through the Kurile Islands (Fig. 1). The southern part of Okhotsk Sea, off Abashiri, is known to be the region where the Soya warm current sinks in early spring (Takizawa, 1982; Copyright c The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB
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