The Origins of Magnetic Minerals and Their Changes in the Sediments from Lake Nakaumi and Lake Shinnji, Western Japan: Effects of Ancient Iron Manufacture

Magnetic properties have been investigated on five sediment cores collected by a Mackereth type corer from Lake Nakaumi and Lake Shinnji, Sanninn district, Japan. Magnetic properties of river sediments taken from typical geological units exposed in the catchment areas of the lakes have been also investigated to study changes of the origin of the magnetic minerals in two lakes. The natural remanent magnetization of these sediments is stable enough to detect paleosecular variations of geomagnetic field. Although these cores have not yet been dated and the sedimentation rates of the cores were tentatively estimated by correlation of the secular variations with the archeomagnetic study for the past 2000 years in southwestern Japan, the AMS radiocarbon dates will be shown at the meeting. The rockmagnetic measurements showed that the magnetic-mineral contents are increased in the uppermost portion of most cores. Especially in two cores from Lake Shinnji, the increase is very rapid and on average they are about ten times as much as those in the lower portion of the cores. XRD analysis for the magnetic extract and stepwise thermal demagnetization of composite IRM for the sediments of two lakes shows that the magnetic mineral in the uppermost part is magnetite and the predominant magnetic mineral in the lower part of the cores is the solid solution between ilmenite and hematite. Granite containing a large amount of magnetite is distributed widely in the catchment areas and it is well known in these areas that after the beginning of the 17th century, weathered granite had been broken down on a large scale in order to extract the iron sand used for iron manufacture. The water of the Hii River has been flowing to the sea through Lake Shinnji and Lake Nakaumi. But, it had been flowing directly into the sea before the two floods in the 17th century. The possible cause of the enrichment of magnetite in the uppermost part of the cores is thought to be this industrial activity and/or the change in the Hii River's flow. The source of magnetic mineral of the solid solution between ilmenite and hematite, which has not observed as main magnetic mineral in the catchments area of the lakes, may be andesite at Mt Sannbe located in the west of this district.