Application of a recently developed method of oriented drill coring to accumulative tephra layers: identification of the Matuyama–Brunhes geomagnetic reversal in Akan caldera-forming eruption deposits

Abstract

Abstract We adapted a recently developed technique of oriented drill coring (100 m in depth) for paleomagnetic measurements, including additional techniques such as half-cutting of the drill cores and accurate cube sampling using an apparatus that precisely guides the cube into the half-cut drill core surface. Our techniques were successfully applied to sequential tephra layers (including loess) at Akan Quaternary caldera volcano in eastern Hokkaido, Japan. Directions of remanent magnetization for a total of 129 specimens were measured by Spinner magnetometer with thermal and alternating field demagnetizations. We detected clear Matuyama–Brunhes geomagnetic reversal between the depths of 20.59 m and 21.30 m, where inclinations change from 65.1 to –44.3°. With these techniques, we can in future obtain useful geologic information (e.g., radiometric ages and absolute paleo-intensities) from volcanic rocks around the Matuyama–Brunhes boundary in Akan volcano. This will clarify more detailed processes of the geomagnetic polarity transition. Our study demonstrates the utility of oriented drill cores from successive tephra and loess layers for high resolution investigate of continuous paleo-geomagnetic records. The new techniques can be adopted to other geological fields on Earth, like in studies of marine sediments, providing continuous paleomagnetic direction data from drill core. Graphical Abstract

Keywords

• Oriented drill coring
• Oriented cube sampling
• Akan caldera
• Paleomagnetism
• Matuyama–Brunhes reversal