Magnetostratigraphy of the Ryukyu Group in Miyakojima Island, Okinawa, Japan

Abstract

The Ryukyu Group in the Ryukyu islands consists of reef limestones of Pleistocene age, and records cycles of marine regression and transgression. Study of the rocks has the potential to constrain precise sea level changes around the middle Pleistocene climate transition from 40 kyr cycles to 100 kyr cycles. To assign a geochronological marker in the age of the transition, we undertook magnetostratigraphic studies of the Ryukyu Group rocks exposed on Miyakojima Island. Paleomagnetic samples were collected at 20 sites from all 5 sequence-stratigraphic units on Miyakojima Island (MY-Units 1 to 5 in ascending stratigraphic order). The conventional thermal demagnetization procedure provided reliable polarity determinations for only seven sites. For the remaining 13 sites, we developed a novel technique of reductive chemical demagnetization (RCD) combined with alternating field demagnetization. This hybrid technique successfully erased the overprinting magnetic components, revealing the primary component. Paleomagnetic directions of 20 sites show that the lower part of the Ryukyu Group (MY-Units 1-3, and the lowest part of MY-Unit 4) records reversed geomagnetic polarity, whereas the upper part (all but the lowest part of MY-Unit 4 and MY-Unit 5) records normal polarity. Combining the magnetostratigraphic data with existing calcareous nannofossil data, we conclude that the reversed-normal geomagnetic polarity transition corresponds to the Matuyama-Brunhes boundary (MBB). These magnetostratigraphic data including the MBB improve the geochronology of the Ryukyu Group, which is useful for the temporal correlation between the Ryukyu Group and the other climate records during the middle Pleistocene climate transition.