1:45 PM - 3:15 PM
[SGL22-P12] Matuyama-Brunhes reversal in oriented drill cores from Akan caldera-forming eruption deposits, east Hokkaido, Japan
Keywords:Akan caldera, Matuyama-Brunhes reversal, Oriented drill cores
Akan caldera (24 km x 13 km), eastern Hokkaido, Japan, was formed by successive pyroclastic eruptions during Calabrian to Chibanian (1.8~0.2 Ma) (Hasegawa et al, 2022). The pyroclastic deposits are divided into at least 40 “eruptive units,” of which some are separated by paleosols that represent significant time intervals between eruptions. The recognized 40 eruptive units can be summarized into 17 eruptive “groups” (Ak1–Ak17, in descending stratigraphic order), each of which is composed of a series of “eruptive units” (Hasegawa and Nakagawa, 2016). In order to estimate the time scale of large caldera-forming eruptions based on pyroclastic deposits using paleomagnetic methods, we obtained oriented-drill core samples (100 m in depth) at Tsurui Village, southeast of Akan caldera (Nishiki et al., 2022).
Fine ash parts were collected by a method we have developed for accurate orientation using an aluminum/plastic sampling cube and an apparatus for precisely guiding the cube into the boring core. Directions of remanent magnetization for a total of 129 cubes were measured by spinner magnetometer with thermal and/or alternating current demagnetization. Demagnetization plots for some samples showed a larger dispersion. Data from samples for which the principal component analysis showing MAD values exceeding 15º were rejected.
In this study, we detected clear geomagnetic reversal between the depth of 20.59 m and 21.30 m, where inclinations change from 65.1° to -44.3°. Based on the stratigraphy, lithofacies and petrological features, the ash layers from the depth of 20.59 m and 21.30 m can be correlated with Ak8 and one of the eruption deposits composing of Ak9, respectively. Previous tephro-chronologic studies reported that Ak8 is older than 0.76 Ma and Ak9 is younger than 1.0 Ma (Hasegawa and Nakagawa, 2012), suggesting that Matuyama-Brunhes geomagnetic reversal exists between the depth of 20.59 m and 21.30 m. No other signals of geomagnetic precursor and rebound, which are reported at Matuyama-Brunhes transition by previous studies, were detected around Ak8 and Ak9. Only a 10 cm thick volcanic ash paleosol lies between the eruption deposits including -20.59 m and -21.30 m ash layers without an unconformity. The sequence from Ak8 to Ak9 can be well observed not only in the cores but also at a lot of outcrops. We will obtain volcanic rock samples for radiometric dating and higher resolution/quality paleomagnetic data around Ak8-Ak9 to investigate more detailed processes (e.g. duration of reversal and temporal change of absolute paleomagnetic intensity) of Matuyama-Brunhes transition.
This study was based on the joint research project "Research on temporal evolution processes of large-scale eruption" by NRA, Ibaraki univ. and JAMSTEC.
Fine ash parts were collected by a method we have developed for accurate orientation using an aluminum/plastic sampling cube and an apparatus for precisely guiding the cube into the boring core. Directions of remanent magnetization for a total of 129 cubes were measured by spinner magnetometer with thermal and/or alternating current demagnetization. Demagnetization plots for some samples showed a larger dispersion. Data from samples for which the principal component analysis showing MAD values exceeding 15º were rejected.
In this study, we detected clear geomagnetic reversal between the depth of 20.59 m and 21.30 m, where inclinations change from 65.1° to -44.3°. Based on the stratigraphy, lithofacies and petrological features, the ash layers from the depth of 20.59 m and 21.30 m can be correlated with Ak8 and one of the eruption deposits composing of Ak9, respectively. Previous tephro-chronologic studies reported that Ak8 is older than 0.76 Ma and Ak9 is younger than 1.0 Ma (Hasegawa and Nakagawa, 2012), suggesting that Matuyama-Brunhes geomagnetic reversal exists between the depth of 20.59 m and 21.30 m. No other signals of geomagnetic precursor and rebound, which are reported at Matuyama-Brunhes transition by previous studies, were detected around Ak8 and Ak9. Only a 10 cm thick volcanic ash paleosol lies between the eruption deposits including -20.59 m and -21.30 m ash layers without an unconformity. The sequence from Ak8 to Ak9 can be well observed not only in the cores but also at a lot of outcrops. We will obtain volcanic rock samples for radiometric dating and higher resolution/quality paleomagnetic data around Ak8-Ak9 to investigate more detailed processes (e.g. duration of reversal and temporal change of absolute paleomagnetic intensity) of Matuyama-Brunhes transition.
This study was based on the joint research project "Research on temporal evolution processes of large-scale eruption" by NRA, Ibaraki univ. and JAMSTEC.