17:15 〜 19:15
[SEM16-P01] 宮古島礁性石灰岩の古地磁気層序および岩石磁気・風成塵の記録
キーワード：宮古島、風成塵、磁鉄鉱、還元化学消磁
Reefal limestones are composed of corals and other organisms’ fossils with significant heterogeneity in the samples and their magnetizations are weak in general. Previous studies on the reefal limestones in Miyakojima Island (e.g. Anai et al., 2018) suggest that magnetite seems to be the main carrier of their weak magnetism, whose origin is fossil magnetotactic bacteria and/or detrital. In the reefal limestones, other magnetic minerals such as hematite or goethite are also present preventing extraction of primary magnetizations carried by magnetite. Thus, standard magnetization measurement using superconducting magnetometer, and AF or thermal demagnetization is not easy for paleomagnetic studies. Anai et al. (2018) was successful in minimizing the influence of secondary magnetizations carried by hematite and/or goethite using reductive chemical demagnetization (RCD) providing reliable age constraints based on magnetostratigraphy. In addition, reefal limestone is known to be an excellent recorder of paleoenvironment, such as sea-level change and/or climate change. Here, we present results of magnetostratigraphic and rock magnetic studies as well as investigations on eolian dusts including grain size analyses using specimens from the outcrops around the southern coastal area of Miyako-jima Island.
We have taken samples from seven (MK3~MK9) and eight (MK11~MK18) sites along the western and eastern consecutive road sections, respectively. We conducted NRM measurements on all paleomagnetic specimens and stepwise AF demagnetization experiments on pilot specimens. The preliminary results suggest that the paleomagnetic polarity boundaries corresponding to Matuyama-Brunhes transition is between MK4 (normal) and MK5 (reversed) for the western section, which has the approximate altitude of 35 m. We explored improvements on RCD by making notches on the specimens in combination with ultrasonic cleaning and stirring in vacuum as well as dripping. Rock magnetic measurements were conducted on a selected specimen for each site using Lake Shore VSM 8604; i.e. hysteresis, back field DC demagnetization, IRM acquisition and first order reversal curve (FORC). FORC analysis suggest that there are varying proportions of single domain (SD), multidomain (MD) and vortex state grains, which is considered as mainly magnetite. Specimens with corals and coral fragments are generally unstable, whereas specimens with rhodolith are stable with prominent central ridge suggesting fossil magnetotactic bacteria origin. IRM unmixing suggests the presence of higher coercivity minerals, which is considered as hematite and/or goethite. The specimens show increase in S-ratios and reduction of higher coercivity minerals in IRM unmixing curves after RCD indicating that secondary magnetic minerals such as hematites and/or goethites are removed by RCD. A grain size analysis with Horiba LA-960 on the specimen from MK1 shows that there are two populations centered around 0.2 μm and 7 μm. Details on the eolian dust particles and grain sizes will be presented.
We have taken samples from seven (MK3~MK9) and eight (MK11~MK18) sites along the western and eastern consecutive road sections, respectively. We conducted NRM measurements on all paleomagnetic specimens and stepwise AF demagnetization experiments on pilot specimens. The preliminary results suggest that the paleomagnetic polarity boundaries corresponding to Matuyama-Brunhes transition is between MK4 (normal) and MK5 (reversed) for the western section, which has the approximate altitude of 35 m. We explored improvements on RCD by making notches on the specimens in combination with ultrasonic cleaning and stirring in vacuum as well as dripping. Rock magnetic measurements were conducted on a selected specimen for each site using Lake Shore VSM 8604; i.e. hysteresis, back field DC demagnetization, IRM acquisition and first order reversal curve (FORC). FORC analysis suggest that there are varying proportions of single domain (SD), multidomain (MD) and vortex state grains, which is considered as mainly magnetite. Specimens with corals and coral fragments are generally unstable, whereas specimens with rhodolith are stable with prominent central ridge suggesting fossil magnetotactic bacteria origin. IRM unmixing suggests the presence of higher coercivity minerals, which is considered as hematite and/or goethite. The specimens show increase in S-ratios and reduction of higher coercivity minerals in IRM unmixing curves after RCD indicating that secondary magnetic minerals such as hematites and/or goethites are removed by RCD. A grain size analysis with Horiba LA-960 on the specimen from MK1 shows that there are two populations centered around 0.2 μm and 7 μm. Details on the eolian dust particles and grain sizes will be presented.