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

Anai, Chisato; Shibuya, Hitoshi; Mochizuki, Nobutatsu

doi:10.5575/geosoc.2017.0052

Cited by 2 publications

(8 citation statements)

References 34 publications

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“…The study site is the Ryukyu Group on Miyakojima Island, Okinawa, Japan. The paleomagnetism of the limestones of the Ryukyu Group has been published by Anai et al (2017). The results of this previous investigation are summarized below.…”

Section: Study Site and Previous Investigationmentioning

confidence: 99%

“…Thus, all specimens were demagnetized by thermal demagnetization and RCD + AF demagnetization. Table 1 shows the paleomagnetic direction data from Anai et al (2017). These directions were selected using the criterion of maximum angular deviation (MAD) ≤ 15°.…”

Section: Study Site and Previous Investigationmentioning

confidence: 99%

“…In contrast, Q-43-2 showed the same polarity with both demagnetization methods. Anai et al (2017) also performed RCD + thermal demagnetization for these sites and obtained the same polarity as the RCD + AF demagnetization. These results suggest that the Ryukyu limestone acquired a secondary magnetization that cannot be demagnetized by thermal or AF treatments alone.…”

Section: Study Site and Previous Investigationmentioning

confidence: 99%

“…The samples used in the present study are the same as those used by Anai et al (2017). The samples are reef limestones composed of particles of various sizes including coral fossils with bioclastic structures.…”

Section: Samplesmentioning

confidence: 99%

“…In this study, we present a new type of chemical demagnetization using a reductive etchant instead of strong acids. As a case study of this chemical demagnetization method, we investigated reef limestone samples from Ryukyu group in Japan (Anai et al 2017). Etchant supply methods and efficient concentration of the etchant for effective chemical demagnetization on remanent magnetization are discussed in this paper.…”

Section: Introductionmentioning

confidence: 99%

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Reductive chemical demagnetization: a new approach to magnetic cleaning and a case study of reef limestones

Anai¹,

Mochizuki²,

Shibuya³

2018

Earth Planets Space

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Chemical demagnetization is not preferred as a demagnetizing method in paleomagnetism because strong acids are cumbersome to handle and require considerable time compared to alternating field and thermal demagnetizations. Particularly, for rocks with carbonate minerals, strong acidic solutions are not applicable. This study presents a new method, termed reductive chemical demagnetization (RCD), using ascorbic acid solution as a reductive etchant. Ascorbic acid is a strong reductive agent and converts Fe 3+ ions of secondary magnetic minerals to water-soluble Fe 2+ ions, which facilitate chemical demagnetization of carbonate rocks. The carbonate frame can remain intact if the pH of the solution is buffered at approximately 7 with sodium bicarbonate. This etchant is more suitable than strong acid in terms of handling in a paleomagnetic laboratory, particularly in a magnetic field free room. To reduce the required time, a technique of dripping the etchant on the sample was also devised. This helps the fresh etchant flow through the voids between the grains of rocks to rapidly remove dissolved Fe 2+ ions. As a case study of RCD, reef limestone samples were examined. The results showed that the dripping experiments with 5% ascorbic acid solution were the most effective. It took 72 h to reach the remaining isothermal remanent magnetization (IRM) constant. Thermal demagnetizations of 3-component IRM indicate that RCD removed the high coercivity remanences carried by hematite and goethite. These magnetic minerals were considered to be precipitated between the grains of the rock, and thus they were dissolved by the RCD treatment. A chemical remanent magnetization (CRM), acquired by secondary magnetic minerals, can easily mask the primary remanence for sedimentary rocks of weak magnetization, and the coercivity or unblocking-temperature spectra of the primary remanence and secondary CRM overlap; however, RCD can effectively remove the secondary CRM. RCD prior to alternating field or thermal demagnetization has the potential to improve paleomagnetic demagnetization of sedimentary rocks.

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Section: Study Site and Previous Investigationmentioning

confidence: 99%

Section: Study Site and Previous Investigationmentioning

confidence: 99%

Section: Study Site and Previous Investigationmentioning

confidence: 99%

Section: Samplesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Reductive chemical demagnetization: a new approach to magnetic cleaning and a case study of reef limestones

Anai¹,

Mochizuki²,

Shibuya³

2018

Earth Planets Space

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Geological history of the land area between Okinawa Jima and Miyako Jima of the Ryukyu Islands, Japan, and its phylogeographical significance for the terrestrial organisms of these and adjacent islands

Watanabe

Arai

Otsubo

et al. 2023

Prog Earth Planet Sci

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The modern and Late Pleistocene terrestrial fauna of Miyako Jima and adjacent islands (the Miyako Islands) in the southern Ryukyu Islands, southwestern Japan, includes some endemic taxa or genetically unique populations that exclusively have closest allies in the more isolated Okinawa Jima and adjacent islands (the Okinawa Islands) than in the Yaeyama Islands, which are located southwest of the Miyako Islands with much narrower intervening straits. Those taxa or populations include representatives of lineages that have physiologically highly limited ability for over-sea dispersal and the Miyako Islands are currently separated from the Okinawa Islands by at least 300 km of open water; therefore, the formation of this phylogeographical pattern is perplexing. In this study, we review the late Cenozoic geology of the Miyako Islands, southern Okinawa Jima, the Okinawa–Miyako submarine plateau (OMSP; a plateau located between Okinawa Jima and Miyako Jima), and the Kerama gap, which is a depression between the OMSP and Okinawa Jima. We then consider the origin of the modern and Late Pleistocene terrestrial animals, including a number of non-volant vertebrates on the Miyako Islands. Finally, we propose a new hypothesis (the OMSP hypothesis) to explain the enigmatic composition of modern and Late Pleistocene terrestrial vertebrate fauna of the islands. Southern Okinawa Jima was uplifted and emerged after ca. 2 Ma and was temporarily connected to the OMSP, which is likely to have emerged earlier than southern Okinawa Jima, to form a large island extending from Okinawa Jima to the Miyako Islands with a NE–SW direction of ~ 400 km. Subsequently, Okinawa Jima became separated from the OMSP when the Ryukyu Group—which is composed of Quaternary reef and associated fore-reef and shelf deposits—began to accumulate around the island at 1.7–1.4 Ma. During the interval from 2.0 to 1.7–1.4 Ma, numerous terrestrial animals, including flightless vertebrates, extended their distribution to the OMSP. Although the Miyako Islands repeatedly underwent complete submergence during deposition of the main part of the Ryukyu Group (1.25–0.4 Ma), they were uplifted and emerged to become a land area after ca. 0.4 Ma. In contrast, the OMSP subsided after ca. 0.4 Ma and was almost completely submerged after 0.27 Ma. During ca. 0.4–0.27 Ma, terrestrial animals migrated from the OMSP to the Miyako Islands.

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

Cited by 2 publications

References 34 publications

Reductive chemical demagnetization: a new approach to magnetic cleaning and a case study of reef limestones

Reductive chemical demagnetization: a new approach to magnetic cleaning and a case study of reef limestones

Geological history of the land area between Okinawa Jima and Miyako Jima of the Ryukyu Islands, Japan, and its phylogeographical significance for the terrestrial organisms of these and adjacent islands

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