Bulletin of the Geological Survey of Japan Top Page

Bulletin of the Geological Survey of Japan Vol.75 No.5/6 (2024)

Special issue on marine geology around Tokara Islands in Kagoshima Prefecture: result of marine geological mapping survey cruises in 2022

Cover photograph | Table of Contents | Abstract

Cover photograph

Marine geological survey around the Tokara Islands, Kagoshima Prefecture, in Fiscal Year 2022

The Geological Survey of Japan (GSJ) has published the Marine Geology Map Series (1:200,000) for Japan since the 1970s. In this special issue, we report the results of seismic reflection surveys, bathymetric surveys, magnetic anomaly surveys, and marine sediment surveys conducted around the Tokara Islands as part of the Northern Okinawa Trough Project.

Upper: Sunrise over the northern Tokara Islands. From left to right: Kuchinoshima Is., Gajashima Is., Nakanoshima Is., Suwanosejima Is., and Tairashima Is.
Lower left: Preparation of a multi-channel streamer cable for a seismic reflection survey conducted by the vessel Shinyo-maru during the GS22 survey cruise.
Lower right: Group photograph on the vessel Bosei-maru (GB22-2 survey cruise) after the completion of sampling around the Tokara Islands.

(Photograph and Caption by INOUE Takahiko)

Table of Contents

All the pages PDF ： 75_05_full.pdf ［40MB］ update: 2025.01.15

Title	Author	PDF
Preface
Special issue on marine geology around Tokara Islands in Kagoshima prefecture: result of marine geological mapping survey cruses in 2022 Fiscal Year	INOUE Takahiko, ITAKI Takuya and AMANO Atsuko (p161–165)	75_05_01.pdf ［3.8MB］
Report
Overview of the seismic survey and dredge in the vicinity of Northern Tokara Islands conducted during geological map research cruises in 2022FY	ISHINO Saki, ISHIZUKA Osamu, HARIGANE Yumiko, ARIMOTO Jun, MISAWA Ayanori and INOUE Takahiko (p167–196)	75_05_02.pdf［17MB］ update: 2025.01.15
Shallow submarine structure around Tokara Islands based on the high-resolution subbottom profiler survey during the GS22 cruise	MISAWA Ayanori, FURUYAMA Seishiro, KOGE Hiroaki, ARIMOTO Jun, ISHINO Saki and SUZUKI Yoshiaki (p197–208)	75_05_03.pdf ［9MB］
Depositional ages of sedimentary rocks obtained from the sea floor around the Tokara Islands during GB22-1, GB22-2 and KH23-1 cruises, based on calcareous microfossil assemblages	ARIMOTO Jun and TANAKA Yuichiro (p209–222)	75_05_04.pdf［1MB］
Article
Submarine sediment distribution and its controlling factors around the western area off Tokara Islands and southern area off Yakushima Island	SUZUKI Yoshiaki, ITAKI Takuya, KATAYAMA Hajime, KANEKO Naotomo, HARIGANE Yumiko, SAITO Naoki, IWATANI Hokuto, MATSUI Hiroki, ISHIZUKA Osamu, YAMASAKI Makoto, ARIMOTO Jun, TOKUDA Yuki, SENTOKU Asuka, IKEUCHI Eri, IGUCHI Akira, SUZUKI Atsushi and SEIKE Koji (p223–248)	75_05_05.pdf［10MB］ update: 2025.01.15
Report
Preliminary results on distribution of benthic foraminifera collected during the GB22-1 and 2 cruises around Tokara Islands	HASEGAWA Shiro (p249–263)	75_05_06.pdf［1.6MB］
Chemical composition of marine surface sediments around Tokara Islands (GB22-1 and 22-2 Cruise), Japan	KUBOTA Ran, TACHIBANA Yoshiko, ITAKI Takuya, KATAYAMA Hajime, SUZUKI Yoshiaki and MANAKA Mitsuo (p265–278)	75_05_07.pdf ［1.3MB］
Marine environment around the Tokara Islands based on CTD observations and ocean general circulation model	SAITO Naoki, SUZUKI Yoshiaki, ITAKI Takuya and SUZUKI Atsushi (p279–297)	75_05_08.pdf［3MB］

errata_75_05.pdf: Updated files in accordance with this errata. （2025.01.15）

Abstract

Overview of the seismic survey and dredge in the vicinity of Northern Tokara Islands conducted during geological map research cruises in 2022FY

ISHINO Saki, ISHIZUKA Osamu, HARIGANE Yumiko, ARIMOTO Jun, MISAWA Ayanori and INOUE Takahiko

Multi-channel seismic and dredge surveys were conducted around the Northern Tokara Islands to construct marine geological maps at a scale of 1:200,000. We present an overview of the seismic survey and dredge results of three cruises (GS22, GB22-1, and GB22-2), and preliminary interpretations of the seismic stratigraphy and geological structures. Four sedimentary units (TY2, TY3, TY4) are continuously traced in the vicinity of Tane-Yaku Spur, which consists of acoustic basement (TY1). Normal faults with NNE-SSW trending develop widely along the western slope of Tane-Yaku spur. To the north of the Tokara Islands, the lowermost sedimentary unit (NT1) occurs under a topographic high that extends from southwest of Kuroshima Island to Gon Sone, at the eastern edge of the Okinawa Trough. The overlying sedimentary units (NT2 and NT3) occur mainly in flat sea floor around the islands and the topographic highs. Unit NT2 is affected by normal faults and folds with NNE-SSW trending, which widely develop north of the Tokara Islands. The geological structures of units TY2, TY3, and NT2 suggest they deposited during formation of the half-graben that extends from the Ryukyu Arc to the western part of the Okinawa Trough. Outcrops of unit TY3, unit TY1, the lower part of unit NT2, and the upper part of unit NT2 were dredged at sites GB22-1-D07, GB22-1-D08, GB22-2-D09, and GB22-2-D10, respectively. We will combine precise correlations among profiles in the surveyed areas with age data from rock analysis to interpret the seismic stratigraphy. The results will be published as marine geological maps of the Tokara Islands.

Shallow submarine structure around Tokara Islands based on the high-resolution subbottom profiler survey during the GS22 cruise

MISAWA Ayanori, FURUYAMA Seishiro, KOGE Hiroaki, ARIMOTO Jun, ISHINO Saki and SUZUKI Yoshiaki

During the GS22 cruise of the R/V Shinyo Maru (The Tokyo University of Marine Science and Technology) from May to June 2022, a high-resolution subbottom profiler (SBP) survey was conducted, focusing on the area of the Tokara Islands and Yakushima Island to obtain geological information on the shallow subbottom structure in the area. This paper summarizes the shallow subbottom structure around Yakushima and Takarajima Islands, as revealed by the SBP surveys. Although the internal structure of the sedimentary sequences could not be determined over most of the continental shelf in the area north of Yakushima Island, we identified partially stratified sedimentary sequences with a maximum thickness of ~20 m below the seafloor. The SBP profiles revealed the internal structure of stratified sedimentary sequences down to a maximum of 40 m below the seafloor in the volcanic front area located west of Yakushima Island. The partially stratified sedimentary sequences occur between the volcanic front area and the western slope of the Tane-Yaku Spur, a characteristic bathymetric high between Yakushima and Tanegashima Islands, although most of the internal structure could not be identified due to poor penetration of acoustic waves. We also surveyed the area north of Takarajima Island, where many earthquakes have occurred in recent years, suggesting the presence of seafloor movement related to seismic activity. However, the data indicate that no faults or intrusion structures have reached the seafloor or shallow part of the sedimentary sequences in this region.

Depositional ages of sedimentary rocks obtained from the sea floor around the Tokara Islands during GB22-1, GB22-2 and KH23-1 cruises, based on calcareous microfossil assemblages

ARIMOTO Jun and TANAKA Yuichiro

Calcareous nannofossil and planktic foraminiferal assemblages were examined to determine the depositional ages of sedimentary rocks obtained during cruises GB22-1, GB22-2 and KH23-1 around the Tokara Islands, northern Ryukyu Arc. The depositional ages of index microfossil-bearing samples are generally well correlated to the standard biozones of the Pliocene and Quaternary: biozones CN10–CN15 for the calcareous nannofossils and biozones PL5–PT1 for the planktic foraminifera. Samples belonging to biozones CN10c–CN13a are from a unit equivalent to the middle–upper part of the Shimajiri Group. Volcaniclastic samples assigned to biozone CN14 indicate that the Pleistocene was a time of intense volcanism in the study area. In addition, the spatial distribution of sedimentary units corresponding to biozones CN14b–CN15 provides essential information to reconstruct the late Quaternary tectonics of the northern Ryukyu Arc.

Submarine sediment distribution and its controlling factors around the western area off Tokara Islands and southern area off Yakushima Island

SUZUKI Yoshiaki, ITAKI Takuya, KATAYAMA Hajime, KANEKO Naotomo, HARIGANE Yumiko, SAITO Naoki,
IWATANI Hokuto, MATSUI Hiroki, ISHIZUKA Osamu, YAMASAKI Makoto, ARIMOTO Jun,
TOKUDA Yuki,SENTOKU Asuka, IKEUCHI Eri, IGUCHI Akira, SUZUKI Atsushi and SEIKE Koji

As part of geological survey cruises GB22-1 and GB22-2 conducted in the area west of the Tokara Islands and the area south of Yaku Island, we conducted surface sediment sampling at 95 sites and sediment core sampling at 1 site. Sediment data, such as sediment samples and photographs of the seafloor, were analyzed for various species (e.g., bryozoa); the distribution of living corals and coral remains; planktic foraminifera assemblages, size distribution, and preservation; and environmental DNA extraction.
In the area west of the Tokara Islands, medium sandy to muddy sediments are distributed on the flat seafloor at depths of 800 m or more, and scattered outcrops occur near the top of a topographic high. We found gravelly sediments and locally rippled sandy sediments around the topographic high. This sediment distribution can be explained by the abundant supply of fine-grained sediment from surrounding land areas such as the Asian continent and Kyushu Island, the influence of the Kuroshio Current flowing northward in this area, and locally generated bottom currents related to topographic effects. In the area south of Yakushima Island, a large area of outcrop occurs along the Tane-Yaku Spur, including Yaku-Shin Sone, and gravelly to muddy sediments are distributed to the east and west of the spur. The sandy sediments contain a large amount of biogenic material, and ripples were observed at several points. The distribution of these sediments reflects the erosion and sediment transport associated with the changing velocity of the Kuroshio Current as it moves eastward across the Tane-Yaku Spur and out to the Pacific Ocean. The analysis of bryozoa, coral distribution, and planktic foraminifera assemblage, size distribution, and preservation suggests that biological production is strongly influenced by the Kuroshio Current in the area west of the Tokara Islands and the area south of Yaku Island.

Preliminary results on distribution of benthic foraminifera collected during the GB22-1 and 2 cruises around Tokara Islands

HASEGAWA Shiro

Basic statistics on the occurrence of benthic foraminifera are investigated using sediment samples collected from east and west of the Tokara Islands (from around the Tane-Yaku Spur and the Amami Spur, and from the eastern margin of the Okinawa Trough, respectively) during cruises GB22-1 and -2. Using 47 samples collected from depths of 257 to 1,438 m, I calculated foraminiferal indices such as the benthic and planktic foraminiferal numbers (FNb and FNp, respectively), the proportions of three types of test composition of benthic foraminifera, and the proportion of planktic foraminifera relative to the total foraminifera (PF%). These indices were almost constant with water depth throughout the Nansei Islands. Four depth zones, corresponding to previously reported Zones II to V around the Nansei Islands, are recognized based on the depth distribution of characteristic species. The proportion of agglutinated forms of benthic foraminifera is reduced at depths of ≥ 700 m in the area “East of Spurs”, suggesting that allochthonous calcareous species of shallow-water origin are widely distributed in the sediments. In addition, a reduction in the planktic/total foraminifera (PF%) value is observed at sites dominated by abiotic clastic sediment or allochthonous benthic foraminiferal remains. Both anomalies are likely to have formed under the influence of the strong Kuroshio Current and the topography surrounding each site.

Chemical composition of marine surface sediments around Tokara Islands (GB22-1 and 22-2 Cruise), Japan

KUBOTA Ran, TACHIBANA Yoshiko, ITAKI Takuya, KATAYAMA Hajime, SUZUKI Yoshiaki and MANAKA Mitsuo

The contents of 24 elements in 71 samples of marine surface sediment collected from the Tokara Islands were determined, and the geochemistry and spatial patterns of the data were investigated. Compared with Okinawa Island, the samples of this study are rich in Al₂O₃ and T-Fe₂O₃, and poor in CaO. The survey region is divided into four areas based on geochemistry. The sediments in the southeastern area are rich in Al₂O₃, TiO₂, and T-Fe₂O₃, and poor in CaO and Sr. Those in the northeastern area are rich in CaO and Sr, and those in the southwestern area are rich in MnO, Ni, and Pb. The sediments in the northwestern area are rich in K₂O, Na₂O, and Rb. Although the relationship between elemental contents and water depth is spatially variable, the marine sediments of this study are estimated to be derived from three sources: bioclasts, mafic clastic material, and felsic clastic material.

Marine environment around the Tokara Islands based on CTD observations and ocean general circulation model

SAITO Naoki, SUZUKI Yoshiaki, ITAKI Takuya and SUZUKI Atsushi

In order to make multifaceted interpretations of the seafloor geology around the Tokara Islands, it is necessary to understand the marine environment, including the Kuroshio Current. In this paper, we analyze CTD observations conducted from the south offshore of Yakushima Island to the north offshore of Takarajima Island during the GB21-2, GB21-3, GB22-1, and GB22-2 cruises. In addition, the flow patterns were analyzed using an ocean general circulation model. The Kuroshio Current generates relatively strong currents between Yakushima Island and Nakanoshima Island and between Nakanoshima Island and Suwanosejima Island, and the strong currents reach the seafloor. In the downstream direction of the Kuroshio Current between Nakanoshima Island and Suwanosejima Island, the surface layer has low water temperature and high salinity. This is thought to be due to the Kuroshio Current passing through the rising topography of the seafloor, causing vertical mixing. At the same location, dissolved oxygen in the surface layer is higher. This may be because vertical mixing supplies nutrients to the surface layer and stimulates primary production. In the vicinity of Kuchino-Erabujima Island, high turbidity layers on the seafloor were observed at many locations. This is possibly due to resuspension caused by strong currents associated with the Kuroshio Current or turbidity currents caused by active volcanic earthquakes.