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Bathymetric surveys using a multibeam echo sounder (MBES) were conducted northwest of Kyushu, Japan, during cruises GS23 and GB23 to create marine geological maps. High-resolution seafloor bathymetry was obtained during these surveys, although the characteristics of the multibeam system made it difficult to cover large areas with shallow water depths during the surveys. As a result, the surveys did not yield sufficient spatial data to advance geological interpretations of the region. This report focuses on the methods used for depth data acquisition and processing, as well as the results, while omitting detailed descriptions of the seafloor topography.

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Geomagnetic surveys were undertaken to compile marine geophysical maps of the area off northwest Kyushu. A total magnetic anomaly map was created based on observations of the total magnetic field. The survey area contains a complex distribution of magnetic anomalies ranging from short to long wavelengths. The area west of the Goto Islands and east of Ikishima Island is marked by complex short- wavelength magnetic anomalies. In contrast, the area from west of the Goto Islands to south of Tsushima Island is characterized by a long-wavelength positive anomaly that trends northeast–southwest. Given the shallow water depth over most of the survey area, bathymetric data are insufficient to assess the seafloor topography. The short-wavelength magnetic anomalies likely reflect the occurrence of igneous rocks on the seafloor, as also observed on nearby islands. The northeast–southwest trending positive magnetic anomaly is presumed to correspond to the northern end of the belt of positive magnetic anomalies that extends from near the Senkaku Islands to the Goto Islands.

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The marine area around Tsushima and Goto islands, northwestern Kyushu, is mainly a shallow-water shelf region and is of interest in terms of disaster prevention and utilization of marine space. To compile 1:200,000 marine geology maps of the region, multichannel seismic reflection surveys with a GI gun source were conducted, and a preliminary investigation of the geological structure was undertaken. From a geophysical perspective, subsurface strata in the study area are generally divided into acoustic basement and overlying sedimentary units. The acoustic basement occurs in shelf areas as well as topographic highs around the islands. The upper surface of the acoustic basement is commonly planar as a result of erosion or is undulating as a result of folding and faulting. The area from south of Tsushima Island to west of the Goto Islands contains several large high-angle normal faults that generally strike NE–SW along zones of basement uplift and subsidence. These faults are thought to have formed local depressions through lateral displacement and influenced the distribution of sedimentary units. The influence of the Tsushima Current is observed throughout the area, including erosion of basin slopes and floors to the south of the Goto Islands, the development of channel structures and channel-fill units around submarine channels and straits, and the development of local sand waves on the seafloor. In the Gotonada Sea area, the lower part of the sedimentary units is characterized by well-stratified parallel reflective surfaces, while the upper part contains clinoform sets. The sedimentary units that fill the Gotonada sedimentary basin are inferred to have been deposited continuously in response to tectonic subsidence. Local intrusive structures, which occur mainly around the Ikishima and Goto islands, might be related to igneous activity.

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Sub-bottom profile (SBP) data around Ikishima Island and the Goto Islands were acquired during the GS23 geophysical research cruise. This paper reports the characteristics of acoustic facies observed beneath the sea surface. Overall smooth bottoms with transparent or no internal reflections are recognized off Ikishima Island, suggesting the widespread distribution of coarse sediments. An eroded, thick-bedded stratified facies is widespread north of the Goto Islands, indicating the presence of the last glacial erosional surface. To the southwest of the Goto Islands, a thin-bedded stratified facies is observed along the terrace and slopes of the Goto Submarine Canyon and Fukue Basin. This facies shows truncated intra-reflections on the slope, suggesting that the stratified facies unit was deposited before the formation of the Goto Canyon.

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As part of the Marine Geological Map Project of the Geological Survey of Japan, the geological survey cruise GB23 was undertaken in the northeastern East China Sea and western Sea of Japan. Although the water depth over most of the survey area is <200 m, deeper areas also exist, such as the Goto Canyon (water depth: 400–600 m) and the area southwest of the Goto islands and the Danjo Basin (maximum water depth: ~800 m). Seafloor sediment was sampled at 198 sites using a Kinoshita grab sediment sampler, a vibro-corer, a rock dredger, and a rock corer. Here we report the characteristics of seafloor sediments collected using the Kinoshita grab sediment sampler. Coarse sediment with a low mud content occurs in the Tsushima Straight. Finer sediment with a high mud content occurs in other parts of the survey area. The seafloor sediments contain bioclasts such as mollusc shells and coral remains, including fossilized shells. This report also presents preliminary results on the calcareous nannoplankton and main planktic foraminifera in the seafloor sediment samples.

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CTD (Conductivity-Temperature-Depth profiler) observations and bottom water sampling were conducted at 192 sites in the waters surrounding the Tsushima Island during cruise GB23 from October to November in 2023. The survey area is located around the Tsushima Strait, through which the Tsushima Warm Current flows into the Sea of Japan. The observed sea surface water temperature ranged from 18.9 to 23.9 ℃ and salinity ranged from 33.7 to 34.6, suggesting the influence of both the Kyushu Coastal Current and the Tsushima Warm Current. The bottom water temperature generally decreased with increasing depth. In the deeper layers south of the Goto Islands, we observed a water mass thought to be North Pacific Subtropical Mode Water, which flowed to this region from the Goto Canyon. A high-turbidity layer was detected in the coastal waters off Kyushu and north of Tsushima Island, likely influenced by the local topography and ocean currents.

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During cruise GB23 off Northwest Kyushu, rock samples were collected using a rock corer and dredger at four sites to determine the lithology and geological ages of the acoustic stratigraphic units. At one site off the west coast of Ikishima Island, samples of columnar sandy sediment and basalt were collected by rock corer. Dredge operations, together with the seafloor observations by underwater camera, were conducted at three sites around the Goto Islands. Basalt was collected at one site to the south of the islands, and sedimentary rocks (e.g., sandstone and conglomerate) and basalt were collected at one site to the west. We examined the calcareous nannofossils in the sedimentary rock samples obtained by grab and dredge sampling. As a result, two samples obtained from the Gotonada Sea and north of Tsushima Island are correlated to the CN15 biozone.

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We determined the concentrations of 23 elements in 186 marine surface sediment samples collected in the area offshore from northwestern Kyushu, and report the chemical composition data and spatial trends. CaO is the most abundant element in the samples, with a wide concentration range of 2.63–50.2 wt%. The study area is divided into three regions (the Gotonada Sea area, the area around the Hiradoshima-Genkainada Sea, and the Tsushima Strait) with distinct chemical compositions. The samples from the Gotonada Sea area have high concentrations of 18 elements and low concentrations of K₂O, CaO, Rb, Sr, and Ba. The relationships among the major elements concentrations in samples from this area differ from those in samples from the other two areas. These results suggest that the Gotonada Sea samples received a large contribution of mafic clastic material. The concentrations of K₂O, Rb, and Ba in the samples from the Hiradoshima-Genkainada Sea are higher than those in samples from the other two areas, indicating that some of the Genkainada Sea samples contain terrigenous particles. Samples from the Tsushima Strait have high concentrations of CaO and Sr, and differ from river sediments in Tsushima due to the dilution effect of the CaO component of biogenic debris particles (carbonate minerals). A comparison of the elemental compositions of river and marine sediments in the coastal zone suggests a low rate of sediment discharge from the present-day terrestrial environment to the marine environment.

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Benthic foraminifera in surface sediments of the Tsushima Strait, collected during the GB23 cruise, were subjected to a preliminary examination. Benthic foraminiferal assemblage compositions at multiple sites are consistent with those of shallow-water (sublittoral) bathymetric zones around southwestern Japan. Spatial variations in the assemblage composition, planktonic-to-total foraminiferal ratio, and preservation (e.g., filled by green marine clay) might reflect the ambient environment and sedimentary processes in the region of the strait.

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The East China Sea is rich in marine biodiversity, although the azooxanthellate scleractinian coral fauna has yet to be characterized in this region. To address this limitation, azooxanthellate scleractinian corals were collected northwest of Kyushu using a Kinoshita-type grab sampler during marine geological survey cruise GB23. A total of 687 azooxanthellate scleractinian specimens were collected, representing 23 genera and 19 species. Peponocyathus folliculus specimens were obtained from 79 of the 163 sampling sites. This infaunal species exhibits automobility and undergoes transverse division (asexual reproduction) during the anthocaulus and anthocyathus stages. This indicates that asexual reproduction and automobility play an important role in increasing the population size of azooxanthellate solitary corals in soft-substrate environments.

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There have been few taxonomic studies of Anthozoa in the Tsushima Strait and the East China Sea. Accordingly, we undertook a taxonomic survey to gather information on the diversity of Anthozoa collected during the GB23 cruise in 2023. As a result, five genera across four orders and five families were identified from five sites off the coast of northwest Kyushu, with a putative species recorded for the first time in this region. It is expected that further surveys will lead to the discovery of other first recorded species and undescribed species.

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Fish specimens were collected during surface sediment sampling as part of a marine geological mapping survey of cruise GB23 in the East China Sea and Japan Sea, northwest of Kyushu, Japan. The specimens are assigned to 7 families and 10 species: Apterichtus sp., Scolecenchelys aoki, Ophichthidae gen. sp. broken specimen, Bregmaceros anchovia (3 specimens), Carapidae gen. sp., Stalix immaculata, Champsodon snyderi (2 specimens), Priolepis winterbottomi, Gobiidae gen. sp., and Samaris sp. Of these, B. anchovia represents the first record from the waters around Saga and Nagasaki prefectures, and P. winterbottomi represents the first record from the waters around Fukuoka and Saga prefectures. In this paper, we report the occurrence of each species and information on the bottom sediment and bycatch at the collection sites to advance our knowledge of the habitat of the fish species.

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