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Abstract Recent silicoflagellate distributions have been examined in 195 surface sediment samples from the North Pacific to establish the silicoflagellate assemblage dataset from surface sediments across the North Pacific for paleo sea surface temperature (SST) reconstruction. The relative abundance data of all seven silicoflagellate taxa identified in this study were used for the modern analog method to reconstruct past SST. The SST reconstruction based on the silicoflagellate modern analog technique was applied to the two sediment cores from the Japan Sea. With the development of ice sheets, the Japan Sea experienced a low salinity isolation event during the Last Glacial Maximum (LGM: 19 ka–26.5 ka), which hampers the SST reconstruction employing geochemical proxies such as alkenone and Mg/Ca thermometry. The reconstructed glacial SST between 25 and 15 ka was ~ 5 °C in the northern and southern Japan Sea, indicating significant southward migration of the glacial subpolar front. The timings of the deglacial SST rises were ~ 14 ka at the south core site and ~ 10 ka at the north core site, indicating a latitudinal lag. This lag likely reflects a gradual increase in the fluxes of the Tsushima Warm Current from the East China Sea through the Tsushima Strait. Compilation of the silicoflagellate assemblages in the North Pacific surface sediments used for the modern analog technique to reconstruct past sea surface temperature (SST), revealing the Japan Sea SST during the last glacial maximum ~20 ka ago.

Abstract The southwestern Ryukyu Trench is an ideal place for investigating sediment transport from the Taiwan mountain belt to the Ryukyu Trench floor. To study the characteristics of trench turbidites and sediment transport along the trench, we utilize two piston cores: KR1518-PC04/PL04 at the southwestern end of Ryukyu Trench and YK1501-PC14/PL14 in the trench floor. We also collect two push cores, YK1611-6K1467MG and 6K1467MR, from undisturbed seabed surface to observe lithology. Our analysis reveals that the lithology of the cores shows homogeneous gray mud layers intercalated with very-thin fine sand layers, which we interpret as hemipelagites intercalated with very-thin turbidites. We identify 60 turbidites (0.3–4.0 cm thick) from YK1501-PC14 and 36 turbidites (0.9–4.2 cm thick) from KR1518-PC04. Based on observation from YK1501-PC14, most turbidites, which were less than 1 cm in thickness, displayed planar and sharp bottom contacts. The turbidite thickness of YK1501-PC14 (6448 m water depth) is thinner than that of KR1518-PC04 (6147 m water depth) in general. Through X-ray fluorescence core scans (ITRAX), we distinguished trench turbidites by elemental patterns and accurately determined the depths of every very-thin turbidite. Most hemipelagites from YK1501-PC14 and KR1518-PC04 show similar ITRAX profiles, indicating a similar source of the sediments. ITRAX analysis also revealed five intervals in high-Ca mud in YK1501-PC14, suggesting another potential source from the Ryukyu forearc. Anisotropy of Magnetic Susceptibility (AMS) results show that both YK1501-PC14/PL14 and KR1518-PC04/PL04 exhibit an oblate fabric indicating a normal sedimentary condition. Magnetic signatures showed the presence of pyrrhotite, supporting evidence that Taiwan-sourced sediments can be transported up to ~ 250 km by turbidity currents and spread from the Taiwan Island to the Ryukyu Trench floor. This study improves our understanding of Ryukyu Trench turbidites and provides valuable insights into active sediment transport along the southwestern Ryukyu Trench.

Abstract Hadal trenches are unique geological and ecological systems located along subduction zones. Earthquake-triggered turbidites act as efficient transport pathways of organic carbon (OC), yet remineralization and transformation of OC in these systems are not comprehensively understood. Here we measure concentrations and stable- and radiocarbon isotope signatures of dissolved organic and inorganic carbon (DOC, DIC) in the subsurface sediment interstitial water along the Japan Trench axis collected during the IODP Expedition 386. We find accumulation and aging of DOC and DIC in the subsurface sediments, which we interpret as enhanced production of labile dissolved carbon owing to earthquake-triggered turbidites, which supports intensive microbial methanogenesis in the trench sediments. The residual dissolved carbon accumulates in deep subsurface sediments and may continue to fuel the deep biosphere. Tectonic events can therefore enhance carbon accumulation and stimulate carbon transformation in plate convergent trench systems, which may accelerate carbon export into the subduction zones.

Abstract Previous studies of sediments recovered from the Japan Trench between 37° 25′ N and 38° 30′ N document distinctive turbidite beds induced by huge earthquakes. We studied two sediment cores at 39°N to investigate the depositional record further north along the Japan Trench. These investigations spatially extend our knowledge of the depositional record of earthquakes in the Japan Trench. We examined specifically the precise stratigraphy of turbidite beds using paleomagnetic secular variation, and a tephra correlation. Additionally, anisotropy of magnetic susceptibility (AMS) was investigated to understand the depositional conditions of each turbidite bed. The inferred ages of turbidite beds in this study closely approximate their previously reported ages, which are correlated with the historical and prehistorical huge earthquakes off Tohoku, northeastern Japan. The paleo current directions during deposition of turbidite are inferred from their grain alignment based on AMS data. The directions of basal part reveal northeastward in the slope-side basin and north-northeast in oceanward basin. The directions of basal and upper thick muddy part of a turbidite bed are not always consistent, which suggests the hydraulic condition in the narrow elongated deep-sea basin. This fact could be essential information to elucidate a unique hydraulic condition during the turbidite deposition in the confined basin in the Japan Trench.

Abstract Deep-sea turbidite has been used to determine the history of occurrence of large earthquakes. Surface-sediment remobilization is a mechanism of the generation of earthquake-induced turbidity currents. However, the detailed mechanism of surface-sediment remobilization caused by earthquake ground shaking is unclear. To understand how high peak ground acceleration (PGA) caused by a large earthquake can remobilize surface sediments, we determined the age of a surface-sediment core recovered from the mid-slope terrace (MST) of the inner slope of the Japan Trench in northern Sanriku to determine turbidites generated by large historical earthquakes and calculate the PGAs of these earthquakes using an empirical attenuation relation commonly used in Japan. Small offsets in radiocarbon ages and excess 210Pb activities between turbidite and hemipelagic muds suggest that the turbidites in the core resulted from surface-sediment remobilization. 137Cs and excess 210Pb chronologies indicate that the three uppermost turbidites in the core are correlated with three large historical earthquakes, namely the 1968 common era (CE) Tokachi-oki, the 1933 CE Showa–Sanriku, and the 1896 CE Meiji–Sanriku earthquakes. Calculation of PGAs for large historical earthquakes along the northern Japan Trench indicates that a PGA of > 0.6 g is necessary for turbidite deposition in the MST basin. This threshold is larger than that reported for central Sanriku and may vary spatially. Moreover, turbidites in the MST deposits are more frequent in the northern Japan Trench than in the central Japan Trench, suggesting that the occurrence of three types of large M8-class earthquakes in the northern Japan Trench might have contributed to the frequent occurrence of large PGAs.

Submarine canyons are efficient sediment transport pathways from shallow marine areas to deep sea. Along active margins, large tsunamis are a trigger for sediment transport to deep sea. However, sediment transport through submarine canyons by such extreme wave events in the carbonate depositional environments has not been fully understood. Large tsunamis have repeatedly struck the coral reef islands of the southern Ryukyu Islands and have transported large boulders composed of coral from the reef to shore. In this study, we examined sediment cores collected near the mouths of submarine canyons and basin floor on the southern Ryukyu arc’s forearc. The presence of coarse calciturbidites containing coral, molluskan fragments, and coral reef benthic foraminifera was limited on the submarine fan formed at the mouth of the reef-connected and shelf-incised submarine canyon. In cores collected near the mouth of shelf-incised submarine canyons with no reef connection and slope-confined canyons, no coarse calciturbidite is observed. Few calciturbidites were found in more downslope cores, implying that most calcareous grains derived from shallow marine areas were deposited on this fan. Depositional intervals of turbidites were calculated to be several hundred–a few thousand years, which agree with the recurrence intervals of large tsunamis estimated from onshore tsunami deposits, based on radiocarbon dates in hemipelagic mud intervals. No temporal change in the depositional intervals of calciturbidites in the cores from the submarine fan since the last glacial maximum. Therefore, the tsunamis may be an important mechanism for surface sediment reworking in coral reefs, and shelf-incised and reef-connected submarine canyons can play an important role in the efficient transport of shallow marine calcareous grains to the deep sea.