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For assessment of the potential of the Beppu Bay sediments as a Global boundary Stratotype Section and Point (GSSP) candidate for the Anthropocene, we have integrated datasets of 99 proxies. The datasets for the sequences date back 100 years for most proxy records and 1300 years for several records. The cumulative number of occurrences of the anthropogenic fingerprint reveal unprecedented increases above the base of the 1953 flood layer at 64.6 cm (1953 CE), which coincides with an initial increase in global fallout of 239Pu+240Pu. The onset of the proliferation of anthropogenic fingerprints was followed by diverse human-associated events, including a rapid increase in percent modern 14C in anchovy scales, changes in nitrogen and carbon cycling as recorded by anchovy δ15N and δ13C, elevated pollution of heavy metals, increased deposition of novel materials (spheroidal carbonaceous particles, microplastics, polychlorinated biphenyls), the occurrence of hypoxia (Re/Mo ratio) and eutrophication (biogenic opal, TOC, TN, diatoms, chlorophyll a), unprecedented microplankton community changes (compositions of carotenoids, diatoms, dinoflagellates), abnormally high spring air temperatures as inferred from diatom fossils, and lithological changes. These lines of evidence indicate that the base of the 1953 layer is the best GSSP level candidate in the stratigraphy at this site.

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.

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.

The distribution and characteristics of marine surface sediments are a basic marine geological information. Large river floods are a frequent natural hazard that transport substantial terrigenous sediments into the marine environment. In August 2003, TY ETAU (0310) caused heavy rainfall in the southern coast of Hokkaido, north Japan, where some mountainous rivers in the Hidaka region flooded. Two deposition modes for the 2003 flood sediments can be identified by comparing the pre- and post-flood surface sediment distribution. Shore-normal shallow depressions off the mouth of the Saru and Atsubetsu rivers served as channels for the discharged floodwater preventing dispersion and maintaining the necessary water density to transport the materials as density bottom currents. This action also promoted long-distance transport of flood materials across the continental shelf. Absence of depression on the inner shelf off the mouth of the Niikappu and Shizunai rivers may have dispersed floodwaters near the river mouth and deposited the flood materials close to the shore. Marine geological mapping suggests that the differences in submarine topography (the presence or absence of shallow depressions) are closely related to the regional geological structure. Thus, submarine geology is a controlling factor of the seafloor environments influenced by the river flood.

Accidental leakage of radionuclides from the Fukushima Nuclear Power Plant (FNPP1) took place in the aftermath of the catastrophic tsunamis associated with the Great East Japan Earthquake that occurred on March 11, 2011. Significant amount of radionuclides released into the atmosphere were reportedly transported and deposited on land located near FNPP1. The Niida River, Fukushima, Japan, has been recognized as a terrestrial source of highly contaminated suspended radiocesium adhering to sediment particles in the ocean through the river mouth as a result of hydrological processes. Remaining scientific questions include the oceanic dispersal and inventories of the sediments and suspended radiocesium in the ocean floor derived from the Niida River. Complementing limited in situ data, we developed a quadruple nested 3D ocean circulation and sediment transport model in an extremely high-resolution configuration to quantify the transport processes of the suspended radiocesium. Particularly, we investigated the storm and subsequent floods associated with Typhoon 201326 (Wipha) that passed off the Fukushima coast in October 2013, and subsequently promoted precipitation to a considerable extent and associated riverine freshwater discharge along with sediment outfluxes to the ocean. Using in situ bed sediment core data obtained from regions near the river mouth, we conducted a quantitative assessment of the accumulation and erosion of the sediments and explored the resultant suspended radiocesium distribution around the river mouth and nearshore areas along the Fukushima coast. We identified three major accumulative areas, near the river mouth within an area 1 km, around the breakwaters in the north of the river mouth, and along the southern coastal area, while offshore and northward transports were minor. The present study clearly exhibits substantial retention of the land-derived radiocesium adsorbed to the sediments in the coastal areas, leading to possible long-term influences on the surrounding marine environment.

The southwestern Ryukyu Trench represents the ultimate sink of sediments shed from Taiwan into the Philippine Sea, which are mainly transported to the trench by turbidity currents via submarine canyons. Here, we present trench turbidites intercalated with hemipelagites in a gravity pilot core and a piston core acquired on the Ryukyu Trench floor at 6147 m water depth. We performed X-ray fluorescence core scans (ITRAX profiles), magnetic measurements, and organic geochemistry analyses to discriminate turbidites from hemipelagites. We identified 36 turbidites (0.9–4.2 cm thick) based on visual core descriptions and Ca/Fe ratios in the ITRAX profiles. Three of these turbidites show magnetic signatures indicating the presence of pyrrhotite and peaks in the magnetic susceptibility profile, suggesting that Taiwan-sourced sediments are transported to the Ryukyu Trench by long-runout turbidity currents. Pyrrhotite is also present in hemipelagites of the upper part of the retrieved cores, indicating a dominant sediment source in Taiwan over the last several thousand years. Ca/Fe and Zr/Rb ratios in the ITRAX profiles mark distal turbidites (about 1–3 cm thick), and Zr/Rb peaks mainly reflect grain size changes. Detailed analyses of a representative turbidite show good correlation between Ca/Fe and Zr/Rb peaks with upward-coarsening and upward-fining trends that delimit the turbidite. Sedimentary organic matter in hemipelagites is characterized by higher total organic carbon and total nitrogen contents and higher δ13C values than that in turbidites. Our multi-proxy approach employing high-resolution XRF core scans to differentiate turbidites from hemipelagites contributes to establishing a comprehensive view of modern trench sedimentation from Taiwan to the southwestern Ryukyu Trench.

Accidental leakage of radionuclides from the Fukushima Nuclear Power Plant (FNPP1) took place in the aftermath of the catastrophic tsunamis associated with the Great East Japan Earthquake that occurred on March 11, 2011. Significant amount of radionuclides released into the atmosphere were reportedly transported and deposited on land located near FNPP1. The Niida River, Fukushima, Japan, has been recognized as a terrestrial source of highly contaminated suspended radiocesium adhering to sediment particles in the ocean through the river mouth as a result of hydrological processes. Remaining scientific questions include the oceanic dispersal and inventories of the sediments and suspended radiocesium in the ocean floor derived from the Niida River. Complementing limited in situ data, we developed a quadruple nested 3D ocean circulation and sediment transport model in an extremely high-resolution configuration to quantify the transport processes of the suspended radiocesium. Particularly, we investigated the storm and subsequent floods associated with Typhoon 201326 (Wipha) that passed off the Fukushima coast in October 2013, and subsequently promoted precipitation to a considerable extent and associated riverine freshwater discharge along with sediment outfluxes to the ocean. Using in situ bed sediment core data obtained from regions near the river mouth, we conducted a quantitative assessment of the accumulation and erosion of the sediments and explored the resultant suspended radiocesium distribution around the river mouth and nearshore areas along the Fukushima coast. We identified three major accumulative areas, near the river mouth within an area

Sediment gravity flow is a density current that moves down a slope due to the gravitational force. Submarine landslides are a trigger mechanism for sediment gravity flows, and the movement of a large amount of sediment mass has the potential for generating tsunami. An oceanic trench is a terminal depression in the hadal environment that receives mass transport deposits (MTDs) from the slope. Thick, acoustically transparent layers were found in small basins along the northern Japan Trench. Sediment cores obtained from these basins were composed of thick, homogeneous mud, suggesting the presence of distal MTDs. Radiocarbon dates obtained using bulk organic carbon from the sediment indicated that these MTDs were formed around 2000 years ago and were correlated with each other. Benthic foraminiferal assemblages in the MTDs contained upper-slope species. There are many submarine landslides distributed along the upper slope of the Hidaka Trough; thus, the upper slope of this trough is important not only as a sediment source but also as a source of carbon for the Japan Trench, which is an effective sink of organic carbon. However, the exact locations of the submarine landslide on the upper slope have not yet been determined. Furthermore, from the geohazard point of view, the upper slope is an area of large sediment movements, which should be considered in the context of tsunami hazard mitigation.

Tsunamis are generally considered to disturb the seafloor, rework surface sediments, and change seafloor environments. However, the response of the seafloor to such extreme wave events has not been fully elucidated. Herein, we compare the surface sediments before and after the 2011 Tohoku-oki tsunami on the Sendai shelf and demonstrate that both sandy and muddy sediments were significantly reworked on the shelf. Muddy sediments (> 10 cm thick) were redeposited as graded mud with no or little bioturbation, characterizing the offshore muddy tsunami deposit, while well-sorted sand was found as the sandy tsunami deposit. This redeposited layer could also be retained in the shelf mud sequence. The results imply that the high friction velocity of the tsunami wave and its long-term effect on Sendai Bay might contribute to the large sediment reworking. Part of the resuspended mud moved offshore to the slope area as turbidity currents. Thus, the tsunami is an important mechanism not only for shelf sedimentation but also for deep-sea sedimentation along active plate margins. The detection of 134Cs derived from the Fukushima Daiichi Nuclear Power Plant accident in the redeposited mud indicates that the suspended shelf water state was maintained for some days after the tsunami.

Understanding the impact of earthquakes on subaqueous environments is key for submarine paleoseismological investigations seeking to provide long‐term records of past earthquakes. For this purpose, event deposits (e.g., turbidites) are, among others, identified and stratigraphically correlated over broad areas to test for synchronous occurrence of gravity flows. Hence, detailed spatiotemporal petrographic and geochemical fingerprints of such deposits are required to advance the knowledge about sediment source and the underlying remobilization processes induced by past earthquakes. In this study, we develop for the first time in paleoseismology a multivariate statistical approach using X‐ray fluorescence core scanning, magnetic susceptibility, and wet bulk density data that allow to test, confirm, and enhance the previous visual and lithostratigraphic correlation across two isolated basins in the central Japan Trench. The statistical correlation is further confirmed by petrographic heavy grain analysis of the turbidites and additionally combined with our novel erosion model based on previously reported bulk organic carbon 14C dates. We find surficial sediment remobilization, a process whereby strong seismic shaking remobilizes the uppermost few centimeters of surficial slope sediment, to be a predominant remobilization process, which partly initiates deeper sediment remobilization downslope during strong earthquakes at the Japan Trench. These findings shed new light on source‐to‐sink transport processes in hadal trenches during earthquakes and help to assess the completeness of the turbidite paleoseismic record. Our results further suggest that shallow‐buried tephra on the slope might significantly influence sediment remobilization and the geochemical and petrographic fingerprints of the resulting event deposits., Geochemistry, Geophysics, Geosystems, 21 (6), ISSN:1525-2027

Recent studies of the sedimentary record of the Japan Trench indicate unique earthquake-induced depositional characteristics. To correlate such deposits to historical earthquakes, we studied the stratigraphy of two sedimentary cores obtained from basins of the Japan Trench at 7400 m and 7600 m water depth using paleomagnetic secular variation (PSV). Because of their high sedimentation rates, results show that excellent geomagnetic records are preserved in the sediments in spite of the great water depth. Paleomagnetic secular variation, especially declination during the past 3000 years revealing a distinctive feature of large oscillation, offers an opportunity to make precise stratigraphic correlations. Identification of boundaries between thick fine turbidite intervals and hemipelagite intervals was made based on rock magnetic properties. Successful applications of extraction of paleomagnetic data solely from the hemipelagic interval demonstrate that stratigraphic positions of turbidite in paleomagnetic secular variation record are mutually well correlated in PSV variations. Even over long distances between sites, the turbidites induced by earthquakes can be correlated exactly. The turbidite ages were determined by referring to the PSV master curve. Especially, the ages of turbidite units regarded as induced by the Jogan earthquake can be inferred from two cores that closely approximate its documented age.

Paleorecords of pelagic fish abundance could better define the nature of fishery productivity dynamics and help understand responses of pelagic fish stocks to long-term climate changes. We report a high-resolution record of sardine and anchovy scale deposition rates (SDRs) from Beppu Bay, Southwest Japan, showing multidecadal and centennial variability in the abundance of Japanese sardine and Japanese anchovy during the last 2850 years. Variations in the sardine SDR showed periodicities at ∼50, ∼100, and ∼300 yr, while variations in the anchovy SDR showed periodicities at ∼30 and ∼260 yr. Comparisons between and correlation analyses of the time series of the sardine and anchovy SDRs demonstrate that there is not a consistent out-of-phase relationship during the last 2850 years. This indicates that the multidecadal alternations in the sardine and anchovy populations commonly seen in the 20th century did not necessarily occur during earlier periods. The Japanese sardine SDR record shows a long-term decreasing trend in the amplitudes of the multidecadal to centennial fluctuations. This decreasing trend may have resulted from an increasing trend in the winter sea surface temperature in the western North Pacific. The multicentennial variability in sardine abundance during the last millennium is consistent with the variabilities in the abnormal snow index in East Asia and the American tree ring-based Pacific Decadal Oscillation index, suggesting a basin-wide or regional climate–marine ecosystem linkage.

Tephra is a product of explosive volcanic eruptions, and is deposited and preserved in both terrestrial and marine environments. Because a tephra is a geologically synchronous deposit, tephras constitute important isochrones in the Quaternary sequences in and around Japan, and can be used to correlate onshore and offshore events. Tephras are considered to be robust time markers connecting the two environments. For this comparison, it is important to establish a marine tephra catalog. We examined a total of 52 marine cores collected from offshore Fukushima to south of Cape Erimo-misaki, Hokkaido, to identify the Holocene tephras in this region. As a result, 11 tephras were identified and correlated with known late Quaternary tephras. Among them, three Holocene tephras are widespread in this region: the To-a (915 CE) and To-Cu (6 ka) tephras from Towada Caldera, North Tohoku, and the Hr-FP (6th Century) tephra from Haruna Volcano, North Kanto. The To-a tephra is distributed in the southern part of the study area. The Hr-FP tephra has a very narrow distribution, and is only found offshore of the central Sanriku coast. The To-Cu tephra, on the other hand, shows a wider distribution, covering most of the study area. These tephras provide important and robust isochrones for correlating offshore and onshore events along the deep Japan Trench.

We investigated the biotic response of benthic foraminifera (Rhizaria) to Holocene paleoceanographic changes in a upper bathyal environment by examining fossil benthic foraminifera younger than ~13...

An 820.4-cm long piston core (YK12-15 PC-01) was obtained from site OT-1B (28°07.15′N, 127°11.97′E, 1050 m water depth), located in the middle of the Okinawa Trough in the southern East China Sea (ECS). For core YK12-15 PC-01, oxygen and carbon isotopes (δ18O and δ13C) of planktic foraminifera Globigerinoides ruber sensu stricto (250–355 μm size fraction) and Globorotalia inflata (>250 μm size fraction) together with total organic carbon (TOC), nitrogen content (TN), and δ13C of organic matter were analyzed for the last 38.5 thousand years (kyr). The δ18O records of G. ruber (δ18Oruber) and G. inflata (δ18Oinflata) were used to estimate the temperature/salinity of the surface and subsurface waters, respectively. In addition, we estimated the vertical temperature gradient between the surface and the subsurface based on the difference in δ18O between G. ruber and G. inflata (Δδ18Oruber-inflata). The δ18Oinflata and Δδ18Oruber-inflata indicate millennial-scale variations in association with the oscillations in Antarctic temperatures during the Marine Isotope Stage (MIS) 3. These results suggest that variations in subsurface temperature and vertical temperature gradient in the upper water column in the ECS were closely related to the Atlantic bipolar seesaw during MIS 3. The subsurface temperature in the ECS was higher at the beginning of Greenland stadials when the Antarctic temperatures showed the lowest values. The warming of the subsurface is likely attributed to the stronger Kuroshio transport. High peaks in the TOC and TN records coincide with the low peaks in δ18Oinflata (high temperature), suggesting higher primary productivity, which is in good agreement with the interpretation of the intensified Kuroshio. However, the subsurface warming was not recognized during Herinch events, indicating different temperature responses in the upper water column from other stadials.

Using high-resolution paleomagnetic data, we examined the potential for obtaining precise ages from sediment core samples recovered from deep-sea basins close to rupture zones of the 2011 and earlier earthquakes off Tohoku, Japan. Obtaining detailed stratigraphic ages from deep-sea sediments below the calcium compensation depth (CCD) is difficult, but we found that the samples contain excellent paleomagnetic secular variation records to constrain age models. Variations in paleomagnetic directions obtained from the sediments reveal systematic changes in the cores. A stacked paleomagnetic profile closely matches the Lake Biwa data sets in southwest Japan for the past 7,000 years, one can establish age models based on secular variations of the geomagnetic field on sediments recovered uniquely below the CCD. Comparison of paleomagnetic directions near a tephra and a paleomagnetic direction of contemporaneous pyroclastic flow deposits acquired by different magnetization processes shows precise depositional ages reflecting the magnetization delay of the marine sediment record.

Changes in water column conditions in the northwestern Pacific during the last 23 ka were reconstructed using geochemical and isotope proxies and redox elemental compositions along with published data (alkenone sea surface temperature (SST) and benthic foraminiferal fauna) at core GH02-1030. Surface water primary productivity in terms of biogenic opal and TOC contents, which mainly represented export production of diatom, was closely related to alkenone (spring-summer) SST and the development of spring-summer mixed layer depth. The different variation patterns of nitrate and silicic acid utilization, estimated by bulk δ 15 N and δ 30 Si diatom values, respectively, are most likely due to the water column denitrification influence on bulk δ 15 N. Dysoxic bottom water conditions occurred during the Bolling-Allerod (BA) and the Pre-Boreal (PB), which was evident by laminated sediments, abundant dysoxic benthic foraminifers, and increased redox elemental compositions. Although surface water productivity increased during the BA and PB, dysoxic bottom water conditions were caused by a combination of enhanced surface water productivity and reduced ventilation of North Pacific Intermediate Water (NPIW) in response to meltwater input from the high latitude areas. Based on records of core GH02-1030 and other cores in the northwestern Pacific, the Okhotsk Sea, and the Bering Sea, which are all proximal to the modern NPIW source region, dissolved oxygen concentrations of bottom water were more depleted during the BA than PB. Such difference was attributed to more sluggish NPIW ventilation due to more meltwater input during the BA than the PB. The opening or closure of the Bering Strait is critical to the direction of meltwater transport to the northwestern Pacific.

The content, optically determined properties, and stable isotope composition of organic carbon in fine-grained sediment cores were analyzed to investigate the origins of deep-sea sediments deposited in the Aceh forearc basin and on the Sunda trench floor off Sumatra from the late Pleistocene to the Holocene. In the Aceh basin, the depositional frequency of turbidite mud decreased as sea level rose during the deglaciation. The terrigenous organic carbon content was high at the end of the last glacial period, whereas during the deglaciation most of the organic carbon was of marine origin. In the Sunda trench, the Holocene turbidites consisted of remobilized slope sediments from two different sources: sediments derived from the old Bengal/Nicobar fan included thermally matured organic fragments, whereas those derived from the trench slope contained little terrigenous organic carbon.

Benthic foraminiferal assemblages within turbidites recovered from two upper slope stations off Sendai Bay, northeast Japan, have been analyzed to estimate the source area of the turbidites resulting from the 2011 Tohoku-oki earthquake and tsunami. The turbidite at one station (St. 5; 893 m water depth) is composed of a surface layer that contains foraminiferal species living on the outer shelf or inner bay, and a lower part with bathyal species. In the turbidite at the second station (St. 6; 1446 m), the assemblage consists of only bathyal species (living in 800–1000 m). The composition of the St. 6 assemblage gradually changes from the base to the top of the turbidite, consistent with the upward-fining graded structure (i.e. thin-walled, tiny species are increased in the upper layer). The lower part of the turbidites at both sites, St. 5 and St. 6, are considered to have originated from turbidity currents triggered by earthquake shaking. In contrast, the most probable generation mechanism for the turbidity current that formed the surface layer at St. 5 is the tsunami-related agitation of sediment in shallow water. The relatively good preservation of benthic foraminiferal tests in the turbidites suggests transport processes with minimal internal friction and/or collision of grains within the turbidity current. These results show that benthic foraminifera are useful in turbidite paleoseismology.

The delivery of organic carbon (OC) to the ocean's deepest trenches in the hadal zone is poorly understood, but may be important for the carbon cycle, contain crucial information on sediment provenance and event-related transport processes, and provide age constraints on stratigraphic sequences in this terminal sink. In this study, we systematically characterize bulk organic matter (OM) and OC signatures (TOC/TN, δ 13 C, 14C), as well as those from application of serial thermal oxidation (ramped pyrolysis/oxidation) of sediment cores recovered along an entire hadal trench encompassing high stratigraphic resolution records spanning nearly 2000 years of deposition. We analyze two cores from the southern and northern Japan Trench, where submarine canyon systems link shelf with trench. We compare results with previously published data from the central Japan Trench, where canyon systems are absent. Our analyses enable refined dating of the stratigraphic record and indicate that event deposits arise from remobilization of relatively surficial sediment coupled with deeper erosion along turbidity current pathways in the southern and central study site and from canyon flushing events in the northern study site. Furthermore, our findings indicate deposition of predominantly marine OC within hemipelagic background sediment as well as associated with event deposits along the entire trench axis. This implies that canyon systems flanking the Japan Trench do not serve as a short-circuit for injection of terrestrial OC to the hadal zone, and that tropical cyclones are not major agents for sediment and carbon transfer into this trench system. These findings further support previous Japan Trench studies interpreting that event deposits originate from the landward trench slope and are earthquake-triggered. The very low terrestrial OC input into the Japan Trench can be explained by the significant distance between trench and hinterland (>180 km), and the physiography of the canyons that do not connect to coast and river systems. We suggest that detailed analyzes of long sedimentary records are essential to understand OC transfer, deposition and burial in hadal trenches.

Deep-sea turbidite deposits are useful tools for submarine paleoseismology. It is fundamental to understand what kind of disturbance occurred owing to the earthquake and which event-induced deposits were formed in a particular area. The 2011 Tohoku-oki earthquake and tsunami were the most destructive geohazards in Japanese history. To understand their influence in the seafloor environment and to characterize the related event deposits, undisturbed sediment cores were collected using a multiple corer along two transects off Sanriku and Sendai, Japan, during July–August 2011. The deposits from the 2011 Tohoku-oki earthquake and tsunami were widely observed along the seafloor, from the outer shelf to the lower trench slope off Sanriku. Their composition indicated multiple sources for the deposited material, reflecting that the seafloor disturbance occurred across a wide area. Two types of sedimentary structures (Types 1 and 2) have been recognized in the event deposits. The Type 1 event deposit is further divided into two subtypes, 1-1 and 1-2. Subtype 1-1 is characterized by a fine-grained turbidite sequence with a sharp basal contact and a parallel and/or cross-laminated basal coarse-grained bed that is a few cm thick, suggesting that sediment particles were transported over relatively long distances by turbidity currents. Relatively short-distance transport by sediment resuspension and settling is hypothesized for subtype 1-2 deposits, which have a thin basal coarse-grained bed without internal structure, and for Type 2, which are composed of homogeneous muddy sediments above a sharp surface but without any basal coarse-grained bed. However, organic geochemical characteristics of the event deposits suggest that long-distance transport was limited even for subtype 1-1 deposits. The wide spatial distribution of the event deposits, which also are evident across a wide range of water depths, reflects the magnitude of the earthquake and size of the tsunami it triggered.

The microstructures of turbiditic and hemipelagic muds and mudstones were investigated using a scanning electron microscope to determine whether there are microstructural features that can differentiate turbiditic from hemipelagic sedimentary processes. Both types of muddy deposits are, in general, characterized by randomly-oriented clay particles. However, turbiditic muds and mudstones also characteristically contain aggregates of ‘edge-to-face’ contacts between clay particles with long-axis lengths of up to 30 μm. Based on observations of the clay fabric of the experimentally-formed muds settled from previously agitated muddy fluids, these types of aggregates, hereafter referred to as ‘aggregates of clay particles’, are interpreted as having been formed by the collision of component flocs in turbulent fluids. Furthermore, some aggregates of clay particles have ‘face-to-face’ contacts between clay particles; this is similar to face-to-face aggregates characteristically developed in fluid-mud deposits that are commonly recognized only in turbiditic mudstones, indicating the possibility of a final stage of deposition under highly-dense conditions, such as temporary fluid muds. In conjunction with earlier proposed lithofacies-based and ichnofacies-based criteria, aggregates of clay particles should be useful for the differentiation of turbiditic and hemipelagic muddy deposits, particularly with limited volumes of non-oriented samples from deep-water successions.

During the Last Glacial Maximum (LGM), ice volume reached a relative maximum and global temperature was lower than today. Understanding continental ice volume change requires accurate reconstruction of relative sea level, but tectonic uplift and isostatic adjustment effects complicate many records. Using marine sediment cores from the Bonaparte Gulf in northwestern Australia, a so-called “far field” tectonically stable site, Yokoyama et al. (2000) reported that the LGM terminated abruptly at 19 ka with a rapid sea-level rise (19 ka event). Their sea-level reconstruction determined the age of the LGM termination, but the timing of its inception remained less well constrained, partly because the number of radiocarbon analyses was insufficient to clarify LGM duration. Here we return to the Bonaparte Gulf and present a new relative sea level that better constrains LGM duration with high-resolution radiocarbon dating of a recently recovered marine sediment core (water depth: 120 m, length: 583 cm). Radiocarbon dating of 23 molluscs and 26 bulk organic-matter samples, together with total organic carbon, total nitrogen, and stable carbon isotopes, provide a record of paleoenvironmental variability in response to sea level change. Our results indicate that the LGM sea-level minimum occurred at 20.8 cal ka BP and the LGM duration was shorter than previously reported.

Strike-slip basins are one of the most important accumulation spaces for sediment of terrigenous, biogenic, and volcanic origins, and generally include large amount of event deposits. Although these event deposits are important basin filling process, research on this topic, particularly the effects of event deposits, is insufficient. In this study, we discuss sedimentation features based on grain composition and other properties for ca. 3000 year periods in Beppu Bay, which is strike-slip basin located at the western end of an arc-bisecting dextral fault known as Median Tectonic Line (MTL) associated with the northwestward subduction of the Philippine Sea Plate. This sediment is composed of hemipelagic clay and coarser event layers of turbidites referred to as types A, B, and C; ash layers referred to as type D; and other referred to as type E. The turbidite event layers, which accounted for 92% of the total major event layer, with >1 cm thickness, consist of particles related to volcanism, including hydrothermal activity. The events control the regional filling rate and transportation of coarse and heavy volcaniclastic materials. In particular, type A, which accounted for 73% of the total major event layer thickness, is likely induced by earthquakes related to the MTL, according to its age. As a result, the basin filling processes are controlled mainly by tectonics related to the subduction of the Philippine Sea Plate.

The 2011 Tohoku-oki earthquake and tsunami was the most destructive geohazard in Japanese history. However, little is known of the past recurrence of large earthquakes along the Japan Trench. Deep-sea turbidites are potential candidates for understanding the history of such earthquakes. Core samples were collected from three thick turbidite units on the Japan Trench floor near the epicenter of the 2011 event. The uppermost unit (Unit TT1) consists of amalgamated diatomaceous mud (30–60 cm thick) that deposited from turbidity currents triggered by shallow subsurface instability on the lower trench slope associated with strong ground motion during the 2011 Tohoku-oki earthquake. Older thick turbidite units (Units TT2 and TT3) also consist of several amalgamated subunits that contain thick sand layers in their lower parts. Sedimentological characteristics and tectonic and bathymetric settings of the Japan Trench floor indicate that these turbidites also originated from two older large earthquakes of potentially similar to the 2011 Tohoku-oki earthquake. A thin tephra layer between Units TT2 and TT3 constrains the age of these earthquakes. Geochemical analysis of volcanic glass shards within the tephra layer indicates that it is correlative to the Towada-a tephra (AD 915) from the Towada volcano in northeastern Japan. The stratigraphy of the Japan Trench turbidites resembles that of onshore tsunami deposits on the Sendai and Ishinomaki plains, indicating that the cored uppermost succession of the Japan Trench comprises a 1500-yr-old record that includes the sedimentary fingerprint of the historical Jogan earthquake of AD 869.

The M9.3 2004 Sumatra and M9.0 2011 Tohoku earthquakes and tsunamis had devastating consequences for the adjacent heavily populated coasts. They had large shallow coseismic slip that reached the trench, which is thought to be a key factor in generating the huge tsunamis. The sedimentary record contains unique spatial and temporal information on the effects of megathrust earthquakes. It can provide a better understanding of earthquake-related tectono-sedimentary processes and of the earthquakes themselves. We present new results obtained from high-resolution multibeam bathymetry, subbottom profiles acquired with Parasound and ten, 10 m long piston cores recently collected from the Japan Trench margin during R/V Sonne SO251 expedition. We identify the 2011 Tohoku event-deposit extending for ~250 km along the Japan Trench basins from the enrichment and concentrations of short-lived radioisotopes (excess) xs210Pb, 137Cs and 134Cs in all ten cores. The high enrichment of xs210Pb points to surficial sediment remobilization as the main source. In three cores from the northern, central and southern segments of the Japan Trench, for the first time, we identify sediment remobilization by its 87/86Sr, eNd and Pb isotope signatures and the composition and texture of the sediment. With this new approach, we fingerprint older sedimentation events, potentially linked to earthquakes and infer processes responsible for their deposition. Thick, acoustically transparent and homogeneous units we name “homogenites”, characterize these event deposits. Physical, geochemical and acoustic properties link homogenites to the 2011 Tohoku and previous earthquakes. We propose a multi-stage entrainment and depositional process related with both the high frequency and the exceptionally large low frequency and long duration motions expected from ruptures of the shallow megathrust below the compliant outer upper-plate. These large long-period motions could propagate along the outer upper plate beyond the rupture zone and could generate widespread entrainment of surficial sediment.

Patton, Jason ... et. al., In order to investigate the possibility of a long-term paleoseismic history from offshore sedimentary records in Sumatra, we collected 144 deep-sea sediment cores in the trench and in lower slope piggyback basins of the Sumatra accretionary prism. We used multibeam bathymetry and seismic reflection data to develop an understanding of catchment basins, turbidity current pathways, and depositional styles, as well as to precisely locate our gravity cores, piston cores, Kasten cores, and multicores. We use detailed physical property data, including computed tomographic X-ray, gamma density, magnetic susceptibility, grain-size analysis, faunal analysis, and smear slides, to evaluate the turbidite stratigraphy and sedimentology at each site. We use radiocarbon age control for piggyback basin sites above the carbonate compensation depth, and use 210Pb and 137Cs to evaluate the timing of the most recent sedimentary deposits. Using well-log correlation methods and radiometric age control, we test for potential correlations between isolated sites in piggyback basins and the trench. [...]

Hokkaido Island in northern Japan is located in a cool temperate to boreal climate zone influenced by summer monsoons and typhoons during summer–autumn, while its eastern area is influenced by the subarctic Oyashio Current. Palynomorph (pollen and spores) distribution was investigated in surface sediments from the shelf and slope (40–2300 m water depth) in the offshore area of Tokachi Plain, central-eastern Hokkaido Island. The objective of this study was to examine the source, transportation, and deposition of palynomorphs in marine environments. The majority of palynomorphs were transported from the Tokachi coastal plain by both strong local winds from the mountain areas during the spring–summer and the floods caused by heavy monsoon and typhoon rainfall. Approximately 10% of all palynomorphs were transported from both southwest and west areas by the wind. Palynomorph transportation by the Oyashio Current could not be discriminated because the same vegetation is present in Hokkaido and the islands in upper streams of the Oyashio Current. The total grain abundance was dependent on water depth. High concentrations were observed at depths of 700–1500 m, which is a region associated with high levels of fine silt and clay, suggesting that the transportation and deposition of palynomorphs are controlled by their grain sizes. This study supported a strong correlation between the distribution of sediments and the concentration of palynomorphs.

We compared meiofaunal assemblages obtained from the landward slope of the Japan Trench off the Sanriku region of Honshu Island before the 2011 off the Pacific coast of Tohoku Earthquake, 4.5 months and 1.5 years after the earthquake. Sediment samples were collected after the earthquake along two transects. Meiofaunal density after the earthquake did not differ from that before the earthquake. However, meiofaunal composition after the earthquake was different from that before the earthquake, and the composition dispersion after the earthquake was lower than that before the earthquake. These results suggested that the turbidity current may have affected meiofaunal composition and reduced its variability. These changes were probably caused by the seismic motion and displacement of the substratum after the earthquake, rather than sediment redeposition induced by the earthquake. To assess the effect of the earthquake more thoroughly, we compared meiofaunal assemblage in the Sanriku region before and after the earthquake with that in the Kuril Trench (comparable productivity to the Sanriku region) and Ryukyu Trench (lower productivity). Differences in meiofaunal composition before and after the earthquake within the Sanriku region were lower than those between the Sanriku and Ryukyu regions. These results suggested that deep-sea meiofaunal assemblages are influenced by large-scale disturbances, but changes are within the range of variation generated by surface productivity levels.

Little is known about offshore tsunami deposits, although they have the potential to improve palaeotsunami reconstructions, especially in areas with limited preservation of onshore tsunami deposits. We analysed 44 sediment cores collected in 2012 at water depths of 14–30 m in Sendai Bay, north-eastern Japan, to identify the deposits of the 2011 Tohoku-oki tsunami with the aid of 134Cs released from the Fukushima-Daiichi Nuclear Plant. The preserved tsunami deposits in the inner shelf are interbedded with very fine sand and mud, showing similar features to storm deposits. In the shoreface, a distinct layer of coarse sand overlies fine sand, appearing to have originated as beach sand transported offshore by the tsunami backwash. The coarse sand fades offshore around the boundary between the shoreface and inner shelf. Many of the core sites show evidence that surface sediment was reworked, probably by post-tsunami storm waves, suggesting that further reworking would make the ultimate preservation of the tsunami deposits unlikely.