Ken Ikehara

The subduction organic carbon (OC) flux is closely linked with seafloor sedimentation processes. In hadal trenches, sediment transport and resuspension caused by turbidity events provide rapid feedback of subducting carbon fate to seafloor sedimentation dynamics. Here we demonstrate that sediment sorting creates distinct OC burial patterns in the Japan Trench: Prolonged seafloor residence enables preferential removal of labile OC components and result in a refractory subsurface OC pool. Conversely, rapid sediment burial shortens seafloor residence time and promotes subsurface anaerobic OC degradation. We further suggest that these mechanisms result in at least 37.6% loss of the OC subduction flux in accretionary margins due to early diagenetic and thermal alteration processes, while erosive margins only loss 11% during subduction and preferentially transfer thermally stable OC into subduction zones. Our work bridges surficial depositional regimes and deep carbon transfer, highlighting the need to consider depositional settings when refining subduction zone carbon budgets.

Sediment cores recovered from the deep sea often cannot be sampled for microbiological analysis immediately due to the need for core splitting and processing for subsequent onboard measurements and core storage. Consequently, sections are often stored at 4°C under anaerobic conditions for extended periods. However, the impact of the temporal storage on microbiological analysis remains uncertain. In this study, using paired sediment samples from the Japan Trench collected during International Ocean Discovery Program (IODP) Expedition 386, we performed comparative analysis of microbial community composition between samples immediately frozen after core recovery onboard and samples stored anaerobically for 9–11 months onshore. Although the overall community structure was broadly preserved, we observed substantial increases in some opportunistic taxa (e.g., Shewanella, Psychrobacter, and Firmicutes) and a marked decrease in archaeal abundance and diversity. Nevertheless, more than 95% of amplicon sequence variants (ASVs) were retained, and key subseafloor bacterial lineages such as Atribacterota and Chloroflexi showed no significant change in abundance. These results indicate that although long‐term cold storage can introduce identifiable preservation‐related biases, DNA‐based analyses of archived cores can still provide meaningful insights into subseafloor microbial communities when interpreted with appropriate caution.

Hadal trenches preserve exceptional sedimentary archives of past geological events, yet their depositional processes remain poorly constrained. The Japan Trench captures complex earthquake‐triggered event beds that record repeated sediment‐gravity flows that deliver terrigenous and biogenic material into the trench, informing palaeoseismic reconstructions. Classical event stratigraphy treats such event beds as products of brief, isochronous processes. This study aims to build on that concept to document hydrodynamic conditions and the relative depositional timing and duration within single event beds. Using four well‐documented historical earthquake‐triggered beds recovered during IODP Expedition 386, high‐resolution analyses of sedimentary structures were conducted—integrating physical, chemical and biogenic characteristics—to develop a systematic, facies‐based framework for identifying event‐internal characteristics. Six event‐internal facies (F1–F6), bounded by hemipelagic facies (F0), form a fining‐upward sequence that records the shift from high‐energy, non‐cohesive, bedload‐dominated flows to low‐energy, cohesive, suspension‐dominated deposition. A 32‐μm grain‐size threshold separates coarse‐grained (F1–F3; >32 μm) from fine‐grained (F4–F6; <32 μm) facies. Event‐internal facies organise into pulses and pulse groups that stack hierarchically into three patterns: single‐pulsed, multi‐pulsed and amalgamated. Amalgamated patterns comprise two or more single‐ or multi‐pulsed successions separated by breaks in the fining‐upward trend. Breaks marked by opportunistic trace fossils or by F6 indicate pauses during deposition (quasi‐synchronous flows), whereas their absence suggests synchronous flows. Most event beds are amalgamated and comprise flows emplaced at different times and sourced from different directions, as reflected by variable composition, facies and palaeo‐flow indicators. Basin physiography strongly influences facies development and thickness: basin highs record erosion–deposition stages, whereas depocentres pond turbid water masses in prolonged suspension, producing thick fine‐grained tops. Transitional facies (F4) forms where new flows interact with a still‐settling suspension cloud, providing a basis to interpret complex mainshock–aftershock sequences. By providing new insights into hydrodynamic conditions and relative depositional timing and duration, this facies‐based framework advances event stratigraphy and improves global understanding of deep‐marine sedimentary dynamics.

Hadal oceanic trenches are the deepest places on our planet. They form due to downward bending of subducting ocean crust along subduction zones, act as terminal sinks for sediment and particulate and dissolved organic carbon, and form high-resolution archives to unravel the history of subduction zone processes including subduction megathrust earthquakes and tsunamis. To fill the gap in long-term paleoseismic records of giant (M_w 9-class) subduction zone earthquakes, such as the Tōhoku-oki earthquake in 2011, International Ocean Discovery Program (IODP) Expedition 386 successfully collected 29 giant piston cores at 15 sites along the hadal Japan Trench at more than 7500 m water depth, recovering up to 37.82 m long, continuous successions spanning a few thousand to ∼ 20 000 years from 11 individual trench-fill basins. The record comprises numerous earthquake-related event deposits and has revealed new findings of earthquake-triggered carbon export to the hadal zone and dissolved carbon cycles stimulating intensive microbial activity in trench sediments. Yet, the record only covers the top 40 m of the up to ∼ 160 m thick trench-fill sequence that is expected to comprise a much longer earthquake record and offer further clues about hadal-zone element cycles. Furthermore, in central Japan Trench (cJT) basins, trench-fill sediment sequences are characterised by several slightly tilted prominent seismic reflections with high amplitudes in multi-channel seismic profiles. Tilts become larger with increasing sub-bottom depths, suggesting periodic tilting events. The respective event time horizons link to buried trench-fill deformation structures and correlative ponded depositional units hypothesised to have resulted from large coseismic slip propagation to the trench and large-scale sediment remobilisation associated with strong seafloor shaking induced by past megathrust earthquakes. The youngest slip-to-the-trench event horizon is correlative with the base of a thick turbidite bed of the 869 CE Jōgan earthquake, which was a major earthquake event before the 2011 CE earthquake. However, due to limited coring depth so far, it is impossible to date the older hypothesised slip-to-the-trench events. International Ocean Drilling Programme (IODP³) Expedition 503 plans to drill a trench basin in the cJT to recover the whole trench-fill sequence for dating and establishing event stratigraphy for paleoseismologic interpretations and further investigations of earthquake-related element cycles in a hadal trench environment. Combining the stratigraphy and chronology of thick event deposits, interstitial-water geochemistry proxy data for past fluid flow pulses, and core-to-seismic correlation with paleo-slip-to-the-trench events, we would like to clarify how often the slip-to-the-trench events have occurred. Unravelling the complete trench-fill sedimentary record and pore water profile will significantly advance our understanding of the nature and recurrence of hadal trench tsunamigenic slip and the underlying megathrust earthquakes and related geohazards, as well as the effects on enhancing carbon accumulation in the hadal trench that may stimulate carbon transformation and eventual export into the subduction zone.

Establishing along‐strike correlations of event deposits in deep‐sea environments allows for the reconstruction of spatial and temporal patterns of geological processes, such as seismically triggered turbidites. However, in settings like the ultra‐deep Japan Trench, highly variable seafloor topography and limited chronological tie points pose significant challenges for tracing individual deposits between semi‐isolated trench basins. Using cores from International Ocean Discovery Program (IODP) Expedition 386 at the central Japan Trench, this study examines how non‐destructive X‐ray fluorescence core scanning (XRF‐CS) combined with multivariate statistics can be used to distinguish and correlate individual event deposits across contrasting depositional settings, based on their geochemical fingerprints. The results of the five‐cluster model demonstrate that the XRF‐CS properties of event deposits are clearly distinguishable from background sediments, as well as from one another. The chemostratigraphic cluster sequence can be correlated between cores from basin depocentres—where event deposits are thicker but the record is shorter, and cores from topographic highs—where the record extends further back in time but event deposits are thin and difficult to distinguish using previously applied event correlation methods. The high‐resolution chemostratigraphy enables a more detailed, geochemically‐enhanced stratigraphic interpretation, identifying several previously unrecognised event deposits with unique internal structures that suggest varying depositional processes. Notably, excluding the 2011 event deposit, the three most recent event deposits are allocated to different clusters, implying compositional heterogeneity probably linked to sediment provenance. The results of this study therefore demonstrate a rapid and reproducible technique for using chemostratigraphy to establish high‐resolution event correlations in submarine sediment cores, leading to several working hypotheses for Japan Trench palaeoseismology to be tested with future work.

Megathrust earthquakes in subduction zones, such as the 2011 Mw 9.1 Tohoku-oki earthquake, are rare but pose significant threats to society. Their long recurrence intervals and limited historical records make reconstructing recurrence models challenging. The International Ocean Discovery Program (IODP) Expedition 386 addressed this by recovering over 800 meters of sediment cores from 11 trench-fill basins along the Japan Trench, providing a unique opportunity to extend paleo-earthquake records. Despite this, achieving reliable spatiotemporal correlations of event deposits remains a complex task. Here we show that high-resolution chemostratigraphic correlations using X-ray Fluorescence Core Scanning (XRF-CS), Principal Component Analysis (PCA), and Cluster Analysis (CA) effectively link event deposits across cores M0083D and M0089D in the northern basin and M0090D in the southern basin of the central Japan Trench. We identify eight event deposits in the northern basin, characterized by higher Ca and Sr with upward-decreasing trends, or elevated Si, Rb, and K without such trends, indicating distinct compositional differences and depositional processes of the turbidity currents. Across basins, M0090D deposits exhibit consistent clustering with M0089D but differ in internal structures and elemental trends, suggesting spatially similar sediment sources but varying erosion and transport mechanisms. Temporal chemical variations further suggest surficial sediment remobilization, rather than landslides, as the dominant trigger for turbidity currents, as it transports slope material that evolves compositionally over time. This insight reinforces the reliability of chemostratigraphy for event-stratigraphic correlation. Moreover, the spatial distribution of event deposits further highlights potential rupture areas and turbidity current pathways. Southward thinning of high Si, Rb, and K deposits suggests a northern source, while thicker Ca and Sr deposits in the southern core may imply a southern rupture zone. These findings establish a robust chemostratigraphic framework, enhancing our understanding of paleo-earthquake dynamics along the Japan Trench. The approach provides a valuable tool for reconstructing earthquake histories in other subduction zones, contributing to global paleoseismology research.

The hadal zone, >6 km deep, remains one of the least understood ecosystems on Earth. We address bioturbational structures in sediment cores from depths exceeding 7.5 km, collected during the IODP Expedition 386 in the Japan Trench. Micro-CT imaging on 20 core sections allowed to identify biogenic sedimentary structures (incipient trace fossils) and their colonization successions within gravity flow deposits. Their frequency, and consequent changes in substrate consistency, oxygenation and organic matter delivery and remineralization controlled the endobenthic colonization. The gravity-flow beds show recurring bioturbation successions: The initial colonization is characterized by deposit-feeding structures such as Phycosiphon, Nereites and Artichnus generating typically 20 cm thick intensively bioturbated fabrics. The final colonization stage comprises slender spiral, lobate and deeply penetrating straight and ramifying burrow systems such as Gyrolithes, Pilichnus and Trichichnus, interpreted to include burrows of microbe farming and chemosymbiotic invertebrates. The main factor precluding colonization is soupy substrate. Organic matter degradation and post-event upward expansion of the anoxic zone drive the change from deposit feeding to microbe-dependent feeding strategies.

The deep oceans are environments of complex carbon dynamics that have the potential to significantly impact the global carbon cycle. However, the role of hadal zones, particularly hadal trenches (water depth > 6 km), in the oceanic dissolved organic carbon (DOC) cycle is not thoroughly investigated. Here we report distinct DOC signatures in the Japan Trench bottom water. We find that up to 34% ± 7% of the DOC in the trench bottom is removed during the northeastward transport of dissolved carbon along the trench axis. This DOC removal increases the overall DOC recalcitrance of the deep Pacific DOC pool, and is potentially enhanced by the earthquake-triggered physical and biogeochemical processes in the hadal trenches. Radiocarbon analysis on representative oceanic transects further reveals that the Pacific deep-water DOC undergoes distinct removal compared to those in the Atlantic and Indian Oceans along the thermohaline transport. Our findings highlight hadal trenches as previously unrecognized DOC sinks in the deep ocean system, with varying dynamics that warrant further investigation.

Deep-sea and subseafloor sedimentary environments host heterotrophic microbial communities that contribute to Earth’s carbon cycling. However, the potential metabolic functions of individual microorganisms and their biogeographical distributions in hadal ocean sediments remain largely unexplored. In this study, we conducted single-cell genome sequencing on sediment samples collected from six sites (7,445–8,023 m water depth) along an approximately 500 km transect of the Japan Trench during the International Ocean Discovery Program Expedition 386. A total of 1,886 single-cell amplified genomes (SAGs) were obtained, offering comprehensive genetic insights into sedimentary microbial communities in surface sediments (in situ methane concentrations and contained genes involved in glycine betaine metabolism. These findings suggest that the phylogenomically diverse and novel Atribacterota JS1 is widely distributed in Japan Trench sediment, playing crucial roles in carbon cycling within the hadal sedimentary biosphere. IMPORTANCE: The Japan Trench represents tectonically active hadal environments associated with Pacific plate subduction beneath the northeastern Japan arc. This study, for the first time, documented a large-scale single-cell and metagenomic survey along an approximately 500 km transect of the Japan Trench, obtaining high-quality genomic information on hadal sedimentary microbial communities. Single-cell genomics revealed the predominance of diverse JS1 lineages not recoverable through conventional metagenomic binning. Their metabolic potential includes genes related to the degradation of organic matter, which contributes to methanogenesis in the deeper layers. Our findings enhance understanding of sedimentary microbial communities at water depths exceeding 7,000 m and provide new insights into the ecological role of biogeochemical carbon cycling in the hadal sedimentary biosphere.