Michael Strasser

Recent climatic change affects sediment transfer in Alpine catchments beyond historical experience, increasing the need for robust quantification of sediment cascades to better anticipate the frequency and magnitude of future climate-driven natural hazards. Despite widespread projections of increased natural hazard activity, many studies lack quantitative volume estimates over more than a decade. Lake Plansee, located in the Northern Calcareous Alps (Tyrol, Austria), represents a quasi-closed geomorphological system in which previous studies have identified an increase in debris flow activity since the 1920s. Here, we revisit historical aerial imagery using recent photogrammetric techniques to quantify sediment transfer on 54 debris fans to the lake between 1952 and 2020, and to establish a first-order quantitative reconciliation between terrestrial sediment-transfer estimates and sediment accumulation inferred from the lacustrine sink archive within a common source-to-sink framework. Fans surrounding Plansee are grouped based on their connection to the lake inferred from the co-occurrence of active channel linkage to the lake and net sediment export (i.e., negative net change). Twenty-one debris flow channels are connected to the lake and actively deliver sediment. Sixteen channels indicate progression towards the lake and are expected to connect in the future. Rainstorm intensity analysis shows that, since 1980, the number of days with precipitation intensities capable of initiating debris flows (>10 mm h-1) has increased by 36%, accompanied by a 12% increase in total erosion volumes and a 47% decrease in deposition volume at connected fans, suggesting an overall decrease in internal storage at the fans and enhanced sediment transport to the lake. The decadal response of sediment dynamics on the fans suggests that the increase in precipitation events results in an overall increase in sediment connectivity between catchments, fans, and the lake, driven by episodic events and diverse individual fan responses. These findings are reflected in land-cover changes: the increase in erosion is accompanied by an increase in non-vegetated areas on 70% of the active fans after the 1980s. Event-layer deposition from the lacustrine record reveals a first-order debris flow volume estimate of 3.69 ± 0.42 × 105 m3 deposited between 1952 and 2018. In comparison, a total of 3.42 ± 0.43 × 105 m3 of sediment was transferred from the fans to the lake in the last seven decades. Here, we provide the first direct agreement between terrestrial sediment-volume estimates derived from historical aerial imagery and lacustrine sediment-core reconstructions over multi-decadal timescales. While reconciliation of debris flow activity over time from sediment cores and terrestrial sediment dynamics is complex, both records agree on the overall increase of debris flow activity over the last decades coinciding with a shift in climatic conditions.

X-ray computed tomography (XCT) scans are one of the first measurements performed after the recovery of cores during expeditions using the drilling vessel (D/V) Chikyu. Despite their completeness and availability, XCT scan data have not been effectively utilized for routine visual core description and shipboard sampling. We designed a Python-based visualization workflow that is compatible with the systems on D/V Chikyu and implemented it in the onboard core workflow.

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.

Hadal trenches uniquely 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, influencing both paleoseismic reconstruction and deep-sea carbon cycling on multi-millennial timescales. Classical event stratigraphy treats these event beds as the product of a brief, isochronous process. This study aims to build on this concept to enable the documentation of relative depositional timing within a single event bed. Within four well documented historical earthquake-triggered beds recovered during IODP Expedition 386 and linked to megathrust earthquakes (CE 2011 Tohoku-Oki, CE 1454 Kyotoku, CE 869 Jogan, and ~2.3 ka), we develop a systematic, facies-based framework to identify event-internal characteristics. This integrated approach resolves sedimentary structures—physical, chemical and biogenic characteristics—by combining visual core description with high-resolution 2D and 3D imaging (X-ray computed tomography and X-ray radiography), grain-size measurements, and geochemical datasets (X-ray fluorescence). 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. This shift is marked by a 32 µm grain-size threshold that separates coarse-grained (F1–F3; >32 µm) from fine-grained (F4–F6;

Hadal trenches at subduction margins represent hotspots where episodic tectonic events can reorganize sedimentation and biogeochemical processing. In particular, earthquake-triggered depositional pulses may redistribute and rapidly bury matter including nitrogen. Despite this potential importance, nitrogen burial in the deepest trench environments is still poorly constrained, and how earthquakes modulate nitrogen inputs, internal cycling, and delivery toward subduction remains unclear. Here, we measured elemental and isotopic geochemical parameters from high-resolution long sediment cores drilled at the Japan Trench during IODP Expedition 386. By integrating high-resolution event stratigraphy with robust chronological constraints, we quantified nitrogen inputs, transformations, and burial under two contrasting depositional regimes: background sedimentation and earthquake-triggered event deposition. We find that earthquake event layers deliver nitrogen-bearing material and are associated with substantially enhanced nitrogen burial relative to background intervals. Estimated nitrogen burial fluxes rise from ~24 to ~300 Tg N yr-1 and the event-driven flux exceeds published deep-sea mean burial estimates by ~1.5×102. In addition, earthquake-triggered event deposition markedly increases the relative proportion of bioavailable nitrogen in trench sediments, from 82.6% in background intervals to 91.4%. Collectively, these changes in the bioavailable nitrogen supply may diversify nitrogen transformation pathways, manifesting as a more complex nitrogen-cycling signal in hadal trench sediments. We propose a "seismic nitrogen pump" in which earthquakes transiently accelerate nitrogen cycling through organic matter activation and mass transport, enhancing sedimentary retention and potentially reshaping the subducting nitrogen reservoir. Our findings challenge the view of trenches as static nitrogen repositories, identifying tectonic forcing as a key driver of subduction-zone biogeochemistry and underscoring the need to incorporate episodic perturbations into global nitrogen budgets and deep carbon–nitrogen coupling frameworks.

Hadal ocean trenches are among the least explored environments on Earth, yet they host the largest and most hazardous earthquakes. Formed at subduction zones where megathrust earthquakes and tsunamis originate, hadal trenches act as terminal sinks for sediment and carbon. Because instrumental and historical records are too short to capture the full range and recurrence of giant (Mw ≥9) earthquakes, hadal trench basins provide unique geological archives to reconstruct long-term earthquake behavior, including rare slip-to-the-trench events that generate large tsunamis. These basins also host extreme subseafloor ecosystems and play an unresolved role in Earth’s carbon cycle, making them key targets for integrated scientific ocean drilling and Earth system research.IODP³ Expedition 503 (November–December 2025) drilled a trench-fill basin in the central Japan Trench at Site C0028 using the D/V Chikyu. Coring in five holes at water depths of up to 7,608.5 m reached a maximum depth of 178 m below seafloor (mbsf), recovering a complete trench-fill succession and providing the first continuous full record from the depositional center of a hadal trench basin. Initial results demonstrate that drilling successfully penetrated the full trench-fill sequence and its base. Lithostratigraphy documents a systematic transition from basal volcaniclastic-rich deposits to mixed detrital sediments and overlying biosiliceous oozes, reflecting basin initiation, growth, and progressive migration toward the trench axis. Structural data show increasing bedding dips and a normal-fault regime in the lowermost section, consistent with horst-and-graben formation related to bend faulting of the incoming Pacific Plate. An angular unconformity at depth, together with paleomagnetic observations and initial stratigraphic correlations to IODP and DSDP sites sampling the sedimentary cover of the Pacific oceanic crust, confirms recovery below the trench-fill base.Event stratigraphy is exceptionally well preserved. Numerous thick turbidites, replicated between holes and tied to seismic reflectors, form a robust framework for paleoseismic interpretation. Distinct variability patterns in radiolarian fossil taxa abundances, together with frequent tephra layers, provide strong potential for high-resolution chronological control. Paleomagnetic data indicate a polarity reversal in the deepest cores, tentatively correlated with the Matuyama–Brunhes boundary (~773 ka), implying that the recovered sequence spans several hundred thousand years.Geochemical analyses largely confirm previous results from giant piston coring during IODP Expedition 386 in 2021 down to a depth of approximately 40 mbsf. A decrease in alkalinity, previously hypothesized from shallow subsurface records, is confirmed, with significant changes in pore-water profiles observed below ~80 mbsf down to the base of the trench-fill sequence. Integrated sedimentological, mineralogical, physical property, headspace gas, and pore-water data document depth-dependent reaction zones, compaction trends, and early diagenesis linked to dynamic element cycling in the hadal subseafloor. Importantly, Expedition 503 successfully recovered high-quality core material suitable for microbiological investigations, enabling assessment of subseafloor microbial activity and its coupling to geochemical processes.Together, these initial results demonstrate that hadal trench basins preserve long, continuous archives of tsunamigenic megathrust behavior and associated biogeochemical processes, opening new perspectives on earthquake recurrence, geohazards, and carbon cycling along subduction zone systems.

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.