Recovering the Long-Term Record of Subduction-Zone Tsunamigenic Slip and Element Cycling in a Hadal Trench Basin at the Japan Trench: Initial Results of IODP³ Expedition 503

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.