Kiichiro Kawamura

Framboidal pyrite is widespread in anoxic marine sediments, but the processes that control the redistribution and preservation of pyrite aggregates after their formation remain poorly constrained. Here, I examine the microfabric and spatial organization of framboidal pyrite in shallowly buried deep-sea sediments from the Sagami Trough and the Sea of Japan using complementary freeze-dried and resin-embedded specimens. Framboidal pyrite occurs preferentially within the cavities of benthic and planktonic microfossils, where aggregates are concentrated along the lower sides of the cavities and form distinct geopetal fabrics. Loosely aggregated framboids composed of sub-spherical microcrystals occur together with densely packed framboids, indicating differences in aggregate packing and crystal development. Filamentous material less than 1 μm in thickness occurs around and between framboids within microfossil cavities, whereas isolated framboids in the surrounding sediment matrix generally lack comparable material. The preferential concentration of framboids along the lower sides of cavities is consistent with gravitational redistribution of mobile pyrite aggregates after their formation, followed by physical stabilization within confined pore spaces. The spatial association between filamentous material and pyrite further suggests that such material may have contributed to the retention of pyrite aggregates, although its composition and origin cannot be determined from the present observations. These observations indicate that the spatial organization of framboidal pyrite in marine sediments can record not only mineral formation but also subsequent physical redistribution and stabilization within sedimentary microenvironments.

Fine-grained shear zones occur in both tectonic faults and gravitational landslides, making their origin difficult to determine from individual microstructural features alone. A potentially useful distinction is provided by deformation history: tectonic faults commonly experience repeated earthquake cycles in which the sense and magnitude of deformation may change, whereas landslide slip zones are primarily governed by gravitational downslope loading. How reversal of shear is recorded in clay-rich microfabric, however, remains poorly understood. Here, I investigate the development of fabric memory during repeated reversal shear using direct-shear experiments on clay-rich materials from the Jin’nosuke-dani landslide, central Japan. Undisturbed and slurry samples were tested under normal stresses of 0.4, 0.8, and 1.6 MPa. Following 1000 min of pre-consolidation, specimens underwent ten predetermined shear cycles, with the shear direction reversed after each cycle. Shear stress and vertical displacement were recorded automatically, and the experimentally deformed materials were examined by scanning electron microscopy (SEM), synchrotron radiation micro-X-ray computed tomography (SP-µCT), and anisotropy of magnetic susceptibility (AMS). SEM and SP-µCT reveal preferred clay-flake orientations, fragmented and locally rotated shale clasts, asymmetric structures, and three-dimensional reorganization of the clay-rich material. AMS provides a complementary bulk-scale constraint, with Kmax approximately horizontal and oriented perpendicular to the imposed shear direction. The transverse Kmax orientation, together with the observed particle and clast rotation, is consistent with rotation and rolling of gouge-like material during reversal shear. We propose that reversal does not simply erase the fabric generated during the preceding shear cycle. Instead, elements of the earlier fabric are retained while being rotated, disrupted, and overprinted by deformation in the opposite direction. Repeated reversal therefore produces a composite fabric that records cumulative deformation history, which I refer to as experimental fabric memory. This rotational fabric memory may be associated with deformation histories involving repeated reversal and could provide a process-based line of evidence for distinguishing tectonic shear zones from gravitational landslide slip zones when combined with independent geological, geomorphological, and kinematic evidence. The present study does not establish that the observed fabric is uniquely tectonic, because the experiments were performed on landslide-derived material and the fabric was measured only after the ten-cycle test. Rather, it demonstrates a testable mechanism by which reversal deformation can leave a distinctive microfabric signature.

Kichiro Kawamura, Shinji Sassa, Yasukuni Okubo, Char-Shine Liu, Gregory F. Moore, Francesco Latino Chiocci, Abdelaziz Abdeldayem. Episodes 2026;49:113-4. https://doi.org/10.18814/epiiugs/2026/02602s00

This study investigates sedimentation processes and associated submarine sliding on the floor of the Ryukyu Trench. Three dive surveys were conducted using the manned submersible Shinkai 6500 (hereafter 6K) in the trench. The AD 1771 Meiwa tsunami was generated by a giant submarine slide on the landward trench slope in this region. Dive survey site 6K#1469 was located on a mouth of the submarine canyon in the forearc basin. Dive survey site 6K#1468 was located on the uppermost part of the submarine slide, where steps and fissures were observed. Dive survey site 6K#1467 was located at the toe of the submarine slide, which is covered by a muddy sediment layer. Core sample 6K#1467 measured 36.3 cm long and comprised muddy sediments interlayered with silt and/or sand laminations. The paleocurrent direction of the laminated sediments is mostly from north to south, indicating that they were not transported along with the trench but were from the landward slope, probably via repeated small submarine slides. The sedimentation rate for sample 6K#1467 was estimated at 2.42 mm/year using a depth profile of ²¹⁰Pb_ex concentration within 10 cm deep and a date for the sediment of ~ 50 year ago. Based on the sedimentation rate and the number of laminae, the typical recurrence interval of the silt/sand layers is calculated to be ~ 6 year. We interpret these results as indicating that repeated small submarine slides have occurred persistently (recurrence interval of ~ 6 year) at the toe of the investigated giant submarine slide in the Ryukyu Trench over the last 50 year.

The deep-sea is considered to be an ultimate sink for marine plastic debris. The abyssal (3500-6500 m) covers the bulk of the deep ocean floor yet little is known about the extent of plastic debris on the abyssal seafloor. We undertook a quantitative assessment of debris presents on the abyssal seafloor (5700-5800 m depth) beneath the Kuroshio Extension current system in the Northwest Pacific, which is one of the major transit points for massive amounts of debris sourced from Asia that are entering the North Pacific Ocean. The dominant type of debris was single-use plastics-mainly bags and food packaging. The density of plastic debris (average 4561 items km ⁻² ) in the abyssal zone was the highest recorded for an abyssal plain, suggesting that the deep-sea basin of the Northwest Pacific is a significant reservoir of plastic debris.