Abstract The giant diatom Ethmodiscus rex exhibited widespread and episodic blooms in tropical and subtropical oceans during the late Quaternary, yet the mechanisms underlying their formation remain uncertain. A previous hypothesis emphasized the role of atmospheric dust input from inland Asia in triggering diatom blooms in the western Pacific. However, this interpretation has largely overlooked the physiological capacities of E. rex , including vertical migration and intracellular nutrient storage. Here, we use geochemical and isotopic analyses of sediment cores from the western North Pacific Gyre to constrain the timing and nitrogen availability of bloom events. Radiocarbon dating reveals that E. rex blooms occurred from the Last Glacial Maximum through the early Holocene, extending the temporal range proposed by earlier studies. Elevated Si/Ti and Ba/Ti ratios, increased biogenic opal concentrations, and higher bulk organic carbon δ 13 C values indicate enhanced primary productivity and rapid consumption of dissolved inorganic carbon during bloom periods. However, limited organic‐matter preservation suggests extensive remineralization during sedimentation. Most notably, a mean depletion of ∼2.4‰ in organic matter δ 15 N relative to background values points to the utilization of isotopically light subsurface nitrate. We infer that upper ocean dynamics such as Rossby waves and internal oscillations intermittently uplifted the deeper nutricline, bringing nitrate‐rich subsurface waters into an effective depth window for E. rex without penetrating the surface mixed layer. Within this window, nutrients could be effectively utilized, facilitating rapid proliferation under oligotrophic surface conditions. This mechanism remains hypothetical and requires further testing through proxy and modeling studies.

Whale falls are biodiversity oases at seabeds1-6, yet their record from the oceans has remained sparse and fragmentary6,7. Here we report the discovery of a vast whale necropolis in the Diamantina Zone (4,616- to 7,001-m depth), extending about 1,200 km along the sea floor of the southeastern Indian Ocean. This area has a deep and extensive accumulation comprising five modern natural whale-fall communities and 476 fossil cetaceans recorded. We show that carcasses host specialized communities dominated by brittle stars, bone-boring worms and chemosynthesis-based bivalves and that the fossil record in this area comprises both extant and extinct deep-diving beaked whales. Isotopic dating shows that whale falls in this region have occurred since at least 5.3 million years ago. These findings reshape the understanding of the limits and biogeography of whale-fall ecosystems and establish some deep sea floors as a fossil archive for tracing cetacean evolution over geological time.

Deep-sea hard substrates host faunal novelties and distinct evolutionary lineages. However, sessile organisms on rocks are difficult to sample and largely unknown at extreme hadal depths. Here, we report a deep hard-substrate fauna (9000 to 10,898 meters), comprising 32 species of six protist and metazoan phyla, most millimeter-sized and new to science, from the Kermadec and Mariana trenches, using the manned submersible Fendouzhe. We show that the filamentous organisms dominating these assemblages are heterotrophic foraminiferans, challenging the earlier chemolithoautotrophic hypothesis. Large-scale seafloor imaging and sampling suggest that similar protistan-dominated sessile communities thrive in seven hadal regions around Oceania. These faunas open new perspectives on biodiversity at the deepest ocean depths and unveil widespread, but previously unrecognized, carbon hotspots in global hadal trenches.

The evolutionary ecology of prokaryotes within oceanic trenches remains largely unexplored. Here, we analyzed 19,194 medium-quality prokaryotic metagenomic assembled genomes (MAGs) from sediments of the Diamantina, Kermadec, Yap and Mariana trenches. Hadal zones hosted relatively ancient phylogenetic lineages across most prokaryotic phyla compared to non-hadal zones. A linear correlation between depth differences and the pairwise patristic distances was observed inter/intra trenches, indicating a depth-dependent evolutionary boundary. This pattern was further reflected by a clear vertical shift of genomic features, including carbon/nitrogen atoms per amino-acid-residue side chain (C/N-ARSC), pseudogene density and non-synonymous to synonymous mutation ratio (pN/pS). Close phylogenomic relationships for MAGs among different trenches suggested a rapidly and continuously exchange, which was very likely facilitated by deep-sea currents. Molecular dating of hadal Nitrososphaeria and Alphaproteobacteria revealed a divergence of respective 2.2 and 1.8 billion years ago, earlier than the emergence of contemporary trenches, emphasized the hadal zone sediments as a crucial ecological reservoir for prokaryotic evolution.

Branched glycerol dialkyl glycerol tetraethers (brGDGTs), originally linked to terrestrial bacteria for paleoenvironmental reconstructions, are now also known to be produced by marine bacteria. However, the distribution patterns and environmental controls of marine-sourced brGDGTs remain poorly understood, limiting their applicability in marine settings. Here, we analyzed sediments from the South China Sea (n = 45, 3-4574 m depth) and compiled a marine dataset (n = 1274, 0-10,840 m depth) to investigate brGDGT distributions in the global ocean. We find that brGDGT cyclization increases with depth <200 m but decreases markedly at greater depths. After excluding terrestrially influenced samples, marine-sourced brGDGTs exhibit a strong, consistent negative correlation between cyclization and water depth-both regionally and globally. In contrast, isomerization and, to a lesser extent, methylation increase with depth. Marine brGDGTs differ fundamentally from their terrestrial counterparts in composition, index relationships, and environmental controls, reflecting distinct bacterial assemblages and/or adaptation strategies shaped by the contrasting ecosystems. Therefore, conceptual frameworks developed for soil brGDGTs cannot be directly applied to marine realms. However, the observed structural shifts-declining cyclization and increasing isomerization and methylation-correlate with water depth, bottom-water temperature, and surface productivity, and likely reflect microbial responses to the cold, high-pressure, and nutrition-limited conditions of the deep ocean.

Anaerobic ammonium oxidation (anammox) bacteria are an important functional guild in the nitrogen cycle and contribute up to 50% of nitrogen loss in the global ocean. Hadal trenches have been recognized as a hotspot of marine biogeochemical cycles; however, the metabolic traits, ecological adaptations, and potential origins of anammox bacteria in this critical habitat remain largely unexplored. Here, we reconstructed eight anammox metagenome-assembled genomes from sediments of four hadal trenches (Diamantina, Kermadec, Mariana, and Yap), which represent four out of the five distinct anammox bacterial families (i.e. Candidatus Scalinduaceae, Ca. Anammoxibacteraceae, Ca. Subterrananammoxibiaceae, and Ca. Bathyanammoxibiaceae). The dominant trench anammox bacteria, affiliated with Ca. Scalindua, were similar to those found in shallow coastal sediments and oxygen-deficient seawaters. Beyond the core anammox metabolism, the hadal Ca. Scalindua genomes contain genes encoding cyanase and urease, indicating that they can utilize cyanate and urea besides ammonium to thrive in the hadal trenches. Compared to trench-derived Ca. Subterrananammoxibiaceae and Ca. Bathyanammoxibiaceae, ABC-type Fe3+ transporter and sulfate transporter CysZ could help trench-derived Ca. Anammoxibacteraceae genomes to uptake Fe3+ and synthesize sulfur-containing amino acids. Molecular clock analysis suggests that the ancestors of the hadal anammox bacterial lineages appeared on Earth 1.46-0.07 billion years ago, significantly earlier than the geological formation of the trenches. The first hadal anammox bacteria were likely derived from shallower sediments and were transported into the trenches via sediment wasting. Overall, our study reveals a remarkable diversity of hadal anammox bacteria and their origin as well as survival strategies in hadal sediments.

Coral biomineralization typically yields either aragonite (in most scleractinians) or calcite (in octocorals), yet the drivers of this polymorph choice remain debated. Here we report a deep-sea gorgonian coral (Callogorgia sp.) with a coaxial calcite-aragonite-calcite skeleton. Growing at 1,400 m under nearly constant temperature and seawater chemistry, this specimen rules out environmental forcing as a control. High-resolution elemental mapping, Raman microscopy, and infrared nano-spectroscopy show that acidic, aspartate-rich macromolecules are enriched in the calcitic layers but depleted in the aragonitic layer. These macromolecules lower the nucleation barrier for calcite while inhibiting aragonite growth, driving abrupt, layer-by-layer phase switches. Temporal shifts in macromolecule distribution correlate with biological imprints, implying regulation via changes in calcifying tissues or symbionts. Our results provide direct in situ evidence that organic molecules, rather than external conditions, govern carbonate polymorph selection in octocorals, offering a natural template for biomimetic control of CaCO3 phase architecture.

Although per- and polyfluoroalkyl substances (PFAS) are widespread in global ecosystems, their presence in the hadal zone, particularly that of novel compounds, remains unexplored. In this study, 15 PFAS were detected in amphipods from the Mariana, Mussau, and New Britain Trenches (ranging from 0.4-37.5 ng g-1 dry weight), whereas all seawater and sediment samples fell below the detection limit. We quantified organism-water partitioning, analyzed structural and concentration similarity via COSMO-RS, applied neural networks to predict bioaccumulation, and prioritized PFAS risks using a persistence-bioaccumulation-toxicity framework. Short-chain novel PFAS (e.g., PFBA and PFPeA) formed the largest share of total PFAS loads (up to 4.2 ng g-1 dw) but contributed minimally to risk. In contrast, long-chain PFAS (PFTrDA and PFUnDA), though less abundant, exhibited substantially higher risk potential. The novel compound F-53B was detected exclusively in Mariana amphipods. Overall, the accumulation patterns across PFAS classes reflect the combined influence of external exposure and internal partitioning constraints. These findings demonstrate that structural modification does not inherently reduce PFAS hazards and highlight the necessity of including hadal organisms in global chemical risk evaluation.

Abstract Halogen incorporation into coral carbonate skeletons serves as a valuable proxy for reconstructing paleoenvironments. However, the underlying mechanisms remain poorly understood. This study employed femtosecond laser ablation‐inductively coupled plasma time‐of‐flight mass spectrometry (fs‐LA‐ICP‐TOFMS) to perform multi‐elemental (I, Br, P, Ca, Mg, Sr, S, Ba, and Fe) imaging of cross‐sections from carbonate skeleton of deep‐sea Callogorgia sp. coral (octocoral) collected from the Mariana Arc at a depth of 1,386 m. Previously unrecognized iodine‐rich ring patterns and bromine‐rich fan‐like distributions were identified, likely influenced by the biological rhythms of polyps, such as polyp apoptosis‐replacement or migration, which refresh the polyp‐skeleton interface and facilitate halogen accumulation. Additionally, higher iodine and bromine concentrations were consistently observed in the calcitic layers compared to the aragonitic layer, suggesting that biological factors, particularly halogenated organics, dominate halogen incorporation, outweighing mineralogical controls. A comparison of halogen distributions between Callogorgia and bamboo corals demonstrated species‐specific halogen incorporation patterns. These findings provide new insights into coral biomineralization, emphasizing the role of biological rhythms in skeletal formation and trace element incorporation.

Hadal trenches, some of the Earth's least explored and understood environments, have long been proposed to harbour chemosynthesis-based communities1,2. Despite increasing attention, actual documentation of such communities has been exceptionally rare3,4. Here we report the discovery of the deepest and the most extensive chemosynthesis-based communities known to exist on Earth during an expedition to the Kuril-Kamchatka Trench and the western Aleutian Trench using the manned submersible Fendouzhe. The communities dominated by siboglinid Polychaeta and Bivalvia span a distance of 2,500 km at depths from 5,800 m to 9,533 m. These communities are sustained by hydrogen sulfide-rich and methane-rich fluids that are transported along faults traversing deep sediment layers in trenches, where methane is produced microbially from deposited organic matter, as indicated by isotopic analysis. Given geological similarities with other hadal trenches, such chemosynthesis-based communities might be more widespread than previously anticipated. These findings challenge current models of life at extreme limits and carbon cycling in the deep ocean.

Scleractinian cold-water corals (CWCs) are unique and prominent geological archives in the deep ocean, offering crucial insights into ocean dynamics and climate changes over time. Using scleractinian CWC skeletons to reconstruct paleoclimate often relies on combining data from multiple solitary cup corals or branching colonial corals. However, biomineralization strategies vary between species, and even within different skeletal structures (e.g. the corallite and the coenosteum) of the same species, leading to complex geochemical characteristics within coral skeletons, complicating our understanding of CWC growth patterns and the reconstruction of past ocean changes. In this study, we investigated the growth patterns of a colonial scleractinian CWC from the South China Sea using the UTh dating technique and measured the geochemical compositions of the corallite and the coenosteum to evaluate their environmental significance. Our results indicate a budding rate approximately 2.8 polyps per decade, with linear extension rates between 0.47 and 0.57 mm/year, and radial thickening rates ranging from 0.014 to 0.022 mm/year. Significantly, the geochemical compositions, such as delta C-13, delta O-18, delta B-11, Sr/Ca, B/Ca, Ba/Ca, and U/Ca, differ between the corallite and coenosteum. Conversely, Mg/Ca and Li/Mg ratios remain consistent across both structures. These disparities likely reflect varying degrees of vital effects during the formation of different skeletal parts. Among the examined geochemical proxies, Ba/Ca and Li/Mg ratios provided clear signals of environmental changes over the past similar to 500 years. Specifically, Ba/Ca ratios in both corallite and coenosteum exhibit a marked secular decline since the 1500s, suggesting a reduction in surface primary productivity or decreased ventilation of North Pacific intermediate waters. Additionally, intermediate water temperatures estimated from the Li/Mg ratios of these two structures collectively show a tendency to increase towards the twentieth century.

Fe-oxidizing microorganisms in deep-sea hydrothermal vent environments are often used as analogs for primordial life on Earth. In fact, Earth's oldest purported microfossils are preserved as hematite filaments in a jasper rock dated between 4,160 and 4,280 million years and are thought to have originated in a seafloor hydrothermal environment. However, the kinds of post-depositional processes that can alter their morphologies are not well-known, which has implications for recognizing morphologies of bona fide microbial origin in the deep-time rock record. Here, we show that more than 10 morphological types of filamentous Fe-oxyhydroxide microstructures occur in Fe-oxide specimens from deep-sea hydrothermal vents in the southwest Indian Ocean, including thick and thin filaments with the following morphologies: parallel-alignments, branching and pectinate-branching, curved and straight, hollow to tubular, twisted and coated with botryoidal silica. Botryoidal silica mineralization is documented on several filament morphotypes and exhibits pattern with spheroidal twins, circular concentricity, and cavities, whereas their elemental composition is dominated by Si, Fe, Mn, C, with minor S and halogens. Such patterns and substances point to an origin from chemically oscillating reactions, which provide a novel abiotic model based on C, Fe, Mn, S, and halogen redox reactions in colloidal silica, to explain occurrences of botryoidal minerals grown onto deep-sea filamentous Fe-oxyhydroxide microstructures. The documented filamentous morphologies and new model for silica botryoid formation help to understand abiotic carbon cycling in marine and lacustrine environments, ancient filaments preserved in the geological record, as well as a basis to seek similar structures in deep-space settings.

Viruses are the most abundant biological entities in marine ecosystems, playing an important role in biogeochemical cycling and the regulation of microbial dynamics. However, their assembly driving force, genomic evolution, and potential ecological functions in the hadal trench remain largely unknown. Here, 32 359 viral operational taxonomic units were derived from metagenomes of 40 sediment samples in the Kermadec and Diamantina trenches. High novelty and habitat-specific endemism of viruses based on the protein-sharing network analysis were demonstrated. Their auxiliary metabolic genes were involved in the biogeochemical cycles and compensatory metabolic process of the host inferring from the virus-host linkage prediction. Distinct viral community assembly in the two trenches and among different sampling depths was mainly driven by the stochastic processes, especially dispersal limitation. This was further proved by the low genomic mutation rates at deeper depths with potentially high hydrostatic pressures. These niche-dependent distribution patterns and genomic features together reflected the survival and adaptative strategy of viruses. This study provided new insights into the high diversity, ecological potentials, evolution, and adaptive mechanism of viruses in the deep biosphere.

To further constrain the contribution of marine siliceous sediments in the subduction zones to the oceanic gallium (Ga) cycle, we investigated Ga geochemical behavior by examining the mineral, elemental and Ga isotopic compositions of siliceous sediments from the southern Mariana Trench (SMT). The results show that Ga contents vary from 15.6 to 17.6 mu g/g (average = 16.7 mu g/g) in the lower part of the sediment core but the lower Ga contents (2.7 to 10.9 mu g/g, average = 5.9 mu g/g) in the upper part. The systematic variation in geochemistry and mineralogical compositions indicates that abundant Ga in the lower part originates from volcanogenic/basaltic rocks, while the addition of biogenic SiO2 to the upper part dilutes the concentration of these source-rock components, resulting in the concomitant decreases in contents of Ga and other elements. Particularly, reverse weathering leads to an increase of Ga in the top section of the upper part by favoring an uptake of Ga in the seawater into aluminosilicates, as evidenced by the range of pH, Al/Nb, Ga/Nb, and Ga/Al variations. In contrast, delta 71Ga values (relative to the Ga-IPGP standard) show a monotonous range from-0.06 to 0.05 parts per thousand (average =-0.01 parts per thousand, n = 17), which is consistent with those for basalts (0.00 +/- 0.05 parts per thousand). We find that the addition of biogenic SiO2 and reverse weathering do not significantly change the Ga isotopic compositions in the SMT marine siliceous sediments. Collectively, our study highlights that marine siliceous sediments in the SMT may act as an isotopically light sink of Ga in the modern ocean via reverse weathering, and could shed new light for understanding the oceanic Ga cycles.

The elemental compositions of deep-sea corals are invaluable proxies in high-resolution reconstructions of deep-sea paleo-oceanographic conditions, but the incorporation of minor and trace elements into their skeletons and associated biomineralization are poorly understood. In this study, multielement (Cd, P, Ba, K, S, Co, Mn, Fe, Sr, Na, and Mg) and fluorescence imaging of skeletal cross-sections of a subapical branch and middle-basal trunk of a deep-sea bamboo coral, Jasonisis sp. (collected from the seafloor in front of the Mariana arc (11.95355 degrees N, 141.47446 degrees E) at a depth of 1,385.9 m in October 2019) were undertaken using femtosecond laser ablation-inductively coupled plasma time-of-flight mass spectrometry and laser scanning confocal microscopy, respectively. The spatial associations of minor and trace elements and their organic compositions, and associated growth patterns (cyclic rings) of early- (the subapical branch and the near-core region of the middle and basal trunk) and late-stage (the middle-basal trunk excluding near-core regions) coral skeletons were determined simultaneously. The consistent distribution patterns of most elements and the presence of distinct rings correspond to growth patterns associated with nutrient-like elements such as Cd, P, and Ba and organic components within the late skeleton. This likely suggest a significant role of nutrients in the biomineralization process during the late stage. The distinct elemental patterns, exhibiting positive correlation among specific elements (e.g., P, S, Mn, Co, and Cd) and negative correlations with others (e.g., Na and Mg), along with the lower organic content observed in the early skeleton, suggest a unique mineralization mechanism during the initial growth stage, with kinetic processes playing a dominant role. Therefore, combined multielement and organic composition mapping of different parts of coral skeletons provides comprehensive insight into coral growth and the incorporation of trace elements.

Ferromanganese oxides that coat dead marine biological remains such as porous sponges, fish bones, and coldwater corals are widespread in deep-sea environments. However, their mineralization mechanisms and the intrinsic connections between ferromanganese oxides and biological remains are poorly understood. In this study, we examine ferromanganese oxide coatings (FMC) on five dead sponge specimens collected from seamounts in the Central Basin of the South China Sea (SCS) and in the West Mariana Ridge (WMR). Sponge FMC are friable and loosely attached precipitates grown onto the reticulate skeletal structure and they have a narrow range of thicknesses. High-resolution microscopic observations show that sponge FMC are composed of porous, spheroidal, and relatively compact sheath-like Mn oxides with variable oxidation state, which possibly imply a different formation pathway from general ferromanganese deposits (FMD). New observations further reveal the position of sponge FMC spatially overlapping with sponge collagen fibers, as well as reveal widespread presence of microbial cells, Nitrogen-enriched OM with abundant amide groups, and nanoscopic apatite in sponge FMC. The observations provide a new insight into the precipitation of sponge FMC that is related to the decomposition of skeleton-associated proteins of the sponge. The degradation of organic matter contributes to localized enrichment of metal cations and micro-scale oxic-suboxic transitions. Meanwhile, small-molecule carbon compounds derived from protein decomposition are favorable to the activity of heterotrophic microorganisms, which modulates localized physicochemical conditions. These redox variations in microenvironments are proposed to trigger the oxidation and reduction of metal cations and their precipitation into FMC around sponge remains. While the composition of sponge FMC resembles that of hydrogenic crusts, their rare element and yttrium (REY) patterns display notable discrepancies with hydrogenic processes, including weak negative or significantly positive Ce anomaly. The discrepancies reflect the unique geochemical characteristics of sponge FMC, possibly attributed to differences in the activity of oxic-suboxic transitions and in the redox behavior of Mn and Ce metals. Biological remains in the deep-sea have a widespread occurrence and most of them similarly serve also as FMC substrates, which may point to a broader implication for marine polymetallic cycling than previously considered.

Diamantina Trench in the southeast Indian Ocean is one of the less unexplored hadal trenches (> 6000 m) of our planet, which develops the second deepest point (Dordrecht Deep, ca. 7019 m depth) in the Indian Ocean. Humans did not visit its ocean floor until the Chinese submersible Fendouzhe reached its deepest point in January 2023. This expedition collected high-resolution multibeam sonar bathymetry data covering about 3000 nautical miles and conducted 28 scientific dives with high-resolution videos and push core sediments of the upper seafloor (max. 40 cm) at a wide range of submarine geomorphology. This study combines these materials to fully assess the morphological variability of the trench and the causative factors and processes determining such characteristics. Bathymetry data indicate a rugged and complex landscape with various seamounts and debris deposits in the Diamantina Trench which could be classified into three sections. Bounded by the Broken Ridge to the north, the western section contains a series of basins and gorges, as well as parallel intruded ridges (WNW striking). The eastern section shows deeper and steeper slopes compared to the western section. The transitional area of the two sections (the Dordrecht Deep area, 270 km2) is the deepest part of the trench. Four push core sediment profiles were analyzed from the most west and east locations, the Dordrecht Deep area, and the western trend with foraminifera oozes. Layers of foraminifera and calcareous nannofossil oozes occur at the western section, whereas brownish pelagic sediments with occasionally coarse-grained Fe-Mn nodules develop at the eastern section. The preliminary results of total carbon (TC) and total nitrogen (TN) suggest distinct differences among and within profiles. TC values reach 12% in foraminifera oozes and less than 1.2% in the pelagic sediments. TC values decrease rapidly at the upper 10 cm and remain low (0.1–0.2%) at the lower part in the profiles from the eastern section and Dordrecht Deep area. An analogous trend applies to the TN graphs. The sediment profile from the western section, however, shows decreasing TC and TN values within depth. This research provides the first knowledge of the highly spatial heterogeneity of submarine geomorphological characteristics and sediment dynamics in the Diamantina Trench. The ongoing measurements of organic matter content, carbon isotope, and grain size from different topographic locations with the potential of dating methods (e.g., 14C and paleontological data) will further aid in reconstructing the spatial variations of paleoenvironmental changes and organic cycling process, as well as in understanding the relationship with tectonic activities and catastrophic events in hadal zones.

Obtaining actual in situ rate of biogeochemical processes is essential for understanding marine biogeochemical cycles. Automated sampling systems capable of conducting both sampling and incubation under ambient conditions are a powerful approach. Here, we present a full-ocean-depth Biogeochemistry Experiment System (BES), designed for in-situ measurements of ammonium (NH4+) and nitrite (NO2-) oxidation rates in hadal zone (>6000 m). The BES was successfully deployed at four stations in the Yap Trench and the Mariana Trench (Depths: 7869-10903 m), demonstrating its robustness and operational feasibility. NH4+ oxidation rates ranged from undetectable to 0.03 nmol L(-1)d(-1), while NO2- oxidation rates ranged from 0.64 to 2.29 nmol L(-1)d(-1). These results underscore the utility of BES for deep-sea research and highlight its potential for revealing spatial variability in biogeochemical processes, particularly in regions where depressurization compromises sample integrity.

Nickel (Ni) is a biologically essential element in marine systems, yet its oceanic sources and sinks remain incompletely quantified. Micro manganese nodules (MMNs) and particles (MMPs) in marine sediment are major authigenic phase that scavenge critical metals though adsorption or incorporation. The accumulation of these minerals depends strongly on the seawater conditions and diagenetic processes within the sediments. Despite their significance, the influence of different manganese phase on the cycling of polymetallic elementsparticularly Ni and its isotopes-has received little attention. This study analyzed MMPs bearing sediments, MMNs, and MMN bearing sediments from the Northwest Pacific Ocean to characterize their geochemical and Ni isotopic signatures. In core JL190, MMNs display Mn/Fe ratio and trace element patterns typical of suboxic-oxic diagenesis. Across all three cores, Ni concentrations track Mn accumulation closely, as shown by strong Mn/ Al-Ni/Al correlations. However, core TS01-B10 from the Mariana Trench, which contains diatom-rich clay, yields weaker correlations than the two Philippine Sea cores. Despite these differences, bulk sediment S60Ni values remain light (0.01-0.79 %o, median 0.27 %o), consistent with most pelagic sediments. In core B10, declining S60Ni alongside rising Mn/Al ratios imply preferential adsorption of lighter Ni isotopes. In contrast, JL189 show simultaneous increase in Mn/Al and S60Ni, suggesting porewater exchange and isotope fractionation on existing Mn oxides. The heavier Ni isotopic signature in diagenetic MMNs relative to bulk sediments likely reflects prolonged porewater interaction, preserving the porwater's isotopic signature. These results provide the first S60Ni data for MMNs and elucidate their role as Ni carriers. These findings highlight the importance of authigenic Mn oxides in influencing the benthic flux of heavier Ni isotopes, which contributes to balancing the oceanic Ni budget.

Diamantina Fracture Zone in the SE Indian Ocean is one of the less unexplored hadal zones (>6000 m) of our planet without human visits until recently. This study incorporates shallow sediments, submarine videos, and multibeam bathymetry data at a wide range of water depth and geomorphology to fully assess the sediment dynamics of the Diamantina Fracture Zone and their major causative factors. Grain size and organic geochemical analyses confirmed a primary marine source. Australian terrigenous input was indicated by an increasing silty contribution to the eastern hadal section of the fracture zone. Importantly, in the western to middle section, angular volcanic-rich sediments with a peak at 200-300 μm covered the underlying fine pelagic sediments and calcareous oozes, which were likely initiated during the Last Glacial Maximum. Susceptibility to slope failure was high due to localized topographical constraints, rather than earthquakes. The occurrence of sediment ripples at the west and densely-covered manganese nodules at the east implied the ocean bottom circulation with increasing current intensity, which also enhanced the possibility of the gravity-driven slope deposition. This research provides the first knowledge of the highly spatial heterogeneity of sediment dynamics in the remote deep Indian Ocean where continuous but fluctuating downslope and alongslope processes were developed.