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.

Deep-sea mining has the potential to significantly impact the structure and biogeochemical function of benthic microbial communities, however, its short-term ecological impacts remained largely unexplored. Here, we investigated microbial responses in seawater and cobalt-rich ferromanganese crust samples collected before and during a mining trial in the South China Sea using metagenomic sequencing. Mining activities induced rapid shifts in microbial functional profiles from redox-sensitive metabolic pathways toward anaerobic denitrification, potentially promoting nitrogen loss and altering nutrient regeneration processes.. Microbial groups in different ecological niches (i.e. waters and rocks) exhibited disparate responses to mining process, reflecting habitat-specific sensitivities to physical disturbance and sediment plume formation. In addition, an increase in viral-related functional categories suggests that mining disturbance may enhance virus-mediated microbial interactions, with potential implications for metabolic plasticity and community stability. Above results reflected the detrimental impact of human-induced disturbances on deep-sea ecosystems, even with short time period, and thus required further investigation to fully assess the ecological consequences of deep-sea mining.

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.

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.

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.

Arsenic accumulation in seafloor cold seeps is poorly understood. Here, we investigate the interplay between arsenic and methane biogeochemical cycles at the Haima cold seep, South China Sea. Geochemical analyses showed elevated arsenic levels in seep sediments, primarily as sulfide-bound forms, with strong correlations between dissolved arsenic and methane oxidation proxies (DIC, R-2 = 0.64, p < 0.05; delta C-13(DIC), R-2 = 0.86, p < 0.05). Metagenomic sequencing revealed diverse functional genes related to arsenic-, methane- and sulfur cycling, including co-occurrence of arrA and dsrA in sulfate-reducing bacteria Desulfobacterota, which are symbionts of Anaerobic Methane Oxidizing Archaea. We propose that anaerobic oxidation of methane (AOM) and the enhanced "metal particle shuttle effect," both associated with methane release, drive arsenic sequestration. Cold seeps may sequester 0.04-1.81 x 10(3) kg arsenic annually. These finding highlights cold seeps are hotspots of arsenic cycling with implications for oceanic arsenic chemistry.

Natural whale falls and falls of smaller-sized food have been reported for more than 30 years and are known to be important sources of nutrients and organic matter for the seafloor community. However, the associated species composition and ecological processes during such events in the hadal zone were unknown. Therefore, we aimed to determine the impact of depth and predation on the hadal community during the early stages of a food-fall event. This is the first study to investigate the preliminary stages of the two deepest artificial dolphin-fall events in the Philippine Basin (PB) (-7729 m) and Mariana Trench (MT) (-8200 m). A total of nine dives were conducted over a period of 86 days (PB) and 50 days (MT) using the deep-sea manned submersible "Fendouzhe." Our observations in the PB indicated that the first stage (mobile-scavenger) was controlled by the feeding ecology of hadal amphipods and snailfish. In contrast, the absence of predatory snailfish in the MT enhanced the degradation rate of the carcass compared to that in the PB. Most soft tissues were entirely consumed by scavenging amphipods within days of the event in the MT, whereas in the PB, amphipods were observed taking a feeding hiatus on Day 10 to escape snailfish predation. The second stage (enrichmentopportunist), which hosted few grazing faunas, began at different times in each location and overlapped with the mobile-scavenger stage. Dolphin carcasses, being smaller than those of full-grown whales, can only sustain a large community of scavenging amphipods, and indirectly, predatory snailfish. After the first stage, the dispersed organic matter and limited lipid content in dolphin bones were likely insufficient to sustain an active grazing community or the chemosynthetic community that typically follows. We concluded that water depth influences the successional stages and decomposition rate of food falls in the hadal zone by controlling the dynamic relationship between prey and predators. Our study elucidates the ecology of food-fall events in the hadal zone and highlights the key differences in food-fall events at different depths.

As a special type of seep carbonate, the many details concerning the formation mode and mechanism of tubular seep carbonates are rarely reported. Here, new geochemical and mineralogical data regarding tubular seep carbonate (SQW-65) are reported. Sample SQW-65 had anomalously negative δ13C values and positive δ18O values, which suggested the dissociation of gas hydrate. Additionally, almost all the sub-samples showed no Ce anomaly (Ce/Ce*average = 0.93), with obvious U enrichment (21.3< UEF <240.3), which indicates that the studied tubular seep carbonate was formed in an anoxic environment. Subsequently, the formation process of the studied tubular seep carbonate is further discussed according to the variability of mineralogical and geochemical characteristics from the rim to the core of the tubular formation. In the early stage of the studied tubular seep carbonate (periphery), owing to the influence of terrigenous components, the quartz and Ti content and Y/Ho ratio were high. However, with the formation of the periphery, the influence of terrigenous components was gradually weakened. In addition, from the rim to the core, the carbon and oxygen isotope values showed a “covariation” coupling relationship, an enrichment of U, and a reduction in total rare earth element content. This is because as the outer wall thickens and the internal fluid channel narrows, the intensity of the sulphate-driven anaerobic oxidation of methane and the associated precipitation rate of carbonate also increase.

The important geological, environmental, and biological information contained in Fe -oxides (oxyhydroxides) in extreme hydrothermal environments has provided unique opportunities for exploring the microbial -driven mechanism of Fe -oxide formation. However, the interactions between Fe -oxides and organic components and their association with in situ geochemical signatures in hydrothermal systems remain poorly understood. This study analyzed Fe -oxide deposits collected from the Longqi hydrothermal field in the Southwest Indian Ridge and the Edmond and Kairei hydrothermal fields in the Central Indian Ridge using the manned submersible "ShenhaiYongshi". Mineralogical and geochemical characteristics of hydrothermal iron oxide deposits indicated that microbial activity may influence trace element distribution. Analysis with laser confocal Raman spectroscopy (LR) revealed biomineralized structures with significant absorption peaks of Fe-oxyhydroxides. This was further confirmed by two typical L-shaped and Y-shaped structures directly measured using a novel scanning probe technique called infrared photo induced force microscopy (IR-PiFM). The strong peak in the range of 1500 to 1680 cm -1 corresponded to the bending vibration of Fe -O -H. X-ray photoelectron spectroscopy (XPS) showed that the oxidation state and coordination of iron strongly correspond to a biogenic origin. Fe -bearing phases in the biomineralized structures occurred mainly as FeOOH. The combined results of in situ LR, IR-PiFM, and XPS showed that organic compounds identified in mineralized structures could be polysaccharides, lipids, and proteins. The results suggested that bioorganic molecules may play a key role in the formation of Fe(III)-deposited nucleation sites, which promoted the development of biomineralized iron structures. These observations may provide new insights for understanding the biomineralization mechanism of Fe in extreme hydrothermal environments.

Cold-water coral (CWC) communities are biodiversity hotspots on the world's deep seafloor. Although deep-sea corals in the South China Sea (SCS) have been reported before, they are only sporadic. A comprehensive and systematic understanding of the CWC in the SCS would forge the basis for future protection. Here we conducted the first systematic survey on the CWCs in the following six broad-scale sub-regions, from the northwest and northeast slopes to the seamounts in the western and central basins of the SCS, through twenty-four dives of the human-occupied vehicle ShenhaiYongshi. Statistical analysis provided detailed information on the distribution, abundance, size, diversity, and density of CWCs and the in situ environmental conditions supporting coral habitats. We found that the SCS hosted highly diversified coral communities, including twelve genera of gorgonians, six genera of black corals, and one genus of stony corals. The differences in the spatial distribution patterns of coral communities suggested that several environmental variables (depth, temperature, salinity, substrate, and geomorphology) might influence the development of CWCs in the SCS. The intermediate water layer of the SCS appeared to provide suitable habitat for deep-sea coral communities and potentially promoted connectivity. Furthermore, differences between sub-regions within the SCS may be an important factor responsible for the biogeographic patterns of CWCs. These sub-regions of CWCs were observed to range from 0.004 to 0.622 corals m(-2), with an average of 0.139 corals m(-2). The mean density of CWCs in the SCS was relatively high compared to well-studied CWC hotspots. Overall, the results revealed the significance of the SCS as an important CWC hotspot in the world. These findings provide a fundamental basis for the protection of deepsea coral assemblages in the SCS.

Deep oceans receive mercury (Hg) from upper oceans, sediment diagenesis, and submarine volcanism; meanwhile, sinking particles shuttle Hg to marine sediments. Recent studies showed that Hg in the trench fauna mostly originated from monomethylmercury (MMHg) of the upper marine photosynthetic food webs. Yet, Hg sources in the deep-sea chemosynthetic food webs are still uncertain. Here, we report Hg concentrations and stable isotopic compositions of indigenous biota living at hydrothermal fields of the Indian Ocean Ridge and a cold seep of the South China Sea along with hydrothermal sulfide deposits. We find that Hg is highly enriched in hydrothermal sulfides, which correlated with varying Hg concentrations in inhabited biota. Both the hydrothermal and cold seep biota have small fractions (<10%) of Hg as MMHg and slightly positive Δ199Hg values. These Δ199Hg values are slightly higher than those in near-field sulfides but are 1 order of magnitude lower than the trench counterparts. We suggest that deep-sea chemosynthetic food webs mainly assimilate Hg from ambient seawater/sediments and hydrothermal fluids formed by percolated seawater through magmatic/mantle rocks. The MMHg transfer from photosynthetic to chemosynthetic food webs is likely limited. The contrasting Hg sources between chemosynthetic and trench food webs highlight Hg isotopes as promising tools to trace the deep-sea Hg biogeochemical cycle.

Volcanic ash is a major component of marine sediment, but its effect on the deep-sea carbon cycle remains enigmatic. Here, we analyzed mineralogical compositions and glycerol dialkyl glycerol tetraether (GDGT) membrane lipids in submarine tuffs from the Mariana Trough, demonstrating a fraction of organic carbon associated with volcanic ash is produced in situ. This likely derives from chemolithotrophic communities supported by alteration of volcanic material. Tuff GDGTs are characterized by enrichment of branched GDGTs, as in chemolithotrophic communities. Scanning electron microscope, Raman spectrum and nano secondary ion mass spectrometry analysis demonstrates organic carbon exists around secondary heamatite veins in the altered mafic minerals, linking mineral alteration to chemolithotrophic biosynthesis. We estimate organic carbon production of between 0.7 − 3.7 × 10 11 g if all the chemical energy produced by ash alteration was fully utilized by microorganisms. Therefore, the chemolithotrophic ecosystem maintained by ash alteration likely contributes considerably to organic carbon production in the seafloor.

The frictional characteristics of sediments control the development of landslides in submarine landslide areas. They are the basic parameters for the evaluation of submarine slope stability, and the study of the geothermal-pressure field evolution, and the law of gas hydrate hosting during the landslide process. The continental slope areas of the northern South China Sea are not only rich in gas hydrate resources, but also have many landslides in geological history. They are still potential submarine landslide areas. In order to understand the in situ frictional characteristics of the sediments in the landslide areas of the northern South China Sea as much as possible, we carried out the triaxial quasi-static frictional sliding experiments for four sediment collected from the Shenhu Canyons, under the conditions of confining pressure P-c = 20 MPa, pore pressure P-p =10 MPa and temperature T approximate to 20 degrees C. The experimental results show that: (1) all the four shallow sediments show the characteristics of velocity strengthening and displacement strengthening. (2) The maximum static friction coefficient (mu(max)) and steady-state friction coefficient (mu(ss)) of these four sediments range from 0. 460 to 0. 510, and from 0. 455 to 0. 554, respectively. Moreover, there is a positive relationship between mu(max) and mu(ss). (3) The ranges of cohesion (c) and friction angle (phi) are 0. 30 similar to 0. 57 MPa and 24. 5 degrees similar to 27. 0 degrees, respectively. In fact, the topographic slope usually lower than 6. 8 degrees in the Shenhu Canyons. It means that the unstable slip could not be induced only by the sediments weight in this area. In addition, based on the multi-stage landslide characteristics of the Shenhu Conyons, the spatial distribution relationship between the landslide bodies and the bottom boundaries of the hydrate stability domain which usually overlap with the bottom simulating reflectors (BSRs), gas chimneys and other structures, we infer that the landslides are predominantly caused by the increase of pore pressure which reduces the formation strength near the BSRs. The increase of pore pressure may be caused by the accumulation of the thermogenic free gas from deep, or/and the hydrate decomposition near the BSRs due to the disturbance of geothermal-pressure fields after the sudden geological events, such as earthquakes.

AbstractThe deposition of different atmospheric mercury (Hg) species into oceans determines the atmospheric Hg lifetime and the production of neurotoxin methylmercury. Yet, the relative contribution of atmospheric Hg(II) and Hg(0) is largely unconstrained. Here, we report the concentrations of total Hg and methylmercury, as well as Hg isotope composition in living corals collected from the tropical South China Sea (SCS). The results show that the Hg in corals is mainly present as inorganic Hg, exhibiting slightly negative δ202Hg and small yet significant Δ199Hg and Δ200Hg. These isotope features closely resemble those of pelagic waters, suggesting that shallow‐water corals that are widely distributed in oligotrophic waters could be used to trace atmospheric Hg deposition pathways. A mixture model based on coral Δ200Hg indicates that approximately 49% of the seawater Hg in the tropical SCS, a region characterized by high rainfall, originates from atmospheric gaseous Hg(0), much larger than previous estimates.

The shallow seas and subduction trenches are not only the main potential areas for mineral and hydrocarbon resource, but also areas of frequent tectonic earthquakes. The shallow heat flow and deep temperature distribution are crucial for understanding the process of plate subduction and magma activity. In these areas, the shallow temperature and heat flow fields are strongly disturbed by the bottom water temperature variation (BTV). Thus, its background heat flow needs to be obtained by long-term observation. After a comprehensive analysis of the technical characteristics of the existing long-term seafloor heat flow observation techniques, we proposed a scheme for tethered long-term seafloor heat flow monitoring system (TLHF), and since 2013, carried out a series of pre-developments and tests in the South China Sea, Xingycou Lake, Huguangyan Maar Lake, and Kangding shallow borehole in the Xianshuihe fault zone. The results show: (1) the self-developed long-period and low-power miniature temperature loggers can continuously work for one year in an environment of 2 similar to 36 degrees C. The tethering-type launch and recovery scheme is still feasible even under the conditions of steep terrain, 1.5 knots of velocity and without dynamic positioning. (2) In northern South China Sea, the BTV generally increases as the water depth becomes shallower, whose disturbance to the shallow temperature field cannot be ignored in shallow area. For example, the BTV is only 0.025 similar to 0.053 degrees C during 17 days in Dongsha waters with a depth of 2600 similar to 3200 m but up to 0.182 similar to 0.417 degrees C within 2 days in Xisha waters with a depth of 850 similar to 1200 m. In summer, the seafloor heat flow on the northern slope of the Taixinan Basin (with a water depth of 763 m) from 0.69 W . m(-2) at the shallow surface to -0.25 similar to-0.05 W . m(-2) at a depth of 0.83 m. (3) In Xingycou Lake and Huguangyan Maar Lake, the BTV amplitude gradually decreases and the phase lags during the process of conduction to the deep. That causes the intensity and direction of the heat flow to vary with change of seasons. In Zhonggu Village of Kangding City, the shallow ground temperature is high to 35 similar to 36 degrees C in winter but low to 28 similar to 32 degrees C in summer due to heavy rainfall in summer, and fluctuates synchronously at different depths. The surface heat flow is 0.504 W . m(-2) at the depth of m, and rises to 0.901 W . m(-2) at the depth of m. That indicates the upwelling of thermal fluid from the deep part of the Xianshuihe fault zone. These preliminary work has laid a solid foundation for the development and application of the TLHF system.

Molybdenum (Mo) isotope signature (Mo-898/95) of marine sediments is a powerful proxy for constraining marine redox conditions and early diagenetic processes. Oxic pelagic sediments have been increasingly acknowledged as an important sink for Mo in its global cycling, however, the mechanisms controlling Mo enrichment and isotope fractionation during early diagenesis in this environment are not fully understood. In this study, four sedimentary cores were sampled by full-ocean-depth lander system in the Mariana Trench, aiming to explore the processes controlling Mo partitioning in the sediments of the deepest ocean in the world. Sediments from four locations exhibit a wide range of authigenic Mo contents (Moauth, relative to continental crust), varying from 1.98 to 43 mu g/g. A positive relation between Mo and Mn contents was identified for these sediments, which suggests that Mo are mainly hosted in Fe-Mn (oxyhydr)oxides. In addition, negative correlations between Mo-898/95 values and Mo/Mn ratios are found at each site. The Mo-898/95 values of the sediments vary from-1.71 +/- 0.05 to -0.15 +/- 0.04%0, revealing similar trends towards lighter values with burial depth from three sites. However, throughout the sediment core TY41 obtained from Mariana Trench, the Mo-898/95 values of sediment remain nearly constant around -0.55%0. Supported by the abundant Fe (oxyhydr)oxides throughout this site, it is likely the sediment is affected by enhanced fluid exchange across sediment-seawater interface, and the Mo-898/95 values of dissolved Mo in pore water resemble seawater signature. Therefore, the Mo isotopic offset between dissolved Mo and sediment is similar to the expected fractionation during the adsorption to Mn oxides (similar to+ 3.0%0) and hematite (similar to+ 2.2%0), which suggests that most of the dissolved Mo is scavenged by Mn oxides and/or hematite. The extremely light Mo-898/95 values of sediments in deeper parts of the cores coincide with enhanced Mo accumulation. This indicates an even lighter isotope signature of dissolved Mo in pore water during the re-precipitation of Fe-Mn (oxyhydr)oxides upon an isotopic equilibrium for Mo adsorption to metal oxides. Both the patterns of both dissolved oxygen and nitrate suggest that the study sites are located within the nitrate reduction zone and Mn reduction occur at deeper depths below the studied interval. Therefore, the source of the isotopically light dissolved Mo in pore water was explained by the preferential release of isotopically lighter Mo from Fe-Mn (oxyhydr)oxides during the enhanced dissolution at deeper depth. In contrast, at the shallower depths of these cores (expect for core TY41), the higher Mo-898/95 values towards seawater-sediment interface suggest the dissolved Mo is more impacted by the seawater exchange. This study provides a deeper insight into the diagenetic Mo cycling in trench environments, with Fe-Mn (oxyhydr)oxides rich sediments as an important sink for Mo.

Hadal trenches have higher microbial carbon turnover rates as compared to adjacent abyssal plains. However, the source of organic carbon in the trench remains enigmatic. In this study, we show that a fraction of organic carbon is possibly derived in situ and correlated with chemoautotrophic communities supported by the fluid discharge of water-rock interaction in the trench wall, based on analysis of glycerol dialkyl glycerol tetraether (GDGT) membrane lipids, including archaeal isoprenoid GDGTs (IsoGDGTs) and bacterial branched GDGTs (BrGDGTs), in sediments and rocks of the Mariana and Yap Trenches, northwest Pacific Ocean. These trench sediments contained relative higher BrGDGTs ratios, which was a rare observation in the open ocean. The BrGDGT-to-IsoGDGT ratios ranged in 0.02–0.88 (mean = 0.10 ± 0.11) in sediments and 0.09–0.38 (mean = 0.17 ± 0.13) in altered rocks. The calculated values of branched and isoprenoid tetraether (BIT) index ranged from 0.02–0.73 (mean = 0.18 ± 0.11) in sediments and from 0.16–0.9 in altered rocks (mean = 0.37 ± 0.27). Moreover, these GDGTs exhibited similar characteristics to those of altered basalt rocks, indicating inputs of organic carbon from the trench subsurface environment. Thus, in addition to organic-rich material settling, we propose chemoautotrophic activity in oceanic crust could be an additional source of organic carbon in the deepest part of the ocean, with an important role in deep-sea carbon cycles.

Low-temperature hydrothermal systems are generally dominated by Fe?Si oxyhydroxide deposits. However, the formation process and mechanism of modern hydrothermal Fe?Si oxyhydroxides at ultra-slow spreading centers remain poorly understood. This study focused on six Fe?Si oxyhydroxide deposits collected from different sites at a typical ultra-slow spreading center, the Southwest Indian Ridge (SWIR). The mineralogical and geochemical evidence showed significant characteristics of a low-temperature hydrothermal origin. Sr and Nd isotope compositions of Fe?Si oxyhydroxide deposits at the SWIR probably reflected a combined signature of the hydro thermal fluids and seawater. Pb in the Fe?Si oxyhydroxides exhibited a close association with the substrate rocks and seawater. The Mo?ssbauer spectra and iron speciation data further provided insights into iron-bearing phases in all deposits. Two different types of biomineralized forms were also discovered in these deposits by scanning electron microscopy (SEM) analysis. Energy-dispersive X-ray spectroscopy (EDS) and nanoscale secondary ion mass spectrometry (nanoSIMS) revealed that distinct biogenic structures were mainly composed of Fe, Si, and O, together with some trace elements. Based on these findings, we propose that microbial activity plays a significant role in the formation of Fe?Si oxyhydroxides at the ultra-slow spreading SWIR.

Trench systems are some of the most important geodynamic settings on Earth and have a substantial influence on the global metal cycle. Trench environments are affected not only by episodic deposition of volcanic ash, but also exhibit a continuous water-rock chemical exchange. Ferromanganese nodules, which are archives for the marine Mn cycle, were first discovered in the southern Mariana Trench during China's major 10,000-meter hadal trench scientific expedition in 2016. Nevertheless, their geochemical characteristics and formation mechanism in sedimentary environments of the trench zone remain enigmatic. In this study, these ferromanganese nodules and surrounding sediments were examined by geochemical and microbial methods. Bulk geochemistry indicated that trench nodules are characterized by high Mn/Fe ratios and nodule textures indicate a rapid growth rate, with Mn mainly from two sources: volcanic ash alteration and fluids discharge from the trench seabed. High-resolution in situ geochemical analysis categorized microlayers of an individual nodule into three types. Type I is the interior part that has a high Mn/Fe ratio (>20) and high growth rate (52.19-3571.73 mm/Myr), and formed by the influence of fluid discharge activities around this region. Type II is laminae from the exterior part that have a hydrogenetic origin with lower Mn/Fe ratio (0.01-2.50) and lower growth rate (4.54-9.86 mm/Myr). Type III is laminae from the exterior part that have a diagenetic origin with moderately high Mn/Fe ratio (2.5-15.0) and moderately high growth rate (12.40-18.93 mm/Myr). 16S rRNA gene sequence analysis revealed that the bacterial community structures in the nodule-bearing sediment samples, dominated by Shewanella and Colwellia, were substantially different from those of reference sediment samples. For the first time, we propose a conceptual model for a geochemical Mn cycle and nodule formation in trench sedimentary environments using both geochemical and microbial data. (C) 2021 Elsevier Ltd. All rights reserved.