Deep-sea Sediments Research Articles

Niche Partitioning and Intraspecific Variation of Thaumarchaeota in Deep Ocean Sediments.

Deep-sea sediments contain a large number of Thaumarchaeota that are phylogenetically distinct from their pelagic counterparts. However, their ecology and evolutionary adaptations are not well understood. Metagenomic analyses were conducted on samples from various depths of a 750-cm sediment core collected from the Mariana Trench Challenger Deep. The abundance of Thaumarchaeota and archaeal amoA generally decreased with depth, except for an unexpected peak midway through the core. The thaumarchaeotal metagenome-assembled genomes were classified into diverse phylogenetic clusters associated with amoA-NP-γ, amoA-NP-θ, and amoA-NP-δ of ammonia-oxidising Thaumarchaeota and non-ammonia-oxidising lineages. The most abundant group was within amoA-NP-γ, which is usually found in coastal and shallow habitats, indicating potential niche expansion from marine shallow to hadal environments. This benthic group showed within-species genomic variations compared to the previously identified Hadal water group, suggesting microdiversification of hadal Thaumarchaeota along with niche separation between benthic and pelagic environments. Evolutionary adaptations associated with the benthic-to-pelagic transition included reduced genome size, loss of motility/cell adhesion, altered energy metabolism, and different mechanisms for substrate acquisition and regulation (e.g., ammonium). These findings offer new insights into the evolution of hadal Thaumarchaeota and demonstrate, for the first time, intraspecies-level genomic variation in Thaumarchaeota related to the benthic-versus-pelagic niche partitioning in the deep ocean.

In-situ strength testing of deep-sea superficial sediments using a high-resolution ball penetrometer

Deep-sea engineering requires precise evaluation of seabed sediments properties. However, traditional in-situ testing instruments struggle to identify low-strength sediment and cannot operate effectively beyond 6000 m. This study introduced a high-resolution ball penetrometer designed to assess in-situ undrained shear strength of deep-sea sediments. Firstly, the penetrometer integrated optical fibers as core sensing components with a pressure differential structure. Subsequently, the resolution and pressure resistance of the penetrometer were thoroughly examined. The penetrometer’s performance was evaluated through the laboratory, harbor basin, and deep-sea tests. Finally, the penetrometer was utilized to investigate sediment properties in a deep-sea mining area. The results demonstrate that: (a) the penetrometer’s design allows sensors to perform under pressure up to 120 MPa; (b) the pore water pressure and penetration resistance sensors maintain resolutions 0.1 kPa and 0.5 N, respectively; (c) it detects subtle changes in sediment properties more effectively than conventional T-bar and CPTu; (d) it is especially suited for testing ultra-soft sediments; (e) the sediment in the Western Pacific mining area exhibits variability, including stratified structures and weak layers.

Coccoliths as Recorders of Paleoceanography and Paleoclimate over the Past 66 Million Years

Coccolithophores are a major group of oceanic calcifying phytoplankton, and their calcite skeletal remains, termed calcareous nannofossils, are a major component of deep-sea sediments accumulating since the Jurassic. Coccolithophores play a role in both the biological pump and the carbonate pump, exporting organic and inorganic carbon, respectively, out of the surface ocean. This means that they are key responders to and recorders of ocean carbon cycle and climate changes over geological and shorter timescales, and studying these responses can help elucidate the uncertain fate of calcifying phytoplankton under projected climate change scenarios. Here, we review established and emerging approaches for reconstructing (a) mixed-layer ocean temperature, (b) marine productivity, and (c) aspects of the ocean carbon cycle, using calcareous nannofossils from deep-sea sediments. For each parameter, we discuss the different proxies that have been proposed, based on abundance or species composition, inorganic geochemistry, and/or coccolith morphology, and explore their applications and limitations in Cenozoic paleoceanography. ▪ Calcareous nannofossils can be used to reconstruct upper ocean conditions and changes over centennial to million-year timescales. ▪ Key coccolith-based proxies for temperature, productivity, and the carbon cycle are reviewed. ▪ Approaches based on assemblages, geochemistry, and morphology provide novel insights into the evolution and adaptation of coccolithophores and past climate.

Bioturbation coefficients and organic carbon degradation rates of deep-sea sediments in the central-eastern tropical Pacific

Bioturbation coefficients and organic carbon degradation rates of deep-sea sediments in the central-eastern tropical Pacific

The Microfungal Communities in Deep-Sea Sediments from the Equatorial Atlantic

Microfungi of deep-sea sediments, and especially those several meters below the water–sediment interface, are poorly studied. In this work, for the first time, microfungal communities isolated by cultivation from deep-sea sediments of the eastern part of the Equatorial Atlantic (the Romanche and Chain Fracture Zones) were investigated. Fungi were isolated from sediments sampled at each of 12 stations from horizons 1.0–4.7 m below the sediment–water interface. To study microscopic fungi, one sediment horizon was isolated from each core. The fungal abundances were within the range of 0.0–3300.0 CFU g-1 sediment dry weight. A total of 19 fungal taxa from the phyla Ascomycota (18) and Basidiomycota (1) were identified, and Mycelia sterilia 1 strain was also isolated. Seven fungal species were encountered only once. In this case, the maximum similarity of species composition, in terms of the Bray – Curtis coefficient, was 57.14% (horizons 1.0 and 3.6 m, four common species). A comparison of the taxonomic structures of fungal communities from the study area was made with those from sediments of the Indian and Pacific Oceans and other areas of the Atlantic. The fungal communities from sediments in the study area were compared with those from the Indian and Pacific Oceans and other areas of the Atlantic. From the literature data and present study results, a list of fungal species with 180 names was compiled. The fungi belonged to 97 genera, 57 families, 32 orders and 13 classes of the phyla Ascomycota, Basidiomycota, and Mucoromycota. The diversity of fungal communities was assessed using indicators of taxonomic richness (number of taxa from different ranks), proportions (genera/families, species/families, species/genera), Average Taxonomic Distinctness index (AvTD, Δ+) and Variation in Taxonomic Distinctness index (VarTD, Λ+). Four and twelve fungal classes were identified in sediments in the Eastern Equatorial Atlantic and the Indian Ocean, respectively. The species/genera proportions in the communities varied from 1.33 (Indian Ocean) to 3.8 (other areas of the Atlantic Ocean). For the fungal communities of the Eastern Equatorial Atlantic, the AvTD index value was minimal (Δ+ = 50.19), the VarTD index was maximal (Λ+ = 945.38), and they were beyond the 95% confidence interval. This was due to the small number of the fungal classes and vertical and horizontal unevenness of species distribution along taxonomic branches, which was manifested in the dominance of species of the family Aspergillaceae (78.9% of the species in the class Saccharomycetes and Eurotiomycetes), only two species belonging to the classes Sordariomycetes and one species belonging to the class Microbotryomycetes (phylum Basidiomycota). Consequently, statistically significant differences were found between the taxonomic structures of the fungal communities of the Eastern Equatorial Atlantic and the other regions of the World Ocean, which are due to the insufficient amount of data obtained on the species composition of fungi in the sediments of this area. The study did not reveal any pattern in the change in the number of fungal species and their abundance in relation to the water characteristics (temperature, pH, and salinity), horizon depth in the sediment core, sediment type, or sampling station location in the Romanche and Chain Fracture Zones.

Phytotoxic and Antifungal Activity of (-)-Penienone Produced by Penicillium palitans (Ascomycota) Isolated from Deep Sea Sediments in the Southern Ocean, Maritime Antarctica.

Metabolites isolated from Penicillium palitans, obtained from deep sea sediments in the maritime Antarctica, were investigated for phytotoxic and antifungal activities. The fungus was submitted to solid state fermentation, and its crude extract was produced. Chromatographic separations of the P. palitans methylene chloride crude extract led to the isolation of two analogous compounds: (-)-penienone (1) and (-)-palitantin (2). The phytotoxic activity of these compounds was assessed against seeds of Lactuca sativa and Agrostis stolonifera. Both compounds demonstrated activity exclusively in the monocotyledonous model, A. stolonifera. At a concentration of 1 mg mL-1, (-)-penienone caused 100 % inhibition of seed germination, while (-)-palitantin had only moderate activity on germination and growth of seedlings at this concentration after 7 days of treatment. Against Lemna paucicostata, at 57 μM, (-)-penienone inhibited growth by 50 % (IC50). (-)-Palitantin was much less active. When evaluated for their antifungal activity against Colletotrichum fragariae, only (-)-penienone had an IC50 value of 0.3 μM. The strain P. palitans was shown to be a promising source of phytotoxic and antifungal compounds, suggesting that extremophilic fungi, such as wild Penicillium strains from Antarctica, have the potential to yield novel prototype molecules for new pesticide development.

Early Ediacaran Xueqiong ophiolite in the East Kunlun Orogen, northern Tibetan Plateau: Insights into the early evolution of the Proto-Tethys Ocean

Reshaping the tectonic evolution of the Proto-Tethys Ocean is of paramount significance for comprehending the ocean-continent transitions since the Neoproterozoic. Nevertheless, the intricacies of this evolution, particularly during its early stage, remain a pivotal issue that needs further deciphering. The discovery of early Ediacaran ophiolites within the East Kunlun Orogen offers a crucial clue for exploring this issue. This study presents intergrade field geology, zircon U–Pb geochronology and whole-rock geochemistry on the related rocks from this specific ophiolite identified in the Xueqiong area, easternmost East Kunlun Orogen. Field investigations reveal the currently remaining dismembered Xueqiong ophiolite merely include gabbro, basalt and chert, which are in the form of tectonic blocks within an ophiolitic mélange. Zircon U–Pb dating results show that the gabbro samples from two near rock slices yield consistent weighted mean ages of 597 ± 5 Ma and 601 ± 2 Ma, whereas the basalt sample gives a similar age of 600 ± 6 Ma as well, indicating the magmatic component of the ophiolitic suite was formed at ca. 600 Ma during the early Ediacaran. Geochemical analysis indicates that all the gabbro and basalt share a common parental magma. Patterns of rare earth and trace elements show their properties between the enriched mid-ocean ridge basalt and oceanic island basalt models, along with their characteristic trace element covariances aligning with enriched mid-ocean ridge basalt, suggesting a mantle source similar to the enriched mid-ocean ridge basalt, with interaction with the residual asthenosphere mantle material. Relatively low SiO2, TFe2O3 and MnO2 contents and flat rare earth element patterns of chert, suggesting its lithogenic property and restricted basin setting near the continental margin. These findings collectively indicate that the Xueqiong ophiolite represents remnants of the oceanic lithosphere and overlying deep-sea sediments from the early-stage evolution of the Proto-Tethys Ocean, when the ocean evolution might not have been fully mature and oceanic floor sedimentation capable of receiving terrigenous detritus. This evidence further supports that the Proto-Tethys Ocean in the East Kunlun domain could be traced back at least to the early Ediacaran.

A review on water jet-sediment coupling and critical techniques during deep-sea polymetallic nodules collection

With the increasing development demand of the modern energy industry, heightened focus has been directed toward the exploitation of deep-sea polymetallic mineral resources. The water jet-sediment coupling under the jet action plays a crucial role in the collection processes. Understanding this mechanism is essential for achieving both efficient collection and minimal environmental disturbance. In this paper, from the deep-sea sediments to which the polymetallic nodules (PMNs) adhere, the characteristics of sediments are studied, focusing on physical and mechanical properties, cementation effects, rheological, and dynamic properties. Upon this foundation, the evolving patterns of flow field structure and characteristics (changes of pressure and velocity), jet-induced sediment erosion and plume generation (erosion pattern, disturbance volume), as well as the characteristics of nodules stripping-movement (such as lift-up force and movement law) are analyzed. Furthermore, following an analysis of the jet-sediment coupling mechanism, the technical applications of jet collection (collection structure design and operation parameter matching) are discussed. Finally, the existing problems and future research directions are discussed. This research reviews the coupling mechanism of water jet and sediment in the process of jet collection, which provides a reference for the design of collection equipment with high efficiency and low-disturbance characteristics.

Effects of climate change and methane-rich fluid activity on sedimentary sulfur geochemical records in the northern South China Sea since mid-Pleistocene

Microbial sulfate reduction (MSR) and associated pyritic sulfide formation are important diagenetic processes in marine sediments. The sulfur isotopic composition of pyrite (δ34Spyr) is proven to be sensitive to changes in sedimentation rates and the content and reactivity of organic carbon, especially on the continental shelves and upper slopes (water depth < 350 m). However, the diagenetic responses of sulfur to variations in climatic and depositional conditions in the deep-sea sediments are still poorly understood. This study combines element contents and isotopes to characterize diagenetic interplays of sulfur, organic carbon, and methane in the continental slope sediments of the northern South China Sea since the mid-Pleistocene. Our data suggest that the total organic carbon (TOC) increased during glacial times, implying enhanced primary productivity due to increased nutrient supply by the East Asian Winter Monsoon, in addition to efficient transfer of organic carbon and better preservation of organic carbon due to reduced bottom water oxygen. Total sulfur and chromium reduction sulfur contents varied concomitantly with the TOC, suggesting an increased burial of organic carbon that enhanced the organoclastic sulfate reduction (OSR) and the formation of authigenic pyrite. The environmental changes did not induce a significant shift in δ34Spyr, due most likely to relatively low sedimentation rates and large fractionation in sulfur isotope through OSR during the glacial-interglacial cycles. Instead, it is hypothesized that the sulfate-driven anaerobic methane oxidation promoted the formation of a higher amount of authigenic pyrite. Consequently, it created a closed diagenetic system leading to positive excursions in δ34Spyr at the sulfate-methane transition zone. Our results suggest the vulnerability of pyrite formation and its sulfur isotopic composition to the changes in monsoon-driven primary productivity and the methane-rich fluid migrations in the continental margin sediments. This study complements the growing evidence for the local diagenetic controls on sedimentary sulfur geochemical records by highlighting the importance of early diagenesis in paleoenvironment reconstruction based on the content and sulfur isotopic composition of pyrite.

On the diversity, phylogeny and biogeography of cable bacteria.

Cable bacteria have acquired a unique metabolism, which induces long-distance electron transport along their centimeter-long multicellular filaments. At present, cable bacteria are thought to form a monophyletic clade with two described genera. However, their diversity has not been systematically investigated. To investigate the phylogenetic relationships within the cable bacteria clade, 16S rRNA gene sequences were compiled from literature and public databases (SILVA 138 SSU and NCBI GenBank). These were complemented with novel sequences obtained from natural sediment enrichments across a wide range of salinities (2-34). To enable taxonomic resolution at the species level, we designed a procedure to attain full-length 16S rRNA gene sequences from individual cable bacterium filaments using an optimized nested PCR protocol and Sanger sequencing. The final database contained 1,876 long 16S rRNA gene sequences (≥800 bp) originating from 92 aquatic locations, ranging from polar to tropical regions and from intertidal to deep sea sediments. The resulting phylogenetic tree reveals 90 potential species-level clades (based on a delineation value of 98.7% 16S rRNA gene sequence identity) that reside within six genus-level clusters. Hence, the diversity of cable bacteria appears to be substantially larger than the two genera and 13 species that have been officially named up to now. Particularly brackish environments with strong salinity fluctuations, as well as sediments with low free sulfide concentrations and deep sea sediments harbor a large pool of novel and undescribed cable bacteria taxa.

Improved resolution of microbial diversity in deep-sea surface sediments using PacBio long-read 16S rRNA gene sequencing.

The PacBio long-read platform, with its exceptional base-level resolution exceeding 99%, has advanced our comprehension of deep-sea microbial diversity. By comparing microbial community analyses conducted using the Illumina short-read and PacBio long-read sequencing platforms, we have provided an enhanced understanding of fine spatial-scale patterns in microbial community diversity with depth across a deep-sea sediment core, as well as methodological insights that will be valuable for future research in this field.

Spatial heterogeneity of sediments on a topographically diverse descending plate at convergent margins: An example from the outer slope of the Mariana Trench

Unraveling deep ocean sediment composition and its controlling mechanisms is crucial to understanding the interplay between the Earth's surface and the deep Earth. Convergent margins are generally featured by various seafloor landforms, but how submarine topography affects sediment accumulation processes remains poorly understood. Here we focus on submarine surface sediments from the outer slope of the Mariana Trench (covering water depths of 1218–6310 m) and combine microscopic observation, clay mineralogy, and elemental geochemistry to characterize sediment composition and to reveal physical, chemical, and biological sedimentary processes at topographically diverse convergent margins. Smear slide observations indicate that foraminifera shells dominate in shallow areas, while pelagic clays and diatom oozes prevail in trench and slope areas. Clay minerals primarily consist of illite (26–98 %) with low crystallinity index values and diverse chemical index values, followed by chlorite (0–45 %) and kaolinite (0–29 %). Smectite (0–61 %, 46 of 70 samples have smectite contents <5 %) of the samples from the outer slope area is low but appears dominant close to the trench. Bulk element compositions are mainly affected by the relative calcareous, siliceous, and lithogenous components that dissolved or accumulated inconsistently as submarine topographic feature changes. Our results imply that submarine topography and sediment provenance lead to significant heterogeneity of sediment composition at topographically diverse convergent margins. The submarine topographic relief, together with associated bathymetry and deep-sea circulation primarily controls carbonate deposition on the Caroline Plateau, and influences the sinking particles (including diatom shells and arc volcanic-sourced materials) accumulation in the trench basin. Continent-derived aeolian dust introduces illite-rich detritus, while localized volcanism potentially contributes to smectite-rich sediment in certain topographic provinces. We propose that the deep sea sedimentation processes in the Mariana convergent system are more diverse and complex than those at the margins with relatively plain topographic characteristics (e.g., the Japan Trench). The significant heterogeneity on descending plates along convergent margins may lead to various subduction behaviors. These findings contribute to a better understanding of deep ocean sediment source-to-sink systems and have implications for Earth's material cycle and multi-sphere interaction studies.

Bacterial Diversity in Deep-Sea Sediment of West Pacific Nodule Province

Dense polymetallic nodule fields are found in different areas of the Pacific and Indian Oceans. However, limited knowledge exists about microbial diversity, processes and functions in deep-sea polymetallic nodule sediments. This study investigated microbial diversity, composition and function in sediments from various locations and depths in a western Pacific polymetallic nodule province. Sediment cores were collected, DNA extracted, and the V3–V4 regions of the 16S rRNA gene were sequenced using Illumina MiSeq. The test results show that the abundance and diversity of microbial communities in sediments from different sites vary significantly. The dominant microbial communities at the family level in the three sediment samples were all mainly Marinobacteraceae and Alcanivoracaceae. Sediment samples from core 1 had similar microbial structures and microbial community functions. Surface sediment had higher species richness, diversity and evenness than the middle layer. The dominant phylum at different depths was consistent. There was significant spatial heterogeneity in the microbial community within sediments from polymetallic nodule regions. This study expands on our knowledge of the spatial and vertical distribution of microbial community diversity beneath polymetallic nodules in deep-sea settings.

Revealing the metabolic potential and environmental adaptation of nematophagous fungus, Purpureocillium lilacinum, derived from hadal sediment.

The extreme environment shapes fungi in deep-sea sediments with novel metabolic capabilities. The ubiquity of fungi in deep-sea habitats supports their significant roles in these ecosystems. However, there is limited research on the metabolic activities and adaptive mechanisms of filamentous fungi in deep-sea ecosystems. In this study, we investigated the biological activities, including antibacterial, antitumor and nematicidal activity of Purpureocillium lilacinum FDZ8Y1, isolated from sediments of the Mariana Trench. A key feature of P. lilacinum FDZ8Y1 was its tolerance to high hydrostatic pressure (HHP), up to 110 MPa. We showed that HHP affected its vegetative growth, development, and production of secondary metabolites, indicating the potential for discovering novel natural products from hadal fungi. Whole-genome sequencing of P. lilacinum FDZ8Y1 revealed the metabolic potential of this piezotolerant fungus in carbon (carbohydrate metabolism), nitrogen (assimilatory nitrate reduction and protein degradation) and sulfur cycling processes (assimilatory sulfate reduction). Transcriptomic analysis under elevated HHP showed that P. lilacinum FDZ8Y1 may activate several metabolic pathways and stress proteins to cope with HHP, including fatty acid metabolism, the antioxidant defense system, the biosynthetic pathway for secondary metabolites, extracellular enzymes and membrane transporters. This study provides valuable insights into the metabolic potential and adaptation mechanisms of hadal fungi to the challenging conditions of the hadal environment.

A baroduric immobilized composite material promoting remediation of oil-polluted sediment at typical deep-sea condition: The performances and potential mechanisms

Contriving immobilized bioreagent is of great significance to enhance bioremediation of marine oil pollution. However, there remains a notable scarcity of correlational study conducted at deep sea condition. Herein, we first developed a baroduric microsphere encasing biotic and chemical materials to remediate oil-contaminated sediments at deep-sea microcosm. Total oil degradation efficiency of microsphere-treated group reached 71% within a month, representing an approximate 35% increase compared to natural remediation. Absorption and biodegradation by microsphere provided a comparable contribution to oil elimination. Together with scanning electron microscope observation, the physical mechanism was that the reticulate structure of microsphere surface facilitating oil adsorption and bacteria attachment. Via metabarcoding analysis for meta and metabolically-active microbes, we demonstrated the primary working center was located at the microsphere. Proteobacteria, Firmicutes, Bacteroidota and Desulfobacterota were the key activated bacteria. More importantly, we revealed the ecological mechanisms were associated with the following aspects: 1) the addition of microsphere significantly improved the metabolic activity of bacteria (particularly including several oil-degrading taxa); 2) the microspheres enhanced ecological stability and microbial functional diversification during bioremediation; 3) expressing activity of pathways involving oil component degradation, biosurfactant production, biofilm architecture, biogeochemical and energy cycling all were observed to be up-regulated in microsphere-treated samples. Altogether, our results provide important theoretical guidance and data support on application of immobilization technology in removing in-situ oil pollution of deep-sea sediment.

Calcite Dissolution-Reprecipitation Reactions Are a Key Control on the Sr/Ca, Mg/Ca and δ88/86Sr Compositions of Himalayan River Waters

Silicate weathering on the continents is thought to play a critical part in regulating global climate on geological time scales but determination of the magnitude of silicate weathering fluxes is frustrated by the complexity of the weathering processes. Here we present analyses of stable Sr-isotopic compositions (𝛿88/86Sr) in a suite of river waters and bedloads from the Himalayas in Nepal to establish the lithological controls on 𝛿88/86Sr values and the secondary processes that impact carbonate weathering. A control on 𝛿88/86Sr values is lithology with the rivers in carbonate-dominated catchments marginally lower (∼0.07 ‰) than in silicate-dominated catchments. However, as for 87Sr/86Sr ratios, 𝛿88/86Sr values of carbonates are altered by silicate-carbonate mineral exchange during metamorphism. The major potential secondary control on 𝛿88/86Sr values in Himalayan catchments is precipitation of secondary calcite responsible for the marked elevation of Sr/Ca and Mg/Ca ratios in the waters. We re-evaluate the mechanisms for secondary calcite formation and conclude that a balanced solution and re-precipitation process, driven by the relative instability of the primary Mg-rich calcite, provides a better explanation for the elevated Sr/Ca ratios than simple precipitation from a highly over saturated solution. This solution-re-precipitation process is akin to that invoked to explain diagenesis of deep-sea sediments. The mechanism of secondary calcite formation impacts the distinction of cation inputs from carbonate and silicate minerals. The correlation between 𝛿88/86Sr water-calcite fractionations, Sr/Ca partition coefficients and precipitation rates allows the calcite re-precipitation rates to be inferred from the covariation of water 𝛿88/86Sr values and Sr/Ca ratios. These rates are very low (&lt;10-8 mol m-2 s-1) but are consistent with those inferred from field estimates of the amount of calcite re-precipitated, the surface area of carbonate exposed to weathering and the calcite weathering flux. The low precipitation rates are also consistent with previously reported 𝛥44/40Ca isotope fractionations of ∼−0.2‰. The calcite reprecipitation rates are comparable to silicate weathering rates previously inferred from Li-isotopic compositions which is consistent with calcite re-precipitation taking place very close to equilibrium following the initial rapid saturation of the fluids by calcite.

Carbonate dissolution fluxes in deep-sea sediments as determined from in situ porewater profiles in a transect across the saturation horizon

Despite their importance for long-term climate regulation, the rates and mechanisms of seafloor carbonate dissolution are poorly understood, especially with respect to calcite saturation and the role of sedimentary metabolic CO2 production. Here, we present results from an in situ porewater sampler deployed at the Cocos Ridge in the eastern equatorial Pacific, where we examine seafloor carbonate dissolution in locations with bottom water Ωcalcite ranging from 1.0 to 0.84 (1600–3200 m). With cm-scale resolution from the sediment–water interface to 35 cm, we present porewater profiles of total alkalinity, pH, dissolved inorganic carbon (DIC), δ13C of DIC, Ωcalcite, [Mn], [Ca], and [Sr], as well as solid phase porosity, % CaCO3, and % organic C. These profiles provide evidence that deep-sea sedimentary carbonate dissolution occurs via sediment-side control, wherein dissolution is dominated by sedimentary processes rather than strictly bottom water saturation state. We estimate dissolution fluxes using three independent approaches: alkalinity fluxes, δ13C of DIC combined with DIC fluxes, and [Ca] fluxes. We report seafloor dissolution fluxes with uncertainties < 38 %: 40 ± 15, 98 ± 20, 100 ± 32, and 89 ± 27 μmol CaCO3/m2/day at sites 3200, 2900, 2700, and 1600 m deep, respectively. The magnitude of dissolution fluxes is a function of bottom water saturation state (Ωcalcite), bottom water dissolved oxygen, and sedimentary CaCO3 content, but not correlated with any of these parameters independently. We observe dissolution occurring at all stations, including where bottom water is saturated with respect to calcite, and present evidence that this occurs through respiration-driven dissolution within the sediment. At all sites, porewater Ωcalcite decreases below bottom water values before increasing toward saturation deeper in the sediment. Using the δ13C of DIC, we partition the DIC fluxes across the sediment–water interface and find 21–48 % of DIC is sourced from CaCO3 dissolution, with the remainder sourced from organic matter respiration. We present a sedimentary mass balance, assembled with dissolution rates and mass accumulation rates obtained through Δ14C of foraminiferal calcite, and calculate CaCO3 burial efficiencies between 2 and 67 %, inversely correlating with water depth. Our results also provide evidence that net chemical erosion of 5,000––10,000 year old carbonate is occurring at the deepest site. Aerobic organic C respiration coupled with sedimentary CaCO3 dissolution, as documented here, will provide more alkalinity to bottom waters than from undersaturation-driven dissolution alone. This process can neutralize anthropogenic CO2 at the seafloor in a larger range of saturation states than previously estimated.

Ichnotaxonomy of microboring traces in marine aphotic depths

Microboring traces in carbonate skeletal fragments deposited in aphotic depths of the oceans are studied, evaluated, and described with respect to their marine ecology and palaeoecology as well as ichnotaxonomy. Sand-size deep sea sediment particles dredged from depths ranging between 600 and 3266 m of the Bermuda Pedestal, Central Atlantic Ocean, the Florida Escarpment, the Mediterranean Sea, the Red Sea and the Indian Ocean were studied. Following ichnological rules, trace fossils are described as ichnogenera and ichnospecies, defined as products of organismal behaviour. This, in our view, refers to the growth habit of microboring organisms in response to environmental stimuli within the substrate they penetrate. The problem of palaeobathymetry is discussed in conjunction with the distinction between light-dependent and light independent microboring organisms, with the emphasis on the latter. We considered this distinction to be important because only the light-dependent microborers have been recognized as indicators of ancient depositional depths, whereas the light-independent ones are expected to occur at any depth, subject to the availability of organic nutrients. Microboring organisms often leave morphologically similar traces due to convergent evolution. Their responses may change during their life cycle; they may produce different traces when pursuing their vegetative vs. reproductive functions. New ichnotaxa are described. All are regarded as organotrophs given their aphotic zone deep sea origin. This work presents the most complete set of deep sea microbial euendolith traces, to date.

Metal regeneration during an ex-situ disturbance experiment on deep-sea sediments from the polymetallic nodule area of western Pacific

Metal regeneration during an ex-situ disturbance experiment on deep-sea sediments from the polymetallic nodule area of western Pacific

Optimization of high-precision Bekker test plate structure for rarefied soils–example of deep-sea subsoils

The Bekker apparatus is an important tool for measuring soil bearing capacity; however, it is not entirely suitable for deep-sea sediments, and the size of the loading plate can affect the accuracy of the Bekker bearing parameters. This paper employs the Coupled Eulerian-Lagrangian (CEL) method to explore how different conditions impact Bekker bearing parameters. As the size of the loading plate gradually increases, the soil compression mechanism and load path reach a relatively stable state, leading to the stabilization of Bekker bearing parameters. The study proposes a range of plate sizes for deep-sea soils with varying strengths, noting that the physical mechanisms of low-strength soils are relatively simple, making them more consistent with the ideal conditions described by Bekker’s theory. Furthermore, an analysis of the relationship between Bekker bearing parameters and penetration depth reveals that the parameters corresponding to different strength soils vary almost parallel with depth. This is attributed to the same settlement path, where the distribution of loads and the geometrical shapes of the supporting structures remain unchanged, resulting in a similar trend in the variation of Bekker parameters. The research findings provide a theoretical basis for analyzing the bearing capacity characteristics of soft deep-sea soils.