Abyssal Plain Research Articles

Assessing the reliability of species distribution models under changing environments: a case study on cetaceans in the North-East Atlantic

1.Species Distribution Models (SDMs) assume stable relationships between species and their environment from which predictions are made. These relationships are likely to vary with changing environments, and predictions might depend more on modelling choices than on empirical data. Reliability assessments of predictions are necessary to support policy-making.2.We identified environmental extrapolations among potential predictions of cetaceans’ distribution from 2005 to 2020 in the North-East Atlantic and calculated the percentages of calibration data with similar environments (nearby data), supporting these predictions. Thus, the assessment of reliability is generic, as evaluated before model fitting.3.Predictions on continental shelves were extensively supported by the calibration data and were more reliable throughout the year than predictions on continental slopes and abyssal plains, which were more supported in summer. Predictions off Portugal were particularly uncertain due to the lack of surveys in this region of deep, warmer waters with seamounts.4.The high effort between May and July led to a southern winter shift of nearby data, following the decrease in temperatures. A large part of the predictions between December and April was extrapolated due to the low coverage of the winter primary productivity drops, spring peaks and cold waters. They were based on data collected during other seasons and regions, and given the large spatial extent of the area, and the seasonality and regionality of the cetacean distributions, reliable winter predictions might be restricted to geographic areas where winter surveys took place. These predictions are more uncertain and warrant caution.5.Synthesis and applications: extrapolations and nearby data highlighted environmental gaps to predict cetacean distributions in the North-East Atlantic, which could be covered by future surveys. This informs model users of regions and periods when predictions reliability becomes uncertain. SDMs are invaluable tools for supporting conservation applications and, despite the warnings that have been issued, the degree of information available for predicting distribution is still rarely reported. We recommend adding this assessment as routine information on the reliability of predictions.

Molecular characteristics of dissolved organic matter in the porewater of "Challenger" Deep sediments, Mariana Trench

Hadal trenches (> 6,000 m water depth) have been revealed as hotspots of organic carbon deposition and mineralization. Here, we present the molecular compositions of porewater dissolved organic matter (DOM) at the “Challenger” Deep (Site MT02; 10,954 m water depth) sediments of the Mariana Trench and the adjacent abyssal plain sediments (Site MT04; 5,800 m water depth) using ultra high-resolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS). The "Challenger" Deep sediments are characterized by higher abundance of recalcitrant compounds, mainly composed of highly unsaturated compounds (79.7%) and carboxyl-rich alicyclic molecules (57.1%), compared to the abyssal plain sediments (68.3%&51.5%). Principal component analysis suggests that TOC content in the sediments exerts an important control on the molecular characteristics of porewater DOM. It is likely that higher TOC content triggers elevated microbial-mediated organic matter mineralization, thereby forming more refractory organic matter compounds. These results improved the knowledge of the poorly-understood DOM compositions and microbial organic matter degradation in the ultra-deep, extreme ocean environment.

Vertical Velocity 3D Estimates from the Linear Vorticity Balance in the North Atlantic Ocean

Abstract Large-scale three-dimensional vertical movements of the world’s oceans play a fundamental role in their functioning, yet they are still overlooked. This study aims to enhance our understanding of the relationship between horizontal and vertical flows as a function of depth to derive some major characteristics of the w field from a relatively well-constrained variable. For this purpose, we analyze an eddy-permitting ocean general circulation model simulation, focusing on the linear vorticity balance (LVB) over the North Atlantic basin. For spatial scales greater than 5°, over more than 50-yr average, the LVB holds with less than 10% error over the whole depth range of most of the subtropical upper tachocline–upper ocean and part of the tropical one, away from western boundary currents and zonal tropical flows. The spatial extent of the balance diminishes in the intermediate ocean, particularly in the western half of the basin. Within the deep tachocline, the balance holds over large portions of the abyssal plains. Based on these results, the β-plane geostrophic vertical velocity is derived from the vertical indefinite integral of the geostrophic LVB from the surface. While its time mean aligns only qualitatively with the model reference w, it accurately captures the interannual variability over most of the basin, even close to eastern boundaries. However, it cannot reproduce the variability in the lower tropics, the western boundary, and the North Atlantic Current system. Our results emphasize the dominance of geostrophy in establishing most of the ocean model vertical flow variability.

Early extensional salt tectonics controls deep-water sediment dispersal

Predicting the distribution of sedimentary facies during the early stages of deformation of salt-detached continental margins is key to constraining the location and stratigraphic architecture of hydrocarbon and CO2 reservoirs, as well as to understanding the oceanic carbon cycle. Despite its importance, we still have a relatively poor understanding of salt-sediment interactions during the early phases of extensional salt tectonics, mainly because subsequent salt-related deformation and/or deep burial of the related stratigraphic succession means the related deposits are poorly imaged in seismic reflection data and/or not penetrated by borehole data. We investigated the interplay between early extension-related salt deformation and deep-water sediment dispersal using 3-D seismic reflection data from the northern Levant Basin offshore Lebanon. Our results indicate that salt tectonics has two contrasting impacts: Whereas slope-parallel faults favor early sediment transfer along downslope-oriented corridors to the abyssal plain, slope-normal faults and ramp-syncline basins trap land-derived sediments, hampering or delaying their transport to the abyssal plain. These results help refine source-to-sink models of turbidite systems developing in young salt basins, highlighting the crucial role of extensional tectonics in controlling sediment dispersal and the development of intra-slope depocenters and emphasizing the impact of fault strike, ramp-syncline basin evolution, and salt thinning. Our study has significant implications for predicting the location of deep-water coarse-grained sediment and the preservation of land-derived organic carbon in mature, more structurally complex, salt basins.

Small topographical variations controlling trace maker community: Combining palaeo- and neoichnological data at the Porcupine Abyssal Plain

Ichnological research has generally assumed that abyssal plains are dominated by quiescent, homogenous environmental conditions. Thus, deep-sea trace fossil assemblage changes have been usually linked to significant spatial and temporal environmental variations. Here, we conducted a comparative ichnological analysis between a small abyssal hill (50 m elevation) and the surrounding abyssal plain; this modest bathymetric variation is known to generate substantial environmental heterogeneity for the benthic fauna community of the Porcupine Abyssal Plain (c. 4850 m depth), Northeast Atlantic. Based on X-ray data from a 5 × 5 core grid emplaced in two box cores, we compared hill and plain bioturbational sedimentary structures, including trace fossil assemblages (e.g., ichnotaxonomy) and biodeformational structures (e.g., mixed-layer depth). We observed that topographically-enhanced near-bottom currents over the hill likely produce significant changes in depositional dynamics and sediment properties (e.g., grain size, organic matter content and degradation), and control specificities of bioturbational sedimentary structures (e.g., trace fossils, mixed layer attributes such as thickness, mottled background, discrete traces). Palaeoichnological data suggested that the abyssal plain had experienced consistent conditions during the last thousands of years while the abyssal hill recorded improving environmental conditions for the trace maker community. Our results highlight the complexity of the deep-sea environment, demonstrating that small changes in bioturbated sedimentary assemblages appear even within the same box core (m-scale), and that substantial changes can occur due to environmental heterogeneity (e.g., subtle topographic variations) at the local scale (km-scale). Considering the vast global extent of abyssal hill terrain, we suggest that their influence on the bioturbational sedimentary record may be significantly under-appreciated and require more attention in palaeoenvironmental reconstructions.

Predicting particle catchment areas of deep-ocean sediment traps using machine learning

Abstract. The ocean's biological carbon pump plays a major role in climate and biogeochemical cycles. Photosynthesis at the surface produces particles that are exported to the deep ocean by gravity. Sediment traps, which measure deep-carbon fluxes, help to quantify the carbon stored by this process. However, it is challenging to precisely identify the surface origin of particles trapped thousands of meters deep due to the influence of ocean circulation on the sinking path of carbon. In this study, we conducted a series of numerical Lagrangian experiments in the Porcupine Abyssal Plain region of the North Atlantic and developed a machine learning approach to predict the surface origin of particles trapped in a deep-ocean sediment trap. Our numerical experiments support the predictive performance of the machine learning approach, and surface conditions appear to provide valuable information for accurately predicting the source area, suggesting a potential application with satellite data. We also identify factors that potentially affect prediction efficiency, and we show that the best predictions are associated with low kinetic energy and the presence of mesoscale eddies above the trap. This new tool could provide a better link between satellite-derived sea surface observations and deep-ocean sediment trap measurements, ultimately improving our understanding of the biological-carbon-pump mechanism.

Detrital zircons record the provenance and source-to-sink process in the northwestern margin of the South China Sea

The northwestern continental margin of the South China Sea, extending from the broad shelf and canyon-growth slope across the Xisha Trough, represents a relic crustal rift that eventually connects to the abyssal plain. However, the provenance and source-to-sink process of sediments in this special topography remains unclear. In this study, we present the detrital zircon U-Pb geochronology of surface sediments from the northwestern margin of the South China Sea to identify the sediment provenance, track the transport pathway, and understand the source-to-sink process in this margin. The results showed that detrital zircons exhibit a wide range of U-Pb ages from 3,062.9 to 41.5 Ma and are characterized by peaks centered on 140–154 Ma, 240–258 Ma, 425–452 Ma, and 738–991 Ma. Statistical analysis indicated that the clastic sediments on the shelf closely resemble those sourced from the Pearl River system. Conversely, sediments on the slope and abyssal plain demonstrated characteristics indicative of multiple sources, primarily originating from the Pearl River, with a minor contribution from the Red River. Further quantitative study revealed a progressive increase (from 8.2% to 43.2%) in the prevalence of Red River-derived sediments with ascending water depth on the shelf and slope, whereas sediment influx in the deepest sector remains predominantly sourced from the Pearl River. This study emphasizes the controls of topography and currents in modulating the transportation of coarse-grained and fine-grained sediments within the source-to-sink system and provides a favorable solution for reconstructing the sedimentary process in continental margins.

Oceanitis abyssalis sp. nov., a new deep-sea fungus from sunken wood collected at the depth of 5707 m in the Northwest Pacific Ocean

A new deep-sea fungus, Oceanitis abyssalis sp. nov. was described based on analyses of the LSU rDNA sequences and morphological characteristics. The new species was found on a branch of Prunus sp. collected at 5707 m depth of the abyssal plain in the Northwest Pacific Ocean, east off the Japan Islands. To the best of our knowledge, our discovery is the deepest record of the genus Oceanitis and also for the obligate deep-sea fungi. Morphologically, Oceanitis abyssalis is closely similar to O. scuticella Kohlmeyer and both were collected from deep-sea water. Oceanitis abyssalis can be distinguished from O. scuticella by having unicellular ascospores, smaller deciduous polar appendages and occasionally tree-like appendages, semi-persistent asci, smaller ascomata that are drop-shaped, and cream-colored. We also reviewed the taxonomic status of O. scuticella. The phylogenetic position of the authentic O. scuticella remains unclear due to the lack of genetic information and the inaccessibility of the type specimen. However, our investigation of the molecular and morphological characteristics of specimens previously identified as O. scuticella suggests that they include several cryptic, undescribed species. Notably, the Kuril-Kamchatka Trench material (M0229768), previously identified as O. scuticella, is suggested to be a representative of Oceanitis abyssalis.

Unveiling the Presence of Short- and Medium-Chain Chlorinated Paraffins in the Hadal Trenches of the Western Pacific Ocean.

This study delves into the unexplored distribution and accumulation of chlorinated paraffins (CPs), pervasive industrial contaminants used as flame retardants and plasticizers, within the hadal trenches, some of Earth's most isolated marine ecosystems. Analysis of sediments from the Mussau (MS) and Mariana trench (MT) reveals notably high total CP concentrations (∑SCCPs + ∑MCCPs) of 10,963 and 14,554 ng g-1 dw, respectively, surpassing those in a reference site in the western Pacific abyssal plain (8533 ng g-1 dw). In contrast, the New Britain Trench (NBT) exhibits the lowest concentrations (2213-5880 ng g-1 dw), where CP distribution correlates with clay content, δ13C and δ15N values, but little with total organic carbon and depth. Additionally, amphipods from these trenches display varying CP levels, with MS amphipods reaching concerning concentrations (8681-16,138 ng g-1 lw), while amphipods in the MT-1 site show the lowest (4414-5010 ng g-1 lw). These bioaccumulation trends appear to be primarily influenced by feeding behaviors (δ13C) and trophic levels (δ15N). Utilizing biota-sediment accumulation factor values and principal component analysis, we discern that CPs in sediment may come from surface-derived particulate organic matters, while those in amphipods may come from the above carrion. Our findings elucidate the profound impacts of the emerging pollutants on the Earth's least explored marine ecosystems.

Provenance shift of the abyssal plains in the Southwest sub-basin of the South China Sea at ∼ 8 Ma: Tectonics & climate changes implication

Tectonic processes and climatic changes are recognized as two major drivers of erosion along the southeastern margin of the Tibetan Plateau and consequently are the main factors controlling depositional patterns along the margins of the South China Sea. However, the role of tectonics and climate in governing the types and patterns of sedimentation in the abyssal plains is relatively little known. The results of International Ocean Discovery Program (IODP) drilling in the abyssal plains of the Southwest sub-basin of the South China Sea show that the multi-trace element and rare earth element (REE) character of sediments older than ∼8 Ma are in disorder. εNd(0) and 87Sr/86Sr values exhibit large fluctuations, which implies temporally variable sediment sources during the early post-spreading stage of the South China Sea. High εNd(0) and low 87Sr/86Sr values and abundant Cenozoic (13–35 Ma) zircon grains in the sediments suggest relatively juvenile sources for their origins, such as the Cagayan Ridge and Palawan Block in the south before ∼8 Ma. Multi-trace element and REE patterns of sediments younger than ∼8 Ma are more uniform. Low εNd(0) and high 87Sr/86Sr values of the sediments indicate that more continentally-derived sediments were transported to the abyssal plain. Coastal rivers in SE Vietnam, and the Mekong and Red Rivers in the west gradually became the major detrital sources of abyssal sediments in the Southwest sub-basin after ∼8 Ma. The switching of the sediment sources from the south to the west is consistent with the progressive uplift of the Vietnamese Central Highlands (VCH) and the SE Tibetan Plateau margin during the Late Miocene. The strengthened summer monsoon since ∼5 Ma affected not only the marginal basins, but also the deep abyssal basins. We argue that the regional tectonics shaped and controlled the shift of provenance and sediment routing system, whereas the East Asian monsoon affected the sediment flux to the abyssal plains of the Southwest sub-basin in the South China Sea.

Diversity and dynamics of bacteria from iron-rich microbial mats and colonizers in the Mediterranean Sea (EMSO-Western Ligurian Sea Observatory): Focus on Zetaproteobacteria.

Autotrophic microaerophilic iron-oxidizing Zetaproteobacteria seem to play an important role in mineral weathering and metal corrosion in different environments. Here, we compare the bacterial and zetaproteobacterial communities of a mature iron-rich mat together with in situ incubations of different Fe-bearing materials at the EMSO-Ligure West seafloor observatory, which is located on the abyssal plain in the NW Mediterranean Sea. Our results on bacterial communities enable us to make a clear distinction between those growing on mild steel anthropic substrata and those developing on basaltic substrata. Moreover, on anthropic substrata we highlight an influence of mat age on the bacterial communities. Regarding zetaproteobacterial communities, our results point to an increase in ZetaOTUs abundance and diversification with the age of the mat. We corroborate the key role of the ZetaOTU 2 in mat construction, whatever the environment, the substrata on which they develop or the age of the mat. We also show that ZetaOTU 28 is specific to anthropogenic substrata. Finally, we demonstrate the advantage of using dPCR to precisely quantify very low abundant targets, as Zetaproteobacteria on our colonizers. Our study, also, allows to enrich our knowledge on the biogeography of Zetaproteobacteria, by adding new information on this class and their role in the Mediterranean Sea.

Morphology and evolution of submarine canyons around the Zhongsha Platform, South China Sea: Implications for sedimentary processes in a modern isolated carbonate setting

Recent high-resolution multibeam bathymetry and seismic data from the platform-top to the abyssal plain of the Zhongsha Platform allow for a detailed investigation of the morphologies, spatial distribution, and trigger mechanisms of submarine canyons, submarine landslides, and associated sedimentary features along modern isolated carbonate slopes. The newly observed Zhongsha Canyon System provides a natural laboratory for reconstructing the source-to-sink sedimentary processes in a pure carbonate setting. This study reveals that there are thirty-four submarine canyons at water depths between 300 and 4100 m on the northern and western slopes of the Zhongsha Platform. Two morphologically different submarine canyon types are identified: (1) dendritic canyons, which exhibit abundant tributaries with scallop-shaped failures at the canyon heads, and (2) linear canyons, which feature rare tributaries with elongated failures at the canyon heads. The dendritic canyons are more complex in morphology than the linear canyons as a result of the interaction among numerous tributaries. Canyon initiation and evolution pass through three phases: (1) initial stage: off-platform sediment transport and platform margin failures contribute to erosive gravity flows; (2) developmental stage: initiation and incision of submarine canyons along platform margin failures; and (3) mature stage: numerous mature canyons along the platform margin. Off-platform sediment transport, density cascading, gravity flows, monsoon currents, and deep circulation play an essential role in shaping the slope morphologies. In addition, submarine landslides are extensively observed along the entire slope of the Zhongsha Platform at water depths of 600 to 4200 m, including canyon-wall failures, slope landslides, canyon-front landslides, and slope-toe failures based on their location and genesis. These processes can steepen the platform slopes by upward retrogressive and downward progressive erosion. On a larger scale, the persistent submarine canyons and occurrence of landslides around the Zhongsha Platform contribute to the uniqueness of this landscape among modern carbonate slopes. The morphologies and evolutionary processes of Zhongsha Canyon System present significant differences from the global carbonate submarine canyons in terms of their dimensions and trigger mechanisms. The findings of this work provide novel insights into the morphological features and sedimentary processes of submarine canyons in modern isolated carbonate platform settings.

Hadal zones of the Southwest Pacific and east Indian oceans

The hadal zone (water depths > 6000 m) are unlike the overlying shallower marine regions (bathyal and abyssal) as it does not follow a continuum from the continental shelves to abyssal plains, but rather exhibits a globally disjunct series of discrete deep-sea habitats confined within geomorphological features. From an ecological perspective, hadal communities are often endemic to individual or adjacent features and are partitioned and isolated by geomorphological structures. To examine the size, shape, depth and degree of isolation of features where hadal fauna inhabit, this study explores the broad seafloor geomorphology, and distinctly partitioned hadal areas, across the Southwest Pacific and East Indian oceans using global bathymetric datasets. This research revealed the area occupied by hadal depths to be 716,915 km2 of which 58% are accounted for by trenches, 37% in basins and troughs, and 5% fracture zones. The largest feature in terms of area > 6000 m depth is the Wharton Basin with 218,030 km2 spanning 376 discrete areas. The largest continuous hadal habitats were the Kermadec and Tonga trenches at 145,103 and 111,951 km2 respectively, whereas features such as the Java Trench comprise two hadal components partitioned by a bathymetric high. Conversely, no physical barrier exists between the New Britain and Bougainville trenches thus any literature pertaining to hadal species or habitats from these trenches can be merged. This study highlights that the hadal zone mainly comprises two main geomorphological features (trenches and basins) that differ in size, depth and complexity. Hadal basins cover vast, generally shallower areas, comparable to abyssal plains, whereas trenches, despite a lesser footprint, represent greater depth ranges and complexity. As such, sampling designs and interpretation of ecological data must differ and hadal basins likely play an increasingly important role in understanding ecological shifts from abyssal to hadal ecosystems.

Biodiversity and community structures across the Magellan seamounts and abyssal plains in the western Pacific Ocean revealed by environmental DNA metabarcoding analysis

Seamounts within deep-sea ecosystems are recognized as biodiversity hotspots, yet they are increasingly threatened by anthropogenic activities such as overfishing, resource exploitation, and climate change. To establish environmental baseline data and assess the impacts of current or future anthropogenic activities, we conducted environmental DNA (eDNA) metabarcoding analyses to comprehensively characterize prokaryotic and eukaryotic diversity across various water depths of the Magellan seamounts and the adjacent abyssal plains. Metabarcoding analysis revealed higher richness and diversity in both prokaryotic and eukaryotic communities within the seamounts compared to the abyssal plains. Overall, the analysis identified 9,068 prokaryotic amplicon sequence variants (ASVs), representing 39 phyla and 47 classes. Similarly, 4,569 eukaryotic ASVs were identified, spanning 34 phyla and 93 classes. Furthermore, our results revealed distinct community structures between the seamounts and abyssal plains, with turnover across different water depths. These findings are crucial as they indicate the unique ecological roles and potential vulnerability of seamount communities, emphasizing the need for targeted conservation strategies. Our study underscores the importance of conducting comprehensive long-term environmental assessments of the impacts of anthropogenic activities on seamount ecosystems and highlights the potential of eDNA metabarcoding as a powerful tool for guiding conservation and management efforts in remote and challenging marine environments.

Toward mending the marine mass balance model for nickel: Experimentally determined isotope fractionation during Ni sorption to birnessite

In fewer than fifteen years, the study of Ni stable isotopes has advanced from early method development to application of a powerful tool for resolving a long-standing question: why does it appear that output fluxes of Ni from the global oceans far exceed input fluxes? The seawater concentration of Ni, a bioessential trace metal, is almost certainly at steady state on timescales comparable to its residence time of ∼20 kyr, so some of the current flux estimates must be inaccurate. Just as the input and output fluxes should balance, so should the flux-weighted isotopic compositions of the inputs and outputs. Thus, isotopic characterization of inputs and outputs provide an additional constraint on a balanced model of the marine Ni budget. Here, we report on experiments designed to elucidate fractionation mechanisms and magnitudes for sorption of Ni to Mn oxyhydroxide (birnessite), because Mn-rich sediments accumulating on abyssal plains represent the largest sink flux of Ni from seawater to marine sediments. Our results show remarkably large fractionations at low ionic strength (average Δ60/58Nidissolved-sorbed = +1.38 ‰). Neither closed-system equilibrium trends nor Rayleigh curves fit the data well. Fractionations are even larger at high ionic strength (Δ60/58Nidissolved-sorbed ranging from +2.0 to +4.0 ‰), and they decrease with experimental duration from 2 d (49 h) to 27 d. The high ionic strength data fit Rayleigh trends well. Here, we use X-ray absorption fine-structure spectroscopy (EXAFS) and results from previous studies to support interpretation of our data as combinations of kinetic and equilibrium isotope effects that vary in their proportional contributions to the total fractionation with time and with surface loading. One important consequence of this study is that none of the experimental results reported thus far, including ours, are directly applicable to building steady-state models of the Ni cycle. Even our longest duration experiments did not achieve equilibrium, which is likely to be manifest in the very slowly accumulating sediments on abyssal plains. Our work constrains further the mechanisms of Ni sorption to birnessite and clearly indicates that determination of equilibrium fractionation in this system, although challenging, will be a crucial step toward resolving the apparent marine Ni imbalance.

Two Effective Degrees of Freedom Can Represent the Dominant Features of Global Rayleigh Wave Dispersion Maps

AbstractObjectively exploring global variations in crust and upper mantle structure helps constrain fundamental aspects of Earth's plate tectonic and convective processes. Here we adopted a Variational Auto‐Encoder to explore the degrees of freedom of global Rayleigh wave dispersion maps at 4–40 mHz. We found that two latent variables sufficiently represent the global variations, suggesting inherent coupling between crustal and mantle seismic properties. We propose that the two extracted latent variables mostly correspond with crustal thickness and upper mantle thermal structure. The first variable shows low values for continental mountain belts and ocean spreading ridges, contrasted by high values for abyssal plains. The second variable shows low values for most oceanic lithosphere and Phanerozoic continental areas, contrasted by high values for Archean cratons. Latent space correlations indicate that continental lithosphere has more strongly coupled depth features than beneath the oceans, which might be a consequence of its longevity.

THE ROLE OF BOTTOM MESO-SCALE DYNAMICS IN CONTOURITE FORMATION IN THE ARGENTINE BASIN

The Argentine Basin is a deep-sea basin located in the South Atlantic Ocean that contains sedimentary deposits derived from different provenances. It is characterized by complex ocean dynamics encompassing diverse spatial and temporal dimensions. The northward subantarctic Malvinas Current and southward subtropical Brazil Current converge at the western margin of the Argentine Basin, resulting in the formation of the Brazil-Malvinas Confluence. Bottom currents, particularly currents flowing alongslope and horizontal eddies, are crucial in shaping the seafloor and in the formation of sedimentary features (e.g. contourites). The poorly understood strength and variability of bottom currents leave the processes that control sedimentation in deep environments unclear. High-resolution (1/12°) reanalysis was used to analyze near-bottom flows and bottom dynamics were compared with seafloor sedimentary characteristics obtained from geophysical datasets and sediment cores. High speeds, up to 3.5 m/s at the surfa ce and up to 1.4 m/s at the bottom, reveal the presence of intense flows in this area. The Zapiola Drift, an approx. 1,200 m high sedimentary deposit located in the central part of the Argentine Basin, is bounded by a zone of high bottom eddy kinetic energy (EKE) that resulted in the erosion of the seafloor and in the accumulation of sandy mud. The Malvinas Current is distinguished by strong and constant currents flowing northwards along the continental slope and by minimal EKE at the bottom. The area of the continental slope along which the Malvinas Current flows corresponds to a contourite terrace, a relatively flat surface composed almost entirely of sandy sediments and with abundant erosional features. The regions of highest EKE activity in the bottom layer is the overshoot of the Brazil-Malvinas Confluence and the Abyssal Plain. Our study highlights the impact of bottom current dynamics on contouritic sedimentation. In some regions, sedimentation is influenced by sporadic processes that occur between intense and weak flow events over time, which are considered intermittent processes. While sedimentation in other areas is controlled by constant flows. A better understanding of the strength and variability of bottom currents will improve paleoceanographic reconstructions based on the sedimentary record.

Microbial respiration in contrasting ocean provinces via high-frequency optode assays

Microbial respiration is a critical component of the marine carbon cycle, determining the proportion of fixed carbon that is subject to remineralization as opposed to being available for export to the ocean depths. Despite its importance, methodological constraints have led to an inadequate understanding of this process, especially in low-activity oligotrophic and mesopelagic regions. Here, we quantify respiration rates as low as 0.2 µmol O2 L-1 d-1 in contrasting ocean productivity provinces using oxygen optode sensors to identify size-fractionated respiration trends. In the low productivity region of the North Pacific Ocean at Station Papa, surface whole water microbial respiration was relatively stable at 1.2 µmol O2 L-1 d-1. Below the surface, there was a decoupling between respiration and bacterial production that coincided with increased phytodetritus and small phytoplankton. Size-fractionated analysis revealed that cells <5 µm were responsible for the majority of the respiration in the Pacific, both at the surface and below the mixed layer. At the North Atlantic Porcupine Abyssal Plain, surface whole water microbial respiration was higher (1.7 µmol O2 L-1 d-1) than in the Pacific and decreased by 3-fold below the euphotic zone. The Atlantic size-fraction contributions to total respiration shifted on the order of days during the evolution of a phytoplankton bloom with regular storm disturbances. The high-resolution optode method used in the Atlantic captured these significant shifts and is consistent with coinciding stain-based respiration methods and historical site estimates. This study highlights the dynamic nature of respiration across vertical, temporal, and size-fractionated factors, emphasizing the need for sensitive, high-throughput techniques to better understand ocean ecosystem metabolism.

Continuous deposition of pre-aged organic carbon in the southern Mariana Trench since the last deglaciation

One of the key challenges in understanding the role of hadal trenches in the marine organic carbon (OC) cycle is the need to distinguish OC sources, assess accumulation rates, and identify controlling processes. To address this, sediment core samples from the southern Mariana Trench were analyzed for their contents and carbon isotopes (13C and 14C) of both total OC (TOC) and saturated long-chain fatty acid (n-LCFA). Our findings indicate that sedimentary OC in the southern Mariana Trench is primarily derived from marine sources, as indicated by TOC-δ13C values ranging from −19.9‰ to −17.0‰ and the ratios of TOC to total nitrogen (TOC/TN) of 5.5–8.4. The 14C ages of TOC and n-LCFA ranged from 7159 to 22,662 yr and 8713 to 22,819 yr, respectively, with both showing gradual down-core increasing trends. This allows for a conservative estimation of sedimentation rate of 17.5–18.7 cm kyr−1 in the southern Mariana Trench, leading to a sedimentary OC accumulation rate of 529–942 g C m−2 kyr−1, which is 1–2 orders of magnitude higher than that observed in the abyssal plain. We propose that the continuous lateral downslope transport played a significant role in focusing pre-aged OC into the southern Mariana Trench. Therefore, hadal trenches are likely to accumulate substantial amounts of sedimentary OC in the deep ocean.

Foreland basin development in response to Proto-Tethyan Ocean closure, western North China Block

Closure timing of the northeastern Proto-Tethyan Ocean between the Qilian-Qinling Terranes and the North China Block remains unsolved, with assumptions ranging from end-Ordovician to the Devonian. To address this issue, integrated studies of stratigraphy and geochronology were conducted on Late Ordovician strata in the southwestern Ordos, which recorded the earliest tectonic transition from passive margin to foreland basin in the westernmost North China Block. Stratigraphic and paleontologic syntheses demonstrate that pre-Katian strata are shallow-marine deposits dominated by benthonic faunas. Meanwhile, Katian successions above a paraconformity are characterized by deep-water debrites and turbidites containing abundant planktonic graptolites. Provenance analysis reveals an evolving source from the North China Block basement to the Qilian-Qinling arc terranes at the beginning of Katian (ca. 450 Ma). Detrital zircons from pre-Katian quartz arenites yield ages of ca. 1600–2800 Ma, significantly older than their depositional timing. In contrast, Katian turbidites in the southwestern Ordos and the North Qilian Orogen display similar age patterns dominated by ca. 450–900 Ma ages. These clues imply a basin-filling shift from passive margin to underfilled foreland, separated by forebulge unconformity formation at Sandbian-Katian boundary. Stratigraphic correlation further suggests that sequence of foreland basin-infill transitions was broadly simultaneous along strike more than ~1200 km across the southwestern North China Block, including forebulge flexural uplift, followed by rapid shallow shelf drowning to abyssal plain, until arrival and superimposition of arc-derived turbidites. Along-strike synchronicity of orogenic activities implies that closure of the northeastern Proto-Tethys was nearly synchronous. The L-shaped orogen-parallel foreland encompassing the southwestern North China Block unveils oroclinal bending of the Qilian-Qinling collision belt, which still dominates the geology of present northeastern Tibet. Our new insights provide a stratigraphic constraint for the timing and mode of the initial elimination of the northeastern Proto-Tethys.