Abyssal Plain Research Articles

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.

Acoustic imaging of stable double diffusion in the Madeira abyssal plain

Sub-mesoscale and mesoscale (i.e., 1–10 km and 10–200 km, respectively) ocean processes are highly relevant for the understanding of global circulation, mixing of water masses and energy exchange between ocean layers. However, the processes happening at these scales are hard to be characterized using direct measurements of temperature and salinity. Direct measurements are obtained from vertical probes and/or autonomous vehicles, which, despite their high vertical resolution, are sparsely located in space and therefore unable to capture spatial details at these scales. Seismic oceanography (SO) data have been successfully used to imaging and characterize the ocean at these spatial scales. These data represent indirect measurements of the ocean temperature and salinity along kilometric transects with high horizontal resolution (i.e., a near-synaptic view of the system under investigation), but lower vertical resolution when compared with direct observations. Despite its complex oceanographic setting, the Madeira Abyssal Plain is still largely uncharacterized due to the lack of direct observations. We show for the first time a comprehensive processing, modelling and interpretation of three 2-D seismic oceanography sections from this region. The data show coherent seismic reflection in space, depth and time and shed light into this oceanographic setting with an unprecedent horizontal resolution. The SO modelling and interpretation are combined with existing direct measurements and a quantitative method to correlate thermohaline staircases interpreted from conductivity-temperature-depth casts and seismic reflections is proposed. The results show the relatively stable presence of thermohaline staircases in simultaneously time and space between 1200 and 2000 m of water depth and their spatial variability and contribute to the generalization of SO in physical oceanography.

A new deep-sea species of Halirages Boeck, 1871 (Crustacea: Amphipoda: Calliopiidae) inhabiting sponges

In the vast abyssal plains northwest of Iceland, white glass sponges of the genus Caulophacus Schulze, 1886 were inhabited by reddish Bythocaris G.O. Sars, 1870 shrimps and pinkish amphipods. After in situ observations at 3700 m depth, in -1°C waters by a remotely operated vehicle, members of this assemblage were collected and preserved for molecular studies. Based on integrative taxonomic analyses, the amphipods were identified as a new species of the genus Halirages Boeck, 1871 – Halirages spongiae sp. nov. Lörz, Nack & Tandberg –, as described in detail below. Part of our integrative approach was to establish reference DNA barcodes for known species of Halirages. However, our investigation of material of Calliopiidae G.O. Sars, 1895 collected around Iceland and Norway revealed slight morphological discrepancies in all the described species of Halirages. Except for Halirages fulvocinctus (M. Sars, 1858), none of the encountered specimens of Calliopiidae fully matched a current species description. We illuminate the morphological characteristics of nine operational taxonomic units, which also represented clades in COI and 28S. We set the Icelandic samples in the context of Halirages from Canada and Norway. A key to the world species of Halirages is provided.

Organic Carbon Sources in Surface Sediments on the Northern South China Sea

AbstractThe burial of organic carbon (OC) in marine sediments regulates CO2 content in the atmosphere. However, the OC sources and their effect on the OC preservation in sediments of the continental marginal sea remain elusive. Here, we survey the abundance, stable carbon and radiocarbon isotopes of OC, as well as mineral surface area and grain size in surface sediments from the shelf to the abyssal plain of the South China Sea. We found that the marine OC comprises the 63 ± 9% and 78 ± 14% of sedimentary OC off South China and Luzon, respectively. The petrogenic OC contributing to sedimentary OC in these sites is generally higher than that of soil OC (22 ± 10% vs. 12 ± 6%). The sedimentary OC off Taiwan is predominantly derived from petrogenic OC, accounting for 57 ± 10%, with remaining consisting of marine OC (36 ± 10%) and soil OC (6 ± 1%). High soil OC contents are found in fine sediments on the inner shelf off South China, and high petrogenic OC contents occur in fine sediments off Taiwan. Marine OC contents are high in fine sediments on the middle shelf and upper slope off South China, but low in coarse sediments on the outer shelf and upper slope, and fine sediments on the lower slope and abyssal plain. The OC sources, mineral surface area, and oxygen exposure time of OC together control the preservation of OC in sediments with their relative importance differing on varying depositional settings of this sea.

Deep-ocean channel-wall collapse order of magnitude larger than any other documented

Submarine channels are the largest conveyors of sediment on Earth, yet little is known about their stability in the deep-ocean. Here, 3D seismic data from the deep-ocean Hikurangi channel-levee system, offshore New Zealand, reveal the largest channel-wall failure yet documented. Collapse of both channel-walls along a 68 km stretch created a mass-transport deposit of 19 km3, containing 4 km long blocks. Channel-walls typically collapse piecemeal, but here synchronous failure of both channel-walls and landslide erosion of the seafloor is documented, requiring a new process model for channel-wall failure. Mass-failure on this scale poses an under-appreciated risk to seafloor infrastructure both within channels and over regions extending twice the channel width into their overbank. Hitherto, channel-wall failures of this size are unrecognised in abyssal plains; its scale changes our understanding of how channel-levee systems are constructed and how they conduct sediment, carbon and pollutants into the deep-ocean.

Geomorphologic and infilling characteristics of the shelf-incising submarine canyon in the eastern Makran accretionary prism, offshore Pakistan

Seven shelf-incising submarine canyon systems are developed in the Makran accretionary prism, offshore Pakistan. This paper uses multibeam bathymetric data and multichannel seismic profiles to study the geomorphologic and infilling characteristics of E Canyon in the east of accretionary prism. The E Canyon extends nearly north–south from the upper slope to lower slope, the bottom of the canyon is located in 1000 to 3000 m water depth, the main axis extends about 55 km, and the width varies from 5 to 9 km. The E Canyon shows U-shaped concave features in the northern cross-section view and V-shaped concave features in the southern cross-section view. Two continuous erosion pools are developed at the fifth and third ridges of the southern segment, respectively. The steepest sidewall (with slopes of 25.75° and 40.47°) of E Canyon was found near the third ridge. The filling sequence of coarse channel lags, slump debris flows, stacked channel sands and mass transported deposits and turbidite deposits from bottom to top is developed in the northern segment of the canyon. Strong erosion is developed in the southern part of the canyon and there is no sedimentation inside. On the abyssal plain, the continuation of the E Canyon is clearly erosive. Our preliminary analysis shows that the factors controlling the evolution of E Canyon may include sediment gravity flow, tectonic activity, sea-level change, climate and onshore rivers activity. The correlation between evolution of E Canyon and gas hydrates was explored, and it is believed that the erosion of E Canyon plays a destructive role in hydrate accumulation. At the same time, the destruction of hydrate-bearing layer induces the escape and leakage of hydrocarbon-bearing fluid, leading to the development of pockmark on the sidewalls of E Canyon. And the evolution model of gas hydrate accumulation in the development area of E Canyon was established.

The Guangya submarine fan in the South China Sea: A distinctive channelized slope-through fan

Submarine fans play a crucial role in understanding deep-sea sedimentary processes, establishing source-to-sink linkages, and identifying deep-water hydrocarbon reservoirs. Various types of submarine fans have been documented. Here we present findings on the Guangya submarine fan in the Southwest subbasin (SWSB) of the South China Sea (SCS), by analyzing multibeam bathymetric and two-dimensional multichannel seismic data. Bathymetric data reveal that the Guangya fan, covering an area of ~22,500 km2, comprises seven leveed channels, leading to a large composite lobe off the channel mouths. Different from typical submarine fans, the Guangya fan exhibits an elongated shape, extending longitudinally from the shelf edge through the slope to the abyssal plain, while laterally confined within a trough-like morphology in the southwestern tip of the SWSB and beyond. Lacking a typical feeding canyon, the fan appears to be self-fed by the leveed channels that serve as the primary depositional element. In contrast to the radial channel pattern in typical fans, the channels within the Guangya fan form a unique braid to dendritic pattern, genetically linked to channel merging and intersecting effects. Long wavelength and low wave height cyclic steps are identified, highlighting the occurrence of energetic supercritical turbidity currents. Seismic stratigraphic analysis shows that the Guangya fan primarily consists of high-amplitude reflections, indicating its coarser-grained or sandy turbidite-dominated nature. The fan has been evolved since the Pliocene, and mainly comprises depositional elements including channel fills, levees, lobes, and mass transport deposits. The specific tectonic context at the convergence of the conjugate margins, bounding the funnel-shaped SWSB, significantly influences the development of the Guangya fan. The Guangya fan could be originated from a shelf-edge staging area predominantly fed by the Mekong River during sea-level lowstands.

Fungal Abundance and Diversity in the Mariana Trench, the Deepest Ecosystem on Earth.

Hadal trenches host abundant and diversified benthic prokaryotic assemblages, but information on benthic fungi is still extremely limited. We investigated the fungal abundance and diversity in the Challenger Deep (at ca. 11,000 m depth) and the slope of the Mariana Trench in comparison with three sites of the adjacent abyssal plain. Our results indicate that trench sediments are a hotspot of fungal abundance in terms of the 18S rRNA gene copy number. The fungal diversity (as the number of amplicon sequence variants, ASVs) was relatively low at all sites (10-31 ASVs) but showed a high turnover diversity among stations due to the presence of exclusive fungal taxa belonging to Aspergillaceae, Trichosphaeriaceae, and Nectriaceae. Fungal abundance and diversity were closely linked to sediment organic matter content and composition (i.e., phytopigments and carbohydrates), suggesting a specialization of different fungal taxa for the exploitation of available resources. Overall, these findings provide new insights into the diversity of deep-sea fungi and the potential ecological role in trench sediments and pave the way for a better understanding of their relevance in one of the most extreme ecosystems on Earth.

The Influence of Seamounts on the Enrichment of Rare Earth Elements in Sediments—A Case Study of the Marcus-Wake Seamounts in the Western Pacific Ocean

Deep-sea sediments enriched in rare earth elements and yttrium (REY-rich sediments) are widely distributed on the deep-sea floor, and their formation mechanism remains elusive. Although studies have recognized the link between seamounts and REY-rich sediments, in-depth analysis of the specific roles and effects of seamounts in the formation of REY-rich sediments is lacking. In this study, we analyzed surface sediments from the Marcus-Wake Seamounts for grain size, geochemistry, and mineral composition and classified the samples into three types: samples with moderate REY content and dominated by terrestrial detritus; samples with high REY and authigenic mineral content; and samples rich in CaCO3 but poor in REY. The REY in the sediments of the study area partly originate from Asian dust input and partly from seawater and/or pore water, and are mainly enriched in REY carrier particles including bioapatite fossils and micronodules. The amount of REY carrier particles influences the REY content in the sediments. The current field, primary productivity, weathering process, and depositional environment around seamounts are different from those of abyssal plains, which are conducive to the formation of REY-rich sediments. Strong bottom currents may exist in the southeastern direction of some large seamounts (e.g., Niulang Guyot), leading to the selective accumulation of REY-rich bioapatite fossils and micronodules, resulting in the formation of REY-rich sediments.

Long-term eddy modulation affects the meridional asymmetry of the halocline in the Beaufort Gyre

Abstract. Against the background of wind-forcing change along with Arctic sea ice retreat, the mesoscale processes undergoing distinct variation in the Beaufort Gyre (BG) region are increasingly important to oceanic transport and energy cascades, and these changes subsequently put oceanic stratification into a new state. Here, the varying number and strength of eddies in the central Canada Basin (CB) and Chukchi–Beaufort continental slope are obtained based on mooring observations (2003–2018), altimetry measurements (1993–2019), and reanalysis data (1980–2020). In this paper, the variability in the BG halocline, representing the adjustment of stratification in the upper layer, is shown in order to analyse how variability occurs under changing mesoscale processes. We find that over almost the last 2 decades the halocline depth has deepened by ∼ 40 m in the south of the central gyre, while that in the north has deepened by ∼ 70 m according to multiple datasets. Surrounding the central gyre, the asymmetry of the halocline, with much steeper and deeper isopycnals over the southern continental slope, reduced after 2014. In the meantime, eddy activities in the upper layer from the southern margin of the BG to the abyssal plain have been enhanced. Moreover, the convergence of the eddy lateral flux has increased as the halocline structures on either side, which is at least 120 km from the central gyre, have reached a nearly identical and stable regime. It has been clarified that long-term dynamic eddy modulation through eddy fluxes, facilitating the freshwater redistribution, affects the meridional asymmetry of the BG halocline. Our results provide a better understanding of the eddy modulation processes and their influence on the halocline structure.

Carbon and silica fluxes during a declining North Atlantic spring bloom as part of the EXPORTS program

The goal of NASA's EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) project is to develop a predictive understanding of the fate of global ocean primary productivity and export of carbon from the surface to the deep ocean. Thorium-234 (234Th, t1/2 = 24.1 d) was used to measure sinking particle export from an anticyclonic eddy during the EXPORTS North Atlantic cruise (May 2021) at the Porcupine Abyssal Plain. The four-week sampling period was broken into three time periods (“epochs”) where 800 234Th seawater samples were collected from over 50 CTD casts with high depth resolution over the upper 500 m. Size-fractioned particulate samples were collected to determine particulate organic carbon (POC) and biogenic silica (bSi) to 234Th ratios using in situ McLane pumps. A 234Th non-steady state model shows an eddy center epoch average progression of increasing 234Th export (∼2800 ± 300 (Epoch 1; standard deviation) to 4500 ± 700 (Epoch 3) dpm m−2 d−1) out of the top 110 m of the water column over the course of the cruise (29 d). This translates into an epoch average progression of ∼11 ± 1 to 14 ± 2 mmol C m−2 d−1 of sinking POC flux, and ∼ 3 ± 1 to 6 ± 1 mmol bSi m−2 d−1 of sinking bSi flux to deeper waters at 110 m. The overall efficiency of the biological carbon pump (amount of net primary production reaching 100 m below the euphotic zone) increases from ∼10% to ∼30% throughout the sampling period. The temporal trends discussed extensively in this paper show that POC and bSi export increase during diatom bloom evolution.

Geochemistry of buried polymetallic nodules from the eastern Pacific Ocean: Implication for the depth-controlled alteration process

Polymetallic nodules are often exposed or semi-buried in surface sediments of abyssal plains, with certain older generations of nodules being deeply buried in the sediment column. Owing to the limitations of sampling, it is unclear whether these nodules have remained intact or have been altered after burial. To explore this question, we carried out a comparative study of the mineralogy and chemistry of polymetallic nodules recovered from the surface sediment and two depth intervals within a sediment column (220–223 cm and 550–555 cm) in the Clarion–Clipperton Zone of the eastern Pacific Ocean. All of the nodules examined have obvious layered structures. The main minerals in the buried nodules are 10 Å vernadite, 7 Å phyllomanganate, and Fe-vernadite, which are characteristic of the hydrogenetic type of polymetallic nodule. However, the in situ Mn/Fe ratios suggest that some microlayers are diagenetic type. With the burial depth, the total content of rare earth elements and yttrium decreased in the bulk nodules, whereas their positive Ce anomalies and diagenetic parts gradually increase. The nodule buried at 220–223 cm sediment depth shows no obvious alterations, whereas the one buried at 550–555 cm has an approximately 200-μm-thick microlayer of continuous diagenetic growth on its surface. Additionally, this GC04_550–555 cm nodule has a microlayer with higher Cu/Ni and Si/Al ratios, located adjacent to its outer diagenetic microlayer, further indicating its alteration after burial. Therefore, the alterations of buried polymetallic nodules may be related to their burial depth. Compared with the sediment layers above and below, the bulk sediment at the 550–555 cm interval has a relatively lower Mn content and higher Fe content, which may be related to the observed alterations in the buried nodule found therein. Our findings on the alterations of polymetallic nodules after burial and the possible impact of the sedimentary environment on those changes provide a theoretical basis for establishing the burial events of these ferromanganese aggregates in the geological history of the study area.

Distribution and genomic variation of ammonia-oxidizing archaea in abyssal and hadal surface sediments

Ammonia-oxidizing archaea of the phylum Thaumarchaeota play a central role in the biogeochemical cycling of nitrogen in benthic sediments, at the interface between pelagic and subsurface ecosystems. However, our understanding of their niche separation and of the processes controlling their population structure in hadal and abyssal surface sediments is still limited. Here, we reconstructed 47 AOA metagenome-assembled genomes (MAGs) from surface sediments of the Atacama and Kermadec trench systems. They formed deep-sea-specific groups within the family Nitrosopumilaceae and were assigned to six amoA gene-based clades. MAGs from different clades had distinct distribution patterns along oxygen-ammonium counter gradients in surface sediments. At the species level, MAGs thus seemed to form different ecotypes and follow deterministic niche-based distributions. In contrast, intraspecific population structure, defined by patterns of Single Nucleotide Variants (SNV), seemed to reflect more complex contributions of both deterministic and stochastic processes. Firstly, the bathymetric range had a strong effect on population structure, with distinct populations in abyssal plains and hadal trenches. Then, hadal populations were clearly separated by trench system, suggesting a strong isolation-by-topography effect, whereas abyssal populations were rather controlled by sediment depth or geographic distances, depending on the clade considered. Interestingly, genetic variability between samples was lowest in sediment layers where the mean MAG coverage was highest, highlighting the importance of selective pressure linked with each AOA clade’s ecological niche. Overall, our results show that deep-sea AOA genome distributions seem to follow both deterministic and stochastic processes, depending on the genomic variability scale considered.

Marine snowstorm during the Permian−Triassic mass extinction

The Permian−Triassic mass extinction (PTME) interval is marked by major excursions in both inorganic and organic carbon (C) isotopes. Carbon cycle models predict that these trends were driven by large increases in productivity, yet organic C−rich rocks are not recorded in most PTME shelf sedimentary successions. Anomalous C-rich facies have been reported from rare abyssal plains records now exposed in Japan and New Zealand, where black shales at the PTME are extraordinarily organic-rich units. We examined organic matter at the Waiheke, New Zealand, section, and results show that these deposits are dominated by lamalginites composed of unicellar solitary or colonial phytoplankton produced during algal blooms that falls as “marine snow.” We modeled the impact of ash fall from eruptions in the Siberian Traps large igneous province and argue that they fertilized the Panthalassa Ocean with P and Fe, leading to a marine “snowstorm” and significant C drawdown marking this major biobloom during the PTME.

Complete mitochondrial genome of the abyssal coral Abyssoprimnoa gemina Cairns, 2015 (Octocorallia, Primnoidae) from the Clarion-Clipperton Zone, Pacific Ocean

The Clarion-Clipperton Zone (CCZ) in the tropical East Pacific is a region of interest for deep-sea mining due to its underwater deposits of polymetallic nodules containing economically important metals such as nickel, copper, and cobalt. It is also a region of extensive baseline studies aiming to describe the state of the environment, including the biodiversity of the benthic fauna. An abundant component of the abyssal plain ecosystem consists of sessile fauna which encrusts polymetallic nodules and are vulnerable to potential impacts arising from exploitation activities, particularly removal of substrate. Therefore, this fauna is often considered to have key species whose genetic connectivity should be studied to assess their ecological resilience. One such species is Abyssoprimnoa gemina Cairns, 2015, a deep-sea coral from the CCZ whose presence in the Interoceanmetal Joint Organization (IOM) claim area has been confirmed during samplings. In this study, we used next-generation sequencing (NGS) to obtain the 18S nuclear rRNA gene and the complete mitochondrial genome of A. gemina from IOM exploration area. The mitogenome is 18,825 bp long and encodes for 14 protein coding genes, 2 rRNAs, and a single tRNA. The two phylogeny reconstructions derived from these data confirm previous studies and display A. gemina within a highly supported cluster of seven species whose mitogenomes are all colinear and of comparable size. This study also demonstrates the suitability of NGS for DNA barcoding of the benthic megafauna of the CCZ, which could become part of the IOM protocol for the assessment of population diversity and genetic connectivity in its claim area.

Acoustic twilight: A year‐long seafloor monitoring unveils phenological patterns in the abyssal soundscape

AbstractDespite the perpetual darkness of the deep sea, contrasting the sunlit epipelagic waters, many deep‐sea organisms exhibit rhythmic activities. To discern environmental cues that may serve as entrainment signals for deep‐sea organisms, this study investigated the soundscape of the abyssal plain south of Minamitorishima Island. Our analysis revealed clear diel and seasonal patterns, primarily driven by evening fish choruses and marine mammal vocalizations. These evening choruses, discernible above the background noise, likely serve as a circadian time cue for organisms capable of perceiving them within the aphotic depths. In addition, the frequent detection of whistles and echolocation clicks suggests this region functions as a foraging ground for marine mammals. These acoustic cues might guide organisms with auditory capabilities toward habitats rich in sinking food debris and whale falls. By elucidating the ecological processes shaping abyssal soundscape dynamics, these findings open new directions for further exploration in deep‐sea chronobiology.

Lack of strong seasonality in macrobenthic communities from the northern Barents Sea shelf and Nansen Basin

The Barents Sea has been coined ‘the Arctic hotspot’ of climate change due to the rapidity with which environmental changes are taking place. This transitional domain from Atlantic to Arctic waters is home to highly productive benthic communities. This system strongly fluctuates on a seasonal basis in its sympagic-pelagic-benthic coupling interactions, with potential effects on benthic standing stocks and production. Recent discoveries have questioned the marked seasonality for several high Arctic seafloor communities in coastal waters of Svalbard. Still, the seasonal variability of benthic process in the extensive Barents Sea open shelf remains poorly understood. Therefore, we studied the seasonality of macrofauna communities along a transect in the northwestern Barents Sea comprising two hydrographic domains (Arctic vs. Atlantic Water, across the Polar Front) and three geomorphological settings (shelf, continental slope and abyssal plain). Overall, we did not find strong signs of seasonal variation in taxonomic community structure and functional diversity. However, we found some weak signs of seasonality when examining each station separately, especially at a station close to the Polar Front, with high seasonal fluctuations in abiotic drivers indicating a stronger pelagic-benthic coupling. The lack of seasonality found both at the shelf stations south and north of the Polar Front could be related to organic matter stored in the sediments, reflected in constant levels of total organic carbon in surface sediment across time for all stations. We did observe, as expected, highly spatially structured environmental regimes and macrofauna communities associated to them from shelf to slope and basin locations. Understanding the underlying spatio-temporal mechanisms by which soft-bottom benthic communities are structured along environmental gradients is necessary to predict future impacts of climate change in this area. Our results indicate that short-term climate driven changes in the phenology of pelagic ecosystem components might not be directly reflected in the Arctic benthic system, as seafloor processes seem to be partially decoupled from those in the overlying water.

Constraints on the expanse of Greater India in the Early Cretaceous from radiolarians

Defining the original size of Greater India is a critical parameter for reconstructing the kinematics and timing of the India–Asia collision. Early Cretaceous paleontology-based paleoceanographic and paleogeographic reconstruction for the northern coast of East Gondwanaland provides independent constraint for the extent of Greater India. In this study, newly discovered Early Cretaceous radiolarian faunas from the Tethys Himalaya were objectively compared with non-Tethyan austral faunas from the Argo Abyssal Plain (AAP) and the typical Tethyan fauna from Italy using the Hayashi's Quantification Method Type III. We found that (1) The occurrence of austral cold-water radiolarian species at the western coast of Australia and northern coast of Greater India is attributable to northward intrusion of circumantarctic cold water via a seaway newly opened during the breakup of East Gondwanaland from ∼136 Ma; (2) Gyangze was under shallow warm-water conditions with typical Tethyan radiolarian fauna (Cluster 3) until the late Valanginian when cold-water species (Cluster 1) arrived from the south. A water mass boundary shift from Tethyan species dominant to austral cold-water species dominant in Gyangze indicates its lower paleolatitude than Ocean Drilling Program (ODP) Leg 123, Site 765 in the AAP; (3) The same austral cold-water radiolarian-bearing intervals from the late Berriasian to Valanginian indicate that Kangma in southern Tibet was proximal to ODP Site 765; (4) Different radiolarian faunas in Italy–Zhongba, Kangma, and the AAP reveal complex water conditions under different degrees of effect by Tethyan warm water and austral cold water at different water depths during the Barremian to Aptian. This radiolarian biogeography is highly consistent with the paleomagnetic reconstruction and indicates that eastern Greater India extended to the Exmouth Plateau (>2400 km) during the Early Cretaceous.

A multi-cubic-kilometre neutrino telescope in the western Pacific Ocean

Next-generation neutrino telescopes with substantially improved sensitivity are required to pinpoint the sources of the diffuse astrophysical neutrino flux detected by IceCube and uncover the century-old puzzle of cosmic-ray origins. A detector near the Equator will provide a unique viewpoint of the neutrino sky, complementing IceCube and other neutrino telescopes in the Northern Hemisphere. Here we present results from an expedition to the northeastern region of the South China Sea, in the western Pacific Ocean. A favourable neutrino telescope site was found on an abyssal plain at a depth of ~3.5 km. At depths below 3 km, the sea current speed, water absorption and scattering lengths for Cherenkov light were measured to be vc < 10 cm s−1, λabs ≈ 27 m and λsca ≈ 63 m, respectively. Accounting for these measurements, we present the design and expected performance of a next-generation neutrino telescope, Tropical Deep-sea Neutrino Telescope (TRIDENT). With its advanced photon-detection technology and large dimensions, TRIDENT expects to observe the IceCube steady source candidate NGC 1068 with 5σ significance within 1 year of operation. This level of sensitivity will open a new arena for diagnosing the origin of cosmic rays and probing fundamental physics over astronomical baselines.

Distribution of microplastics in bathyal- to hadal-depth sediments and transport process along the deep-sea canyon and the Kuroshio Extension in the Northwest Pacific

Understanding microplastic (MP) behavior in oceans is crucial for reducing marine plastic pollution. However, the complex process underlying MP transportation to the deep seafloor remains unknown despite the deep sea being considered its major sink. We focused on MP distribution in Sagami Bay (adjacent to highly populated areas of Japan), the plate triple junction connected through the Sagami Trough, and the abyssal plain immediately below the Kuroshio Extension. We observed the highest number of MPs in the abyssal stations, more than previously reported. The polymer types and aspect ratio of MPs in the abyssal stations significantly differed from those in the bathyal/hadal stations. The study suggests that MPs accumulated in the open ocean surface layer sink to the abyssal plains immediately below it, while MPs from land sources accumulate in the bathyal depth and are transported to the hadal depth near the coast through turbidity currents along the submarine canyon.