Deep Ocean Basins Research Articles

Serendipitous Internal Wave Signals in Deep Argo Data

AbstractSerendipitous measurements of deep internal wave signatures are evident in oscillatory variations around the background descent rates reported by one model of Deep Argo float. For the 10,045 profiles analyzed here, the average root‐mean‐square of vertical velocity variances, , from 1,000 m to the seafloor, is 0.0045 m s−1, with a 5%–95% range of 0.0028–0.0067 m s−1. Dominant vertical wavelengths, λz, estimated from the integrals of lagged autocorrelation sequences have an average value of 757 m, with a 5%–95% range of 493–1,108 m. Both and λz exhibit regional variations among and within some deep ocean basins, with generally larger and shorter λz in regions of rougher bathymetry or stronger deep currents. These correlations are both expected, since larger and shorter λz should be found near internal wave generation regions.

Ideas and perspectives: Sea-level change, anaerobic methane oxidation, and the glacial–interglacial phosphorus cycle

Abstract. The oceanic phosphorus cycle describes how phosphorus moves through the ocean, accumulates with the sediments on the seafloor, and participates in biogeochemical reactions. We propose a new two-reservoir scenario of the glacial–interglacial phosphorus cycle. It relies on diagenesis in methane hydrate-bearing sediments to mobilize sedimentary phosphorus and transfer it to the oceanic reservoir during times when falling sea level lowers the hydrostatic pressure on the seafloor and destabilizes methane hydrates. The stock of solid phase phosphorus mobilizable by this process is of the same order of magnitude as the dissolved phosphate inventory of the current oceanic reservoir. The potential additional flux of phosphate during the glacial period is of the same order of magnitude as pre-agricultural, riverine dissolved phosphate fluxes to the ocean. Throughout the cycle, primary production assimilates phosphorus and inorganic carbon into biomass, which, upon settling and burial, returns phosphorus to the sedimentary reservoir. Primary production also lowers the partial pressure of CO2 in the surface ocean, potentially drawing down CO2 from the atmosphere. Concurrent with this slow “biological pump”, but operating in the opposite direction, a “physical pump” brings metabolic CO2-enriched waters from deep-ocean basins to the upper ocean. The two pumps compete, but the direction of the CO2 flux at the air–sea interface depends on the nutrient content of the deep waters. Because of the transfer of reactive phosphorus to the sedimentary reservoir throughout a glaciation cycle, low-phosphorus and high-CO2 deep waters reign at the beginning of a deglaciation, resulting in rapid transfer of CO2 to the atmosphere. The new scenario provides another element to the suite of processes that may have contributed to the rapid glacial–interglacial climate transitions documented in paleo-records.

Study on Extension of the 6-DOF Measurement Area for a Model Ship by Developing Auto-tracking Technology for Towing Carriage in Deep Ocean Engineering Tank

The deep ocean engineering basin (DOEB) of the Korea Research Institute of Ship and Ocean Engineering (KRISO) is equipped with an extreme-environment reproduction facility that can analyze the motion characteristics of offshore structures and ships. In recent years, there have been requirements for a wide range of six-degree-of-freedom (6-DOF) motion measurements for performing maneuvering tests and free-running tests of target objects (offshore structures or ships). This study introduces the process of developing a wide-area motion measurement technology by incorporating the auto-tracking technology of the towing carriage system to overcome the existing 6-DOF motion measurement limitation. To realize a wide range of motion measurements, the automatic tracking control system of the towing carriage in the DOEB was designed as a speed control method. To verify the control performance, the characteristics of the towing carriage according to the variation in control gain were analyzed. Finally, a wide range of motions was tested using a model test object (a remotely operated vehicle (ROV)), and the wide-area motion measurement technology was implemented using an automatic tracking control system for a towing carriage.

The Arctic tern Sterna paradisaea: consistency and variability in spatial use at a global oceanographic scale

Elucidating the ecological factors underpinning migratory strategies of seabirds is necessary for understanding resilience to environmental change. Arctic terns Sterna paradisaea breed in the Northern Hemisphere and are unique for the global scale of their migration. Geolocator data from 37 Arctic terns breeding in a low-latitude colony, 10 of which were re-tagged in successive years, were analysed to characterise their migratory behaviour and to test the hypothesis that individuals have repeatable migration strategies. Seawater immersion data suggested a fly-forage strategy, with birds remaining on the wing at night and only foraging during daylight. Southward movement was focused initially along Atlantic eastern-boundary upwelling systems. Most terns then reoriented eastwards, crossing the southern Indian Ocean before moving south to the Antarctic. Foraging intensity differed between migration phases. Indian Ocean foraging locations were diverse, and less frequent over deep ocean basins. Foraging intensity was highest in the later stages of return migration, particularly in and around the Azores Confluence Zone. High movement speeds and foraging intensity on return migration may be adaptations to optimise reproductive success. Some aspects of migration phenology were repeatable between years, but trajectories were displaced by wind. Repeat birds did not use the same foraging areas in different years, and their trajectories across the Indian Ocean also differed. The results of this study suggest that the Indian Ocean crossing is a behaviour pattern, surviving since the last ice age, enabling Arctic terns breeding at low-latitude northwest European colonies to arrive at fragmenting Antarctic sea ice when foraging conditions are suitable.

Establishing the Foundation for the Global Observing System for Marine Life

Maintaining healthy, productive ecosystems in the face of pervasive and accelerating human impacts including climate change requires globally coordinated and sustained observations of marine biodiversity. Global coordination is predicated on an understanding of the scope and capacity of existing monitoring programs, and the extent to which they use standardized, interoperable practices for data management. Global coordination also requires identification of gaps in spatial and ecosystem coverage, and how these gaps correspond to management priorities and information needs. We undertook such an assessment by conducting an audit and gap analysis from global databases and structured surveys of experts. Of 371 survey respondents, 203 active, long-term (>5 years) observing programs systematically sampled marine life. These programs spanned about 7% of the ocean surface area, mostly concentrated in coastal regions of the United States, Canada, Europe, and Australia. Seagrasses, mangroves, hard corals, and macroalgae were sampled in 6% of the entire global coastal zone. Two-thirds of all observing programs offered accessible data, but methods and conditions for access were highly variable. Our assessment indicates that the global observing system is largely uncoordinated which results in a failure to deliver critical information required for informed decision-making such as, status and trends, for the conservation and sustainability of marine ecosystems and provision of ecosystem services. Based on our study, we suggest four key steps that can increase the sustainability, connectivity and spatial coverage of biological Essential Ocean Variables in the global ocean: (1) sustaining existing observing programs and encouraging coordination among these; (2) continuing to strive for data strategies that follow FAIR principles (findable, accessible, interoperable, and reusable); (3) utilizing existing ocean observing platforms and enhancing support to expand observing along coasts of developing countries, in deep ocean basins, and near the poles; and (4) targeting capacity building efforts. Following these suggestions could help create a coordinated marine biodiversity observing system enabling ecological forecasting and better planning for a sustainable use of ocean resources.

Numerical modelling of a Macrotidal Bay over the last 9,000 years: An interdisciplinary methodology to understand the influence of sea-level variations on tidal currents in the Bay of Brest

Estuaries play a major role in the transfer of sediments from the continents to the shelves and deep ocean basins. Their position at the interface between land and sea promotes them as a key area for the understanding of ocean sediment supply, but yet long-term evolution remains poorly understood. The main reasons of the lack of knowledge about estuaries filling are the lack of hydrodynamic data in the past and the temporal application of numerical models. Oceanographers and geologists have developed numerical models to simulate currents and sedimentation. On one hand, hydro-sediment models allow a good physical representation of estuarine hydrodynamic processes and their impact on sedimentation, but only over time-scale spanning years to decades. On the other hand, stratigraphic diffusive models aim to study the impact of various geological processes on sedimentary basins over millions of years, but they are unable to describe in detail the tidal hydrodynamic processes that govern estuaries.In response to this timescale issue, this study presents a first step attempt to explore the evolution of tidal current distribution in relation with Holocene eustatic variations and seafloor evolution. Here we focus on a macro-tidal estuary, the bay of Brest, where tidal processes dominate, as the estuary is naturally protected from ocean swells. This paper aims to set up a methodology to simulate the (past) tidal currents over a long time period and correlate them with sedimentary data. Major changes in deposit dynamics are first identified from cores and seismic data, and the corresponding paleo-topographies and paleo-sea-levels are rebuilt. A process-based hydrodynamic model (MARS3D) is then used to test the impacts of these paleo-bathymetries on hydrodynamics over a 1-year time span. Four scenarii have been considered, representing four key stages of the Holocene transgression in the Bay of Brest.The simulated barotropic currents distributions were analysed and bottom currents impact on the seafloor compared with sedimentary records to understand past hydrodynamic context and associated sediment spatial distribution over geological time scale. Hydrodynamic simulations and sediments records are linked, in order to propose a reconstruction of the tidal influence on sediments over the last 9000 years. The results show changes of the tidal patterns related to the paleoenvironmental evolution (bathymetry and sea-level variations). Even if a hydro-sediment model would be needed to make a direct correlation between simulated currents and sediment records, this successful application in the Bay of Brest shows that discontinuous modelling can help to understand tidal current evolution and their impact on sediment distribution over long periods.

The effect of COVID-19 on underwater sound

The sounds of the turning machinery and propulsion systems from commercial ships are ubiquitous in the temperate deep ocean basins. The vertical structure of seawater temperature, salinity, and pressure conspire to create nearly lossless underwater propagation conditions at 10’s and 100’s of Hz, the frequency band where diesel engines and generators, cavitating propellers, and turning gearboxes produce their peak acoustic energy. In the shallow waters of the continental margins, the human use of the ocean for transport, fishing, recreation, and construction has led to increasing levels of anthropogenic noise over the last century. As the COVID-19 pandemic spread across the globe, the reduction of economic activity and marine traffic resulted in a decrease in underwater noise, with observations made at a number of underwater listening stations and autonomous recording devices across the ocean basins. These measurements, along with ship track data provided by the automated identification system (AIS) and trade and shipping statistics, improve our knowledge of the relationship between acoustic energy, the marine acoustic habitat, and the various human uses of the ocean.

New Constraints on the Physical and Biological Controls on the Silicon Isotopic Composition of the Arctic Ocean

The silicon isotope composition of silicic acid, δ30Si(OH)4, in the deep Arctic Ocean is anomalously heavy compared to all other deep ocean basins. To further evaluate the mechanisms leading to this condition, δ30Si(OH)4 was examined on US GEOTRACES section GN01 from the Bering Strait to the North Pole. Isotope values in the polar mixed layer showed a strong influence of the transpolar drift. Drift waters contained relatively high [Si(OH)4] with heavy δ30Si(OH)4 consistent with the high silicate of riverine source waters and strong biological Si(OH)4 consumption on the Eurasian shelves. The maximum in silicic acid concentration, [Si(OH)4], within the double halocline of the Canada Basin formed a local minimum in δ30Si(OH)4 that extended across the Canada Basin, reflecting the high-[Si(OH)4] Pacific source waters and benthic inputs of Si(OH)4 in the Chukchi Sea. δ30Si(OH)4 became lighter with the increase in [Si(OH)4] in intermediate and deep waters; however, both Canada Basin deep water and Eurasian Basin deep water were heavier than deep waters from other ocean basins. A preliminary isotope budget incorporating all available Arctic δ30Si(OH)4 data confirms the importance of isotopically heavy inflows in creating the anomalous deep Arctic Si isotope signature, but also reveals a surprising similarity in the isotopic composition of the major inflows compared to outflows across the main gateways connecting the Arctic with the Pacific and the Atlantic. This similarity implies a major role of biological productivity and opal burial in removing light isotopes entering the Arctic Ocean from rivers.

Dating North Pacific Abyssal Sediments by Geomagnetic Paleointensity: Implications of Magnetization Carriers, Plio-Pleistocene Climate Change, and Benthic Redox Conditions

Non-carbonaceous abyssal fine-grained sediments cover vast parts of the North Pacific’s deep oceanic basins and gain increasing interests as glacial carbon traps. They are, however, difficult to date at an orbital-scale temporal resolution and still rarely used for paleoceanographic reconstructions. Here, we show that sedimentary records of past geomagnetic field intensity have high potential to improve reversal-based magnetostratigraphic age models. Five sediment cores from Central North Pacific mid-latitudes (39–47°N) and abyssal water depths ranging from 3,900 to 6,100 m were cube-sampled at 23 mm resolution and analyzed by automated standard paleo- and rock magnetic methods, XRF scanning, and electron microscopy. Relative Paleointensity (RPI) records were determined by comparing natural vs. anhysteretic remanent magnetization losses during alternating field demagnetization using a slope method within optimized coercivity windows. The paleomagnetic record delivered well interpretable geomagnetic reversal sequences back to 3 Ma. This age span covers the climate-induced transition from a biogenic magnetite prevalence in the Late Pliocene and Early Pleistocene to a dust-dominated detrital magnetic mineral assemblage since the Mid-Pleistocene. Volcaniclastic materials from concurrent eruptions and gravitational or contouritic sediment re-deposition along extinct seamount flanks provide a further important source of fine- to coarse-grained magnetic carriers. Surprisingly, higher proportions of biogenic vs. detrital magnetite in the late Pliocene correlate with systematically lowered RPI values, which seems to be a consequence of magnetofossil oxidation rather than reductive depletion. Our abyssal RPI records match the astronomically tuned stack of the mostly bathyal Pacific RPI records. While a stratigraphic correlation of rock magnetic and element ratio logs with standard oxygen isotope records was sporadically possible, the RPI minima allowed to establish further stratigraphic tie points at ∼50 kyr intervals. Thus, this RPI-enhanced magnetostratigraphy appears to be a major step forward to reliably date unaltered abyssal North Pacific sediments close to orbital-scale resolution.

Turbidity currents at polar latitudes: A case study of NP-28 channel in the Amundsen Basin, Arctic Ocean

Turbidity currents are fundamental marine sedimentary processes that deliver sediments from continental margins to deep ocean basins. They develop and maintain deep-sea channels that act as principal conduits for this sediment delivery. While turbidity current channels are frequently considered as analogous to river channel systems, there are distinct differences in the mechanisms that govern their formation. This is particularly true at high-latitudes, where glacial margin processes and Coriolis effects have direct consequences on the characteristics of channel-forming turbidity currents. Understanding the latitudinal controls that influence the structure of these channels is thus a key objective of this study.The geomorphology of the glacial-fed NP-28 Channel in the Amundsen Basin of the Arctic Ocean, the highest-latitude deep-sea channel on Earth, is assessed. Compilation of available bathymetry shows that the channel follows a low-gradient, low-sinuosity path roughly paralleling the base of Lomonosov Ridge. The channel has a broad cross-section comparable to other glacially-fed and tributary-fed deep-ocean channel systems. High-resolution subbottom profiles reveal intrachannel terraces and an incised channel thalweg that were likely formed through erosion by traction-dominated currents; stratified levees and sediment waves were likely deposited by overbank flows. Coriolis deflection of channel-filling currents is evident from consistent levee asymmetry along the entire channel path. Within the channel, the channel floor geometry suggests local dominance of both Coriolis and centrifugal forces at the base of the channel; the latter in particular where the channel is deflected by external bedrock. Overall, channel morphology is largely consistent with existing models of Coriolis force effects on turbidity currents at high latitudes. This influence of Coriolis force, in addition to the presumed glacial style of the sediment input and bedrock interaction with the channel path, are considered the major factors that affected channel morphology.

Early Cretaceous radiolarians and chert geochemistry from western Yarlung Tsangpo suture zone in Jiangyema section, Purang county, SW Tibet

Moderately to well-preserved radiolarian faunas belonging to 51 genera and 81 species have been identified in a continuous bedded chert succession in the Jiangyema section in the west segment of the Yarlung Tsangpo suture zone (YTSZ), Purang county, SW Tibet. Based on the occurrence of some characteristic species, especially the evolutionary appearance of age-diagnostic species two radiolarian zones and two subzones are recognized and respectively named as the Cecrops septemporatus zone, Aurisaturnalis carinatus zone, Aurisaturnalis carinatus carinatus subzone and Aurisaturnalis carinatus perforatus subzone. These can be correlated with coeval Hauterivian to upper Barremian radiolarian biozones in both the western Tethys and Japan. They provide a better biostratigraphical framework and reliable age constraints for parts of the Neotethyan Ocean floor that were subducted beneath supra-subduction zone (SSZ) ophiolites in the western part of the YTSZ.Geochemical data indicate that SiO2 contents of bedded cherts range from 79.84 to 94.65% with an average of 89.39%. Comparison of geochemical characteristics on discriminant graphs suggests that the bedded cherts were deposited in a deep oceanic basin near a continental margin. Our results and integration of available radiolarian studies from bedded cherts along the YTSZ suggest that the Neo-Tethys was a deep ocean between the Indian and Eurasian continents in which pelagic sedimentation was on-going until at least the late Barremian.

Comparison of Two 16S rRNA Primers (V3-V4 and V4-V5) for Studies of Arctic Microbial Communities.

Microbial communities of the Arctic Ocean are poorly characterized in comparison to other aquatic environments as to their horizontal, vertical, and temporal turnover. Yet, recent studies showed that the Arctic marine ecosystem harbors unique microbial community members that are adapted to harsh environmental conditions, such as near-freezing temperatures and extreme seasonality. The gene for the small ribosomal subunit (16S rRNA) is commonly used to study the taxonomic composition of microbial communities in their natural environment. Several primer sets for this marker gene have been extensively tested across various sample sets, but these typically originated from low-latitude environments. An explicit evaluation of primer-set performances in representing the microbial communities of the Arctic Ocean is currently lacking. To select a suitable primer set for studying microbiomes of various Arctic marine habitats (sea ice, surface water, marine snow, deep ocean basin, and deep-sea sediment), we have conducted a performance comparison between two widely used primer sets, targeting different hypervariable regions of the 16S rRNA gene (V3–V4 and V4–V5). We observed that both primer sets were highly similar in representing the total microbial community composition down to genus rank, which was also confirmed independently by subgroup-specific catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) counts. Each primer set revealed higher internal diversity within certain bacterial taxonomic groups (e.g., the class Bacteroidia by V3–V4, and the phylum Planctomycetes by V4–V5). However, the V4–V5 primer set provides concurrent coverage of the archaeal domain, a relevant component comprising 10–20% of the community in Arctic deep waters and the sediment. Although both primer sets perform similarly, we suggest the use of the V4–V5 primer set for the integration of both bacterial and archaeal community dynamics in the Arctic marine environment.

Multifrequency inversion of global ambient seismic sources

SUMMARYWe develop and apply a method to constrain the space- and frequency-dependent location of ambient noise sources. This is based on ambient noise cross-correlation inversion using numerical wavefield simulations, which honour 3-D crustal and mantle structure, ocean loading and finite-frequency effects. In the frequency range from 3 to 20 mHz, our results constrain the global source distribution of the Earth’s hum, averaged over the Southern Hemisphere winter season of 9 yr. During Southern Hemisphere winter, the dominant sources are largely confined to the Southern Hemisphere, the most prominent exception being the Izu-Bonin-Mariana arc, which is the most active source region between 12 and 20 mHz. Generally, strong hum sources seem to be associated with either coastlines or bathymetric highs. In contrast, deep ocean basins are devoid of hum sources. While being based on the relatively small number of STS-1 broad-band stations that have been recording continuously from 2004 to 2013, our results demonstrate the practical feasibility of a frequency-dependent noise source inversion that accounts for the complexities of 3-D wave propagation. It may thereby improve full-waveform ambient noise inversions and our understanding of the physics of noise generation.

Permian and Triassic radiolarians from chert breccia in the Nong Prue area, western Thailand: its origin and depositional setting in the Paleotethys

Bedded chert and siliceous shale successions previously regarded as the Silurian–Devonian rock units, distributed in the Nong Prue area, northwest of Kanchanaburi, western Thailand, yielded Lopingian (upper Permian) and Lower–Middle Triassic radiolarians. We found chert breccia layers in northern Nong Prue area, mainly consisting of angular to sub-angular chert clasts with matrices of silt-sized chert grains and clay minerals. We discriminated uppermost Pennsylvanian–Lopingian (upper Carboniferous–upper Permian) and Middle Triassic radiolarian-bearing chert clasts from four different levels of the chert breccia; 28 species of 15 genera with one radiolarian gen. et sp. indet. are identified. On the basis of sedimentary characteristics of the chert breccia, we suggest that the chert breccia is of sedimentary origin. The radiolarian assemblages reported here, together with previously known lithological and paleontological evidence, further indicate that the chert breccia was deposited in the Paleotethys with chert clasts derived from fine grained siliceous rocks in the continental margin to deep ocean basin of the Sibumasu Terrane.

Advanced mosasaurs from the Upper Cretaceous Nakaminato Group in Japan

AbstractPlotosaurus is a highly aquatically adapted mosasaur, which is supposed to inhabit the deep ocean basin. The geographic occurrence of this genus has been limited only to the west coast of North America. In this study, two Plotosaurus‐type mosasaur caudal vertebrae derived from the Upper Cretaceous Nakaminato Group in Ibaraki Prefecture, Japan, are described with discussion on the paleobiogeographic significance of the Late Cretaceous mosasaur fauna in the Northwestern Pacific region. The two specimens are an intermediate caudal vertebra found in a beach cobble, which presumably originated from the Hiraiso Formation (upper Campanian), and a terminal caudal vertebra found in situ in the lower Isoai Formation (lower Maastrichtian). Because their relative centrum lengths (ratio of centrum length/centrum height, ~0.7) are very close to that of Plotosaurus, the specimens are referred to cf. Plotosaurus sp. The two specimens provide the first evidence that highly specialized Plotosaurus‐type mosasaurs inhabited the Northwestern Pacific Ocean, suggesting that such forms had a wider distribution than previously recognized and might have existed since the late Campanian in the Northwestern Pacific Ocean.

Stress Drop Analysis on Banda Sea

Stress drop is a fundamental parameter of earthquake source that describes stress before and after an earthquake. The purpose of this study was to determine the tectonic characteristics of the Banda sea region. The research method used is a mathematical analysis and Nelder Mead Simplex nonlinear inversion methods. The results show that the Banda Sea is the area with complex tectonic conditions and large earthquake impacts. The Banda sea earthquake generated a stress drop of between 2 MPa -10 MPa from small to medium, it can be concluded that the rocks in the Banda Sea are relatively harder because the Banda Sea has a complex and varied oceanographic profile. The Banda Sea contains many continental fragments and has very deep ocean basins in the North Banda and Weber Deep, as well as plate rollbacks that form geometry in the shape of a spoon. The strength of rocks in the Banda Sea is also strengthened by the results of gravitational anomalies, there are significant differences in the gravity anomalies around the Banda ocean which indicate high density. This information is very important to know the amount of pressure released shortly after the earthquake which has a very large impact as a disaster mitigation measure.          Â

The Siah Cheshmeh-Khoy-Misho-Tabriz fault (NW Iran) is a cryptic neotethys suture: evidence from detrital zircon geochronology, Hf isotopes, and provenance analysis

ABSTRACT Provenance analysis including sandstone modal compositions, heavy mineral assemblages, and LA-ICP-MS U-Pb dating with in-situ Hf isotopic compositions of detrital zircons defines two types of Late Cretaceous-Eocene turbiditic sequences separated by the Siah Cheshmeh-Khoy-Misho-Tabriz Fault in the Azerbaijan Province, Northwest Iran: (1) Volcaniclastic turbidites to the southwest and (2) terrigenous turbidites to the northeast. The first appearance of Cr-spinel in Upper Cretaceous sandstones of both sequences indicates the erosion of ophiolites at that time. The southwestern volcaniclastic sandstones contain few Late Triassic and dominantly Late Cretaceous zircon grains whose Hf isotopic compositions suggest a continental magmatic arc provenance. In the northeastern terrigenous sandstones, detrital zircon U-Pb ages cluster in five main age groups from Paleoproterozoic-Neoproterozoic with Hf isotopic compositions typical of continental crust to Late Cretaceous-Palaeocene zircon grains with Hf isotopic compositions similar to those of the southwest turbidites. The provenance characteristics define, along with sedimentary structures, two separate basins, a marginal basin to the northeast, and a deep oceanic basin to the southwest. We conclude that the Siah Cheshmeh-Khoy-Misho-Tabriz Fault overprints a boundary between two lithospheres separated by ophiolite outcrops; therefore, the Siah Cheshmeh-Khoy-Misho-Tabriz Fault is a reworked suture. The Cretaceous arc, witnessed by clastic material, has been largely eroded and/or subducted.

Lower Cretaceous Provenance and Sedimentary Deposition in the Eastern Carpathians: Inferences for the Evolution of the Subducted Oceanic Domain and its European Passive Continental Margin

AbstractReconstructing orogenic systems made up dominantly by sediments accreted in trenches is challenging because of the incomplete lithological record of the subducted oceanic domain and its attached passive continental margin thrusted by collisional processes. In this respect, the remarkable ~600 km long continuity of sediments exposed in the Eastern Carpathian thin‐skinned thrust and fold belt and the availability of quantitative reconstructions for adjacent continental units provide excellent conditions for a paleogeographical study by provenance and sedimentological techniques constraining sediment routing and depositional systems. These sediments were deposited in the Ceahlău‐Severin branch of the Alpine Tethys Ocean and over its European passive continental margin. We report sedimentological, paleomagnetic, petrographic, and detrital zircon U‐Pb data of Lower Cretaceous sediments from several thin‐skinned tectonic units presumably deposited in the Moldavides domain of the Eastern Carpathians. Sedimentological observations in the innermost studied unit demonstrate that deposition took place in a deepwater basin floor sheets to sandy turbidite system. Detrital zircon age data demonstrate sourcing from internal Carpathian basement units. The sediment routing changes in more external units, where black shales basin floor sheets to sandy mud turbidites were sourced from an external, European continental area. Although some degree of mixing between sources located on both margins of the ocean occurred, constraining a relatively narrow width of the deep oceanic basin, these results demonstrate that the internal‐most studied unit was deposited near an Early Cretaceous accretionary wedge, located on the opposite internal side relative to the passive continental margin domain of other Moldavides units.

Microbial ecology and biogeochemistry of hypersaline sediments in Orca Basin

In deep ocean hypersaline basins, the combination of high salinity, unusual ionic composition and anoxic conditions represents significant challenges for microbial life. We used geochemical porewater characterization and DNA sequencing based taxonomic surveys to enable environmental and microbial characterization of anoxic hypersaline sediments and brines in the Orca Basin, the largest brine basin in the Gulf of Mexico. Full-length bacterial 16S rRNA gene clone libraries from hypersaline sediments and the overlying brine were dominated by the uncultured halophilic KB1 lineage, Deltaproteobacteria related to cultured sulfate-reducing halophilic genera, and specific lineages of heterotrophic Bacteroidetes. Archaeal clones were dominated by members of the halophilic methanogen genus Methanohalophilus, and the ammonia-oxidizing Marine Group I (MG-I) within the Thaumarchaeota. Illumina sequencing revealed higher phylum- and subphylum-level complexity, especially in lower-salinity sediments from the Orca Basin slope. Illumina and clone library surveys consistently detected MG-I Thaumarchaeota and halotolerant Deltaproteobacteria in the hypersaline anoxic sediments, but relative abundances of the KB1 lineage differed between the two sequencing methods. The stable isotopic composition of dissolved inorganic carbon and methane in porewater, and sulfate concentrations decreasing downcore indicated methanogenesis and sulfate reduction in the anoxic sediments. While anaerobic microbial processes likely occur at low rates near their maximal salinity thresholds in Orca Basin, long-term accumulation of reaction products leads to high methane concentrations and reducing conditions within the Orca Basin brine and sediments.

Structure and tectonics of the continental margins of India and the adjacent deep ocean basins: current status of knowledge and some unresolved problems

Structure and tectonics of the continental margins of India and the adjacent deep ocean basins: current status of knowledge and some unresolved problems