Bottom Currents Research Articles

FLUIDOGENIC LANDFORMS WITHIN THE PERMAFROST ZONE ON THE SHELF OF THE PECHORA AND KARA SEAS

The study is based on the results of multibeam echo-sounding and high-frequency seismic profiling during the 2018-2022 cruises of the R/V “Akademik Nikolai Strakhov” and “Akademik Boris Petrov”. Regular changes of morphometric parameters and the internal structure of pingo-like formations of the shelves of the Pechora and Kara seas were revealed. A morphometric analysis of pingo-like formations was carried out, which made it possible to draw conclusions about their relative age, as well as the role of near-bottom currents and slope processes in their modern dynamics. It was found that the density and morphological variety of pingo-like formations depend on the geological and tectonic features of the bottom area, the presence and nature of permafrost, the intensity of degassing, and the time of shelf flooding during the Holocene transgression. Pingo-like formations on the shelf, where the depth exceeds 70-80 m, emerged at the early stages of the Holocene transgression, and by now the permafrost there has largely thawed out. At the same time, pingo-like formations are still prominent in the relief and actively transformed by bottom currents, slope and, possibly, pseudovolcanic processes associated with ongoing degassing. Pingo-like formations are rare within shallow (up to 20-30 m) shelf areas close to the shore, and, apparently, continue their evolution at present. At the same time, the large thickness and continuity of permafrost prevent active fluid flow, acting as a seal. Pingo-like formations in the shallow-water zones are mainly cone-shaped mounds without intensive degassing. The density of pingo-like formations is maximum at the intermediate depths (from 20-30 to 70-80 m), in the presence of insular or discontinuous permafrost, under high fluid flow intensity within the fault zones and oil- and gas-bearing structures. Near-surface sediments in such areas are characterized by a combination of localized processes of heaving and active degassing. It predetermines a wide variety of the morphological types of pingo-like formations.Keywords: Arctic, bottom topography, degassing, pingo-like features, acoustic anomalies, multibeam echo sounding, seismoacoustic profiling

Link between Lesser Himalayan Neoproterozoic Krol succession and the Adelaide rift complex of Australia ‒ evidence from the mudstone deposits of Krol succession, Himachal Pradesh, India

The mudstone deposits, a major constituent of the Lesser Himalayan Krol succession, represents the terminal phase of Marinoan snowball melting, an important part of the most significant events of the Proterozoic time, the Cryogenian Snowball Earth. The Facies analysis suggests that the sediments were supplied to the shelf by meltwater flows, and were later dispersed to the deeper part of the shelf by current/wave-enhanced bottom currents. The upper part of the deposit bears the signature of gradual shallowing of the basin caused due to glacioisostatic rebound. Petrographic study of the samples, randomly collected from different parts of the deposit, reveals that the mud deposit is composed of silt-sized particles of quartz and dolomite, and argillaceous material. Besides the variation in proportion of these constituent components, no other material was found within this deposit. This compositional homogeneity of this huge deposit primarily suggest derivation from some preexisting sediments. Mineralogical similarity and perfect match in trace and Rare Earth elements composition suggest that this mud was derived from the periglacial loess deposits of the Nuccaleena Formation of South Australia. This is in agreement with the provenance of the underlying Krol Sandstone. The mudstones, along with the underlying Krol Sandstone, thus suggest that the Lesser Himalayan Krol succession represent an extended part of the Adelaide rift system of South Australia. The relative position of India and Australia during Neoproterozoic, as per the reconstructed configuration of the Rodinia supercontinent, corroborates this inference.

Late Pleistocene boreal molluscs in the Gulf of Cadiz: Past and current oceanographic implications

Remains of molluscs were collected from the seafloor on the north-eastern margin of the Gulf of Cadiz, between 300 and 1000 m water depth, using different sampling methods (e.g. dredging, trawling and box-coring), during several deep-sea expeditions. Samples contained a suite of species which nowadays mostly occur northwards of the English Channel, together with other widespread species. Species now locally extinct in the Gulf of Cadiz and restricted to northern latitudes, which unequivocally indicate a faunal shift, include the gastropods Buccinum undatum, Colus gracilis, Liomesus ovum and Neptunea antiqua, the bivalves Arctica islandica, Chlamys islandica, Modiolus modiolus, Mya truncata and Nuculana pernula and the scaphopod Antalis entalis. These species represent “Boreal Guests” of marked palaeoclimatic significance, some of which are reported for the first time in the Gulf of Cadiz. The boreal species collected were mostly large (>5 cm) whereas smaller boreal species were extremely scarce, probably winnowed away by strong bottom currents. The pteropod Limacina retroversa, at present restricted to water masses northwards of the Iberian Peninsula but widespread in Mediterranean sediments of the last glaciation, was also recorded. Accelerator mass spectrometry (AMS) 14C dates obtained from nine specimens of molluscs ranged between 26.1 and 14.6 kyr B.P., thus confirming their attribution to a last glacial assemblage. The abundance of these molluscan remains in the present Mediterranean Outflow Water pathway could be explained if this outflow was reduced in intensity or more likely shifted to a deeper level, leaving the upper slope in contact with suitable Atlantic intermediate waters. The findings of Boreal Guests in the Gulf of Cadiz document the continuity of the faunal shift which is well-known in the Mediterranean basin. Species still living in the Gulf of Cadiz and the Alboran Sea nevertheless account for 84.6% of specimens among the larger species.

Predicting bottom current deposition and erosion on the ocean floor

AbstractMapping sediment deposition and erosion by thermohaline ocean bottom currents is important for the development of ocean infrastructure, future geo resources and understanding the sedimentology of contourites and abyssal sediment wavefields. However, only a limited percentage (estimated 20%) of the ocean floor has been mapped directly through seismic or sonar imaging. To better delineate where zones of bottom current deposition and erosion exist, we develop a prediction from numerical model solutions and sedimentological measurements of the ocean floor. This is achieved by integrating three types of data, which include the following: (1) bottom current shear stress from a model run of the HYCOM numerical ocean model; (2) sedimentation rates from ocean lithospheric age and sediment thickness from the GlobSed Model; (3) the measured extents of bottom current deposits from sonar observations. Shear stresses and sedimentation rates inside and outside the mapped extents of bottom current deposits allow us to quantify the conditions that are conducive for bottom current deposition. These conditions are then extrapolated and displayed on a 1/12° arcsecond resolution map of the world's oceans and validated through comparison with known, mapped systems. Based on our prediction, around 12% of the ocean has significant deposition by bottom currents while only 1% has erosion. Most bottom current activity occurs where thermohaline currents impinge upon the ocean floor like on continental slopes or some areas of the abyssal plain. Deposition and erosion also occur where constriction of ocean bottom currents takes place as in straits and seaways. Inland basins (i.e. seas) and continental shelves are mostly disconnected from global‐ocean thermohaline bottom current conveyors and, therefore, have limited bottom current deposition and erosion. Mid‐ocean ridges also have little bottom current deposition due to low sediment supply.

The Modern Nile Delta Continental Shelf, with an Evolving Record of Relict Deposits Displaced and Altered by Sediment Dynamics

The most extensive coverage of surficial sediment samples collected to date on Egypt’s Nile Delta coast and shelf is needed to better define sediment dispersal patterns across this setting’s rapidly eroding margin. Changes in time are now induced by River Nile sediment cutoff by dams, sea level rise, marked shelf subsidence, and regional climate changes, which have altered the amounts and components of sediments; these require replacement, along with the implementation of more effective coastal protection measures. Multiple computer-generated offshore maps depict the distributions and proportions of sand, silt, and mud; the mean grain size and standard deviation (sorting); heavy mineral concentrations; and carbonate content. Heavy mineral lobes at the coast and offshore identify former Nile branch sites. Channel lobes extending seaward resulted from their progradational phase and from the delta’s altered sedimentation from the early to late Holocene. The progressive deposition and erosion of these fossil fluvial lobes, and of two active Nile channels, selectively removed their quartz and less dense minerals, thus concentrating heavy minerals on the coast and inner shelf. The prolonged dispersal of original sediment effluence from relict and recent Nile tributaries induced variable depositional patterns on the present shelf. These coastal depocenters, along with extensive sand, silt, and mud from shelf sediments, were reworked further seaward and dispersed by bottom currents, thus masking most previous onshore-to-offshore transport patterns. The major surficial features document long-term responses to the diverse dispersal that influenced the shoreline to the outer shelf deposits from the Pleistocene to the present.

Submarine Geomorphology and Sedimentary Features around the Egadi Islands (Western Mediterranean Sea)

In this paper, the physiography, geomorphological features, and sedimentary bedforms of the offshore Egadi Islands (Italy) have been illustrated and mapped through an integrated analysis of high-resolution bathymetric, seismo-acoustic, and sedimentological data. The study area is characterized by a wide, up to 25 km, continental shelf which is separated by a NNW-trending linear incision, the Marettimo Channel, along which several erosional and depositional features have been detected and mapped. Sedimentary prograding wedges were detected at water depths between 100–125 m along the shelf margin, which accumulated during the sea-level fall and lowstand stages of the last glacio-eustatic cycle (post- MIS 5.5). This study detected several slope breaks defining scarps across the continental shelf, which were interpreted as coastal cliffs that originated during the post-LGM eustatic sea-level rise. Several fields of different types of sedimentary bedforms, including 2-D and 3-D hydraulic dunes and sorted bedforms, were found across the continental shelf, providing evidence of a high hydrodynamic regime affecting the seafloor. Further on, this study recognized erosive and depositional features related to bottom currents (contourites) in the Marettimo Channel. These findings provide a better understanding of the morpho-sedimentary evolution of the Egadi Islands offshore in the latest Quaternary. Moreover, they offer essential scientific support for effectively managing the most valuable priority habitats for conservation, such as the Posidonia oceanica meadow and coralline algae bioconstructions (Coralligenous habitat).

Early life strategies and juvenile mortality in Favosites (Anthozoa, Tabulata) from the Middle Devonian of the Mader Basin (Anti-Atlas, Morocco)

An exceptionally well-preserved population of numerous tiny, juvenile colonies of the tabulate coral Favosites goldfussi was discovered in the lower Eifelian of the Ouihlane section, Anti-Atlas, Morocco. The studied coralla provide a unique opportunity to reconstruct the details of the astogeny, i.e., the early growth of a colony, including the ontogeny of the protocorallite, and the process of offsetting of the first hysterocorallites. The investigated material afforded also an opportunity for the so far most detailed analysis of the early colony development in favositid corals. The studied population inhabited a relatively deep-water environment, with the colonies growing on a soft, muddy substrate exposed to constant unidirectional bottom currents. These environmental factors resulted in asymmetrical development of the colonies, which, starting from the early growth stages, developed specific, mud-sticker or mud-catcher growth strategies that stabilised their position on the seafloor. Despite their adaptation to the relatively unstable environment, including the dominance of the soft muddy substrate and exposure to unidirectional bottom currents, the studied favositid colonies were, however, apparently unable to cope with irregular events of high sediment influx, which resulted in their high juvenile mortality.

Observation and Parameterization of Bottom Shear Stress and Sediment Resuspension in a Large Shallow Lake

AbstractParameterizations for bottom shear stress are required to predict sediment resuspension from field observations and within numerical models that do not resolve flow within the viscous sublayer. This study assessed three observation‐based bottom shear stress (τb) parameterizations, including (a) the sum of surface wave stress and mean current (quadratic) stress (); (b) the log‐law (τb = τL); and (c) the turbulent kinetic energy (τb = τTKE); using 2 years of observations from a large shallow lake. For this system, the parameterization τb = τw + τc was sufficient to qualitatively predict resuspension, since bottom currents and surface wave orbitals were the two major processes found to resuspend bottom sediments. However, the τL and τTKE parameterizations also captured the development of a nepheloid layer within the hypolimnion associated with high‐frequency internal waves. Reynolds‐averaged Navier‐Stokes (RANS) equation models parameterize τb as the summation of modeled current‐induced bottom stress (τc,m) and modeled surface wave‐induced bottom stress (τw,m). The performance of different parameterizations for τw,m and τc,m in RANS models was assessed against the observations. The optimal parameterizations yielded root‐mean‐square errors of 0.031 and 0.025 Pa, respectively, when τw,m, and τc,m were set using a constant canonical drag coefficient. A RANS‐based τL parameterization was developed; however, the grid‐averaged modeled dissipation did not always match local observations, leading to O(10) errors in prediction of bottom stress. Turbulence‐based parameterizations should be further developed for application to flows with mean shear‐free boundary turbulence.

Analysis of mooring-observed bottom current on the northern continental shelf of the South China Sea

The bottom current is an important component of the three-dimensional ocean circulation, which is of significance for the safeties of ocean bottom engineering and facilities, the research on sediment and pollutant transports, and the ecological environment. Due to the lack of observation data, research on the bottom current in the South China Sea (SCS) has been limited. This study systematically analyzes bottom currents from 0.5 to 5 m above the seafloor based on 20-month-long mooring observations on the shelf slope west of the Dongsha Atoll. The spectral analysis results indicate that currents induced by the internal tides and the internal solitary waves (ISWs) comprise dominant constituents of bottom currents with velocity amplitudes up to O(50–90) cm/s. Dominant internal tide-induced bottom currents of the velocity amplitude of O(50) cm/s occupy 53% of the total horizontal kinetic energy. The pulsating ISW-induced bottom currents reach a maximum amplitude of 93 cm/s, which has intrinsic relation to the amplitude of the ISWs, according to the soliton propagation speed solutions of the Korteweg–De Vries (KdV) equation. The background bottom current is strongest in winter, followed by spring, and weak in summer and autumn, which is closely correlated with the behavior of mesoscale eddies. These results are of significance for understanding the dynamics of the bottom boundary layer.

Sedimentary response to the early Silurian (Rhuddanian-Aeronian) post-glacial transgression and Kwangsian Orogeny in the Upper Yangtze region, South China

Unlike the lowermost Silurian ‘hot’ shales deposited in many other regions of the world, the early Silurian (Rhuddanian-Aeronian) succession in South China exhibits a distinctive lithofacies transition from organic-rich black siliceous mudstone in the early Rhuddanian, to interlaminated siltstone and mudstone with relatively low total organic carbon content in the late Rhuddanian, and finally to organic-lean argillaceous mudstone or calcareous mudstone in the late Aeronian. The interlaminated siltstone and mudstone interval has been the subject of a long-standing dispute over its nature, whether it belongs to turbidite or contourite. In this study, we employed a range of approaches, including petrographic analyses, UPb dating of detrital zircons, and measurements of paleocurrent directions to generate a dataset that reveals: (1) four typical fine-grained facies developed within the interlaminated siltstone and mudstone interval, (2) detrital zircon UPb ages yielding a wide age range from Archean (∼2.50 Ga) to early Paleozoic (0.42 Ga), with a predominant peak of ca. 447 Ma based on 516 detrital zircon UPb analyses, and (3) paleocurrent directions indicate the presence of current circulation in the Upper Yangtze region. The results of this study demonstrated that the detritus of the interlaminated siltstone and mudstone interval originated from concurrent explosive volcanic eruptions along the southeastern margin of the South China Craton. It was deposited and reworked by bottom currents under the background of the early Silurian post-glacial transgression. We suggest that the occurrence of the interlaminated siltstone and mudstone interval is indicative of a far-field sedimentary response to the onset and progradation of the Kwangsian Orogeny in the Upper Yangtze region.

Dynamic response of water flow and sediment transport off the Yellow River mouth to tides and waves in winter

Freshwater and sediments are transported from the Yellow River mouth downstream along the coast into Laizhou Bay under the northeasterly wind in winter. Numerical experiments indicate that sediment transport shows the tendency of convergence in the river mouth, divergence in the downstream area, and convergence in the north of Laizhou Bay. Tides and waves are the two main forcings affecting the transport of water and sediments off river mouths. For the high-turbidity Yellow River mouth and the adjacent sea, tidal forcing enhances the subtidal downstream transport of water and sediments off the river mouth into Laizhou Bay, whereas wave forcing has little effect on the advection of water and sediments. Sediment resuspension is controlled by the bottom shear stress induced by tides and waves. The tide-induced bottom shear stress is higher in the north of Laizhou Bay and south of Bohai Bay due to the stronger bottom tidal current. The wave-induced bottom shear stress plays a more important role in sediment resuspension, which is higher in the nearshore region along the Yellow River Delta away from the coast to some extent on account of the maximum near-bottom wave orbital velocity. Tidal mixing strengthens the upward diffusion of the bottom suspended sediments. Without tidal forcing, there is an interesting phenomenon along the Yellow River Delta. In the nearshore region, the decreased bottom shear stress suspends less sediment above the bed. However, in the offshore region, the enhanced stratification hinders the upward diffusion of the bottom sediment due to the lack of tidal mixing, resulting in higher suspended sediment concentration (SSC) in the bottom layer.

Channel inception through bottom‐current erosion of pockmarks revealed by numerical simulation

AbstractIn deep‐marine environments, the inception of channels can be induced by the interaction between bottom currents and rough topography. However, it is still unclear under which conditions such features can form and what happens in the earliest phase of channel development. In this study, based on the morphological, sedimentary and oceanographic settings of a pockmark field in the NW South China Sea, we reveal the process of channel inception through the erosion of pockmarks by bottom currents. Using numerical simulations, we show that an appropriate current velocity can induce the erosion of pockmark trains in cohesive sediments, leading to the coalescence of discrete pockmarks and the formation of a channel with a rough thalweg. The interaction of bottom currents with the pockmarks induces a significant erosion along the pockmarks axis. Bottom‐current erosion is strongest at the downstream edges of pockmarks, where the horizontal velocity reaches a maximum and an upwelling forms. Erosion increases as the distance between pockmarks reduces. In our simulation results, a channel is only formed by the coalescence of pockmarks if the distance between pockmarks is <6 times the diameter of the pockmark. This study provides evidence of the formation of channels by bottom currents, which helps reconstruct palaeoceanographic conditions based on sediment architecture. It also shows the complex hydrodynamics at these structures that strongly control sedimentary processes and may affect distribution of benthic ecosystems in marine environments.

The Role of Tropical Cyclone on Changjiang River Subaqueous Delta Geomorphology: A Numerical Investigation of Tropical Cyclone Danas (2019)

AbstractSubaqueous deltas are facing growing threats of erosion. Tropical cyclones (TCs), which are predicted to affect mid‐latitude estuaries more frequently with higher intensity in the future, can play an important role in subaqueous delta geomorphology change. Nevertheless, direct in situ measurement of TC‐induced bed elevation change (BEC) was rarely reported before. In this study, four tripods were deployed at different parts of the Changjiang River subaqueous delta (CRSD) to capture BEC during TCs in July 2019. Rapid erosion (20 mm day−1) during TC Danas (2019) was observed by high precision bed elevation measurement, over 10 times faster than that during calm weather conditions. Moreover, the recorded rates of BEC varied substantially among these stations. A coupled three‐dimensional wave‐current‐sediment model was then developed for the CRSD to help understand the spatial variability of observed BEC. The model results showed two striking features of BEC during TC Danas: (a) enhanced sediment accretion near the sharp bottom salinity front offshore of the north channel; (b) marked erosion near the front of CRSD. Further analysis showed that the converging bottom currents in concert with enhanced sediment resuspension during TC Danas led to significant convergence of sediment flux near the bottom salinity front. In addition, the diverging isobaths along the front of the CRSD resulted in the onshore deviation of bottom current and diverging sediment flux under the downwelling‐favorable winds of Danas. Many TCs in the CRSD generate similar downwelling‐favorable winds and currents, growing TC threats in the future may therefore exacerbate erosion in this region.

Post-phosphogenesis processes and the natural beneficiation of phosphates: Geochemical evidence from the Moroccan High Atlas phosphate-rich sediments

The Upper Cretaceous-Paleogene sedimentary series of the High Atlas hosts underexplored phosphate-rich sediments and presents an excellent example of how complex interactions between various geological processes control the accumulation and distribution of phosphates. Genetically-linked phosphatic lithofacies from this series were investigated for their mineralogical and geochemical compositions with the purpose of assessing the impact of post-phosphogenesis sedimentary processes on the geochemical behavior of genetically-linked phosphatic lithofacies and understanding the distribution of chemical elements. Sampled phosphatic horizons from five representative sections along the Marrakesh High Atlas borders were investigated using optical and scanning electron microscopies (OM and SEM), X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), and inductively coupled plasma mass spectrometry (ICP-MS) techniques. Results indicated that the studied lithofacies are composed of a mixture of phosphatic and non-phosphatic particles in variable proportions depending on the facies type. However, the phosphatic fraction is dominated by carbonate fluorapatite (CFA), showing an average CO32− content of 8.40 ± 0.37 wt%. Geochemical results revealed variations in the chemical composition of the different phosphate facies. Low-grade pristine phosphatic sediment (P2O5 < 12%wt.) can be turned by the effects of storms and bottom currents into high-grade granular phosphate beds containing up to 24 wt% P2O5. This natural enrichment is mirrored in the bulk rock by increasing P2O5 concentrations against decreasing detrital phase-associated chemical elements (K, Al, Si, and Ti). Among the investigated lithofacies, karst-filling phosphate show relatively lower uranium and cadmium contents as it experienced a polyphase evolution, including winnowing, submarine reworking, transport, and subaerial weathering, traced using some redox-sensitive proxies such as cerium and uranium. Despite this polyphase sedimentary differentiation, the apatite resisted chemical changes and preserved its original geochemical signature, specifically the rare earth elements signal that reflects oxic seawater conditions. However, the differentiation processes modified the bulk rock composition of phosphates through the preferential leaching of gangue phases (e.g., carbonates). Furthermore, these processes triggered the oxidation of organic matter and sulfides, removing some associated elements of environmental concern, such as uranium and cadmium. Conclusively, the combination of mechanical winnowing/reworking and subaerial chemical weathering is the most effective way of the natural beneficiation of phosphate, both economically and environmentally.

Tidal to decadal scale hydrodynamics at two contrasting cold-water coral sites in the Northeast Atlantic

Cold-water corals (CWCs) thrive in areas with complex and rough topography favoring the development of highly diverse benthic communities. Several biotic and abiotic factors including organic matter supply, temperature, bottom roughness and currents are important drivers of ecosystem structure and functioning in deep-sea environments at different spatial and temporal scales. Little is known, however, how basin-scale changes in the ocean climate affect these drivers at local scales. Here, we use high-resolution implementations of the hydrodynamic model ROMS-AGRIF for estimating characteristic spatial and temporal scales of local hydrodynamics in response to variations of basin-scale currents imposed by distinct changes of the Atlantic Meridional Overturning Circulation (AMOC) in the past century. We focus on two CWC communities on the SE Rockall Bank slope and at Condor Seamount. We considered two contrasting AMOC states that were identified from the 1958–2009 hindcast of the 1/20° resolution VIKING20 North Atlantic basin-scale ocean circulation model and used as boundary conditions for the high-resolution local area models. At SE Rockall Bank, variability of near-bottom currents in both regions was largely dominated by tidal dynamics, but strongly modified by AMOC induced basin-scale variations of water mass properties and bottom currents. During strong AMOC years, waters in the main CWC depth corridor (600–1200 m) were cooler and less saline but were dominated by stronger bottom currents when compared with conditions during weak AMOC years. At Condor Seamount, bottom currents were largely unaffected by AMOC related changes close to the summit at water depths < 400 m. Kinetic energy dissipation rates derived from the 3D near-bottom velocity field appeared to positively relate with the in-situ CWC distribution. Kinetic energy dissipation is therefore proposed as a mechanistic descriptor of CWC presence as it provides a more mechanistic view of hydrodynamics driving organic matter supply to filter and suspension-feeding communities.

Response of the benthic biomass-size structure to a high-energy submarine canyon

IntroductionBody size regulates all biological processes, including growth, reproduction, metabolism, trophic interactions, etc., and is the master trait across organisms, populations, and communities. Despite a rich literature on the impacts of human and natural disturbances on body size, a clear knowledge gap is the effect of the submarine canyons on the benthic size structures in the deep sea, hindering our understanding of the ecological processes of these dominant ecosystems on the continental margin.MethodsTherefore, we conducted repeated sediment sampling to compare meiofauna and macrofauna biomass body-size spectrum, growth, metabolism, and size composition from a high-energy submarine canyon, Gaoping Submarine Canyon (GPSC), and the adjacent continental slope off SW Taiwan. The GPSC is a dynamic ecosystem connected to a high sediment-yield small mountain river subjected to strong internal-tide energy, swift bottom currents, frequent mass wasting events, and high terrestrial sediment inputs.ResultsWe found that the meiofauna and macrofauna were characterized by relatively larger individuals dominating on the slope to smaller ones dominating in the canyon. As a result, the community biomass, secondary production, and respiration were depressed with distinctive biomass-size composition in the canyon compared to the non-canyon slope. The environmental factors related to internal tide disturbance (i.e., bottom current velocity, duration of sediment erosion, or low light transmission) substantially influence the body size composition of the canyon benthos, while food supplies (i.e., TOC and C/N ratio) and sediment characters (i.e., grain size and porosity) correlated closely with the slope communities.DiscussionWe concluded that the disturbed condition in the GPSC may have wiped out or depressed the local benthic assemblages, and only the smaller, more resilient species could persist. Our results also highlight that the alterations of the canyon benthic community could be a reference to deep-sea ecosystems under anthropogenic disturbances or global climate change.

Occurrence and behavioral rhythms of the endangered Acadian redfish (Sebastes fasciatus) in the Sambro Bank (Scotian Shelf)

The genus Sebastes is a morphologically and ecologically diverse genus of rockfish characterized by high longevity, late-maturity and low natural mortality. On the northwest Atlantic continental shelf, the Acadian redfish (Sebastes fasciatus) is the most common rockfish species above 300 m depth. This species has been widely exploited resulting in the depletion or collapse of most of its stocks. Management of long-lived species with intricate life-history characteristics is challenging and requires highly integrated biological and oceanographic monitoring, which allow the identification of environmental drivers and demographic and behavioral trends. The present study uses high-temporal resolution imaging and environmental data, acquired with an autonomous lander deployed for 10-months at the Sambro Bank Sponge Conservation Area (Scotian Shelf) to elucidate S. fasciatus temporal dynamics and behavioral trends in response to near-bed environmental conditions. S. fasciatus, mostly displayed passive locomotion and static behaviors, in common with other shelf-dwelling Sebastes species. Structural complexity provided by sponges positively influenced S. fasciatus presence. Fish used sponges to avoid being dragged by bottom currents. Hydrodynamics appear to act as a synchronizing factor conditioning its swimming behavior. S. fasciatus total counts exhibited a seasonal shift in rhythm’s phase likely reflecting changes in lifestyle requirements. This study provides new insights on S. fasciatus dynamics and behavior. Nonintrusive monitoring approaches, such as the one used in this study, will be key to monitor this threatened species populations. Especially, since it is expected that S. fasciatus will experience distribution shifts to higher latitudes due to future climate stressors.

The effect of deep ocean currents on ocean- bottom seismometers records

Abstract. Ocean-bottom seismometers (OBSs) are usually deployed for seismological investigations, but these objectives are impaired by noise resulting from the ocean environment. We split the OBS-recorded seismic noise into three bands: short periods, microseisms and long periods, also known as tilt noise. We show that bottom currents control the first and third bands, but these are not always a function of the tidal forcing. Instead, we suggest that the ocean bottom has a flow regime resulting from two possible contributions: the permanent low-frequency bottom current and the tidal current. The recorded noise displays the balance between these currents along the entire tidal cycle, between neap and spring tides. In the short-period noise band, the ocean current generates harmonic tremors corrupting seismic dataset records. We show that, in the investigated cases, the harmonic tremors result from the interaction between the ocean current and mechanical elements of the OBS that are not essential during the sea bottom recording and thus have no geological origin. The data from a new broadband OBS type, designed and built at Instituto Dom Luiz (IDL – University of Lisbon)/Centre of Engineering and Product Development (CEIIA), hiding non-essential components from the current flow, show how utmost harmonic noise can be eliminated.

Foraminiferal sandy contourite of the Limpopo Corridor (Mozambique margin): Facies characterization and paleoceanographic record

Contourites encompass a wide variety of sedimentary facies. Some of them show common facies with others deep-water deposits, such as turbidites and hemipelagites. Sedimentological characterization at macro- and microscales is valuable to discriminate those close facies but the distinction is not always clear. Contourites are increasingly used in paleoceanographic and paleoclimatic reconstructions. Improving their characterization is therefore essential to their interpretations.This study aims at characterizing a foraminiferal sandy contourite facies from a sediment core collected on top of a contourite drift under the influence of the Antarctic Intermediate Water/North Atlantic Deep-Water interface located in the Limpopo Corridor (Mozambique margin, Indian Ocean) at 2000 m depth. This work is based on a detailed analysis of the sedimentary record using physical properties (gamma density and magnetic susceptibility) as well as laser grain size and X-Ray Fluorescence core scanning data.Our results show that: (1) the foraminiferal sand is vertically continuous and homogenous over 633 kyr with an average sedimentation rate of 0.26 cm.kyr−1; (2) this contourite facies results from sedimentation with low terrigenous inputs under stable hydrodynamic conditions over time, with relatively strong bottom currents for this water depth (∼16 cm.s−1); (3) during glacial stages, sedimentation rates are lower and bottom current speed is higher than during interglacial stages. We also propose the concept of “Contourite Graphical Chart” which summaries the theoretical distribution of contourite endmembers from cross-plot of “sorting versus grain-size median D50” and their related sedimentary processes.This study highlights the relevance of condensed foraminiferal sandy contourites as long time-scale paleoceanographic archives and provides new insights for studies related to paleoclimatology and paleoecology.

The Late Miocene‐Early Pliocene Biogenic Bloom: An Integrated Study in the Tasman Sea

AbstractThe Late Miocene‐Early Pliocene Biogenic Bloom (∼9–3.5 Ma) was a paleoceanographic phenomenon defined by anomalously high accumulations of biological components at multiple open ocean sites, especially in certain regions of the Indian, and Pacific oceans. Its temporal and spatial extent with available information leaves fundamental questions about driving forces and responses unanswered. In this work, we focus on the middle part of the Biogenic Bloom (7.4–4.5 Ma) at International Ocean Discovery Program Site U1506 in the Tasman Sea, where we provide an integrated age model based on orbital tuning of the Natural Gamma Radiation, benthic foraminiferal oxygen isotopes, and calcareous nannofossil biostratigraphy. Benthic foraminiferal assemblages suggest changes in deep water oxygen concentration and seafloor nutrient supply during generally high export productivity conditions. From 7.4 to 6.7 Ma, seafloor conditions were characterized by episodic nutrient supply, perhaps related to seasonal phytoplankton blooms. From 6.7 to 4.5 Ma, the regime shifted to a more stable interval characterized by eutrophic and dysoxic conditions. Combined with seismic data, a regional change in paleoceanography is inferred at around 6.7 Ma, from stronger and well‐oxygenated bottom currents to weaker, oxygen‐depleted bottom currents. Our results support the hypothesis that the Biogenic Bloom was a complex, multiphase phenomenon driven by changes in ocean currents, rather than a single uniform period of sustained sea surface water productivity. Highly resolved studies are thus fundamental to its understanding and the disentanglement of local, regional, and global imprints.