Oceanic Bottom Research Articles

The Evolution and Arrest of a Turbulent Stratified Oceanic Bottom Boundary Layer over a Slope: Upslope Regime and PV Dynamics

AbstractThe influence of a sloping bottom and stratification on the evolution of an oceanic bottom boundary layer (BBL) in the presence of a mean flow is explored. As a complement to an earlier study by Ruan et al. (https://doi.org/10.1175/JPO-D-18-0079.1) examining Ekman arrest in a downslope regime, this paper describes turbulence and BBL dynamics during Ekman arrest in the upslope regime. In the upslope regime, an enhanced stratification develops in response to the upslope Ekman transport and suppresses turbulence. Using a suite of large-eddy simulations, we show that the BBL evolution can be described in a self-similar framework based on a nondimensional number X/Xa. This nondimensional number is defined as the ratio between the lateral displacement of density surfaces across the slope X and a displacement Xa required for Ekman arrest; the latter can be predicted from external parameters. Additionally, the evolution of the depth-integrated potential vorticity is considered in both upslope and downslope regimes. The PV destruction rate in the downslope regime is found to be twice the production rate in the upslope regime, using the same definition for the bottom mixed layer thickness. It is shown that this asymmetry is associated with the depth scale over which turbulent stresses are active. These results are a step toward improving parameterizations of BBL properties and evolution over sloping topography in coarse-resolution ocean models.

Underwater Wireless Sensor Networks: Estimation of Acoustic Channel in Shallow Water

Underwater sensor networks (UWSN) include a large number of devices and sensors which are positioned in a specific area to carry out monitoring in cooperation with each other as well as data collection. In this paper it has been studied and simulated the performance of an extremely important parameter for communication in UWSN such as the acoustic channel capacity as function of water temperature and salinity arise. The performance’s knowledge on acoustic channel may be improved with a deep study of its dependence by season, weather conditions or environmental parameters variation. If an accurate estimation of the acoustic communication capacity utilization for a given area is required, we must consider also the bottom materials of this area. The simulation results presented in this study through an improved algorithm, will help to understand better the underwater acoustic channel performance as a function of all these factors. This is of particular importance for acoustic modems designing, in order to implement suitable functionalities able to adapt the data transmission capacity of the acoustic link to the structure of the oceanic bottom and its component material.

The Evolution and Arrest of a Turbulent Stratified Oceanic Bottom Boundary Layer over a Slope: Downslope Regime

AbstractThe dynamics of a stratified oceanic bottom boundary layer (BBL) over an insulating, sloping surface depend critically on the intersection of density surfaces with the bottom. For an imposed along-slope flow, the cross-slope Ekman transport advects density surfaces and generates a near-bottom geostrophic thermal wind shear that opposes the background flow. A limiting case occurs when a momentum balance is achieved between the Coriolis force and a restoring buoyancy force in response to the displacement of stratified fluid over the slope: this is known as Ekman arrest. However, the turbulent characteristics that accompany this adjustment have received less attention. We present two estimates to characterize the state of the BBL based on the mixed layer thickness: Ha and HL. The former characterizes the steady Ekman arrested state, and the latter characterizes a relaminarized state. The derivation of HL makes use of a newly defined slope Obukhov length Ls that characterizes the relative importance of shear production and cross-slope buoyancy advection. The value of Ha can be combined with the temporally evolving depth of the mixed layer H to form a nondimensional variable H/Ha that provides a similarity prediction of the BBL evolution across different turbulent regimes. The length scale Ls can also be used to obtain an expression for the wall stress when the BBL relaminarizes. We validate these relationships using output from a suite of three-dimensional large-eddy simulations. We conclude that the BBL reaches the relaminarized state before the steady Ekman arrested state. Calculating H/Ha and H/HL from measurements will provide information on the stage of oceanic BBL development being observed. These diagnostics may also help to improve numerical parameterizations of stratified BBL dynamics over sloping topography.

An Integrated Method for Determining the Oceanic Bottom Mixed Layer Thickness Based on WOCE Potential Temperature Profiles

AbstractIn this study we examined the applicability of the threshold, curvature, maximum angle, and relative variance methods for identifying the oceanic bottom mixed layer (BML) thickness . Using full-depth temperature profiles along 17 WOCE sections covering the Atlantic, Indian, and Pacific Oceans, we found that the BML thicknesses determined based on the threshold, curvature, and maximum angle methods had wider 95% confidence intervals and much lower quality indexes compared with those based on the visual inspection (). The relative variance method appeared to perform better than the other methods because the 95% confidence interval and (0.60) values were closer to those determined based on the visual inspection, although differences were still present. We then proposed an integrated method by optimizing the possible values obtained from the four methods. The BML thicknesses determined using the integrated method were closest to those based on the visual inspection according to the higher (0.64) and more stations (71%) with . Compared with the results in previous studies, the integrated method determined the consistent BML thicknesses in most regions (e.g., the northern Atlantic), and it also effectively identified the BML thicknesses in some regions where the BML was considered to be not readily detectable (e.g., the Madeira Abyssal Plain).

Wave boundary layers in rotating stratified fluid and near-inertial oscillations

Theory of wave boundary layers (WBLs) developed by Reznik (J Mar Res 71: 253–288, 2013, J Fluid Mech 747: 605–634, 2014, J Fluid Mech 833: 512–537, 2017) is extended to a rotating stratified fluid. In this case, the WBLs arise in the field of near-inertial oscillations (NIOs) driven by a tangential wind stress of finite duration. Near-surface Ekman layer is specified in the most general form; tangential stresses are zero at the lower boundary of Ekman layer and viscosity is neglected below the boundary. After the wind ceases, the Ekman pumping at the boundary becomes a linear superposition of inertial oscillations with coefficients dependent on the horizontal coordinates. The solution under the Ekman layer is obtained in the form of expansions in the vertical wave modes. We separate from the solution a part representing NIO and demonstrate development of a WBL near the Ekman layer boundary. With increasing time t, the WBL width decays inversely proportional to $$ \sqrt{t} $$ and gradients of fields in the WBL grow proportionally to $$ \sqrt{t} $$ ; the most part of NIO is concentrated in the WBL. Structure of the WBL depends strongly on its horizontal scale L determined by scale of the wind stress. The shorter the NIO is, the thinner and sharper the WBL is; the short-wave NIO with L smaller than the baroclinic Rossby scale LR does not penetrate deep into the ocean. On the contrary, for L ≥ LR, the WBL has a smoother vertical structure; a significant long-wave NIO signal is able to reach the oceanic bottom. An asymptotic theory of the WBL in rotating stratified fluid is suggested.

GLOBAL SUSPENDED SEDIMENT YIELD: ITS GEOMORPHOLOGICAL AND GEOECOLOGICAL INTERPRETATION

The present-day suspended sediment yield into the Ocean is estimated at 14.3 · 109 t/year or 147 t/km2 · year, which consists 1.3 times of its natural value. Average river water turbidity amounts to 327 g/m3. The maximum of suspended sediment yield sites in the Southeast of Asia; the minimum - in the plains of Eurasia and North America adjoining the Arctic Ocean. Oceanic sediment accumulation equals to 40 t/km2 · year. Decrease of land height averages 1 m in 19 400 years and rise of oceanic bottom - 1 m in 40 300 years. The greatest possible value of suspended sediment yield may reach 44-45 · 109 t/year under the condition of complete agricultural land conversion.

Genesis of massive sulfide deposits in the Verkhneural’sk ore district, the South Urals, Russia: Evidence for magmatic contribution of metals and fluids

Melt inclusions and aqueous fluid inclusions in quartz phenocrysts from host felsic volcanics, as well as fluid inclusions in minerals of ores and wall rocks were studied at the Cu-Zn massive sulfide deposits in the Verkhneural’sk ore district, the South Urals. The high-temperature (850–1210°C) magmatic melts of volcanic rocks are normal in alkalinity and correspond to rhyolites of the tholeiitic series. The groups of predominant K-Na-type (K2O/Na2O = 0.3–1.0), less abundant Na-type (K2O/Na2O = 0.15–0.3), and K-type (K2O/Na2O = 1.9–9.3) rhyolites are distinguished. The average concentrations (wt %) of volatile components in the melts are as follows: 2.9 H2O (up to 6.5), 0.13 Cl (up to 0.28), and 0.09 F (up to 0.42). When quartz was crystallizing, the melt was heterogeneous, contained magnetite crystals and sulfide globules (pyrrhotite, pentlandite, chalcopyrite, bornite). High-density aqueous fluid inclusions, which were identified for the first time in quartz phenocrysts from felsic volcanics of the South Urals, provide evidence for real participation of magmatic water in hydrothermal ore formation. The fluids were homogenized at 124–245°C in the liquid phase; the salinity of the aqueous solution is 1.2–6.2 wt % NaCl equiv. The calculated fluid pressure is very high: 7.0–8.7 kbar at 850°C and 5.1–6.8 kbar at 700°C. The LA-ICP-MS analysis of melt and aqueous fluid inclusions in quartz phenocrysts shows a high saturation of primary magmatic fluid and melt with metals. This indicates ore potential of island-arc volcanic complexes spatially associated with massive sulfide deposits. The systematic study of fluid inclusions in minerals of ores and wall rocks at five massive sulfide deposits of the Verkhneural’sk district furnished evidence that ore-forming fluids had temperature of 375–115°C, pressure up to 1.0–0.5 kbar, chloride composition, and salinity of 0.8–11.2 (occasionally up to 22.8) wt % NaCl equiv. The H and O isotopic compositions of sericite from host metasomatic rocks suggest a substantial contribution of seawater to the composition of mineral-forming fluids. The role of magmatic water increases in the central zones of the feeding conduit and with depth. The dual nature of fluids with the prevalence of their magmatic source is supported by S, C, O, and Sr isotopic compositions. The TC parameters of the formation of massive sulfide deposits are consistent with the data on fluid inclusions from contemporary sulfide mounds on the oceanic bottom.

Diffuse flows of high-temperature fluids and their role in the formation of hydrothermal deposits

The compositions of the hydrothermal deposits formed at the oceanic bottom by diffuse and concentrated high-temperature hydrothermal flows are highly variable. The role of these flows in the formation of hydrothermal ores varies depending on the maturity of the hydrothermal mounds, the volcanism, and other factors. The average composition of the mature hydrothermal deposits is largely determined by the diffuse fluxes of hydrothermal fluids rather than by their concentrated flows.

The origin of Bahia seamounts (Brazil Basin, South Atlantic) in connection to new data on their age

Micropalentological studies of limestones and isotopic-geochronological determinations of the age of zircons found in vulcanites dredged from the region of the northern chain of the group of Bahia seamounts in the 28th cruise of the R/V Akademik Vavilov were undertaken. The recognized age difference of the northern chain of the Bahia seamounts of northwestern strike from 75 to 84 Ma from the southeast to the northwest indicates the fact that this chain was formed as a result of hot spot activity. The age of vulcanites is in good correlation with the Maastricht and Coniacian-Santonian age of limestones occurring on the slopes of the seamounts. The similarity of the seamounts age and the age of the oceanic bottom indicates the fact that the hot spot was localized near the spreading axial zone of the MAR. Along the strike of the chain, the centers of volcanic activity are correlated to the zones of transform faults, which are important in the hot spot activation. The formed chain is a weak zone in the lithosphere and the focus of younger stages of volcanic activity; there were at least two of stages within the northern chain of the Bahia seamounts, and these occurred about 75 and 59 Ma ago.

Tectonic types of deepwater basins and structural segmentation of the North Atlantic

Typification of tectonic structures is one of the important lines of tectonic research. Recently, I have published several articles, which are concerned with deepwater oceanic basins. This paper is focused on tectonic typification of deepwater basins of the North Atlantic. They are attributed to three types: perispreading, central thalassogenic, and pericontinental. The first type comprises the Irminger, Iceland, Greenland, and Lofoten basins. The first two basins are associated with the Reykjanes Ridge and the two others, with the Mohns Ridge. The central thalassogenic type is exemplified in the Norwegian Basin, while the pericontinental type in the Rockall Trough. Two systems of basins are distinguished by morphostructural and historical-geological features: the northern system of the Oligocene-Quaternary structures and the southern system of the Paleocene-Quaternary structures. The Greenland-Faroe tectonovolcanic zone serves as their tectonic interface. In the tectonic typology of their deepwater basins, the North Atlantic is closer to the Indian than to other oceans. The present-day configuration of the northern basins is determined by neotectonics. The tectonic movements in the northern system of basins at this stage were more contrasting than in the southern system. This explains the greater depth of the former basins. The spatial position of the deepwater basins belonging to different types determines the tectonic segmentation of the oceanic bottom. The southern, central, and northern latitudinal segments correspond to different geodynamic states of the Earth’s interior.

Records of sedimentary dynamics in the continental shelf and upper slope between Aveiro–Espinho (N Portugal)

The sedimentary unconsolidated cover of the Aveiro–Espinho continental shelf and upper slope (NW Portugal) records a complex interplay of processes including wave energy and currents, fluvial input, sediment transport alongshore and cross-shelf, geological and oceanographic processes and sediment sources and sinks. In order to study this record, a set of surface sediment samples was studied. Sediment grain size and composition, as well as the mineralogical composition (by XRD) of the fine (<63μm) and clay (<2μm) fractions and benthic microfaunal (foraminifera) data were analysed. Cluster analysis applied to the sedimentological data (grain size, sediment composition and mineralogy) allowed the establishment of three main zones corresponding to the: inner-, mid- and outer-shelf/upper slope.On the inner-shelf, the sedimentary coverture is composed of siliciclastic fine to very fine sand, essentially comprising modern (immature) terrigenous particles. The sediment grain size, as well as mineralogical and microfaunal composition, denote the high energetic conditions of this sector in which the alongshore transport of sand is predominantly southward and occurs mostly during the spring–summer oceanographic regime, when the main river providing sediments to this area, the River Douro, undergoes periods of drought. This effect may emphasize the erosive character of this coastal sector at present, since the Ria de Aveiro provides the shelf with few sediments.On the mid-shelf, an alongshore siliciclastic band of coarse sand and gravel can be found between the 40m and 60m isobaths. This gravelly deposit includes relic sediments deposited during lower sea-level stands. This structure stays on the surface due to the high bottom energy, which promotes the remobilization of the fine-grained sediments, and/or events of sediments bypassing.Benthic foraminifera density and “Benthic Foraminifera High Productivity” (BFHP) proxy values are in general low, which is consistent with the overall small supply of organic matter to the oceanic bottom in the inner- and mid-shelf. However, the Ria de Aveiro outflow, which delivers organic matter to the shelf, leaves its imprint mainly on the mid-shelf, identifiable by the increase in foraminifera density and BFHP values in front of the lagoon mouth. The higher values of BFHP along the 100m isobath trace the present position of an oceanic thermal front whose situation may have changed in the last 3/5ka BP. This zone marks a clear difference in the density, diversity and composition of benthic foraminifera assemblages. Here, in addition, sediment composition changes significantly, giving rise to carbonate-rich fine to medium sand in the deeper sector. The low bottom energy and the small sedimentation rate of the outer-shelf contributed to the preservation of a discontinuous carbonate-rich gravel band, between the 100m and 140m isobaths, also related to paleo-littorals, following the transgression that has occurred since the Last Glacial Maximum.The winter oceanographic regime favours the transport of fine grained sediments to the outer-shelf and upper slope. The inner- and mid-shelf, however, have low amounts of this kind of sediment and the Cretacic carbonated complexes Pontal da Galega and Pontal da Cartola, rocky outcrops located at the mid- and outer-shelf, act as morphological barriers to the cross-shelf transport of sediments. Thus a reduced sedimentation rate occurs in these deeper sectors, as indicated by the lower abundance of detrital minerals, which is compensated for the high sedimentary content of biogenic carbonates. The relatively high BFHP and Shannon Index values indicate water column stratification, high supply of organic matter and environmental stability, which provide favourable conditions for a diversified benthic fauna to flourish. These conditions also encourage authigenic chemical changes, favourable to glauconite formation, as well as illite and kaolinite degradation. Benthic foraminifera and clay mineral assemblages also reveal the effect of the internal waves pushing upward, and downslope losses of the sediments on the outer-shelf and upper slope.

Thermal thickness of the Earth’s lithosphere: a numerical model

A technique is suggested and the thermal thickness of the lithosphere is calculated, as well as the temperature distribution in the lithosphere on the basis of data on topography, the age of the oceanic bottom, crustal composition and structure, gravity anomalies, and mean annual surface temperatures. The bottom of the lithosphere is determined as the 1300°C isotherm. The calculation resolution is 0.5°×0.5°. All first-order tectonic structures, such as mid-ocean ridges and plume areas in oceans, continental rifts, cratons, and orogenic belts, are expressed in the computed thermal thickness. The comparative analysis of the thermal thickness of oceanic and continental lithosphere, lithosphere of cratons and young platforms, ancient and young orogens, remnant oceanic basins and adjacent continental areas can be used in geodynamical analysis of the corresponding regions.

New data on neotectonic bottom deformations in the western Brazil Basin, south Atlantic

Neotectonic deformations of the oceanic bottom are widespread in the Central Atlantic (1) developing in the MidAtlantic Ridge, deep oceanic basins, and even its marginal areas (2). The problems of the origin, scales, and mechanisms of these tectonic movements remain debatable. In this connection, of great interest are new data on tectonic deformations developed in the upper sedimentary layers of the western Brazil Basin. They were obtained during the acoustic study of the oceanic bottom with the profilograph Parasound in Cruise 24 of the R/V Akademik Sergey Vavilov car� ried out in 2008. The bottom structure of the western Brazil Basin is

Miocene volcanism of the Ninetyeast Ridge

The East Indian or Ninetyeast Ridge is one of the most unique and enigmatic structures of the oceanic bottom. It is characterized by a linear shape and extends in the meridional direction via the eastern part of the Indian Ocean for almost 5000 km and is approx� imately 200 km wide (Fig. 1). The ridge was discovered in the 1920s and first studied in detail in 1963 during the R/V Vityaz scientific expedition. In subsequent years, geological–geophysical studies of the ridge were conducted in cruises of research vessels from the Shir� shov Institute of Oceanology, RAN [1] and other Rus� sian and foreign scientific organizations. Most of the geological–geophysical data were obtained there in the 1970s–1980s.Since that time, the scientific equip� ment and study methods have become substantially better. Despite the sufficient knowledge of the ridge structure owing to numerous oceanological expedi�

Structure and geological history of sedimentary petroliferous basins in the North Atlantic

Pericontinental sedimentary basins in the northern Atlantic Ocean contain the largest reserves of oil and gas. In the Paleozoic-initial Paleogene period, they were developing as rift grabens subjected to repeated tectonic dislocations. Spreading of the oceanic bottom that started in this region in the Paleocene-Eocene was accompanied by rapid subsidence with accumulation of volcanogenic and turbiditic sediments. All these processes resulted in the formation of a thick sedimentary cover with both oil source and reservoir rocks at its different levels. The region under consideration is unique by the diverse composition and facies types of sediments.

Evaluation of palaeo-oxygenation of the ocean bottom across the Permian–Triassic boundary

The ‘long-lasted anoxia’ of a deep ocean bottom which may be related to the Permian–Triassic (PT) mass extinction event has been accepted without critical evidence. The degree of bioturbation is examined for radiolarian chert successions in central Japan with the reassessment of geochemical proxies to evaluate the degree of palaeo-oxygenation of a pelagic oceanic bottom. No evidence for anoxia is found from Wuchiapingian and lower Changhsingian chert by the reassessment of geochemical proxies. Anoxia started from the late Changhsingian stage that was interrupted by an oxygenation event at the supposed PT boundary. Griesbachian to the earliest Smithian, being characterised by the occurrence of black carbonaceous claystone, is an anoxic interval with rare bioturbation in the early part and is oxygenated in the later part. Succeeding grey siliceous claystone of early Smithian represents largely an anoxic interval, but it is oxygenated in the later part of this stage. Grey siliceous claystone with black carbonaceous claystone interbeds of Spathian was suboxic to oxic, and temporal anoxia being caused mainly by high productivity intermittently was established within a short time interval. Red chert is rather common in Anisian, but unusual sedimentation of organic matter still continued until the latest Anisian.

Tank experiments of sound propagation over a tilted bottom: Comparison with a 3‐D PE model

We present results of tank experiments of long‐range acoustic propagation over a wedge‐shaped oceanic bottom. Previous work investigated the propagation of broadband pulses in a range‐independent configuration showing good agreement between experimental results and model predictions [Korakas et al., Proceedings of UAM 2007 (Heraklion, Crete, Greece, 25‐29 June 2007)]. As a follow‐up, preliminary experiments were carried out considering a tilted bottom. Non‐negligible 3‐D effects were observed and proved to compare favorably with numerical predictions obtained running a parabolic equation based code [Sturm et al., Proceedings of UAM 2007]. In the present work we continue the investigation of the wedge‐like environment. Additional series of experimental measurements are performed. Received signals are recorded on a very fine spatial grid. Then, the measured data are processed using matched field processing. In our inversion algorithm, the replica are provided by a fully three‐dimensional parabolic equation code. The technical aspects of the inversion procedure used (e.g., CPU time) are discussed.

Tectonic types of deepwater basins in the Indian Ocean

Among 16 deepwater basins located in the central Indian Ocean and along its western, eastern, and southern margins, the central, perioceanic, and perispreading tectonic types are recognized. The Central, Cocos, Wharton, and Crozet basins belong to the first type. The second type comprises the Somalia, Mascarene, Madagascar, Mozambique, and Agulhas basins localized along the western margin of the ocean; the Argo, Gascoyne, Cuvier, and Perth basins that are situated along its eastern periphery; and the African-Antarctic Basin in the southern periphery. The South Australian and Australian-Antarctic basins pertain to the third type. Spatially and tectonically, the pericontinental basins are conjugated with continental blocks in the ocean (rises, plateaus, microcontinents). Together, they make up specific tectonic systems that extend parallel to the continents. The formation of such systems is controlled by horizontal movement of continental blocks and tectonic subsidence of the oceanic bottom.

Tectonic types of the Pacific abyssal basins

This paper continues a series of publications on tectonics, structural evolution, and formation mechanism of abyssal basins. The Pacific abyssal basins of the Western thalassogen are subdivided into three types: intermontane, interfault, and syneclise-type. The new term thalassosyneclise is proposed for the third type. Basins of the first, second, and third types are developed in the western, northeastern, and southern Pacific, respectively. The main mechanism responsible for the formation of basins belonging to the first two types is a stepwise subsidence of the oceanic bottom. The formation of a thalassosyneclise abyssal basin is probably related to energy flux of the South Pacific plume.

Energetic analysis of global topography

Diagrams of the various distributions of the potential energy of the continental and oceanic-bottom topography as a quantitative indices of energy consumption by tectonic processes have been calculated and plotted for the first time on the basis of high-resolution ETOPO 2′ digital data on a 2′ × 2 ′ uniform network with height intervals of 100 m. The potential energy of the continents and oceanic bottom is mainly stored in mountainous regions of Asia and South America with an anomalous type of energy distribution and in mid-oceanic ridges, respectively. The topography of the oceanic bottom bears five times more energy than the continental topography. With allowance for the isostatic compensation of denudation, the bulk reserve of the potential energy of the global topography is estimated at 1033 erg, a value which amounts to only an insignificant fraction of other items in the energetic budget of the Earth. Fifty percent of this energy has been gained over the last 70 Ma. Currently, the rate of accumulation of the potential energy of the topography is 1025 erg/yr, i.e., 1000 times less than the bulk geothermal flow and 10 times less than energy released by all earthquakes during a year. So far, there are no grounds to bind the potential energy of the Earth’s topography with a specific source.