Bottom Current Flow Research Articles

Ichnological record in gateways and other high-energy deepwater environments: a palaeo- and neoichnological approach

The evolution of gateways determines variations in palaeoenvironmental (ecological and depositional) conditions affecting tracemaker communities, and hence biogenic structures. With respect to palaeogateways, bottom currents and associated deposits (i.e. contourites) may be approached through trace-fossil studies. Ichnological analysis – individual ichnotaxa and changes in ichnofacies – of Late Miocene contourites and associated facies from the Rifian Corridor palaeogateway (Morocco) reveals a major impact of food supply, flow velocity, seafloor heterogeneity and hydrodynamic energy. The fact that Eocene to middle Miocene deep-marine deposits, mainly contourites, from the nearby Indian palaeogateway (Cyprus) show ichnofacies replacement is evidence of increased bottom-current flow velocity associated with sea-level variations. Moreover, ichnofabric changes reflect pelagic, gravitational and bottom-current processes that simultaneously influence sedimentation. Changes in the trace-fossil composition of both areas, and in the shape of burrows, reflect intermittent bottom-current processes that governed contourite deposition. Neoichnological studies complement and validate the trace-fossil record of high-energy deep-sea environments that can occur in palaeogateways. Modern traces ( lebensspuren ) observed on the seafloor show variable features (e.g. diversity, density, morphology) depending on the type of disturbance that high-energy conditions exert on the trace-making benthic fauna. Parameters such as substrate consistency, nutrient availability/distribution and the duration and intensity of energetic events play a major role in determining lebensspuren features. We conclude that an integrative palaeo- and neoichnological approach is a powerful tool when used to improve our knowledge of benthic ecosystems in high-energy deep-sea environments such as gateways.

Dolomite formation in the Miocene Kardiva platform, Maldives archipelago: a tale of closed-system and open-system dolomitization by current pumping of seawater

Dolomites from two IODP Expedition 359 sites on the northeastern margin of the Miocene Kardiva platform, Maldives archipelago, were examined to explore the question of open versus closed system dolomitization in a drowned carbonate platform. The uppermost approx. 130 m of platform margin carbonate at site U1465 contains less than or equal to 12% dolomite except for five, meters-thick intervals with up to 65% dolomite. All U1465 dolomites consists of decimicron-sized euhedral cement crystals and mimetically replaced peloids and coralline red algal clasts. The abundance and the petrographic features of these dolomites are similar to periplatform and slope dolomite in many other settings that have been interpreted as the product of hydrologically closed-system diagenesis. In contrast, the recovered platform margin deposits at site U1469 are greater than 99% dolomite. Those dolomites are partially fabric retentive with fine-to-medium crystalline, planar subhedral to euhedral crystal mosaics of replacive dolomite and dolomite cement, all with Sr contents that average 256 ppm. Their characteristics are comparable to Neogene platform carbonates universally interpreted to signify hydrologically open-system dolomitization. Sr-isotope ages indicate Miocene dolomitization at both sites after platform drowning, and 18Odolomite values are compatible with dolomitization by cold (10⁰ to 15⁰ C) seawater when the platform margin was, on average, approx..400 m below sea level (mbsl). A current pumping mechanism for the advection of seawater into the top of the platform at site U1469 is proposed and tested with a computational fluid-flow simulation. Current pumping occurs when strong ocean bottom currents flow over sedimentary bedforms and generate lateral pressure differences along the sediment-water interface. The pressure differentials drive seawater through the underlying sediments. The flow simulation shows that the ocean currents that swept large sediment sand waves over and off the drowned Kardiva platform for many millions of years could have vigorously pumped Miocene seawater to sub-seafloor depths of many tens of meters . Mass balance considerations suggest complete dolomitization of the upper 20 m of the platform within 500 Ky or less. The current pumping mechanism could drive dolomitization and mineralogical stabilization below hiatal surfaces, or very slowly accumulating sediments, in any marine setting characterized by strong bottom currents. The greater acidity of Mioc ene seawater relative to younger seawater possibly made the pumping, by any means, of Miocene seawater of normal salinity an effective dolomitizing agent at site U1469 and in undrowned Miocene platforms globally.

Hydrodynamic analysis of the inner part of the Gulf of Nicoya, Costa Rica: Updating field measurements for coastal users

Although extensively studied, the inner region of the gulf of Nicoya (one of the main estuaries of Costa Rica) has a relatively low amount of research focused on its hydrodynamic. This represents a limitation for establishing hydrodynamical models and doing complex analysis of the water movement, which are vital for the proper management of the multiple economic, social, and ecological actors present at the estuary. The aim of this study is to generate and update hydrodynamic and bathymetric data of the inner region of the Gulf of Nicoya, with measurement periods and temporal resolutions that could be used for better understanding the hydrodynamic patterns and the development of future numerical models. Water level, current velocity, and bathymetry were measured in two separate field surveys, almost two weeks each, during dry and rainy seasons. Time series of each variable were collected with temporal resolutions of 20 min or less. Pressure and acoustic gauges were used for measuring the variables in representative sites of the hydrodynamics of the estuary. The measured data supports the hydrodynamic pattern described in previous studies, but shows some differences derived from the longer and finer data series. The most inner part of the estuary presents a time lag of up to 25 min and 30% amplification of the tide wave in comparison to the estuary’s entrance. Maximum currents show values of 1.3 m/s, and mostly around the change of tides, there are moments in which bottom and superficial currents flow in almost opposite direction with velocities less than 0.2 m/s. Bathymetry shows various areas where the new and historical data do not coincide. The present study contributes to update and extend the available data on hydrodynamic related variables, delivering updated time series with higher temporal resolution for future numerical models and general analysis from other coastal users.

Dispersion and fate of methane emissions from cold seeps on Hikurangi Margin, New Zealand

The influence of cold seep methane on the surrounding benthos is well-documented but the fate of dissolved methane and its impact on water column biogeochemistry remains less understood. To address this, the distribution of dissolved methane was determined around three seeps on the south-east Hikurangi Margin, south-east of New Zealand, by combining data from discrete water column sampling and a towed methane sensor. Integrating this with bottom water current flow data in a dynamic Gerris model determined an annual methane flux of 3 x 105 kg at the main seep. This source was then applied in a Regional Ocean Modelling System (ROMS) simulation to visualize lateral transport of the dissolved methane plume, which dispersed over ∼100 km in bottom water within 1 year. Extrapolation of this approach to four other regional seeps identified a combined plume volume of 3,500 km3 and annual methane emission of 0.4–3.2 x 106 kg CH4 y-1. This suggests a regional methane flux of 1.1–10.9 x 107 kg CH4 y-1 for the entire Hikurangi Margin, which is lower than previous hydroacoustic estimates. Carbon stable isotope values in dissolved methane indicated that lateral mixing was the primary determinant of methane in bottom water, with potential methane oxidation rates orders of magnitude lower than the dilution rate. Calculations indicate that oxidation of the annual total methane emitted from the five seeps would not significantly alter bottom water dissolved carbon dioxide, oxygen or pH; however, superimposition of methane plumes from different seeps, which was evident in the ROMS simulation, may have localized impacts. These findings highlight the value of characterizing methane release from multiple seeps within a hydrodynamic model framework to determine the biogeochemical impact, climate feedbacks and connectivity of cold seeps on continental shelf margins.

Slope instability and submarine-moat initiation: Insights from the northern South China sea margin

Submarine landslides that occur on continental margins can reshape the seafloor morphology and form steep slide scarps. Although the emplacement processes of submarine landslides have been well studied, the development of post-slide deposition has long been overlooked. Here, we use high-resolution multibeam bathymetric and three-dimensional seismic data from the northern South China Sea margin to decode how slide scarps affect bottom currents and generate associated bottom current-related features. Distinct features have been identified within the slide scars, such as sediments waves and drifts. In parallel, multiple longitudinal negative topographies are distinguished along the scarps, undercutting into underlying strata with width and depth up to 240 m and 20 m, respectively. These incisional structures are interpreted as moats, formed following the emplacement of landslides, and running along the foot of scarps that parallel to contours. The origin of these moats could thus represent an interaction between slide scars and contourite deposition. We suggested that escarpments act as morphological barriers, regionally affecting the velocity and direction of bottom currents, leading to an intensification of their velocities and their erosive ability. Furthermore, we propose a co-evolution model of submarine landslides and related moats, which elucidates and emphasizes the significance of scarps in moat initiation and evolution. Our results demonstrate that the interaction between escarpments and bottom current flow can lead to topographic remodeling and sedimentary processes on varying continental margins.

Lithofacies and Shale Oil Potential of Fine-Grained Sedimentary Rocks in Lacustrine Basin (Upper Cretaceous Qingshankou Formation, Songliao Basin, Northeast China)

Shale oil has become a global hotspot of unconventional exploration and development. In this study, the latest drill core and experiment analyses of the Qingshankou Formation in the northern Songliao Basin were used to evaluate its lithofacies classification, sedimentary environment, pore types, pore-throat structure characterization, and shale oil potential. Lithofacies classification was determined according to the total organic carbon (TOC) content, sedimentary structure, and rock mineral content. Laminae genesis and micro-sedimentary structures indicate the deposition of fine-grained sedimentary rocks (FGSRs) in a semi-deep to deep lacustrine environment; however, evidence also suggests partial reworking by storm events and bottom current flows. FGSRs mostly comprise type I kerogen, with small amounts of type II1. The average vitrinite reflectance of the FGSRs was 1.37%, indicating middle to high stages of thermal maturation within the oil generation window. The N2 adsorption experiment indicated that silty mudstone (SM), silty fine mixed sedimentary rock (SFMR), and argillaceous fine mixed sedimentary rock (AFMR) had ink-bottle-shaped and slit-shaped pores, and the lithofacies were dominated by mesopores, accounting for 77.4%, 71.9%, and 80.8% of the total pore volume, respectively. Mercury injection capillary pressure analysis indicated that SM and SFMR had an average pore-throat radius of 0.01–0.04 μm, whereas AFMR and CM were dominated by nanopores, mainly distributed in the range of 0.004–0.0063 μm. Based on the comprehensive studies of TOC content, pore development, and brittleness, we concluded that organic-rich laminated SM and SFMR should be the focus of shale oil exploration of the Qingshankou Formation in the northern Songliao Basin, followed by organic-rich or organic-moderate laminated and layered AFMR, as well as calcareous fine mixed sedimentary rocks.

Cold-water coral framework architecture is selectively shaped by bottom current flow

The three-dimensional (3D) structure of habitat-forming corals has profound impacts on reef ecosystem processes. Elucidating coral structural responses to the environment is therefore crucial to understand changes in these ecosystems. However, little is known of how environmental factors shape coral structure in deep and dark waters, where cold-water coral (CWC) reefs thrive. Here, we attempt to infer the influence of current flow on CWC framework architecture, using 3D scanning to quantify colony shape traits (volume compactness and surface complexity) in the reef-building CWC Desmophyllum pertusum from adjacent fjord and offshore habitats with contrasting flow regimes. We find substantial architectural variability both between and within habitats. We show that corals are generally more compact in the fjord habitat, reflecting the prevailing higher current speeds, although differences in volume compactness between fjord and offshore corals are more subtle when comparing the fjord with the more exposed side of the offshore setting, probably due to locally intensified currents. Conversely, we observe no clear disparity in coral surface complexity between habitats (despite its positive correlation with volume compactness), suggesting it is not affected by current speed. Unlike volume compactness, surface complexity is similarly variable within a single colony as it is between colonies within the same habitat or between habitats and is therefore perhaps more dependent than volume compactness on microenvironmental conditions. These findings suggest a highly plastic, trait-specific and functionally relevant structural response of CWCs to current flow and underscore the importance of multiple concurrent sources of hydrodynamic forcing on CWC growth.

Bedforms of Bonifacio Strait (Western Mediterranean): hydrodynamics, coastal outline, supply and sediment distribution

Abstract This paper shows the results of high-resolution survey on the continental shelf of Bonifacio Strait (north Sardinia). Multisensor and multiscale data obtained by means of sub-bottom profiler CHIRP and multibeam and side-scan sonar made it possible to identify the morphological features of bedforms. The analysis of both side-scan sonar sonograms mosaic and high-resolution seismic data allowed us to perform a detailed mapping of various substrates highlighting the presence of complex bedform associations. These bedform associations are typical of continental shelf environments affected by high-speed bottom currents. Multisource data analysis integrated by sedimentological study allowed us to identify the directions of sediment movements on the seabed and the progradation of bedforms. Locally, it was found that the bedforms of mobile bottoms are still active and they persist for more than a decade. The path of the currents’ circulation revealed that in the Bonifacio Strait, the bottom currents flow westwards, opposed to the prevailing sea surface flows.

Internal Lee Waves Generated by Shear Flow Over Small‐Scale Topography

AbstractWhen the bottom current flows over a small‐scale topographic feature (with characteristic horizontal scale of 0.1–10.0 km), it generates internal lee waves. These lee waves furnish the energy cascade from mesoscale to dissipative scale, resulting in enhanced turbulent mixing. The existing lee wave theory and parameterization are mostly based on uniform background flow. We improve the linear lee wave theory by considering shear background flow and viscous and diffusive effects, which are ubiquitous in the ocean. In particular, the introduction of vertical viscous and diffusive terms effectively improves the problem of critical layer (where the wave frequency is close to the Coriolis frequency) for linear lee waves under weakly dissipative limit. Then, the influences of different types of shear flow on the frequency, vertical wavelength, group velocity, energy flux, energy density, energy dissipation and irreversible mixing of lee waves are explored through both theory and numerical simulation. A bottom‐up decreasing background flow can enhance energy dissipation and irreversible mixing below the critical layer. However, in the real ocean, the dissipation and mixing caused by a decreasing background flow may be weaker than that caused by an increasing background flow in a depth integrated sense, especially when considering a bounded domain. Additionally, the theory of wave action conservation in the absence of dissipation, which is commonly used as a criterion in research of internal waves propagating in varying background fields, is extended to a “modified” wave action equation considering dissipation, which may potentially improve the lee wave parameterization in the future.

Late Miocene contourite depositional system of the Gulf of Cádiz: The sedimentary signature of the paleo-Mediterranean Outflow Water

Late Miocene contourite deposits related to the paleo-Mediterranean Outflow Water (MOW) were identified in the Betic and Rifian corridors prior to the restriction of the Mediterranean-Atlantic gateway during the latest Miocene. In this study, we identified for the first time their downstream continuation in the Gulf of Cádiz through seismic stratigraphic analysis and the interpretation of contourite diagnostic features. The late Miocene contourite depositional system (CDS) consists of three stages (initial-, growth-, and maintenance-drift) which recorded the late Tortonian to early Messinian evolution from weak to vigorous bottom current flow in the Gulf of Cádiz prior to its cessation in the middle to late Messinian, represented by the buried-drift stage. Development of the CDS in the Gulf of Cádiz is coeval to a period when the continental margins were affected by regional deformation. Tectonism and diapirism, on top of sedimentary and climatic factors, exerted control on drift distribution and dimensions. However, dataset limitations hindered detailed analysis on the effect of deformation on CDS evolution. Overall, the long-term evolution of the late Miocene CDS across the Gulf of Cádiz towards the West Iberian margin suggests strengthening of paleo-MOW during the late Miocene which has significant impact on the North Atlantic ocean circulation and the late Miocene global cooling trend.

Deep-water sedimentation controlled by interaction between bottom current and gravity flow: A case study of Rovuma Basin, East Africa

By using 3D seismic dataset recently acquired in offshore Mozambique, we address the distribution pattern of an Upper Eocene channel-levee-lobe complex in deep-water of Rovuma Basin. The unique levee of this complex asymmetrically developed only on one side of the channel course. We attribute this asymmetric levee development to the interaction between the western slope bottom current from south to north and the gravity flow from west down slope. Due to the winnowing and sorting by the western slope bottom current, the sorted fine grain sediments of the gravity flow was blew and deposited to aside remaining clean sand from the gravity flow deposited within the channel as quality reservoir. The stacking and expanding of the levee forced the channels windward migrated to the south forming channel-1 to 6 from north to south.The corresponding lobe did not keep its feeder channel's axis direction, but bended from northeast to east or southeast, forming a U-turn from the channel to lobe. We believe that in addition to the paleotopography effect, the bottom current in Querimbus Graben might also lead to the bending of the lobe. The Querimbus Graben bottom current in this region flows from north to the south driven by the southward-drifting eddy. We suppose that a southward bottom current also existed at the Eocene time in this region and contributed to the southward bending of the lobes.

Deepwater sedimentary bedforms in a mobile substrate terrain: Examples from the central Gulf of Mexico Basin

We have used high-resolution geophysical data to investigate depositional and erosional bedforms in two geomorphologic provinces of the deepwater central Gulf of Mexico Basin: (1) the Mad Dog and Atlantis areas in the Sigsbee Escarpment region and (2) the Holstein minibasin within the salt canopy in the slope. Multibeam bathymetry indicates that the seafloor relief in the study areas is highly irregular because it is influenced by the dynamic behavior of underlying salt bodies resulting in the development of diverse bathymetric features. Side-scan images reveal erosional furrows of different morphologies at the base of the Sigsbee Escarpment that are oriented subparallel to the escarpment. Wide and sinuous furrows overlie mass transport deposits (MTDs), whereas, in other areas along strike, narrow rectilinear furrows are found beneath MTDs. The furrow fields in the Sigsbee Escarpment are located within a large series of erosional features that are linked to the action of westward flowing bottom currents associated with topographic Rossby waves that manage to rework sediments at water depths up to 2000 m. The interaction between the bottom current flow and the seafloor is likely influenced by the MTD’s irregular top surface relief and lateral changes in the escarpment’s morphology resulting in the development of complex sinuous furrow morphologies. North of the escarpment, subbottom profiles indicate a series of buried sediment waves found in the southern rim of the Holstein minibasin. Sediment wave morphometry indicates wavelengths ranging from 116 to 339 m and wave heights between approximately 0.8 and 2.4 m. Sediment waves were likely formed by turbidity currents as they exited the minibasin. The vertical change in topographic relief from the minibasin to the salt high led to variations in flow thickness and flow velocity of turbidity currents passing over the minibasin’s open rim. Consequently, these changes in flow regime led to the formation of sediment waves.

A mixed turbidite – contourite system related to a major submarine canyon: The Marquês de Pombal Drift (south‐west Iberian margin)

AbstractSynchronous interaction between bottom currents and turbidity currents has been reported often in channel–levée systems where the thickness of the turbidity currents exceeds that of the levées. Such interplay between along‐slope and down‐slope sedimentary processes is one of the mechanisms by which ‘mixed turbidite–contourite systems’ can originate. However, bottom currents flow over large areas of the seafloor, including continental slopes characterized by deeply incised submarine canyons rather than channel levées. In these cases, a direct interaction between along‐slope and down‐slope currents is, theoretically, unlikely to take place. In this study, oceanographic, swath bathymetry, multichannel seismic data and sediment cores are used to investigate a 25 km long, 10 km wide and up to 0.5 km thick deep‐sea late Quaternary deposit that sits adjacent to the north‐west flank of one of the major canyons in the North Atlantic, the São Vicente Canyon, in the Alentejo Basin (south‐west Iberian margin). The area receives the influence of a strong bottom current, the Mediterranean Outflow Water, which has swept the continental slope at different water depth ranges during glacial and interglacial periods. Architectural patterns and sediment characteristics suggest that this sedimentary body, named Marquês de Pombal Drift, is the result of the interaction between the Mediterranean Outflow Water (particularly during cold periods) and turbidity currents flowing along the São Vicente Canyon. Because the canyon is incised significantly deeper (ca 1.5 km) than the thickness of turbidity currents, an additional process, in comparison to earlier models, is needed to allow the interaction with the Mediterranean Outflow Water and transport sediment out of the canyon. In the São Vicente Canyon, and likely in other canyons worldwide, interaction of turbidity currents with contour currents requires intermediate nepheloid layers that export the finer‐grained fraction of turbidity currents out of the canyon at the boundary between major water masses.

Ocean surface and bottom water conditions, iceberg drift and sediment transport on the North Iceland margin during MIS 3 and MIS 2

Radiocarbon dates and marine tephra suggest that the upper 10 m of core MD99-2274 off North Iceland extends from ∼0 to ∼65 ka BP. A multi-proxy sediment and biomarker study at a ∼0.5 ky resolution is used to derive a paleoclimate scenario for this area of the southwestern Nordic Seas, which during the Holocene had intermittent excursions of icebergs and a seasonal cover of drifting sea ice across the site. The sortable silt mean size (S̅S̅) suggests a bottom current (1000 m depth) flow speed maximum to minimum range of ∼8 cm/s during Marine Isotope Stages 2–3, but the data are unreliable for the Holocene. Slow-down in flow speeds may be associated with massive ice and water discharges linked to the Hudson Strait ice stream (H-events) and to melt of icebergs from Greenland in the Nordic seas where convection would have been suppressed. Five pulses of sediment with a distinct felsic component are associated with iceberg transport from E/NE Greenland. Sea ice, open water and sea surface temperature (SST) biomarker proxies (i.e. IP25, HBI III, brassicasterol and alkenones) all point towards near-perennial sea ice cover during MIS 3 and 2, rather than seasonal sea ice or open water conditions. Indeed, our biomarker and sediment data require that the seas north of Iceland experienced a nearly continuous cover of sea ice, together with icebergs calved from ice stream termini, which drifted southward. The cross-correlation of the quartz % records between MD99-2274 and the well-dated core PS2644 in Blosseville Basin indicates significant coherence in the records at a multi-millennial (∼8 ky) timescale. A transition to open ocean conditions is evident from the early Holocene onwards, albeit with the occurrence of some drift ice and icebergs.

Factors controlling margin instability during the Plio-Quaternary in the Gela Basin (Strait of Sicily, Mediterranean Sea)

This study presents novel findings on the Pliocene and Quaternary evolution of the Gela Basin (Strait of Sicily, Mediterranean Sea), an area recording the interaction between tectonics, climate change at Milankovitch and sub-Milankovitch timescales, and dynamic water masses exchange between the eastern and western Mediterranean Sea. The calibration of seismic profiles with exploration boreholes allowed for the refining of the chronostratigraphic framework of the Gela Basin and highlighted the main phases of margin growth. Since the Pliocene, the margin has recorded the deposition of 100 m high shelf-edge clinothems, accompanied by sediment drifts on the slope and mass-transport deposits (MTDs), possibly triggered by seismic activity. Through the Plio-Quaternary the locus of deposition of sediment drifts migrated upslope due to a progressive shift of bottom currents. After the Middle Pleistocene Transition (MPT) the margin experienced an accelerated outbuilding with the deposition of a 700 m thick succession in only 0.8 Myr. At this time, a marked change in sedimentary architecture reflects the growth of shelf-edge clinothems and associated MTDs on the slope, and the spreading of contourite deposits over a broader and generally shallower area. Sediment flux to the basin and the intensity of bottom currents appear both paced at 100 kyr eccentricity orbital cycles. The growth of bottom current deposits on a large portion of the upper slope and outer shelf likely reflects constraints to the bottom current flow by the margin morphology, inherited from Miocene and Pliocene tectonics. Overall, a combination of long-term tectonic activity, climate change and shifts in oceanographic regime resulted in a complex along-strike variability of the margin morphology and stratigraphic architecture and affected where and when MTDs were emplaced. These conclusions show how climate cyclicity influence sediment supply which combined with margin morphology can promote slope instability in continental margins.

Interaction between seabed morphology and water mass in the South Eastern Arabian Sea: deductions of water mass flow

Geomorphic analysis utilizing multibeam bathymetry and sediment profiler data along with conductivity, temperature and depth (CTD) measurements was undertaken to investigate morphological aspects of bottom current features that prevail in the area. The aim of the study was to deduce the prevailing water mass in the area and to infer interactions between water mass and seabed of South Eastern Arabian Sea. Morphological and bottom current features were identified and analysed. Based on morphological components and their structural orientation, deduction of flow pattern and direction of current flow in the area were attempted. Deconstructing structural aspects of typical bottom current features such as contourite drifts and scours reveal origin under influence of consistent exogenic bottom current having definitive flow pattern. Sub-surface seismic profiles were analysed to infer the presence of a consistently aggrading seafloor. The aspect distribution computed exhibits a prominent trend ranging between 200° and 275° indicating NNW-SSE orientation of erosive features which is usually synonymous with flow direction in the area. Backscatter intensity analysis undertaken for the area revealed seafloor with patches of loose to compact surface sediment distribution. Variation in reflection intensity level from surface sediments was utilized to demarcate zones that are susceptible in future to consistent turbulence-induced erosion. Temperature salinity (TS) diagram for values from CTD measurements was plotted overlaying isopycnals and the resulting plots tagged for various water masses that prevail in the Arabian Sea. Owing to the depth of the study area (~ 1800 m) and structural orientation of bottom current features, dominant oceanographic agent from TS plot was deduced to be the North Indian Deep Water (NIDW). Surface and sub-surface morphology of existing features were interpreted for influence of bottom current action. Geomorphic analysis of seabed is an efficient method to understand the flow pattern and of prevailing water masses and can be applied as passive indicators of bottom current flow. This approach of deducing flow pattern and intensity of water mass movement by analysing morphology of the undersea features can be effectively used globally especially in areas where adequate data and observations for deep water masses are scarce.

Hybrid turbidite-drift channel complexes: An integrated multiscale model

Abstract The interaction of deep-marine bottom currents with episodic, unsteady sediment gravity flows affects global sediment transport, forms climate archives, and controls the evolution of continental slopes. Despite their importance, contradictory hypotheses for reconstructing past flow regimes have arisen from a paucity of studies and the lack of direct monitoring of such hybrid systems. Here, we address this controversy by analyzing deposits, high-resolution seafloor data, and near-bed current measurements from two sites where eastward-flowing gravity flows interact(ed) with northward-flowing bottom currents. Extensive seismic and core data from offshore Tanzania reveal a 1650-m-thick asymmetric hybrid channel levee-drift system, deposited over a period of ∼20 m.y. (Upper Cretaceous to Paleocene). High-resolution modern seafloor data from offshore Mozambique reveal similar asymmetric channel geometries, which are related to northward-flowing near-bed currents with measured velocities of up to 1.4 m/s. Higher sediment accumulation occurs on the downstream flank of channel margins (with respect to bottom currents), with inhibited deposition or scouring on the upstream flank (where velocities are highest). Toes of the drift deposits, consisting of thick laminated muddy siltstone, which progressively step back into the channel axis over time, result in an interfingering relationship with the sandstone-dominated channel fill. Bottom-current flow directions contrast with those of previous models, which lacked direct current measurements or paleoflow indicators. We finally show how large-scale depositional architecture is built through the temporally variable coupling of these two globally important sediment transport processes. Our findings enable more-robust reconstructions of past oceanic circulation and diagnosis of ancient hybrid turbidite-drift systems.

Deep-sea litter in the Gulf of Cadiz (Northeastern Atlantic, Spain)

This study describes the distribution and composition of litter from the Gulf of Cadiz (Northeastern Atlantic, Spain), a region of confluence between the Atlantic and Mediterranean, with intense maritime traffic. Several geological features, such as canyons, open slopes and contourite furrows and channels, were surveyed by remotely operated vehicle (ROV) observations between depths of 220 and 1000 m. Marine litter was quantified by grouping the observations into six categories. Our results indicate the presence of markedly different habitats in which a complex collection of different types of litter accumulate in relation to bottom current flows and maritime and fishing routes. This result justifies a seascape approach in further anthropogenic impact studies within deep-sea areas.

A NEW TEICHICHNOID TRACE FOSSIL SYRINGOMORPHA CYPRENSIS FROM THE MIOCENE OF CYPRUS

ABSTRACTA new teichichnoid trace fossil, Syringomorpha cyprensis from the Miocene of Cyprus, is proposed as a vertical burrow composed of an arcuate-like tube with horizontal parts to subhorizontal distally and vertical to subvertical parts proximally and triangular spreiten in the inner corner of the tube. Previously, this ichnogenus was represented only by the lower Paleozoic, shallow marine S. nilssoni, which disappeared after the Cambrian. Syringomorpha cyprensis marks the reappearance of similar behavior, in a deep-sea environment with pelagic, contouritic, and turbiditic sedimentation, influenced by frequent turbiditic and bottom current flows. Both ichnospecies of Syringomorpha could be produced by the same taxonomic group of probable worm like organisms (polychaetes?). Energy conditions were a stronger influence on the distribution of S. cyprensis tracemaker rather depth.

Neogene palaeoceanographic changes recorded in a carbonate contourite drift (Santaren Channel, Bahamas)

AbstractThe Santaren Drift between the Great Bahama Bank and Cay Sal Bank (Bahamas) is closely linked to the development of the Gulf Stream and its shape and geometry record the local to global oceanographic, climatic and tectonic events since the Miocene. High‐resolution multichannel seismic data from the Santaren Channel allow detailed insight into the growth phases of the contourite drift, and by using the stratigraphic information from Ocean Drilling Program Site 1006 to infer its sedimentation rates. The results bring new understanding to this region and to interpretation of carbonate drifts. The data document that the signatures of a bottom current flow in the Santaren Channel initiated about 12·3 Ma, as indicated by the first occurrence of sheeted drifts and moat development at the northern part of the Santaren Channel. Narrowing and steepening of moat flanks as well as the pronounced upslope migration of the moat reflects a sustained current acceleration of the bottom currents until 5·5 Ma, associated with a transformation into mounded elongated drifts. Between 5·5 Ma and 3·1 Ma, bottom current intensity reached its maximum probably caused by the final closure of the Central American Seaway. The last 3·1 Myr were characterized by a marked increase in volume through flow reaching a maximum during the past 900 kyr. Drift growth was driven by the combined sources of export from the shallow‐water carbonate factory and by pelagic rain. The Middle Miocene channel‐related sheeted drift of the inner Santaren Channel is characterized by low accumulation rates, but a rapid increase of accumulation rates occurred during the Early Pliocene. The contourite drift buildup was disturbed by minor erosional phases with narrow moats in the Late Pliocene due to increasing bottom‐current velocities forced by strengthened Atlantic Ocean ventilation. The Early Pleistocene was dominated by increased periplatform sedimentation and margin progradation facilitated by a reduction in along‐slope current flow speed and a concurrent widening and flattening of the moats.