Microplastic transport, deposition and burial in seafloor sediments by turbidity currents

2020 ◽  
Author(s):  
Florian Pohl ◽  
Joris Eggenhuisen ◽  
Ian Kane ◽  
Michael Clare
Keyword(s):  
Critical Role ◽  
Turbidity Currents ◽  
Plastic Pollution ◽  
Submarine Canyons ◽  
Flume Experiments ◽  
Sediment Bed ◽  
Opposite Trend ◽  
Order Of Magnitude ◽  
Seafloor Sediments ◽  
Annual Flux

<p>Plastic pollution of the world’s oceans represents a threat to marine eco-systems and human health and has come under increasing scrutiny from the general public. Today the global input of plastic waste into the oceans is in the order of 10 million tons per year and predicted to rise by an order of magnitude by 2025; much of this plastic ends up on the seafloor. Plastics, and microplastics, are known to be concentrated in submarine canyons due to their proximity to terrestrial plastic sources, i.e. rivers. Plastics are transported in canyons by turbidity currents, mixtures of sediment and water which flow down-canyon due to their density; these flows can also ‘flush’ canyons, eroding and entraining the sediment lining the canyon walls and bottom. A single turbidity current can last for weeks and transport more sediment than the annual flux of all terrestrial rivers combined. Although it is known that these flows play a critical role in delivering terrestrial sediment and organic carbon to the seafloor, their ability to transport and bury plastics is poorly-understood. Using flume experiments we investigate turbidity currents as agents for the transport and burial of microplastic fragments and fibers. Microplastic fragments are focused at the flow base, whereas fibers are more homogeneously distributed throughout the flow. Surprisingly though, the resultant deposits show the opposite trend with fibers having a higher concentration that fragments. We explain this observation with a depositional mechanism whereby fibers are dragged out of suspension by settling sand grains, are trapped in the aggrading sediment bed and are buried in the deposits. Conversely, fragments may remain suspended in the flow and are less likely to be trapped on the bed. Our results suggest that turbidity currents can transport microplastics over long distances across the ocean floor, and that turbidity currents potentially distribute and bury large quantities of microplastics in seafloor sediments.</p>

Surface modification of ZnO on the Zn-polar 0001 face by self-assembled triptycene-based polar molecules along with an increase of electric conductance

2022 ◽  
Author(s):  
Hiroki Shioya ◽  
Naoko Inoue ◽  
Masaro Yoshida ◽  
Yoshihiro IWASA
Keyword(s):  
Surface Modification ◽  
Critical Role ◽  
Polar Molecules ◽  
Hydroxyl Groups ◽  
Oxide Semiconductors ◽  
Assembled Monolayers ◽  
Temperature Dependences ◽  
Order Of Magnitude ◽  
Sheet Conductance ◽  
Self Assembled

Abstract Application of self-assembled monolayers (SAMs) is a representative method of surface modification for tuning material properties. In this study we examine the influence of the surface modification by coating the Zn-polar 0001 surface of ZnO single crystal with a SAM of triptycene-based polar molecules in our own technique and investigated temperature dependences of the sheet conductance of the surface with and without the SAM. The sheet conductance at 70 K with the SAM is increased by an order of magnitude, compared to the case without the SAM. We infer that the additional electrons are introduced at the surface by the polar triptycene molecules, whose electropositive hydroxyl groups are supposed to face toward the Zn-polar surface of ZnO. The present result implies that the molecular orientation of the triptycene SAM plays a critical role on the surface properties of oxide semiconductors.

scholarly journals Direct estimation of electron density in the Orion Bar PDR from mm-wave carbon recombination lines

2019 ◽  
Vol 625 ◽  
pp. L3 ◽  
Author(s):  
S. Cuadrado ◽  
P. Salas ◽  
J. R. Goicoechea ◽  
J. Cernicharo ◽  
A. G. G. M. Tielens ◽  
...  
Keyword(s):  
Electron Density ◽  
Critical Role ◽  
Elemental Abundance ◽  
Hyperfine Line ◽  
Line Profiles ◽  
Star Forming ◽  
Star Forming Regions ◽  
Trace Species ◽  
Order Of Magnitude ◽  
Uv Photons

Context. A significant fraction of the molecular gas in star-forming regions is irradiated by stellar UV photons. In these environments, the electron density (ne) plays a critical role in the gas dynamics, chemistry, and collisional excitation of certain molecules. Aims. We determine ne in the prototypical strongly irradiated photodissociation region (PDR), the Orion Bar, from the detection of new millimeter-wave carbon recombination lines (mmCRLs) and existing far-IR [13C II] hyperfine line observations. Methods. We detect 12 mmCRLs (including α, β, and γ transitions) observed with the IRAM 30 m telescope, at ∼25″ angular resolution, toward the H/H2 dissociation front (DF) of the Bar. We also present a mmCRL emission cut across the PDR. Results. These lines trace the C+/C/CO gas transition layer. As the much lower frequency carbon radio recombination lines, mmCRLs arise from neutral PDR gas and not from ionized gas in the adjacent H II region. This is readily seen from their narrow line profiles (Δv = 2.6 ± 0.4 km s−1) and line peak velocities (vLSR = +10.7 ± 0.2 km s−1). Optically thin [13C II] hyperfine lines and molecular lines – emitted close to the DF by trace species such as reactive ions CO+ and HOC+ – show the same line profiles. We use non-LTE excitation models of [13C II] and mmCRLs and derive ne = 60–100 cm−3 and Te = 500–600 K toward the DF. Conclusions. The inferred electron densities are high, up to an order of magnitude higher than previously thought. They provide a lower limit to the gas thermal pressure at the PDR edge without using molecular tracers. We obtain Pth ≥ (2−4) × 108 cm−3 K assuming that the electron abundance is equal to or lower than the gas-phase elemental abundance of carbon. Such elevated thermal pressures leave little room for magnetic pressure support and agree with a scenario in which the PDR photoevaporates.

scholarly journals How do tectonics influence the initiation and evolution of submarine canyons A case study from the Otway Basin, SE Australia

2021 ◽  
Author(s):  
Nan Wu ◽  
Harya Nugraha ◽  
Michael Steventon ◽  
Fa Zhong
Keyword(s):  
Late Miocene ◽  
Submarine Canyon ◽  
Turbidity Currents ◽  
Submarine Canyons ◽  
Seismic Reflection Data ◽  
Reflection Data ◽  
Depositional Processes ◽  
Seabed Topography ◽  
Gravity Driven ◽  
Unconformity Surface

The architecture of canyon-fills can provide a valuable record of the link between tectonics, sedimentation, and depositional processes in submarine settings. We integrate 3D and 2D seismic reflection data to investigate the dominant tectonics and sedimentary processes involved in the formation of two deeply buried (c. 500 m below seafloor), and large (c. 3-6 km wide, >35 km long) Late Miocene submarine canyons. We found the plate tectonic-scale events (i.e. continental breakup and shortening) have a first-order influence on the submarine canyon initiation and evolution. Initially, the Late Cretaceous (c. 65 Ma) separation of Australia and Antarctica resulted in extensional fault systems, which then formed stair-shaped paleo-seabed. This inherited seabed topography allowed gravity-driven processes (i.e. turbidity currents and mass-transport complexes) to occur. Subsequently, the Late Miocene (c. 5 Ma) collision of Australia and Eurasia, and the resulting uplift and exhumation, have resulted in a prominent unconformity surface that coincides with the base of the canyons. We suggest that the Late Miocene intensive tectonics and associated seismicity have resulted in instability in the upper slope that consequently gave rise to emplacement of MTCs, initiating the canyons formation. Therefore, we indicate that regional tectonics play a key role in the initiation and development of submarine canyons.

scholarly journals A two-dimensional layer-averaged numerical model for turbidity currents

2018 ◽  
Vol 477 (1) ◽  
pp. 439-454 ◽  
Author(s):  
Shihao Yang ◽  
Yi An ◽  
Qingquan Liu
Keyword(s):  
Numerical Model ◽  
Large Scale ◽  
Current Model ◽  
Turbidity Currents ◽  
Two Dimensional ◽  
Dynamic Features ◽  
Field Scale ◽  
Sediment Bed ◽  
Sediment Transportation ◽  
Flow Features

AbstractTurbidity currents occur widely in submarine environments, but field-scale numerical simulations of the flow features have not been applied extensively. Here, we present a two-dimensional layer-averaged numerical model to simulate turbidity currents over an erodible sediment bed, and taking into consideration deposition, entrainment and friction. The numerical model was developed based on the open-source code, Basilisk, ensuring well-balanced and positivity-preserving properties. An adaptive spatial discretization was used, which allows multi-level refinement. The adaptive criterion is based on the dynamic features of the flow and sediment concentrations. The numerical scheme has a relatively high computational efficiency compared with models based on the Cartesian mesh. A hypothetical case based on a true large-scale landform (the Moroccan Turbidite System, offshore NW Africa) was studied. Compared with previous models, the current model accounted for the coupling between flow, sediment transportation and bed evolution. This approach may improve simulation results and also allow the simulation of complex field-scale landforms, while preserving the flow details.

scholarly journals How typhoons trigger turbidity currents in submarine canyons

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
Octavio E. Sequeiros ◽  
Michele Bolla Pittaluga ◽  
Alessandro Frascati ◽  
Carlos Pirmez ◽  
Douglas G. Masson ◽  
...  
Keyword(s):  
Turbidity Currents ◽  
Submarine Canyons

Recurrence of turbidity currents on glaciated continental margins: A conceptual model from eastern Canada

2020 ◽  
Vol 90 (10) ◽  
pp. 1305-1321
Author(s):  
Alexandre Normandeau ◽  
D. Calvin Campbell
Keyword(s):  
Conceptual Model ◽  
Sea Level ◽  
Deep Water ◽  
Turbidity Current ◽  
Sediment Supply ◽  
Turbidity Currents ◽  
Continental Margins ◽  
Submarine Canyons ◽  
Eastern Canada ◽  
Current Activity

ABSTRACT Turbidity currents in submarine canyons transport large volumes of sediment and carbon to the deep sea and are known to present a major risk to submarine infrastructure. Understanding the origin, the triggers, the recurrence, and the timing of these events is important for predicting future events and mitigating their impact. Depending on the morphological and latitudinal setting of submarine canyons, different external controls will govern the recurrence of turbidity currents. Here, we assess the recurrence of turbidity currents in shelf-incising submarine canyons off eastern Canada in order to examine the effects of external forcings such as glacier retreat and sea level on the deep-water sedimentary record. We used multibeam bathymetry, sub-bottom profiles, and the analysis of turbidites in sediment cores to infer the triggers of turbidity currents over time and propose a conceptual model for the activity of turbidity currents during glacial retreat. The chronostratigraphy of turbidites shows that turbidity current activity in the glaciated The Gully submarine canyon (eastern Canada) was highest between 24 ka cal BP (LGM) and 17 ka cal BP, with > 100 turbidites per 1,000 yr, when the ice sheet was directly delivering sediment to submarine canyons. As the ice margin retreated, the dominant sediment supply switched to glaciofluvial and then to longshore drift, while RSL remained low. The recurrence of turbidity currents nonetheless decreased drastically to < 10 per 1000 yr during that time, pre-dating the rise in RSL. This timing suggests that the reduction of turbidity-current activity is closely linked to retreating glaciers rather than to sea-level rise, which occurred later. Following the retreat of the ice sheet, sea level rose progressively to drown the shallow banks on the continental shelf, and turbidity currents ceased being active after 13 ka cal BP. In the late Holocene, landslide and concomitant turbidity-current recurrence increased to 1 per 1,000 yrs, with at least four new events recorded in deep water. This study shows that glacial sediment supply and sea level controlled the type of sediment supply to the continental slope, which in turn controlled the triggers of turbidity currents over time and the flushing of sediment to the deep water. By comparing with other glaciated margins, we propose a conceptual model explaining the recurrence of turbidity currents, taking into account RSL change and the position of the ice margin relative to the shelf edge. This conceptual model can help predict turbidity-current activity and offshore geohazards on other ancient and modern glaciated continental margins.

Darwin Harbour – a partnership approach to understanding and evaluating environmental challenges

2019 ◽  
Vol 59 (2) ◽  
pp. 523
Author(s):  
Richard Brinkman ◽  
Edward Butler ◽  
Terry O'Connor ◽  
Claire Streten
Keyword(s):  
Critical Role ◽  
Scientific Basis ◽  
Northern Australia ◽  
Development Activity ◽  
Operational Risks ◽  
Marine Industry ◽  
Environmental Attributes ◽  
Order Of Magnitude ◽  
Development Plans ◽  
Partnership Approach

Darwin Port plays a critical role in the Northern Territory’s export-oriented economy and in supporting the economy of northern Australia more generally. Darwin Port is the only true multi-modal port in northern Australia, with direct connectivity to southern Australia and beyond via the southern rail link. The port has developed rapidly over the past decades, with redevelopment of the Waterfront Precinct, development of numerous residential marinas, construction of the East Arm Wharf and marine supply base, and construction and operation of liquefied gas processing and export facilities. Future development plans include Middle Arm Industrial Precinct and Marine Industry Park. These developments have resulted in an order of magnitude increase in dredging requirements, an increase in vessel traffic and an increase in public focus on environmental quality within the harbour. A coordinated, multi-agency approach to understand the complex marine environment of Darwin Harbour has underpinned much of the development activity and provided a sound scientific basis to evaluate potential environmental impacts associate with past and future developments. Through a hierarchical process of environmental assessment and identification and mitigation potential risks, the Australian Institute of Marine Science has partnered with industry, government and other research providers to deliver projects to map and assess habitats, monitor environmental attributes and evaluate potential impacts, and provide environmental information in novel and accessible ways to support harbour operations and inform strategic planning. The partnership approach adopted for Darwin Harbour serves as a model for leveraging resources to understand and manage environmental and operational risks in northern ports.

Transport of adsorbing metals from stream water to a stationary sand-bed in a laboratory flume

1995 ◽  
Vol 46 (1) ◽  
pp. 209 ◽  
Author(s):  
H Eylers ◽  
NH Brooks ◽  
JJ Morgan
Keyword(s):  
Metal Ions ◽  
Stream Water ◽  
Flow Conditions ◽  
Advective Flow ◽  
Flume Experiments ◽  
Lithium Ions ◽  
Overlying Water ◽  
Experimental Conditions ◽  
Sediment Bed ◽  
Laboratory Flume

The transport of zinc and lithium ions between the overlying water column and the stationary sand-bed in a laboratory flume with bottom bedforms is investigated. Experiments have been performed under simplified conditions in a recirculating laboratory flume with straight impermeable walls and a sand-bed. The sand is well sorted and acid-washed to provide reproducible experimental conditions. The chemical composition of the recirculating water is controlled and steady flow conditions are maintained in the experiments. The concentrations of initially added metal ions are monitored both in the circulating overlying water and in the pore water of the sediment bed. Batch experiments were performed to investigate the chemical partitioning of the metal ions to the sand grain surfaces, and the data were compared with adsorption values obtained from the flume experiments. A model based on pressure-driven advective flow and linear partitioning of the pollutant to the sediment has been developed and accurately predicts the rate of transfer of the metal ions (zinc and lithium) into the bed in the case of stationary bedforms.

A new mechanism for the triggering of turbidity currents offshore tropical river deltas

2020 ◽  
Author(s):  
Gaetano Porcile ◽  
Michele Bolla Pittaluga ◽  
Alessandro Frascati ◽  
Octavio Sequeiros
Keyword(s):  
Sediment Transport ◽  
Meteorological Data ◽  
Extreme Weather Events ◽  
Turbidity Currents ◽  
Coastal Circulation ◽  
Complex Flow ◽  
Submarine Canyons ◽  
Tropical River ◽  
River Deltas ◽  
Water And Sediment

<p>When narrow continental shelves are stressed by extreme weather events, nearshore currents dominate the coastal circulation leading to complex flow patterns that can result in previously unforeseen cross-shelf exchange of water and sediment. Here we present a series of detailed studies carried out to investigate the nature of turbidity currents that impacted upon a submarine pipeline offshore Philippines, nearby tropical river deltas, after the landfall of intense typhoons. These rivers debouch into a shelf only a few hundreds of meters wide that is interrupted by steeper continental slopes carved by multiple submarine canyons. Turbidity currents were detected through regular pipeline monitoring, which showed lateral displacements and sea-floor erosion where the pipeline crosses some of these canyons. Seabed assessments indicated signatures of the occurrence of turbidity currents as opposed to landslides or ground motion due to earthquakes. Particularly, the submarine canyons were covered with regular sediment patterns that indicated the passage of deep-water turbulent flows, suggesting the local occurrence of turbidity currents. Meteorological data pointed at river floods and meteocean conditions, and associated fluvial sediment delivery and coastal sediment transport, as the most likely leading mechanisms for the triggering of turbidity currents. Hydrological modelling and related sediment transport calculations show these rivers were not capable to debouch into the sea with sediment concentrations high enough to generate hyperpycnal flows. Nevertheless, river plumes played an active role as source of sediment available on the shelf. Conversely, the role of the coastal circulation was found to be crucial for the triggering of turbidity currents. Our simulations show the development of exceptional rip currents (megarips) that flush out water and sediment from the inner shelf in the cross-shore direction towards the canyons’ heads, ultimately triggering turbidity currents into deep ocean waters. Such extreme nearshore circulations require the passage of intense typhoons in proximity to the trigger area inducing shore-normal incoming waves at peak conditions that in association with shoreline concavity at the river deltas favour the formation of erosional megarips, whose dynamics strongly depends on typhoon's approach latitude. The turbidity current modelling confirmed such an interpretation, matching field observations in the form of pipeline displacements. These evidences support our hypothesis that typhoon-induced megarip circulations could be responsible for the triggering of turbidity currents in submarine canyon systems offshore tropical river deltas. This newly identified mechanism has wide implications on the threatening of seafloor infrastructures and the assessment of frequency and duration of turbidity currents.</p>

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