Sediment resuspension and the generation of intermediate nepheloid layers by shoaling internal bores

Masunaga, Eiji; Arthur, Robert S.; Fringer, Oliver B.; Yamazaki, Hidekatsu

doi:10.1016/j.jmarsys.2017.01.017

Cited by 35 publications

(39 citation statements)

References 39 publications

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“…Strong resuspension events have been reported elsewhere related to internal waves and tidal bores impinging onto the continental shelf and slope, producing bottom and intermediate nepheloid layers originating either at the shelf break or along the continental slope (Van Raaphorst et al, 1998;van Haren, 2009;Walter et al, 2012;Masunaga et al, 2017). A case of BNL and INL formation in deep water by a fall storm was presented by Miles et al (2013), and for coastal waters by Warner et al (2008).…”

Section: Episodic Large-scale Resuspension Eventsmentioning

confidence: 68%

Scales of seafloor sediment resuspension in the northern Gulf of Mexico

Diercks

Dike

Asper

et al. 2018

Elementa: Science of the Anthropocene

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Diercks, A-R, et al. 2018 Scales of seafloor sediment resuspension in the northern Gulf of Mexico. Elem Sci Anth, 6: 32. DOI: https://doi. org/10.1525/elementa.285 IntroductionThe sedimentation of large amounts of oil via marine snow and its accumulation on the deep seafloor (>1,200 m) during and after the Deepwater Horizon (DwH) oil spill (Passow et al., 2012;Valentine et al., 2014;Brooks et al., 2015;Chanton et al., 2015;Daly et al., 2016;Joye, 2016;Joye et al., 2016;Passow, 2016) raised questions regarding the distribution and re-distribution processes of freshly sedimented material (marine snow) on the seafloor. Once on the seafloor, marine snow contributes to unconsolidated fluffy sediment layers (Gardner, 1978;Gardner et al., 1984;1985;Walsh et al., 1988;Pilskaln et al., 1998;Newell et al., 2005) that are subject to resuspension and the production of benthic nepheloid layers (BNLs).Resuspension leads to the re-invigoration of degradation processes, which would impact the degradation rates of the oil associated with marine snow following the DwH accident (Ziervogel et al., 2016). Additionally, resuspension leads to lateral transport and redistribution of the material that sank to the seafloor. After the DwH accident such re-distribution processes make it especially difficult to estimate the total amount of Macondo oil that reached the seafloor (Passow and Hetland, 2016).BNLs, which are formed when the frictional stress of water motion strips sediment off the seafloor, therewith carrying particles into the overlying water layer, exist near the seafloor, but may reach tens to hundreds of meters upward into the water column (McCave et al., 1976). The thickness of the BNL extending above the seafloor scales with the strength of the bottom currents and the particle composition. Bottom currents >10 cm s -1 may cause resuspension events (Gardner et al., 2017), especially when low density phytodetritus or fine silt covers sediments, but large benthic storms reach 20 cm s -1 (Gardner et al., 1985). Besides locally resuspended material, particles in the BNL also include aggregates settling from the upper ocean as Time-series data of size-specific in-situ settling speeds of marine snow in the benthic nepheloid layer (moored flux cameras), particle size distributions (profiling camera), currents (various current meters) and stacked time-series flux data (sediment traps) were combined to recognize resuspension events ranging from small-scale local, to small-scale far-field to hurricane-scale. One smallscale local resuspension event caused by inertial currents was identified based on local high current speeds (>10 cm s -1) and trap data. Low POC content combined with high lithogenic silica flux at 30 m above bottom (mab) compared to the flux at 120 mab, suggested local resuspension reaching 30 mab, but not 120 mab. Another similar event was detected by the changes in particle size distribution and settling speeds of particles in the benthic nepheloid layer. Flux data indicated two other small-scale events, which occurre...

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Section: Episodic Large-scale Resuspension Eventsmentioning

confidence: 68%

Scales of seafloor sediment resuspension in the northern Gulf of Mexico

Diercks

Dike

Asper

et al. 2018

Elementa: Science of the Anthropocene

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“…Transport of heat, nutrients, and organic materials in the ocean is essential to the understanding of global ocean circulation and ecosystems (Munk & Wunsch, 1998; Walsh, 1991), and recent studies suggest that internal waves play an important role in mass transport in coastal regions (Arthur & Fringer, 2016; Bourgault et al, 2014; Masunaga, Arthur, et al, 2017). It is well known that internal tides are ubiquitous phenomena in the coastal ocean and are associated with strong currents and turbulent mixing.…”

Section: Introductionmentioning

confidence: 99%

“…The rate of turbulent kinetic energy dissipation due to internal tides can reach 10 −5 W kg −1 in coastal shallow regions (Masunaga et al, 2016; Walter et al, 2014). In addition, mixing induced by internal tide breaking enhances sediment/mass transport between coastal regions and the open ocean (Bourgault et al, 2014; Masunaga, Arthur, et al, 2017); thus, internal tides must be considered to better understand mass transport in ocean ecosystems.…”

Section: Introductionmentioning

confidence: 99%

“…(a) Schematic image of BC circulation and (b) intermediate nepheloid layer generated by internal wave breaking provided by Masunaga, Arthur, et al (2017). …”

Section: Introductionmentioning

confidence: 99%

“…Arthur and Fringer (2016) investigated breaking of internal solitary waves using a direct numerical simulation method and showed an intrusion of mixed water propagating toward the offshore resulting in INLs. Masunaga, Arthur, et al (2017) proposed a generation mechanism of INLs caused by internal tide breaking. They suggested that INLs spread over 10 km from the coast within 3 days (Figure 2b) and that the growth of INLs is largely influenced by the topographic slope angle.…”

Section: Introductionmentioning

confidence: 99%

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Baroclinic Residual Circulation and Mass Transport Due to Internal Tides

2020

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Baroclinic (BC) tidal residual circulation due to internal tides is investigated around islands over a shallow ridge using a numerical ocean model. Internal tides enhance vertical mixing over shallow slopes, leading to horizontal density gradients that drive BC residual circulation along the main thermocline. For a strongly stratified summer case, the vertical diffusivity estimated by the Mellor and Yamada turbulence closure model exceeds 1 × 10 −2 m 2 s −1 , and the velocity of BC residual circulations reaches 0.2 m s −1 . The magnitude of BC residual circulation is larger than that of barotropic residual tidal circulation, implying that BC residual circulation due to internal tides plays an important role in coastal ocean circulation. Furthermore, BC residual flow accounts for an equal percentage (5%) of the total tidal kinetic energy as the barotropic residual flow under summer stratification conditions. Results from a coupled sediment resuspension and transport model show the growth of intermediate nepheloid layers formed by strong bottom shear stress and BC residual flow. The residual component contributes largely (23% of the total in summer) to the total suspended sediment flux. Seasonal variability is explored, with weaker winter stratification leading to reduced mixing, and thus weaker BC residual circulation and sediment flux. The magnitude of the BC residual circulation is also shown to be proportional to the square of the tidal amplitude.Plain Language Summary Through the transport and mixing of heat, nutrients, and sediment, ocean flows affect marine ecosystems as well as the global climate. However, because they are both inherently complex and difficult to model or observe, ocean transport and mixing processes are not fully understood. This study uses a numerical model to examine transport by internal tides, focusing on coastal regions where transport processes are especially important for marine ecosystems. Internal tides are tidal-scale internal waves (with semidiurnal or diurnal period) that propagate in regions with density stratification, and are usually generated by tidal flow over topography. Model results show that as internal waves interact with the coastal slope, a time-averaged residual flow develops with a maximum velocity near the main thermocline. This circulation, known as "baroclinic residual circulation," is generated by mixing associated with internal tides and contributes to cross-shore flows. Furthermore, by coupling a sediment resuspension and transport model to the internal tide simulation, it is shown that residual circulation transports sediment offshore, forming subsurface turbidity layers known as "intermediate nepheloid layers." These results imply that baroclinic residual circulation due to internal tides is an important contributor to cross-shore transport in coastal ecosystems.

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Dynamics of Internal Tides Over a Shallow Ridge Investigated With a High‐Resolution Downscaling Regional Ocean Model

Masunaga

Uchiyama

Suzue

et al. 2018

Geophysical Research Letters

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This study investigates the dynamics of tidally induced internal waves over a shallow ridge, the Izu‐Ogasawara Ridge off the Japanese mainland, using a downscaled high‐resolution regional ocean numerical model. Both the Kuroshio and tides contribute to the field of currents in the study area. The model results show strong internal tidal energy fluxes over the ridge, exceeding 3.5 kW m−1, which are higher than the fluxes along the Japanese mainland. The flux in the upstream side of the Kuroshio is enhanced by an interaction of internal waves and currents. The tidal forcing induces 92% of the total internal wave energy flux, exhibiting the considerable dominance of tides in internal waves. The tidal forcing enhances the kinetic energy, particularly in the northern area of the ridge where the Kuroshio Current is not a direct influence. The tidal forcing contributes to roughly 30% of the total kinetic energy in the study area.

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Sediment resuspension and the generation of intermediate nepheloid layers by shoaling internal bores

Cited by 35 publications

References 39 publications

Scales of seafloor sediment resuspension in the northern Gulf of Mexico

Scales of seafloor sediment resuspension in the northern Gulf of Mexico

Baroclinic Residual Circulation and Mass Transport Due to Internal Tides

Dynamics of Internal Tides Over a Shallow Ridge Investigated With a High‐Resolution Downscaling Regional Ocean Model

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