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Transport of Particles Across Continental Shelves

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Transport of Particles Across Continental Shelves W&M ScholarWorks VIMS Articles 2-1994 Transport Of Particles Across Continental Shelves CA Nittrouer LD Wright Virginia Institute of Marine Science Follow this and additional works at: https://scholarworks.wm.edu/vimsarticles Part of the Geochemistry Commons, and the Geophysics and Seismology Commons Recommended Citation Nittrouer, CA and Wright, LD, "Transport Of Particles Across Continental Shelves" (1994). VIMS Articles. 1410. https://scholarworks.wm.edu/vimsarticles/1410 This Article is brought to you for free and open access by W&M ScholarWorks. It has been accepted for inclusion in VIMS Articles by an authorized administrator of W&M ScholarWorks. For more information, please contact [email protected]. TRANSPORT OF PARTICLES ACROSS CONTINENTAL SHELVES Charles A. Nittrouer L. Donelson Wright Marine Sciences Research Center VirginiaInstitute of Marine Science State Universityof New York, Stony Brook College of William and Mary, Gloucester Point Abstract. Transport of particulate material across cesses at the seabed play important roles within the continental shelves is well demonstrated by the distri- boundary layer. The coupling of hydrodynamic forces butions on the seabed and in the water column of from currents and surface gravity waves has a partic- geological, chemical, or biological components,whose ularly strong influence on across-shelf transport; dur- sources are found farther landward or farther seaward. ing storm events, the combined effect can transport This paper addressespassive (incapable of swimming) particles tens of kilometers seaward. Several impor- particles and their transport across (not necessarily tant mechanisms can cause bidirectional (seaward and off) continental shelvesduring high standsof sea level. landward) transport, and estimates of the net flux are Among the general factors that influence across-shelf difficult to obtain. Also, measurements of across-shelf transport are shelf geometry, latitudinal constraints, transport are made difficult by the dominance of along- and the timescale of interest. Research studies have shelf transport. Geological parameters are often the investigatedthe physical mechanismsof transport and best indicators of net across-shelftransport integrated have made quantitative estimates of mass flux across over time scales longer than a month. For example, continental shelves. Important mechanisms include fiuvially dischargedparticles with distinct composition wind-driven flows, internal waves, wave-orbital flows, commonly accumulate in the midshelf region. Across- infragravity phenomena, buoyant plumes, and surf shelf transport of particulate material has important zone processes. Most particulate transport occurs in implications for basic and applied oceanographic re- the portion of the water column closest to the seabed. search (e.g., dispersal of planktonic larvae and parti- Therefore physical processesare effective where and cle-reactive pollutants). Continued research is needed when they influence the bottom boundary layer, caus- to understand the salient mechanisms and to monitor ing shear stresses sufficient to erode and transport them over a range of timescales. particulate material. Biological and geological pro- INTRODUCTION ticles is toward and possibly into estuaries, lagoons, beaches, or other coastal environments. The continentsare the largest sourcefor most types The importance and diverse aspects of across-shelf of particulate material entering the world ocean. The transport for all materials (water, solutes, and parti- fraction of material that is carried to the ocean beyond cles) have led to much research, and this paper re- continental shelves is dependent on mechanismsthat views some of the more recent studies. The present transport particles across shelves. This is the funda- synthesisis confined to the following considerations. mental importance of across-shelftransport, but it also 1. Discussions focus upon passive particles that has several corollary considerations. (1) Many of the consist primarily of inorganic sediments but also in- dissolved components supplied by continents are clude dead organic solid phases. Living organisms are transformed into particles on shelves (e.g., particle- relevant only if they reside on particles (e.g., some reactive metals, biological nutrients), and therefore bacteria) or are incapable of horizontal or vertical the fates of these materials also depend on mecha- motion (e.g., some planktonic larvae). The flux of nisms of particulate transport. (2) Mechanisms pre- water is considered only as the transporting medium venting across-shelf transport (i.e., accumulation of for particles. Some dissolved phases are relevant, but particles on continental shelves)are equally important only after their transformation to a particulate form. because they also control the quantity and quality of 2. Particulate transport across continental shelves material delivered to the deeper ocean. (3) Some oce- (e.g., toward the shelf break) will be considered. The anic mechanismscan transport particles landward. In mechanismsfor transport off shelves are not necessar- areas where these mechanisms operate and during ily the same and will not be discussedexplicitly. periods when they are operating, net transport of par- 3. Continental shelves are geologically ephemeral Copyright 1994 by the American GeophysicalUnion. Reviewsof Geophysics,32, 1 / February1994 pages 85-113 8755-1209/94/93 RG-02603515.00 Paper number 93RG02603 e85e 86 ß Nittrouer and Wright: ACROSS-SHELFTRANSPORT 32, 1 / REVIEWSOF GEOPHYSICS features. They do not exist during glacial low stands, They commonly have infragravity frequencies (peri- or during long periods of high stand where shorelines ods between 20 and 300 s). are able to prograde to the shelf break. In addition, Epicontinentalversuspericontinentalshelves'Epi- across-shelf transport probably behaves differently continental shelves are semienclosedand lie partially during periods of rapid rise or fall of sea level. Discus- within the confines of continents (e.g., the Yellow sions will be limited to relatively stable high-stand Sea). Pericontinental shelvesare those off open coasts conditions. (e.g., the Atlantic or Pacific coast of North America). The following sections provide a synthesis of the Gravity waves: As used in this review, gravity general considerations for across-shelf transport of waves imply water waves (such as wind-induced sur- particles. Physical processesdrive the transport, and face waves or tides) in which gravity provides the most of the mass flux occurs in the bottom boundary restoring force that sustainswave propagation. Grav- layer. The longer-term evidence for the transport is ity waves may be either barotropic (surface) or ba- demonstratedfrom geological observations. Consider- roclinic (internal). ations for future research are discussed in the final Geostrophic currents' Steady currents that are section. sustained by a balance between a pressure gradient force and the Coriolis force (e.g., the Gulf Stream). Groupy waves: Wind-driven surface gravity GLOSSARY OF TECHNICAL TERMS waves in nature commonly arrive at any point as a successionof alternating packets or "groups" of high Active versuspassive continental margins: Active and low waves. The resulting wave train of varying margins are tectonically active in the sense that they wave amplitude is referred to as groupy. are in the immediate vicinity of a convergence(colli- Hydraulic roughness: The nominal height of the sion) between lithospheric plates or near an active bottom roughnesselements in terms of their effects on spreadingcenter. Passive margins are relatively inac- near-bottom flows. The greater the hydraulic rough- tive tectonically and are distant from spreading or nesslength, the higher will be the elevation above the convergence. bed at which friction causes the current velocity to go Baroclinic' Pertaining to processes that involve to zero. pressuregradients related to density differences(strat- Infragravity waves: Surface gravity waves with ification) within the water column. oscillation frequencies lower than those of the wind- Barotropic: Pertaining to processesthat involve generatedsurface waves. The periods of infragravity pressure gradients related to variations in sea surface waves are in the range of 30 to 300 s. Infragravity elevation. waves have multiple origins: edge waves, leaky mode Bottom boundary layer: The layer of the water standingwaves, or groups of wind-generated surface columnjust above the seafloorwhere flows are signif- waves. icantly retarded by bed friction and where vertical leaky mode standingwaves: Standingwaves that momentum transfers are most intense. are not trapped near the coast. In contrast to edge Buoyant plumes: Relatively thin layers of posi- waves, reflected energy associated with leaky mode tively and negatively buoyant sediment-laden water standing waves is radiated ("leaked") back to sea. issuing from a point source such as a river mouth. Typically, these waves have infragravity frequencies. Positively buoyant plumes disperse in the surface Radiation stress: An excess flux of momentum layer, negatively buoyant plumes disperse over the (associated with surface gravity waves) in the direc- bed. tion of wave propagation. Coastal-trappedwaves (continental shelf waves): Shear stress: A friction force per unit area. As Long-period (several days) oscillations that are topo- appliedin the context of this paper, shear stressrefers graphicallytrapped near the coastand propagatealong
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