The fate of nitrogen and phosphorus at the land-sea margin of the North Atlantic Ocean
S. W. NixonJames W. AmmermanLarry P. AtkinsonV. M. BerounskyGilles BillenWilliam C. BoicourtWalter R. BoyntonThomas M. ChurchDominic M. DiToroRagnar ElmgrenJonathan GarberAnne E. GiblinRichard A. JahnkeNick OwensMichael E. Q. PilsonSybil P. Seitzinger
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Continental Margin
Continental Margin
Mass wasting
Passive margin
Tectonic subsidence
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Seafloor Spreading
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The continental slope and plateau off northern California are underlain by a well-defined and extensive acoustic reflector that crosses other reflectors and mimics the surface of the sea floor. This bottom-simulating reflector (BSR) lies at an average subsurface depth of 250 m and is laterally continuous beneath the Klamath Plateau (water depths of 800-1200 m) off northernmost California. Limited data show that it extends northward into offshore Oregon and seaward at least to the base of the slope (3000 m water depth). The BSR has been mapped along the continental margin for a distance of over 130 km and underlies an area of at least 3000 km/sup 2/. The water depths and sediment depths of the BSR, as well as its pervasiveness, all suggest that it represents the base of a natural-gas hydrate. Using standard phase-boundary diagrams for hydrate stability, the authors estimate that the geothermal gradient in this area is about 50/sup 0//km. Such a gradient is higher than found along most subduction margins, possibly because the subducting oceanic crust in this region is young. This acoustically inferred gas hydrate is the first mapped along the western US conterminous continental margin.
Continental Margin
Clathrate hydrate
Passive margin
Seabed
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During the past decade many geo-scientific discoveries suggest that continental margins are regions of extensive thrusting. Seismologic data establish that earthquake hypocenters are contained in a tabular volume of rocks that plunges steeply landward below the seaward edges of continents. First-motion studies of these earthquakes indicate a convergence of continental and oceanic crusts. On land, coastal areas of extensive thrust faulting are becoming known. These data support the hypothesis of ocean-floor spreading which requires extensive thrusting of oceanic crust beneath the continents. This hypothesis and the seismologically defined thrust zone imply that profound compressional deformation should take place at the base of continental slopes. Structures produced by compressional forces are not observed in seismic-reflection records of the sediments filling marginal trenches or in sediments tilted against the continental slope during development of the trenches. Continental rises also consist of undeformed strata. Only deformation from subsidence and slumping has been seen at the foot of the continental slope from southern Chile to the outer Aleutian Islands. The observations of little or no thrusting at the juncture of the upper continental and oceanic crusts are now numerous and well established. These data must also be End_Page 747------------------------------ considered in hypotheses that explain the development of continental margins. End_of_Article - Last_Page 748------------
Continental Margin
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Continental Margin
Slumping
Submarine canyon
Terrigenous sediment
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An abrupt change in slope marks the boundary between the continental shelf and the continental slope. The slope change previously has been attributed to (1) sedimentation at wave base and in equilibrium with present sea-level, (2) sedimentation at wave base but related to some past lowered sea-level, (3) wave abrasion in equilibrium with present sea-level, and (4) wave abrasion related to some past lowered sea-level. To test these various possible origins, the concept of wave base is examined theoretically. It is found that wave base appears unimportant in the regimen of the sea, and it does not account for the observed shelf-break. The break appears to be related to the depth at which breakers form which, excluding rare cases, is at a maximum of about 5 fathoms. The wr ters term this the depth of vigorous abrasion. On theoretical grounds it appears, therefore, that the break-of-slope is caused by wave abrasion related to a sea-level lowered to within about 5 fathoms of the break. This theory is tested with several field facts and these appear to confirm it. For example, the depth of the shelf-break is approximately the same on the windward and the leeward side of islands which is in agreement with the concept of the break being formed at the depth of vigorous abrasion, but is in disagreement with the concept of wave base. Some geological implications of this theory are briefly discussed. These are: (1) sea-level has recently (geologically speaking) risen eustatically, (2) this recent rise has taken place more rapidly than changes of land-level effected by erosion, sedimentation, or diastrophism.
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Yangtze river
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The continental margin west of Oregon consists of a generally convex-upward surface 35 to 60 mi. (nautical) wide. The continental shelf, which forms the upper part of the surface, slopes seaward at less than 1° and ranges irregularly in width from 9 to 35 mi. Several elongate hills or banks rise above the general shelf level. The lower portion of the continental margin (continental slope) increases in average declivity from about 1°-10°, and is modified by numerous ridges, hills, benches, and submarine canyons. Off the central part of the Oregon coast the shelf widens from 13 mi. at 45°00^primeN. to 35 mi. at 43°58^primeN., and then narrows abruptly to 16 mi. at 43°55^primeN. Two essentially north-trending shoals, Stonewall Bank and Heceta Bank, dominate the topography of the shelf in this area. The apparent offlap relationship of late Eocene to middle Miocene marine sedimentary rocks along the shore and the occurrence of Pliocene marine sedimentary rocks on the two banks suggest that one or more Tertiary sedimentary basins exist beneath the continental shelf and slope. Gravity measurements indicate that thick sections of sedimentary rocks may be present. From echo soundings, Stonewall and Heceta Banks are interpreted to be the surface expressions of structures associated with the Tertiary basins. Fine to very fine detrital sands and glauconite-rich silts and clays cover the shelf in areas between the gravel and rock outcrops common on and in the vicinity of the shoals. End_of_Article - Last_Page 1758------------
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Continuous seismic profiler recordings over the continental shelf northeast of Newfoundland show an accumulation of unconsolidated sediments ranging in thickness from 30 to 150 m, unconformably over-lying bedrock that dips gently eastward toward the continental slope. The distribution of these unconsolidated sediments indicates that the banks that compose the 'shelf-edge rise' are not primarily morainal material, but rather they are areas of positive relief on the bedrock surface. Anomalous strike-dip relationships associated with these bank areas suggest that they may also have structural significance. The underlying bedrock layers are truncated by the continental slope, and there is reversal in the dip of these layers from eastward to westward beneath the slope. The axial zone of the resulting fold apparently coincides very nearly with the axis of a large magnetic anomaly that extends along the continental margin in this region. A Tertiary to post-Tertiary age is suggested for the fold, as well as the structural disturbance associated with the topographic relief on the continental shelf. Within the continental rise an area of irregular bottom topography is considered to be due to the accumulation of large masses of sediment by slumping. Subbottom reflectors at depths beyond the continental rise show crumpling, which is assumed to be further evidence of mass-movement due to gravity. Superficially, the continental margin northeast of Newfoundland appears similar to parts of the continental margin to the south, on the flank of the Appalachian structural complex.
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Slumping
Bedrock
Passive margin
Seafloor Spreading
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