Near-bottom currents

Current regimes are also correlated with feeding mode. Very slow currents yield low horizontal particle flux and allow even fine particles to settle. This, combined with the absence of primary production in the dark abyss, yield very low concentrations of suspended food particles, often making suspension 

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Some amounts of sand riverine sediments are transported in channels in the form of sand ripples, waves and dunes formed on the river bed under the influence of the near-bottom currents. This sediment load (called bedload) usually comprises about 10% of the suspended sediment load. Taking into account this fact, we find that the total river sediment input to the Black Sea may range up to 84 x 106 tyear-1. 

On an average, the flow in the Fehmambelt below 20 m depth is governed by topography. Salinity data indicate that most of the time the near-bottom currents observed in the deep... 

We hypothesize that the development of nonuniform distribution of both linear and sediment accumulation rates is related to the course of the rim current at the Gotland Deep. Empirical measLuements of near-bottom currents are rare (e.g., Hagen and Feistel, 2001 Hagen and Feistel, 2004). Hence, near-bottom velocities were derived from a 3D model (Fig. 14.9 Schmidt, personnel communication). 

Besides the decreasing trend of Hg contents in surface sediments, this hot spot is still active, that is, serving as a source for Hg-rich suspensions participating in a basinwide natural tracer experiment. The 2005 mapping shows a very vivid example of a mercury distribution pattern (geochemical aureole) reflecting the sediment dynamics, including the effects of near-bottom currents. 

The surface circulation of the western basin apparently remains anticyclonic, and the deep circulation cyclonic, under the predominant winds. As far as the relatively fast surface currents are concerned, the transversal spatial scale of today s sea is too small for the Coriollis force to be significant, so that the direct wind drag matters rather than the Ekman transport. On the other hand, the bottom layer circulation seems to immediately follow the sea surface slopes in the classic barotropic manner, so the Coriollis force is still effective for the slower, near-bottom currents. 

Nevertheless, few attempts have been made to calculate the rates of sediment transport from observations of the near-bottom current velocities (McCave, 1971 Ludwick, 1975b Gadd et al., 1978). 

Gadd, P. E., Lavelle, J. W., and Swift, D. J. P. (1978). Estimates of sand transport on the New York shelf using near-bottom current meter observations. J. Sediment. Petrol. 48, 239-252. 

Shelves that face prevailing westerly winds and are open to oceanic waves are most affected by storms Johnson and Baldwin, 1986). The strong near-bottom currents are... 

FIGURE 2.11 Major surface currents in the oceans. Surface currents are mainly wind-driven, but are also influenced by the oceans boundaries and by the Coriolis force. Not illustrated here are deep-water currents, mainly driven by differences in water density. For example, water carried northward by the Gulf Stream sinks after giving up much of its heat, and flows southward in the Atlantic Ocean as a near-bottom current. On the planetary scale, one net effect of ocean currents is to transport large amounts of heat from warm equatorial regions toward the poles (Earth Science Reference Tables, 2011). 

By combining the findings of Cacchione, Drake and the results reported here, a coherent model can be proposed to explain the deposition inventory of the radionuclides. The down-canyon current transports large quantities of sediment toward the radioactive waste disposal site at 4000 m. Within the upper canyon, fine material is transported the furthest. Near the mouth of the canyon, sediment erosion of the walls occurs due to the down-canyon currents meeting a proposed opposing on-shore bottom current. The eroded material from the walls is transported and the finer material is deposited in eddies formed where the two currents meet. 

The near-surface currents at about 10 m depth predominantly flow toward northwesterly directions (outflow), and the flow 3—4 m above the bottom is predominantly directed toward east and southeast (inflow). The magnitude in the near-bottom layer is smaller than the magnitude within the surface layer. The net mean currents (speed) are comparable, but in opposite directions. Opposite pressure gradient forces determine the water exchange through the Fehmambelt within both layers. 

The ADCP current profiles at the same site revealed an interesting structure. Between the surface layer (net outflow) and the near-bottom layer (net inflow) there is an intermediate layer with an upper boundary fluctuating between 12 and 18 m (Table 6.5). At this upper boundary, the mean vector speed is minimum and exhibits a great directional variability (sf). At its lower boundary between 16 and 23 m depth, the intermediate layer exhibits relatively high vector speeds and directional stability. No matter whether the mean values indicate outflow within the entire water column (the opposite case—inflow within the entire water column—was never observed on the long run) or outflow and inflow prevail in the upper and... 

At the eastern flank of the relatively shallow and topographically even Darss SiU is the MARNET station Darss Sill (DS, see Chapter 3), where operational ADCP current measurements have been carried out since 2004. This is the region from where the most important proportion of saline and oxygenated water masses from the Kattegat occasionally spreads within the near-bottom layer down the slope into the deep of the Arkona Sea. Table 6.8 shows the annual and monthly means of the year 2005. 

Wind and density induced currents near the bottom temporarily are very weak. If there were no tidal currents, which are weak but are always ( ) present oxygen depletion in the near-bottom layers of the Baltic Sea probably would occur more frequently. Land uplift and subsidence play a major role in current studies of sea level rise. However, when evaluating gauge data, also the tides have to be taken into account, especially in historical datasets (Liebsch, 1997),... 

When comparing the maximum current velocities with the distribution map of linear sedimentation rates (Fig. 14.8), it becomes evident that the highest rates appear in the NE part of the study area where the steepest gradient in maximum near-bottom velocities occurs. The idea is that the intense currents at the eastern margin lead to an import of large amounts of suspended matter into the basin, and this material sinks to the seafloor and produces high accumulation rates at the sudden reduction of turbulence. 

FIGURE 14.26 Model situation for mean current velocities of >5.5 cm/s in the near-bottom layer of the Arkona Basin. The current directions and velocities are described by black arrows (The figure was provided by Seifert, lOW). 

Over the remainder of the region, concentrations of methane ranged from 300-500 nL/L. The small surface maximum observed near Unimak Pass arises from vertical turbulence in Unimak Pass. Near-bottom waters south of the Alaska Peninsula are enriched in methane, which becomes entrained in the northerly flow through the pass. Again, the surface concentrations of methane indicate the mean surface current drift as water moves into the Bering Sea. 

The lower concentrations of methane correlate with lower sea surface temperatures (SST) and near-bottom temperatures over the middle shelf. Bottom temperatures over the middle shelf were < -1 C in October 1975 and July 1976, compared to temperatures of 3.5-4.5 C observed in May 1981 (39). Water temperatures this low indicate local ice formation the previous winter. On the other hand, bottom temperatures in St. George Basin in May 1981 were about the same as those observed in October 1975 and July 1976 (17) indicating that these waters were less influenced by the abnormally cold winters than the middle shelf region. This is expected because the north-setting coastal current brings relatively warm Gulf of Alaska water across the outer shelf (19). 

Season distributions of methane also show that methane is a qualitative descriptor of mean current flows in Unimak Pass, along the North Aleutian shelf, and in the near-bottom waters of St. 

The third case, a pressure gradient induced by the bottom current and a supposed advective movement of the pore water, is of importance mainly on the shelf where shallow waters, fast currents, an uneven underground, and high permeability values in coarse sediments are encountered. Ziebis et al. (1996) as well as Forster et al. (1996) were able to demonstrate by in situ measurements and in flume experiments that the influence of bottom currents may be indeed crucial for the superficial pore water of coarse sand sediments near to the coast. 

Sharpies (1997) made observations on the intrusion of subtropical water into the coastal zone of northeast New Zealand from late winter to midsummer 1994-1995, 1 year after the Karenia spp. bloom. He showed that the intrusion of subtropical water into the gulf was associated with high salinity nutrient depleted surface layers and near bottom waters with high nitrate N levels (3-5 mmol/m ). Southeasterly winds that occurred in late-November and mid-December 1992, prior to the bloom, are consistent with the hypothesis that Ekman transport effects on the East Auckland current induced the movement of a subtropical water mass, possibly carrying an established dinoflagellate community, into the Hauraki Gulf in late-December 1992. 

The ECS circulation is dominated by the northward flow of two loops of the Kuroshio Current the Taiwan Warm Water in the west and the Yellow Sea Warm Water in the east. Both water masses are characterized by high salinity and warm water temperatures. In contrast, the southward flow in near bottom water occurs from the flow of the Changjiang River and Jiangsu Coastal Waters along the Chinese coast, the Korean Coastal Waters in the east, and the Yellow Sea Cold Water in the north (Fig. 4.1, Lee and Chao, 2003). The coastal currents in particular appear as seasonally cold and brackish water... 

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