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et al.25 for E (North East Pacific margin).

layer mainly coincide with the occurrence of pycnoclines, where rapid changes in seawater-density (and thus excess-density of aggregates) can reduce or even stop the vertical flux of the aggregates. The physical forcing in the mixed layer creates changes in the biological processes, which depend on them. Here, organic rich aggregates are formed which are derived from gelatinous housing of zooplankton species, mucous feeding webs used by others, faecal material, and from phytoplankton cells and their component particles.26 The proportion of free water within the aggregates, its porosity, determines how fast the internal environment changes in response to varying external conditions; the porosity also influences the rate at which small particles accumulate on the aggregates. After their slow descend through the pycnocline, differential settling is mainly responsible for additional aggregate formation as well as the migrating zooplankton, which consume the aggregates to depth of up to 1000 m.

11.4 TRANSPORT WITHIN THE BENTHIC BOUNDARY LAYER: THE RESUSPENSION LOOP

Once on the sea floor, the aggregates are more easily remobilized into the benthic boundary layer than the bulk sediments beneath,23 and are resuspended back into the water column, being again subjected to aggregation and disaggregation processes.27 Long-term studies at different continental margins revealed that the bottom sediments consist of a thin surface layer of organic rich aggregates (mean diameter 100 to 2500 /m). Theseresuspend under critical shear velocities [M*c] of 0.4 to 1.2 cm sec-1 (mean m*c of 0.8 ± 0.1 cm sec-1) and have median diameters of 140 to 450 /m and settling velocities of 0.05 to 0.35 cm sec-1 (Figure 11.1). The aggregates consist of up to 75% of organic matter, which is mostly refractory with a carbon/nitrogen ratio exceeding 10, and the lithogenic material is embedded in the amorphous matrix of the organic matter. The BBL aggregates contain remnants of faecal pellets, meiofauna organisms, and shell debris of foraminifera. Approximately 35 to 65% of the bacteria of the BBL are particle attached and live within the organic matrix of the aggregates and approximately 1% of the organic fraction is labile bacterial organic carbon.23,24 The BBL aggregates in >100 /m size range are resuspended under similar flow conditions as particles of similar size but higher density (sand). However, they are transported over much greater distances due to their lower density and porous structure, which reduces their settling velocity (Figure 11.2, compare Figure 11.1). These aggregates can subsequently be transported in tide-related resuspension-deposition loops over long distances. Table 11.1 summarizes typical particle characteristics from continental margin BBLs. A cohesion effect for the aggregates is visible at about 30 /m (Figure 11.2). Thus, organomineral aggregates with average sizes <30 /m behave in the same way as clay (<2 /m), and particles coarser than 30 /m should display size sorting behaviour. The last result is different from the calculations of McCave et al., who propose 10 /m as threshold between noncohesive and cohesive sediment behavior.31 This difference seems due to particle stabilization from micro-bial exudates.32 The erosion threshold data were mainly obtained in summer, when biological activity in surface sediments at the study site is high.33 Evidence for the importance of biological adhesion on critical stress for incipient transport has been

FIGURE 11.2 Critical bed shear stress for erosion of continental margin sediments showing the onset of a cohesion effect at about 30 ^m. The black curve represents the modified Shields curve after Unsold.28 The dashed lines refer to the limits of available high quality data evaluated by Miller et al.29 and Self et al.30

Particle size d50 (^m)

FIGURE 11.2 Critical bed shear stress for erosion of continental margin sediments showing the onset of a cohesion effect at about 30 ^m. The black curve represents the modified Shields curve after Unsold.28 The dashed lines refer to the limits of available high quality data evaluated by Miller et al.29 and Self et al.30

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