Local and global aspects of the bottom nepheloid layers in the world ocean

Summary Fine-grained marine sediments are cohesive but their degree of cohesion is not simply determined by grain size. Cohesion controls erodibility, and water content, mineralogy, cation exchange capacity, salinity of interstitial and eroding fluid, organic mucus content and Bingham yield strength are all parameters relating to cohesion that have been proposed. No unique relation to erodibility has emerged and for erosion of slowly deposited sediment, modified by biota, it seems that measures of surface properties such as aggregate strength should be more relevant than bulk properties such as yield strength. The latter may be more appropriate for rapidly deposited estuarine muds. Once eroded, suspended sediment is subject to flocculation and biological aggregation, which alters size and settling velocity distributions, thereby controlling distribution in the flow and rate of deposition. Disaggregation may also occur through turbulent straining of particles, but in most marine situations this will permit stable particles over 1 mm in diameter, though under higher shear close to the bed the upper limit may be 100–200 μm. For deposition where τ0 < 0.1 Pa the maximum stable aggregate size is probably ≥ 100 μm. Deposition of fine sediment on smooth beds probably occurs by entrapment in, and settling through, the viscous sublayer. On rough beds particles are trapped in the interstices between roughness elements. We have only a rough idea of critical deposition conditions but can give a fairly good estimate of deposition rate in given conditions. The longer term net deposition rate as measured by radiometric methods over weeks to months probably owes as much to frequency of erosion as to frequency and rate of deposition and is not yet well predicted from fluid and sediment parameters.

Erosion, transport and deposition of fine-grained marine sediments

Current-controlled, abyssal microtopography and sedimentation in Mozambique Basin, southwest Indian Ocean

An attempt is made to outline a satisfactory general theory for the formation of Pacific deep-sea manganese nodules based on a consideration of the time of initiation of formation of the nodules, a mechanism of maintenance of the nodules at the sediment surface and the role of biological productivity of the surface waters in influencing nodule characteristics. Using this model, the principal physical, chemical and distributional features of the nodules and reasons for their differences on a regional scale in the Pacific can be interpreted.

Tim P. Barnett

Papers

Ocean Temperatures: Precursors of Climate Change?.