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Processes |
US Geological Survey, Menlo Park, California 94025 USA
Forward light scattering and light transmission are the two best techniques, coupled with direct sampling, to measure the spatial and temporal variations in suspended sediments of the ocean. It has been difficult to calibrate these methods with satisfactory precision in terms of suspended particulate matter (SPM) concentration; the best precision is approximately ± 10 µgL–1. The poor resolution is due to the many unknown factors in the relationship between optical effects and concentration of SPM, in addition to errors in direct sampling of SPM concentration. Because typical deep-sea concentrations are in the range of 5 to 50 µgL–1, extreme care must be taken to avoid contamination by airborne dust and salt filter residues. As a short-lived tracer of water motions, SPM is useful because its settling rates range from about 10–1 to 102 m day–1, rates giving half-life time-scales roughly comparable to time-scales of deep-water eddy motions and boundary layer events. More needs to be learned, however, to define these settling rates with some confidence. Much progress has recently been made in studies with sediment traps which directly measure the downward vertical flux of particles. The work with sediment traps is prompted by the realization that large fast-settling aggregates and faecal pellets are important in the delivery of material to the bottom, and due to their short residence time and hence low concentrations in the water column, they are not sampled adequately by water bottles.
