Interaction of rising frazil with suspended particles: Tank experiments with applications to nature
Widespread occurrence of sediment-laden (turbid) sea ice and high concentrations of diatoms and foraminifers in ice have recently been reported from both polar regions. Many possible mechanisms of particle entrainment into ice have been postulated, among which scavenging by rising frazil ice and nucleation or adhesion of ice onto suspended particles appear to be the most likely ones. No reliable experimental data on the mechanisms, however, are available. Because of the importance of turbid ice for sediment transport, tanks for laboratory-scale experiments were constructed, in which frazil crystals produced at the base were monitored rising through water column laden with various types of particulate matter, including plankton. Observations made in salt water are reported here. Over a distance of 1.5 m, frazil < 1 mm in diameter grew to crystals or flocs several cm in diameter, rising at average velocities of 2 to 3 cm/s. Rise velocities were a function of frazil size, but varied greatly due to interactions of ice particles of different size and velocity and the resulting turbulence. Sand-size particles could be either trapped permanently by rising frazil, or were temporarily supported and again released. With live plankton, a several-fold enrichment of ice occurred, suggesting that their irregular shapes or appendages were caught by ice flocs. Diatom- and foram tests were also relatively effectively trapped. The concentration of silt- and clay-size terrigenous detritus in frazil tended to increase relative to the water. We found no preferential sorting by ice in this size range. Various kinds of evidence showed that ice does not nucleate onto foreign particles, and has no adhesive properties. Foreign material resided in the interstices of crystal aggregates, and particles denser than water could be released by agitation, suggesting that scavenging is a mechanical process. With rising frazil, the settling of particulate matter therefore is either retarded or reversed, resulting in a net upward sediment flux and a sediment-laden ice cover from this process of suspension freezing.