Transcript for Drift Ice as a Geologic Agent, segment 09 of 11


{{{Turbid Ice}}}

A thick upper layer of turbid ice, commonly found in first-year ice, results from a second, far more important entrainment mechanism. Turbid ice contains evenly disseminated fine sediments. In some winters it is widespread over the Beaufort Sea shelf. The originally disseminated sediment soon becomes concentrated on the surface by summer melting. Brown discoloration in this July, nineteen seventy-nine satellite image shows the regional extent of sediment-covered ice in that year. The region of discolored ice carried sixteen times more sediment than the local rivers supply each year. According to free drift simulations, the ice with its sediment load is displaced from the shallow shelf to the area covered by the colored ellipse within one year. These findings have prompted detailed studies of the entrainment mechanism for turbid ice.

Turbid ice is produced by a process called suspension freezing. It requires a combination of subfreezing air temperatures and turbulence, leading to water temperatures slightly below its freezing point. When water is supercooled and turbulent, frazil ice crystals occur in the water column and on its surface as streaks seen here on a shallow Arctic lagoon. Frazil interlocks with itself and with suspended matter in the water. Frazil crystals such as these on a sandbed attach to various fixed objects forming anchor ice, seen here on a current meter and here on a net. During and after a freezing storm, frazil in the water column and anchor ice rises with its sediment to the surface to form a layer of slush ice which can become several meters thick. This dirty slush layer congeals with time as it freezes from the surface down. Afterwards clean ice grows below the dirty slush. Details of the interaction of frazil with suspended matter and with the bed have been examined in many laboratory experiments. A small racetrack flume placed in a freezer was used in some of these experiments. The following freshwater sequence shows the action of frazil ice on a sand substrate with currents moving at sixty centimeters per second. A lack of ice on mineral specimens suspended in the water at the left and on the sand bottom is observed until just before maximum supercooling is reached. As the water temperature drops to slightly over one-tenth of a degree Celsius below its freezing point, frazil flocks are forming and can be seen moving along the bed. These are mostly too heavy with sand to rise.

Classic theory of sediment dynamics which considers stress of the bed, threshold velocity for grain movement, saltation and so on, breaks down in supercooled water.

Finally, at the point of maximum supercooling, ice is beginning to build up on mineral specimens on the left while flocks roll along the bed sluggishly and occasionally come to rest in the shelter of a ripple where they are buried by the advancing sand. The frazil moved by the currents is enriched in particulate matter above levels in the surrounding water. This is what ultimately leads to the formation of turbid ice.

Other sets of experiments were conducted in two-meter deep tanks placed in freezers to learn about the interaction of frazil rising through a sediment-charged water column. Frazil forms in the base of these tanks which are stirred mechanically. In this view frazil crystals are seen at the sixty centimeter level in seawater. Frazil crystals rapidly grow and flocculate as they rise from the region of basal turbulence into quiet water at ninety centimeters and here at one hundred ten centimeters. Rise rates vary with flock size. Large flocks rise much faster than small ones. Fast-rising frazil flocks entrain particulate matter into their wake and slam into slow ones, often destroying them. The result is a considerable differential-motion-induced small-scale turbulence in the water, shown here as the camera follows rising frazil flocks over a distance of thirty centimeters.

In most experiments the slush ice accumulating at the surface has higher sediment concentrations than the underlying water. This indicates turbid ice is produced by frazil scavenging of sedimentary particles from the water. The scavenging is made visible here by introduction of fluorescent sand grains, appearing in ultraviolet light as bright spots. As some grains continue to settle downward, others are trapped by frazil flocks and are carried upward.

{{{Background noise}}}

During freezing storms and frazil production, concentrations of suspended particulate matter in the Beaufort Sea are elevated to an order of magnitude above those during otherwise similar, nonfreezing storms. Particles are also subjected to scavenging by resuspension from ice wallow. At such times, wind-driven currents of two knots occur on the shallow shelf, resulting in major sediment movement through the action of still-submerged sediment-charged frazil, before true ice rafting on the sea surface begins.

Major entrainment events may ultimately introduce sediment loads of over one thousand tons per square kilometer of ice and are therefore highly significant for the shelf and coast sediment budget.

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