Favorable Conditions for Suspension Freezing in an Arctic Coastal Polynya

Abstract

Arctic sea ice incorporates and transports sediment, releasing it back into the water column during the melting season. This process constitutes an important aspect of marine sediment transport and biogeochemical cycling. Sediment incorporation into sea ice is considered to occur mainly through underwater interaction between frazil ice and resuspended sediment, referred to as suspension freezing. However, harsh environmental conditions have greatly limited field observations of this phenomenon. Analysis of mooring data from a coastal polynya in the northeastern Chukchi Sea, in conjunction with coastal ice radar and meteorological data, indicates that suspension freezing is a key mechanism for sediment entrainment into sea ice. During polynya episodes, acoustic backscatter data obtained by an Acoustic Doppler Current Profiler showed the presence of frazil ice from the surface down to 20-to 25-m depth, coinciding with in situ and potential supercooling. Underwater frazil ice persisted over 1 week under windy, turbulent water column conditions. A combination of the turbidity and Acoustic Doppler Current Profiler backscatter data revealed upward sediment dispersion associated with strong currents during the polynya episodes. The fact that frazil ice and resuspended sediment were detected at the same depth and time strongly suggests the interaction between ice crystals and sediment particles, that is, suspension freezing.Plain Language Summary Sea ice incorporates, transports, and releases particulate matter.These processes constitute an important aspect of the biology, biogeochemical cycling, and pollutant transport in polar oceans. Seafloor sediments serve as the most important source of such particulate matter; however, the process of sediment incorporation into sea ice remains poorly explored. We conducted a year-long study of sediment resuspension and entrainment processes, using underwater sensors deployed in the Chukchi Sea. During winter, wind-driven offshore transport of sea ice created area of open water and newly grown thin ice that persisted for several days, so-called coastal polynya or flaw lead system. Our sensors recorded small ice crystals, so-called frazil ice, that formed in the water column when water temperatures were below freezing point (supercooling). During some of these episodes, sediment was resuspended from the seafloor and dispersed upward by the strong currents, bringing it into water depths at which frazil ice was encountered. Such conditions provide for opportunities that allow frazil ice crystals or aggregates to capture resuspended sediment, a process referred to as suspension freezing. Based on this study, we propose that suspension freezing commonly occurs in shallow Arctic polynyas, serving as a key process of sediment incorporation into sea ice.

Sediment-laden sea ice in southern Hudson Bay: Entrainment, transport, and biogeochemical implications

“…While these are the first in situ observations of sediment-laden sea ice in Hudson Bay, similar sediment-laden sea-ice types have been observed previously in nearby James Bay and Foxe Basin (Pelletier, 1986), and in several areas throughout the Arctic, including the Chukchi Sea (Barnes et al, 1982;Kempema et al, 1989;Reimnitz et al, 1993;Ito et al, 2019), Laptev Sea (Larssen et al, 1987), Sea of Okhotsk (Nomura et al, 2009), Baltic Sea (Granskog, 1999), and Central Arctic (Pfirman, 1987;Nurnberg et al, 1994). Given that this unique ice type can present hazardous conditions in a region with an active open-water shipping season (Babb et al, 2019) and impact local biogeochemical cycles within Hudson Bay in a way not previously considered (Eicken et al, 2005), an indepth analysis of the formation, transport, and melt of sediment-laden sea ice in southern Hudson Bay is pertinent.…”

“…Rates of Aeolian transport are not well constrained for sub-Arctic seas. Suspension freezing occurs when frazil ice is formed rapidly in turbid waters on sediment particle surfaces (Ledley and Pfirman, 1997;Darby et al, 2011;Ito et al, 2019) or due to double-diffusion at a temperature/ salinity interface (Golovin et al, 1999), both of which subsequently incorporate/aggregate suspended sediment-laden ice crystals into sea ice. This process occurs most commonly when ice forms in shallow areas of open water, as in coastal flaw leads or polynyas, where enhanced momentum transfer from the atmosphere to the ocean fosters strong water column mixing and sediment resuspension coupled with cold air temperatures that drive rapid ice growth (Pelletier, 1986;Ito et al, 2019).…”

“…Suspension freezing occurs when frazil ice is formed rapidly in turbid waters on sediment particle surfaces (Ledley and Pfirman, 1997;Darby et al, 2011;Ito et al, 2019) or due to double-diffusion at a temperature/ salinity interface (Golovin et al, 1999), both of which subsequently incorporate/aggregate suspended sediment-laden ice crystals into sea ice. This process occurs most commonly when ice forms in shallow areas of open water, as in coastal flaw leads or polynyas, where enhanced momentum transfer from the atmosphere to the ocean fosters strong water column mixing and sediment resuspension coupled with cold air temperatures that drive rapid ice growth (Pelletier, 1986;Ito et al, 2019). Finally, anchor rafting occurs when an existing ice cover entrains sediment ranging in size from silts to boulders (Reimnitz et al, 1987) through direct contact with the seafloor, due to tides or wind-induced ridging (Barnes et al, 1982;Reimnitz et al, 1987;Ledley and Pfirman, 1997;Héquette et al, 1999).…”

“…Although commonly formed in coastal waters, sediment-laden sea ice can be transported great distances if released into the mobile ice pack (Kempema et al, 1989;Pfirman et al, 1990;Dethleff et al, 2000;Darby et al, 2011;Krumpen et al, 2019). Sediment-laden sea ice from coastal flaw leads in the Beaufort and Chukchi seas has been observed over 100-km offshore (Barnes et al, 1982;Kempema et al, 1989;Reimnitz et al, 1993;Ito et al, 2019), while the Transpolar Drift Stream advects sediment-laden sea ice from flaw leads of the Laptev Sea across the North Pole to exit the Arctic Ocean through Fram Strait (Larssen et al, 1987;Wegner et al, 2017) into the Greenland Sea (Pfirman et al, 1990;Darby, 2003). Sediment-laden sea ice may not be evident from optical remote-sensing platforms during winter due to the snow cover, but as the snow and ice surface melt begins in spring, sediment concentrates at the ice surface and becomes evident from optical remote-sensing platforms (Huck et al, 2007).…”

High-resolution simulations of interactions between surface ocean dynamics and frazil ice

“…In general, the effectiveness of convective dissipation, and thus the contribution of convection to the total dissipation, increases with increasing h. Observations show that under typical conditions during strong katabatic wind events, the mixed layer thickness often reaches a few hundreds of meters. Shallow mixed layers are not realistic under those conditions unless, of course, the total water depth is small, as in some coastal polynyas in the Arctic (e.g., Dethleff, 2005;Ito et al, 2019).…”