Sediment-laden sea ice — so-called dirty sea ice — is often observed in the Arctic Ocean, especially its marginal seas. This sediment has an important role in polar region biogeochemical cycling; for example, iron and other micro-nutrients released during sea-ice melt are known to trigger phytoplankton blooms. Seafloor sediments are believed to be the primary source of particles entrained in sea ice, incorporated via the process of suspension freezing: mm-to-cm sized frazil ice crystals in the water column encase suspended sediments (usually silts or clays), which are subsequently rafted to the surface and consolidated to form the ice pack. Field-based evidence for this process, however, is limited.
Masato Ito from Hokkaido University, Japan, and colleagues use acoustic and optical mooring data from a coastal polynya — an area of open water surrounded by sea ice — located near Utqiaġvik (Barrow), Alaska, to provide observational support for suspension freezing. During episodic wintertime polynya episodes, acoustic Doppler current profiler (ADCP) measurements revealed frazil ice presence to depths of 20–25 m. ADCP and turbidity data further exposed re-suspended bottom sediments at similar depths, dispersed upward when current speeds exceeded ~0.4 ms−1, providing enough energy to re-mobilize fine particles (<40 µm) from the seafloor. This observed spatial and temporal co-occurrence of frazil ice and re-suspended sediment facilitates their interaction and entrainment. Although these results are obtained from one mooring in the Chukchi Sea, the characteristics are typical of Arctic coastal polynyas, lending support for suspension freezing being a key mechanism Arctic-wide.
With increasing Arctic storminess projected under anthropogenic warming, a corresponding increase in turbulence and sediment re-suspension may suggest greater sea ice sediment concentrations in the future, with subsequent biogeochemical and physical feedbacks. However, before future projections can be made, longer-term observational experiments are needed to first verify contemporary processes, including the spatial scales and rates of suspension freezing.
Ito, M. et al. Favorable conditions for suspension freezing in an Arctic coastal polynya. JGR Oceans. https://doi.org/10.1029/2019JC015536 (2019)