Platelet Ice under Arctic Pack Ice in Winter

Abstract

The formation of platelet ice is well known to occur under Antarctic sea ice, where subice platelet layers form from supercooled ice shelf water. In the Arctic, however, platelet ice formation has not been extensively observed, and its formation and morphology currently remain enigmatic. Here, we present the first comprehensive, long-term in situ observations of a decimeter thick subice platelet layer under free-drifting pack ice of the Central Arctic in winter. Observations carried out with a remotely operated underwater vehicle (ROV) during the midwinter leg of the MOSAiC drift expedition provide clear evidence of the growth of platelet ice layers from supercooled water present in the ocean mixed layer. This platelet formation takes place under all ice types present during the surveys. Oceanographic data from autonomous observing platforms lead us to the conclusion that platelet ice formation is a widespread but yet overlooked feature of Arctic winter sea ice growth. Plain Language Summary Platelet ice is a particular type of ice that consists of decimeter sized thin ice plates that grow and collect on the underside of sea ice. It is most often related to Antarctic ice shelves and forms from supercooled water with a temperature below the local freezing point. Here we present the first comprehensive observation of platelet ice formation in freely drifting pack ice in the Arctic in winter during the international drift expedition MOSAiC. We investigate its occurrence under the ice with a remotely controlled underice diving robot. Measurements of water temperature from automatic measurement devices distributed around the central MOSAiC ice floe show that supercooled water and thus platelet ice occur widely in the winter Arctic. This way of ice formation in the Arctic has been overlooked during the last century, as direct observations under winter sea ice were not available and contrary to typical Antarctic observations; manifestation of platelet ice in Arctic ice core stratigraphy has been more challenging to identify.

Methane pumping by rapidly refreezing lead ice in the ice-covered Arctic Ocean

“…In particular, we suggest that this process is important in the beginning of sea ice formation in leads before the time of complete brine entrapment, where brine is basically seawater of salinity 32 PSU in contact with the very cold atmosphere. Supercooling was also observed in seawater underneath Arctic winter sea ice (Katlein et al, 2020). The observed water temperatures of only 0.01-0.02 K below the respective seawater freezing point (Katlein et al, 2020) probably represent a final residual supercooled state at the end of ice floe formation.…”

“…Supercooling was also observed in seawater underneath Arctic winter sea ice (Katlein et al, 2020). The observed water temperatures of only 0.01-0.02 K below the respective seawater freezing point (Katlein et al, 2020) probably represent a final residual supercooled state at the end of ice floe formation.…”

Sea-Ice Permittivity Derived From GNSS Reflection Profiles: Results of the MOSAiC Expedition

“…Winter temperature might be slightly lower with negligible effect on . For sea ice two different types are considered: first-year ice (3) and multiyear ice (4). The corresponding ice parameter are: salinity of 4.0 psu for FY, 0.5 psu for MY and temperature as specified in the table.…”

Turbulent Mixing During Late Summer in the Ice–Ocean Boundary Layer in the Central Arctic Ocean: Results From the MOSAiC Expedition

“…Horizontal distribution of sea ice draft, including the depth of sea ice keels, was obtained from a multi‐beam sonar installed on a remotely operated vehicle (ROV; Katlein et al., 2020; 2022) (Figure 1b). During MOSAiC Leg 5, the ROV was operated within a distance of ∼200 m through a hydrohole, located at x = 50 m and y = −110 m, toward the ship.…”