Observation and modeling of platelet ice fabric in McMurdo Sound, Antarctica

Abstract

[1] During the annual growth of landfast ice in McMurdo Sound, Antarctica, an episodic flux of platelet ice crystals from the ocean contributes to the build up of a porous subice platelet layer, which is steadily incorporated into the sea ice cover as it thickens over winter. In November 2007, we examined the spatial variability of these processes by collecting sea ice cores, with simultaneous oceanographic observations, along an east-west transect in the sound. Previously identified draped and bladed platelet ice types were observed. In addition, we identify resumed columnar growth which appears to be a result of geometric selection from the subice platelet layer after the arrival of new platelet crystals from the ocean has ceased. A numerical model of mechanical platelet ice processes is developed that predicts crystal texture and c axis distributions, producing virtual incorporated platelet ice with known growth history. This model demonstrates how a disordered subice platelet layer arises from an initially flat interface and suggests that such a layer is more likely to form later in the growth season. The model also suggests how the grain boundary density in incorporated platelet ice responds to changes in the flux of loose platelet crystals from the ocean. Application of this result to our 2007 platelet ice observations indicates that sea ice in western McMurdo Sound is subject to larger and more persistent platelet fluxes than the ice in the east. This is consistent with the pattern of in situ supercooling just beneath the ocean surface.

Observations of turbulence beneath sea ice in southern McMurdo Sound, Antarctica

“…If the large scales of turbulence dominate over smaller scales then it may promote rapid growth of even larger frazil crystals Dempsey et al, 2009) by sweeping supercooled water, originally at the base of the ice shelf-affected fluid layer, closer to the surface and so increasing the thermal deficit even more. However, this would require some form of covariance measurement in order to identify these upwards bursts of newly-supercooled water.…”

“…Improved flux and exchange estimates will ultimately enhance our understanding of the way the McMurdo and Ross Ice Shelf Cavities interact with the wider Ross Sea and the implications this has for sea ice growth, ventilation of the sub-ice shelf cavity and the impact on ice shelf basal melt. But this large scale picture also requires work at the frazil crystal scale to understand how turbulence influences the actual growth, downward resuspension and aggregation processes of this ice, which forms a significant fraction of the sea ice cover in McMurdo Sound (Dempsey et al, 2009;Leonard et al, 2006;McGuiness et al, 2009). …”

The sub-ice platelet layer and its influence on freeboard to thickness conversion of Antarctic sea ice

“…Of further interest, it is well known that the outflow of supercooled water from the ice shelf cavity creates an additional heat sink to the ocean promoting sea ice growth (Trodahl et al, 2000;Hellmer, 2004;Purdie et al, 2006;Gough et al, 2012), which increases sea ice thickness in close proximity to ice shelves (Hellmer, 2004;Hughes et al, 2014). This additional ice that forms as a direct result of oceanic heat flux driven by the availability of supercooled water can be split into three components: platelet (or frazil) crystals suspended in the water column, an unconsolidated porous layer of sub-ice platelets directly beneath the sea ice and a layer of consolidated platelet ice incorporated into the sea ice (Dempsey et al, 2010). The subice platelet layer, which does not contribute to the mechanical integrity of the sea ice cover, and has a very different density than consolidated ice, creates an additional source of buoyancy, resulting in an increase in sea ice freeboard.…”

Flow and mixing near a glacier tongue: a pilot study

“…Smith et al, 2001). This phenomenon is readily observed in the region (Dempsey et al, 2010) including during the very first of such measurements by Edward Nelson in 1911 during Scott's second expedition (Deacon, 1975).…”