Electromagnetic Mapping of the Thickness of the Sub-Ice Platelet Layer Under Landfast Sea Sce in McMurdo Sound, Antarctica

Abstract:

The landfast sea ice around the fringes of Antarctica is often characterized by the presence of platelet ice which forms from the outflow of supercooled ice shelf water from the cavities under ice shelves. The spatial distribution and thickness of the unconsolidated sub-ice platelet is therefore an indicator of oceanic processes at the underside of usually inaccessible ice shelves. Here we show that the presence and thickness of the sub-ice platelet layer can be easily mapped by means of ground-based electromagnetic induction (EM) sounding using both the Inphase and Quadrature components of the induced, secondary electromagnetic field. From comparisons between drill-hole and EM measurements our analysis indicates a mean conductivity of the sub-ice platelet layer of around 600 mS/m. Under the assumption of this conductivity, we show that both the thickness of the sub-ice platelet layer as well as the thickness of the consolidated sea ice above can be concurrently inverted, in good agreement with drill-hole measurements. This inversion is based on the evaluation of two independent look-up tables calculated from a forward model. During extensive surveys across McMurdo Sound in 2011 and 2013 we have mapped the regional distribution and thickness of the sub-ice platelet layers, showing a narrow zone of the thickest platelet ice indicative of the main outflow of supercooled Ice Shelf Water in agreement with previous measurements. However these new data are more detailed and show little variation between the two years. The data also demonstrate the spreading and thinning of the platelet layer with increasing distance from the ice shelf front.