Airborne mapping of the sub-ice platelet layer under fast ice in McMurdo Sound, Antarctica

Abstract

Abstract. Basal melting of ice shelves can result in the outflow of supercooled ice shelf water, which can lead to the formation of a sub-ice platelet layer (SIPL) below adjacent sea ice. McMurdo Sound, located in the southern Ross Sea, Antarctica, is well known for the occurrence of a SIPL linked to ice shelf water outflow from under the McMurdo Ice Shelf. Airborne, single frequency, frequency-domain electromagnetic induction (AEM) surveys were performed in November of 2009, 2011, 2013, 2016, and 2017 to map the thickness and spatial distribution of the landfast sea ice and underlying, porous SIPL. We developed a simple method to retrieve the thickness of the consolidated ice and SIPL from the EM inphase and quadrature components, supported by EM forward modeling, and calibrated and validated by drill-hole measurements. Linear regression of EM inphase measurements of apparent SIPL thickness and drill-hole measurements of true SIPL thickness yields a scaling factor of 0.3 to 0.4, and rms error of 0.47 m. EM forward modeling suggests that this corresponds to SIPL conductivities between 900 and 1800 mS/m, with associated SIPL solid fractions between 0.09 and 0.47. The AEM surveys showed the spatial distribution and thickness of the SIPL well, with SIPL thicknesses of up to 8 m near the ice shelf front. They indicate interannual SIPL thickness variability of up to 2 m. In addition, they reveal high-resolution spatial information about the small-scale SIPL thickness variability, and indicate the presence of persistent peaks in SIPL thickness that may be linked to the geometry of the outflow from under the ice shelf.

Winter growth and tidal variability of the sub-ice platelet layer observed with electromagnetic induction soundings

“…We considered how changes in the tidal currents could affect the conductive properties of the SIPL and seawater with respect to the EM observations. The spatial distributions of the thickness of ice shelf-influenced fast ice and the sub-ice platelet layer were additionally assessed in McMurdo Sound in November 2018 with ground-based EM and drill hole surveys, and concurred with a previous four-year dataset (Brett et al, 2020). In high resolution (~0.3-0.4 m sample spacing) small-scale spatial surveys, prominent undulations were observed in the draft of the SIPL with ~30-50 m wavelengths and amplitudes of ~0.3-0.6 m for the first time with EM.…”

“…On the Southern transect, the thickest SY ice and SIPL were observed at ~165.6° E in the main region of supercooled ISW outflow from the MIS cavity. This thicker fast ice and SIPL were previously observed with EM (Brett et al, 2020b;Haas et al, 2020), and in multiple other studies ranging from oceanographic profiling (Hughes et al, 2014;Lewis and Perkin, 1985;Robinson et al, 2014) to satellite altimetry assessments of fast ice freeboard (Brett et al, 2020a;. The SY ice regime likely contributed to the very thick SIPL of 11 m observed in November 2018.…”

“…In November 2018, we assessed the late spring spatial distributions of the thicknesses of fast ice and SIPL (refer to sect. 2.1) with EM and drill hole surveys and re-surveyed the EM transects previously assessed by Brett et al, 2020in November 2011, and 2017 Additionally, a high-resolution gridded EM survey was carried out in the west in November 2018 to assess if the EM instrument could resolve metre-scale features in the SIPL draft (refer to sect. 2.1).…”

“…Thinner or thicker total ice and SIPL result in larger or smaller I and Q responses, respectively, and both responses are non-linear. Importantly, increases in SIPL thickness drive a larger response in I relative to Q (Haas et al, 2020;Irvin, 2018). Over thicker total ice and SIPL, the signalto-noise ratio is lower and the ability to detect changes in thickness is reduced.…”

“…The spatial distributions of ice shelf-influenced fast ice and the SIPL have been well observed in McMurdo Sound using a variety of techniques including ice coring (Dempsey et al, 2010;Gough et al, 2012), drill hole measurements (Brett et al, 2020b;Price et al, 2014), electromagnetic induction (EM) surveying (Brett et al, 2020b;Haas et al, 2020; and satellite altimetry (Brett et al, 2020a;Price et al, 2015;. The development of a supercooled ISW plume over winter, and the effects on fast ice formation in McMurdo Sound have been investigated in previous studies (Hunt et al, 2003;Leonard et al, 2011;Leonard et al, 2006;Mahoney et al, 2011;Gough et al, 2012).…”