Deriving Melt Rates at a Complex Ice Shelf Base Using In Situ Radar: Application to Totten Ice Shelf

Irena Vaňková,Benjamin K Galton‐Fenzi,J Paul Winberry,Keith W Nicholls,Sue Cook

doi:10.1029/2021gl092692

Abstract

AbstractA phase‐sensitive radar (ApRES) was deployed on Totten Ice Shelf to provide the first in situ basal melt estimate at this dynamic East Antarctic ice shelf. Observations of internal ice dynamics at tidal time scales showed that early arrivals from off‐nadir reflectors obscure the true depth of the ice shelf base. Using the observed tidal deformation, the true base was found to lie at 1,910–1,950‐m depth, at 350–400 m greater range than the first reflection from an ice‐ocean interface. The robustness of the basal melt rate estimate was increased by using multiple basal reflections over the radar footprint, yielding a melt rate of 22 ± 2.1 m a−1. The ApRES estimate is over 40% lower than the three existing satellite estimates covering Totten Ice Shelf. This difference in basal melt is dynamically significant and highlights the need for independent melt rate estimates using complementary instrumentation and techniques that rely on different sets of assumptions.

Highlights

Accurate knowledge of the rate at which ice shelves melt at their base is essential for quantifying the immediate effect of ocean water properties on ice-shelf thickness, a dynamically important quantity for the ice sheet as a whole
We developed a method that accounts for basal geometry complexities and derived a melt rate estimate of ∼22 m per year, which is lower than previous estimates, but it confirms that the basal melt rate Totten Ice Shelf experiences is unusually high for East Antarctica
Under the assumption of incompressibility, when ice is deformed in the vertical, a compensating horizontal deformation takes place in order to conserve volume

Summary

Introduction

Accurate knowledge of the rate at which ice shelves melt at their base is essential for quantifying the immediate effect of ocean water properties on ice-shelf thickness, a dynamically important quantity for the ice sheet as a whole. The basal melt rate derivation typically assumes a flat ice shelf base This assumption is likely to fail in the proximity of grounding lines of thick and fast flowing glaciers, where large basal slopes and basal crevassing are expected to occur. Time series of internal reflector vertical displacements were constructed by cross-correlating the complex signal for each pair of consecutive time shots (Stewart, 2018; Vaňková et al, 2020) These time series were used to obtain time-mean vertical velocity of the internal reflectors, which are typically used to estimate the vertical strain rate profile needed for a basal melt rate estimate (Nicholls et al, 2015). The recording was made using a 24 channel Geometrics Geode data logger at 4 kHz sampling rate with 10-m geophone spacing

Results

Melt and Strain From Off-Nadir Reflections

Basal Geometry From Tides

Basal Melt Rate

Implications