Yearly evolution of Basal Terraces in the cold-cavity of Ekström Ice Shelf in East Antarctica

Basal terraces occur at the base of Antarctic ice shelves. They are characterized by near-vertical walls, often several tens of meters high, which are interconnected by planar, quasi-horizontal, smooth interfaces. Basal terraces have been observed on numerous warm-cavity ice shelves, particularly close to the grounding zone. Their formation has been linked to preferential, ocean-induced horizontal melting at the vertical walls and subdued melting at the horizontal interfaces. Often they are identified as basal melting hot-spot with melt rates much higher than the ice-shelf wide average. However, direct confirmation of these processes on seasonal to yearly timescales do not yet exist.

Here, we present a comprehensive ground-based radar dataset that images the three-dimensional geometry of a basal-terrace field near the grounding zone of the cold-cavity Ekström Ice Shelf. The dataset consists of two time slices spaced one year apart and is analyzed in an Eulerian framework. The radar data are complemented by continuously measuring ApRES thickness measurements, which are integrated into the 3D geometry.

We find that basal melt rates at the horizontal ice face in the nadir direction are approximately one order of magnitude smaller than melt rates inferred from off-nadir reflections, which originate from a nearby inclined interface. All melt rates are with a max of several meters per year small compared to other studies. There is little subseasonal to seasonal variability. Apart from overal thinning, virtually no discernible changes in the 3D geometry are observed over the annual timescale. In airborne radar data, basal terraces occur preferentially near the grounding zone and disappear further seaward.

Taken together, our data support findings from previous studies that ocean-induced melt rates vary significantly over sub-kilometer distances. However, our results also suggest that basal terraces can enter a dormant mode in which they passively advect seaward and maintain a stable geometry without the need for persistently high basal melt rates.