How quickly the world’s ice sheets melt is influenced by the shape of the seafloor below them.
Beneath the Borchgrevink and Roi Baudouin ice shelves in East Antarctica could lie the answers to how accurately – or mistakenly – we predict future ice melt and all the dangers it presents.
The melting of Antarctica’s ice sheets is a recognised side effect of global warming and a disturbing threat to the world’s climate stability. Rising sea could inflict an immeasurable number of disasters on future generations.
Less well understood is the role of floating ice shelves that stand sentinel at the ocean’s edge, stabilising the ice sheets’ flow off the continent. How effectively they do that depends on many factors, in particular the balance between ice gain from snowfall, and loss from melting at the base of the shelves.
The interplay of these processes is complex. For example, basal melting strongly depends on the flow of warm ocean water into the cavity between the ice shelf and the seafloor below. That in turn is influenced by the shape of the seabed.
Unfortunately, while knowledge of the bathymetry beneath the ice shelves is crucial to accurately predicting melt rates, data is often sparse, largely thanks to the logistical difficulties involved.
This paper details how an independent team of geoscience and marine researchers set out to find a better, cost-effective way to generate more reliable estimates of the sub-ice bathymetry, using an inversion of airborne gravity data, plus Oasis montaj, to create 3D models of the bathymetry. (Gravity data reflects the subsurface mass distribution related to density contrasts between bodies of ice, water, and bedrock, and can therefore be used to model the shapes of their interfaces.)
The workflow includes:
- Filtering the gravity data to produce a gravity anomaly grid whose mid-wavelength variability resembles that of mid-wavelength bathymetric patterns from existing depth data.
- Tackling error estimation and anomalies that creep into the data from other source body configurations, such as continental crusting and denser mantle rocks.
- Deriving bathymetric models with cross sections showing the troughs across the sea floor to divine what impact they might have on basal melting.
Read the academic paper to see the full set of charts and conclusions, and what the team’s key discovery – that the volumes of cavities beneath Borchgrevink and Roi Baudouin’s ice shelves had been under-estimated by around 100% – could imply for the region’s ice shelf retreat.
Example extract:
“Gravity data was acquired with the ZLS Ultrasys modified LaCoste & Romberg Air/Sea gravimeter (S/N56) mounted on a gimbal-stabilised platform close to the airplanes’ centres of gravity, at a sampling rate of 1 Hz. The navigation data, necessary for deriving gravity anomalies from the accelerations recorded by the gravimeter, was processed using precise point positioning. The ice-penetrating radar we used operates with a 150-MHz signal generated by a synthesizer with burst durations of 60 or 600 ns and uses two short backfire antennas, one mounted underneath each aircraft wing.”
Figure 6 (left) Bathymetric model with cross-sections. The overview depicts four 2D profiles (AA, BB, CC, and DD) showing cross-sections of the bathymetric model with its inherent resolution of 5 km. These 2-D sections share the same scale and vertical exaggeration of 20:1. Bedrock in the recent topographic compilation Bed Machine Antarctica by Morlighem (2020) is marked with yellow dashed lines. Profiles AA and BB show the shallowest point of the gateway leading into the RBIS. This point and one other are marked red in the overview and 2D sections for orientation. BB also shows the course of the trough with a plateau close to the grounding line. Cross-section CC exhibits the central BIS overhanging the continental shelf by approximately 50 km and traverses a bathymetric high. The section DD displays the two distinct gateways neighbouring this bathymetric high.
Illustration:
