Today we say goodbye to Cavity Camp on the Thwaites Glacier and move onto the Dotson Ice Shelf. We are very interested in this particular area, because several ‘ice rises’ have here become ungrounded in the last decade. Ice rises are high points in the ocean floor (like underwater mountains) on which the ice can rest. The additional drag from resting on these pinning points provides additional stability to the feeder glaciers (good). However, our satellite measurements show that all ice rises on the Dotson Ice Shelf become smaller and some have even detached already (bad). This is evident in the map, where grounding line rings from the early 1990s (blue) have become smaller over the millennium (yellow lines) and almost disappeared in 2014 (red). So why have they detached?
The reason is most likely, that the ice has thinned so much from basal melting, that it is not thick enough anymore to rest on the tops of the underwater mountains. Our measurements will (a) show that this is the explanation for the disappearance of pinning points, and (b) highlight areas of rapid basal melt. So how do we measure basal melt in the field from the surface of the ice shelf?
With an Auto-phase sensitive radio echo sounder (or short, the ApRES). This state-of-the-art instrument has been developed by the British Antarctic Survey and is our Starship Enterprise in the fleet of glaciological instruments. What makes it so special? The instrument can measure ice thickness at its location to millimeter accuracy. This means that if we take a measurement and return after one week, we can measure the change in ice thickness at this location. But here is an example:
Let’s say we take a measurement today (light blue curve in the graph) and the depth of the ice base below the surface is 650.42 m. We then return after one week and take the measurement again on exactly the same location and it is 650.0 m (yellow curve). This means that 42 centimeters will have melted away in 7 days at this location – this corresponds to a rate of 25 m (82 ft) per year ! Holy penguin.
But the scientist in you says that 7 days isn’t long enough to represent an entire year, right? For this reason we first measure as many of these points as possible (light blue circles in the map) and then repeat them after 7 days. We can now tell the areas with high from the areas with low basal melt. We then modify the ApRES from attended mode to unattended mode and leave the instrument sitting in the field – continuously transmitting its measurements via satellite link to my warm office in Oregon. With this time series we can now tell if our map of basal melt rates from the repeat measurements is representative, or if there are seasonal trends of increased basal melting. I’m particularly interested if the basal melt is connected to ocean tides which might play an important role in this crucial area.