by Dr Craig Stevens, Physical Oceanographer at the National Institute of Water and Atmospheric Research (NIWA)
We completed our first helicopter survey of the field location this morning. It was around a 30 minute flight south of the Station.
In that time we stopped off for some brief tourism at Inexpressible Island where a number of Scott’s team were forced to spend the winter of 1912 in a snow cave surviving on seals before walking back to Cape Evans very early in the spring. This brought home the changes of the last century as we had barely finished our breakfast that would have been a week’s nutrients for those fellows – and quite a bit tastier than freeze-dried seal.
I must admit to some (much!) trepidation before the flight. There are competing needs here — we need ice that’s in a useful location, not too thick, not too thin and not all broken up and melting. The only ice likely to fit this bill is held in place by the seaward extension of the massive David Glacier — the Drygalski Ice Tongue. This icy behemoth some 80 km long, 15 km wide, 200 m thick and travelling at a couple of km a year is the last giant floating glacier left — for the time being.
If features like this or its close relative the ice shelf, break up and allow increased flow of ice off the polar ice cap then sea level will likely increase rapidly. So fast might this increase be that it will not only become detectable in a human lifetime, it may actually dominate the activities of our species for centuries to come.
Erich von Drygalski led a substantial German Antarctic voyage in the first years of the 1900s. They worked over in the regions south of the Indian Ocean and Australia. They intentionally froze their ship, The Gauss, into the sea ice and wintered successfully. Unlike in the arctic, where the ice breaks up and refreezes in a constant grinding jumble of floes, they found the sea ice around their ship remained in one contiguous piece which would have been some comfort as they had to deal with some appalling weather. This sea ice condition sounds similar to the piece of ice we landed on south of the Ice Tongue — nice and flat.
My viewing of the satellite imagery suggested there was quite a bit of fast ice real estate just south of the Drygalski but this gave little indication of its thickness or condition. We flew about 5 km south of the southern edge of the massive ice tongue. Upon landing we shovelled off the small layer of crusty snow and started drilling using a kovacs. This thin drill is simply designed to measure ice thickness.
I couldn’t watch as they drilled away — fully expecting them to get to 4 m and still not have broken through — well beyond the capabilities of our lightweight gear. But good news came soon enough when our Italian Navy Seal guide Davide broke through the bottom of the ice at around 2.6 m! This was a little thicker than we’d worked through in the past but possible. We drilled a few more holes in a 10 m vicinity to confirm this thickness, took a GPS location, and departed.
I felt that the ice north of us towards the ice tongue would likely be thicker but we didn’t see any obvious rifting or steps so we flew a kilometre north and drilled again as this might be slightly more be protected from breakout in the coming weeks. Ice breakout can happen in a matter of hours — ocean swell from storms sometimes 1000s of km away literally flexes the sea ice to pieces.
A little surprisingly it turned out to be thinner — just a shade under 2 m, again confirmed with a few repeat holes nearby. This should be well within our drilling capabilities. Essentially, I think the region is made up of a jumble of large sea ice pieces that have broken out and refrozen. We took a wide sweeping departure with the helicopter as we left, showing fresh cracks seaward of our site and clear steps up to much thicker multi-year ice to the north and west. I think we’ve chosen as best we can and we’ll keep a close watch as the days progress.