Day 23 Cumberland Bay

Cumberland Bay, South Georgia

Wednesday, 18th April 2012. While the weather was pretty spectacular yesterday in bringing to life the icy splendour of the glacier in Royal Bay, it had a lot more up its sleeve when we got to Cumberland Bay at South Georgia. The bay is actually two bays, or more precisely two fjords cutting deep into the island. Most of the clouds that had lingered over the breathtaking mountain scenery at first light had now disappeared. We had arrived just in time for a day of glorious blue sky and sunshine. At the end of the fjord we were treated again to a dazzling display of sunlight bouncing off a gleaming white glacier tongue. Deeply fissured columns of ice lit up in all shades of aquamarine, like a giant jewel reaching out to sea. I would have loved to see the glacier calve that day, but nothing of the sort happened. It was only the light that changed constantly.

Glacier meets ocean

Throughout the day we conducted a CTD survey of the waters inside the fjord and at its mouth (measuring Conductivity/Temperature/Ddepth to calculate temperature/salinity/density of water). The depth was reasonably shallow, between 130 and 260m, so each cast was done in fairly rapid progression. Down, up, move a bit, down, up, down, move a bit and so on. The ship's path prescribed a transect from the start of glacier out to sea for the first part and then a cross-section over the mouth of the fjord in the second part.

View towards King Edward Point

CTD deployment

The measurements of temperature and salinity revealed some interesting, if not unexpected, features. At the surface of the water is a distinct layer of about 10m thickness which is noticably fresher (less saline) than the deeper waters. This brought about some interesting problems. As the ship was moving the fresh surface layer could be seen on the underway salinity sampler. As it's mounted on the bow of the ship it measures the first bit of new water that the ship pierces into when it's steaming along. But every time as we came to a stop, the salinity rose again to that of normal sea water. What was going on? Well, the ship has powerful propellers which when on reverse thrust produce huge whirls of turbulence that act as brakes and maneuver it into an exact position. But the thrusters were also destroying the surface layer of fresher glacial outflow water. As we came to a stop, the special surface layer was already nicely blended into the deeper ocean water, and the original salinity signature was gone in the exact patch of water around the ship where we were about to do our measurements. As a result, the captain adopted a much gentler approach to stopping the ship at the designated station. We approach the station slower and the reverse trhuster were engaged earlier and then turned off just before the target position. The ship then gently drifted into a patch of undisturbed water where we could get some excellent measurements of the waters within the fjord.

Fred at the CTD control terminal

During the day I had the chance to take the helm at the CTD station on the science deck and look after the instruments in the CTD enclosure on the work deck. On the control terminal I could see the data streaming in via the cable in real time. I could observe the different layers of water in the fjord as the probe descended. The actual winch is operated by experienced crew, but the CTD "driver" checks the depth and gauges the distance to the sea bed to give precise instructions to the winch operator. It's the aim on each cast to send the CTD to within 10m of the bottom. While the winch operator only knows how much cable has been paid out, the CTD operator can see the output from an echo-sounder mounted on the the CTD. This echo-sounder, called the altimeter, measures how far the bottom of the CTD frame is off the bottom, so the winch can be stopped in time. It is considered pretty bad form to bring up a broken and dented CTD that is covered in mud.


When I wasn't at the CTD control terminal, I was on the work deck and checked the instruments every time they cam back on board. The currents meters mounted on the frame don't have their own data cable going back to the ship, so it must be plugged back into the ship-board powersupply every time it cames back up. So the time between station gives time for recharging but also for downloading and archiving of the data from the current meter. Once processed this data can reveal how fast the different water masses are moving - useful information to reveal the dynamics of the flows within the fjord.

After the CTD transect there was still some time for taking more sediment cores before the weather moved in. I had a great day admiring the scenery and they say that good things comes to those who wait, but good things also don't last long. As the sun was setting, the wind had already whipped up to a tidy 40 knots. The previously calm waters started to boil, big waves with white caps rolled in, the clouds returned and before long the magic scenery of Cumberland Bay was hidden once again behind fog and sideways rain showers. But it was good while
it lasted!