SIPEX Update: 28 September – 10 October
We have bid a fond farewell to the sea ice as we have reached the edge of the ice zone and are now in the open ocean heading for Hobart and home, so it is time for a short review of the last couple of weeks. When I last wrote, we were pretty much stationary in an area of heavily deformed ice, waiting for the ice pack to break up a bit and make travelling easier.
Some of the biologists on board had noticed that the ice we were breaking through in that area was very brown on the underside. The brown colouring comes from the algae that live in and on the underside of the ice and are an important part of the sea ice ecosystem. There had been little algae in the sea ice we had sampled so far on this voyage and the biologists were anxious to get more samples, so we had a short ice station there on the morning of the 28th before moving on.
By midafternoon the next day, we had made it out of the worst of the heavy ice and were moving along much more quickly and smoothly toward the northeast through a series of leads.
Some of the biologists who collected their brown ice the previous day were doing experiments in the ship laboratories to determine how the algae in their ice samples respond to different levels of light. This is an important ecological question if climate change results in thinning of Antarctic sea ice, because the algae form the base of the Antarctic marine food web. Sea ice acts as a sunshade for the algae – a piece of ice one metre thick blocks out about 99% of the light that reaches the ice surface (assuming there is no snow cover) – so thinner ice would mean that the algae would receive more light.
On Sunday, we finally found some flat, stable ice and had a calm sunny day as well. Everyone was out on the ice right after breakfast to take advantage of the brief window of opportunity before the storm that was forecast for the following day. The helicopters kept busy deploying another array of drifting buoys to measure ice movement and deformation, as well as doing more laser altimetry and aerial photography. The oceanographers set up their Conductivity, Temperature and Depth (CTD) meter in their continuing effort to learn more about the Antarctic ocean currents that help drive global ocean circulation. After drilling a hole in the ice and attaching the CTD and rope to a large metal tripod, they lowered the CTD slowly to 1,000 metres before bringing it up again.
Surface currents are caused primarily by wind blowing across the water, a process that is shut off over large areas in the Antarctic by winter sea ice cover. Deeper water currents, on the other hand, are driven by differences in water density in different parts of the ocean. The density of the ocean water in a given area is determined by its salinity (salt water is denser than fresh), temperature (cold water is denser than warm), and pressure (which increases with depth).
The CTD calculates the salinity indirectly by measuring electrical conductivity (how easily electricity passes through the water). Electricity passes through salty water more easily than it does through fresh water, so the more saline the water, the higher the conductivity. Pressure is determined by depth. As scuba divers learn, the pressure increases by about 1 atmosphere – that is, equal to the weight or pressure of Earth’s entire atmosphere at sea level – for every 10.4 metres of water depth.
The krill biologists moved in as soon as the oceanography team had finished their CTD measurements. They took advantage of the tripod, rope and ice hole to lower a high-definition video camera to 100 metres to spy on any krill that might be in the area.
We spent the night at this ice station and woke up the next day to a clear bright morning, but it didn’t last long. By 8:30 am, the clouds had come in and the wind started to pick up. Everyone was packed up and off the ice by 10:30 – a good thing because by noon the wind was gusting up to 36 knots and snow was starting to fall. We started moving again, this time to the west, both to get into an area of thinner ice to wait out the storm and to position ourselves to be under the path of the NASA ICESat (a satellite equipped with laser altimeter) that would be passing overhead on Thursday morning.
The following day we were buffeted by 50-60 knot winds and blowing snow all day long in the third and strongest blizzard we had yet experienced. No one was even allowed to go out on the decks, so lots of videos were watched, books read, and equipment cleaned and organised.
You can only keep a group of Antarctic scientists off the ice for so long, however. The following morning the winds had died down to about 30 knots and you could see for approximately 100 metres. We had only travelled about 2 nautical miles from our last ice station, although we had drifted nearly 30 miles with the ice. Being unable to move the ship because of the poor visibility, we decided we might as well have an ice station right where we were. It certainly beat sitting around on the ship all day.
The transect line was limited to 100 metres, instead of the usual 200 metres, because everyone had to stay close enough to the ship to see the flags at the bottom of the brow (the gangplank that we use to get from the ship to the ice). Someone up on the bridge of the ship keeps a constant eye on everyone working on the ice, kind of like a lifeguard at the beach. If we travelled so far away that we couldn’t see the flags, the bridge lookout would also not be able to see us.
Contrary to our hopes and the meteorologists’ expectations, the weather did not improve substantially the following day. The wind gradually died down, but we still had no horizon and no visibility in the morning, so we were unable to make any progress toward our next destination. We started moving shortly after noon, when visibility improved a bit, only to have the snow start again as soon as we did. At least this time the snow was falling vertically instead of horizontally.
The going after that was very slow. The recent storm had left a heavy coating of snow on the ice that made it much more difficult for the ship to break through. The snow absorbed much of the ship’s energy and stuck to the hull, increasing the friction. Movement was in fits and starts – forward a short distance until we were hung up in the snow and ice, then reversing a longer distance to get momentum for the next assault. By Friday October 5th, we had been cocooned inside a giant marshmallow for four days. Nothing around us but white – white snow, white ice, white sky. After hours of persistent bashing against the ice and snow the previous day, while making little progress, we had decided to stop where we were for the night and try again in the morning. We were hoping that conditions would clear enough for the helicopters to check out the ice situation and help find the best route.
The following morning was warm (-9°C with very little wind) but the cloud ceiling was still too low for the helicopters to fly. The biologists couldn’t be restrained, however, so we had a mini-ice station for them to collect some cores while waiting for the clouds to lift. The ice was very blocky and deformed, and covered with up to a metre of snow. That, combined with an icy layer under some of the snow, made for interesting walking (as well as sliding and falling). The heavy snow had created areas of ‘negative freeboard’ in many places, meaning that the ice surface was below sea level and there was a layer of water between the bottom of the snow and the top of the ice.
We made good progress through the ice that night and the next morning (Sunday, October 7) saw a strange colour in the sky – blue! The clouds had finally lifted and the sun was sparkling off the crystals in the snow as we poured off the ship to set up ice station 13. Unfortunately, the sunshine didn’t last long and the clouds were back by noon. Still, the winds were light, the temperature fairly warm and the ice was good, so all of the teams spent the day out gathering more data.
Sunday night was extremely busy and many of the ship’s crew and researchers worked through until dawn. The oceanography team wanted to calibrate their CTD and double-check the measurements they had been getting, so they decided to deploy it together with the ship CTD. The ship CTD is a larger and more complex version that is usually used to gather measurements in the open ocean. It is deployed through a special hatch off the trawl deck and lowered to a depth of 2,000 metres. In addition to measuring conductivity, temperature and depth, it is armed with Niskin bottles – tubes that can be opened and closed at specific depths to gather water samples.
After the CTD manoeuvres, the biologists conducted two Rectangular Midwater Trawls (RMTs) in a lead near the day’s ice station. They had seen krill on the video footage taken by the remotely operated vehicle (ROV) earlier in the day and had lowered a net in the ROV hole to collect samples. They wanted to compare what they saw in the under-ice environment with what might be in the nearby water, so they did two trawls – one that sampled from 0 to 200 metres depth and a second one from 200 to 400 metres. Their findings were surprising – they found a fair number of krill in the shallower trawl, but fewer in the deeper water. In addition, the krill gathered from under the ice had stomachs full of ice algae, while those in the nearby open water had empty stomachs. Observations like this help them understand how different conditions and different habitats might affect the krill population.
We travelled about 20 nautical miles north during the night before arriving at ice station 14 the following morning. Cloudy conditions prevented the helicopters from flying, but everyone else was out on the ice for most of the day taking advantage of the chance to maximise their samples. We spent the night at this ice station, so the oceanography team kept their equipment running all night (clearing newly formed ice out of the holes every 2 hours). The krill team also had another chance to drop their video camera down the CTD hole and see what the krill get up to after dark.
The morning marked the end of our time on the ice. A few of the biologists collected some last minute cores and the ROV went for its final journey beneath the ice. After that, we started packing sleds and other field equipment that would not be needed for the rest of our journey back into the blue container that is usually put out on the ice when we work. The packing process continued over the next three days as we prepared the ship for the waves and swells of the open ocean.
We got one last chance to collect a few ice cores in the smaller ice floes of the marginal ice zone the day before yesterday. We spent the previous night heading south to get away from a fairly large swell that was going through the ice pack and, by morning, were about 60 miles from the northern edge of the sea ice. The swell had died down to the point where it was safe to use the crane and basket to lower three people, a generator and an ice corer onto a small floe. The drillers quickly extracted five ice cores, put them in plastic tubes and were hoisted back onto the ship. The biologists were particularly pleased to find that the bottom part of the cores had a high concentration of sea ice algae.
Now the hard work begins – analysing all the samples, examining the data collected and trying to figure out what it all means.
By Sandy Zicus, Antarctic Climate & Ecosystems Cooperative Research Centre
About SIPEX: The Sea Ice Physics and Ecosystems eXperiment (SIPEX) research is an international project that is jointly organised by the Antarctic Climate & Ecosystems Cooperative Research Centre and the Australian Antarctic Division. It involves 45 researchers from ten different countries and is part of a larger International Polar Year (IPY) project looking at sea ice in the Antarctic. SIPEX is an interdisciplinary program that is examining interactions between sea ice structure, sea ice biology and the ocean food web. It departed Hobart, Tasmania on 4 September and will return on 17 October.
What is IPY
Monday, 15 October 2007 01:33
Exploring sea ice off AntarcticaWritten by ASPECT
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