Earth Institute News
Contact: Mary Tobin
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Reports From the Field
The Antarctic CORC/ARCHES Expedition
April 5 through May 8, 2003
April 27, 2003
Afternoon, Sunday, April 27, 2003, from Dr. Gerd Krahmann
Aboard the R/V LAURENCE M. GOULD...
We are back at Palmer Station after a 12-day trip into the northern Weddell Sea, our study area. During those twelve days we experienced beautiful days, foggy nights, some stormy weather, long work days and nights, and a huge disappointment.
In report number 3, I reported that we had just finished a test CTD station. What I had not described then was the other part of our work. Out there in Weddell Sea, we have several instruments deployed in so-called moorings. These moorings consist of a big weight (used railroad wheels) as an anchor holding it in place, several large glass balls which provide buoyancy and are able to withstand the enormous pressure at the bottom of the ocean, and several instruments which record temperature, salinity, and the direction and speed of the water passing the instrument. All these components are attached to a steel wire. We typically drop these moorings, let them record data for a year or two and then come back to pick them up again.
As we are mainly interested in the part of the water very close to the ocean floor these moorings cover only the lowest 500m of the water column of 2000 to 4000m. The process of picking them is particularly tricky as there is no way to physically get to them. We instead send an acoustic signal, which sounds somewhat like a bird's song, to a special instrument in the mooring. This instrument, the release, permanently listens for a particular melody. As soon as it hears it, it opens up a lever and drops the mooring weight. The buoyancy of the glass balls then pulls the mooring to the sea surface where we pick it up with the ship.
In our case the three moorings we have in the area, have been there for two to two and a half years. This is very much at the limit of the lifetime of batteries in the releases. We were thus very eager to get to them and recover the instruments with all their invaluable data.
Unfortunately for us, Mother Nature had something different in mind.
When we were approaching the study region, we received satellite images of the ice extent in the area. And they showed nothing good. The southernmost mooring was clearly in a place where the ice cover was nearly total. Even in the case that our ship could have reached the location and we could have triggered the release, the mooring would have likely come up under ice, invisible and non-recoverable for us.
But we still had hopes to reach the two northern moorings, which appeared to be in a region of partial ice cover. When we finally arrived in the general area, we soon recognized that this partial ice cover was at the limits of the ship's possibilities (see image at left). Since we arrived during night hours, we decided to do two CTD stations a little further to the north and attempt to reach the northernmost mooring during daylight hours. Daylight gives the distinct advantage that the ship's captain can see and follow open areas in the ice (called leads). In the morning of April 18, we headed southward for 5 hours, until the increasing thickness and density of the ice forced us to stop, do a final CTD station, and turn around. When we turned around, we were still some 60 nautical miles away from the mooring location.
Since we could not proceed with our plans, we had to come up with some other useful way of using the ship's and our time in the area. We decided to head further north into areas where there would be less ice and do an extensive CTD station survey. The locations of the CTD stations was chosen in a way that we were able to trace the spreading pathway of the Weddell Bottom Water (WBW) north of the South Orkney Islands. At some of the locations other scientists from Lamont-Doherty Earth Observatory did a survey in 1997. We will be able to compare the data and look for changes in temperature, salinity, and oxygen content. We will also be able to illuminate the fate of the WBW once it leaves our original experiment area.
As for the moorings, we are likely to come back next winter and attempt to recover them. Though the release batteries will be beyond their nominal life time, they are known to last several months longer.