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Earth Institute News

posted 03/24/03

Contact: Mary Tobin
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Reports From the Field

The Antarctic AnSlope Expedition
February 25 through April 11, 2003

AnSlope Cruise 1, Week 4
Monday, March 24, 2003, from Arnold L. Gordon, Chief Scientist
Aboard the R/V NATHANIEL B. PALMER, Ross Sea, Antarctica...

Recovery of a mooring. The yellow balls are floats that bring the mooring line with its instruments to the sea surface after the mooring is released from the anchor. Care is taken to pop the mooring up when there is a clearing in the ice cover.

For the last week we have been fighting the sea ice, working our way west where we earlier set out a number of moorings with instruments to measure the changing ocean temperature, salinity, and ocean currents over a one-year period. Our path westward crossed the continental margin of the Ross Sea many times, where we investigated how different parts of the Ross Sea shelf, with its varied banks and troughs, affect exchange of waters with the adjacent deep ocean. We have found that there is quite a bit of spatial (or temporal) variability, though not as impressive as the export of saline shelf water observed to the west.

Early Sunday morning on 23 March, we were at the mooring site of Central E [71° 57.090'S; 173° 12.757' E]. Central E was deployed on 3 March. During deployment the mooring line was snagged on an ice floe, which when finally released, landed the mooring on the steeply sloped sea floor as intended, but a bit too shallow. The top of the mooring ended up at 104 m depth, rather than near 300 m, making the mooring vulnerable to destruction by passing icebergs. Icebergs have drafts of 200 to maybe 300 m, and there are many icebergs in the region. There is a risk in the recovery and redeployment; both will have to be carried out in nearly full cover of sea ice. Recovery requires a hole in the ice cover so that the mooring can surface once the release mechanism is triggered.  Risks have to be weighted: the risk of leaving the mooring be, exposed to icebergs, against the risk of recovery and redeployment. Central E being a tall mooring, about 1300 m long, carrying the most instruments of all of the AnSlope moorings, requires a substantial clearing of sea ice to be safely recovered. In the end we decided the iceberg risk was to great to ignore, something had to be done. During the wee hours of Sunday morning we streamed towards Central E, through heavy ice cover. Hopes were not great that a break in the ice cover of just the right size would be found at just the right place over the mooring, but against odds, it was, and Central E mooring was recovered!

The second mooring experiencing launch problems that could have damaged instruments is Central B [72° 03.849'S; 173° 06.176' E].  It also was not quite ideally positioned relative to Central C. It has a shorter mooring line, 300 m, meaning a smaller clearing is needed, but none-the-less represents a challenge. Unfortunately a nearby hole in the ice was not very large, and though the ship substantially enlarged it, and we timed the hole drift so it would be over the mooring upon release, the mooring surfaced under the ice. The search began about 1:30 PM on Sunday through range finding and ice crunching. By 7:00 PM Sunday, the detective work was successful and the mooring was recovered, albeit in three pieces. The pieces will be inspected on Monday, and the data of the moorings downloaded.

Redeployment of Central E and B is anticipated for tomorrow, after the data is downloaded, the instruments refurbished, and new mooring wire prepared (the used wire is too jumbled for re-use). An added benefit of the correction of the mooring depth is that we now have our first glimpse of AnSlope time series data from the moorings, albeit only three weeks worth.

We will remain in the mooring area for a bit over a week.  Our objective is to rotate the two moorings (and perhaps a bottom moored pressure gauge) and obtain, by lowering CTD/acoustical doppler current profiler (CTD/LADCP) instrumentation, a snapshot view of the temperature and salinity stratification and currents from the sea surface to the sea floor.  Around 31 March we begin our travel to New Zealand with a series of CTD/LADCP stations along what we think is the export pathway of the saline bottom waters formed in the western Ross Sea.

A bit about life aboard the Palmer:  Food on the ship is very good, and plentiful.  There is a selection of deserts available all day.  There is lots of 'talk' of going to the ship gym. There is also talk about how the clothes dryers are running too hot, shrinking our clothes (accounting for their tightness). People are getting along very well, lots of good humor, though we are getting rather tired of the ice. The mess hall is close to the ship's bow, at the water line, so as we crunch through the ice the noise stifles conversation. As with most US flag research ships, the Palmer is dry, no alcohol. Lots of talk about wine, and its health bearing qualities (the 'medicinal' purpose approach -- its not working). We receive and send electronic mail three times a day, via satellite. In addition to the contact with the folks back home, we received the news.  We all eagerly await updates during this most stressful time.

With the snow-capped peaks near Cape Adare visible (on Sunday) to our west, and air temperature hovering near -20C, I sign off until next week.

Arnold L. Gordon