04 March 2001 - Rothera and Boldly Going to new Places
RRS James Clark Ross Diary
Noon Position : 66° 37' South, 70° 53' West.
Distance travelled since Grimsby: 22,832 nautical miles
Air temperature @ noon: 2.6 degrees Celsius
Sea temperature @ noon: 3.4 degrees Celsius
On Monday we arrived in Rothera to disembark Kjetil the Kongsberg-Simrad engineer after he had completed his work on board. He was due to catch our Dash-7 aircraft the following day to Stanley. And seeing as we were in Rothera it was a good opportunity to take some cargo in and take some waste out. It was a fair exchange, we gave them three containers, a sewage tank and some roof panels and in return we received 13 containers, three old trailers and two skips full of waste. Ultimately this meant that our clear aft deck was no more. Well it was too good to last !
We
always show the usual photos of Rothera but on this occasion we thought
we would show what may possibly be the most southerly tea shack in the world.
As
we were due to be sailing into uncharted waters we also took the
opportunity to test the echo sounder in the workboat. Should we have cause to
use it we decided it would be wise to try it beforehand. Here we have Norman
Thomas, our Electrical Officer undertaking his dual role as Chief in Charge of
Echosounder. For all you salty seadogs out there the sounder is an old Kelvin
Hughes - Old but reliable
So there we had Rothera, in and out in 6 hours, as the call was in the middle of a science cruise. Next stop Wordie Ice Shelf. Read on.........
THE SCIENCY BIT IN THE MIDDLE by Dr Carol Pudsey (BAS)
Ice streams on the Antarctic Continental Shelf.
Some 20,000 years ago the Antarctic continent was even more heavily
glaciated than it is today. Ice sheets not only covered the present land
surface, but extended tens to hundreds of km out across the continental shelf.
During past glacial periods, the shelf was eroded by these great ice sheets so
that it now lies some 400-500 m below sea level. By contrast, the continental
shelf off the UK is mostly less than 100 m deep. Rock debris eroded from the
continent was transported offshore at the base of the ice sheet. Most of it
ended up beyond the continental slope, but some remained on the shelf as a
deforming layer of till (see fig 1).
As the last glacial period ended, ice sheets melted in the northern and southern hemispheres and global sea level rose. Some 8-11,000 years ago the Antarctic ice sheets lifted off the continental shelf to become floating ice shelves, which in due course fragmented into icebergs until the ice margin had retreated to the present-day coastline. Large ice shelves remain in the Ross Sea and Weddell Sea. At present, glaciomarine sediment is accumulating on the continental shelf (fig. 2). Clues to its cold-water origin include the remains of organisms that live only in polar waters, and isolated pebbles which melted out of drifting icebergs. This layer of sediment, a few metres thick, is draped over the topography left by the ice sheet.
Ice flow out from the continent was not uniform, but was concentrated into ice streams. Modern-day examples can be seen in satellite images of the Ross Sea and Weddell Sea. The position of ice streams was controlled by geological structure and by the size of glacial drainage basins. On cruise JR59 we are studying a former ice stream west of the Antarctic Peninsula. Ice flowed north from glaciers on Alexander Island and the adjacent Antarctic Peninsula, west from Neny Fjord and southwest from Laubeuf Fjord, to join one large ice stream. This flowed north, deepening the shelf to form Marguerite Trough (fig. 3).
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Using the new swath bathymetry system installed on the ship last summer's refit (see diaries for 30 May 2000 to 2 July 2000) we have obtained beautiful images of the seabed, revealing detailed evidence for glacial erosion and transport. The ship sailed to within 1 km of the Wordie Ice Shelf and the end of Hampton Glacier. See picture right above.
We can clearly distinguish (A) regions of convergent flow in the E and SE, (B) uniform northward flow in a deep trough at about 68°S, (C) an area of the outer continental shelf which still shows northward flow but where the ice sheet appears to have been much thinner and less erosive. (Figs A, B and C).
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Figure A. |
Figure B. |
Figure C. |
Click on images to enlarge. | ||
We will complete the survey by looking at an area just to the west of the main trough (i.e. compare the ice stream with the slower-flowing ice to one side). This will be the first high-resolution map of an Antarctic shelf from glacial source to the shelf edge. Next year we plan to return with coring equipment to sample the shelf sediments. The sedimentary structures, ice-rafted pebbles and organic remains within the sediments will all provide clues to the glacial history of this area over the last 20,000 years.
NAVIGATING IN THE WHITE BITS ON THE CHARTS - The Driver's eye view
Normally when navigating the oceans of the world a mariner can look at the soundings marked on charts and know that these are the highest points (shallows) that they can expect to encounter. This is all fine and good in a well-surveyed area, such as around the British Isles. Down here however, things are a little different due to the lack of surveys carried out. For example the chart we are navigating with was surveyed in 1948 with occasional amendments since, and that is the only part which has actually been surveyed. As you can imagine there is a lot of erroneous data around, as finding your position before the advent of GPS could sometimes be a little hit and miss. So we have proper white bits where the actual depth of the water is unknown. The whole Antarctic Peninsula is mountainous so it stands to reason that the seabed will be of the same nature. That is, lots of very sharp ups and downs.
The chart below is one used to collect new soundings as the ship penetrates new waters. Hopefully you can see the ships track moving from top centre down to the area marked as Wordie Ice Shelf. This ice shelf covered the area of the end of the ships track as little as thirty years ago. You can also see large areas of white, hence no soundings.
The
collector chart showing the new soundings. As we are driving in such a precipitous area we have several modern tools
to help us.
Summary - Uncharted blank areas on charts are filled in using the tools below :
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Picture 1. This screen shows the extent of the SWATH coverage of the seabed. We have to maintain an overlap so the images can be digitally stitched together successfully. |
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Picture 2. Our Hydrographic echo sounder good to depths of 10 km shows us what is going on directly below the ship. This is recorded and will make its way back to the Hydrographic Office for inclusion on Admiralty Charts. |
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Picture 3. Our 360 degree fish finder can be angled so that it hits the seabed ahead of the ship. In 300 meters of water you can see the depth of the seabed at 300 meters distant. i.e.. Enough time to stop the ship !!! A most useful tool. |
And should any of these tools fail and we get into a real sticky situation we still have the workboat with its echosounder to go ahead of the vessel and pick a safe passage through.
SNOW NEWT AND FINISHING SHOT
We just thought we had better prove that those of you back in the UK are not the only ones with a bit of snow.
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Two Davids at play !! Left we have David Gooberman (2nd Officer) practising to build snow men for his son and Right we have Dave Cutting (Chief Engineer) helping to put the finishing touches to the snow newt on the aft deck. It was going to be a crocodile, but the designer encountered problems ! |
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Here
we see the sunrise over Adelaide Island some 50 miles distant as
seen on Saturday morning by Pippa the Doc and the 4-8 watch.
