25 March 2002 - The End of JR71 Science Cruise
RRS James Clark Ross Diary
Position at 1200: 48° 13.9'S, 54° 14.2'W - 245 nautical miles North-east of Stanley on the Western end of the Falkland Escarpment
Distance Travelled since Grimsby: 37388 Nautical Miles
Air temperature: 12.7°C; Sea temperature: 13.4°C
Weather : Cloudy but fine and clear with good visibility and plenty of sunshine, wind 5 increasing 6, Increasing sea and low swell
Current, frequent weather observations reported back to BAS Headquarters in Cambridge is used to plot the ship's current position and recent track. Meteorological data are also available from this page. The callsign of RRS James Clark Ross is ZDLP.
North (a little) at last
We are (Eds note, this was written last week!) now only a few hours out of Stanley and the end of the JR71 science cruise. We finished off the cruise at midday on Saturday and turned North for the run back to the Falklands. Up until midday we had been swathing like crazy to try and get as much done as possible before the time was up. There has to be a cut off time to each cruise to give us time to get back before the start of the next one. This has to take into account things such as bad weather on the way back which can slow us down, on the other hand if we leave too early and the weather is nice then we have lost some valuable science time, this calls for some tricky negotiations between the PSO and the Master until a deal is struck.
On this occasion we struck it lucky with the weather and although it was not very nice, 50+ knot winds and a rather large accompanying swell, it was chasing us from behind and so pushing us towards the Falklands. This was accompanied by the occasional whiteout and heavy snow showers! It is fast getting into the time of year when you do not really want to be down in the Antarctic on a ship.
View from the bridge.....
Above: Three views from the bridge, showing that some days are definitely better than others!! Click on the images for a better view.
The final science bit........Ice sheet dynamics and sedimentation on the NE Antarctic Peninsula continental shelf and slope
By Jeff Evans and Colm Ó Cofaigh
This week’s final science summary compliments the work on palaeo-ice streams on the western Antarctic Peninsula shelf (see Week 24 diary), and builds on last week’s science page by Carol Pudsey. The main objective of this research is to relate sedimentation patterns and processes on the continental shelf and slope around the Antarctic Peninsula to changes in the extent and volume of the Antarctic Peninsula ice sheet over the last 30,000 years. Here, we mainly focus on the northeast side of the Antarctic Peninsula, the Larsen-A region.
We have used a variety of different geophysical techniques ranging from swath bathymetric mapping of the sea floor, to TOPAS sub-bottom profiler records (which gives us a picture of the top few tens of metres below the seabed) to sediment coring of the seabed. Swath bathymetric data indicate that the seafloor in the Larsen-A region is characterised by elongate landforms that were formed beneath an easterly flowing ice sheet that extended to the continental shelf break during the last glacial maxima (around 15,000 years ago) (Image 1). The distribution of these subglacial landform shows that ice sheet flow is more concentrated and faster through bathymetric troughs than across adjacent shallower regions of the continental shelf. The TOPAS records show that subglacial till (sediment deposited under an ice sheet) is thin and discontinuous across the region.
Sediment vibrocores collected by the British Geological Survey (BGS) team contain at their base a consolidated till overlain by a much softer and water-rich subglacially deformed till. Collectively these sediments provide direct evidence for the advance of a grounded ice sheet across the northeast Antarctic Peninsula shelf and marked changes in ice sheet dynamics through time. Coarse-grained sediments overlying these tills indicate that during deglaciation, the grounded ice sheet lifted off the seabed and formed a floating ice shelf. These sediments further suggest that the subsequent retreat of the ice margin towards the present coastline was relatively slow. Provisionally, this contrasts with the deglacial record in Marguerite bay where glacier retreat appears to have been relatively rapid.
Geophysical and geological investigations were also carried out on the continental slope bordering the Larsen-A region. In this area, the continental slope is much gentler compared to the western Antarctic Peninsula slope. Sediments recovered in cores from the slope directly in front of the shelf trough are dominated by coarse-grained debris flow deposits derived from the remobilisation of subglacial sediment fed directly to the slope from the faster flowing part of the former ice sheet. Preliminary investigations suggest that the slope regions away from the shelf trough are more gullied with less downslope remobilisation reflecting lower sediment supply to the slope in response to slower ice sheet flow on the shelf.
Image 1 – Swath map of the outer Larsen-A continental shelf and slope.
The final JR71 photo gallery.......
Clockwise from top left: The Day Shift, (L-R; John, Ian, Graham, Jenny and Colin), The Night Shift, The ship entering the outer harbour at Stanley, a little diatom being studied, a tooth(?!?) and a friendly minke whale. Click on the images to enlarge them
Photos by John Derrick
Thank-yous this week......
To all of the scientists from the last cruise (JR71) who have been with us for six weeks now, have a safe journey home and especially to those who have contributed to the web page with text, images or photos, thankyou.
Coming up next week......
We were in Stanley from last Tuesday morning until Friday morning. During this time we de-mobilised all of the last cruise before sailing for a 40 day swath cruise around the Falklands continental shelf, at the moment we are running swath lines up and down the shelf break on the Falkland escarpment.....more on this next week.