13 March 2002 - Born to Swath!
RRS James Clark Ross Diary
Position at 1200: 65° 05.8'S, 54° 40.3'W - 86 nautical miles SExE from James Ross Island
Distance Travelled since Grimsby: 35657 Nautical Miles
Air temperature: 0.2°C; Sea temperature: 1.0°C
Weather : Overcast but bright; fine and clear; good visibility; wind WNW force 4; slight 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.
Born to swath.....!
Well first apologies for no update last week, this was because we were busy doing the science (described later) Because of the nature of the job (i.e. 24 hr working) and the particular hazards of the job in this part of the world, the deck officers have all been on different back to back shifts for the last couple of weeks. The Master and Chief Officer have been working 12 hr shifts and the Second Officer and myself have been doing 6 hr shifts, this is because there has been (and still is) a lot of ice about. The visibility is quite often poor and then it gets dark, and on top of all that, we are in unsurveyed waters! With two Officers on the bridge, plus a lookout, there are plenty of eyes looking out the window as well as ensuring the safe navigation of the ship, and making sure we are following the science plan as best we can. This does however mean that things like our web page have to take a lower priority.
So picking up where we left off at the last update, we passed around the top of the peninsula and headed out to complete a line of CTD's, Keith Nicholls has written about this for us this week. Then we headed into the Prince Gustav Channel, between James Ross Island and the mainland, to begin the swath survey and coring.
After this we spent some time swathing and coring in the Larsen Bay area before heading East to the edge of the continental shelf for a few days, after running a seismic line on the way back to the coast we eventually shifted around the corner to the Larsen 'B' and 'C'. After a couple more days here we moved back into the Larsen Bay area to tidy up some holes in the swath that had been covered by ice before and finally we headed out to the shelf break again for yet more swath and coring, this is where we are today.....that is the last couple of weeks in a very short nutshell, it sounds easy if you say it quickly!!
Rogues' Gallery: photos by Dave Smith
Click on the images to enlarge them
The science bit........Part 1....Oceanography in the NW Weddell Sea – or, What happened to the bottom water in Drake Passage?
Dr Keith Nicholls and Dr Carol Pudsey
For several years now, RRS James Clark Ross has been involved in monitoring the structure and total transport of water within the Antarctic Circumpolar Current. Each year, during one of the early-season crossings of the Drake Passage from the Falkland Islands, a team has taken a sequence of vertical profiles of conductivity and temperature (CTD profiles) along a line from the southern edge of the Falkland Plateau to the northern tip of the Antarctic Peninsula. Sheldon Bacon and co-workers describe the background to the work and the most recent set of measurements in this weekly diary. They point out that, considering the Polar Front is well to the north this year, there appears to be an unprecedented lack of the cold deep water that usually flows from where it is formed in the Weddell Sea into Drake Passage.
So where's it gone? Is it because the cold deep water of the type that usually escapes from the Weddell Basin into Drake Passage is simply no longer being produced? Or is there some mechanism preventing it from flowing out into Drake Passage? As part of an attempt to address these questions time was made available during cruise JR71 to determine whether the supply of Weddell Sea Deep and Bottom Water remains intact. Eleven CTD stations were occupied along a 230 km line that runs from the continental shelf into the deep Weddell sea (see map). There would have been twelve, except that a very large iceberg stationed within a few hundred metres of our target location had spawned many offspring that were cluttering up the entire area. As well as CTD profiles, water samples were taken from several depths on each profile. The samples will be analysed in the UK to provide clues to the mechanism that produced the various water masses.
The JR71 CTD section was positioned so that it should cut across the flow of newly formed deep and bottom water as it travels northward along the Antarctic Peninsula. The temperature data have been plotted for the lower 400 m of the water column, with the Antarctic Peninsula to the left (see contour plot). This is a distorted picture, as the sea floor in reality drops steeply away down the continental slope, but it clearly shows that the plume of cold water remains as healthy as ever. The –0.7°C contour is highlighted, as this represents the boundary between the Weddell Sea Deep Water (above) and the colder Weddell Sea Bottom Water (below).
So what does it mean? There are several possibilities that need to be investigated. It has certainly been a very strange year as far as conditions in the Weddell Sea are concerned. The unprecedented lack of sea ice in the north of the basin (very good for JR71), and the very heavy pack ice to the south (very bad for re-supplying Halley Base on the Brunt Ice Shelf!) might suggest that there is some new mechanism coming into play to prevent the water from flowing out into Drake Passage. But it's likely that we will need to look more closely at what's been happening in the Weddell Sea over the last year or so before we will unravel the puzzle. Very little newly formed cold deep water was seen flowing north in the Weddell Sea when the same CTD section was occupied during a Brazilian cruise in 2000. Perhaps we are seeing in the Drake Passage this year the downstream result of that depletion. If so, we would expect all to be returned to normal during next season's transect of the Drake Passage. We will have to wait and see.
The science bit......Part 2..... Tracing the pathways of ice on the continental shelf
By Carol Pudsey, Principal Scientist
Although the Weddell Sea pack ice has receded unusually far south this year, the sea where we are is still full of drifting icebergs of all sizes, from a few tens of metres across up to giant bergs 70 x 40 km. Many of them contain rock fragments which will fall to the seabed when the ice melts. Thirty years ago, floating ice shelves extended a considerable distance from the east coast of the Antarctic Peninsula, covering many of the areas where we have been surveying. They have receded during the 100-year period of historical observations. We found from the cores we collected on cruise JR48 that the ice shelves receded in a similar way several thousand years ago and then re-formed.
Some 20,000 years ago at the Last Glacial Maximum ice was even more extensive and thicker; in fact a grounded ice sheet covered not only the present-day land surface but much of the continental shelf. It flowed slowly outwards from the mountainous spine of the Peninsula towards the coast on either side. The evidence for this is twofold: the morphology of the seabed, and the types of pebbles present in sediment cores
A. The detailed bathymetric images from the swath mapping system show linear features on the seabed, which are identical to glacial bedforms seen on land where ice sheets are known to have existed. These features provide clear evidence of the direction of ice flow, Over the last 10 days we have been surveying glacial troughs near the coast. The image below shows glacially-carved bedforms in the area of the former Larsen-A ice shelf. Ice converged from areas to the southwest, west and north of the deep area; it then flowed eastwards across the continental shelf.
B. Sediment cores collected on the continental shelf many miles from land contain pebbles up to several centimetres in size. These are much too big to have been moved by ocean currents, so they must have been transported by ice. In our sediment cores the uppermost 1-2 m generally consists of mud with isolated ice-rafted pebbles; all the pebbles can be matched with rock outcrops on land. The lower part of each core is a mixture of mud, sand and pebbles and is interpreted as a till deposited from the base of an ice sheet. Interestingly, the pebbles in the till cannot all be matched with local outcrops. The tills contain a higher proportion of Jurassic and Cretaceous sedimentary rocks (shales and sandstones) than we would expect. These sedimentary rocks underlie the continental shelf only a few metres below the surface.
The science bit.....Part 3......The Larsen B ice shelf disintegration
By Carol Pudsey, Principal Scientist
We had been planning to survey and core in the area of the Larsen-B ice shelf where some large icebergs have calved in recent years. Early in the cruise we heard that there had been another calving event, about 8 km of icefront amounting to 600 sq. km, between 31/1/02 and 17/2/02. In the memorable words of our colleague David Vaughan at BAS Cambridge, "And if it's still warm and you're near the icefront keep your eyes open and note anything that goes bump in the night. There is no reason to think that it's all over for this season".
On March 5th we received a satellite image revealing the complete disintegration of the northern half of Larsen-B. From this and the ship's Dartcom HRPT satellite images, the mass of disintegration debris seemed to be expanding eastwards and southwards, as the Larsen-A debris was observed to do in 1995. On March 7th the ship made a considered approach in between the southern edge of the debris and a row of four very large tabular bergs extending NE of Jason Peninsula. These four bergs were drifting northwards towards the edge of the debris, and in the limited time available a systematic swath survey was not possible. We were able to observe and photograph the debris and take one core before escaping to the south at nightfall. The large tabular bergs which we knew to be freshly broken were an amazing sight, beautiful clean ice cliffs with a vertical or overhanging profile. The main mass of debris looked like the aftermath of a huge explosion - certainly brought home the fact that disintegration is a different process from iceberg calving. It was exciting to be there, though a little frustrating given that we couldn't see very far from the top of the ship.
The Larsen Ice-Shelf area photo gallery
The colour image is from the ships Dartcom HRPT system, which we have talked about before, the black and white image was one sent from Cambridge and the image showing where we have been, was created on board by Peter Morris.
Thank-yous this week......
Carol Pudsey and Peter Morris for words and images and Keith Nicholls for his somewhat delayed piece on the CTD's, Photos from Steve, Angus, Keith Nicholls and Dave Smith
Coming up next week......
STANLEY!!! Oh and the end of the cruise, de-mobilising, with some seismics maybe before that as well as more swath and coring!