Feb 9 - Our Next Week in the Amundsen Sea
Our Next Week in the Amundsen Sea
Well, the James Clark Ross hasn't moved all that far since you last read about her exploits. When you last left the ship, we had just finished mapping out the sea floor, coring and casting CTDs in the region of the Getz Ice Shelf just to the west of the Pine Island Bay area. After finishing work in the Getz area, the ship headed further east to the Dotson Ice Shelf, the next ice shelf in the direction of Pine Island on the coast bordering the Amundsen Sea. A schedule of work similar to that of the Getz Ice Shelf was under taken here, but this was as far to the east in the region as was possible for the ship to go due to the dangerous ice conditions described last week. Thus, after undertaking a number of cores for sediment on our way north, the JCR headed back into the pack ice at 4:30pm on 2nd Feb, leaving the polynya along the route by which we had entered some 10 days previously.
The trip out of the polynya through the pack ice, was not to be as easy as the journey in, which had taken less than 24 hours. The JCR had only been back in the pack ice for a matter of hours when it started to become much tighter, and required "backing and ramming" to break specific areas of heavier ice. With such slow progress the decision was made to call a halt to proceedings by midnight, ready for another attempt in the morning. Overnight the ship drifted in a long arc to the west. Next morning the pack was still too thick and tight due to the wind conditions and it was decided to call it a day after just 3 hours of working the pack, with no more than a few hundred yards of actual progress made through the ice. As expected, on 4th February, the wind came around to ESE, easing the pressure within the pack ice, and from 0800 good progress was made. During the evening meal at about 7pm the ship broke free from the pack ice, coming into open water. The ice edge, having been blown down by the fresh Northerly winds of the pas few days, was much further south than where we much further south than where we had entered the pack just a couple of weeks previously.
As described in the first web page from down here in the Amundsen Sea, the usual communication systems have had to be significantly modified, and are still to this day. Direct internet access and use of the ships usual satellite phone system has now been unavailable since we passed Peter 1st Island over two and a half weeks ago. Individuals on board, though, have still been able to contact their family if required, using the much more expensive Iridium system available. The ability of the ship to send email messages, for both work and personal purposes has been highly variable also. Whilst in the polynya the ability of the Radio Officer to get a link to the Inmarsat satellite has depended very much on the exact position of the ship, as well as its heading at the time. This has led to many an early morning/late evening wake-up call to Mike from the bridge in an attempt to get information to and from Cambridge. The last few web diaries have been sent during these very random and often brief periods when the satellite has been "in view" from the ship.
This cruise, although having all scientists on board permanently working on a shift schedule, has been the first this season where space has allowed the use of the holds for fitness pursuits. Up until leaving Rothera just over a month ago every area of the storage space was jammed full with cargo of one form or another. Now that the forward holds are almost completely free of cargo, they can be put back into their secondary use by members of the ships complement. Since the start of this cruise there have been a turbo trainer, exercise mats, table-tennis table and punch bags set up, all securely tied down so as to endure the rigours of the open ocean. This new acquisition of space has also meant the start of the traditional keep fit circuits run by this crew's Purser, Rich, which have been undertaken three times per week by many of the visiting science staff. Many have certainly used this long science cruise as a chance to get fit, rather than relax into a more sedentary ship's lifestyle.
On to the actual science
(with Rob Larter and Tara Deen)
The current research cruise on JCR involves work in a remote part of the southernmost Pacific Ocean known as the Amundsen Sea (see map in diary entry three weeks ago). As explained in previous diary entries, the main objectives of this cruise are to investigate the glacial history of the continental shelf in this region and to study present day interactions between the ocean and Antarctic ice sheet.
Remote sensing and scientific work this week
Last week we reported that we were working in a ‘polynya’ (a large area of open water enclosed by sea ice) in the western part of the Amundsen Sea. This week we completed our work there, then made our way back north to the open ocean.
Much of our work involves looking at the sea floor, which lies between 400 and 2000 m below sea level. Physically getting to the sea floor is very difficult, so we have to use a form of remote sensing. On this cruise, we are using three types of remote sensing - multibeam echo sounding, sub-bottom profiling and seismic profiling. Each method gives us a different type of information about the sea floor and the processes that have formed it.
Multibeam echo sounding gives us detailed imagery of only the sea floor surface. As this method has a wide beam we can overlap lines of data to give us a large map of the surface of the sea floor. Last week we finished collecting multibeam imagery of the sea floor north of the Dotson and Getz ice shelves. These images show features formed by the ice sheet scouring and dragging sediment across the sea floor when it extending onto the shelf during the last ice age. From such features we can interpret the flow directions of the past ice sheet and also get a general idea of how fast the ice flowed.
Sub-bottom profiling only gives us a single line of data rather than a wide beam, but it allows us to see a high-resolution image of the shallow sub-surface (tens of metres) of the sea floor. This is useful to gain an immediate idea of what sediments lie on the sea floor, which allows us to choose coring sites. Last week we collected several sediment cores in the same area as the multibeam survey to determine when, and how quickly, the ice sheet retreated from the shelf. The sediment cores show a clear change between the type of sediment deposited underneath the ice and the type deposited in open water once the ice had retreated. Once the cores have been transported back to Cambridge we will take samples from the sediments near this change and use radiocarbon dating to determine its age.
The third type of remote sensing data we have acquired is seismic data. To do this we inject a pulse of compressed air into the water, and measure how long it takes the pressure wave generated by this to bounce from the sea floor and sub-sea floor to our receivers - much like an echo coming back from the side of a building. This gives us information about the structure of the sea floor at a much deeper level (tens to thousands of metres) than the previous two types of remote sensing, which allows us to determine the processes that have formed the continental shelf over time.
Last week we reported a study of the physical properties of seawater along the front of the Getz Ice Shelf, using conductivity, temperature and depth (CTD) recorders. This week we carried out a similar study along the front of the Dotson Ice Shelf, which is further east. Studies using data from satellites have indicated that the Dotson Ice Shelf is thinning by about 4 m per year, so the purpose of this study was to find out how much of this thinning is due to melting at the base of the floating ice shelf as a result of relatively warm water flowing underneath it.
All of the different types of data we are acquiring provide extra pieces of information to help us better understand the glacial history of this region, both in the past and the present.