Feb 1 - In the Amundsen Sea
In the Amundsen Sea
Well, when you left us last week we had just broken through the pack ice into the open polynya around the area the James Clark Ross had been aiming to do most of it's work during this geophysics cruise. After just a taster of coring on the outer rim of the shelf we had, at last, got closer inshore towards the various ice fronts within this area of the Amundsen Sea. Pine Island Glacier is obviously the area that has been mentioned continually over the last several web pages but the ice conditions, as will be described later, just meant it was not to be. Although we were in one, more western, polynya, and Pine Island Bay Glacier appeared accessible via an eastern polynya, passage between the two was simply not a safe option for the ship. It will thus be described later on exactly what science was undertaken instead.
The drop in temperature had been obvious during the week prior to the ship arriving this far south. Amazingly, up until now, the JCR had actually experienced no more than a few days of temperatures below freezing prior to this week. Now, temperatures were to drop permanently below zero. Even with this drop in temperatures the weather was still highly variable. Early on during the week the wind was no more than a few knots and the ship was also bathed in sunshine. Even though the temperature was actually -5oC during these few days it certainly didn't feel it with the warmth of direct sunshine on deck. Things did deteriorate over the week though and by the weekend the wind was up to around 40 knots, making the effect of the wind-chill a different matter entirely. A sortie to shore by boat, in an attempt to obtain rock samples, for a small number of scientists and crew, the day prior to this highly unpleasant weather, was certainly a less than enjoyable experience.
Since it has been a week of solid science work, I will hand you quickly over to the scientists for a full explanation of what we have been doing.
Science overview with Rob Larter
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 two weeks ago). 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.
Why have we come to the Amundsen Sea?
The Amundsen Sea lies on the side of the West Antarctic Ice Sheet (WAIS) facing the Pacific Ocean. Although the WAIS contains a much smaller amount of ice than the neighbouring East Antarctic Ice Sheet, there is greater concern about its possible instability in the face of global warming and rising sea levels. This is because the base of much of the WAIS lies below sea level, raising fears that thinning of the ice could ultimately lead to its collapse. Complete collapse of the WAIS would result in a rise of about 5 m in global sea level. Most scientists working in the area think that complete collapse within the next few hundred years is unlikely, but even loss of one sector of the ice sheet would necessitate a drastic increase in current projections of sea level rise over the present century.
Although an ice sheet appears not to change much over a period of a few days, the ice is actually slowly deforming and flowing. Where ice flow directions converge, the flow accelerates, eventually feeding into fast flowing glaciers or “ice streams”. Calving of icebergs from these ice streams is the main way in which ice is lost from the Antarctic ice sheets. More than 40% of the area of the WAIS feeds into ice streams that flow to the Pacific side of Antarctica, but because of the remoteness of this part of the Antarctic coast, and hence the time it takes a ship to get here, it is relatively little studied. The largest ice streams that flow to the Amundsen Sea coast come right from the centre of the WAIS, and the part where its base is furthest below sea level [insert link to BEDMAP webpages]. These factors make this part of the WAIS the most vulnerable to change and also the part where changes are likely to have the greatest effect on other sectors of the ice sheet.
A further cause for concern about this part of the WAIS is that results of studies using data from satellites have shown that the glaciers flowing to the Amundsen Sea have thinned rapidly over the past 15 years. The thinning is greatest in the parts of the ice sheet and glaciers that extend into and float on the sea, known as “ice shelves” and “glacier tongues”. As ice shelves and glacier tongues all along the Amundsen Sea coast have been thinning simultaneously, it has been proposed that the cause lies in the ocean. Specifically, the cause has been proposed to be melting of the base of the floating ice as a result of upwelling of relatively warm water from the deep ocean onto the continental shelf.
How are we studying the ice sheet from JCR?
In past ice ages the WAIS extended onto the continental shelf. By imaging glacial features on the sea floor with the ships advanced sonar systems and collecting sediment cores we will be able to reconstruct the patterns of flow in the past ice sheet and determine the history of glacial retreat since the last ice age. This information will then be used to refine computer models of how ice sheets change in response to changes in sea level and climate, which in turn can be used to predict how the WAIS will change in the future.
By lowering sensors into the ocean to measure variations in its temperature and salinity we can measure the flow of relatively warm water from the deep ocean onto the continental shelf. The same sensors can be used close to the edges of ice shelves to calculate how much water in flowing beneath them and how much melting is occurring at their base.
What have we done so far?
Ironically the locations in which we have been able to work so far have been constrained by the distribution of ice. The area we were most interested to work in, offshore from the large Pine Island and Thwaites glaciers, is still covered by a high concentration of sea ice. JCR managed to reach the edge of the continental shelf in two places, but the restricted size of areas of open water placed severe limitations on the work we could do.
Satellite images showed a large area of open water landward of the sea ice in the western part of the study area. Such open water areas within sea ice are known as “polynyas”. We decided to try to get into this polynya, and after spending most of Monday pushing through ice floes, we entered a vast expanse of open water. We had initially hoped to find a way to reach Thwaites and Pine Island glaciers by going eastward from the polynya, but its eastern edge is barred by a line of large tabular icebergs, smaller icebergs that have broken from them, and sea ice. After finding out that we could not go further east, we decided to concentrate our efforts on the ice shelves and past glacial systems along the coast where we were.
Dartcom satellite image of the southern Amundsen Sea on 28th January, showing the polynya where we are now working and the dense concentration of sea ice over our intended work area. Yellow areas are ice (ice sheet, glaciers, icebergs and sea ice), black areas are open water and white areas are high cloud. Red lines mark the approximate positions of the coast and ice shelf edges. Orange dot marks the position of JCR at the time the image was collected.
During the past five days we have imaged a large area of the sea-floor using the JCR’s multibeam echo sounder. The images we have collected show where past glaciers scoured the sea floor and the directions in which they flowed. The picture emerging is that glaciers from the inlets where the Dotson and Getz ice shelves are now located joined together to form a large ice stream, and that this flowed further out to the northwest. We have collected several sediment cores, from which we will be able to date samples to find out when ice retreated from the shelf. We will also analyse the composition of the sediment to find out where in West Antarctica the ice flowed from. We have collected seismic reflection profiles, which provide cross-sections showing the geological structure to a depth of several hundred metres below the sea floor. These data show where glacial troughs have changed their position in the past and the nature of the substrate below the sea floor, allowing us to see how this affected the flow of large glaciers in the past.
Oceanography with Deb Shoosmith
The open water at the edge of the Getz Ice Shelf has given us the opportunity to make ocean measurements which will help us gain an idea of how the ocean may be driving the rapid ice shelf thinning that has been observed by satellites. The Getz Ice Shelf is constrained at either side by land, leaving a vast underwater “cavern” beneath the ice into which seawater can flow. If this seawater is sufficiently warmer than the freezing point then it may well be able to explain the high melt rates currently measured in this region.
To measure the ocean impact we sailed alongside the Getz Ice Shelf front as close as the Captain would allow (approximately 500 metres from the ice edge). Every 6 km or so, we stopped and deployed a conductivity, temperature and depth (CTD) recorder. This measures very accurately the sea temperature and salinity (along with a number of other properties such as dissolved oxygen concentration) from the sea surface to the sea floor. By spanning these measurements across the entire ice front we obtain a very clear picture of what kind of water is flowing underneath the ice shelf. This showed us that the deeper waters here are over 2 degrees warmer than the in-situ freezing point of seawater which means that the ocean is indeed able to melt the underside of the Getz Ice Shelf.
Although this result is perhaps unsurprising to many of us who work in this field, we have been able to go one step further by attempting to measure exactly how fast this melting is happening. The ship is fitted with an instrument called an acoustic doppler current profiler which measures the ocean currents down to a kilometre beneath the ship. Using this information combined with the CTD data, we are able to determine how much warm water flows in underneath the ice shelf and also how much cooler ‘melt’ water flows back out, and hence to calculate how much melting is actually occurring. Such results as these are very important as we can then attempt to evaluate how much ice shelf melting is likely to contribute to sea level rise.