Feb 08 - Bellingshausen Sea
Date: 8th February 2004
Noon Position: lat 69 15.0 S long 79 12.8 W
Distance Travelled since Immingham: 25835 Nautical Miles
Air temperature: 0.6°C
Sea temperature: 1.5°C
The JCR this Week.
This week as we head north and west it has got colder, the nights have drawn in and we have been getting passionate about our mud. A dropped core, an audience with a leopard seal, an amazing coincidence and a twist in the wire have all been highlights of a week which has inspired even the engineers to poetry. As we have headed north and been subjected to increasing periods of darkness at night, it has felt colder. The snow settles more on the deck, the sea has started to freeze just a little and of course the night shift is undertaken, partly, in the dark.
Not all of the cores go exactly to plan. A swell means that extra wire must be paid out to allow the corer to reach the sea bed. This has the disadvantage of leaving a loop of wire free when the swell subsides, and may result in this loop becoming caught. This was the case on a particular core late one evening. The wire became wrapped around the head of the corer and could not be disentangled. Some very skillful driving, both of the ship and the winch, meant that the corer was lowered onto a convenient icefloe. Once the weight was off the wire, this released and the corer could be recovered. Below is an image of the icefloe, complete with the imprint of the corer, following successful disentanglement. There was a wee bit of ice inside the corer as well, but this didn't worry our sediment experts, who were just pleased to have their quota of mud!
The corer is recovered after being landed on an icefloe. The imprint can clearly be seen.
Carol and Claus relieved to have recovered their mud and not quite convincing everyone else they have found a new method of ice coring.
A bit about the Bellingshausen Sea.
The Bellinsghausen Sea is described as the area north, northwest and west of Alexander Island. It was first discovered during the Russian Antarctic Expedition of 1819- 1821, and named after the leader of that expedition, Thaddeus Thaddevich Bellingshausen (1779- 1852). The sea was not given its name until the French Antarctic Expedition to the same area in 1908 to 1910. Bellingshausen and his team circumnavigated the world through the southern ocean in two sailing ships, the Vostok and the Mirnyy. They conducted a survey of the South Shetland Islands, South Georgia and the South Sandwich Islands but are best known for their discovery of Alexander Island.
While we are gathering data about the geological properties of the sea bed and how it was formed, we are sailing through uncharted waters. The data we collect is not only useful scientifically but is also sent back to the Hydrographic Office in Taunton. This new data may then be used to create a new chart or to update the exiisting ones, depending on the amount of new information gathered and the accuracy of the previous information.
The track of the JCR during JR104. Note the very blank areas of the chart, some of which will be filled in using the data obtained during this cruise.
We have now completed the programme of scientific work on this cruise and are starting the 4½ day passage back to the Falkland Islands. This week's science update from Rob Larter explains how the data we have collected will be used.
Science bit in the middle.
By Rob Larter.
The purpose of this cruise was to study the changes that have taken place in the Bellingshausen Sea since the last ice age. The area where we have been working is marked by the grey-filled box on the map below.
We have targeted the Bellingshausen Sea for this work because previous data suggest that it contains one of the major pathways through which ice drained from the West Antarctic Ice Sheet during the last ice age (see the "Science Bit in the Middle" in the two previous diary entries for more details).
There are a lot of icebergs and ice floes in the Bellingshausen Sea today, but during the last ice age this area was filled with ice that extended to the sea floor all across the continental shelf.
What have we done?
At the height of the last ice age, 20,000 years ago, grounded ice extended to the edge of the continental shelf around most of Antarctica. To find out how ice flowed across the continental shelf in the Bellingshausen Sea, we have used one of the ship's advanced sonar systems, known as a "multibeam echo sounder" or "swath bathymetry system". This is capable of revealing elongated or linear features on the sea floor that were produced by the flow of grounded ice. In places where such features have remained undisturbed by later events they enable us to interpret past ice flow directions and approximate flow rates. On the continental slope this system reveals features formed by the sediment transport processes that carried away sediment delivered to the shelf edge by the ice.
This is a more detailed map of the area enclosed by the box on the location map at the start of the science section. The red lines are our survey tracks. The dotted black lines mark the sides of the glacial trough that runs across the continental shelf. Open-headed arrows indicate approximate directions of past ice flow interpreted from the multibeam echo sounder data we have collected. The red dot lablelled 'R' is the BAS base at Rothera on Adelaide Island.
Data from another sonar system, the "sub-bottom acoustic profiler", reveals whether sea-floor features are moulded out of sediments or carved out of bedrock. It also shows where topographic features on the sea floor are buried by a drape of younger sediments. On the continental slope it reveals the shape and thickness of the most recent sedimentary units deposited by the "mass flow" processes that have carried material down the slope.
To find out more about the processes by which ice flowed across the shelf, when it retreated, and the nature of the sediment it delivered to the slope, we have collected sediment cores. We have used a gravity corer to penetrate up to 6 m into the sea floor and collect cylindrical sediment cores. These represent a time period extending back from the present to when grounded ice last covered each core site. We have also used a box corer to collect samples of the undisturbed sediment surface, which may be missing in gravity cores.
Four scientists, a Doc and a core. The group (from left: Carol, Emma, Claus-Dieter, Jeff and Colm) are proudly holding a recently-collected gravity core after it has been cut into 1m-long sections.
What have we found?
On the continental shelf the multibeam echo sounder has revealed linear features in many places, providing us with a good idea of the pattern of ice flow during the last ice age (see map above and examples below). The multibeam echo sounder images below are unprocessed and include artefacts both parallel and perpendicular to survey lines. The survey lines are shown in white.
This image shows multibeam echo sounder data over a continental shelf trough in Eltanin Bay (see map above) that is about 1000 m deep. The sea-floor topography has been illuminated with a "false sun", shining from the east (right), to emphasize fine resolution features. The data reveal a number of features called "drumlins", indicated by white arrows. Drumlins are well known features in many areas on land that have been glaciated. They are mounds of glacial sediment ("till") that are elonagted parallel to the direction of ice flow, the upflow end being rounded and the downflow end tapering to a point. The black, open-headed arrows indicate the direction that ice flowed across this area.
This image shows multibeam echo sounder data covering the eastern margin of the main glacial trough on the continental shelf (see map above). As in the previous image, the sea-floor topography has been illuminated with a "false sun", this time shining from the northeast (top right), to emphasize fine resolution features. In this image colour has been used as well, to represent depth (warmer colours represent shallower depths, red areas are 450 m or less and dark blue areas are greater than 650 m. A number of linear features are seen ("mega-scale glacial lineations") that typically form at the base of fast flowing ice. Some of these are marked with white arrows. The black, open-headed arrows indicate the direction that ice is interpreted as having flowed across this area. The image also shows a number of furrows carved by the keels of icebergs both in the trough and on the bank on the right of the image (marked by black, solid arrows).
Gravity cores are normally split along their length to examine the sediment within them. We have not split the gravity cores on board because this is a relatively short cruise. Moreover there are now a number of analytical methods that can be applied to whole cores before splitting them, providing much information on the physical properties of the sediment. Looking through the transparent core liners we can make out enough to know that our cores generally show a transition from creamy brown mud near the surface to grey glacial sediments further down. The same pattern was observed in some cores collected on a previous reconnaissance cruise to this area on the German vessel R/V Polarstern.
Above is one Claus-Dieter did earlier - these two photos show the split surface of a sediment core collected previously in this area on R/V Polarstern. The upper photo shows the sediment from the top 20 cm below the sea floor. The creamy brown mud at the top of the core contains a high proportion of a type of calcareous microfossils called foraminifera. The next photo shows the part of the core from 20 - 120 cm below the sea floor. This includes a transition to grey sediment that contains a mixture of grain sizes ranging from pebbles, through sand and silt down to mud. This mixture of grain sizes is typical of glacial sediments ("till") deposited beneath or near to grounded ice.
Our box cores show that the foraminifera are only present in the top layer of sediment on the outer part of the continental shelf and the slope. However, different microfossils, known as diatoms, are found in the top layer of sediment on the inner part of the shelf. Diatoms are generally more common around Antarctica. The reason why foraminfera are abundant in the recent sediments over part of this area remains a mystery at the moment. However, it is fortunate for us that they are present here because their calcareous shells are excellent material for radiocarbon dating.
A photo of the surface of one of our box cores from the outer continental shelf, showing a thick layer of creamy brown mud that is typically rich in foraminifera. The sample has tilted slightly, so the smooth surface on the lower part of the photograph is part of the side of the core. On the surface of all our box cores there are pebbles that have been dropped by icebergs. Many of these pebbles have a veneer of manganese on their surface. As manganese crusts grow very slowly, this suggests rates of sediment accumulation have been slow over the past few thousand years.
The slow rates of sediment accumulation implied by the manganese veneer on pebbles at the surface of the box core may explain why the thickness of post-glacial sediment is less in this area than around many other parts of Antarctica. However, an alternative possibility is that deglaciation of the shelf in the Bellingshausen Sea was later than on some other places around Antarctica. We should be able to distinguish between these two possibilities when samples from our cores have been dated.
What does it all mean?
The preliminary results we have obtained on board leave no doubt that the Bellingshausen Sea did indeed contain one of the major pathways through which ice drained from the West Antarctic Ice Sheet during the last ice age. The features we have observed in the multibeam echo sounder data also confirm that ice flowed to the continental shelf edge in a very wide, fast flowing "ice stream". Once samples from the sediment cores have been dated we will be able to estimate when ice retreated from the continental shelf in the Bellingshausen Sea, and then we will be able to look at records of global climate and sea level change to see if these give any hints about what might have triggered deglaciation. Finally, to reconstruct the ice sheet during the last ice age and improve understanding of what caused its retreat, we will collaborate with scientists who work on computer models of ice sheets.
There is life beyond mud...
Each week one or two crew training sessions are given. This week, after checking there were no other ships in the area, Dave Gooberman the mate gave a demonstration of how to deploy flares and allowed members of the crew to practise for themselves.
Dave Gooberman the mate demonstrating handheld flares.
Will the 2nd cook lets off a parachute flare.
Ian Raper lets off the smallest flare he can find!
...but ultimately not much...
George Dale provides entertainment for the engineers from BAS.
Claus trustingly giving the bosun a mud core.
Today is 2nd Engineer Colin Smith's 50th birthday.
HAPPY BIRTHDAY COLIN
A final thought until next week...
Our last sight of the pink ice as we head north.
Next week... Stanley again, and the dreaded Drake passage.