Feb 01 - Swath Survey
Date: 1st February 2004
Noon Position: lat 71 58.2' S, long 77 43.1' W
Distance Travelled since Immingham: 24469 Nautical Miles
Air temperature: -1.1°C
Sea temperature: -0.5°C
The JCR this Week.
This week finds us in the same area as last, continuing our swath survey of previously unsurveyed areas of the Bellingshausen Sea. The landscape is continuously changing, with variation in the amount and thickness of the ice, the wildlife and, of course the sky. The daylight is continuous and it seems as light at 1am as it is at 1pm.
On Friday we had the privilege of seeing the sun for the first time in a week, and it came out in style. Beautiful, vast and deep blue skies were reflected in the pools of water between almost continuous ice floes. Seals- mostly crabeaters but also some leopard seals bask on the floes like dogs lying in the sunshine. They show no sign of alarm until we are almost upon them when they turn and growl furiously at the big red object disturbing their peaceful sleep! The penguins are less fierce and simply flap their wings and then run around in panicked circles before diving into the sea.
The JCR ploughs effortlessly through the ice. At this time of the year the floes are fragile and many are breaking up. The ice we have encountered so far is mostly young and thin first year sea ice. Formed as the sea froze at the beginning of the winter last year, it is less than 2m thick and breaks easily. Where the pack is less dense, the floes are simply pushed aside by the ship. Where it is more compact, it takes more effort for the ship to break or push aside. At the moment, for the sake of good swath data, we are trying to avoid this sort of ice. However, we may need to work our way through some of it to get to the Ronne entrance later in the cruise.
For a full description of the different types of ice conditions, see last year's diary from December 29th 2002.
|The view at 4pm. Click to enlarge.||The view at 4am. Click to enlarge.|
On Friday we finally found the bottom of the world. Land appeared on the horizon, a huge mass of creamy-coloured ice extending into a perfect dome beneath the low-lying clouds. This was Smyley Island, part of Antarctica known as the English Coast. It looked as forbidding as it does in the photograph below, with nothing to break the continuous line of the ice cap. The lack of features makes it difficult to work out how high or how far away it is.
In spite of the starkness of this landscape, there is an abundance of wildlife. The sea ice is often green and brown on its under-surface, due to the presence of phytoplankton. These form the very bottom of the food chain at the top of which are the seals and penguins we see every day.
Science Bit In The Middle.
By Rob Larter with photographs courtesy of Steve Bremner.
We are presently in the middle of a research cruise to investigate the glacial history of the Bellingshausen Sea, which lies to the west of the Antarctic Peninsula. The area where we are working is marked on the map below by a rectangular box.
The first four and a half days of the cruise were spent just getting from the Falkland Islands to the study area. The remoteness of the Bellingshausen Sea is one of the reasons that few research vessels have visited it. Another is that it is covered by sea ice for most of the year. Even with an ice-strengthened vessel like JCR, it is only possible to work here for about two months in most years, between mid-January and mid-March.
Sea ice like this covers some parts of our study area even in February, which is usually the month when there is least sea ice cover.
What are we doing in the Bellingshausen Sea?
As explained in last week's "Science Bit in the Middle", our main objective is to find out how ice flowed across the continental shelf in the Bellingshausen Sea during the last ice age, and when it retreated from the shelf. To achieve this we are using the ship's advanced sonar systems and collecting sediment cores from the sea floor. Once we had reached the study area, we spent most of the next two days collecting sonar data to define the detailed topography of the sea floor in an area where an enormous glacier, or "ice stream", flowed to the edge of the continental shelf. These data confirmed that the shelf edge in this area is unusually deep. Much of the continental shelf around Antarctica lies at water depths of 400-500 m, but the shelf edge is deeper than 600 m all across the 150 km-wide mouth of the former glacial trough. Even at such great depths the swath bathymetry data revealed furrows carved by the keels of giant icebergs.
Part of the swath bathymetry survey of the shelf edge showing furrows (arrowed) carved by icebergs in water depths of more than 600 m. No modern icebergs are thick enough to have grounded in this water depth, so the furrows seen here were probably produced by the action of giant icebergs as the ice sheet retreated after the last ice age. These data have not yet been processed and include some artefacts both parallel to and perpendicular to the survey tracks, which run from lower left to upper right across the area. The darker blue area along the northwest edge of the area surveyed is the upper part of the continental slope.
We collected sediment cores at three locations on the upper part of the continental slope. Analysis of these cores will reveal the characteristics of the sediment transported to the shelf edge and released onto the slope by the ice stream. We will try to find out when the ice stream stopped supplying sediments to the slope by radiocarbon dating samples from the thin layer of marine sediments that overlies the glacially-derived sediments.
The gravity corer is deployed over the stern of the ship. The corer is lowered into the sea floor to collect a cylinder of sediment that may be up to 6 m long.
The box corer is deployed using the amidships gantry, and is used to recover an undisturbed sediment surface, which may be missing in gravity cores. It samples an area 30 cm square. This picture shows the box corer being recovered after collecting a sediment sample.
An undisturbed sea floor sample, recovered in a box core from the upper continental slope. The pebbles on the surface were transported and dropped by icebergs. The dark colour of many of the pebbles is due to the growth of manganese crusts. These develop very slowly, and their presence here indicates that recent rates of sediment deposition on the slope have been very low - not more than a few centimetres per thousand years.
Searching for the source of the ice stream
We plan to return to the shelf edge region later in the cruise to extend the area surveyed. Another part of the project is to find the source areas of the ice that formed the ice stream, and estimate the size of the former ice drainage basin. One important potential tributary to the ice stream may have been a glacier flowing out of the broad fjord known as the Ronne Entrance, which lies to the south of Alexander Island. These locations are marked on the map below.
This is a more detailed map of the area enclosed by the box on the location map at the start of the science section.
Satellite images we had received showed that the Ronne Entrance was virtually free of sea ice, but to get there the JCR had to push through some quite densely concentrated sea ice for a distance of about 50 km. The ship succeeded in getting through the ice and reaching relatively open water. This means we can now collect the first high-resolution bathymetry data ever obtained in the Ronne Entrance. Linear features on the sea floor, scoured by the base of the grounded ice that once emerged from this fjord, record the direction of past ice flow. We will also collect sediment cores in this area to characterize the glacial sediments derived from this fjord, and compare them with the glacial sediments deposited in the shelf edge region. If we find a marine sediment layer overlying the glacial sediments, we will date samples from this to estimate when grounded ice retreated into the fjord.
A few images of ship life.
|An unusual view of John Summers driving the winches from the warmth of the UIC room.||A treat we couldn't allow you to miss was the sight of 2nd Officer Dave King mopping the floor in the surgery, after he had trodden mud all over it!|
|Doug Trevvet, Tom Elliott and Jim Stevenson seal and penguin spotting during a break from the engine room.||
The view from our office windows.
A final thought ...
The midnight sun skirting the horizon. At this time of year, this far south, the sun never sets.
Next week ... more mud and maybe a bit of ice too.