11 February, 2011 RRS James Clark Ross

RRS James Clark Ross Cruise JR244 to the southern Weddell Sea

British Antarctic Survey scientists on this cruise are carrying out marine geological and geophysical studies to determine the long-term history of the Antarctic Ice Sheet and climate in the southern Weddell Sea.

The map shows the track of RRS James Clark Ross from the Falkland Islands to the southern Weddell Sea. The yellow oval outlines the main study area. Red dots mark BAS research stations. The area shaded in blue is mostly covered by sea ice.

Objectives

Specific questions that we are aiming to answer through work on this cruise include:

• how far the Antarctic Ice Sheet advanced onto the continental shelf in this region

during the last glacial period (about 20,000 years ago).

• the history of glacial retreat as the climate warmed after the last glacial period.
• what processes occurred beneath the ice that enabled it to flow across the shelf,

and how the type of material at the sea floor (e.g. hard rock or soft sediment) affected these processes.

• whether or not the ice sheet on West Antarctica collapsed during previous interglacial periods, which have occurred at intervals of about 100,000 years over the past 800,000 years. There is particular concern about the stability of the West Antarctic Ice Sheet because most of its bed is below sea level. If this ice sheet did collapse it would have caused global sea level to rise by more than 3 m.

• how the ocean temperature and sea ice cover in the region have changed during glacial-interglacial cycles, and in particular since the last glacial period.

The results of these studies will be used to test and refine computer models of ice sheets that will be used to predict how much the Antarctic Ice Sheet will contribute to sea level rise in a warming climate.

Equipment

The main tools being used for this research are coring devices that collect sediments from the sea floor. A gravity corer is used to collect sediments that have accumulated over many thousands of years. In some places where sediment has accumulated more slowly gravity cores contain records that extend back hundreds of thousands of years.

The picture above shows the gravity being recovered in the Weddell Sea.

Mud smeared along the outside of the core barrels shows how far the corer penetrated into the sea floor.

A box corer is used to sample soft surface sediments, which may be disturbed by the gravity corer. Box core samples show what sort of sediments have accumulated under recent conditions.

The picture above shows the box corer being recovered earlier during the cruise, near the South Orkney Islands.

Sophisticated sonar devices are being used to map the shape of the sea floor and the thickness of soft sediments. The results of these surveys are essential for selecting the best core sites. The sonar data also provide a lot of information about the pattern of ice flow and the processes that operated beneath the ice when it advanced onto the shelf.

The ship’s multibeam echo sounder reveals the water depth and the shape of the sea floor over a zone up to four times as wide as the water depth under the ship, as illustrated in the picture below. It transmits 191 narrow beams of high frequency sound (near the upper limit of the frequency range detectable to the human ear) from the bottom of the ship and detects the echoes from the sea floor.

A sub-bottom acoustic profiler is used to show the thickness of soft sediments beneath the sea floor and the layering within them.  It transmits intermediate frequency sound pulses (quite high-pitched to the human ear) from the bottom of the ship and detects the echoes from the sea-floor and soft sediment layers beneath the sea floor.

The image below shows an example of part of a sub-bottom profile.

The sub-bottom profiler only “sees” through very soft muddy sediments. Even some relatively young glacial sediments cannot be imaged with this device. For this reason we sometimes need a way of looking through harder sediments or sedimentary rocks to find out what is beneath the sea floor and understand the origin of sediment deposits. On this cruise we are occasionally using a small seismic reflection profiling system consisting a single “airgun” and a short “hydrophone streamer”. The airgun is towed behind the ship and creates a low frequency sound signal (in the range of the bass notes on a musical instrument) by releasing a burst of high pressure air into the water. The hydrophone streamer, which is also towed behind the ship, detects the sound reflected from the sea floor and boundaries between layers of sedimentary rocks beneath the sea floor.

The picture below shows the seismic airgun being deployed over the stern of the ship.