BAS cruise JR 275 departed the Falkland Islands on the James Clark Ross in early February and will sample the South Scotia Ridge, southern South Sandwich Islands and Weddell Sea before returning to Stanley in late March. This Antarctic research cruise is running three scientific programs: a biological program collecting animal samples and video footage, a geophysical program including seafloor mapping and collection of rock samples, and an oceanographic program using CTDs and gliders.

Week 1 – Sailing from Stanley to the South Sandwich Islands – Feb 13 2012
Mel Mackenzie (Museum Victoria)

It has been a surprisingly busy first week for all aboard the James Clark Ross. From the excitement of the first wildlife sightings of whales, dolphins, penguins and albatross as we left the Falkland Islands, to the less-joyous attempts to gain our sea legs as we crossed Drake Passage, the EvolHist (biology) team have been busy adjusting to life and work onboard a moving ship.

Breaching Dwarf Minke whale. Photo: Jimmy Rudd – BAS

On Thursday as we moved into the calmer waters of the Scotia Sea our resident geologist Phil was able to deploy his first ‘rock dredge’ near some seafloor volcanoes. The dredge is a simple but useful tool. Rocks are dislodged by the sharp frame around the first net as it is dragged along the sea floor. This is followed by the effective ‘back up’ of a metal cylinder with a solid base, which can also collect rock (and sediment samples) if the first net is not successful.

James Clark Ross crewmen Dave and Seamus, assisting with deployment of the rock dredge. Photo: Melanie Mackenzie - Museum Victoria

On our second dredge for the day – success! There was a buzz on deck as Phil realized we’d struck what he was after – magnesium-rich rocks called ‘peridotite’. These particular peridotite rocks have been carried by lava all the way from the earth’s mantle, a phenomenon not known from any other place on the major spreading ridges of the ocean floor. These amazing little rocks are still being studied by Phil and his colleagues and have only been collected from this site once before.

British Antarctic Survey (BAS) geologist Phil Leat with his proud haul of 'peridotite' rock. Photo: Melanie Mackenzie - Museum Victoria

On Friday night a lucky few braved the cold night air to glimpse their first iceberg from ‘Monkey Island’ at the top of the ship. My first sighting was just after 5.30 am the following day as we neared the South Orkney Islands. It was a beautiful sight to see these huge bright white plateaus bobbing by, and a sure sign that we’d moved into true Antarctic waters.
The marine geology team had been busy ‘swathing’ our path, bouncing sound waves off the ocean floor to create a ‘virtual map’ of its depth and make-up, and allowing us to choose a suitable site for deployment of our scientific trawl equipment.

BAS marine geologist Gwen Buys with ‘swath’ showing current depth in metres. Photo: Melanie Mackenzie - Museum Victoria

We started our test sampling with the Agassiz trawl (AGT), a sturdy piece of collecting equipment, made up of a metal frame and thick net, and less likely to be damaged if we happen to find ourselves in a rocky area.

The frame and net of the Agassiz trawl (AGT). Photo: Melanie Mackenzie - Museum Victoria

A two minute trawl yielding many of the animal groups we had hoped to see including sea spiders, sponges, brittle stars and many other creatures, even my own favourite sea cucumbers! With only small rocks brought up with the catch it was then safe to deploy the more sensitive epibenthic sledge (EBS) while we sorted through the first catch.

Brittle stars were abundant on our first trawl, often found clinging to sponges. Photo: Pete Lens – British Antarctic Survey

Brittle stars were abundant on our first trawl, often found clinging to sponges. Photo: Pete Lens – British Antarctic Survey

The team quickly sort through the catch, preserving and storing the material for later taxonomic and genetic work. Photo: Pete Lens – British Antarctic Survey

The epibenthic sledge (EBS) with two nets at back and camera at front. Photo: Melanie Mackenzie - Museum Victoria

The epibenthic sledge uses two nets to collect animals from the ocean floor, and this particular sledge has also been fitted with camera equipment, allowing us to glimpse the animals in their environment. Being able to link the preserved animals back to this footage later will help build a more complete picture of abundance, habitat, and interaction between animals. Deploying the equipment requires skill, a good harness, and a little bravery as the trawl is lowered over the back deck.

Crew and scientists deploying the epibenthic sledge. Photo: Melanie Mackenzie - Museum Victoria

Tonight we’re heading towards some volcanic calderas near the South Sandwich Islands. With a little luck we’ll miss the worst of the coming storm and will be able to start our next deployments early tomorrow. But for now, as the ship has started rolling again, I think it would be wise for me to sign-off and grab a hand rail before I get too dizzy!

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.