Weddell Sea Expedition: The Search for Endurance

The Weddell Sea Expedition was one of the largest expeditions ever to have been to the Weddell Sea and took place from 3 January to 20 February 2019. It's aims were threefold:

1. To take measurements of the Larsen C ice shelf.
2. To examine the oceanography, biology and ecosystem in the area.
3. To find the wreck of Shackleton’s ship Endurance.

The Weddell Sea is a body of water between the Antarctic Peninsula and East Antarctica that is famously difficult to access – Endurance was the only ship that had managed to penetrate this area of the Weddell Sea before. The Western Weddell Sea has sea ice up to 2 metres thick, but the concentration of ice is the major problem. Scott Polar Research Institute (SPRI) researchers looking at imagery of the Larsen C and wreck site area, discovered that the area usually has 100% sea ice cover. The expedition had to be timed precisely to make sure that the ship would arrive when the sea ice was at its minimum, at the end of January/beginning of February.

There are only four or five ships in the world that can handle these ice conditions and Dr Shears chose the South African vessel Agulhas II. Built in Finland in 2012, she’s polar class 5, which means she’s not an ice-breaker but can break through 1 metre thick ice at a speed of 5 knots. What the 134m ship lacked in extreme ice-breaking ability, she made up for with the equipment onboard, including onboard science laboratories, a big aft deck, helicopter hangar and a moon pool – a hole in the centre of the hull which can be used to deploy instruments. Also unique to this ship was a range of pressure and stress sensors which allowed onboard engineers to measure and understand what effect the ice pressure has on the ship’s hull.

Dr Shears said that the South African crew were the best he has ever worked with in 30 years of polar exploration. South Africa is not widely known as a polar nation and many people don’t know about their Antarctic programme, so they felt a huge amount of national pride at being chosen for the expedition. The combination of the South African captain, Knowledge Bengu – the first black ice pilot in Africa – Capt Freddie Lighthelm, the specialist ice pilot that SPRI brought in to the crew, and the experience of Dr Shears and Professor Dowdeswell allowed them to make this unique voyage to such a difficult location.

Rather than sailing from South Africa or South America, the whole crew were flown into the Thimble ice shelf, 130km from the South African station SANAE. The ship had just completed the annual station resupply before the SPRI team came aboard and started navigating the sea ice to Larsen C. They planned to send robotics down to take measurements of the ice shelf which is breaking up, but also to study the iceberg A68 which is one of the largest icebergs ever to have calved in Antarctica. This iceberg proved crucial to the ship’s successful passage to the Endurance wreck site as it helped to dam some of the ice to the south of it, leaving a route through.

The crew successfully made it to the Larsen C site at the end of January, but then hit a setback that lost them vital time. The pressure housing on one of the Remotely Operated Vehicles (ROV) that they sent down for a test dive exploded at a depth of 3,000 metres. The crew headed back to King George Island in the hope of collecting replacement parts but, due to the weather, the parts couldn’t be flown in. They knew they had a very narrow window to reach the wreck site before the sea ice closed in, so they headed straight there to search for Shackleton’s ship.

Professor Julian Dowdeswell – Chief Scientist

The science team was not only interdisciplinary, but also very international with scientists from the UK, Norway, South Africa, Russia, New Zealand and the Netherlands.

Some of the themes they were studying include:

• Bathymetry and marine geophysics
• Ice shelf change
• Sea ice
• Marine biology
• Polar oceanography

Prof Dowdeswell said that the root of the expedition’s research can be traced directly to the interdisciplinary scientific work carried out by researchers on Scott and Shackleton’s expeditions in the early 1900s. But why the Weddell Sea? In 1995 and in 2002 two of the largest ice shelves on the Eastern side of the Antarctic Peninsula – Larsen A and Larsen B – collapsed catastrophically. The scientists wanted to determine whether this collapse has only started happening now, or whether the collapse was part of a regular process with several such events occurring over the last 10,000 – 15,000 years. Prof Dowdeswell said that if the collapses were part of a regular process the scientists weren’t too worried, but if this is the first time they have collapsed in the last 10,000 years (the present inter-glacial) it would be a sign of negative human influence on the planet. Larsen C, which is the fourth biggest ice shelf in the whole of Antarctica, also calved an enormous 150km long iceberg (A68) in July 2017. Is it a sign that Larsen C is starting to collapse, or is it natural for an ice shelf to calve a large iceberg every few decades?

Ice shelves are important because they act as buttresses, using back pressure to keep the ice on the land. If the ice shelves are gone, land ice will flow faster, adding significantly to global sea level rise. This ice shelf research is therefore a critical part of inter-governmental predictions and assessments of future sea level rise which, in turn, has an impact on London and the Thames Barrier’s ability to cope with rising sea levels.

The second reason for choosing the Weddell Sea is that it is one of two major areas that produce Antarctic bottom water - the densest water that is formed in the southern hemisphere. As the sea surface freezes to form the thin cover of sea ice (just a few millimetres thick), the sea ice becomes less salty. This is because salts are rejected during the freezing process resulting in a layer of very cold, very salty, very dense water immediately beneath the sea ice. Because of its density, it sinks to the base of the water column and flows northward. In the north the same happens in the Labrador and Norwegian Seas and it flows southward as the gulf stream, driving a large part of the southern ocean circulation. Therefore, the amount of sea ice that forms, the rate at which it forms, and the way it changes through the decades has a direct impact on the circulation of the entire ocean.

Prof Dowdeswell explained some of the marine geophysical, bathymetric and echo sounding data that was collected on the ship. The equipment they used included two state-of-the-art autonomous underwater vehicles (AUVs), one of which looked upwards to map the sea ice canopy, and another that looked downwards to map the sea bed. As they can travel just tens of metres above the sea floor or below the ice canopy, their spatial resolution is much greater than conventional ship-mounted echo sounders that could be hundreds or thousands of metres above the sea floor. The improved resolution means that a one metre long object on the sea bed can be identified, whereas it may not even be picked up by a conventional ship a few hundred metres above at a lower frequency. This means that the shape of the sea floor can be seen in great detail. As the water shallows, icebergs that have been floating impinge on the sea floor and plough through the sediment, cutting an ‘iceberg plough mark’, just like a plough in a field. This very high resolution mapping of the sea floor can confirm the presence or absence of iceberg plough

marks, while a series of string-like features on the sea floor indicate the direction of past ice flows. This is important to the reconstruction of past ice sheets.

The other thing the scientists looked at was ice shelf change. Using US defence satellite imagery dating back to the 1960s (which has only just been made available) and more current satellite data up to the present day, they were able to map how the ice shelf terminus position has changed. This gives a good indication of how fast the ice has advanced or retreated since the 1960s, but they wanted to look further back to the past 15,000 years. Analysing the marine geology and geophysics of the sea floor offered some clues as to whether the ice sheet collapse is an aberration of today’s warmer world, or whether it’s a repetitive process.

From the historical echo sounding data it was clear that only one ship had operated in this area before, CA Larsen’s ship Jason when the ice shelf was discovered in the 1890s. He was incredibly lucky to gain access because no ships have been able to get there since. Other vessels tried last year and Polar Stern, the big German ice-breaker, attempted to follow in after the WSE team, but was unsuccessful. The 2019 WSE expedition collected more bathymetric data on the region than all previous ships combined. This data has since been sent to JEDCO, the international organisation that create maps, and the UK Hydrographic Office to increase their data set on the Weddell Sea area.

The fortunate positioning of the A68 iceberg held back the sea ice for two or three weeks, giving enough shelter for the team to slip in and do some sampling. Together with British Antarctic Survey, they utilised existing radar and digital elevation models to understand, not just the behaviour of the margin of the last ice shelf, but also the shape of the underwater cavity beneath it. The shape of the cavity, which could be hundreds of metres thick, has an impact on the water flow beneath, which in turn affects the melting rate of the ice shelf.

The A68 iceberg continued to move while the team were in situ and they found that the iceberg now covered some of the areas they had been sampling just a week before. Looking at sea ice coverage data going back to 2002, the only year that had good ice conditions for ships was 2002 itself. Even 2019 didn’t have good sea ice conditions, but the presence of the protective iceberg, as well as the skill of the ice pilots and crew, contributed to the success of the expedition.

The team wanted to gauge the thickness of the sea ice by measuring it simultaneously from both below and above. They used drones to fly over the surface of the sea ice while, at the same time, the AUV measured the three-dimensional shape of the underneath. The sea ice is just a few metres thick, with a layer of winter snowfall on top and the satellites can measure the free board (above-water surface) of the ice up to 15cm of accuracy. But the satellites can’t distinguish between snow and ice, so the team dropped people onto the ice floe to get in-situ measurements that would help calibrate the satellites. This information was important because, in order to work out how much dense bottom water is being produced by sea ice freezing around Antarctica, scientists need to know not just the free board, but the density and therefore mass of the ice and this is the only way calibrate it.

The drone is fitted with a snow radar which is able to measure the depth of the snow. That is calibrated with the density measurements taken by people digging snow pits on the floe. Combine it with the AUV’s three-dimensional submarine shape of the sea ice and the team were able to understand how much dense bottom water is being formed in the Weddell Sea, and hopefully how that changes from year to year. In a collaboration with the German Antarctic research station, the scientists also dropped buoys onto the ice to measure its drift. Some of the buoys have

drifted hundreds of miles and are still going, which allows them to track the rate of sea ice drift all year round.

Researchers were also examining the marine biology in the Larsen C area – some parts of the sea floor that had been covered for 10,000 years have only been revealed in the last few years as the ice shelf retreats. What impact has the collapse of the ice shelf had on its biological communities? The scientists used AUVs and an ROV (tethered remotely operated vehicle) which can touch down on the sea floor and collect biological samples. In areas as remote as this, new species are almost always discovered.

Throughout the expedition, they looked at the oceanographic biology and the paleogeography - how the oceans have changed in the area. Oceanographers used a series of water samples and drift nets to sample the biota, temperature and salinity both at the sea surface and between the surface and the deep sea. In different areas and different water masses and salinities they found different kinds of phytoplankton and organisms. Examining the marine biota from a sediment core will give clues as to the temperature and salinity of the water over hundreds or even tens of thousands of years. At the time of Prof Dowdeswell’s talk, the sediment cores were only just starting to be analysed, but they will carbon date the cores and create an exact timeline of the changes in water temperature, mass and salinity over time. This will help them to understand the retreat and advance of the ice shelves in the region and hopefully show whether the Weddell Sea ice has broken up repetitively over the last 15,000 years, or whether the collapse is being triggered now for the first time.

The layer cake structure of the core material gives us a key to the past and that’s why interdisciplinary work is so effective. Understanding the biology, sedimentology, glaciology, geophysics etc allows a group of scientists to work on the complex interaction between ice and ocean and put a time frame on those interactions. The scientific data provided by the Weddell Sea Expedition will also be written up and delivered to policy makers, in the hope that it will help make the case for the Weddell Sea becoming the world’s largest marine protected area (MPA).

Prior to the expedition the scientists used the limited geophysical data available for the Weddell Sea to see if they could work out the likelihood of finding the Endurance wreck. If there were iceberg plough marks in the approximate location of the wreck it could be assumed that the ship had been broken up and dispersed by the icebergs. But the mapping showed no plough marks for hundreds of kilometres around the Endurance location. The sea floor was very flat with evenly distributed, very slowly accumulating sedimentation, which suggests the area has lain undisturbed for a long time. The scientists therefore made the following hypotheses:

1. The sea floor is likely to be very flat.
2. There have been no big landslides in the area.
3. Iceberg keels rarely graze deeper than 700 metres and the wreck is at a depth of 3,000 metres.
4. The sedimentation rate is probably only a few millimetres per year.
5. The ship should still be there, it's just a matter of finding it.

Dr John Shears – The search for Endurance

It is often called the most challenging shipwreck in the world to find, but not because its location isn’t known. Working through the extensive SPRI archives, Dr Shears consulted the Endurance captain and expert navigator, Frank Worsley’s diary, and other expedition sources which provide a very accurate record of exactly where the ship sank. Consulting with oceanographers, Dr Shears

worked out how far the wreck was likely to have drifted in 3,000 metres of water and believed she should still be within a few nautical miles of Worsley’s marked location. The challenge was trying to get to the site and this is where the South African crew came into their own, ‘puddle hopping’ across from one area of open water to the next, breaking the ice as they went and making very slow progress at just 3-5 knots.

The South Africans have an unusual technique for loosening a ship in ice. They swing the ship’s crane from one side to the other which keels the ship and loosens the pack ice so that she doesn’t get stuck. Whenever the ship came to a grinding halt, it would be surrounded by up to 300 crab-eater seals (mis-named by early explorers as there are no crabs in Antarctica). In open water they were also accompanied by minke whales and emperor penguins – the team didn’t expect to see so much wildlife in such a remote area.

Ships mariners who work in Antarctica rely on satellites for mapping routes through the sea ice and Dr Shears and his team partnered with the German space agency, DLR, which has a cloud penetrating satellite that gave them really detailed imagery. They created a 12 nautical mile by 6 nautical mile search box over the wreck site and, within four hours of arriving, had dropped an AUV to start searching the sea bed for Endurance. The AUV had been programmed to carry out 11 sea bed transects in a 44 hour mission, but after 30 hours and 7 transects, disaster struck and they lost communication with the AUV. A large search operation was launched to find the AUV, but the temperature was dropping from -5C to -20C and the sea ice was closing in. With safety the crew’s top priority they were forced to abandon the search for Endurance and the AUV, which may have held the clue to the wreck’s location. With current technology it’s not possible to transmit the data live from the AUV, so the results of the search and the final resting place of Endurance remain trapped beneath the ice on the sea floor.

Questions

How was the expedition funded?

The expedition was fully funded by a Dutch charity - the Flotilla Foundation – which was recently set up to help protect the world’s oceans. The charity’s primary aim was to find the wreck of Endurance, but Dr Shears explained that, as this area of the Weddell Sea was so difficult to get to, it was essential to include a scientific element to the expedition as well.

Will there be a Weddell Sea Expedition II?

Dr Shears said that they’d love to go back as there’s still unfinished business with the wreck, and they’ve only scratched the surface of the scientific work that could be done. He says he is quietly confident that in two or three years, there might be a Weddell Sea Expedition II. He added that a condition of the wreck search permit from the FCO was that the expedition wouldn’t touch the wreck or remove anything from it (not that they planned to anyway).

In a future expedition they could search for the lost AUV and hope to recover the data, but there is always a risk that they wouldn’t be able to get there another year, due to the sea ice. Analysing data from previous years, Prof Dowdeswell’s team looked for patterns that would help them predict future sea ice cover, but found no scientific correlations that would help them predict the ice conditions for the next expedition. They always had a back-up plan in case they couldn’t get access to the West this year, and that was to carry out scientific work in the East Weddell Sea instead.

Could the recently launched RRS Sir David Attenborough be used for the next expedition?

The RRS Sir David Attenborough is a very similar vessel to the Agulhas II so it would be capable, but Dr Shears said that it would be up to British Antarctic Survey to decide whether they wanted to use their new ship for an expedition of this kind. Prof Dowdeswell added that in an ideal world it would be a two ship operation, with one breaking the ice and the other following and doing the research, but that would obviously increase the cost dramatically.

When will the scientific results be published?

Post-doctoral money was already secured to be able to process all of the data which was a great help. The marine geophysics research can be turned around fairly quickly and Prof Dowdeswell submitted an article to the journal Nature very soon after delivering this talk. But the marine geology takes a lot longer as the cores have only just arrived in the UK and are being analysed now. Realistically the results of the carbon dating should be available in early- to mid-2020.

Why were you unable to recover the AUV?

The AUV had a beacon on it and, had it surfaced on the water, the team should have been able to find it. But the acoustic detection system doesn’t work in the same way when there is a layer of sea ice - the acoustic waves spread out horizontally beneath the ice, making it very difficult to detect them from the surface. Originally there was a back-up plan which was to use the ROV to find the AUV beneath the ice but, unfortunately, the ROV had already been damaged and was out of action. This expedition had more kit than a polar research vessel has ever had before, so they were incredibly lucky, but it would have been a big ask to have requested more.