Blown away by the eons of time

By: Chief Scientist, Bramley Murton.

The week-end has been dogged by lumpy weather. A nasty storm cyclone has hit us and we have had to run away, 200 miles to the North. The storm is unusual for this time of year, and is exceptionally deep. It has tracked from the west right across the Rio Grande Rise, lashing the ship and stopping science. Luckily we recovered all our gear before it hit.

A nasty wee storm finds us and sends us packing for the weekend.

The Autosub6000 being recovered after its second mission over the Rio Grande Rise.

Just after recovery, within a couple of hours, the sea picked up and we decided to postpone the next HyBIS dive. This was a pity as we have made some interesting discoveries on the last dive. However, without the heave compensation working on our main winch, we can’t use HyBIS in heavy seas as it is too hard to control. The break will give the HyBIS team time to make some minor adjustments to the motors and lights to optimize the next dive after the storm passes.

The sea get a wee bit lumpy shortly after recovering the Autosub6000.

The last HyBIS dive was made over the southwestern side of the Great Rift. Here we started at the top edge of the scarp and made our way slowly down a few terraces. The sight was amazing: ferromanganse crusts were there, as we predicted, but they have a polygonal pattern on the surface, resembling a large honeycomb. Closer inspection shows the crusts are being eroded by patches of sand in the centre of the polygons leaving them to resemble cracked dried mud.

Polygonal pattern to ferromanganese crusts covering lavas on the edge of the Great Rift.

Further down the slope we found lavas exposed. These were also polytonally jointed. The jointing on the lava surface turns into vertical joints (cracks) underneath, forming columns. This type of structure is well known in lava flows from land – a classic example is the Giant’s Causeway in County Antrim, Ireland. The difference here is that the lavas we are looking at are 900m below sea-level, yet they look like subaerial eruptions. In contrast, submarine lavas form distinctive pillow shapes. This is because the water quenches the lava and forms a skin, like an amniotic sack, that stretches as the lava fills it until it forms a bulbous shape. Where we get sheet flows on the seafloor, they rarely, if ever, form columnar jointing. Is this evidence that we are looking at eruptions on the Rio Grande Rise when it was an island?

Columnar-jointed lavas on the edge of the Great Rift, Rio Grande Rise.

The vertical face of the scarp exposing the lava flow. The scarps is 30m high and forms the edge of the Great Rift.

Red clay and mud underneath the columnar jointed lava, and on top of another lava flow, 900 metres below sea level.

Our suspicions were confirmed at the bottom of the terrace where the columnar-jointed lava gave way to an underlying bed of red clay. The clay deposit merged into the top of another lava flow beneath the first. Geologists recognize this mud as a bole, the top of a lava flow that has been weathered by sub-tropical and humid conditions, oxidizing the lava and turning its iron content red, and turning the rock into clay minerals. This is in effect an ancient soil, or as we call it, a palaeosoil. Millions of years ago, volcanoes erupted lavas on the Rio Grande Rise when it was an island. Sun and rain eroded the top and transformed it into soil. Plants probably grew in the sunlight and who knows what animals grazed on them. Then, after thousands of years, the volcanoes erupted again and buried the landscape in another flow of white-hot lava and everything was incinerated. As it cooled, the lava contracted forming the columnar jointing we see today. This was probably followed by further cycles of weathering, soil formation, plants growing, animals grazing, lava eruptions and incineration. We are just seeing a snap shot in the eons of time that have formed the Rio Grande Rise when it was the equivalent of a sub-tropical Iceland, and long before it was drowned and submerged beneath the South Atlantic.

Back up on the top of the scarp we see the lavas give way to a sediment-covered flat seafloor strewn with large rounded boulders. These are made of the black lava, but are smooth and round. Only in areas where there is very high energy can such boulders form. Areas such as the beds of fast flowing rivers, or on the sea shore where waves can crash and tear at the cliffs, dislodge chunks of rock and roll them around until they form smooth and round boulders. Certainly not at the bottom of the ocean, 700m below sea level, where such waves can’t affect the seafloor. Here the energy is low, and currents are slow, moving only the smallest of sand grains. Here, then, we are seeing evidence for the drowning of the Rio Grande Rise, when the sub aerial lavas became sea cliffs and the waves churned the rocks into a boulder strewn shore.

Boulders strewn over the seafloor close to the edge of the Great Rift are evidence that this was once a sea shore where waves crashed and tore at the cliffs, long ago as the Rio Grande Rise was drowned.

Further back from the edge of the Great Rift, the boulders give way to a hard seafloor covered in calcareous sands. These sands are derived from the skeletal remains of plankton in the overlying ocean. The sediments are thin, only a few tens of centimeters, and the seafloor underneath is hard when HyBIS lands to take a scrape. The underlying seabed is also calcareous, made of crushed carbonate shells forming a sandstone. Was this once a sandy lagoon behind the cliffs and boulder strewn seashore? Almost a kilometer back from the edge of the Great Rift we come across an increasing number of chunks of black ferromanganese crust lying on the white pelagic sediment.  Where have they come from? The answer lies another 100m west, where we come across a ledge that cuts across the seascape.

Terrace with ferromanganese crust on the top overlying a calcareous sandstone.

Only 2 metres tall, the ledge is capped by 20cm thick layer of black crust. Underneath the crusts is the hard calcareous sandstone seafloor, here eroded and exposing the layers of crushed shelly sand. The edge of the ledge is undercut and pieces, some a metres across, of the crust have fallen off or are overhanging the ledge. HyBIS tries to take a sample, but it is too big. Eventually we get a couple of pieces that fill the basket. Another beach terrace? Perhaps, but this one has been cemented and covered by the ferromanganese crust that we are searching for. We know that the crusts grow at about 1 to 3 mm per million years, so again, we are confronted by a scene that has been frozen in time for tens of millions of years; a fossil shore line from the distant past, now drowned and lost beneath the stormy South Atlantic ocean.

First sight of the Rio Grande Rise

By: Chief Scientist, Bramley Murton.

The past couple of days we have been diving with HyBIS over the northern side of the great rift that divides the Rio Grande Rise in two. The Rio Grande Rise is a volcanic plateau formed 80 million years ago when a hot spot in the mantle underlay the Mid-Atlantic Ridge. As the African and South American tectonic plates separated, the hotspot caused an excess of volcanism generating an island rather like Iceland. The trail of the hotspot can be seen on each side of the Mid-Atlantic Ridge as the Walvis Ridge in the east and the Rio Grande Rise in the west. With time, the island plateau subsided beneath sea level to its present position we see today.

The tectonic position of the Rio Grande Rise and its conjugate, the Walvis Ridge.

The Rio Grande Rise, the Great Rift, and the location of our study area.

Prior to diving, we have to carefully plan the dive so that we know exactly what we are aiming to see – and each dive aims to test a hypothesis. In our case, we are looking to see if the cobalt rich crusts vary in outcrop and thickness with increasing distance from the edge of the Great Rift. The reasoning is simple: the Great Rift is over 1400m deep and guides the direction of currents and tides within it. As these moving water masses approach the walls of the rift, there is friction in the form of turbulence and eddies that create a more energetic environment, suppressing sediment accumulation and allowing the crusts to grow. This energetic environment is likely to be different to the north and south of the rift, as a result of geostrophic effects that cause the currents to turn. Further away from the edge of the rifts, the energy is likely to be less, and we expect to see an increase in the extent and thickness of the sediment cover. Hence our dives are designed to survey and sample at different locations with increasing distance from the rift walls.

Tim Le Bas, our survey and data manager, plots the locations of the dives on our new bathymetry maps we have made with our multibeam echo sounder system. We then transfer the maps to the HyBIS team. After launching the HyBIS, we start-out across the seafloor making a detailed video survey and sampling the rocks and biology.

Co-chief scientists Paul Lusty (standing) advises Tim Le Bas where to place the tracks for the next HyBIS dive mission.

 Launching the HyBIS is a quick procedure. The vehicle is prepared for the dive in a covered hanger inside the ship and wheeled out under the gantry where it is lifted into the air and dropped into the water. Floats are attached to the cable to keep it above the HyBIS when it lands on the seafloor. The whole operation takes about ten minutes. Recovery is the reverse, and is equally as fast.

The deck crew launch our HyBIS robotic underwater vehicle over a calm sea from the strarboard side of the side of the RRS Discovery as evening draws in.

The science team watch as the HyBIS ‘flies’ across the seafloor.

Meanwhile, the AUV team have been struggling to prepare the Autosub6000 robotic submarine for its first dive. One of the risks with shipping delicate equipment across the world is that things can be bumped in transit. We think this had happened to our internal navigation module, and as a result the team have spend a lot of late nights integrating a spare system in the vehicle. It is now ready for its first mission.

The robotic submarine Autosub6000 being prepared for its first mission over the Rio Grande Rise.

ENDs

Discovery voyage DY094: Minerals and life at the Rio Grande Rise

By Bramley Murton, chief scientist

The Rio Grande Rise is a lost land of dinosaurs, ravines and plateaus the size of Wales that formed 72 million years ago by huge eruptions of volcanic lava and drowned 22 million years ago. Now 700 m below sea level, the Rio Grande Rise lies 1400 km east of Brazil, in the South Atlantic. Surrounded by water over 3000 m deep, the relatively shallow Rio Grande Rise is of interest for seafloor mineral deposits rich in iron, manganese and other metals that are important to modern society.

Map showing the location of the Rio Grande Rise and mineral exploration blocks (in red) licensed to the Brazilian Geological Survey (courtesy of GEBCO).

Two of these metals in particular are critical to any future effort to reduce our dependence on hydrocarbons: cobalt and tellurium. Cobalt is essential in rechargeable batteries that are needed if we are to move to electric vehicles. Tellurium is essential for high-efficiency solar-electric power generation. Our voyage aims to enhance understanding of the processes controlling the formation and composition of these deep-ocean mineral deposits and the biology that colonises them.

Cobalt-rich crusts recovered during the MarineE-tech programme in 2016 by the robotic submarine Isis, North Atlantic (courtesy, B Murton).

Cruise DY094 sailed from Santos, Brazil, on the 20^th of October with a scientific team from the National Oceanography Centre, British Geological Survey, University of Edinburgh and the University of Sao Paulo. We have with us the autonomous robotic submarine Austosub6000 and the remotely operated submarine HyBIS. With these machines, we will explore the Rio Grande Rise, mapping it in great detail with our sonars and filming and sampling the seabed mineral deposits and their biology.

RRS Discovery in Santos with the yellow Autosub6000 on the after deck (courtesy: Paul Lusty).

Entering the secure dock areas and boarding the RRS Discovery at the port of Santos, Brazil. RRS Discovery in Santos with the yellow Autosub6000 on the after deck (courtesy: Paul Lusty).

Science team discuss their plans for the voyage while on their way to the Rio Grande Rise, on board the RRS Discovery  (courtesy: Paul Lusty).

Map showing the location of one of our study areas where we have started exploring the sea floor (courtesy of our partners at the University of Sao Paulo).

So far we have seen a varied and fascinating seafloor includes a huge rift over 1500 m deep and 250 km long that cuts the Rio Grande Rise in two, mysterious sinkholes, and the ancient remains of beaches long since drowned under hundreds of metres of water. Although ours is purely a scientific voyage of discovery, our results will tell us a lot about the potential value of the mineral deposits to future renewable energy industries and how vulnerable the life on the seafloor is to mining.