Rivers of the Deep

This is largely new science that is waiting for an effective mapping technology that can be put on a satellite.  Yet it is also pretty obvious that the system must be huge and extensive to properly rebalance the natural inflows, temperature changes and as stated even the Cariolus effect.

Mud flows are more random but also far more able to alter terrain.  Deep structures likely exist everywhere. 

The region I would like to see mapped and continuously monitored is the South Atlantic.  It happens to be the choke point and key transfer point between the two major oceans and it is large enough to avoid it going in one direction.

Cryptic river: The torrents that flow on the seabed

24 February 2014 by Katia Moskvitch

They gush along the ocean floor and can wipe out the internet. Need another reason to understand the planet's underwater rivers?

http://www.newscientist.com/article/mg22129570.700-cryptic-river-the-torrents-that-flow-on-the-seabed.html?full=true#.UwzgtPldUtQ

UNDERNEATH the Bosporus Strait flows a mysterious river. It has banks and rapids and in places is a kilometre across. If it snaked across the land, the volume of water careering through it per second would make it our sixth largest river after the Amazon, Ganges, Congo, Yangtze and Orinoco. Yet the crews on board the ships that ply the strait between the Sea of Marmara and the Black Sea don't even know the river exists. It flows silently 70 metres beneath them before reaching the edge of the sea shelf and disappearing into the deep.

The hidden river has no name, yet is by no means unique. Myriad underwater rivers criss-cross the ocean floor, some many thousands of kilometres long, tens of kilometres wide and hundreds of metres deep. They are the arteries of our planet. They shunt sediments into the deep, carrying with them the oxygen and nutrients that allow life to thrive at great depths. They also seem to be a vital part of the world's carbon cycle, burying organic matter carried from the shore.

How these rivers work is a mystery, though. Satellites cannot reach so far underwater and sonar and radar from the surface are little help. Now at last, underwater submarines and lab experiments are opening a window on these enigmatic rivers under the sea. Their findings could be a boon for everyone from climate scientists to telecommunications and oil companies.

The seabed is home to the strangest rivers on the planet (Image: Peter Adams/AWL Images/Corbis)

Drain all Earth's oceans and you would find that underwater rivers have gouged a maze of conduits known as abyssal channels. The rivers that rush through them may look like terrestrial rivers, but they behave much more like avalanches, dust storms or the pyroclastic flows from a volcano. Their destructive power makes them a major hazard for the telecommunication cables that snake along the ocean floor carrying most of our transoceanic voice, data and internet traffic. "If you dangle a cable across a channel that is hundreds of metres deep and several kilometres across, and an underwater flow comes down, the cable will snap," says James Gardner of the Center for Coastal and Ocean Mapping at the University of New Hampshire in Durham.

Today, companies hope to avoid abyssal channels by surveying the ocean floor before laying any cable. If a channel's banks are steep, any cable laid across it will be suspended mid-water and more prone to accidental damage from fishing equipment and anchors, says Stuart Wilson, cable route engineering manager at Global Marine Systems in Chelmsford, UK. Put a cable in an underwater river and it will strum like a guitar string so much that there is a risk of abrasion as it rubs away at the banks, he says.

Yet it took some time for cable-laying firms to become aware of underwater rivers and their destructive potential. When Queen Victoria sent a congratulatory telegram to US president James Buchanan on 16 August 1858, in honour of the first-ever transatlantic cable, long telegraph cables were already zigzagging across the bottom of the Mediterranean and the Red Sea. The ships lowering them to the seabed were blissfully unaware of any treacherous underwater flows. And for several decades, luck was with them.

All that changed at 5.02 pm on 18 November 1929 when a powerful earthquake struck 250 kilometres off the south coast of Newfoundland, Canada. At magnitude of 7.2, it was a strong one and yet the damage on land was minimal – a few chimneys fell and some roads were blocked by minor landslides.

At the bottom of the ocean, it was a different story. The Grand Banks earthquake switched on an underwater river. Some 200 cubic kilometres of sediment came tumbling down a slope, snapping a dozen transatlantic telegraph cables and depriving the Canadian coast of communication. Twenty-eight people perished as the subsequent tsunami swept the shore.

Mapping the snapping

At the time, the cable breaks were blamed on the earthquake. It was another two decades before researchers identified the real culprit. Geologists Bruce Heezen and Maurice Ewing at Columbia University in New York analysed records from Grand Banks, noting where and when each submarine cable snapped. Their conclusion? The earthquake had sent sediment tumbling off the continental shelf. A mixture of mud and seawater swept across the seabed at tremendous speed, snapping one cable after another. Heezen and Ewing's calculations showed that the flow of this "turbidity current" reached speeds up to 100 kilometres per hour, sculpting banks in the seabed and travelling 600 kilometres down the continental slope from the earthquake's epicentre.

At the time of Heezen and Ewing's discovery, geologists had begun to suspect that the ocean floor was criss-crossed by abyssal channels. Early, crude underwater mapping in the late 1930s and 1940s had uncovered deep lacerations etched out by erosion at the world's continental margins, just as the Grand Canyon bites into the land. Geologists suggested that these features were the result of underwater erosion caused by sediment-laden turbidity currents. But it took Heezen and Ewing's research to work out what could trigger and shape these currents.

Turbidity currents are still snapping submarine cables. With 95 per cent of international phone, internet and data transmissions buzzing through them, a break can leave an entire region incommunicado. In 2006, for example, an earthquake near Taiwan sparked a violent turbidity flow that plunged the island into a temporary communication blackout. At least 16 cables broke causing havoc across South-East Asia.

With so much at stake, it's perhaps surprising that we have better maps of the channels criss-crossing Mars and Venus than we do of the abyssal channels on Earth. "It would take hundreds of years of ship-time to find and map them all," says Gardner. "We need to wait for future fleets of remote vehicles to do it." Ironically, it's often a break in a submarine cable that alerts a cable-laying company to a channel they'd failed to spot.

Getting them on the map is one thing; understanding them is quite another. Strangely, it turns out that underwater rivers "dry up", just as terrestrial rivers do in a drought. They are still filled with water, of course, but there is no flow of mud and sand through them. Instead, it takes a powerful event to get a turbidity current going again. The trigger could be an earthquake, or sediment that has accumulated at the top of a canyon and suddenly collapses under its own weight.

Or it could be a terrestrial river running into the sea. Take the river Congo, for example. By the time it reaches the Atlantic, its water is rich in sediment. So much so that it can trigger an underwater river that flows far into the ocean, carving an abyssal channel in the seabed as it goes. Similarly, the Yellow river in China tumbles into Xiaolangdi reservoir and gives birth to an abyssal channel. The sediment in the river makes its flow denser than the water in the reservoir – so if you were to sit in a boat in the middle of the reservoir, you would see the Yellow river slide by underneath.

Although most abyssal channels seem to have been formed by rivers and can be traced back to them, some have been spotted right in the middle of an ocean. Gardner, who routinely maps the Pacific, has discovered gigantic channels nowhere near land. "It's hard to explain why they're there," he says, "but they're there."

Whether they are in an ocean or in a lake, muddy, rapid turbidity currents are slippery customers. "People have tried to put instruments down into them to measure velocity and sediment concentrations, and the currents usually destroy all the tools," says Brian Romans, a geologist at Virginia Tech in Blacksburg.

One underwater river, however, is friendlier than others. The huge one under the Bosporus Strait is less muddy than most. Instead of sediment, it consists of much saltier water that has flowed from the Mediterranean through the Sea of Marmara. This salty water makes the current denser than the surrounding waters of the Black Sea. And the density difference is enough to drive the flow along the sea floor. Although the composition of the current is different from its muddy submarine cousins, the dynamics of the flow are still the same, says Jeff Peakall, a sedimentologist at the University of Leeds, UK. And being in relatively shallow waters, the channel is much easier to study.

Round the bend

Peakall took a torpedo-shaped robotic yellow submarine to the Bosporus in 2010, and again in July 2013. His team recorded the first-ever detailed measurements of the flow through a abyssal channel – and were surprised by what they found. For starters, the channel twisted and turned like a disturbed rattlesnake. Terrestrial rivers, of course, can also be wiggly or straight depending on the surrounding geography and geology. But underwater rivers are different. Peakall has analysed abyssal channels around the world. "All the wiggly ones are close to the equator and when you go to the poles, they all become straight," he says.

But why? Peakall suspected that the culprit might be the Coriolis force, which deflects objects moving in a rotating frame of reference. It's the Coriolis force that largely controls the circulation of Earth's atmosphere and oceans – without it, we wouldn't have hurricanes or the Gulf Stream. Its effects are strongest at high latitudes and weaken towards the equator. So could the Coriolis force affect turbidity currents too?

Unable to check out different abyssal channels directly, Peakall turned to experiments in the lab. Together with Mathew Wells and Remo Cossu of the University of Toronto, Scarborough, he mounted a 2-metre-long tank on a spinning table and filled it with water. On the bottom of the tank they built a meandering acrylic channel, and introduced a dense saline flow to simulate a muddy sea-floor turbidity current. Then they spun the tank at different speeds to mimic Earth's rotation at different latitudes and measured the flow along the channel.

What they found showed that underwater rivers are dramatically different to their terrestrial counterparts. In both cases, the movement of water around a bend is controlled by a combination of forces. The main ones on land are the centrifugal force that pushes water outwards when it flows quickly around a bend, and the force from the weight of the water.

But for turbidity currents, Cossu, Wells and Peakall found that the Coriolis force has an impact too. To understand why, think of how much lighter a brick feels underwater than it does on land. That's because the upward force of the water partly counters the downward pull of gravity on the brick.

Similarly, the force from the weight of a turbidity current is weakened by the surrounding water. With the weight of the water no longer the dominant force in a turbidity current, the Coriolis force plays a far greater role than it does on land. It pushes the flow to one side and leads to a huge height difference between the sides.

This altered flow leads to underwater rivers taking a different shape and depositing their sediment differently too. Because the Coriolis force is larger nearer the poles, the combined forces produce an underwater river that flows much straighter than it does near the equator.

What's more, channels that zigzag typically do so in a very different manner from rivers on land, says Peakall. When water in a river flows around a bend, the forces conspire so that water close to the riverbed flows from the outside of the bend to the inside as it moves downstream. Close to the water surface, there is an overall movement of flow from the inside of the bend to the outside. In an underwater river, however, the balance of forces is altered. "It was quite amazing to see the channels do just the opposite," says Peakall.

There are more oddities. Terrestrial rivers never stay in one place, because their banks constantly erode they migrate and move slowly sideways across their floodplains. Submarine channels don't do this. Once they reach a certain level of "wiggliness", they build up vertically for hundreds of metres instead.

There is more to finding and studying abyssal channels than pure scientific endeavour. There are also massive economic incentives. When oil companies look for drilling sites offshore, their goal is ancient channel structures 2 to 4 kilometres below the sea floor. When these channels were still active, turbidity currents brought in sands and muds. Many of these sands are now reservoirs keeping oil and gas trapped in the pores between the grains. The oil and gas migrate upwards from more deeply buried muds, where ancient plants and animals decayed thousands of years ago.

Once a channel is found, "our understanding of the sediments filling the channel becomes very important", says Mike Mayall, a sedimentologist at energy company BP. Advances in seismic data are helping us get to grips with the nature of sediment. Existing technology allows oil companies to visualise the distribution of sands and muds, with a resolution better than 10 metres in some places.

Then there is the global carbon cycle. A constant flow of organic material from land to the ocean depths may play a key role in it. Although organisms on the sea floor consume some of the transported carbon, part of it gets buried in marine sedimentary deposits, never to re-enter the atmosphere. "It's a hot area of research now," says Romans. "People are trying to quantify how much carbon is transported and buried, at what rate, and how it affects the global carbon cycle." A better understanding of abyssal channels will help to model climate more accurately.

Each time a submarine dives into an underwater river, we learn a little more. Every river modelled in the lab helps us plumb the unknown depths of what happens under the waves. The underwater rivers are slowly but surely giving up their secrets.

This article appeared in print under the headline "Cryptic river"