Science communication

Rivers under the sea

A guest post from Professor Jeff Peakall, University of Leeds

Professor Peakall’s Shell Lecture, ‘Rivers Under the Sea’, can be viewed on our YouTube channel:

Satellite image of the Lena delta (Image courtesy of Fugro)

Satellite image of the Lena delta (Image courtesy of Fugro)

Channel networks are known from across the solar system. Rivers on Earth have long been key to human civilisation, with many of our great cities developing around them, and their regular flooding continuing to attract abundant media attention.

Recently, a series of space probes have brought us spectacular images and increased knowledge of channels from elsewhere across the Solar System, from the giant (up to 3500 km long) enigmatic channels of Mars, through to the sinuous channels of Venus that appear to be the product of lava flows, to the most recent discovery of flowing channels on Titan, Saturn’s largest moon. Titan’s channels are almost unimaginably cold, transporting liquid methane across the surface of the moon.

Martian channels. © ESA/DLR/FU Berlin (G.Neukum)

Martian channels. © ESA/DLR/FU Berlin (G.Neukum)

There are another set of channels in the Solar System almost unknown to popular science, albeit increasingly studied by geologists. They traverse the ocean floor of our own planet, and outside of rivers are the longest channels we know (up to 3800 km long). They form in a whole series of different locations, from passive margins where they form the arteries of submarine fans (the largest sedimentary deposits on Earth), through deep-ocean channels, to channels that form away from ocean margins. These channels are important for moving sediment to the deep ocean, play an as yet poorly known role in global carbon budgets through the burial of onshore derived organic matter, and their ancient deposits are important reservoirs for hydrocarbons.

We know surprisingly little about the flows that traverse these channels, but what we do know is that some of these channels are still active today, fed by mixtures of sediment particles (mainly sands and mud) and water, that are denser than the surrounding seawater. Terrestrial cousins to these deep-water density currents include snow avalanches and pyroclastic flows. These flows pose major hazards to sea-floor engineering, in particular the sea-floor cables that account for ~95% of all trans-oceanic voice, data and internet traffic.

A 200 kilometre (125 mile) long section of the modern Amazon Fan, showing segments of several highly sinuous submarine channels up to 4 km wide, with banks 100 m high. From Amos and Peakall (2006) courtesy of IFREMER.

A 200 kilometre (125 mile) long section of the modern Amazon Fan, showing segments of several highly sinuous submarine channels up to 4 km wide, with banks 100 m high. From Amos and Peakall (2006) courtesy of IFREMER.

So why are these behemoths of the deep oceans almost unknown in popular science? Why, when we have such fascination for channels on other planets and moons – even when all we have are dim images – is there so little media coverage for these giant channels on Planet ‘Ocean’? The only recent popular science articles that I am aware of are one I co-authored in NERC’s Planet Earth Magazine (2006), and one in the US-based Earth magazine in 2012, looking at global controls on submarine channels. Compared with the recent widespread coverage of Titan’s channels this is remarkably limited. Perhaps readers can help me out with other examples.

Perhaps part of the problem is that until recently we had relatively poor imagery of these channels, buried as they are under kilometres of water which provides an effective shield to many airborne and satellite based measurement techniques. However, advances in acoustic multibeam echosounding techniques, and deployment of these not just from surface ships but remote controlled submarines is changing this, and providing some stunning imagery. See the Coastal and Ocean Mapping/Joint Hydrographic Center for some examples.

A three-dimensional view, in false colour, of the underwater Black Sea channel. Adapted from Parsons et al. (2010), Geology, 38, 1063-1066.

A three-dimensional view, in false colour, of the underwater Black Sea channel. Adapted from Parsons et al. (2010), Geology, 38, 1063-1066.

Until recently, there has also been a belief that submarine channels were rather similar to rivers. Recent work suggests that the channels can in many cases grow and evolve very differently, leading to systems perched hundreds of metres above the sea-floor. Advances in our understanding of the flows within these channels has revealed a wide range of flow types, from dilute and turbulent through to high concentration cohesive, low turbulent flows. The three-dimensional flow fields can also be very different and almost certainly change downstream, so that in places they are similar to rivers, yet elsewhere are dramatically different. Finally, unlike river channels, the sinuosity (wiggliness) of submarine channels appears to change with latitude, so that all the highly sinuous examples are close to the equator whilst at higher latitudes channels are essentially straight.

The origin of this variation is debated, but appears to be due to either the spin of the Earth (Coriolis force) or latitudinal changes in sediment and flow type. In either case, it suggests that the sinuosity of submarine channels varies not just with latitude but also over geological time. If Coriolis force is the key then as the spin of the Earth has been decreasing over geological time, ancient high sinuosity channels would have been restricted to an even narrower zone around the equator. Alternately, if climate is the main driver then we might expect to see quite different submarine channel sinuosity distributions between glacially influenced and non-glacially influenced periods of Earth history.

With much improved imagery, and the realisation that these giant sea-floor channels are at times every bit as unusual as channel networks elsewhere in the Solar System, perhaps some of the excitement felt by those of us studying these dramatic systems will be extended to a broader audience.

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