Mapping the Seafloor: how sound revolutionized our understanding of the bottom of the ocean

People have been mapping the seafloor for ages.  A statue found in an Egyptian tomb from 2000 BC shows a seaman using a weighted line to measure the depth of the water ahead of his vessel.  For the next almost 4000 years, seafarers used variations of the same technique – dropping a weighted “lead-line” to measure the depths of the oceans.  Most of these efforts were for the same reason the Egyptian seafarer was measuring depth – to ensure the safe passage of vessels.   As the age of exploration evolved, curious mariners would also “heave the lead-line” as they crossed the deep oceans.  Sometimes they were able to measure the depth but other times, as was the case for Magellan in the middle of the Pacific, their lines were just not long enough and the waters termed “unfathomable.”   As you can imagine, trying to tell when a line hits the bottom in thousands of feet of water is VERY time consuming and VERY inaccurate.   Nonetheless, using a lead-line, Fridjof Nansen made seven depth measurements as his vessel FRAM drifted in the ice across the Arctic from 1893-1896, determining for the first time that a deep ocean existed beneath the Arctic ice.

 

NANSEN_SOUNDING

(Nansen’s Team taking a depth sounding. Courtesy of Arctic Museum Tromso, Norway)

 

In the early 20th century, we developed echo-sounders to measure the depth of the ocean. Light does not travel very far in water, which is why we can’t use cameras to map the deep sea floor like we do for land on earth and on other planets.  However, sound travels very far in water and if we measure the time it takes for a sound to leave the ship, bounce off the seafloor, and come back, we can measure the depth much more rapidly and accurately.  Echo sounding techniques were developed during the Second World War to find submarines.  After the war, research vessels began to carry echo-sounders but this measurement represented an average depth over a relatively large area of the seafloor.  With these measurements, a broad picture of the shape of the ocean floors began emerged and knowledge of vast ocean-spanning mountain chains (ocean-ridges) and deep trenches evolved leading to the development of the theory of plate tectonics that fundamentally changed our view of earth processes.

 

MBES_PIC

(Courtesy of Atlas Hydrographic)

 

In the late 20th century, multibeam echo-sounders were developed.  These new echo-sounders were able to provide many (often hundreds) of very accurate measurements over a wide (several miles in deep water) swath of the seafloor all at once.  These, combined with new 3-D computer visualization techniques, have revolutionized the way we explore the seafloor; we now have the resolution to see small features that can provide important insight into the processes at work at the bottom of the ocean.  This capability is particularly important for looking at the history of glaciation.  Glaciers are remarkably powerful, acting as rivers of ice that carve the landscape they traverse.  For many years, land geologists have traced the history of the ice ages by hiking around and using aerial photos to map the impact of these glaciers on the landscape.  The history of glaciers does not stop at the water’s edge, however, and with modern multibeam echo sounders, we can map features on the seafloor that show us the direction of ice movement and where it stopped advancing offshore.

 

3DBATHY_PETERMANN

(A multibeam image of Petermann Fjord – looking from Ice Shelf out to entrance of fjord)

 

Petermann Fjord is one of the key routes by which ice from the Greenland Ice Cap finds its way to the ocean and melts (the ocean waters are warmer than the ice). We set out to understand the processes by which this melting occurs because large losses of the Greenland ice can lead to increased sea levels. We are using our multibeam echo-sounderto map features of the Petermann Fjord and its surroundings that indicate how far the ice sheet has advanced and retreated in the past. By comparing these findings to sediment cores collected by others in our team (that record a history of environmental conditions) we can relate the advances and retreats of the ice shelf to changes in both the regional and local climate conditions and thus better understand the forces that are controlling the build-up and melting of the glaciers. With this increased understanding of the relationship between climate and ice sheet behavior, we will hopefully be able to better understand the impact of future climate change.

 

Written by: Larry Mayer

 

 

Searching for the best seafloor muds in the Arctic

 

Right now, in the middle of the British summer, I am sitting surrounded by ice offshore Petermann Glacier in northwest Greenland trying to find the best place to grab mud samples from the seafloor. So, how do we know where to look for the best muds?  Well, we map the seafloor using instruments mounted on the bottom of our icebreaking vessel – you can think of this as remotely-sensing the seafloor in the same way that satellites orbiting the Earth acquire data about sea surface temperatures or take images of the Earth’s surface. Using multibeam echo-sounders we can produce a map of the seafloor in 3D that shows us where forms sculpted by glacier ice might be, allowing us to determine where we should extract our precious muds.

 

We also use another instrument to image the layers of mud that are present beneath the seafloor. The acoustic pulse sent into the water by that instrument is called a “Chirp”, so-called because the sound it makes is a loud, discrete “chirping” noise similar to a perfect, repeating bird tweet. On the lower decks you can clearly hear this sound being transmitted every few seconds – chirp chirp chirp chirp…  Although this sounds annoying, you get used to it, and some of my shipmates even say they can’t fall asleep without it. In fact, a few nights ago, I woke up in the middle of the night worried and wondering what was going on because I couldn’t hear the chirping!

 

Subbotprofile_PetermannFjd

(Sub-bottom profile of thick layered seafloor muds. The total thickness of the layers shown here is about 35 m)

 

Once we have the 3D maps of the seafloor and images of the mud layers below we choose the best locations to recover what are called marine sediment cores. These are long cylinders of marine mud – imagine a drainpipe pushed into the soil in your garden and then pulled back out again giving you a perfect tube of the different soil layers. We do exactly the same on the seafloor using different types of coring instruments depending on whether we want the longest core possible or to, say, only recover the uppermost muds and a sample of the overlying water from the very bottom of the ocean. These layers of mud, sands, and gravels are laid down over time with the youngest material at the surface so that the deeper you penetrate with your “drain pipe” the older the material you recover.

 

MarineMud

 

Up here, in Petermann Fjord and Nares Strait, we are collecting many cores to tell us about how the massive Petermann Glacier behaved in the past. We can look at the types of muds to tell us how close the glacier was to that location, and we can radiocarbon date the shells of some of the critters that used to live on the seafloor at the time to tell us when that happened. The species of critters – usually bottom-dwelling plankton – also tell us about the water masses that the animals lived in, i.e. was the fjord full of fresh, glacial meltwater or was it full of warmer waters that travelled in with ocean currents?  In this way, we can build a picture of how the glacier grew and melted in response to things like climatic change and to past changes in ocean circulation.

 

PetermannFjord1

(Petermann Fjord & Glacier)

 

One of the most amazing things about this project is that we have been able to collect sediments from BENEATH the floating tongue – or shelf – of ice that sticks out from the end of the Petermann Glacier. This is the first time EVER that muds have been recovered from beneath an ice shelf in Greenland. This means that we can find out what types of muds are found beneath floating ice shelves today and then we can look for examples of these in our dated marine cores. You might wonder how we can get sediments from the seafloor beneath 100 metres of floating ice… the answer is to drill a hole through the ice shelf using hot water and then lower a coring device through that hole to access the seabed. The corer is similar to what we use from the ship except that it is slightly smaller to fit down the drilled hole. I was the lucky one who got to cut the “drain pipe” full of mud up into sections and pack it up to bring back to the ship by helicopter from the ice shelf drill site. And, what is it like to have your working laboratory on an ice shelf? It’s a beautiful mixture of white, grey and the purest blue you’ve ever seen in the meltwaters in surprisingly hilly terrain dotted with sneaky round meltpools hiding under a skin of ice. Perhaps, unsurprisingly, it also gets very cold if the sun is not shining on you. However, it’s also one of the quietest, purest and most peaceful places I have ever been. Not your average laboratory space, that’s for sure.

 

Alan_at_iceshelf

(Alan visiting the ice shelf operations)

 

08-09_KellyCores

(Kelly brings back core samples, the first ever extracted from beneath an ice shelf in Greenland)

 

Written by: Kelly Hogan

Marine Geophysicist, British Antarctic Survey, UK

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