Summer Under Arctic Ice
A conversation with WHOI geophysicist Rob Reves-Sohn
This month, an international team of scientists and engineers are exploring the seafloor of the Arctic Ocean while cruising aboard the Swedish icebreaker Oden. The science team is sending three experimental vehicles beneath the ice cap to search the Gakkel Ridge for what would be the first hydrothermal vents ever found in the Arctic Ocean. The cruise involves many disciplines, people, tools, ideas, and perspectives—not to mention many years of planning. But the origins of the expedition to take robots beneath the ice are playfully simple. Chief scientist Rob Reves-Sohn, a geophysicist at Woods Hole Oceanographic Institution, drew inspiration for the cruise in the bathtub. He wasn’t in the bathtub; his daughter was.
A bath inspired you to explore the Arctic? You’re making that up, right?
I know. It doesn’t sound like it should be a true story. But one day about 10 years ago, I was giving my daughter (who was a couple of months old) a bath and trying to make sure she was entertained. We had all this white soapy foam and all these toys in the bathtub. We had a Sesame Street submarine (with Elmo as the captain) that went under the foam. And somehow the thought just kind of clicked: “That’s what we need—robot submarines that can go under the ice to explore the Arctic Ocean.”
At the time, I was naïve. I didn’t realize that people had been developing robotic underwater vehicles for many years. I had no idea they even existed. And then, eventually I ended up at Woods Hole Oceanographic Institution, which has several robotic underwater vehicle labs. So, I definitely ended up in the right place.
Why have you targeted the Gakkel Ridge for your mission below the Arctic Ocean?
In 2001, a research cruise went to the Gakkel Ridge to dredge rocks from the seafloor. The ridge is 3 or 4 miles under water, so to get those rocks you use a long wire and basically lower a huge bucket with teeth and chains and drag it on the bottom. To collect as much data as possible, the scientists also put sensors on the wire to measure the temperature and clarity of the water on the way down.
In the deep ocean, the water is cold and clear. But on mid-ocean ridges, where Earth’s tectonic plates are spreading apart, you get a lot of heat and volcanism. Seawater seeps through cracks in the seafloor, gets heated up, and rises and vents out of the seafloor. You get plumes of slightly warmer, cloudy water that spread out like smoke from a smokestack.
To everyone’s complete surprise on that 2001 cruise, almost every time they lowered the wire into the water, they found evidence for hydrothermal plumes. This was really stunning, and nobody really knew quite what to make of it.
Why was that so surprising?
Because the theory was that mid-ocean ridges are kind of like car engines: The faster they go, the hotter they get, and the more water you need to cool the system.
When you have two tectonic plates that are moving apart very rapidly—for example, in the Pacific—that’s like a Ferrari. It’s a well-lubricated, smooth-running machine. You get a lot of heat, a lot of volcanism, and a lot of seawater percolating through cracks in the seafloor.
The Gakkel Ridge, on the other hand, is like the Chitty Chitty Bang Bang of mid-ocean ridges. It’s spreading apart ultra slowly. The whole process is herky-jerky. You don’t have much heat or volcanic activity, so you ought not to have hydrothermal venting, or not much. And yet all the sensor evidence from the 2001 cruise screamed: “Holy cow! It looks like there’s hydrothermal plumes all over the Gakkel Ridge!” And that’s pretty exciting.
There’s a lot of anticipation—if we can find these vents—about what kinds of animals are going to live there. We have no idea.
Why might animals be different from those at other vent sites around the world?
For one, the Arctic Ocean is semi-enclosed. The deep parts of the Arctic seafloor, where the vents would be, have been separated from the deep regions of the Pacific and Atlantic Oceans for about 26 million years. So whatever life there is at Gakkel Ridge vents has evolved over all that time in isolation from all other deep-sea life. They may have evolved novel adaptations to conditions on the Gakkel Ridge, much the way you find animals in Australia that you find nowhere else.
Why would conditions be different on the Gakkel Ridge?
We expect to find a type of rock on the Gakkel Ridge that’s very different from mid-ocean ridge rocks in other places on Earth. When plates are moving apart rapidly, you get lots of spectacular volcanic eruptions on the seafloor that produce a type of rock called basalt. But when you slow down the spreading process to where the two plates are kind of just barely opening apart, you get sporadic volcanism and not much basalt. Instead, you expose big slabs of material from Earth's mantle. This material is called peridotite, which has a very different chemical composition than basalt. So when seawater chemically reacts with peridotite in a hydrothermal vent system, it generates hydrothermal fluids with a distinctive chemistry.
Why is that important?
Microbes convert the chemicals in vent fluids into energy, so if you have different chemistry, you’ll get different microbes at the base of the food chain and perhaps different animal communities around the vents.
The other interesting thing about peridotite-based hydrothermal systems is that they produce the kind of conditions that scientists think would have existed billions of years ago on Earth, when life was first emerging on our planet. So, who knows, we may see living “fossils” of animals that existed in primordial times.
Beyond the search for life, what else are you looking for?
Massive mineral deposits. When hydrothermal vent fluids react with the hot rocks beneath the seafloor, they form copper, zinc, and iron sulfides, and other minerals. We know that on mid-ocean ridges that are spreading apart slowly, hydrothermal fluids can circulate for a long, long time, to generate and build up massive mineral deposits on the seafloor. Since the Gakkel Ridge is the ultimate example of a slow-spreading ridge, there could be really large mineral deposits on the seafloor.
But exploring all these fundamental questions about the creation of our planet and life on it ultimately depends on being able to find hydrothermal vents in the first place, right?
Absolutely. The first thing we have to do is to determine if the circumstantial evidence of hydrothermal activity that was observed in 2001 is real. To do that, we have to actually find hydrothermal vents on the Gakkel Ridge. And to do that, we have to deploy and recover robotic vehicles through the ice. That is a difficult technological task. Nobody’s ever done that before, so we don’t have much experience.
The truth is that sending vehicles off for a day or two under the ice is a terrifying experience. When those things sink down through the ice and go a couple of miles down...Man! You’re just praying you get them back!
We have a rare opportunity to go to a place on our planet where measurements have never been made before, so we want to bring every piece of sensing and sampling equipment we possibly can because we don’t know if and when we’re going to get back again. We’re trying the best we can, but obviously there are no guarantees of success.