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Scientist pioneered tracer to reveal hidden ocean flows

by David Pacchioli on 20 Jul 2014
WHOI scientists Kurt Polzin, Ray Schmitt, Lou St. Laurent, and John Toole used an instrument called the High Resolution Profiler to track how waters mixed in the deep ocean in the Brazil Basin, where the flat abyssal plain abuts the undersea mountain chain of the Mid-Atlantic Ridge. The experiment revealed much more turbulence and mixing above the seafloor in areas with mountainous terrain than in areas with a relatively smooth seafloor because internal tides running over the abyssal hills generate energy that propagtes upward to drive mixing in the waters well above the hills. The experiment clearly established the importance of seafloor topography in mixing deep water masses back toward the surface. (Gray bars indicate places where data were not collected.) Kurt Polzin, Woods Hole Oceanographic Institution
How can you follow a wisp of water within the turbulent immensity of the ocean? Jim Ledwell figured out a way.

He developed a method to inject a harmless chemical tracer into the ocean and to track it over days, weeks, or months as it spreads through the swirling sea. Over the decades, he has used it throughout the world to illuminate the opaque depths.

Ledwell’s pioneering experiments have unraveled how separated layers of ocean waters mix, revealing clues to one of the enduring mysteries of oceanography: How it is that cold dense waters that have sunk to the abyss can then rise and return to the surface, thus completing the great overturning conveyor belt that drives the ocean’s global circulation and regulates Earth’s climate.

During his career, mostly at Woods Hole Oceanographic Institution (WHOI), Ledwell has been on more than fifty ocean research cruises and spent more than three years of his life at sea. But he never intended to be an oceanographer.

One of seven children of a policeman and a homemaker in the shoe-factory town of Rockland, Massachusetts, he was taught first by Catholic nuns. 'They were very dedicated teachers,' he remembered, 'but not much help in science.'

Young Ledwell’s interests ran to model airplanes and microscopes and by the time he got into eleventh-grade physics at Cardinal Spellman High School in nearby Brockton, he had the chemistry bug. 'We didn’t have a big house for seven kids, but I had a little pantry upstairs where I did experiments,' he said. 'I didn’t blow anything up,' he added drily. 'By that time you couldn’t get the necessary chemicals at the hardware store.'

An ocean experimentalist
With the surge of interest in American science after Sputnik, he went to college on a National Defense scholarship. As an undergraduate at Boston College and then a graduate student in physics at the University of Massachusetts at Amherst, however, 'I was inclined toward theoretical, high-energy particle physics, cosmology, and all that stuff,' Ledwell said. 'It was a wrenching thing to give up that ambition to go into earth science.'

Jobs in theoretical physics were scarce, he reasoned. The employment prospects in earth science looked more promising. And he had already begun to be interested in the burgeoning study of climate change. Encouraged by his UMass professors, Ledwell was accepted at Harvard’s Center for Earth and Planetary Physics, where he settled down to the study of atmospheric chemistry.

Maybe 'settled down' isn’t quite the right phrase. 'Mike McElroy and Steve Wofsey, my mentors, were the kind of guys who would do anything to solve a problem,' Ledwell said. 'They were interested in gas transfer across the air-water interface. And they both had active field programs. So my first field experiment was measuring gases coming out of the Delaware and Potomac Rivers. I can remember running up and down the Potomac in a fifteen-foot motorboat in the heat of August, doing those measurements. I loved it.'

That experience helped convince him he wanted to be an experimentalist, not a theorist. It also gave him skills in chromatography—separating complex mixtures of gases or liquids into their constituent parts—which he would later put to fertile use. Meanwhile, Ledwell was nursing a brand-new passion. 'While I was procrastinating on writing my thesis,' he remembered, 'I read [legendary WHOI oceanographer] Henry Stommel’s book, The Gulf Stream. That got me interested in the ocean.'

The overturning ocean
In particular, Ledwell found, he was interested in the great and complex movement of waters that comprises the ocean’s circulation. A vast 'conveyor belt' of ocean currents flows around our globe. It starts in the North Atlantic and around Antartica, where cold winter winds chill surface waters, making them denser and sending those waters plunging down to the abyss.

The North Atlantic water flows southward in the depths toward Antarctica where it blends with Antarctic deep water, goes into the Antarctic Circumpolar Current, and is carried into the Pacific and Indian Oceans. Somehow, somewhere along the way, this dense cold water, buried deep in the ocean, mixes with warmer waters, becomes lighter and more buoyant, rises back to the surface, and flows back northward toward its original starting points.

At the surface, evaporation leaves behind salt, and saltier waters are denser. When these salty waters travel to high latitudes, they cool, becoming even denser. They sink once again, keeping the conveyor running.

This planetary heating and ventilation system transfers heat throughout the globe. Surface waters absorb large amounts of the greenhouse gas carbon dioxide from the atmosphere. When those surface waters later sink to the depths, they carry the carbon dioxide with them. This helps regulate Earth’s climate.

That much is well established, at least in broad outline. But the global overturning cycle, which takes roughly a thousand years to run its course, depends almost entirely on mixing processes that occur on smaller scales and that are not well understood, especially in the vast regions of the ocean’s depth.

'The ocean is stratified,' Ledwell explained. 'It’s made up of stable layers, determined by temperature and salinity. It gets denser as you go down.' Mixing within these density layers—called horizontal, or isopycnal, mixing—is fast and easy, he said. But mixing across the layers, called vertical or diapycnal mixing, is much slower and more difficult.

Even so, vertical mixing was long thought to play a dominant role in the ocean’s overturning, especially in the depths. At the sea surface, winds, cooling, and evaporation all create turbulence that hasten mixing. In the deep ocean, however, these 'external' forces are mostly nonexistent. That leaves only vertical mixing to churn things up, driven by the surface forces and by internal tides.

Vertical mixing in the ocean
In the 1960s, the influential oceanographer Walter Munk posited that a certain level of vertical mixing was required to maintain the global circulation. Over the ensuing decades, however, there were no observations that confirmed Munk’s estimate.

For starters, it’s a difficult feat to measure turbulence in the deep ocean. A handful of oceanographers in the U.S. and Canada, including WHOI oceanographers John Toole and Ray Schmitt, developed highly sensitive probes in the late 1970s that could free-fall through the ocean and capture turbulent fluctuations in temperature and velocity as they descended.

With these 'snapshot' measurements, scientists calculated an average level of mixing. But the numbers yielded by this process didn’t match the theory. In fact, the early turbulence measurements suggested that vertical mixing in the deep ocean was ten times less than scientists had predicted.

Not surprisingly, these measurements met with healthy skepticism. 'There were always questions about the interpretation of the data,' Schmitt said. Without a better way to measure mixing in the deep ocean, 'there was a lot of motivation within the oceanographic community for a more direct approach.'

That’s about where Ledwell comes in. He was finishing up at Harvard when, three weeks before his thesis defense, one of his committee members suggested a visit to Columbia’s Lamont-Doherty Geological Observatory to see Wally Broecker, a dean of climate change scientists. Broecker, a chemist and an expert on gas exchange, might offer fresh insights on Ledwell’s work on the air-ocean interface. When the two men met, however, Broecker wanted to talk about the deep ocean.

'Wally was interested in this problem of vertical mixing,' Ledwell remembered. 'He had this idea that we could find a tracer that would be sensitive enough to measure it.' The tracer, a harmless chemical that didn’t react with anything, could be dropped into a parcel of water at a given depth and could be tracked as it spread vertically over long periods of time. The result would be a direct measurement of how that parcel diffused through the ocean, and perhaps a way to test the validity of the turbulence data.

'Wally saw my work, and he asked me to come down and get involved in tracer experiments,' Ledwell said of that early meeting. 'So I did.'

Finding a tracer
It was, he concedes, a pretty bold move. For one thing, he and Broecker had an idea for deploying a tracer, but no actual tracer—and no permission to put this as-yet hypothetical tracer into the ocean. For another, the experiments the big-thinking Broecker was proposing would take years to complete.

'It was crazy for a young scientist to commit to something where he couldn’t get a publication in a scientific journal for years,' Ledwell said. 'Also, a lot of people said it wouldn’t work.'

James Lovelock would help solve the first problem. The singular British chemist, father of the Gaia hypothesis, had invented a gas chromatograph capable of detecting fluorocarbons at extremely low levels. Why, he asked Broecker, don’t you use a fluorocarbon for a tracer?

Ledwell began working with one of Lovelock’s protégés, Andrew Watson, to identify a likely candidate. Eventually the two settled on sulfur hexafluoride, a compound that is virtually inert in the marine environment and detectable in concentrations as low as one in 1017 parts, roughly the equivalent of a drop of milk in an Olympic-size swimming pool. Very small amounts of this stuff, they reasoned, would remain traceable in the ocean for years.

They refined their technique through a series of pilot experiments off the coast of Southern California. 'For the first one, in the Santa Monica Basin, we just dissolved the tracer in drums of seawater, then laid the barrels down and pumped them out into the basin,' Ledwell said, 'but because sulfur hexafluoride is not very soluble, it took days to get 200 grams of tracer dispersed into the water.'

Meanwhile, the two had designed a sampler array that could be towed along behind a ship to pick up the thin filaments of tracer. 'That part worked well,' Ledwell said.

To solve the dispersal problem, he and Watson talked a dairy salesman into letting them borrow a milk homogenizer. They figured out how to emulsify the tracer and squirt it through the homogenizer nozzle at high pressure.

A new way to track ocean dynamics
After a second test off Santa Cruz, California, it was time for the open ocean. The North Atlantic Tracer Release Experiment, or NATRE, commenced with release of a patch of sulfur hexafluoride in the northeast Atlantic in May 1992. Ledwell and Watson followed its winding, widening course over the next 30 months.

In NATRE, for the first time, the technique was combined with turbulence measurements taken by the WHOI ocean mixing group led by Schmitt, Toole, and Lou St. Laurent. It was, according to the British oceanographer S.A. Thorpe, writing in his book, The Turbulent Ocean, 'Perhaps the most inventive and imaginative experiment to be made to measure vertical diffusion.' And the two techniques gave results that agreed convincingly: The amount of vertical mixing observable in the deep ocean was indeed ten times less than theory had predicted.

'NATRE did a couple of things,' said St. Laurent. 'It showed that the upper ocean is not mixing at the same rate as the deep ocean, and it confirmed that the answers we were getting from turbulence measurements were real. A longstanding paradigm was broken, and it was broken by Jim. What he did was quite profound.'

There followed a string of high-profile experiments, in which Ledwell worked closely with his turbulence colleagues. 'They all seemed to result in a paper in Nature or Science,' said Schmitt. 'We were breaking new ground.'

Experiments off Brazil and Barbados
The second major effort, in the Brazil Basin, was 'probably the most successful we’ve ever done,' Ledwell said. 'Not so much because of the tracer, but on the turbulence side.' The turbulence data showed a dramatic difference in the rate of mixing in the deep ocean across the flat abyssal plain compared to the rough-bottomed area near the undersea mountain chain of the Mid-Atlantic Ridge. Deep waters collided with abyssal hills and were directed upward where they mixed with warmer, less dense waters higher up. The experiment clearly established the role of seafloor topography in deep-ocean mixing. The resulting scientific paper, with WHOI’s Kurt Polzin as lead author, remains one of the most cited in oceanography.

In 2001, in a third major experiment, Schmitt asked Ledwell to deploy his tracer in the so-called 'salt-finger' staircases off Barbados, where a curious ocean phenomenon occurs.

Cold water is denser than warm water and saltier water is denser than fresher water. In much of the ocean, colder water is saltier than warmer water, so the density differences reinforce each other: Cold, saltier water remains below the warmer, less salty, less dense water above it. But off Barbados and many other regions of the ocean, warm water near the surface is unusually salty and lies above cooler, fresher water. Both the heat and the salt will tend to diffuse into the cooler, less-salty waters, but the warm layer loses heat by conduction much faster than the salt can diffuse. In small irregularities in the border between layers, top-layer water becomes denser than the water around it and sinks in narrow plumes called salt fingers. These downward fingers displace and supply heat to cooler, fresher waters below. That makes those waters less dense and cause fresher fingers to float upward.

'If the density layers were perfectly flat, they could stay that way and not move,' said Ledwell. 'But that situation is not stable. The tiniest perturbation—a little wiggle—can get the fingers growing. And the ocean is always wiggling.' Perturbations can arise from many sources, primarily internal waves driven by the wind and tides.

These small-scale fingers, each only a few centimeters wide and about a meter long, mix the waters into uniform layers of water above and below the fingering interface. These mixed layers are tens of meters thick and can extend horizontally for hundreds of miles, creating a staircase-like pattern in temperature and salinity profiles that descends into the depths.

'Jim’s tracer data confirmed that the staircases strongly enhance the vertical mixing,' Schmitt said, 'and that standard turbulence formulas with minimal mixing don’t apply in locations where this happens.'

Over the years, Ledwell has successfully adapted his tracer technique to help answer questions ranging from how currents along the seafloor disperse microscopic larvae between hydrothermal vents to how swirling bodies of water called eddies in the Sargasso Sea bring up nutrients from the deep to the surface, where they nourish photosynthetic algae and bacteria.

Immune to the weather
In 2007, in recognition for three decades of work, he was awarded the Agassiz Medal, given every three years by the National Academy of Sciences for 'original and fundamental contributions to oceanography.' The list of previous winners reads like a who’s who of the field and includes the names of three of Ledwell’s early influences: Henry Stommel Wally Broecker, and Walter Munk.

It’s a towering achievement, but Ledwell is characteristically modest when the subject is raised. 'When I won it, a bunch of my old high-school friends all went down to Washington to help me celebrate,' he said simply. 'That was the best part.'

Among his colleagues, Ledwell has built a reputation not just for the creativity and precision of his approach, but for a rare combination of intrepidity and grace in the field.

'What we do gets intense,' said St. Laurent. 'The trips we take, the life on the ships, the people, are crazy. Jim is this kind of anchor. He’s the ultimate gentleman scientist.'

'He’s a glutton for punishment,' said Schmitt, of Ledwell’s continued appetite, as a 65-year-old grandfather, for going to sea. Then, more seriously, 'He has the vision and energy to sell a big experiment—and it takes a lot—but he also has the detailed knowledge and the perseverance to carry it out.'

Ledwell, the policeman’s son, 'is immune to weather, seasickness, he just works right through the worst of it,' said the much younger St. Laurent. 'And he can push his technology to crazy levels of detection.'

Turning on DIMES
All of these attributes, and more, would be needed to pull off the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean. DIMES, as the project has been dubbed, is a five-year effort including some 30 scientists from the United States and United Kingdom, deploying both tracer and turbulence techniques in one of the fiercest patches of ocean in the world.

As the project’s chief scientist, Ledwell began planning for DIMES in 2002. The first tracer was deployed in 2009, 1,200 miles west of Drake Passage in the Antarctic Circumpolar Current, or ACC. There have been sampling cruises every six months since, with American and British teams and research vessels taking turns. A final cruise took place in March 2014.

The ACC runs perpetually east through the Southern Ocean and unimpeded around Antarctica. All of the deep water in the world’s ocean goes there, St. Laurent said, and a great deal of mixing occurs. That brings deep water back to the surface, helping to complete the global ocean overturning that regulates Earth’s climate.

It’s a rugged place to conduct research. 'The timescales, space scales, and sheer intensity,' St. Laurent said, 'are at the limit of our observing capabilities.' But the payoff could be huge. Understanding mixing in the Southern Ocean would powerfully impact projections of climate change for the next century.

The most recent DIMES results, published last September 2013 in the journal Nature, show a 20- to 30-fold increase in mixing as water passes over the gnarly mountainous seafloor of Drake Passage. That suggests that this rough topography accounts for most or all of the mixing in the Southern Ocean, and, to some of his colleagues, that this mixing may be large enough to be of global significance.

Ledwell, for his part, is reluctant to extrapolate. But he said the trend is clear. 'What we have been finding over the last thirty years is that vertical mixing is too small to be the governing process for the whole overturning of the ocean. Instead, there are a couple of things going on.

'Bigger mixing occurs near the boundaries, along rough bottoms on the seafloor. The other thing is a lot of circulation actually moves along the isopycnal layers to the poles, instead of across them.' Horizontal mixing, in other words, plays a larger part than previously acknowledged, because, in fact, it isn’t altogether horizontal. There’s enough vertical rise in these layers, especially along the ACC, to allow the returning deep water to eventually reach the surface.

'So it’s a bit more complicated than we once thought,' Ledwell concluded. 'But isn’t everything?'

Culmination of a career
'DIMES is a pretty impressive tour de force of how far you can push this technology,' said St. Laurent. And while Ledwell would like to see a DIMES-like experiment done in the Arctic, he himself thinks that big open-ocean mixing experiments might be over for a while.

On a smaller scale, he has recently used his technique in the Gulf of Mexico as part of an effort to improve responses to accidents like the Deepwater Horizon oil spill. 'We put the tracer in at 1,100 meters deep, about the level of the deep plume from the spill, and watched it for a year,' he said. The data gathered will feed models built to estimate the spread of future spills.

'There are all kinds of perturbation experiments you could do using the tracer as a control,' he added. 'The other thing you can use it for is measuring gas exchange across an interface.' Accordingly, with colleagues from Hawaii and Rhode Island, he has submitted a proposal to study the mixing and exchange of gases across the air-ocean interface in the Arctic’s marginal ice zone. If funded, Ledwell said, that project would take him back to his roots in atmospheric chemistry at Harvard.

Then again, getting quite to the roots will likely have to wait for his retirement. That’s when, Ledwell said, 'I’m going back to being a student. I have a whole room full of books on a broad range of subjects, from cosmology to climate change, and I’m going to see if I can catch up on what’s been going on for the last 40 years.'

Ledwell's studies of mixing in the deep ocean have been supported from their inception primarily by a series of grants from the National Science Foundation Division of Ocean Sciences, with important contributions over the years from a National Research Council Fellowship at the Goddard Institute of Space Studies, from Lamont-Doherty Earth Observatory of Columbia University, including an ARCO Fellowship, from the Edward W. and Betty J. Scripps Senior Scientist Chair at WHOI, from BP through the Gulf of Mexico Research Initiative, and from the Office of Naval Research.

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