|Sampling the South Atlantic|
|Contact: Paul Preuss, [email protected]|
In the summer of 2004 oceanographer Jim Bishop of Berkeley Lab's Earth Sciences Division learned that the National Oceanic and Atmospheric Administration's Research Vessel Ronald H. Brown planned a voyage the coming January and February, through the South Atlantic from the Antarctic to the equator, aimed at measuring increased ocean CO2 from human activity and its effects on ocean biogeochemistry. Says Bishop, "It seemed a perfect opportunity to torture-test some new instruments."
Bishop has pioneered the use of instruments to study particulate carbon in the oceans, most of which is biological in origin. To study the different varieties of these carbon particles and their sedimentation how they settle in the water he and his colleagues developed autonomous Carbon Explorers, deep-diving SOLO floats originally designed by Russ Davis at the Scripps Institution of Oceanography to report on salinity, temperature, and currents, but with added particulate organic carbon detectors and sophisticated global-positioning and rapid-communications systems.
One instrument Bishop wanted to test on the RV Ron Brown was a new Particulate Inorganic Carbon (PIC) detector developed at Berkeley Lab, equipped with polarizing filters to identify the distinctive optical signatures of calcium carbonate from shell-forming plankton species like coccolithophores. The South Atlantic voyage would also provide data to augment information from a voyage Bishop and ESD's Phoebe Lam had made aboard the same ship in the North Atlantic in 2003, during which they collected an unprecedented set of optical carbon observations along an 8,000-kilometer track from Iceland to Brazil.
Just one catch: Bishop and his team were hard at work developing new optical carbon sedimentation recorders for a 45-day expedition in the North Pacific in July 2005, a process none could risk interrupting for the 50-day voyage of the Ron Brown.
A volunteer to the rescue
The summer of 2004 found Alexandra Thompson entering the second year of a research fellowship from the Miller Institute at UC Berkeley one of those awards to young people of "great talent or promise" for which, like a MacArthur Fellowship, no restrictions were placed on the research theme. Alex Thompson has a passionate interest in how carbon dioxide, nitrous oxide, and other greenhouse gases from human activities are warming Earth's climate.
A New Zealander with a Ph.D. in atmospheric chemistry from York University in Toronto, Thompson came to Berkeley to study with renowned soil scientist Ronald Amundson. To the question what does soil have to do with the atmosphere, Thompson answers, "Plenty": "A major question in atmospheric chemistry is the relationship of the atmosphere and the biosphere."
While much of the biosphere is land-based, even more life resides in the oceans. So when Thompson heard through the postdocs' grapevine that an oceanographer at Berkeley Lab was looking for an independent, fast-learning scientist who thrives on little sleep, ready to spend six weeks in stormy seas measuring carbon concentrations in the South Atlantic, she jumped at the chance.
Working with Bishop and his group, Thompson learned how to measure particulate-matter contents filtered from samples of seawater. The RV Ron Brown would collect samples at various depths by lowering a "rosette" of bottles overboard almost all the way to the bottom of the ocean typically some five kilometers deep along the ship's route, except near shorelines.
She would store the filtered material to take back to Berkeley Lab; there, the actual contents would be compared with readings taken simultaneously by the Lab's three optical instruments, which were carried underneath the rig holding the collection bottles and the CTD, an instrument for measuring conductivity, temperature, and depth. Data from the optical instruments would be sent up the wire to the ship along with data from the CTD. Each day Thompson would email the raw data back to Jim Bishop in Berkeley by satellite.
Of the three instruments, those measuring light scattering and particle organic carbon (POC) had been successfully used on Carbon Explorers in far-ranging experiments in the Southern Ocean, the North Pacific, and the North Atlantic; results included the first direct observation of a plankton bloom naturally fertilized by iron-rich dust from a storm in the Gobi Desert, as well as the first evidence indicating that carbon fixed by a plankton bloom artificially fertilized with iron would sink below 100 meters.
The third instrument, that for measuring particle inorganic carbon (PIC), had not yet been successfully deployed on a Carbon Explorer. "The logic of sensor development is that first you get the concept right in the laboratory, then you try it out by lowering it on a CTD to see that it's measuring what it should over a wide range of ocean conditions from freezing near the poles to hot at the equator, from the surface down to 6,000 meters deep," says Bishop. "Then and only then can we justify a transfer of the sensor onto a Carbon Explorer. The Ron Brown expedition was a perfect torture test."
But shortly before Alex Thompson was scheduled to get on a plane to Patagonia, where she would join the scientists and crew aboard the Ron Brown, a serious flaw showed up in a component of the PIC detector; a re-engineered sensor was rushed to Berkeley Lab after the Christmas holiday delays, and ESD's Todd Wood stayed up all night to calibrate the PIC sensor and make sure it worked, then rode a morning Bay Area Rapid Transit train from Berkeley to the San Francisco airport and handed the sensor to Thompson as she waited to board the plane.
Under a southern sun
Thompson stepped off the plane in Punta Arenas, Chile, midway through the Straits of Magellan. It was the height of midsummer, the sun didn't go down until 11 p.m., and the weather was balmy: "freezing and sleeting, with occasional hot spots when the sun comes out," as she emailed her family in New Zealand. She spent the next four days rounding up equipment previously sent from Berkeley and getting it loaded and stowed aboard ship, meanwhile dodging tourists headed for Antarctica who filled the little town "easy to spot," she wrote, because they "travel in gaggles and buy up everything in sight."
Although Thompson is a seasoned open-ocean sailboat enthusiast, she had never been aboard a large research vessel. Intent on teaching herself navigation, she followed with map and compass as the RV Ron Brown left Punta Arenas on January 11. Expecting a turn eastward toward the Atlantic, she was puzzled when her compass indicated they were heading nearly due south. It developed that an Atlantic storm had forced the captain to go around Tierra del Fuego, through the Beagle Channel. Soon the ship was passing "narrow fjords, blue glaciers coming down to the water, towering, incredibly abrupt cliffs," a landscape of "deep green foliage and rainbows."
Farther south the waters teemed with seals, penguins, and whales, and the air with albatrosses and "tons of tiny seabirds, which kept up with the boat even though we were belting along at 12 knots." Then came Iceberg Alley and days of horrific weather, and Thompson started working 20 hours a day.
The first sampling station was at 60 degrees south latitude, 31 degrees west longitude, south and east of South Georgia Island. The ship was surrounded by icebergs, "huge and beautiful, like cathedrals," the winds were howling at over 30 knots, and swells towered over the deck. Conditions dictated that only a small set of bottles could be lowered over the side, and it wasn't possible to collect from near the bottom. As the expedition's Chief Scientists laconically noted, "frustration mounted when groups found the bottles emptied by the samplers before them."
Ditto for Thompson, who wrote, "My job is to filter the water and make sure my optics are working. They aren't. And in the middle of the worst storm my filtration pump stops working, requiring me to rebuild the vacuum part of it. At 3:30 a.m. with everything not tied down rolling from one side of the lab to the other. Still, at least it's daylight." A sun that sets at 11:00 p.m. promptly rises again at 1:00 a.m.
Thompson's assignment was to collect sensor data every half-degree of latitude and samples every one degree of latitude, up the middle of the South Atlantic almost all the way to the equator. "As it is they do a cast, which takes three hours, sail to the next station, with three hours in transit, and do the next cast," Thompson wrote. "I am flat out keeping up with samples...."
Approaching the forbidding peaks of South Georgia Island, Thompson recalled that "Shackleton rowed and sailed his lifeboat from Elephant Island to South Georgia on what has got to be one of the world's most epic journeys." In 1914 the explorer, seeking help for the crew of his ill-fated Antarctic expedition, their ship crushed by the ice, had sailed 850 miles through the world's stormiest waters in a 16-foot, half-open life boat.
As the Ron Brown headed toward the equator, the weather was improving but the workload was increasing. According to the Chief Scientists' report, "There was a dense sampling schedule up and over the shelf of South Georgia Island, with stations close to 10 miles apart to capture the boundary features." What they called "a dense sampling schedule," Thompson called "beaverish."
Meanwhile she stayed in touch with Bishop by email. Daily he worked up simulations of what her instruments should be reading, partly based on sea color in NASA satellite images indicating biomass. "Alex literally didn't have time to sleep," Bishop says. "One thing I wanted to do was let her know when there was blue water ahead" indicating less productive waters "so she could slow down."
One reason Bishop was so eager to collect data in the South Atlantic was his suspicion that the same iron deposits that appear to fertilize blooms of coccolithophores in the North Pacific might be at work there as well. "In the North Pacific Phoebe Lam and I found that the major source of iron was not wind-borne dust but came from the continental shelf. Now, the largest continental shelves on the planet are in the South Atlantic." So it's not coincidental that the South Atlantic is one of the most biologically productive regions on the planet; Alex Thompson's sampling and filtering kept her awake virtually around the clock.
Into the tropics
But as the weeks passed and the RV Ron Brown steamed into the biologically less active tropics, Thompson finally had a chance to get some rest. Indeed, by early February she found herself almost growing a little bored, and when the ship finally made port in Fortaleza, Brazil, she took a couple of weeks off to give herself a break by promptly heading up the Amazon for a quick look around the rain forest.
Back at Berkeley Lab, the new optical measurements, as designed by Bishop and his colleagues and supervised by Thompson aboard the RV Ron Brown's South Atlantic transit, promise to provide a new perspective to the long history of traditional measures of dissolved carbon in seawater.
"It turns out that if you have calcium carbonate particles in the water, it adds to the alkalinity and total carbon measured," Bishop explains. "Most of the time the extra particle component is tiny, but on our 2003 voyage in the North Atlantic we found very significant amounts" presumably from vigorous blooms of coccolithophores. "Now, when geochemists saw those anomalously high signals, they'd been throwing them out. But in fact the data indicates the presence of calcium carbonate fragments from the phytoplankton that did not sink. This could be an important, heretofore-overlooked factor in calculating how much carbon the oceans are actually absorbing."
As more sophisticated data comes in from many sources, new patterns emerge and new questions arise. For Alex Thompson, it's all grist for the mill. "I'm an Earth scientist," she says. "That means everything I do is multidisciplinary."