Secrets of the Southern Ocean

Marine geochemistry specialist Robert Key doesn’t consider himself particularly prone to depression. Yet emails to his wife from a research vessel on the freezing waters of the Southern Ocean depicted an emotional slump amid harsh conditions and brutal working hours.

“It’s wet and windy and miserable, and if you’re down there in the winter, then it’s dark the whole time as well,” said Key, a research oceanographer in the Program in Atmospheric and Oceanic Sciences. “You’re away from contact with people. Essentially all you do is eat and work.”

But Key and other Princeton researchers push through the challenging conditions because they want to learn more about the waters at the bottom of the globe, which have a significant impact on the Earth’s ecosystems and climate. By collecting and analyzing samples of seawater, and using the results to help construct computer models of the ocean and atmosphere, the scientists aim to understand the Southern Ocean’s major influence on the world’s carbon and nutrient cycles. In doing so, they hope to provide insight into what our planet will look like in an era of human-driven climate change.

Though it makes up less than a third of the world’s ocean coverage, the Southern Ocean surrounding Antarctica soaks up about half of the man-made carbon dioxide absorbed by the world’s oceans from the atmosphere each year. Its waters act as a giant pump, with currents that carry carbon dioxide down into the deep recesses of the ocean where the carbon can remain for roughly 1,000 years. In return, currents bring up frigid water from the deep, water that has never been exposed to today’s elevated levels of carbon dioxide and therefore is able to absorb more of the gas than today’s surface waters.

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This frigid water also absorbs heat: the Southern Ocean has helped prevent the planet from warming up as much as it might have by now from human activity, according to Jorge Sarmiento, the George J. Magee Professor of Geoscience and Geological Engineering. Because its waters are so cold, the Southern Ocean absorbs about 60 percent of the excess heat that moves annually from the atmosphere into the ocean.

Along with absorbing carbon dioxide and heat, the Southern Ocean regulates the movement of nutrients such as nitrogen and phosphorus. The ocean’s patterns of circulation transport nutrient-rich water from the deep ocean back to the surface, where currents carry the nutrients to the north. These nutrients provide three-quarters of the ocean’s biological productivity, spurring the production of plant matter called phytoplankton that serves as the basis of the aquatic food chain.

Slideshow of the summer voyage of the research vessel R/V S.A. Agulhas II:

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“It’s old, it’s cold and it’s rich,” said Sarmiento. These traits, he explained, enable the Southern Ocean to have the influence that it does over global climate and nutrient regulation — and challenges scientists to find out how this massive storage vessel for carbon, nutrients and heat might react to rising carbon emissions and climate change.

A key question is whether rising carbon emissions will boost or hamper the Southern Ocean’s ability to sponge up carbon dioxide. Changing wind and rainfall patterns due to a warming Earth could shift how much carbon and heat the Southern Ocean can store in either direction, according to Daniel Sigman, the Dusenbury Professor of Geological and Geophysical Sciences. If winds pick up, for example, then mixing between the Southern Ocean’s deep and shallow waters may pick up as well. If high-latitude rainfall increases, more freshwater on the polar ocean’s surface may mean a higher density difference between the surface and deep waters, leading to less mixing. Depending on the response of ocean biology to this range of possible changes in ocean circulation, the rate of carbon uptake may either rise or fall.

Scientists have built models to predict how the Southern Ocean’s carbon sink will behave over the next several decades. But these researchers lack sufficient observations of the Southern Ocean to adequately inform high-resolution models of the ocean’s circulation, to assess the predictive powers of their models, or even to understand which processes are the most important for the models to provide accurate simulations. Data for winter at the Southern Ocean is especially sparse. A major reason: the brutal working conditions in the winter.

“I thought I knew something about winds and bad waters and blizzards and storms,” said climate modeler Joellen Russell, who grew up in an Eskimo village 31 miles above the Arctic Circle. “I couldn’t get my head around it.” Russell, an associate professor at the University of Arizona who collaborates with Sarmiento, remembers being thrown across her cabin one night when a wave slammed against the side of the ship, and how buckets of water would crash onto the deck in such great volume that the water looked black. “You’re like, ‘I want to go home now,’” she said.

Slideshow of the winter voyage of the research vessel R/V S.A. Agulhas II:

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Field research in the Southern Ocean, which typically lasts between 30 to 70 days, holds scientists to a punishing schedule. They can work up to 18 hours a day, seven days a week. Russell described a typical night working in a chemistry lab during one of her research cruises: she slept on a bean bag on the lab’s floor, waking up every 15 minutes to a beeping noise reminding her to perform certain laboratory tasks. The workhorse nature of the job stems mainly from the high cost of being out on the water: tens of thousands of dollars per day. “Because it’s so expensive to get out there, you feel the pressure,” Russell said. “You want every last bit of data you can get.”

Fortunately, researchers now can use robotic battery-powered floats that provide salinity and temperature measurements for up to five years. About 3,500 of these floats, called Argo floats, are making measurements around the world’s seas. Funding and management for the floats come from the contributions of 23 countries.

But there are measurements that Argo floats leave out, and Associate Professor of Geosciences Frederik Simons hopes to fill in some of the gaps with an instrument of his own. Simons has spent the past few years developing an autonomous buoy, in collaboration with University of Rhode Island professor Harold Vincent, that detects GPS position, time and ocean depth while measuring seismic waves generated by distant earthquakes that are converted to water pressure waves at the ocean floor. They call their instrument the Son-O-Mermaid.
Simons hopes the Son-O-Mermaid will overcome the paucity of oceanic data, particularly in the Southern Ocean. “We are making pictures of the interior of the Earth using waves recorded through the Earth,” said Simons. “When we do that, we can learn about mantle plumes, subduction zones, mid-ocean ridge earthquakes — basic questions that people wonder about.”

Though the Son-O-Mermaid, which can stay in the water in a range of ocean conditions, is set up for use in seismology, researchers could easily adapt it for their own use in physical, chemical or biological data collection, according to Simons.

A three-decade legacy

Much of the groundwork for today’s understanding of the Southern Ocean comes from earlier work by Sarmiento, whose interest in the Earth’s carbon cycle began around 1984. This was when he co-wrote — with J. Robert Toggweiler, an oceanographer at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory in Princeton — one of the first papers to point out that the Southern Ocean’s waters dictate the carbon dioxide content of the atmosphere. His paper was one of three to become so influential in the field as to be given a nickname: the “Harvardton Bears” papers, so named because their authors were affiliated with Princeton, Harvard or the University of Bern.

This early work revealed that the Southern Ocean acts as a leak in the ocean’s biological pump. The Southern Ocean actually releases some carbon dioxide into the atmosphere because phytoplankton, which consume carbon dioxide as they grow, cannot keep up with the rapid supply of nutrients and carbon from the underlying deep ocean and leave much of it unused.

Sarmiento has continued to investigate the ocean’s role in global climate over the three decades since, with a particular focus on the Southern Ocean. In 1996, he constructed a model that predicted how future global warming would affect the ocean’s ability to absorb carbon dioxide: he suggested that warming would weaken the ocean’s circulation, which would in turn compromise the movement of carbon dioxide to deep waters. His further research proposed new ideas about why atmospheric carbon dioxide was lower during ice ages, and he identified pathways that nutrients follow as they migrate from the Southern Ocean to seas farther north.

“You just fall into something, a fascinating problem that you can’t let go of,” said Sarmiento. “Suddenly things come together and you get an answer, and it’s different from anything that anyone else has come up with.”

Sarmiento, who began his Princeton career as the University’s only biogeochemist, is now surrounded by several colleagues examining the oceans’ biogeochemical processes: Michael Bender, professor of geosciences; François Morel, the Albert G. Blanke Jr. Professor of Geosciences; Stephen Pacala, the Frederick D. Petrie Professor in Ecology and Evolutionary Biology; and Bess Ward, the William J. Sinclair Professor of Geosciences.

Sarmiento also has good company in researchers such as Sigman, who has made major contributions to scientists’ knowledge about the Southern Ocean. Sigman is trying to understand the role of the Southern Ocean in global climate by studying past climate changes and reconstructing the strength of the carbon dioxide leak back through Earth history.

In particular, Sigman is exploring the hypothesis that the Southern Ocean carbon dioxide leak was reduced during the ice ages.

Sigman uses marine sediments to identify how iron — which is carried to the Southern Ocean in dust originating in Africa, Australia and South America — has affected phytoplankton growth in the Southern Ocean and contributed to the global climate cycles of the Earth’s last 1 million years.

An essential nutrient for phytoplankton, iron is relatively scarce in the Southern Ocean. The lack of iron prevents phytoplankton populations from growing to numbers large enough for them to fully consume the ocean’s nutrients, including carbon dioxide. This means that a lot of carbon dioxide can leak right back into the atmosphere.

This may not have always been the case, however. By studying marine sediments, Sigman and his colleagues found evidence that more dust deposits in the past may have enabled phytoplankton in the subAntarctic zone — the Southern Ocean region roughly 40 to 50 degrees above the South Pole — to consume more carbon dioxide, which may have helped hold down the global temperature. That lower temperature would have caused stronger ice ages starting 1 million years ago, the scientists wrote in a 2011 paper published in Nature. With more dust — and thus more iron — in the water, phytoplankton growth escalated and ensured that excess carbon in the water was consumed before it escaped into the atmosphere as carbon dioxide gas. This allowed heat-trapping carbon dioxide to stay in the ocean.

“Our hypothesis is that iron supply to the sub- Antarctic zone is one of the two key ingredients that lowers atmospheric carbon dioxide during ice ages,” Sigman said.

The other ingredient is changes in the mixing of water between the surface and the deep ocean. With colleagues from the Swiss Federal Institute of Technology in Zurich, Sigman found evidence that the subAntarctic zone changes were complemented by circulation changes in the Antarctic zone, the Southern Ocean region adjacent to the Antarctic continent and south of the subAntarctic zone. At the beginning of each ice age of the last million years, the Antarctic zone appeared to reduce its vertical mixing, which may have slowed the leakage of carbon dioxide to the atmosphere and trapped more of it in the ocean, providing the first cooling step of the impending ice age. The cooling, in turn, encouraged more dust to be deposited in the ocean, in part because continents became drier and dustier. And the phytoplankton fertilized by this dust may then have caused further carbon dioxide drawdown and global cooling.

These two factors — more dust-borne iron and more mixing of waters — could have allowed the Southern Ocean to absorb more carbon dioxide in the past. These findings may hint at how the Southern Ocean will change as the Earth warms in the next few years, Sigman said, with the possibility of more mixing and less iron input.

With the Southern Ocean playing a considerable role in what the climate might look like in years to come, scientists such as Sarmiento see it as their responsibility to uncover knowledge that will enable others to understand the systems at work. But though Sarmiento and Princeton’s other Southern Ocean specialists have made considerable strides in investigating the ocean’s place in the global climate, challenges remain — from increasing public awareness of the Southern Ocean’s critical importance to uncovering cost-effective, automated methods of obtaining more data from this poorly sampled region.

Key especially looks forward to the latter: he hopes that new technologies will afford him a break from the rigors of Southern Ocean fieldwork.

“It gets harder and harder to work a 72-hour week,” he said. “You get too old to go out to sea as much as I used to.”

-By Tara Thean