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Cold corals

Scientists venture into the Drake to learn more about benthic ecosystem and climate change

 

Visiting the underwater world of corals in the Southern Ocean isn’t exactly like a tropical vacation.

Consider that the stretch of water between the tips of South America and the Antarctic Peninsula, where some of these cold-water corals are found, is one of the most inhospitable in the world. Frequent storms and strong seafloor currents make it a difficult place for researchers to retrieve samples. The marine organisms exist at depths from hundreds to thousands of meters — a deep and dark realm rarely explored until recently.

“Down in the Southern Ocean, very little is known about the cold-water corals that live there,” said Rhian Waller, a co-principal investigator on a project to study the spatial and temporal diversity, distribution and propagation of coral species in the frigid seas between continents.

But learning more about the biogeography of the different coral species was only part of the goal for a five-week research expedition to the Drake Passage earlier this year. It turns out that deep-water corals are also an excellent way to learn about the past conditions of the ocean and overall climate as it existed thousands of years ago.

“The nice thing about corals … for studying climate is that we can date them very accurately,” said Laura Robinson, chief scientist on the research cruise aboard the Nathaniel B. Palmer and co-principal investigator on the project with Waller.

Together, the two scientists and their team of students and international collaborators spent the early weeks of the Antarctic winter trawling and dredging for samples of both live and dead corals from seamounts rising up from the ocean floor or along the relatively shallow continental slopes.

The biologists aboard the ship recovered more than 11,000 individual samples, mainly in the order octocoralia (also known as soft corals), including what may turn out to be a few new species.

Just as importantly, the scientists deployed a couple of different camera systems from the ship to image the seafloor. The TowCam, pulled behind the Palmer, “flies” only about five meters off the seafloor, recording about 2,000 images at each deployment. The DropCam, as the name implies, falls straight to the seafloor while the ship is stopped, snapping pictures only two meters from the bottom.

Waller’s team will analyze the images back in the lab to identify species, information the scientists can use to map distribution and diversity between South America and the Antarctic Peninsula.

“We’ll be able to track where certain species end — whether they fully cross the Drake Passage, or whether they’re only found at certain locations,” explained Waller, an assistant research professor at the University of Maine’s Darling Marine Center.

“The photography and sample sections go side-by-side, really, for the biology side of this project,” she added. “We see so much more in the photographs than we do in the collections, but often we can’t really identify what we have in the photograph unless we have a sample.”

The paleoceanographers, led by Robinson, are more interested in the fossil corals brought aboard the ship for the information they can provide about past ocean conditions and even ocean circulation.

Cold-water corals build hard skeletons made of calcium carbonate that contain chemical signatures that “record” some of the chemical and physical properties of the waters in which they grew. The uranium in the calcium carbonate decays into thorium, a process that allows the researchers to date the coral material very accurately — almost as well as glaciologists who use ice cores to reconstruct past climate.

That accuracy allows them to not only determine what the ocean chemistry was like in the past but exactly when changes occurred. “Using this coral archive is very powerful in that respect,” explained Robinson, an associate scientist at Woods Hole Oceanographic Institution.

Her team is particularly interested in traveling back in time to the last glacial period, more commonly known as the last ice age, about 20,000 years ago and the transition into a warmer period, the modern Holocene. (Some corals that have been collected in the Drake boast ages hundreds of thousands of years old.)

The transition from glacial to interglacial in the Southern Ocean, as told in the chemistry of the corals, could offer insight into how the ocean and the rest of the climate system respond to such environmental swings.

“The more we can understand about how the ocean interacts with the atmosphere, and the timescales of change and the magnitudes of change in all parts of the climate system, the better position we are [in] to say something about changing climate conditions in the future,” Robinson said.

Tied to that understanding is ocean circulation and how it responds to climate change. Again, the corals provide clues in the form of carbon-14, a naturally occurring radioactive isotope of carbon that forms in the atmosphere and dissolves into ocean surface waters. Higher carbon-14 content in the coral material means more ocean mixing between the surface and the deep. The longer deep water is isolated from the surface, the less radiocarbon a sample contains.

“It’s a powerful way of looking at ocean circulation,” Robinson noted.

The dating methods are also important to the biogeographical research, helping the scientists map diversity and distribution of coral populations over time – a unique way of looking at changing ecosystems that may help maybe predict how they will behave in the future.

“It’s definitely a puzzle. A species that seems to have been doing well tens to hundreds of thousands of years ago, we only find one or two live ones on these seamounts,” Waller said.

Shallow sediment cores from the seafloor, which provide far less time resolution than other methods, are important for mapping this historical distribution and diversity. Bits of coral skeletons embedded in the cores can help pinpoint when corals were growing in certain areas of the past.

The vessel traveled more than 5,000 kilometers, visiting the continental slopes of both the Antarctic Peninsula and South America, including infamously stormy Cape Horn. Trips to various seamounts — ancient underwater volcanoes — in the Drake offered an opportunity to collect samples from a variety of depths.

“It’s a pretty wild, featureless part of the world, as you look out from the ship,” Robinson said. “But then [you] go down and take photos of the incredible abundance and diversity of life on these seamounts and shelf areas, which is pretty impressive. You already know it, but when you see it, it’s a good reminder of how diverse these seemingly isolated places are.”

The 2011 expedition follows an exploratory cruise by Robinson and Waller in 2008 that proved the viability of their collection and photo techniques in the Southern Ocean. Preliminary results from that cruise suggested that geological and environmental variability, particularly in temperature, between six sample sites may influence cold-water coral biogeography. No one area resembled the others in terms of animal diversity.

“Very often these cruises come back with more questions than they do answers. It hopefully won’t be the last time we head into the Drake Passage,” Waller said.

NSF-funded research in this story: Laura Robinson, Woods Hole Oceanographic Institution, Award No. 0944474, 0902957; and Rhian Waller, University of Maine, Award No. 1127582.

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Curator: Peter Rejcek, Antarctic Support Contract | NSF Official: Winifred Reuning, Division of Polar Programs