Journey to Antarctica: The Dark Art of Coring
The sea pig just happened to be in the wrong place at the wrong time. Sea pigs (a.k.a. Scotoplanes, a relative of the sea cucumber) look like caterpillars in drag, with flashy horns and bright colors. They live on the deep ocean floor and feast on whale corpses, among other things. This particular little red sea pig was crawling around on the bottom of the Southern Ocean when a device called a multicorer — imagine a futuristic chandelier from Star Wars with four-inch-wide plastic tubes attached — crashed down around it. Within moments, it was trapped in one of the tubes, along with the mud below it and a few inches of the water above it, then hoisted 1,800 feet through the sea to the deck of the Nathaniel B. Palmer, the icebreaker I’m aboard in Antarctica.
A few minutes later, one of the scientists removed the tube from the multicore and stored it in a refrigerator on the ship for future study. It wasn’t until the next morning that Becky Totten Minzoni, a paleoclimatologist at the University of Alabama and head of the ocean core geology team, noticed that in addition to mud and water, the tube contained a sea pig. And it was still very much alive.
“It’s cute!” Minzoni told me as she showed me the bizarre creature in the refrigerator. Minzoni is a lively presence on the ship, flashing pictures of her three kids on her iPhone and cranking out New Orleans funk on her old Apple Shuffle. “It’s also a sign that we got a good core. Because if we’re pulling up sea pigs, it means we’re grabbing the top layer of mud, too. And that’s what we want.”
The purpose of this two-month-long journey on the Palmer is to better understand the risk of collapse of Thwaites glacier (a.k.a. the Doomsday Glacier). As Minzoni puts it, “When we look at the recent history of Thwaites glacier, what I want to know is, how much did it melt, and how fast?” On the Palmer, scientists are using a variety of instruments to answer those questions, but no single tool can tell them as much as ocean coring. By reading the sediments in the ocean cores, which are layered like tree rings, each one containing clues to lost worlds, scientists can interpret the temperature and salinity of the water in past oceans, as well as the movement of currents, all of which have a profound influence on the stability of Thwaites glacier. They can also detect pulses of meltwater that pushed sediments out to sea, which is helpful in understanding if, and when, Thwaites has collapsed before.
Deciphering the past in a tube of ocean mud is not a simple task, however. “Coring is a dark art,” says Ali Graham, a geophysicist who is part of the coring team on the cruise. In order for a core to be useful, it not only has to bring back mud, but bring back the right kind of mud — which, in most cases, means it contains sediments that are nicely layered during the time intervals they are interested in studying, and which contains the shells of dead critters who lived in those intervals, which scientists can date by using multi-million dollar accelerated mass spectrometers to measure the carbon isotope ratio in the fragments of shells.
But there is a fundamental randomness to ocean coring that is unlike many other scientific endeavors. When you drop a big tube down to the bottom of the sea, you are never quite sure what you’ll get when you pull it up. Scientists like Graham use multibeam images and other data to locate areas where sediments are likely to accumulate on the seabed, but they never know for sure what’s down there. Sometimes the coring device hits a rock and bends or breaks. Sometimes the mud falls out before the device reaches the surface. And sometimes they pull up a sea pig.
The other night, I stood at the map table in the forward dry lab on the Palmer while Rob Larter, the chief scientist on the trip, and Kelly Hogan, a geophysicist with the British Antarctica Survey, debated with Graham and Minzoni where the next coring site should be. They wanted a site that was close to the grounding line of Thwaites glacier, so they could capture sediments washed out by recent glacier meltwater; they needed a place on the rugged seabed that was relatively flat, where they knew sediments would accumulate; and they needed it to be deeper than about 1,500 feet, because they were looking for the signature of warm Circumpolar Deep Water currents at that level, but not beyond about 2,500 feet, which is where shells and biological remains are dissolved by the increasing acidity of the water. They consulted bathymetry charts and sonar data that gave them some idea of the terrain but, in the end, it was an educated guess. Larter put his finger on the map and said, “Let’s do it there.” The coordinates were relayed up to the bridge of the ship, and before long, we were on our way.
The next big question: Which coring device should they use at this site? On this cruise, the Palmer is equipped with three basic coring tools: a jumbo gravity corer, a Kasten corer and a multicorer. All of them are basically tubes that you drop into the ocean floor to pull up mud, but they serve different purposes. The jumbo gravity corer is capable of pulling out a 20-foot-long tube of mud if sediments are right; the Kasten corer, which is made of steel and weighted at the top, is the sturdiest of the three devices, but only capable of pulling up about 10 feet of mud; and the multicorer — which is the chandelier-like device that trapped the sea pig — has 12 small tubes arranged in a square, and is designed to capture only the top few inches of sediment.
Graham and Minzoni had hoped to send the multicore down because, on the Palmer, scientists are mostly interested in reconstructing the character of the ocean over the last 100 to 1,000 years, a time period that usually lives in the top layers of the sediment. But looking at an instrument in the lab called a sub-bottom profiler — which uses sonar to paint a rough image of sediment layers on the seabed — they weren’t sure about that. So instead, he decided to send down the sturdier, more reliable Kasten corer.
Out on the main deck of the ship, it was snowing and the wind was blowing 30 knots. Marine techs, the unsung heroes of Antarctic science research, rigged the heavy Kasten corer to the winch on the stern and sent it plunging down into the sea. It took about 45 minutes to hit bottom, then another 45 minutes to return to the deck of the Palmer. The corer only captured a few feet of mud, but it had some living organisms in it, so that was good. The jumbo gravity corer, which they sent down next, went down into subglacial till and returned with a big rock wedged in it. The coring team thought they could do better elsewhere, so the ship moved to another site a few miles away, which Larter, Minzoni and Graham thought looked promising. The whole process was repeated. But this time, when they pulled up the jumbo corer, it was jammed full of about 25 feet of beautiful gray-brown mud. Jackpot! In fact, because the sediments were so nicely layered, they also sent down the multicorer. It came back with two short tubes of excellent mud from the surface of the ocean floor — and the sea pig.
Targeting and retrieving good mud is only part of the dark art of coring. Far more complex is dissecting the core itself and figuring out what the layers of mud mean. Most of that analysis is done later, when the cores are shipped to various university labs that are equipped with tools like mass spectrometers, which are used to date biological materials in the cores. But a lot of initial analysis and preparation is done on the ship, too. A few days earlier, I watched Minzoni go over a fresh Kasten core: She read it like a book, tracing the sediments that were layered into the core, pointing out the worms that burrowed their way down, and creating a compelling — and, at this point, entirely speculative — narrative of the story this core told. “To me, the layers of sediment are like chapters in a book,” she said.
On the Palmer, the lab where the cores are dissected feels like a cross between an autopsy room in the morgue and a kindergarten classroom. The cores are laid out on long tables, where the coring team spends hours dissecting the sediments. Muddy coats and pants are piled up on another table, and the sinks are stained with dyes. Crosby, Stills & Nash blasts from cheap speakers. The other day, I watched Victoria Fitzgerald, who is one of Minzoni’s Ph.D students at the University of Alabama, carefully dig gray mud out of a core with a plastic spoon and load it into small plastic bags, which are labeled with the core number and the exact distance from the top of the core where the mud was removed. She excitedly showed me a tiny clam, called a pelecypod, that they had found in another core — under the microscope, it glowed pink from the dye used to highlight biological material in the core.
When we return to Punta Arenas, Chile, on March 25th, all this mud — carefully sorted, catalogued and packed — will be sent off to various universities, where every tiny shell in the sediment will be dated, trace metals will be analyzed, oxygen isotopes in the water will be measured. Eventually, the mud will be transformed into the data, and that data will be re-assembled and interpreted in scientific papers written by the team members. It will also be incorporated into climate models to help create a more accurate understanding of the future of our rapidly warming world.
“Eventually, these cores will tell a story about Antarctica,” Minzoni told me the other day while she was, as usual, doing a thousand things at once. “The question that fascinates me is, what story will they tell?”
As for the sea pig, it survived its adventure in science. After about 24 hours on the ship, one of the crew members lifted it out of the core water with two plastic spoons. Rachel Clark, a Ph.D student from the University of Houston, carried it to the side of the ship. “Its horns glowed bioluminescent as I carried it,” Clark says. “It was beautiful.” At about 3 a.m., in the Antarctic darkness, Clark dumped it overboard, returning it to the privacy of the deep sea.