WHOI Waypoints: X-Ray Visions of Climate Change


Late in the evening, after traffic in the radiology lab at Falmouth (Mass.) Hospital has slowed, Mea Cook wheels some unusual patients into an empty X-ray room. Ten PVC pipes full of mud—sediment cores from the bottom of the Bering Sea—lay stacked on her cart. As radiologists in neighboring rooms peer inside human patients for signs of broken bones, Cook scans her cores for different signs. Somewhere in the pale layers of sediment cut by bright slashes of ash, she hopes to find secrets of 70,000 years of climate change.

In climatic terms, the Earth is currently basking in a relatively warm and stable period. But jump back 20,000 years to the last ice age, and the climate data curve gets bumpy. During the ice age and the 40,000-year warm period that preceded it, Earth endured a barrage of major climate shifts, including some sudden and drastic changes in the planet’s average temperature.

Paleoclimatologists have deduced from marine sediments and from terrestrial ice that these climate changes were frequent and fast, but just how they occurred remains a mystery. In particular, they are intrigued by the role played by the Pacific Ocean, Earth’s largest water mass. The view into the climate history of that vast ocean has been obscured by a scarcity of adequate sediments. Cook’s cores might provide an important glimpse.

Getting to the core of the matter

The new cores—extracted from floor of the Bering Sea by WHOI Senior Scientist Lloyd Keigwin and colleagues in June 2002—don’t look particularly instructive. In fact, the 20-meter plugs of sediment look like something you might find in a construction ditch after a storm. But these mud-filled PVC and steel pipes are full of information.

Like growth rings in a tree trunk, the layers of sediment are built over time, laying down a geologic record of the passing years. Each layer is laced with the chalk-white calcium carbonate shells of tiny animals called foraminifera. By analyzing the chemical composition of these shells, paleogeologists like Cook can determine the temperature and the amounts of salt, oxygen, and nutrients in the waters when the animals lived. This
data can be used to model past climate conditions and ocean circu-lation patterns.

But not all sediment cores are created equal, and Pacific Ocean cores are notoriously unhelpful. Deep Pacific water tends to be corrosive, dissolving the carbonate shells that geologists seek in their cores. As a result, the Pacific’s role in climate change is poorly understood.

As the recent disaster film The Day After Tomorrow portrayed (with ample Hollywood hyperbole), rapid climate changes are likely triggered, or at least accompanied, by changes in ocean circulation. Modern ocean circulation is dominated by the Atlantic Ocean, but some paleoceanographers theorize that the Pacific may have dominated at times in the past. “In the climate record there is evidence that the circulation in the Atlantic has changed, but no one knows much about what happened in the Pacific,” Cook said. “This is what I’m trying to find out.”

Luckily, Cook’s sediments are an exception to the Pacific problem: They are rich with calcium carbonate shells for chemical analysis. “These are great cores,” she said. “I can resolve events as short as decades and centuries.” Some of the cores include finely layered sections of sediment with which she can resolve three to five year spans.

Three years or even three decades out of 70,000 years may seem like an unnecessary level of detail. But
should the climate begin to change rapidly in the future, it would be useful to know whether past temperatures shifts, ocean circulation flips, and sea level changes happened over 10 years or 100.

“It’s probably going to be the highest-resolution record from the North Pacific,” said Dan McCorkle, an Associate Scientist in the WHOI Department of Geology and Geophysics. The Bering Sea cores won’t be the first to yield a record, as sediments collected off Santa Barbara, Calif., have already been used to create a climate history of the Pacific. But a single location does not a global picture make.

“People have spun a lot of stories about Pacific Ocean paleoceanography from the Santa Barbara record,” said McCorkle, “but we really haven’t had any way to test that.” The Bering Sea cores could finally confirm or refute the Santa Barbara climate story.

Though it is too early to predict her results, Mea hopes she can make a contribution to geologists’ understanding of Earth’s climatic history. “The Pacific could have an important role in global climate that we don’t yet understand. We might be able to use this information to understand what may happen in the future.”

Did you know...

» Mea has been playing the cello since she was eight years old. While other kids were baby-sitting as teenagers, she earned money as a freelance musician, giving lessons, and playing for weddings. She still plays each week in an MIT-based chamber music group.

» She loves movies and, with friends, hosts a classic-movie night every Wednesday for other WHOI film buffs.