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Climate: Does the Southern Ocean hold the ice age key?

Abysmal waters play huge role in global carbon cycles

The water in the Antarctic Sound can be smooth as glass, and sometimes look thick and oily, probably because it's so cold. Click on the photo to learn about some of the environmental issues in Antarctica.

The water in the Antarctic Sound can be smooth as glass. bberwyn photo.

By Summit Voice

FRISCO — The remote Southern Ocean, encircling Antarctica, may be a key driver of the carbon cycle, inhaling and exhaling enough carbon to help shift the global climate in and out of ice ages.

For decades, scientists have been trying to figure out what exactly, along with the known wobbles in Earth’s journey around the sun, may cause the huge shifts that lead to vast ice sheets covering many of the planet’s land masses.

Many of the puzzle pieces are known, for example from the paleoclimate record for the last ice age. Chemical traces from plankton fossils in deep-sea sediments reveal rearranged ocean water masses, as well as extended sea ice coverage off Antarctica. Air bubbles in ice cores show that carbon dioxide in the atmosphere was far below levels seen before the Industrial Revolution.

“We have all these scattered pieces of information about changes in the ocean, atmosphere, and ice cover,” says Raffaele Ferrari, the Breene M. Kerr Professor of Physical Oceanography in MIT’s Department of Earth, Atmospheric and Planetary Sciences, “and what we really want to see is how they all fit together.”

Ferrari is part of a team that tried to track the shifts by modeling wind-driven currents in the Southern Ocean, where abysmal waters rise up to the surface to alternately inhale or exhale carbon dioxide. It’s a logical place to look because researchers know that oceans are powerful regulators of Earth’s climate, storing vast amounts of organic carbon for thousands of years, keeping it from escaping into the atmosphere as CO2. Seawater also takes up CO2 from the atmosphere via photosynthesizing microbes at the surface, and via circulation patterns.

The Southern Ocean is one of the only places where the deepest carbon-rich waters ever rise to the surface, to “breathe” CO2 in and out, and the modern-day Southern Ocean has a lot of room to breathe: Deeper, carbon-rich waters are constantly mixing into the waters above, a process enhanced by turbulence as water runs over jagged, deep-ocean ridges.

But during the peak of the last ice age, permanent sea ice covered much more of the Southern Ocean’s surface. Ferrari and colleagues decided to explore how that extended sea ice would have affected the Southern Ocean’s ability to exchange CO2 with the atmosphere.

Using their model, they found that the shock to the entire Earth from this added ice cover was massive: The ice covered the only spot where the deep ocean ever got to breathe. Since the sea ice capped these deep waters, the Southern Ocean’s CO2 was never exhaled to the atmosphere.

The researchers then saw a link between the sea ice change and the massive rearrangement of ocean waters that is evident in the paleoclimate record. Under the expanded sea ice, a greater amount of upwelled deep water sank back downward. Southern Ocean abyssal water eventually filled a greater volume of the entire midlevel and lower ocean — lifting the interface between upper and lower waters to a shallower depth, such that the deep, carbon-rich waters lost contact with the upper ocean. Breathing less, the ocean could store a lot more carbon.

A Southern Ocean suffocated by sea ice, the researchers say, helps explain the big drop in atmospheric CO2 during the Last Glacial Maximum.

The study suggests a dynamic link between sea-ice expansion and the increase of ocean water insulated from the atmosphere, which the field has long treated as independent events. This insight takes on extra relevance in light of the fact that paleoclimatologists need to explain not just the very low levels of atmospheric CO2 during the last ice age, but also the fact that this happened during each of the last four glacial periods, as the paleoclimate record reveals.

Ferrari says that it never made sense to argue that independent changes drew down CO2 by the exact same amount in every ice age. “To me, that means that all the events that co-occurred must be incredibly tightly linked, without much freedom to drift beyond a narrow margin,” he says. “If there is a causality effect among the events at the start of an ice age, then they could happen in the same ratio.”

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