explaining the Last Glacial Maximum
June 3, 2014

Southern Ocean Research Provides New Insight Into Ice Age Climates

redOrbit Staff & Wire Reports - Your Universe Online

A team of MIT scientists has turned to the Southern Ocean in search of an explanation for the Last Glacial Maximum (LGM), a period in the Earth's climate history during which the northern continents were covered by ice sheets, according to new research appearing in the Proceedings of the National Academy of Sciences.

Plankton fossils in deep-sea sediments contain chemical traces that reveal rearranged ocean water masses, as well as extended sea ice coverage off Antarctica, during the LGM, the researchers explained in a statement. In addition, air bubbles in ice cores demonstrate atmospheric carbon dioxide levels far below those seen prior to the Industrial Revolution.

Ice ages are caused by the Earth’s precession, or slow wobble on its axis of rotation during its transit around the sun. However, researchers believe that the decrease in solar energy by itself would not have been enough to cause the LGM, which took place approximately 21,000 years ago.

“We have all these scattered pieces of information about changes in the ocean, atmosphere, and ice cover, and what we really want to see is how they all fit together,” explained Raffaele Ferrari, the MIT professor of physical oceanography who co-authored the paper with colleagues from Princeton University and the California Institute of Technology (Caltech).

Ferrari and his colleagues have long suspected that oceans held the key to unraveling the mystery, since they store tremendous amounts of organic carbon for thousands of years. In addition to keeping the element from reaching the atmosphere as CO2, it also absorbs carbon dioxide from the air through microbial photosynthesis at the surface level, as well as via circulation patterns, the study authors explained.

The investigative team went to the Southern Ocean, also known as the Antarctic Ocean, in search of answers. The ocean is an essential part of the carbon cycle, providing a link between the atmosphere and the deep ocean. It is one of the few regions where the deepest carbon-rich waters make it to the surface to take-in and release CO2.

“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,” the Institute said. “But during the LGM, permanent sea ice covered much more of the Southern Ocean’s surface.”

Ferrari’s team set out to investigate how the extended sea ice would have impacted the ocean’s ability to exchange carbon dioxide with the atmosphere. Their investigation required them to use a mathematical equation that describes wind-driven circulation patterns around Antarctica to calculate how much water would have been trapped beneath the sea ice during the LGM.

They discovered that the massive amount of ice covered the only region where the deep ocean was able to breathe, preventing the Southern Ocean’s CO2 from being exhaled into the atmosphere. Furthermore, the study authors found a link between sea ice change and a large-scale ocean water rearrangement contained in the LGM’s 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,” the Institute said. “Breathing less, the ocean could store a lot more carbon.”

The fact that the Southern Ocean was covered with ice, unable to release its carbon dioxide, helps explain the sizable drop in atmospheric CO2 during the LGM, they noted. The National Science Foundation-funded study also demonstrates a possible dynamic link between sea-ice expansion and the increase in ocean water insulated from the atmosphere – two events that scientists had long viewed as separate and distinct phenomena.

“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,” the Institute explained.

In contrast to the previous belief that independent changes caused CO2 levels to be reduced by the same amount in every ice age, Ferrari explained that the new study indicates “that all the events that co-occurred must be incredibly tightly linked, without much freedom to drift beyond a narrow margin. If there is a causality effect among the events at the start of an ice age, then they could happen in the same ratio.”