• Atlantic;
  • abyssal;
  • circulation

[1] Scientists have known for 30 years that atmospheric carbon dioxide (CO2) levels varied substantially during the ice ages, yet the cause remains uncertain. Given that the deep ocean contains 50 times more carbon than the atmosphere, variable air-sea exchange may have played a primary role in regulating atmospheric CO2. One of the primary goals of paleoceanography is to determine whether the deep ocean sequestered more carbon during glacial times. Today the deep Atlantic is filled by water masses that form in the North Atlantic near Greenland and in the Southern Ocean near Antarctica. In the South Atlantic the boundary between these water masses sits approximately 4 kilometers below the ocean's surface. Analyses of microfossils from deep-sea sediments show the pattern was very different during the Last Glacial Maximum (LGM, ∼20,000 years ago). The carbon isotopic ratio of these fossils, which reflects the chemistry of the water in which they were formed, indicates there was a stronger isotopic gradient with water depth during the LGM. Water from the Southern Ocean also occupied a much larger proportion of the deep Atlantic, filling it from the bottom to an approximately 2-kilometer water depth. (Paleoceanography, doi:10.1029/2010PA001938, 2011)