Antarctic ice sheets first began to form some 34 million years ago, during a period of sharply declining atmospheric carbon dioxide levels, a new study of ancient algae suggests.
Jefferson Beck/Goddard Space Flight Center/NASA/AP
Antarctica's vast ice sheets first grew when carbon dioxide levels in the Earth's atmosphere sharply declined millions of years ago, scientists now find.
Carbon dioxide is a greenhouse gas — it traps heat radiating away from the Earth's surface. High levels of it in the atmosphere are linked with global warming, while low levels are linked with global cooling. Many such periods of warming and cooling have occurred in the Earth's history, with repercussions for climate around the planet.
But reconstructions of what atmospheric carbon dioxide levels were like back when glaciers began to cover Antarctica nearly 34 million years ago had appeared contradictory. Some research actually suggested carbon dioxide levels rose just before and across this time, a period known as the Eocene-Oligocene climate transition, which is the opposite of what would be expected as prime glacier-growing conditions.
Now research suggests that a sharp decline of atmospheric carbon dioxide levels may have played a major role in seeding Antarctica's glaciers.
Scientists investigated alkenones — tough organic compounds only produced by certain types of algae — to find the carbon dioxide signatures of this period. These photosynthetic organisms would have used carbon dioxide that entered the water from the air, so looking at the chemical makeup of ancient deposits of alkenones can give an idea of what levels of the gas were like in the past.
Paleoclimatologist and geochemist Mark Pagani at Yale University and his team collected alkenones at six deep sea locations across the planet. They sampled spots both near and far from the poles, to get a better sense of what global atmospheric carbon dioxide levels were like during this particular period.