RUTGERS (US)—A 3.2 million-year temperature record from the deep ocean is a reminder that greenhouse gases—and their effect on the earth’s climate—have been around for hundreds of millennia, playing a key role in the start of the ice ages in the Northern Hemisphere millions of years ago.
Yair Rosenthal, a professor of marine science in Rutger’s Institute of Marine and Coastal Sciences, and Sindia Sosdian, Rosenthal’s former student and now a postdoctoral scholar at the Australian National University, established the temperature record and have discovered that, beginning about 900,000 years ago, the nature of the ice sheets themselves made ice ages longer and colder. Their findings were published in a recent edition of Science.
Sosdian and Rosenthal demonstrated that progressive cooling of the deep North Atlantic by about 2 degrees Celsius helped bring on the initial Northern Hemisphere ice ages 2.7 million years ago. They also concluded that the way ice sheets grew and shrank influenced the length and intensity of ice ages.
The large continental, Northern Hemisphere ice sheets formed on top of relatively loose sediments during what geologists call the Pliocene-Pleistocene Epoch, before 900,000 years ago. These sediments, saturated with meltwater, made the ice sheets resting on them unstable and unable to grow to great heights.
About 900,000 years ago, at the middle of the Pleistocene Epoch, successive ice ages had worn away so much sediment that ice sheets were forming on bare bedrock because so much sediment had worn away. The bedrock was very stable, and the ice sheets were able to grow much thicker. The tall ice sheets were able to survive through relatively warm periods between ice ages, and because the ice sheets didn’t decay as quickly, ice ages became longer.
Ice ages typically wax and wane, Rosenthal explains. In the late Pleiocene, the Northern Hemisphere cycled in and out of ice ages every 40,000 years or so. But about 900,000 years ago, the cycle extended to 100,000 years.
“Scientists used to argue about whether those transitional enhancements were associated with major cooling, and if so, why?” Rosenthal says. “But before you had a temperature record, you couldn’t really tell. We know now that each of these transitions is associated with a potential cooling of the planet.”
Sosdian adds: “We now also know that, although the beginning of ice ages (in the Northern Hemisphere) is linked to greenhouse gases, the change in intensity is related to how ice sheets grow and decay.”
The researchers based their work on the study of the fossilized calcareous shells of foraminifera—tiny marine animals that live on and in the sea floor, and whose shells are buried in the bottom sediment. The deeper in the mud the shells are buried, the older they are. Because the shells’ content of magnesium and calcium varies with temperature, scientists can trace the temperature changes over time by analyzing their chemical composition.
The fossils look bit like sand or salt to the naked eye. Under a microscope, however, they reveal themselves as shells. Sosdian put those fossils in a mass spectrometer, which helped her determine the chemical composition of each sample—the amount of magnesium and calcium.
The samples examined by Sosdian and Rosenthal were taken in July 1983 by scientists on a ship, the Glomar Explorer, belonging to what is now the Integrated Ocean Drilling Program, an international scientific effort that yearly sends specialized drilling ships to collect sediment samples from the ocean bottom.
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