Maureen Raymo puts what looks like a pinch of sand on a glass slide and powers up her microscope. Under magnification, the grains are revealed to be fossilized shells of tiny ocean creatures that existed millions of years ago. Raymo, a College of Arts and Sciences earth sciences research professor, is a paleoclimatologist, and these remains are to her what dinosaur bones are to paleontologists: keys to the past.
Raymo studies these shells to better understand the ice ages that have waxed and waned over millions of years on our planet. At the moment, she’s searching for evidence to support a theory she and several colleagues recently proposed to explain a conundrum that has puzzled researchers for years: why the timing of ice growth and decay was different between one million and three million years ago compared to the pattern observed for the last million years.
Scientists know that irregularities in the Earth’s orbit, which occur every 23,000, 41,000, and 100,000 years, affect global climate cycles. Those deviations can nudge the northern hemisphere farther from the sun, causing ice to remain through the summer and auguring a new ice age, like the one that ended 10,000 years ago in North America. But starting in the late Pliocene era, some three million years ago, evidence of the 23,000-year cycle of climate change disappeared from the climate record.
That brings us back to those tiny ocean dwellers, any of the 40 species of foraminifera, or forams, single-celled organisms with calcium carbonate shells. When forams die, their shells fall to the bottom of the ocean. Over millions of years, these deposits have become part of the ocean floor, and they happen to be an excellent indicator of the ocean’s temperature at the time of their demise.
“Oxygen has two stable isotopes, oxygen-16 and oxygen-18, the ratio of which in the ocean is controlled by the amount of ice on land,” Raymo explains. “When ice sheets grow on land, water evaporates off the ocean, and the oxygen-16 molecule, because it’s lighter, evaporates at a greater rate.” Thus, when ocean sediment strata are heavy with oxygen-18, it’s a sign that there is more ice on land. “The whole ocean was relatively enriched in the heavy isotope of oxygen 20,000 years ago, and we know independently from fossil coral reefs that the sea level was 130 meters lower,” says Raymo. “All that water was in an ice sheet, and we can see the evidence for ice on land.”
To analyze the oxygen content of ancient ocean water, scientists drill cores deep in the ocean floor at sites around the globe and take long tubes of the sediment — which includes the forams, whose calcite shells reflect the oxygen content — back to the lab. Fossils extracted from the cores are dissolved in acid and tested.
Based on what she found in the sedimentary record, Raymo theorized in a Science paper last year that from one million to three million years ago, a significant change in ice volume was taking place in Antarctica, much more so than conventional wisdom holds, as the tilt and rotation axis of the Earth changed. She proposed that Antarctic ice sheets were melting while ice was forming in the northern hemisphere, canceling the effect of the 23,000-year cycle in the ocean water record as the light and heavy isotopes of oxygen mixed.
If her hypothesis is true, the ice sheet covering North America “should be waxing and waning with both 41,000- and 23,000-year periodicity, not just the 41,000-year period inferred from the oxygen isotopic record of seawater,” Raymo says. To test this idea, she’s examining samples from cores drilled more than 20 years ago just off the mouth of the Mississippi River.
Initial results show water with lighter oxygen isotopes at the predicted intervals, says Raymo. Her theory, which she is writing about in an upcoming issue of Nature, is garnering interest in the paleoclimatology community.