because it’s lighter, evaporates at a greater rate.” Thus, when sediment strata are Explorations heavy with oxygen-18, it’s a sign RESEARCH AND DISCOVERY that there is more ice on land. “Twenty thousand years ago, the whole ocean was relatively enriched in the heavy isotope of oxygen, and we know indepen- dently from fossil coral reefs that the sea level was 130 me- ters 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 con- tent of ancient ocean water, scientists drill cores deep in the ocean fl oor at sites around the globe and take long tubes of the sediment — which includes the The Search for forams, whose calcite shells re- fl ect the oxygen content — back to the lab. Fossils extracted from the cores are dissolved in the Missing acid and tested. Based on what she found in TINY SHELLS ON THE OCEAN FLOOR HOLD THE the sedimentary record, Raymo theorized in a Science paper ANSWER TO AN ANCIENT CONUNDRUM last year that from one million to three million years ago, a MAUREEN RAYMO puts what looks climate cycles. Those devia tions signifi cant change in ice volume like a pinch of sand on a glass can nudge the northern hemi- was taking place in Antarctica, slide and powers up her micro- sphere farther from the sun, much more so than conven- scope. Under magnifi cation, causing ice to remain through tional wisdom holds, as the tilt the grains are revealed to be the summer and auguring a and rotation axis of the Earth fossilized shells of tiny ocean new ice age, like the one that changed. She proposed that creatures that existed millions ended 10,000 years ago in North Antarctic ice sheets were melt- of years ago. Raymo, a College America. But starting in the Changes in ing while ice was forming in the of Arts and Sciences earth sci- late era, some three northern hemisphere, cancel- ences research professor, is a million years ago, evidence of the Earth’s ing the effect of the 23,000-year paleoclimatologist, and these the 23,000-year cycle of climate orbit can cycle in the ocean water record remains are to her what dino- change disappeared from the nudge the as the light and heavy isotopes saur bones are to paleontolo- climate record. northern of oxygen mixed. gists: keys to the past. That brings us back to those hemisphere If her hypothesis is true, Raymo studies these shells to tiny ocean dwellers, any of the farther from the ice sheet covering North better understand the ice ages forty species of , or America “should be waxing that have waxed and waned forams, single-celled organisms the sun, and waning with both 41,000- over millions of years on our with calcium carbonate shells. causing ice and 23,000-year periodicity, planet. At the moment, she’s When forams die, their shells to remain not just the 41,000-year period searching for evidence to sup- fall to the bottom of the ocean. through the inferred from the oxygen isoto- port a theory she and several Over millions of years, these summer and pic record of seawater,” Raymo colleagues recently proposed deposits have become part of says. To test this idea, she’s to explain a conundrum that the ocean fl oor, and they happen auguring a examining samples from cores has puzzled researchers for to be an excellent indicator of new ice age. drilled more than twenty years years: why the timing of ice the ocean’s temperature at the ago just off the mouth of the growth and decay was different time of their demise. Mississippi River. between one million and three “Oxygen has two stable iso- Initial results show water million years ago compared to topes, oxygen-16 and oxygen-18, with lighter oxygen isotopes at the pattern observed for the last the ratio of which in the ocean the predicted intervals, says million years. is controlled by the amount of Raymo. Her theory, which she Scientists know that irregu- ice on land,” Raymo explains. is writing about in an upcom- larities in the Earth’s orbit, “When ice sheets grow on land, ing issue of Nature, is garner- which occur every 23,000, 41,000, water evaporates off the ocean, ing interest in the paleoclima-

and 100,000 years, affect global and the oxygen-16 molecule, tology community. TAYLOR MCNEIL KALMAN ZABARSKY BY PHOTO

16 BOSTONIA WINTER 2007–2008

16-19 Explorations.indd Sec1:1 12/13/07 1:56:17 PM Maureen Raymo is looking in ocean sediment for evidence of a periodic ice age that disappeared from the climate record some three million years ago.

17 BOSTONIA WINTER 2007–2008

16-19 Explorations.indd Sec1:2 12/13/07 1:56:31 PM