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A date for the ice ages.

For nearly 2 million years, Earth has been moving to the rhythm of the ice ages, bopping back and forth between long glacial epochs and short, balmy spans known as interglacials. To understand what choreographs this global dance, researchers need to determine the timing of the ice ages-a subject of much debate in recent years. A team of oceanographers has now developed a technique, using radioactive elements in seafloor sediments, to pin down the dates of the most recent interglacials.

The shells of ancient marine algae played a pivotal role in the research, explains Niall C. Slowey of Texas A&M University in College Station. As the algae were growing thousands of years ago, their calcium carbonate shells incorporated atoms of uranium from the seawater. After the algae died, their shells blanketed the seafloor, and the enclosed uranium began to decay into thorium.

By measuring the ratios of these radioactive elements in algal shells collected near the Bahamas, Slowey and his colleagues determined the age of the sediments. This enabled them to date dramatic shifts in the sediments' oxygen isotope ratios, which record the ice ages.

Oceanographers had previously relied on the carbon-14 dating technique to judge the age of deep-sea sediments, but this method reaches back only 45,000 years. Because uranium decays more slowly, it provides a means of dating sediments going back several hundred thousand years.

According to the new study, the last interglacial began sometime before 127,000 years ago and ended 120,000 years ago, the scientists report in the Sept. 19 Nature. These findings match dates taken from shallow-water coral, but they contradict substantially older dates obtained from minerals growing on the wall of a Nevada cave called Devils Hole (SN: 10/10/92, p. 228).

Since the late 1800s, scientists have theorized that subtle shifts in Earth's orbit orchestrate the glacial cycle. The Devils Hole record, however, suggested that the last interglacial warming occurred during an orbital phase that should have been associated with cooling. The new dates from Bahamian sediments support the orbital theory, says Slowey.

Other researchers disagree. Jurate M. Landwehr of the U.S. Geological Survey in Reston, Va., notes that Slowey and his coworkers failed to date the beginning of the last interglacial, which makes it impossible to compare their findings with the Devils Hole record. "They are asking the right questions.

Everybody wants to be able to date sediments directly. But they really have not done the critical experiments yet," she says.
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Title Annotation:Earth Science; new technique discovered for determining the dates of the most recent interglacial periods
Publication:Science News
Article Type:Brief Article
Date:Oct 12, 1996
Words:414
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