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. …