National Geographic : 2004 Sep
momentarily ended, Ireland's glaciers began to retreat-rapidly. Meltwater coursed over the land, cutting deep river-size channels and pouring a slurry of mud into the sea. "These were high energy events," says McCabe. As the mud settled, tiny organisms called zooplankton were buried in the sediments. Today, with relative sea level far lower than in the past because the land is no longer weighted with ice, those muddy deposits are up to several hun dred feet above the ocean, and a geologist who knows where to look can find in them the fossils of the shell-covered zooplankton called forami nifera (forams for short). Forams are an inte gral part of paleoclimatological research because their calcareous shells can be dated. And that's why McCabe and Clark have come to this pas ture: to dig about 50 pounds of foram-filled mud for dating. With precise dates for the rapid retreat of the ice in hand, the two will be able to ATIANTAGFORCGIA link Ireland's glacial history with that of North America and Scandinavia. By dating forams from mud on the Irish Sea coast, McCabe and Clark found evidence for a rapid 35-foot rise in global sea level about 19,000 years ago. "That was a Northern Hemisphere melting, a pulling back of the entire ice margin," says Clark. "It wasn't just a little local event. We figure that the equivalent of two ice sheets the size of Greenland's today must have melted within a few hundred years." What could have triggered such a large-scale event? McCabe and Clark argue that it could have been the weight of the ice itself. As the ice sheets grew, their increasing weight pushed down on the underlying land. Where the glaciers sank far enough to reach sea level, the ice began to float, breaking up into icebergs. "That would have added more fresh water to the ocean, changing its salinity and deepwater currents," says Clark.