The question could not be settled by a second set of ice cores drilled at a nearby site by the U.S. Greenland Ice Sheet Project 2. These cores showed the same rapid climate shifts as the GRIP cores from 100,000 years ago to the present. Ice layers corresponding to times before 100,000 years ago were too folded and scrambled to provide reliable information. The sediment cores showed evidence of rapid climate swings between 100,000 and 65,000 years ago, which matched those seen in both the European and American ice cores. The Lamont-Doherty scientists examined the cores for preserved shells of microscopic animal life that thrive in cold surface waters and for limestone fragments scraped off continents by advancing glaciers, carried out to sea by icebergs and deposited on the ocean floor.
Icebergs are further indicators of cold climates and sea temperatures.
They found high amounts of shells and ice-rafted materials at times corresponding to cold air temperatures revealed by the ice cores. But those materials nearly disappeared in layers older than 110,000 years. Until seafloor cores were correlated with the newly drilled ice cores, scientists did not think marine records could provide such detailed information. For some 10,000 years, the earth has enjoyed steady and dependable climate, an interglacial period known as the Holocene. For the preceding 100,000 years, the earth experienced an ice age marked by wildly fluctuating air and ocean temperatures and waxing and waning ice sheets.