Clues From Dust Particles

Dust particles and other bits of debris are also common in both the Greenland and Antarctic ice cores, and they represent the third major source of information on past climates found in ice cores. The amount of dust in a layer of ice is an indication of how dry the region was and how strong the winds were at the time the ice formed. Dust is far more prevalent in ice deposited during glacial periods than in ice formed during interglacials. For example, layers of the Vostok ice core that were laid down in glacial periods contain 20 to 40 times as much dust as the layers from interglacials.

At about the same time that Dansgaard-Oeschger events were first being described, evidence from both ice and sediment cores led to the discovery of another kind of rapid climatic shift. While studying sediment cores from the floor of the North Atlantic Ocean, paleoclimatologist Hartmut Heinrich of the University of Gottingen in Germany noticed unusual layers of debris in some of the cores. These layers, which became known as Heinrich layers, consisted of pebbles, soil, and other material sandwiched between more typical sediment layers. However, this type of debris could only have come from land. How did it get to the bottom of the sea?

By comparing his sediment cores with ice-core data, Heinrich determined that the layers were associated with periods of rapid regional cooling, which are now known as “Heinrich events.” At least six Heinrich events, each lasting 1,000 to 2,000 years, have occurred at irregular intervals during the last 70,000 years. Heinrich theorized that the debris was carried into the oceans by icebergs, mostly from the ice sheet that covered what is now northeastern North America. As the ice sheet expanded during the periods of cooling, it picked up large amounts of soil and rock. Icebergs broke away from the edge of the ice sheet and drifted into the North Atlantic Ocean. The icebergs cooled the surface water of the entire North Atlantic, like ice cubes in a bowl of water. That led to further regional cooling of the atmosphere as the water absorbed heat from the air. As the icebergs melted, the soil and rocks trapped inside them came free and sank, forming layers of debris on the sea floor. The reason for the Heinrich events, each of which ended as abruptly as it began, remained a mystery in 2000.