Drilling Through Ice Sheets for a Record of the Past

On July 1, 1993, a drill penetrating the thickest part of Greenland's ice sheet struck rock. This ended drilling for an American project that retrieved just over 3 kilometers of ice core-a complete cross-sectional sampling of the ice sheet. Thirty-two kilometers away, European scientists had finished drilling through the ice sheet in a similar project a year earlier. Field work began for both projects in 1989. The two projects were closely coordinated. It was decided that data from the two cores would complement one another and determine possible influences from local flow patterns of the glacier.

The holes were drilled using a hollow bit from which a cylindrical core of ice 10 meters in diameter was retrieved in segments around 3 m long. As the cores were retrieved, a portion of each was analyzed by scientists at each site. The remaining ice was shipped to laboratories for more detailed studies.

The reason for drilling at the highest part of the ice sheet is that here the snow converts to glacier ice and moves straight downward as it becomes buried. Thus, drilling here assures a complete record of past snowfall and eliminates complications from lateral flow. The ice record here extends back 250,000 years.

Among other investigations, chemical analyses were made to determine precisely the composition of air in bubbles within the ice. The bubbles in each layer contain air trapped at the time of snowfall. Other analyses determined the composition of dust and minute particles that settled out of the atmosphere with the snow.

Results from these cores added to information obtained from previous drilling near the edges of the Greenland ice sheet and from cores obtained by Russians at Vostok, the central and thickest part of the East Antarctic Ice Sheet.
Layers representing annual snowfall are well-defined for the upper part of the glacier. A high quality, continuous record of annual snowfall and clues to the climate for each year can be determined for at least as far back as 13,000 years ago. This means we have a precise record of the transition from the last ice age to the present warmer climate. Climate changes are inferred in part by determining variations in annual snowfall as represented by the thickness of ice layers. Other techniques for determining climatic conditions involve correlating particular oxygen and hydrogen isotopic ratios to variations in global climate.

One surprising result is how quickly the climate changed at the end of the last ice age. Research on Greenland cores indicates that about 11,700 years ago the temperature increased by 6°C within just a few years' time. Scientists had previously thought the warming that ended an ice age was gradual. This and other discoveries should give us new insight on what factors are most significant for climate changes.

Moreover, researchers have also been able to correlate major volcanic eruptions with specific past years based on the concentration of sulfates in layers of ice. The sulfates are fallout from SO2 blasted into the high atmosphere by volcanoes (e.g., as at Mount Pinatubo, described in chapters 1 and 4). For cores dating from the present to 2,000 years ago, most of the ice layers having relatively high concentrations of sulfates can be correlated to known historical eruptions. Going further back in time, the ice cores give us a detailed record of when eruptions occurred before significant records were kept by humans. The ice cores provide evidence that increased volcanic activity 8,000 to 9,000 years ago correlates with cooler temperatures for that period, indicating that heavy volcanic activity is a significant factor in changing the earth's climate.