Rock of Ages
Ancient sediment holds key to understanding regional climate history
Deep beneath the surface of ancient glacial lake beds in the Connecticut River Valley, the Earth has written its autobiography. A record of thousands of winters and summers lies in layer upon layer of sediment. To a geologist, the annual bands of clay, silt and sand seen in core samples drilled down dozens of feet tell the story of the last ice age.
Much the way that the rings inside a tree trunk yield clues about the rainfall during a tree’s life, the layers of lake sediment provide evidence of past floods, fluctuating temperatures and other natural events. By examining tiers of earth, researchers during the 20th century were able to establish an account of the geological history of New England during the closing phase of the last ice age.
But there are gaps in the scientific record—missing pages in Earth’s story. Now Tufts geologist Jack Ridge has embarked on a three-year project to fill those gaps. He and two undergraduate students spent this past summer drilling as deep as 100 feet into the ground at various spots in south-central New Hampshire. The resulting core samples should help complete a detailed record of glacial and climate change events that occurred thousands of years ago.
“This represents a rare opportunity,” says Ridge, to compile a “continuous, highly detailed record of what happened during that time period in a land environment.” With the “missing pages” restored, geologists will be able to calibrate the data more accurately, and figure out when specific events occurred with great precision. While records from marine sediment and modern ice caps reveal much about past changes, records from the once-glaciated parts of continents are just as meaningful, but more difficult to obtain.
Over the next two summers, Ridge and his students will collect and analyze samples from other areas in the Connecticut River Valley; the work is funded by a $340,000 grant from the National Science Foundation. In addition, the undergraduates will use the summer work as the basis for their senior theses, and a lecturer in the geology department, Jacob Benner, will help develop a Web site that will also have an educational section for K-12 students. This past summer, geology majors Catherine Beck and Emily Voytek, both A08, participated in the project.
The long, cylindrical core samples, which are dried and stored in plastic sleeves about two-and-a-half feet long, are shades of gray or brown, depending on whether they are predominantly clay or sand. They resemble a slice of geologic spumoni, with alternating layers of sediment of varying widths, punctuated by a cross-section of the occasional animal or plant fossil. Each pair of lighter and darker layers, or couplet, signifies a year—the lighter layer for the summer; the darker for the winter. The winter layers are darker because they contain finer clay particles; the summer layers are sandier, and thus lighter in color.
Geologists call the annual couplets “varves,” and the accumulated record of regional geologic history is known as the New England Varve Chronology.
Once Ridge and his team fill in the missing pieces of the chronology, they will have the “longest continuous, high-resolution record” of the changes wrought by the end of the last ice age—that is, the period 11,000 to 18,000 years ago, when the giant ice sheet that had covered New England began receding northward as climate began to warm, eventually shaping the face of the land we inhabit today.
Ridge’s project has its origins in the work of Swedish geologist Ernst Antevs, who in the 1920s collected and measured varves at surface exposures in the Connecticut River Valley, from Hartford, Conn., to St. Johnsbury. Vt.
“But one major gap has existed in all that time,” Ridge says. “When we consolidate the whole varve sequence, we can calibrate [the age of the various varves] much more accurately.”
The reason the varve sequence is important “is that it connects climate effects, relative to other global events,” he adds. “We can figure out when the events occurred.” For instance, the thinner varve layers indicate colder temperatures—“because when it was colder, the snow and ice did not melt as fast,” and thus, less sediment was washed into the ancient lake.
“When it warmed up—and it can warm up quickly—you would expect to see real thick summer layers,” Ridge says. “We can put the patterns together and figure out what the climate was doing. And what’s important to understand is not only the pattern, but what caused that pattern of climate and how fast these changes occurred.”
Another goal of the project is to fine-tune the existing varve chronology, and to better calibrate the existing data. Using fossils found in the varves, Ridge’s team can use radiocarbon dating—a technique that did not exist when Antevs was doing his work in the early 20th century—to place them at a specific time in the past.
Inevitably, those outside Ridge’s field who learn about the project ask him how it relates to today’s climate change controversies. Those who expect an answer that will bolster a particular opinion are likely to be disappointed. “The record of past climate back 11,000 or 18,000 years ago is not a good analog for what’s happening now,” he says.
“Then, there were huge ice caps that covered North America. The climate systems were running differently, and ocean circulation was different. In North America, the edge of the ice cap covered almost all of Canada and extended into Ohio and Pennsylvania, and was a major influence on climate. Studying ancient patterns can give you an understanding of how climate operates, and that may give you insight [into today’s situation], but it’s a completely different beast.”
Helene Ragovin is a senior writer in Tufts’ Office of Publications. She can be reached at firstname.lastname@example.org. This story ran in the December 2007 issue of the Tufts Journal.