In a kind of geological mystery, scientists have known for decades that a massive ice sheet stretched to cover most of Canada and much of the northeastern U.S. 25,000 years ago. What’s been trickier to pin down is how — and especially how quickly — it reached its ultimate size.
One clue to answering that, Tamara Pico said, may involve changes to the Hudson River.
Pico, who is a Graduate School of Arts and Sciences Ph.D. student working in the group led by Jerry Mitrovica, the Frank B. Baird Jr. Professor of Science, is the lead author of a study that estimates how glaciers moved by examining how the weight of the ice sheet altered topography and led to changes in the river’s course. The study is described in a July paper published in Geology.
“The Hudson River has changed course multiple times over the last million years,” Pico said. “The last time was about 30,000 years ago, just before the last glacial maximum, when it moved to the east.
“That ancestral channel has been dated and mapped … and the way the ice sheet connects to this is: As it is growing, it’s loading the crust it’s sitting on. The Earth is like bread dough on these time scales, so as it gets depressed under the ice sheet, the region around it bulges upward. In fact, we call it the peripheral bulge. The Hudson is sitting on this bulge, and as it’s lifted up and tilted, the river can be forced to change directions.”
To develop a system that could connect the growth of the ice sheet with changes in the Hudson’s direction, Pico began with a model for how the Earth deforms in response to various loads.
“So we can say, if there’s an ice sheet over Canada, I can predict the land in New York City to be uplifted by X many meters,” she said. “What we did was create a number of different ice histories that show how the ice sheet might have grown, each of which predicts a certain pattern of uplift, and then we can model how the river might have evolved in response to that upwelling.”
The result, Pico said, is a model that may for the first time be able to use the changes in natural features in the landscape to measure the growth of ice sheets.
“This is the first time a study has used the change in a river’s direction to understand which ice history is most likely,” she said. “There’s very little data about how the ice sheet grew because as it grows it acts like a bulldozer and scrapes everything away to the edges. We have plenty of information about how the ice retreats, because it deposits debris as it melts back, but we don’t get that type of record as the ice is advancing.”