How catastrophic floods may have carved Greenland’s ‘grand canyon’

AMHERST, Mass. – For years, geologists have debated how and when a network of canyons formed under the Greenland ice sheet, especially one that is so deep and long that it is called the “Grand Canyon of Greenland.” Its shape suggests that it was carved by running water followed by glaciation, but until now, “the genesis of this canyon, and similar characteristics in northern Greenland, remain unknown,” say the authors of a new article.

Scientists from the University of Massachusetts Amherst and the University of Copenhagen Ice and Climate Center are now proposing a new mechanism for how the megacannon formed, from a series of catastrophic ‘outburst’ floods that suddenly and repeatedly drained lakes of water melted over time. . Based on simulations of the ice sheet model of the history of the first ice sheet, they show that the climate and topography of the rock bed have “exerted strong controls” on the ice sheet since its inception.

First author Benjamin Keisling, now a postdoctoral fellow at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, New York, did the job as a graduate student with lead author and advisor Rob DeConto at UMass Amherst. He collaborated with colleagues in Denmark, where Keisling had a GROW grant from the National Science Foundation. Details now appear in the diary geology.

Keisling explains that before, repeated flooding seemed to be the mechanism by which the Columbia River and other canyon networks in North America were formed, but they had not been considered to play a role in the formation of the tortured landscape under the ice cap. from Greenland.

He says: “If the floods we proposed occurred, they could have influenced ocean circulation, causing abrupt climate changes of regional and perhaps global significance. The megacannon below northern Greenland also influences the way ice and water flow in today’s subglacial environment, affecting the present -day stability of the ice sheet. ”

He recalls that in most Greenland studies, researchers use the modern ice cap as a starting point to understand how it has changed over time. But Keisling and his co-authors took a different approach, investigating what Greenland was like before the widespread ice age. “We wanted to better understand the dynamics of the” glacial start: how, where and why the ice cap first grew on an ice-free island, “he says.

The team also wanted to better understand how the ice cap grew back after it melted. “We know from previous work that this has happened multiple times in the past and could happen again in the future, given enough global warming,” says Keisling.

They used coupled climate and ice sheet models to simulate the evolution of the Greenland ice sheet over many glacial-interglacial cycles in the last few million years. They found that after long periods with stable temperatures, an exceptionally warm period could cause the ice sheet to recede rapidly. This led to the formation of large ice lakes in areas where the bedrock was still depressed by the weight of the old ice cap.

Their simulations show that the ice dams eventually give way as major floods. “Over time,” says Keisling, “it appears that the filling and drainage of these lakes as the ice receded and repeatedly advanced carved into the Greenland mega-houses.” Similar flooding has been documented at the edge of other retreating ice sheets, he says.

Comparing Greenland to modern floods, the researchers estimate that as many as hundreds of floods carved out its Grand Canyon. The results suggest testable hypotheses for future research that could resolve the lengthy debate over whether the stability of the ice sheet has changed over time, they point out.

“Knowing the history of Greenland’s bedrock provides a context for understanding the long-term behavior of the ice sheet,” says Keisling. “This helps paint a picture of what happened during the last warm periods when the melting ice sheet caused the world’s sea level rise, a phenomenon that we are also seeing today.”

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This work was supported by NASA, an NSF grant and a GROW grant, and the Danish National Research Foundation.