The changing 'history' of a global ice sheet

Imagine that a massive ice sheet covered Canada and oozed down over a large part of the northern United States, like icing spilling down the side of a cake.

That was the situation, somewhere between 19,000 and 26,000 years ago. The ice sheet covered land all the way south to modern-day Pennsylvania, Ohio, Indiana, Michigan, and Wisconsin.

It’s fascinating to picture, but the critical aspect to us today is what happened to the land when that ice sheet melted. And how does that affect modern-day sea-level rise and sinking land.

Ph.D. candidate Karen Williams embarked on computer modeling research to find out about the Earth’s changes after the ice melted. Working with her advisor, Associate Professor D. Sarah Stamps in the Department of Geosciences and collaborators Daniele Melini of the Istituto Nazionale di Geofisica e Vulcanologia in Italy and Giorgio Spada of the Dipartimento di Fisica e Astronomia at the Università di Bologna, Italy, their results were recently published in the Journal of Geophysical Research Solid Earth.

What they found: Ups and downs

Williams used computational modeling to assess the impact of the melting of the Laurentide ice sheet on present-day vertical land motions.

She tested various assumptions about how the ice sheet’s melting affected the Earth, oceans, and gravitational field. The scientific term for how the solid Earth responds to this melting is “glacial isostatic adjustment.” And by “various assumptions,” they simulated how their results depend upon the internal structure of the Earth through nearly 130,000 computational simulations.

“We see a general pattern of downward movements (causing relative sea-level rise) in the eastern United States region,” the authors said.

In eastern Canada, it was the opposite: upward movements causing a lowering of the relative sea-level.

Why land rises or sinks

The reasons for sinking land really start to make sense if you look at what the models suggest versus what you see happening today.

• Williams explained: By getting more accurate model estimates of what is contributing to the land either rising or sinking, “we can identify areas undergoing localized vertical displacements driven by natural and/or anthropogenic sources such as excessive groundwater withdrawal in the Gulf of Mexico or Chesapeake Bay.”
• For example, Stamps said: “Some of the greatest differences between modeled influence of glacial isostatic adjustment and observations occur where there is known groundwater extraction, like Houston, Texas.”

Helping communities plan

The results of the study will help generate maps for researchers, who guide decisions on aquifer management, Williams said. In the end, the research will be incorporated into a comprehensive report for the U.S. Geological Survey that will help Chesapeake Bay area stakeholders better understand the financial, ecological, and social effects of sea-level rise.

“With the improved estimates of vertical displacement driven by glacial isostatic adjustment,” Williams said, “we can better predict land subsidence and relative sea-level changes, which will help address the impacts of present-day coastal hazards.”

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