May 24, 2013
Shoreline Shift May Be Caused By Interactions Between Earth’s Crust And Mantle
Brett Smith for redOrbit.com - Your Universe Online
Many climate models that are based on ancient sea levels warn about the impending dangers of modern-day sea level rise, but what if the Earth beneath our shorelines has shifted over millions of years?
According to a new paper in Science Express, the North American shoreline of over 3 million years ago sits more than 280 feet above the modern sea level from Virginia to Florida.
Using computer modeling to account for sediment buildup and glacial retreats, a team of geophysicists showed that the shoreline shift may have been caused by the interaction between the Earth´s crust and its mantle in a process known as dynamic topography.
“Dynamic topography is a very important contributor to Earth´s surface evolution,” said co-author David Rowley, professor of geophysical sciences at the University of Chicago. “With this work, we can demonstrate that even small-scale features, long considered outside the realm of mantle influence, are reflective of mantle contributions.”
The latest study is partially based on a 2008 paper from co-author Robert Moucha, an assistant professor of Earth Sciences at Syracuse University. In that earlier study, Moucha used evidence from the North American east coast and African west coast to argue against the existence of stable continental platforms.
“The North American East Coast has always been thought of as a passive margin,” or large area free of tectonic activity, Moucha said. “[With Rowley], we´ve challenged the traditional view of passive margins by showing that through observations and numerical simulations, they are subject to long-term deformation, in response to mantle flow.”
Moucha asserted that the viscous mantle flows constantly in every direction, resulting in a lack of what he calls “stable reference points” on the Earth´s surface.
“If one incorrectly assumed that a particular margin is a stable reference frame when, in actuality, it has subsided, his or her assumption would lead to a sea-level rise and, ultimately, to an increase in ice-sheet melt,” Moucha explained
He credits the study´s findings to tomography imaging techniques performed by Nathan Simmons at California´s Lawrence Livermore National Laboratory.
“Nathan, who co-authored the paper, provided me with seismic tomography data, from which I used high-performance computing to model mantle flow,” Moucha said. “A few million years may have taken us a day to render, but a billion years may have taken several weeks or more."
The team said they hope to apply their East Coast model to the Appalachian Mountains, which are also considered a type of passive geology, yet have shown the signs of erosion shaped by dynamic topography.
Team members Rowley and Jerry Mitrovica, a geophysicist from Harvard University, credit Moucha for his groundbreaking approach to dynamic topography.
“Scientists, such as Rob, who produce increasingly accurate models of dynamic topography for the past, are going to be at the front line of this important research area,” Mitrovica said.
“Rob Moucha has demonstrated that dynamic topography is a very important contributor to Earth's surface evolution,” Rowley added. “His study of mantle contributions is appealing on a large number of fronts that I, among others of our collaboration, hope to pursue."
Image 2 (below): The East Coast shoreline, also known as the Orangeburg Scarp, as it may have appeared 3 million years ago. Credit: Syracuse University