Earth`s mantle affects long-term sea-level rise estimates

Last Updated: Friday, May 24, 2013 - 13:58

Washington: A Syracuse University researcher has argued that the Earth`s mantle affects long-term sea-level rise estimates.

From Virginia to Florida, there is a prehistoric shoreline that, in some parts, rests more than 280 feet above modern sea level. The shoreline was carved by waves more than 3 million years ago-possible evidence of a once higher sea level, triggered by ice-sheet melting.

But new findings by a team of researchers, including Robert Moucha, assistant professor of Earth Sciences in The College of Arts and Sciences, reveal that the shoreline has been uplifted by more than 210 feet, meaning less ice melted than expected.

Equally compelling is the fact that the shoreline is not flat, as it should be, but is distorted, reflecting the pushing motion of the Earth`s mantle.

This is big news, said Moucha, for scientists who use the coastline to predict future sea-level rise. It`s also a cautionary tale for those who rely almost exclusively on cycles of glacial advance and retreat to study sea-level changes.

Moucha and his colleagues-led by David Rowley, professor of geophysical sciences at the University of Chicago-have been using computer modeling to pinpoint exactly what melted during this interglacial period, some 3 million years ago. So far, evidenced is stacked in favor of Greenland, West Antarctica and the sprawling East Antarctica ice sheet, but the new shoreline uplift implies that East Antarctica may have melted some or not at all.

"It`s less than previous estimates had implied," said Rowley.

Moucha`s findings show that the jagged shoreline may have been caused by the interplay between the Earth`s surface and its mantle-a process known as dynamic topography. Advanced modeling suggests that the shoreline, referred to as the Orangeburg Scarp, may have shifted as much as 196 feet. Modeling also accounts for other effects, such as the build-up of offshore sediments and glacial retreats.

"Dynamic topography is a very important contributor to Earth`s surface evolution. With this work, we can demonstrate that even small-scale features, long considered outside the realm of mantle influence, are reflective of mantle contributions," said Rowley.

Central to Moucha`s argument is the fact that viscous mantle flows everywhere, all the time. As a result, it`s nearly impossible to find what he calls "stable reference points" on the Earth`s surface to accurately measure global sea-level rise.

"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," said Moucha, who joined SU`s faculty in 2011.

Another consideration is the size of the ice sheet. Between periods of glacial activity (such as the one from 3 million years ago and the one we are in now), ice sheets are generally smaller. Jerry Mitrovica, professor of geophysics at Harvard University who also contributed to the paper, said the same mantle processes that drive plate tectonics also deform elevations of ancient shorelines.

"You can`t ignore this, or your estimate of the size of the ancient ice sheets will be wrong," he aserted.

Moucha puts it this way: "Because ice sheets have mass and mass results in gravitational attraction, the sea level actually falls near the melting ice sheet and rises when it`s further away. This variability has enabled us to unravel which ice sheet contributed to sea-level rise and how much of [the sheet] melted."

The SU geophysicist credits much of the group`s success to state-of-the-art seismic tomography, a geological imaging technique led by Nathan Simmons at California`s Lawrence Livermore National Laboratory.

A paper on their research recently appeared in Science Express.


First Published: Friday, May 24, 2013 - 13:57

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