Studies of under-ice lakes in Antarctica first alerted scientists to the capability of pooled melt-water to refreeze on the bottom of ice sheets and deform the upper layers. But this accretion ice was considered an anomaly "a weird thing that happened over sub-glacial lakes," not over the entire ice sheet, Antarctic geophysicist Robin E. Bell of Lamont-Doherty Earth Observatory
But in examining bright spots found at the bottom of the Antarctic ice sheet, Bell and colleagues have since discovered water is interacting with over a quarter of the bottom of the ice sheet - freezing and pushing the entire ice sheet up in a way that is surprisingly similar to the lake effect. "In fact, I'd forgotten the connection until last night," she said in an interview today.
The new discovery is adding an unknown dimension to the overall layer-cake growth model for ice sheets: that ice sheets gain height one layer at a time as the amount of snow falling on the top outpaces the amount of ice melting at the bottom. Bell and her team using ice-penetrating radar atop Antarctica have just turned this idea upside down.
"In some places up to half the ice thickness has been added from below," Bell and her international team of colleagues, reported in the new issue of the online journal Sciencexpress.
Working with new radars and other imaging techniques at the crown of the East Antarctic Ice Sheet, the researchers identify two mechanisms that lead to the formation of "freeze-on" ice at the base of the ice sheet. In one case, water at the bottom of ice-filled valleys, refreezes to the base of the mile-thick ice because of the water's close proximity to the overlying ice and fluid flow into the above ice matrix where instead of melting the ice around it, the convective thermal transfer of energy results in the water freezing and buoying up the ice sheet, leading to bending and modifying of the layers to the point of top layer erosion.
Along the mountain ridges that border the valleys, the team found a different mechanism occurs: the internal flow of the ice sheet forces water to flow up the mountainsides where the already super-cooled liquid experiences a reduction in pressure and quickly phase-changes into ice. In both instances, the freeze-on ice "packages" expand upward into the internal ice layers, changing their flow patterns, raising the height of the ice sheet, and causing destruction.
"The addition of 100’s of meters ice to the base of an ice sheet deforms the overlying ice upward," writes Bell. "This upwarping modifies the ice sheet stratigraphy and may impact the surface accumulation by changing the surface slope."
While the old model of top-down surface accumulation still accounts for most changes in the size of ice sheets, researchers are going to have to come to terms with the new bottom-up mechanisms. The new idea is important for climate modelers in general, but also for ice-core specialists reconstructing past climates from isotopes locked away in ancient ice.
The team writes:
"The upwarping of internal layers over accretion sites implies active interaction between basal accretion and the entire ice sheet. The accretion-induced upwarping of basal ice will move old ice to a higher elevation in the ice sheet increasing the potential of preserving very old ice. Alternatively the widespread melt required to support the freeze-on process may have destroyed the ice containing the ancient paleoclimate records. Without inclusion of basal processes, simple models of ice sheet temperatures cannot accurately predict the location of the oldest ice."
"It's an extremely important observation for us because this is potentially lifting the very oldest ice off the bed," geologist Jeff Severinghaus of the Scripps Institution of Oceanography in San Diego who was not involved in the study said in a press release. The lifting he said could either better preserve the older ice higher up in the ice sheet, or "make it harder to interpret the record, if it's shuffled like a deck of cards."
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