A new study by researchers at Brown University, published in *Nature Geoscience* and backed by the National Science Foundation, sheds light on the mystery. By refining a physical model of sea-level behavior, the team pieced together a clearer picture of how this ancient flooding event played out.
Their analysis indicates that initial melting of the North American Laurentide Ice Sheet triggered a global chain reaction, prompting widespread ice loss across Eurasia, Asia, and Antarctica. This interlinked behavior among far-flung ice sheets could hold valuable insights for understanding modern sea level threats.
"We see a distinct interhemispheric pattern of melting associated with this catastrophic sea level rise in the past," said Allie Coonin, a Ph.D. candidate at Brown's Department of Earth, Environmental and Planetary Sciences and the study's lead author. "That tells us that there's some sort of mechanism that is responsible for linking these ice sheets across hemispheres, and that's important for how we understand the stability of the Greenland and West Antarctic ice sheets today."
To reconstruct past sea levels, scientists rely on geological clues preserved in ancient shorelines and marine sediments. Fossil corals and other biological markers help anchor the timing and magnitude of sea level changes. From there, they employ a method called sea level fingerprinting to pinpoint meltwater sources. Because meltwater from different regions affects global sea levels unevenly due to gravitational and geophysical factors, regional patterns in sea level change serve as diagnostic tools.
The physics behind these changes are complex. Massive ice sheets exert gravitational pull that draws seawater toward them. When they melt, this pull weakens, allowing local sea levels to drop even as they rise elsewhere. Meanwhile, the crust beneath the ice rebounds from the reduced weight, redistributing water even further.
Past models primarily accounted for fast, elastic rebound of Earth's crust. In contrast, the new study incorporates viscous deformation, a slower mantle flow process akin to honey spreading on a slanted surface. While traditionally considered too sluggish to matter for events like Meltwater Pulse 1a, recent lab findings suggest that viscous flow can respond on century-level timescales.
"People have shown that this viscous deformation can be important on timescales of decades or centuries," noted study co-author Harriet Lau, assistant professor in the same department. "Allie was able to incorporate that into her modeling of solid Earth deformation in the context of sea level physics."
The team's updated model presents a significantly different reconstruction of Meltwater Pulse 1a. It suggests that an initial 10-foot sea level rise from North American melting was followed by larger contributions from Eurasian and West Antarctic ice sheets, adding approximately 23 feet and 15 feet respectively. Earlier models often favored a dominant single-source scenario, split between either North America or Antarctica, without recognizing intercontinental interactions.
"We show that using the appropriate physics makes a big difference in sea level predictions," Coonin said.
The findings imply that current changes in the Greenland Ice Sheet could have cascading effects on Antarctic ice dynamics, despite the geographic divide. The researchers emphasize the need for further investigation into the mechanisms linking distant ice sheets, particularly in the context of today's accelerating climate change.
Research Report:Meltwater Pulse 1A sea-level-rise patterns explained by global cascade of ice loss
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Beyond the Ice Age
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