By determining which ice sheets melted to create a colossal increase in sea levels 14,500 years ago, scientists hope to enable better predictions of how current ice melting will affect levels around the world.
PROVIDENCE, R.I. [Brown University] - Around 14,500 years ago, toward the end of the last ice age, melting continental ice sheets drove a sudden and cataclysmic sea level rise of up to 65 feet in just 500 years or less. Despite the scale of the event, known as Meltwater Pulse 1a, scientists still aren't sure which ice sheets were responsible for shedding all that water.
Now, researchers from Brown University have used an updated physical model of sea-level dynamics to reconstruct Meltwater Pulse 1a. Their work, which was supported by a federal grant from the National Science Foundation, was published in Nature Geoscience.
The research team found that an initially modest melting of ice over North America set off a global cascade of ice loss extending to Europe, Asia and Antarctica. The results reveal surprising linkages between ice sheets around the world and could help scientists make better predictions of future sea level rise, according to the researchers.
"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 in Brown's Department of Earth, Environmental and Planetary Sciences, who led the research. "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."
Reconstructing sea level change
To reconstruct events like Meltwater Pulse 1a, scientists start with sea level records preserved in ancient shorelines and ocean sediments. The sediments contain fossil coral and other biological indicators that help establish the timing and magnitude of past sea level fluctuations.
Once they have a sense of how much sea levels changed and where, scientists use a technique called sea level fingerprinting to figure out which ice sheets contributed the meltwater. When massive ice sheets melt, the resulting increase in sea level is not evenly distributed around the globe. Waters rise in some places more than others - and may even fall in some places - depending upon the location of the meltwater source. The pattern of sea level rise and fall at different places around the globe can be used to trace where the meltwater originated.
Sea level fingerprinting requires properly accounting for the physics of a melting ice sheet. Gravity, for example, plays in important role. Ice sheets are so massive that they exert significant gravitational pull, drawing surrounding ocean water toward them. As an ice sheet melts and loses mass, its gravitational pull weakens, allowing water to move away. This means that sea level near the ice sheet may actually decrease in response to melting, while waters rise elsewhere.
Another important factor is how the solid Earth reacts to melting events. Heavy ice sheets press down on the Earth's crust. When an ice sheet's mass decreases due to melting, the crust beneath rebounds. This crustal rebound can also push water away from the meltwater source, redistributing sea level change across the globe.
In this new study, the researchers used a more complete model of these crustal deformation processes. Previous research had only modeled elastic deformation - the rapid, trampoline-like response to changes in surface mass. However, Coonin and her colleagues also considered a second response known as viscous deformation, in which the mantle, the layer of material beneath Earth's crust, "flows" a bit like honey across a tilted plate. It had long been assumed that viscous responses occurred over thousands of years and weren't important for short-duration events like Meltwater Pulse 1a. But results from recent rock deformation experiments at Brown University and elsewhere are changing that view.
"People have shown that this viscous deformation can be important on timescales of decades or centuries," said Harriet Lau, an assistant professor in Brown's Department of Earth, Environmental and Planetary Sciences and study co-author. "Allie was able to incorporate that into her modeling of solid Earth deformation in the context of sea level physics."
A new scenario
The result is a scenario for Meltwater Pulse 1a that differs substantially from previous reconstructions and aligns more closely with available paleo sea level data. The new scenario suggests that the event began with modest melting of the Laurentide ice sheet over North America, which contributed about 10 feet of sea level rise. That was followed by more dramatic melting of ice sheets over Eurasia and West Antarctica, which contributed around 23 and 15 feet respectively. Previous research findings had attributed Meltwater Pulse 1a mostly to a single source - though they didn't always agree on which one. Some scientists attributed it mostly to North America, while others pointed toward Antarctica. None, however, had detected the potential of a causal link across hemispheres.
"We show that using the appropriate physics makes a big difference in sea level predictions," Coonin said.
More research is needed to fully understand how disparate ice sheets are linked, according to the researchers, but the new findings suggest that today's rapid melting of the Greenland Ice Sheet could influence the dynamics of the much larger Antarctic Ice Sheet - even though the two are half a world apart.
The study was co-authored by Sophie Coulson of the University of New Hampshire and supported by the National Science Foundation (NSF-EAR 2311897).
