The past is prologue
Brown University
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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).