Catastrophic Outburst Floods Carved Greenland’s “Grand Canyon”
Boulder, Colo., USA: Buried a mile beneath Greenland’s thick ice sheet is a
network of canyons so deep and long that the largest of these has been
called Greenland’s “Grand Canyon.” This megacanyon’s shape suggests it was
carved by running water prior to widespread glaciation, but exactly when
and how the island’s grandest canyon formed are topics of intense debate.
Now scientists from the U.S. and Denmark are proposing a surprising new
hypothesis for the megacanyon’s formation: catastrophic “outburst” floods
that suddenly and repeatedly drained large meltwater-filled lakes. The
findings, published this week in the journal Geology, also suggest
that Greenland’s subglacial canyon network has influenced the island’s ice
sheet since its inception.
Although repeated outburst floods have been suggested as the mechanism by
which the Columbia River and other North America canyon networks formed,
they had not previously been considered as the source of the remarkable
landscape hidden beneath the Greenland Ice Sheet, says Benjamin Keisling,
the study’s lead author and a former graduate student at the University of
the Massachusetts, who also collaborated with researchers at Denmark’s
Centre for Ice and Climate during a National Science Foundation GROW
fellowship.
“If the floods we propose occurred, they could have influenced ocean
circulation, causing abrupt climate changes with regional and perhaps
global significance,” says Keisling, now a postdoctoral fellow at
Lamont-Doherty Earth Observatory. “The megacanyon beneath northern
Greenland also influences how ice and water flow in the subglacial
environment today, which affects present-day ice-sheet stability,” he says.
A Different Approach
In most studies of Greenland, researchers use the modern ice sheet as a
starting point for understanding how it has changed over time. But for this
study, Keisling and his co-authors decided on a different approach:
investigating what Greenland looked like prior to widespread glaciation.
“We wanted to better understand the dynamics of ‘glacial inception’—how,
where, and why the ice sheet first grew on an ice-free island,” he says.
The team also wanted to gain insights into how the ice sheet grew back
after melting. “We know from prior work this has happened multiple times in
the past and could again in the future, given enough global warming,” says
Keisling.
The researchers used coupled ice-sheet and climate models to simulate the
Greenland Ice Sheet’s evolution over multiple glacial-interglacial cycles
during the global cooling from the Pliocene into the Pleistocene, 2.58
million years ago. They found that following long periods with stable
temperatures, an exceptionally warm period could cause the ice sheet to
retreat rapidly. This melting led to the development of large, ice-dammed
lakes in areas where the bedrock was still depressed due to the former ice
sheet’s weight.
The simulations eventually show the ice dams give way, leading to large
outburst floods. “Over time,” says Keisling, “it appears that the filling
and draining of these lakes as the ice repeatedly retreated and advanced
carved Greenland’s megacanyons.” Similar floods have been documented at the
edge of other retreating ice sheets, he says.
Ice-Sheet Stability: Past, Present, and Future
Based on comparisons with modern outburst floods, the researchers estimate
that it took tens to hundreds of these events to carve Greenland’s largest
canyon. According to Keisling, widespread sediment deposition in the
water-filled basins may have also impacted the ice sheet’s behavior each
time it grew back.
Ultimately, Keisling says, the study results point to testable hypotheses
that can guide future research to finally settle the ongoing debate about
whether the Greenland Ice Sheet’s stability has changed over time. “Knowing
the history of Greenland’s bedrock provides context for understanding the
ice sheet’s long-term behavior,” he says. “This helps paint a picture of
what happened during past warm periods when the melting ice sheet caused
global sea levels to rise—a phenomenon we are also seeing today.”
FEATURED ARTICLE
Pliocene-Pleistocene megafloods as a mechanism for Greenlandic
megacanyon formation
Benjamin Keisling and colleagues, keisling@ldeo.columbia.edu
URL:
https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G47253.1/584570/Pliocene-Pleistocene-megafloods-as-a-mechanism-for
GEOLOGY articles are online at
http://geology.geoscienceworld.org/content/early/recent
. Representatives of the media may obtain complimentary articles by
contacting Kea Giles at the e-mail address above. Please discuss articles
of interest with the authors before publishing stories on their work, and
please make reference to GEOLOGY in articles published. Non-media requests
for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.
https://www.geosociety.org
# # #