The discovery of an impact crater in remote northwestern Greenland may resolve a major climate history question: what caused the planet to suddenly cool around 12,800 years ago? In a new study published last month in the journal Science Advances, the researchers are careful not to make claims about the larger implications of the find. But details, including the size and approximate timing of the impact, offer much to consider about what triggered Earth’s last sudden climate change.
The impact crater was discovered beneath Hiawatha Glacier, under more than a kilometer of ice. Hiawatha is among the largest impact craters ever discovered on Earth, as well as the northernmost and first to be located under ice. Modern geospatial technology has enabled the Earth’s surface to be thoroughly mapped, leaving significant undiscovered features either deep under the sea or beneath ice, like Hiawatha. That a striking and visible geologic feature of Hiawatha crater’s magnitude had yet to be located makes the find even more remarkable.
An aerial view zooms into a sub-ice crater perspective with overlays of Washington, DC and Paris for relative size (Source: Cindy Starr/NASA Scientific Visualization Studio).
A Huge Crater and a Hunch
The initiative to map the crater was led by the intuition of principal author, Kurt Kjær, a glacial geologist at the University of Copenhagen and curator at the Natural History Museum of Denmark. Kjær wondered whether a connection might exist between an anomalous circular ice pattern he observed in satellite images of the Greenland ice sheet and an iron meteorite on display at the museum where he parks his bicycle.
To pursue his hunch, Kjær needed to know what was under the ice. Joe MacGregor, a glaciologist with the NASA Goddard Space Flight Center, unearthed archival imagery from Operation IceBridge. The temporary mission collected critical data used to predict the response of the Earth’s polar ice to climate change and sea-level rise. NASA assembled the operation after an ice monitoring satellite malfunctioned in 2009, bridging the gap until the successor satellite could be launched in September 2018. The aircraft often operated out of Thule Air Base, near Hiawatha. It often activated its instruments in test mode and happened to overfly the impact site on its flight path to the polar ice cap, adding a layer of serendipity to Kjær’s discovery. “Without Operation IceBridge the crater might’ve gone undiscovered for even longer than it did,” MacGregor told GlacierHub. Lucky or not, Kjær had mounted enough evidence to make his case.
A foundation backed by Copenhagen brewery, Carlsberg, funded the mission. A Basler BT-67 aircraft with a state-of-the-art ice-penetrating radar made three flights in May 2016, to map the suspected location. Kjær’s hunch was correct. The radar revealed a massive crater under the ice, suggesting an extraterrestrial impact. Measuring over 31 kilometers in diameter, the imprint left by the impact is among the largest on the planet, big enough to comfortably hold the city of Paris. Most similar-sized craters on Earth have changed much over time, many eroded to the point of unrecognizability. While ice tends to preserve organic material well, the pressure and grinding weight of ice scours topography. Beneath Hiawatha, the disheveled ice still bore signs of the cataclysm. At the bottom of the crater, classic impact characteristics, like central uplift features, were also apparent.
Recognizing the need for conclusive evidence to solidify his impact finding, Kjær visited Hiawatha later in the summer of 2016. In the outflow of the glacier, he found what he was looking for; tektites, a natural glass formed by meteoric impacts, and shocked quartz. Shocked quartz is only found in post-nuclear blast craters or extraterrestrial impact sites, like the Yucatan’s Chicxulub crater, whose impactor caused the mass extinction that killed off the dinosaurs. The Hiawatha crater’s crisp impact features and disrupted ice indicate it collided with the Earth at a much more recent date, perhaps as recent as the last Ice Age.
Could the Impact Have Triggered Sudden Climate Change?
The potential timing of the impact might be the greatest significance of the discovery. The Earth’s climate fluctuates between glacial and relatively warm interglacial periods, like the present. But as the planet thawed from the last ice age, it abruptly stopped warming, and cooled for over a millennium. For decades, climatologists theorized possible causes for this return to near-glaciation, known as the Younger Dryas. The period is named for an Arctic-alpine flower, Dryas octopetala, whose pollen is found in abundance in ice cores from the era. Some scientists believe Younger Dryas climate reversal may have been triggered by an event around 13,000 years ago. But the lack of physical evidence to support an impact hypothesis left the door open for a variety of theories.
A popular hypothesis for the cause of the Younger Dryas period is a sudden influx of melt water into the North Atlantic Ocean. The fresh water would create a stable surface layer, that would both slow the ocean circulation and freeze easily. An impact like the one that caused the Hiawatha crater would turn enough ice into fresh water to suppress the North Atlantic cycle and halt the warming. The timing seems about right.
Broecker Unconvinced Impact Triggered Younger Dryas
Wally Broecker, known as the “Grandfather of Climate Science,” is a geoscientist at Columbia University’s Lamont-Doherty Earth Observatory. Among many climate firsts, Broecker coined the term “global warming” and was the first to recognize the global Ocean Conveyor Belt, a temperature and salinity-driven cycling of deep ocean water. In a 1989 paper published in Nature, Broecker theorized that the Younger Dryas period, and other periods of cooling like it, was triggered by the reorganization of deep ocean circulation — a critical process for modulating the Earth’s climate.
James Kennett is a marine geologist at the University of California, Santa Barbara, and one of Broecker’s co-authors of the 1989 paper. Kennett told Science, “I’d unequivocally predict that this crater is the same age as the Younger Dryas.” The impact would align with Kennett’s theory that a cosmic event precipitated the Younger Dryas cooling period. But, according to Broecker, the slowdowns of the conveyor belt are the effect of internal oscillation of the ocean system, independent of any impact event. In other words, though a meteor collision may have pre-triggered a cooling period, the Younger Dryas would have happened with or without an impact.
Broecker explained to GlacierHub, “I’m not convinced this caused the Younger Dryas. If you look at the record of Greenland ice cores, they happen over and over again,” Broecker said, referring to the Earth’s cycles of glaciation. “You can say the Younger Dryas was unique — it was triggered by an impact and all the others were just an internal oscillation.”
The location of the crater on the edge of Greenland also gave Broecker reason to doubt the impact-trigger for Younger Dryas, “I don’t think it could have melted that much ice,” he said. There are also other uncertainties regarding the impact, for example, the lack of evidence in deep ice cores taken elsewhere in Greenland. “That’s a problem,” Broecker said, referring to the absence of ejecta in the ice cores.
Whether or not ejecta would be present, however, depends on the angle of impact. Jay Melosh, from Purdue University’s Department of Earth, Atmospheric and Planetary Sciences, approached the question with similar restraint. He cautioned against making conclusions about the impact before a core is drilled and recovered, telling GlacierHub, “It will only be proved by drilling through the ice and demonstrating that the basin contains impact metamorphosed rock.”
While the slowdown of ocean circulation may have occurred independent of an impact, effects on biodiversity and humans would be tied to an impact. The Paleo-Indian Clovis culture and megafauna, like the woolly mammoth, are believed to have disappeared around the onset of the Younger Dryas. Until a core can be taken from Hiawatha, down to the impact-melted rocks, uncertainty regarding the timing will remain.
The study remains silent on questions about ocean circulation, providing the more general conclusion, “based on the size of the Hiawatha impact crater, this impact very likely had significant environmental consequences in the Northern Hemisphere and possibly globally.” It hints at forthcoming research and potentially a global quest for further evidence of the Hiawatha impact. Referring to the Younger Dryas impact theorists, Broecker said, “now people will renew the hunt.” In the quest to cross-reference the impact crater with paleoclimate evidence around the world, Hiawatha glacier might become one of the planet’s most significant. As mankind pushes Earth’s system toward the brink, understanding the planet’s most documented, sudden climate change, the Younger Dryas, becomes ever more urgent.