May 19, 1998 DURHAM., N.C. -- A Duke University geological study proposes a novel cause for the massive and puzzling swarms of icebergs believed to have separated, or "calved," from the Canadian ice sheet to roam the North Atlantic Ocean six different times during the last ice age.
Some scientists have attributed these so called "Heinrich events" to brief thaws in the ice age climate, perhaps caused by shifts in Earth's orbital axis. But Peter Malin and his former graduate student, Allen Hunt, hypothesize a succession of earthquakes were the cause instead.
The scientists published their "iceload-induced" earthquake scenario in the May 14 issue of the scientific journal Nature. Malin is an associate professor of Earth and ocean sciences at Duke's Nicholas School of the Environment, and Hunt is a Ph.D. physicist who studied geology in the school's Earth and ocean sciences division before moving to the University of California at Riverside.
The growing weight of the ice sheet would have triggered those quakes by causing periodic crustal failure along what is now the eastern Canadian coast, according to the researchers' calculations. Each temblor could then shake loose large numbers of rubble-laden icebergs from the looming sheet's edge.
In 1988, German oceanographer Hartmut Heinrich proposed cyclic "ice rafting" as an explanation for the eastern Canadian rock fragments he discovered deposited as rubble layers on the floor of the northeast Atlantic. Geological evidence suggests the rubble patches were deposited in six events during the planet's most recent glacial era.
To explain these Heinrich events, as they are now known, oceanographers proposed the ice sheet originally scraped the rubble from the Canadian continental bedrock. These rock fragments then went to sea on the undersides of icebergs that the seafloor evidence indicated were launched in large numbers. By that scenario, the rubble trails mark the paths where melting icebergs drifted.
Some scientists have suggested the iceberg binge was precipitated by brief warm spells caused by the Milankovich solar cycle. Yugoslavian mathematician and physicist Milutin Milankovich calculated early in the 20th century that cyclical shifts in Earth's orbital axis or orbital position could prompt such thaws by altering seasonal sunlight levels.
Other researchers proposed that, instead of thawing, Canada's Laurentide ice sheet may have been steadily building itself up at that time. In the process, the sheet could have undergone "binge-purge cycles," becoming top heavy enough to periodically shed some of its mass as icebergs. The bergs themselves might have then caused the global climate change that other evidence suggests occurred during that era.
The Duke team began its study after Hunt questioned the thesis that the timing of Heinrich events was directly tied to periodic solar cycles, Malin said in an interview.
Dating techniques based on the radioactive decay of a form of carbon in the rubble layers show the events occurred in six different episodes between 70,000 and 16,000 years ago. But Hunt noted they did not occur at regularly-spaced intervals, as might be expected with a solar-induced phenomenon.
Instead, "their intervals decrease over time," Malin added. "And you can't decrease solar cycles by making the Earth rotate faster around the sun or the seasons somehow shorten."
The opposing idea -- that an ice buildup caused the icebergs to break off -- was supported by evidence the Canadian ice sheet was steadily accumulating between roughly 110,000 and 14,000 years ago. Cores of ancient material, taken from today's Greenland's ice sheet and from ocean sediments, revealed higher amounts at that time of a form of oxygen that increases as the climate cools and ice deposits thicken.
While such an ice buildup could lead to the "binge-purge" cycle scenario, Malin and Hunt argued in their Nature paper that those cycles would have occurred at more evenly-spaced intervals and at a slower rate than the recorded Heinrich events did.
Each theorized iceberg purge would take an estimated 750 years to complete, too long to agree with other evidence that the iceberg swarms calved off in very short intervals, the Duke researchers added. They noted the rubble layers contain almost no foraminifera, tiny shell bearing animals that naturally accumulate on the seafloor at a slow and predictable rate over time. That absence suggest these layers were deposited suddenly.
So, Malin, an earthquake expert, developed evidence for a different explanation for the iceberg flotillas. His calculations showed the ice sheet's growing mass could have caused the underlying Canadian crust to fail at approximately the same intervals as the Heinrich event.
Each of those quakes would have been major ones, registering between Magnitude 7.2 and 8.2 on the Richter scale, which would cause the ground to move suddenly at accelerations greater than one-third Earth's gravity. As the crust shifted, the bottom of the ice sheet's edge would move with it.
"If you take a building, and you move its base at such accelerations, the roof weight will lag behind," Malin said. "So you basically have an object that is off its center of gravity, and it will fall. As the crust shifted, the bottom of the ice sheet's edge would move with it. It doesn't take much imagination to see that if you move the base of a glacier's edge far enough from its top, it's going to begin breaking apart and calving into the ocean."
Earthquakes normally occur along zones where Earth's crustal plates are colliding, sliding underneath each other, or separating. But Malin and Hunt cited evidence for quakes outside of these so-called "tectonic plate boundaries" too.
The two wrote in Nature that in 1929 a magnitude 7.2 temblor centered on the continental shelf off Newfoundland unleashed a tsunami sea wave that killed 27 people. The quake was attributed to uplift, as the crust continues a slow rebound from the now-melted ice sheet's weight.
There is geological evidence for previous "rebound" quakes near the ice sheet's former edges, the Nature authors added, notably the presence of raised shorelines where beaches were suddenly uplifted.
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