Quakes may have launched iceberg flotillas

By Environmental News Network staff

Duke University scientists hypothesize that earthquakes caused a flotilla of icebergs during the last ice age.

Scientists have wondered for a long time what caused the massive swarms of icebergs that peeled off from the Canadian ice sheet and roamed the North Atlantic Ocean six different times during the last ice age.

The standard scientific explanation has attributed these giant roaming ice cubes to brief thaws in the ice age climate. But two scientists from Duke University have hypothesized that a succession of earthquakes was the cause.

Geologist Peter Malin and his former graduate student, Allen Hunt, suggest that the growing weight of the ice sheet would have triggered quakes by causing periodic crustal failure along what is now the eastern Canadian coast. Each temblor could then shake loose large numbers of rock encrusted icebergs from the ice 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 argue 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.

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 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.

Copyright 1998, Environmental News Network, All Rights Reserved

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Related ENN Stories:

• Earth cools in persistent 1,500-year rythmn
• Global warming could trigger ocean current collapse

Related sites:

• Nature
• Duke University

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