Deep-Sea Cores Show Rapid Shifts

Photograph: Gerard Bond
Photograph: Rusty Lotti

New clues from deep-sea sediment cores have revealed that earth's climate has shifted frequently and dramatically in a cycle of 2,000 to 3,000 years--abruptly dropping average air temperatures 5 to 10 degrees F and launching icebergs into the North Atlantic.

Evidence for such rapid and recurrent climate change was reported Feb. 17 by scientists from Columbia's Lamont-Doherty Earth Observatory in the journal Science and at a news conference at the annual meeting of the American Association for the Advancement of Science in Atlanta.

"We believe that this cycle has been in effect for tens of thousands of years and may still be operating today; if so, it's important to know where we now stand in it," Gerard Bond, senior scientist at Lamont-Doherty, said in an interview. "There is clear evidence that significant climate shifts can occur within a lifetime. It's important to determine the mechanisms that trigger them if we are to prepare for the potential effects."

Bond conducted the research with Rusty Lotti, curator of the Ocean Sediment Core Laboratory at Lamont-Doherty.

Analyzing sediments cored from the North Atlantic, the scientists found layers of tiny stones that had been frozen into the bases of advancing glaciers, carried out to sea, deposited on the ocean floor and buried by subsequent sediments. Regularly spaced layers of pebbly debris showed that between 38,000 and 10,500 years ago, "the amount of glacial ice discharged into the North Atlantic increased suddenly every 2,000 to 3,000 years," they wrote in Science.

Bond and Lotti discovered that these iceberg flotillas occurred much more frequently than previously believed and were launched simultaneously from more than one glacier surrounding the North Atlantic. The 2-3,000-year iceberg cycles also coincided precisely with rapid oscillations of regional air temperatures--as recorded in ice cores recently extracted on Greenland. Taken together, the evidence points to an as-yet unknown trigger that regularly caused regional air temperatures to drop and induced ice sheets to discharge icebergs at the same time.

The findings provide an intriguing new clue in the current, rapidly developing research effort to piece together how the earth's atmosphere, oceans and ice sheets interact to produce global climate change.

Evidence for iceberg armadas first appeared in 1988 when German scientist Hartmut Heinrich found six layers in North Atlantic sediment cores of white carbonate-rich stones that originated from North America. Each layer represented a massive discharge of icebergs that occurred every 7-10,000 years between 10,500 and 70,000 years ago. Each discharge is known as a Heinrich event.

In 1993, Bond and colleagues linked Heinrich events to rapid shifts in air and ocean temperatures, documenting a previously unrecognized link among ice sheets, ocean and atmosphere, leading to a theory that internal dynamics of the Laurentide Ice Sheet, which covered North America, created a cycle of growth and collapse that periodically launched iceberg armadas into Hudson Bay.

But last May, Bond and Lotti reported that glaciers in the northern and separate southern portion of the Laurentide Ice Sheet and in Iceland all discharged icebergs at nearly the same times. That challenged the theory that the Laurentide alone triggered the icebergs.

In their new research, Bond and Lotti did high-resolution analyses of the debris rafted out to sea by icebergs. They found small amounts of dark, glassy rock that they traced to volcanoes in Iceland, as well as layers of red-stained rocky particles that originated predominantly from the region around the Gulf of St. Lawrence. Traces of these dark and reddish particles appeared in Heinrich layers, but they were overwhelmed by much larger quantities of white carbonate grains and overlooked. Focusing on the layers of dark and red grains of rock, the Lamont-Doherty scientists found that icebergs were discharged into the Denmark Strait and Gulf of St. Lawrence at the same time every 2-3,000 years.

"Although Heinrich events were originally thought to be the dominant features of the North Atlantic's deep-sea record, our findings demonstrated that they are superimposed on the rhythm of the [2-3,000-year] ice-rafting events," the scientists wrote in Science. "The Heinrich events reflect another, slower rhythm tied to the massive [iceberg] discharges in Hudson Strait."

Searching for a trigger for the iceberg armadas, Bond and Lotti offered two hypotheses. In the first, another large sheet, whose dynamics have not yet been explored, may collapse every 2-3,000 years. Possible culprits are the sheets in Greenland, Barents Sea, Scandinavia, or even Antarctica, which still exist today. Icebergs cast into the North Atlantic would melt, creating a large influx of freshwater that changed ocean circulation patterns and caused abrupt shifts in the region's sea surface and air temperatures and spurred North American and Icelandic ice sheets to surge. Alternatively, a mechanism operating within the atmosphere-ocean system, such as an internal oscillation of the ocean's circulation, may have caused cyclical drops in air temperatures, which launched the iceberg armadas every 2,000 to 3,000 years.

The research was supported by the NSF and NOAA.

Columbia University Record -- February 24, 1995 -- Vol. 20, No. 18