During the last North American ice age, large numbers of icebergs calved from the North American ice sheet and plunged into the North Atlantic Ocean. Scientists have thought that these iceberg surges disrupted a key ocean circulation pattern, but a new study found little evidence of a disruption during some of the major iceberg surges.

Understanding the controls on and the stability of this ocean circulation system could help scientists better assess this system’s vulnerability to future climate change.

These major iceberg surges are known as Heinrich events. While the study found evidence that some Heinrich events had a major impact on the system of ocean currents that transfer heat from the Southern to the Northern Hemisphere (Atlantic Meridional Overturning Circulation, or AMOC), the study found little evidence of major changes to the AMOC for two of the Heinrich Events that occurred earlier, at the height of the last ice age, at about 24,000 and 31,000 years ago.

“That’s surprising, you would expect to see disruption for all of the Heinrich events,” said Jean Lynch-Stieglitz, a professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology in Atlanta, and the study’s lead author. “We just didn’t see any indication that anything out of the ordinary was going on during those previous Heinrich events.”

The study was published January 12 in the journal Nature Geoscience and was supported by the National Science Foundation (NSF). One of the paper’s co-authors, L. Gene Henry, was an undergraduate at Georgia Tech during the research project.

The study’s findings suggest that the broader climate changes seen at the time of the Heinrich events — changes in precipitation at lower latitudes, for example — may not be linked in a direct, simple way to the AMOC.

To examine the impact of Heinrich events on this ocean circulation, the researchers travelled to the Florida Straits, where they took sediment cores from the ocean floor. They analyzed the fossilized shells of a bottom dwelling single celled organisms (foraminifera) for specific oxygen and carbon isotope ratios. These isotopes provide valuable information about the properties of the seawater in the Florida Straits at the time when these small shells were formed over the past 35,000 years.

“Surprisingly, it’s been very, very difficult to get good records of the past variability of the overturning circulation on these time scales,” Lynch-Stieglitz said.

Previous studies have examined the influence of Heinrich events on the AMOC, but the results have been conflicting. Most of the previous studies examined carbon isotopes in deep water foraminifera. Lynch-Stieglitz’s team used a new approach to the problem by examining the upper part of the overturning circulation. The research team also used computer models to show the sensitivity of the seawater properties at these shallower depths to changes in ocean circulation.

“Understanding the controls on and the stability of this circulation system will ultimately help scientists to better assess the AMOC’s vulnerability to future climate change,” Lynch-Stieglitz said.

This research is supported by the National Science Foundation (NSF) under award number OCE-0096472, OCE-0648258, and OCE-1102743. Any conclusions or opinions are those of the authors and do not necessarily represent the official views of the NSF.

CITATION: Jean Lynch-Stieglitz, et al., "Muted change in Atlantic overturning circulation over some glacial-aged Heinrich Events," (Nature Geoscience, January 2014). (http://dx.doi.org/10.1038/NGEO2045).

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