Geotimes - November 2003 - Stuck between a rock and a cold place

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Paleoclimate
Stuck between a rock and a cold place

A stalagmite mined from an island cave in the Indian Ocean suggests that the ages currently assigned to the gold standard of ancient climate records — the Greenland ice cores — need revision for the period between 55,000 and 42,000 years ago.

Stephen Burns, a geochemist at the University of Massachusetts in Amherst, and colleagues report in the Sept. 5 Science that the climate record of the stalagmite, as recorded by oxygen isotope ratios in the calcite, “bears a striking resemblance” to that of the Greenland ice cores. However, they say, the dates of a well-known series of warm spikes are off. According to Burns, this discrepancy means the ages currently assigned to the two Greenland ice cores are too young by 3,000 and 5,000 years. The age offset could cause geologists to rethink the connections between important events in the climate record.

Dominik Fleitmann and Diego Sanz explore the Moomi Cave on Socotra Island as part of their research, collecting the cave’s stalagmites, which recorded ancient climate conditions. Photo by Stephen Burns.

The stalagmite grew during a period of time containing five of the 23 Dansgaard/
Oeschger (D/O) events that occurred during the last ice age. These climate oscillation events, also called interstadials, began with an abrupt warming, lasted hundreds to thousands of years, and then ended with a subsequent cooling.

The D/O events create easily recognizable spikes in the climate record, making them valuable marker tools in the study of climate change and particularly of abrupt climate change. The 3- to 5-degree Celsius increase at the onset of D/O events often happened in less than a decade. The events also appear to be related to the concentrations of greenhouse gases in the atmosphere.

Determining the absolute ages of D/O events, Burn says, has implications for the study of teleconnections — how and when climate change is communicated between different regions of the world and which region is responsible for climate forcing.

The stalagmite, collected from Moomi Cave on Socotra Island off the coast of Yemen, was dated using uranium-thorium dating techniques at 22 points along a 1.73-meter-long core. “The two big advantages stalagmites have are high enough resolution to resolve climate at a decadal scale and that they can be radiometrically dated,” Burns says.

Because the tropics drive Earth’s heat exchange processes between the ocean and the atmosphere, understanding the relationship between tropical precipitation — as seen in the monsoon record of the stalagmite — and D/O events is key, Burns says.

The tropics, however, lack a continuous climate record on the scale of the ice cores, drilled in the early 1990s by the Greenland Ice Sheet Project (GISP) and the Greenland Ice Core Project (GRIP). The more than 3,000-meter-long cores represent a continuous climate record reaching back more than 110,000 years.

“The GISP and GRIP records are probably the best-dated paleoclimate records on Earth because, in general, they’re based on annually dated records,” says Paul Mayewski, former member of the GISP team and now director of the Climate Change Institute at the University of Maine in Orono. “We counted down year by year just like a giant tree ring.”

With speleothems, Mayewski says, the deposition record is not necessarily continuous and complete. Attempts at matching the absolute dates of climate events from one record to another with current technology are difficult, he says. “All these things are the best that we have, but the fact that the debate continues means that nobody’s come up with a definitive way of comparing these events,” he says.

In the stalagmite study, the pattern in the tropical climate record matches that of the Greenland ice cores well once corrected for the age offset. When warm periods occurred in Greenland, the tropics experienced increased precipitation. With a revision to the ice core ages to correct for the offset, the study supports the idea that the climate of the tropics was closely coupled to that of the northern latitudes.

The offset, Burns says, does not represent a possible lag time in the communication of climate change from the tropics to the northern latitudes. “Basically the atmosphere is not going to let that happen,” he says. “Any large rapid climate change in one area of Earth is going to be propagated via the atmosphere to other parts of Earth.”

Burns says more stalagmite studies are needed to determine the absolute ages of D/O events. “It may be that only other stalagmite records will be able to corroborate our results or refine them, or even negate them.”

Sara Pratt
Geotimes contributing writer

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