Pack Rat Piles: Rodent rubbish provides ice age thermometer

For a person, life as a pack rat is one of obsessively collecting, say, newspapers, computer parts, food containers, or maybe all of these. But a literal pack rat gathers plant fragments, bone bits, fecal pellets, and even, occasionally, eyewear.

“A friend of mine lost his glasses to a pack rat,” says Kenneth Cole of the U.S. Geological Survey in Flagstaff, Ariz. In the September Geology, Cole and a colleague report that pack rats’ fossilized collections, secreted away for millennia in caves and rocky overhangs, can improve the portrait of global temperatures at the end of the last ice age.

Known as the Younger Dryas, this portion of the ice age lasted from about 12,900 to 11,600 years ago. Temperatures in Europe, Greenland, and the North Atlantic Ocean during this time averaged 10°C below today’s average temperatures. Scientists have relied on many lines of evidence to reconstruct climate trends. Layers of ice and sea sediment, for example, indicate precipitation and atmospheric composition.

These techniques can’t be used everywhere, however. So, in the arid deserts that surround the Grand Canyon, Cole and Samantha Arundel of Northern Arizona University in Flagstaff have turned to pack rats’ fossilized collections, or middens.

In their study, the researchers found that Younger Dryas winters in the region around the Grand Canyon averaged as much as 8.7°C cooler than winters there do today. That’s about 4°C below previous estimates.

Cole and Arundel revealed the local ice age climate by considering the unique temperature gradient of the Grand Canyon along with clues from pack rat scat and fossilized pieces of a plant called Utah agave that turn up in middens.

“If you walk down the canyon,” Cole explains, “it’s like walking from Oregon to Las Vegas.” That temperature trend was also present during the last ice age. Cole and Arundel reasoned that if they could determine an ancient temperature within the canyon, they could extrapolate to the temperature at the rim and in the surrounding area.

That’s where the Utah agave comes in. It can’t grow where temperatures fall below –8°C. Assuming that the pack rats have a limited range, when the researchers found agave in a midden within the Grand Canyon, they proposed that the location had been above this temperature.

To determine whether an agave-containing midden had originated during the Younger Dryas years, the researchers applied radiocarbon dating to fecal pellets in the same midden. From the location of Younger Dryas middens containing agave and the known temperature gradient, the scientists could infer the ice age temperatures around the canyon.

“This is remarkable detail that more or less matches, in timing and magnitude,” the temperature changes found from studying layers of ice in Greenland, comments Julio Betancourt of the U.S. Geological Survey in Tucson. Betancourt notes, however, that the data from pack rat middens are “messy and subject to large uncertainties” because radiocarbon techniques can pinpoint a date only to within roughly 100 years.

Nevertheless, Cole predicts that the new temperatures will be used in computer simulations to give researchers a better global picture of past temperatures and perhaps to project temperatures into the future.

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