Rodent rubbish as an Ice-Age thermometer

If you want to know the temperature outside, you look at a thermometer. If you want to know what temperatures were like thousands of years ago, you have to be more creative.

In the area around the Grand Canyon, two researchers from Arizona have been piecing together ancient climate histories in an unusual way. They dig up fossilized collections of objects left behind by pack rats.

Pack rats collect all sorts of things, including plant fragments, bone bits, poop pellets, and even objects such as candy wrappers left behind by people. The animals put the objects in piles and go to the bathroom on them, which helps the pile stick together. Such collections are called middens.

Undisturbed, fossilized middens in caves and under rocky ledges can last for thousands of years. They preserve information about what the environment was like when they were first collected.

The researchers were interested in an era called the Younger Dryas, which lasted from about 12,900 to 11,600 years ago. During this time, temperatures in Europe, Greenland, and the North Atlantic Ocean were about 10 degrees C below today’s average temperatures.

To find out what was going on in the Grand Canyon at the time, the scientists identified middens that had been gathered during the Younger Dryas era.

The Grand Canyon is so deep that the weather becomes warmer as you get nearer to the bottom.

U.S. Geological Survey

In these middens, they looked for the remains of a plant called the Utah agave. The plant can’t grow in places where temperatures drop below –8 degrees C (17.6 degrees F). The Grand Canyon is so deep that the weather gets warmer as you drop down into it. So, the line where middens start containing agave shows where temperatures hovered around –8 degrees C.

Using knowledge about how temperatures vary throughout the canyon today and the location of middens containing agave, the scientists were then able to piece together a picture of climate patterns during the Younger Dryas. They found that winters in the region around the Grand Canyon then were as much as 8.7 degrees C cooler than winters there are today. That’s about 4 degrees C below previous estimates.

It’s amazing what a pile of garbage can tell you about ancient times!—E. Sohn

Going Deeper:

Greene, Katie. 2005. Pack rat piles: Rodent rubbish provides ice age thermometer. Science News 168(Sept. 24):198. Available at http://www.sciencenews.org/articles/20050924/fob7.asp .

You can learn more about pack rat middens and their role in studying climate at www.cpluhna.nau.edu/Tools/packrat_middens.htm (Northern Arizona University) and www.ncdc.noaa.gov/paleo/slides/slideset/#packrat (NOAA).