Isotopes reveal sources of ancient timbers

Analysis of some of the architectural timbers in ancient dwellings of the American Southwest has shown from which distant forests the massive logs came. This information could shed new light on trade relationships of one of the most complex societies in pre-Columbian North America.

Between A.D. 900 and 1150, Chaco Canyon was a thriving cultural center for the Anasazi. During that period, residents in what would later become northwestern New Mexico used hand-hewn sandstone and more than 200,000 logs to construct a dozen large, multiroom dwellings. Now, these so-called great houses sit abandoned amid an arid, treeless landscape, says Nathan B. English, a geochemist at the University of Arizona in Tucson. Chaco Anasazi culture collapsed during a regional drought that lasted 50 years.

Where the Chaco Anasazi builders obtained their large timbers, which they used mostly as roof beams, has been a mystery, says English. On average, the timbers are straight logs about 5 meters long and 22 centimeters in diameter, and they weigh about 275 kilograms. They don’t show any external damage that would indicate they were dragged, rolled, or floated to the construction site. That means that the loggers lifted and carried the timbers–a substantial investment of effort, English notes.

Today, the nearest stands of suitable trees are found more than 80 km from the canyon, in the upper elevations of three mountain ranges–the Chuskas to the west, the San Mateos to the south, and the San Pedros to the east. Other groups of researchers had previously tried to identify specific sources of the timbers by matching the proportions of their trace elements to those in soils in the mountain ranges. These efforts failed because the overall soil chemistries are too similar.

In the new research, English and his colleagues turned to the analysis of a single element, strontium. They found that samples of living trees in the three different mountain ranges exhibit distinct ratios of two strontium isotopes. Using those ratios, the researchers pinned down the sources of the ancient roof beams. Chaco Anasazi obtained spruce and fir timbers from the Chuska and San Mateo mountain ranges but not from the San Pedros, the scientists report in the Oct. 9 Proceedings of the National Academy of Sciences.

Most of the roof beams are ponderosa pine, which grew in the canyon and on the distant mountainsides. However, the spruce and fir, which make up about 20 percent of the beams, have grown only high in the mountains.

The work upends the views of some archaeologists who had held that the Chaco Canyon residents first cut down all of the local trees and then ranged farther afield as construction on the great houses proceeded. The new findings back recent research suggesting that spruce and fir logs from distant mountaintops were incorporated into the buildings from the get-go.

Analysis of rings in the timbers indicates that trees were cut down in the spring, says English. By matching the thicknesses of tree rings in various logs with a database of regional climate, the researchers showed that logging occurred every year. The consistent size of timbers demonstrates that loggers were selectively harvesting trees over broad regions rather than clearcutting one area before moving on.

William D. Lipe, an anthropologist at Washington State University in Pullman, says the team’s findings are “promising but preliminary” because the researchers didn’t analyze the strontium-isotope ratios in wood samples from other nearby mountain ranges. He and other anthropologists are curious about how the residents of Chaco Canyon motivated outlying Anasazi communities to provide large amounts of resources and manpower. The origins of the Chaco timbers may provide new clues.

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