Anthropologists have long wondered where the ancestors of the Pueblo Indians found the more than 200,000 trees they used to build elaborate structures in barren Chaco Canyon, New Mexico, around the turn of the last millennium.
Now researchers at the University of Arizona are finding that the solution to this mystery is elemental. They used the element strontium as a fingerprint to match some of the ancient logs with two of three likely suspects -- forested mountaintops more than 50 miles from Chaco Canyon.
Their results were published in the Sept. 25 issue of the Proceedings of the National Academy of Sciences.
"It's essentially a paternity test. We asked, 'Which mountains did these trees grow in?'" explained UA research specialist Nathan English, lead author of the article. The research team also includes Julio Betancourt of the U.S. Geological Survey and an adjunct UA geoscientist, UA geologist Jay Quade, and dendroarcheologist Jeffrey Dean of the Laboratory of Tree-Ring Research.
The strontium signature answered their question: The wood in the Chaco Canyon structures came from the Chuska and the San Mateo Mountains, but not from the San Pedro Mountains.
The technique also helped them answer other questions, such as the likely patterns of resource use by this culture that thrived from the 9th century through the middle of the 12th century in Chaco Canyon.
Anthropologists and other scientists have long been fascinated by the structures built by ancestral Puebloans, known as the Anasazi, partly because they represented such an immense undertaking.
Chaco Canyon has 12 "great houses," each containing several hundred rooms in multiple stories. The rock for the intricate masonry in these great houses came from the stepped sandstone cliffs that border the canyon.
But the big conifer trees that were used for roof support -- mainly ponderosa pine, Douglas fir, spruce and fir -- were brought in from afar.
In the mid-1980s, Betancourt and Dean used images from scanning electron microscopes to show that about a quarter of the architectural timber was spruce and fir. These species have not grown near Chaco Canyon since the last ice age, which ended more than 10,000 years ago.
So researchers had figured the source of timber for the Chaco Canyon construction must have been the distant mountaintops. Now they know exactly which mountaintops were harvested.
In addition, the UA research team has used information from this study and previous efforts to piece together a larger picture of resource use patterns.
"Until now it was assumed that the Chacoans depleted resources in an outward growing circle, constantly having to go further for timber as nearby stands of ponderosa and juniper were consumed," English explained.
Yet the local construction included logs from both the Chuska and San Mateo Mountains that Dean and other tree-ring scientists dated to as early as 974.
"This means they were already importing timber from far away even though there may have been nearby stands of less desirable timber," English continued. "It's much like we harvest timber today -- we find the richest, most appropriate stands and log them rather than moving outward in distance and time from the lumber mill."
Betancourt agreed that the evidence called for a shift in thinking away from a model of resource depletion.
"In reality, the human impact on these forests was minimized, though unintentionally. They were going after saplings, so this was more like thinning than clearcutting," Betancourt explained. Many modern-day foresters encourage thinning of western forests to prevent the build-up of small trees and dead wood that can fuel large-scale fires.
But moving relatively young trees required a coordinated effort. The Anasazi, advanced as they were, lacked vehicles and even horses. A prehistoric system of roads indicates that thousands of logs, typically 15 feet long and weighing about 600 pounds, were carried by hand to Chaco Canyon, Betancourt said.
Another indication that these ancient people collaborated on construction efforts comes from this study. Trees from the same year and the same mountain were used to build different great houses, for instance, English noted.
An unforeseen implication of the strontium study emerged, to the delight of Betancourt. In the future, these isotopes might help track variations of atmospheric dust sources in time and space, he said.
Ecological studies elsewhere in New Mexico indicate that 80 percent of the strontium taken up by trees comes from dust in the atmosphere rather than the bedrock underlying the forest.
This supports the findings from the Chaco Canyon study that trees from each of the mountaintops are isotopically similar despite local variations in bedrock, meaning that the variation between mountaintops must be due to regionally mixed atmospheric dust rather than the local bedrock in the immediate vicinity of the trees.
"The scale of variability is probably on the order of tens of kilometers," Betancourt said. "It opens the field to so many applications."
For instance, researchers might be able to reconstruct prevailing wind circulation patterns of the past if they are captured by variations in strontium ratios in old wood or other datable plant parts.
English refined the laboratory technique to compare strontium ratios in wood, but it could apply to a variety of plant materials, including the ancient corn cobs sometimes found at the Chaco Canyon site in piles assumed to be garbage dumps.
The technique requires dissolving normally insoluble samples of the wood or plant and then capturing the strontium with an ion-specific resin. Then the sample can be analyzed on a mass spectrometer.
The UA mass spectrometer used for this study reports -- to a precision of 1 in 100,000 -- the ratio of the heavier strontium isotope to the lighter strontium isotopes. (Isotopes are atoms of the same element that have slightly different weights because some have an extra neutron or two.)
[Contact: Nathan English, Julio Betancourt, Jeffrey Dean, Jay Quade]