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Strontium Isotope Analyses Reveal the Source of
Released: 9/24/2001

Contact Information:
U.S. Department of the Interior, U.S. Geological Survey
Office of Communication
119 National Center
Reston, VA 20192
Julio Betancourt 1-click interview
Phone: 520-670-6821

Geochemical analyses using strontium isotopes show that many of the timbers used to build the prehistoric great houses of Chaco Canyon, N.M., between A.D. 900 and 1150, were hand-carried to the building site from isolated mountaintops 50 to 60 miles away, according to four Arizona scientists.

Writing in the September 25 issue of the Journal of the National Academy of Sciences, Julio Betancourt of the U.S. Geological Survey in Tucson, and Nathan English, Jeffrey Dean and Jay Quade, all of the University of Arizona at Tucson, say that more than 200,000 conifer trees were used as building timbers 1,000 years ago, in what is now a treeless landscape. Strontium isotopes helped the scientists identify, for the first time, the source of the timbers and correlate them with the time periods in which they were cut.

Because strontium from local dust, water and underlying bedrock is incorporated by growing trees, the scientists were able to identify specific logging sites by comparing strontium-86 and strontium–87 ratios in construction beams from different ruins and building periods to ratios in living trees from the surrounding mountains. These ratios show that the beams came from both the Chuska and San Mateo Mountains, but not from the San Pedro Mountains, which are equally close. Incorporation of logs from two sources in the same room, house and year suggest stockpiling and intercommunity collaboration at Chaco Canyon. The use of trees from both the Chuska and San Mateo Mountains, but not from the San Pedro Mountains, as early as 974, suggests that selection of timber sources was driven more by regional socioeconomic ties than by a simple model of resource depletion with distance and time.

The geochemistry of strontium isotopes is relatively well known and strontium –86 and –87 ratios have been used routinely as environmental tracers in geology, hydrology, ecology, and archaeology, since the 1960s. Strontium-87 is derived from the radioactive decay of rubidium-87. Rocks that are older or have higher initial concentrations of rubidium-87, such as granites, have higher strontium–86 and –87 ratios than younger volcanic rocks derived from the earth’s mantle. Sedimentary rocks generally have intermediate values.

Located near the middle of the San Juan Basin in northwestern New Mexico, Chaco Canyon was the focus of a spectacular blooming of the Anasazi culture. Between 900 and 1150, the Chaco Anasazi developed a complex culture, characterized by monumental architecture, advanced agricultural and water control systems, and elaborate road, trail, and signaling networks that integrated numerous communities into a regional exchange, communication and resource- procurement system. This regional system was in full swing in the 11th century, but collapsed during a regional drought that lasted from 1130 to 1180. The site today is a national monument, administered by the National Park Service of the U.S. Department of the Interior.

Twelve multistoried masonry pueblos of several hundred rooms each occupy the Chaco Canyon core of the regional system. A single great house incorporated millions of sandstone fragments from surrounding cliffs and thousands of wood timbers, used as primary and secondary roof beams and door and window lintels. More than 200,000 timbers, with the primary beams averaging 15 feet in length, seven inches in diameter, and 60 pounds in weight, were used in the great houses. Most of this lumber was acquired in predetermined lengths and diameters and came from trees that had to be felled, processed and hauled from distant and widely separated mountaintops. Potential source areas include the La Plata-San Juan, San Pedro-Nacimiento, San Mateo and Chuska Mountains. The absence of appreciable gaps in the sequence of cutting dates indicates that tree felling was a year-around activity. Continual repairs and piecemeal additions were interrupted by flurries in large-scale construction.

Tree species available for construction were, in order of increasing distance from the canyon: cottonwood, along Chaco Wash; pinyon pine and juniper in nearby scarp woodlands; isolated stands of Douglas-fir in shady alcoves; ponderosa pine on high mesas and the lower slopes of mountains; and spruce, fir and aspen on mountaintops more than 45 miles away. Empirical evidence and modeling indicate that by the year 1000 construction and fuel wood harvesting had eradicated local pinyon-juniper woodlands that have yet to recover. After 1000 the Anasazi relied increasingly on conifers from the surrounding mountains. The distance and direction of these montane forests from the Canyon is a measure of the energy expended to harvest and move the timbers, of the organization, ability and determination of the Chaco Anasazi to build monumental architecture, and of economic, political and social relationships across the San Juan Basin.

Because mountain ranges in the San Juan Basin are geologically diverse, geochemical methods could be used to determine the provenance of Chacoan timbers. The scientists compared the strontium-86 and –87 ratios of bedrock, soil and stream water, and spruce and fir growing at possible logging sites in the San Juan Basin to those of select timbers from six of the great houses in Chaco Canyon. Live trees, rocks, stream and soil waters were sampled from the three most accessible localities for prehistoric logging of spruce-fir stands, the San Pedro Mountains about 53 miles to the east; the San Mateo Mountains, about 50 miles to the south, and the Chuska Mountains, about 46 miles to the west.

Cores were extracted from trees of six species growing at various elevations and in a variety of settings separated by about six miles in the San Pedro Mountains, three miles in the San Mateo Mountains and 15 miles in the Chuska Mountains. In March 2000 and May 2001, using a quarter-inch increment bore, the scientists collected more than 200 tree, rock and water samples for strontium analyses.

Dated architectural wood from six of the 12 Chaco Canyon great houses was obtained from the collections of the Laboratory of Tree-Ring Research (LTRR) at the University of Arizona, Tucson, which allowed the scientists to analyze samples from both modern and ancient trees. The dated collection of architectural timber at Chaco Canyon included a high proportion of fast-growing and straight saplings that were used as secondary roof beams.

The Arizona scientists speculate that ponderosa pine stands within 30 miles of Chaco Canyon may not have provided the large numbers of small trees required for construction of the great houses, but that the crests of the Chuska and San Mateo Mountains would have been ideal sites for logging a great variety of conifer species and size classes. "The Anasazi may have focused on both mountain ranges because no single forest could satisfy their need for small trees of particular species and dimensions," said Betancourt. "Timber sources may also have been determined by preexisting sociopolitical ties between residents of Chaco Canyon and outlying communities at the base of the Chuska and San Mateo Mountains, and the paucity of Chacoan "outliers" or roads east of Chaco Canyon may explain why the San Pedro-Nacimiento Mountains were never logged, despite being the same distance from the Canyon as the other mountain ranges."

"The planning and vast distances involved in procuring 200,000 construction beams are a testament to the Chacoans’ ability to organize large intercommunity labor forces to extract timbers from distant mountains or to motivate the inhabitants of these distant areas to fell timbers for use in Chaco Canyon," concluded Betancourt.

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