June/July 2005
Use of Strontium Isotopic Ratios to Determine Sources of Architectural Timber in Chaco Canyon, New Mexico
Julio L. Betancourt, USGS, Desert Laboratory, Tucson AZ
Amanda C. Reynolds, Dept. of Geosciences, NSF-IGERT Program
in Archeometry, University of Arizona
Nathan B. English, Dept. of Geosciences, University of
Arizona
Jay Quade, Desert Laboratory and Dept. of Geosciences,
University of Arizona
P. Jonathan Patchett, Dept. of Geosciences, University of
Arizona
Jeffrey S. Dean, Laboratory of Tree-Ring Research, University
of Arizona
In two papers published in 2001 and 2005, a team of scientists from USGS and the University of Arizona's Desert Laboratory, Department of Geosciences, and Laboratory of Tree Ring-Research collaborated to use strontium (Sr) isotopic analysis to determine distant logging sources for the spectacular pueblos built in Chaco Canyon between ~AD 900-1200.
In the southwestern U.S., 87Sr/86Sr ratios in archeological bone and plant tissue are being used increasingly to decipher prehistoric migration patterns, residential shifts in population, and long-distance procurement of timber and foodstuffs. Strontium exists in predictable and measurable quantities in rocks and sediment, as well as in plant matter, bones, and teeth recoverable from archeological sites. 87Sr/86Sr ratios are maintained from soil water, through plants, and up the trophic chain, making it ideal for provenance studies of plant and animal materials in geologically diverse regions.
Architectural timber common in southwestern ruins is particularly well suited for 87Sr/86Sr analysis. In most cases, the abundant and well-preserved logs have been identified to species, their architectural function and placement has been established, and their cutting dates have been determined to the exact year, depending on the condition of the timber. This dated and well-provenienced wood has been archived at the Laboratory of Tree-Ring Research, University of Arizona, and is readily accessible for geochemical and other studies. Probably the richest such archive is associated with more than a century of excavations at Chaco Canyon, New Mexico. Construction of Chacoan great houses such as Pueblo Bonito and Chetro Ketl required ~200,000 highland conifers, including spruce (Picea spp.), fir (Abies spp.), Douglas-fir (Pseudotsuga menziesii), and ponderosa pine (Pinus ponderosa)(Fig. 1).
Figure 1. Photograph taken by archeologist Earl Morris in the 1920's
of a cribbed log roof of a small kiva (Kiva L), a ceremonial room, at
Pueblo Bonito.
Paradoxically, Chaco Canyon is set in desertscrub-grassland in the middle of the San Juan Basin and at least 75 km removed from the nearest potential timber sources (Fig. 2). The packrat midden record indicates that highland conifers were either absent locally, as in the case of spruce and fir, or too rare, as in the case of Douglas fir and ponderosa pine, to have provisioned the Anasazi (prehistoric Puebloans) builders of the Chacoan great houses. Most of this wood had to be felled, processed, harvested and hauled from one or more distant highland areas. The distances and directions of sources for timber procurement are one measure of economic, political and social relationships across the San Juan Basin. Location of the source stands can be narrowed somewhat by species (i.e., spruce and fir grow only on some mountaintops), but more specific locations require application of geochemical tracers.
Figure 2. Map of San Juan Basin, northwestern New Mexico, showing location
of Chaco Canyon relative to spruce/fir and ponderosa pine forests that
could have provisioned pueblo construction at Chaco Canyon.
Diverse rock types with diverse chemical signatures characterize the San Juan Basin and surrounding highlands, particularly the mountaintops. A few hundred living spruce, fir and ponderosa across these ranges were cored and analyzed for 87Sr/86Sr ratios, and were used in "fingerprinting" potential sources for the Chacoan timbers. Our first study (English et al. 2001) showed that spruce and fir in six of the great houses came mostly from the Chuska Mountains, with some contributions from the San Mateo Mountains to the south (Fig. 3). [NOTE: The English et al. (2001) was featured in a story by Jared Diamond in the journal Nature (access story below), as well as in Jared's new book, Collapse: How Societies Choose to Fail or Succeed (Viking Books, 2005)]. Our second study (Reynolds et al. 2005) indicates that the architectural ponderosa used in three of the Chacoan great houses likely came mostly from the Chuska Mountains to the west and the La Plata and San Juan Mountains to the north and possibly Hosta Butte to the south. Comparison between ponderosa and spruce/fir architectural samples indicates that great house construction may have been provisioned from three general directions, but not from the east. There are no systematic patterns in spruce/fir or ponderosa provenance by great house or time, suggesting the use of common stockpiles from a few preferred sources.
Figure 3. Strontium isotopic ratios in spruce and fir (triangle) and ponderosa
(circle) growing in mountain ranges that rim the San Juan Basin and preserved
as architectural timber in the great houses at Chaco Canyon.
The multiple and distant sources for food and timber, now based on hundreds of isotopic values from modern and archeological samples, confirm conventional wisdom about the geographic scope of the larger Chacoan system. The complexity of this procurement warns against simple generalizations based on just one species, a single class of botanical artifact, or a few isotopic values.
Diamond, J. 2001. Tree trail to Chaco Canyon. Nature 413, 687-690.
English, N.B., Betancourt, J.L., Dean, J.S., Quade, J. 2001. Strontium isotopes reveal source of architectural timber at Chaco Canyon, New Mexico. Proceedings of the National Academy of Sciences 98, 11891-11896.
Reynolds, A. C., Betancourt, J. L., Quade, J., Patchett, P. J., Dean, J. S., and Stein, J. 2005. 87Sr/86Sr sourcing of ponderosa pine used in Anasazi Great House construction at Chaco Canyon, New Mexico. Journal of Archaeological Science doi:10.1016/j.jas.2005.01.016.
For more information contact:
Julio Betancourt (jlbetanc@usgs.gov)