Strontium isotopes and the reconstruction of the Chaco regional system: evaluating uncertainty with Bayesian mixing models.

Brandon Lee Drake,Wetherbee Dorshow,Wirt H Wills,Marian I Hamilton,John P Hart

doi:10.1371/journal.pone.0095580

Abstract

Strontium isotope sourcing has become a common and useful method for assigning sources to archaeological artifacts. In Chaco Canyon, an Ancestral Pueblo regional center in New Mexico, previous studies using these methods have suggested that significant portion of maize and wood originate in the Chuska Mountains region, 75 km to the East. In the present manuscript, these results were tested using both frequentist methods (to determine if geochemical sources can truly be differentiated) and Bayesian methods (to address uncertainty in geochemical source attribution). It was found that Chaco Canyon and the Chuska Mountain region are not easily distinguishable based on radiogenic strontium isotope values. The strontium profiles of many geochemical sources in the region overlap, making it difficult to definitively identify any one particular geochemical source for the canyon's pre-historic maize. Bayesian mixing models support the argument that some spruce and fir wood originated in the San Mateo Mountains, but that this cannot explain all 87Sr/86Sr values in Chaco timber. Overall radiogenic strontium isotope data do not clearly identify a single major geochemical source for maize, ponderosa, and most spruce/fir timber. As such, the degree to which Chaco Canyon relied upon outside support for both food and construction material is still ambiguous.

Highlights

IntroductionBonito Phase food production was based on the cultivation of maize, beans and squash, and possibly non-domesticates such as sunflowers, and there is evidence for water control features designed to direct seasonal runoff into fields [1]
Soil and water samples grouped by Benson et al (2009) as either Chaco Canyon or the Upper Rio Chaco region were combined to form a single strontium isotope ratio set because the canyon is located in the hydrological center of the samples identified as the Upper Rio Chaco (Figure 2)
Maize Sources Our kernel density plots yielded a series of overlapping probability curves that reveal considerable uncertainty in source attribution for maize. 87Sr/86Sr values from Chaco Canyon maize radiocarbon-dated to Pueblo I and II periods largely overlap with the 87Sr/86Sr values of the Chaco watershed source, as well as many of the other defined sources (Figure 3a, 3b)

Summary

Introduction

Bonito Phase food production was based on the cultivation of maize, beans and squash, and possibly non-domesticates such as sunflowers, and there is evidence for water control features designed to direct seasonal runoff into fields [1]. These crops were grown in Chaco despite constraints imposed by aridity and high elevation, but there is considerable debate among specialists about whether the canyon could have produced enough food to support ‘‘monumental’’ Great House construction efforts or even sustain a relatively small residential population [2]. Plants acquire the 87Sr/86Sr value of the soils that they grow in, and numerous studies have confirmed that a plant’s 87Sr/86Sr value is consistently a less variable, more averaged version of the local soil’s [39] [40]

Methods

Results

Conclusion