One of the most interesting and potentially productive lines of research in Southwestern archaeology these days involves the use of chemical analyses of various archaeological materials to extract more information about the societies that used them than is apparent just from looking at them. The oldest and most established type of research like this is radiocarbon dating, which has historically been used less in the Southwest than elsewhere because it’s both expensive and less precise than tree-ring dating, which was invented in the Southwest and has been extremely important in the study of its prehistory. Lately, however, archaeologists in the Southwest have been using radiocarbon more and more, since it can be used on anything organic (useful for sites which produce no datable wood but plenty of other organic material) and it’s been around for so long that the dates are considered very reliable. They’ve also begun to use some other techniques that are newer but have enormous potential, which is already starting to be realized, to illuminate aspects of the past that have been the cause of much debate.
The most important of these is strontium isotope analysis, which we’ve seen before in the analysis of the wood brought to Chaco for architectural use. Like radiocarbon dating, strontium analysis is based on looking at the ratio of two isotopes of an element, one of which is stable and the other of which is produced by the radioactive decay of another element and therefore varies. Unlike radiocarbon, however, strontium cannot be used for dating on archaeological timeframes, since the half-life of the radioactive decay process involved (the conversion of rubidium-87 to strontium-87) is 48.8 billion years. It can, however, be used to identify locations, since the amounts of strontium and rubidium in different areas vary a lot and strontium is absorbed unchanged by organisms from their environment. Thus, in theory, one could test an organic artifact for its strontium ratio, then compare that to the strontium ratios of the water or soil in various places where the artifact may have originated and figure out where it came from. This would then allow all sorts of archaeological conclusions.
Of course, it’s never quite that simple, as the case of the wood shows. It was relatively easy to use this analysis for the high-elevation types of wood that occur in relatively few places in the Southwest, but when the technique was extended to the very common ponderosa pine beams the number of possible origins increased so much that few definite conclusions could be reached. There is also the problem of making sure that the strontium ratios found in the archaeological material actually resulted from growth processes rather than contamination by later mineral deposits. Since this technique is relatively new, the methodology for it is not yet totally worked out, and not every attempt to use it ends up working.
Both the promise and the pitfalls of strontium analysis are shown clearly by a new paper by Larry Benson of the United States Geological Survey. Benson has made something of a name for himself as the main player in the increasingly important analysis of corncobs found in Southwestern archaeological sites. Corn is a useful plant to use for this sort of thing for a number of reasons:
- It’s pretty common, especially in sites like cliff dwellings and Chacoan great houses with especially good preservation of organic material. The Anasazi depended heavily on corn for their diet, so there are corncobs all over the place.
- It grows quickly. This is not important from the perspective of strontium analysis, but it means that radiocarbon dating can provide a very accurate range of dates within which the corn was grown and eaten. This is in contrast to slow-growing plants, such as trees, which have the problem that the part tested may happen to be much older than the date of use. The combination of accurate dating with strontium-based source determination makes corn a very powerful source of information.
- It bears directly on a variety of important cultural questions. Since corn was the main source of food for the Anasazi, finding out if they were growing it themselves or importing it from elsewhere has major implications for models of cultural systems and their means of support. This is a longstanding issue in the study of Chaco specifically.
This particular paper addresses several issues, both substantive and methodological. Substantively, Benson analyzes a set of corncobs excavated from the Gallo dwelling in the Chaco campground in the 1950s and adds the data derived from them to the data from earlier studies of cobs from this site as well as from Pueblo Bonito and Chetro Ketl. He also reports on strontium isotope ratios from several agriculturally productive areas of the Zuni Reservation and adds them to the previously reported data from other parts of the Colorado Plateau. He then combines this new information with the previously reported data to draw some specific conclusions about the sources of some of the cobs. Importantly, however, he does not come to any conclusions about the sources of the newly analyzed Gallo cobs.
The reason for this lies in the methodological side of the paper, which may be the most important in the context of overall research on this topic. The cobs Benson reports for the first time here, unlike the previously analyzed cobs, were not burned, and part of the purpose of this research was to see if the procedures used to prepare and analyze the burned cobs could be used for unburned cobs as well. As it turns out, they can’t, and the strontium ratios from the unburned cobs appear to come from post-depositional mineral contamination rather than growth conditions. This seems to be because the act of burning effectively “seals in” the trace minerals in the cobs, protecting them from contamination. While this result is somewhat disappointing, in that it means that the strontium data from the new cobs can’t be used to draw any conclusions, it is important in informing others that if they want to do this kind of research on unburned corn cobs they need to come up with new procedures. In the course of doing this analysis Benson also uses some data on recent experimental growing of Pueblo varieties of corn in Farmington that provides valuable reference material on just how closely strontium ratios in corncobs can be expected to correspond to the ratios in the soil and water in the area. The answer is closely, but not perfectly, which is also useful information for future researchers.
Despite those issues, however, this paper does include some important substantive conclusions. Although the new cobs couldn’t be used for strontium analysis, they did produce radiocarbon dates, which correspond very closely to the dates on the earlier Gallo cobs as well as some of the ones from Pueblo Bonito and Chetro Ketl. Interestingly, these dates all cluster tightly around the AD 1180s. As Benson points out, this is after the major drought of the mid-twelfth-century, which is generally interpreted as marking the “collapse” of the Chaco system and the possible depopulation of Chaco Canyon. It has long been known that the canyon was occupied later, from the late twelfth century until the total abandonment of the region during the “Great Drought” of AD 1276 to 1299, but it’s unclear if the population at that time consisted of a remnant from the earlier Chacoan occupation or a reoccupation by people from elsewhere who may or may not have been descended from the earlier Chacoans. In any case, whoever the people were who lived in the canyon in the 1180s, these are their corn cobs.
They didn’t grow them, though. In what is probably the most interesting conclusion of Benson’s paper, and certainly the most surprising, he goes through a careful analysis of the strontium data, excluding the data from the unburned cobs, and finds that the values from the cobs do not overlap with any of the locations in the Chaco area, either in the canyon or around it, that have been tested. It’s certainly possible that they come from somewhere nearby that hasn’t been tested, but at this point a lot of potential growing locations in and around the canyon have been analyzed, so there aren’t a whole lot of additional options. It’s not a very promising area for agriculture, after all, and pretty much all of the obvious places have now been tested for strontium ratios.
So if these cobs didn’t come from Chaco, where were they grown? Benson compares their strontium ratios to data from several areas in and around the San Juan Basin: in addition to the newly reported Zuni sites, these include Lobo Mesa, the Red Mesa Valley, the Rio Puerco of the West, the Defiance Plateau, Chinle Wash, the Four Corners area, Mesa Verde, the Totah, and the Dinetah. This covers almost the whole area once occupied by Chacoan outliers, and several places beyond. The cob ratios turn out to overlap considerably with one of the Zuni areas, the Mesa Verde/McElmo Dome area, the Totah, the Defiance Plateau, Lobo Mesa, and the Rio Puerco valley. For some reason Benson doesn’t mention the Puerco in the text of the article, but in the figure showing the boxplots of the values for the various regions it clearly overlaps a bit with the cob values.
Unfortunately, the strontium analysis itself doesn’t provide any way to choose which of these areas is the most likely source of the corn. Any of them is consistent with the evidence. Benson therefore turns to other lines of evidence to narrow down the choice. He eliminates Lobo Mesa and the Defiance Plateau because of evidence that they were not occupied during this period; he doesn’t go into a whole lot of detail on what this evidence is, which is unfortunate. As I mentioned above, he doesn’t discuss the Puerco at all, which is also unfortunate. This leaves Zuni, the Totah, and Mesa Verde as the remaining options. These are all areas that had Chacoan outliers during the height of the Chaco system and probably experienced immigration of people from Chaco after the system’s collapse, and they were all home to significant populations during this relatively wet period, so they are all plausible sources of corn imported to Chaco. Benson concludes that the Totah is the most likely source based on the fact that it is the closest of the three areas and the one that seems to have had the strongest connections to Chaco, and while he acknowledges that this is little more than a guess, it sounds plausible enough to me. Certainly Aztec, which is often interpreted as a successor to Chaco in some sense, was a major center in the late twelfth century, as was Salmon, and the material culture of the people living in Chaco at the time shows considerable influence from areas to the north (although it’s not entirely clear how to interpret this).
This paper is part of a growing corpus of data, much of it contributed by Benson, showing that the inhabitants of Chaco at various times did in fact import corn to the canyon. This seems to largely settle one of the longstanding disputes in Chacoan archaeology, and it further points out the pointlessness of trying to estimate the population of the canyon by first estimating its agricultural potential. What remains puzzling is how this system would have worked, and why. Beyond the obvious question of who was supplying the corn, which is partially addressed in this paper, the question of what leverage the canyon inhabitants would have had to get those people to supply them remains open. This paper, in fact, seems to raise more questions than it answers in this respect. While during the height of the Chacoan system it is relatively easy to come up with theories for how the canyon inhabitants could have acquired supplies from the surrounding area, in the post-collapse period, when the canyon population was tiny and regional importance had clearly shifted elsewhere, explaining how the few people left at Chaco managed to get others to grow food for them becomes a daunting task. It’s this sort of challenge, however, that I think makes Chaco so fascinating and ensures that it will continue to be a place worth studying for a long time to come.
Benson, L. (2010). Who provided maize to Chaco Canyon after the mid-12th-century drought? Journal of Archaeological Science, 37 (3), 621-629 DOI: 10.1016/j.jas.2009.10.027