Charles D. Frederick

Investigations into the Potential Effects of Pedoturbation on Luminescence Dating

Much effort has been focussed on understanding the luminescence properties of natural minerals to achieve a reliable, accurate and precise dating technique. However, some field related aspects, such as the influence or effect of post-depositional disturbance on luminescence dates, are as yet underexplored. In the case of pedoturbation, depending on its intensity, the rate of sedimentation and unit thicknesses, potentially the whole sedimentary record at a site can be affected. This may lead to distorted OSL chronologies and erroneous sediment burial ages. Pedoturbation can result in sediment mixing and/or exhumation that affect luminescence both at the bulk and single grain level. Effects of these two principle processes on luminescence ages are examined using standard multigrain and single grain protocols. High resolution sampling of surface gopher mounds was used to determine the efficiency of bio-exhumation in resetting luminescence signal. Results show this is an inefficient mechanism for onsite sediment bleaching. The effects on luminescence signal of bio-mixing were explored by comparing a sample collected from within a krotovina (infilled burrow) to an adjacent undisturbed sample. Results show the difficulties in identifying pedoturbated samples at the single aliquot level and the possible inaccuracies in using the lowest palaeodose values to calculate OSL ages. Where pedoturbation of samples is suspected, use of probability plots of palaeodoses data is recommended. From these plots it is proposed that only data falling within a normal distribution centred on the peak probability be used to calculated OSL ages and to mitigate problems arising from pedoturbation.

Chronology, Subsistence, and the Earliest Formative of Central Tlaxcala, Mexico

We propose that pottery-using villages did not appear in the upland Apizaco region of central Tlaxcala, Mexico, until after 1000 B.C., centuries after such developments in choice locations for maize agriculture. We excavated at two of the earliest known Formative sites in the region. That work revealed abundant intact refuse deposits, allowing us to evaluate an exist ing ceramic chronology with new radiocarbon dates as well as characterize Formative subsistence. Our results support a more general model of emerging sedentism in central Mexico involving population dispersions from prime agricultural areas to zones of higher elevation. The earliest pottery-using agriculturalists in Apizaco were probably migrants from adjacent regions.

Phytoliths as indicators of prehistoric maize (Zea mays subsp. mays, Poaceae) cultivation

Maize (Zea mays L. subsp.mays) has been identified in archaeological contexts by a high proportion of large cross-shaped phytoliths. Given the numerous races of maize, this study was undertaken to determine if differences below the species level could be noted. It was also designed to see if phytoliths differed in various plant parts at various stages of growth. Several races were grown under experimental conditions. No significant differences were found. Furthermore, few phytoliths alleged to be diagnostic of maize were discovered. Systemic studies of maize and analyses of prehistoric cultivation by means of phytoliths seem not to be as promising as some researchers have argued.

Magnetic Prospection of Prehistoric Sites in an Alluvial Environment: Examples From NW and West-central Texas

AbstractPortable proton magnetometers were employed at a number of sites dating to the Archaic and Late Prehistoric periods at several different localities in NW and west-central Texas in an attempt to locate subsurface prehistoric features for excavation. Eight magnetic surveys were performed, resulting in the identification and recovery of several fired features, clusters of ceramics, and numerous metal objects of historical age. The results demonstrate that thermoremanent magnetization offered artifacts and features in this environment is often strong enough to allow their detection by magnetic prospection, and suggest that this technique is a valuable approach for maximizing data recovery during testing and mitigation of shallowly-buried alluvial sites in the region. Negative results, however, do not necessarily mean that features are absent; therefore, the method should be considered an adjunct to, rather than a substitute for, traditional testing procedures.

The chronology and collapse of pre-Aztec raised field (chinampa) agriculture in the northern Basin of Mexico

Raised field agriculture in the Basin of Mexico was a highly sustainable farming method that did not depend upon centralised political control. Study of the chinampa system around the Early and Middle Postclassic city of Xaltocan through a combination of remote sensing, GIS, targeted excavation and AMS dating has revealed an extensive area of raised fields that was abandoned when Xaltocan was conquered by an alliance of powerful neighbours during the fourteenth century AD. The rise and abandonment of the chinampa system were thus directly linked to the political economy of the city-state. The failure to revive the raised field systems in the following Aztec period can also be attributed to the impact of political, economic and ecological factors.

SWIDDEN AGRICULTURE IN THE TIERRA FRÍA ? EVIDENCE FROM SEDIMENTARY RECORDS IN TLAXCALA

Abstract Swidden agriculture in Mesoamerica is commonly associated with the hot and humid lowlands and with small isolated communities. Charcoal-rich sediments discovered in Tlaxcala, however, suggest that it was practiced in the cold highlands in the Formative and Classic periods. The headwaters of the Xilomantla drainage incised a nine-meter deep channel shortly before 200 b.c., in response to increased runoff from slopes degraded by agriculture. It was filled back within a few hundred years with sands and muds containing recurrent laminae of charred plant matter that reflect the annual burning of secondary scrub in fallowed fields. A gully in the La Ladera drainage received high inputs of charcoal from the surroundings of a nearby settlement between ca. a.d. 400 and 900. The farming practices inferred from these deposits have no exact ethnographic analog. They inflicted lasting environmental damage, but were upheld for several centuries despite changes in settlement patterns.

Proton magnetometer investigations of burned rock middens in west-central Texas: clues to formation processes

Abstract Burned rock middens are a common type of archaeological feature in Texas west of the Balcones Fault zone, and occur in several different morphologic variants with overlapping spatial and temporal ranges. Although these features are generally recognized as the remains of aboriginal food processing activities, many questions remain regarding the specifics of burned rock midden functions and formation processes. Proton magnetometer investigations of four extant burned rock middens in west-central Texas demonstrate that variations in physical midden morphology are accompanied by consistent differences in the morphology of magnetic anomalies generated by the features. It is argued that this consistent variation reflects differences in the cultural formation processes responsible for the various midden types. The nature of this variation is documented and tentative interpretations of respective formation processes are advanced for specific examples of two different morphologic types of burned rock middens.

Chinampa Cultivation in the Basin of Mexico

Chinampa agriculture is one of the best known, yet least studied forms of intensive agriculture in Latin America. Despite the voluminous literature concerning this productive means of cultivation (e.g. Rojas 1983, 1985; Jimenez-Osorino et al., 1990) archaeological investigations of chinampa agriculture are virtually nonexistent. A few researchers have examined chinampas directly (most notably Avila 1991; 1992; Parsons et al., 1982; 1985), but their emphasis was generally on settlement, rather than the fields themselves (e.g. Parsons et al., 1982; 1985; Javier 1996; Corona 1996). Other than Avila’s precocious work, large scale studies of chinampas (as opposed to chinampa settlements) do not exist. The richness of the written record concerning chinampa agriculture has probably restrained field inquiry to some extent. However, as I intend to show, existing studies rarely exploit the fertile ground that lies between the written accounts and the archaeological realities. The history of chinampa agriculture is largely unknown, and awaits focussed and detailed study. Although we think we understand how these fields were constructed and maintained and what plants were grown on them, archaeological evidence of these attributes is minimal. The data summarized in this paper indicate that chinampa construction was considerably more variable than is traditionally recognized. Furthermore, the claims extending chinampa agriculture into the period before the Postclassic are currently unsupported by direct field evidence. In essence, my point is that we know precious little about the prehistory of chinampa agriculture.

Evaluating Causality of Landscape Change

Compiling and summarizing regional and local evidence of geomorphic change are among the more common tasks of field geoarchaeologists in North America, where they have become almost routine aspects of archaeological survey. The scale of geoarchaeological projects in America is often regional, owing to a considerable number of large development projects such as reservoirs, open cast mines, and highways. In Europe, the scale of development is often (but clearly not always) of smaller scale, and it is my impression that the majority of geoarchaeological projects are performed at the scale of individual sites. The principal exceptions to this generalization are often academic surveys, which in some regions of Europe routinely incorporate geoarchaeological studies. Perhaps the best example of this may be found in Greece (e.g., Pope and van Andel, 1984; Wells et al., 1990; Zangger, 1994; Zangger et al., 1997).

Archeological Testing of TxDOT Right-of-Way through Site 41BL278, Bell County, Texas

In January 2004, Blanton & Associates, Inc., conducted an archeological survey of 8.5 hectares (21.1 acres) for a proposed roadway improvement and bridge replacement project (CSJ: 0396-04-059) along roughly 1,800 meters (m) of the Texas Department of Transportation (TxDOT) right-ofway where State Highway (SH) 317 crosses the Leon River in Bell County, Texas (Ringstaff 2004). That investigation consisted of three shovel tests, ten backhoe trenches, and five hand-excavated columns. Five backhoe trenches and the screened trench columns were excavated in the alluvial terrace on the southern side of the Leon River and encountered a single cultural component buried between 130 and 180 centimeters (cm) below surface (bs). Wood charcoal from burned rock Feature 1 yielded an accelerated mass spectrometer date of 2490 ± 50 B.P. As a result, boundaries of a previously documented prehistoric cultural resource site to the southeast, 41BL278 were extended northwestward to include the river terrace on the southern side of the Leon River where the bridge development, area of potential effect (APE) is proposed. Ringstaff (2004) recommended archeological testing/evaluation in the area of the APE at site 41BL278, if that part of the site could not be avoided by planned bridge expansion. The Texas Historical Commission concurred with this recommendation.